the mineralogy and petrology of the volcanic rocks from...

Contr. Mineral. and Petrol. 15, 24--66 (1967)

The Mineralogy and Petrology of the Volcanic Rocks from the Leucite Hills, Wyoming h ~ S. E . CARMICHAEI~

Department of Geology and Geophysics, University of California, Berkeley

Received January 24, 1967

Con ten t s

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 A. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 B. Petrographic Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1. Volcanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2. Xenoliths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 3. Igneous Rocks of Comparable Type . . . . . . . . . . . . . . . . . . . . 31

C. Analytical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 D. Mineralogy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

1. Leucite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2. Sanidine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3. Olivine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 4. Pyroxene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 5. Phlogopite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 6. Amphibole (Magnophorite) . . . . . . . . . . . . . . . . . . . . . . . . 41 7. Priderite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 8. Wadeite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 9. Apatite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

10. Perovskite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 11. Magnetite and Spinel . . . . . . . . . . . . . . . . . . . . . . . . . . 46 12. Strontiobarytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

E. Petrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 1. The Absence of Iron-Titanium Oxides and Oxygen Fugacity . . . . . . . . . . 48 2. The Alumina Deficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 49 3. Rock Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 4. Normative Alkali Metasilicate . . . . . . . . . . . . . . . . . . . . . . . 53 5. Normative Components and Synthetic Systems . . . . . . . . . . . . . . . . 54 6. Liquidus Temperatures at 1 Atmosphere . . . . . . . . . . . . . . . . . . . 57

F. Petrogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 G. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Abstract. Examples of the three volcanic rock types, wyomingite, orendite and madupite from the Leucite Hills have been examined with the electron microprobe. The results show tha t leucite is non-stoichiometric as predicted by CROSS (1897), having an excess of potassium and silicon, and tha t the only feldspar found, a sanidine, contains up to 18 percent of the iron-feldspar molecule. The co-existing pMogopite, diopside and olivine together with the groundmass amphibole arc all highly magnesian. Of the varied accessory minerals, priderite (K2TisO~8) and wadeite (K4Zr2SisOls) have been identified and analyzed together with ubiquitous apatite and perovskite, both of which contain rare earths in abundance. Com- parative mineralogical data has been obtained on a few representative specimens from West Australia and on the jumillite from Spain.

Volcanic l~ocks from the Leucite Hills, Wyoming 25

The new rock analyses t~gether wi~h the existing da~a h'om ghe Leucite Hills show the rare but characteristic molecular excess of potassium over aluminium; this excess is considered to account for the absence of the iron-titanium oxides in the orendites and wyomingites, and of course for the unusual composition and species of the minerals. ExpIoratory melting experiments show that these potassic lavas have a comparable melting range to magnesian basalts, and a crustal origin is thereby considered precluded. There is no evidence that sialic contamination contributed notably to the composition of the oversaturated orendites and wyomingites, and their relationship by any process of crystal fractionation to the undersaturated madupite is obscure. The generation of madupite could be achieved by crystal fractionation at high pressure of a liquid derived by partial fusion of mantle material.

A. Introduction The occurrence of po tass ium-r ich lavas and plugs in the Leuei te Hills , Wyoming , has been known since before the t u rn of the cen tury , bu t a p a r t f rom the extens ive s t u d y by Cxoss (1897), there has been no recent mineralogical examina t ion of these rocks of such unusua l and ex t reme composi t ion. Cross ( o p cit.) recognized three ma in rock types in the Leuci te Hil ls volcanic assemblage: wyomingite composed of phenocrys t s of phlogopi te in a g roundmass of leueite, diopside, apa t i t e and poss ibly glass; orendite which again has phlogopi te phenocrys ts , bu t a g roundmass of sanidine and leucite, toge ther wi th diopside and apa t i t e ; and madupite with diopside phenocrys t s enclosed b y ragged poiki l i t ic crys ta ls of phlogopi tc , the whole set in a mierocrys ta l l ine and t u r b i d groundmass of diopside, apa t i t e and leucite. I n the W e s t K i m b e r l e y division of Wes te rn Aust ra l ia , there are volcanic plugs and small in t rus iens which have a general s imi la r i ty to those of the Leuci te Hil ls ; these Aus t r a l i an rocks and thei r minerals have been ex tens ive ly s tud ied by WAD~ and Pa ID~a (1940), PaIDER (1939, 1960, 1965), NORaISH (1951) and HE,SHAW (1955). All the Wes te rn Aus t ra l i an rocks are sanidine-free, and in t e rms of C~oss ' s classif icat ion are wyomingi tes ; the i r composi t ion is so ex t reme miner- a logical ly t h a t four new rock types , wolgidi te , f i tzroyi te , cedrici te, and mamil l i t e have been recognized.

Through the kindness of Dr. S. O. AaRELL Of Cambridge University, the writer was tent a suite of specimens collected by him from the Leueite Hills. Professor F. J. Tvm~ER kindly made available some chips of specimens from Western Australia (supplied to him by Professor I~. T. P~IDE~) for a comparative study. Although these two localities provide the only examples known to the writer of highly potassie volcanic rocks, the coarse-grained jumillitic lava of Spain (0 sA~ , 1906) is petrographicMly similar, and several specimens were collected for ~he writer by Dr. I. L. G[Bso~. ~any of these rocks are very fine grained, especially those from the Leueite Hi[[s, Wyoming, and as inclusions of one mineral in another are characteristic of the majority of all these specimens, the electron-probe is an ideal weapon to study their mineralogy.

B. Petrographic Summary

1. Volcanics

Descr ip t ions of the p e t r o g r a p h y of the rocks f rom the Leuci te Hil ls have been given b y CRoss (1897), K~MP (1897) and KEMP and KNIGHT (1903), SO t h a t only a brief general ized descr ip t ion of the ma jo r rock types and the xenol i ths is given here. I n all the Leuei te Hills rocks, phlogopi tc is an ubiqui tous phenocrys t , and ~o t u n c o m m o n l y m a y also be p resen t as a clot of crys ta ls forming a reac t ion

26 I ~ S. E. CA~MIOnA~L:

rim around olivine or spinel phenocrysts (Fig. 1). Diopside is rare as a phenoeryst except in the madupites, but it is a common groundmass constituent in all the Leucite Hills volcanics. In the wyomingites, the groundmass is predominantly made up of small leucite crystals (Fig. 2), with interstitial amphibole (magnophorite), diopside, apati te and rare glass which may be replaced by chlorite. One of the wyomingites (LI-I. 7)

Fig. 1. Olivine with a reaction rim of phlogopite in a very fine grained groundmass of leucite (wyomingite LH. 3). Abundant phenocrysts of phlogopite with fewer and smaller diopside phenocrysts. Magnification • 39

has a very extensive fresh dark glassy groundmass which surrounds phenocrysts of phlogopite, diopside and apatite together with microphenocrysts of leucite. The glass contains numerous clusters of diopside microlites (Fig. 3), which are also concentrated around the margins of the phlogopite phenoerysts. Many of the wyomingite lavas have an extremely fine-grMned vesicular groundmass (LH. 3, LH. 4 and LH. 6) which contains very little glass. The orendites show much more variety in their petrography than the wyomingites, having a groundmass of sanidine and leueite, with sometimes one and sometimes

Volcanic Rocks from the Leucite Hills, Wyoming 27

the other predominating and crystallizing earlier. Some orendites are interbanded with thin layers of wyomingite (LH. 14) and thereby provide an unsolved problem of their origin. Diopside, magnophorite, apatite and rare glass or its alteration product are the other dominant groundmass constituents in the orendites. A most striking coarse-grained orendite with sanidine phenoerysts has been described by C~oss (op. cir.) (Fig. 4) and analyzed by HmLEB~A~D; the writer is grateful

Fig. 2. Wyomingite (LtI. 1) with abundant leucite crystals, interstitial amphibole (magnophorite) and micro-phenocrysts of diopside, apatite and phlogopite. Magnification X 86

to Professor O. F. TUTTLE for a small chip of this analyzed specimen (LI-I. 9). Two orendites have small scattered olivine phenoerysts without any reaction rim of phlogopite (Table 1, LH. 10 and LH. 11); these olivine orendites are new to the described volcanic assemblage of the Leucite Hills (C~oss, op. eit.). The Wyoming madupites are very distinctive and have no counterpart in the west Australian assemblages. They contain abundant small phenoerysts of diopside, often flow aligned, some of which are enclosed in large poikilitie crystals of phlogopite. The crystals of phlogopite are characteristically, but not invariably, irregular in outline, and are set in a turbid groundmass of diopside, chlorite and

28 IA~ S. E. CARMICIIAEL:

glass together with apati te and accessories. This diagnostic texture (Fig. 5) of phlogopite enclosing diopside, together with small crystals of magnetite and perovskite is identical to tha t found in the Arkansas kimberlite; olivine or its pseudomorph has never been found in a madupite however. The accessory minerals found in the Leucite Hills voleanics are varied in amount and unusual in type. Spinel (chrome-bearing), with a reaction rim of phlogopite,

Fig. 3. Glassy wyomingite (LH. 7) showing leucite with fluid inclusions and smaller crystals of diopside in a glassy (dark grey) groundmass. Phlogopite phenocrysts (elongate) have myriads of small diopside crystals attached to their margins. Magnification X 86

is common; priderite in small intensely pleochroic aeicu]ar crystals may show extreme variation in abundance, being either virtually absent or comparatively common. Wadeite is always associated with or moulded onto apati te and has been seen in almost all of the Leucitc Hills specimens. Iron-t i tanium oxides are very rare; ilmenite has never been found, but magnetite is fairly abundant in the madupite (LH. 16) and is absent in all the wyomingites and orendites. Perovskite has been found in a part ly glassy leucite-madupite (LH. 8) and madupite (LH. 16). Other


accessories of sporadic occurrence are chalcedony, carbonate, chlorite, zeolites, and strontiobarytes.

2. Xenoliths

These are of two kinds, foreign and cognate. Although xenoliths are not frequent (CRoss, 1897), they may have considerable relevance to the origin or development

Fig. 4. Orendite (LH. 9) with large cystals of iron-sanidine containing numerous small diopside crystals. Magnophori~e (medium grey) is intcrstitiM to the sanidine, and encloses numerous leucite erysta.ls in the centre of the field. Small black acicular crystals are priderite. Magnifica- tion • 86

of these volcanic rocks. Granitic xenoliths are relatively common, and are typi- cally partially fused and friable. Biotite is completely replaced by trails of iron- oxides, and brownish glass (often devitrified) is formed at the boundaries of quartz and feldspar. The response of the host rock to the incorporation of a partly fused xenolith is variable; adjacent to an outer rim of glass, a wyomingite has developed a coarse-grained facies of orendite in a narrow zone, but in other examples of granitic inclusions there i s n o reaction or change in the mineralogy

30 IA~ S. E. C~IC~A~L:

of the host wyomingite. Xenoliths of partially fused arkose (with abundant introduced carbonate), tuffaceous sandstone, argillite and siltstone have been found by Dr. S. O. AO~ELL. Xenoliths of a gabbro accumulate are also present. One variety is an anorthosite with each of the plagioelase crystals (Ans0.gAbls.~Or0.5; microprobe analysis) showing reaction at the margins, and presenting a dusty appearance. A pyroxene-

Fig. 5. Madupite (LH. 16) with abundant mierophenoerysts of diopside together with irregular phlogopite crystals (light patches). Small black crystals are magnetite and perovskite. The matrix between the phlogopite crystals is of turbid, partially ehloritic material. Magnification ?<39

bearing variety has brown glass as stringers and pools in the plagioclase component, the ferromagnesian minerals (pyroxene and olivine) being extensively recrystallized to iron-oxide, amphibole and glass. Xenoerysts are quite common; a dusty augite is the most frequent and is easily identified by its sieve texture. A recrystallized barkevikitic amphibole and orthoelase or microcline made over to potassic sanidine are rarely found, the amphibole having been seen only once.

Volcanic t~ocks from the Leucite Hills, Wyoming 31

Cognate xenoliths are coarse-grained intergrowths of phlogopite, diopsidc, apatite, priderite and magnophorite with a rather curious ocelli texture. These have a polygonal form due to the phlogopite crystals being arranged tangentially, and the phlogopite shows a dark red-brown outer zone only on the side adjacent to the ocelli. The filling is composed of chlorite (glass ?), zeolite, amphibole (magnophorite), and sanidine (Fig. 6).

Fig. 6. A cognate xenolith of phlogopite, diopside, apatite and magnophorite. Ocellus composed of chlorite, apatite, sanidine, zeolites and magnophorite (dark rims) with phlogopite crystals arranged tangentially. Phlogopite shows iron-rich rims on the edges fucing the ocellus. Magnification •

Cognate xenocrysts are olivine with a reaction rim of phlogopite, chromite with a similar reaction rim, and a green spinel with no evidence of reaction; this last is very rare.

3. Igneous Rocks o/ Comparable Type The Spanish jumillite is essentially a coarse-grained orendite, with abundant phenocrysts of olivine, red-brown biotite, and diopside (Table 1), the last named

32 h ~ S. E. CAR3II(JI~IAEL:

commonly being enclosed in ve ry large anhedra l crysta ls of sanidine. Magno- phori te , leucite rep laced b y a t u r b i d a l t e ra t ion produc t , carbonate , wadei te , apa t i t e , pr ider i te , and poss ibly magne t i t e are accessory. The Aus t ra l i an examples are all var ie t ies of wyoming i te insofar as sanidine is absen t (WADE and PRIDIng, 1940), bu t t h e y are more ex t reme in mineralogical composi t ion and typ i ca l ly coarser gra ined t han the Leuci te Hil ls voleanies. F ive Aus t ra l i an specimens have been s tudied, a f i tz royi te (phlogopi te-a l te red leucite assemblage), a eedriei te (diopside-al tered leucite), two wolgidi tes (phlogopite- d iops ide-magnophor i te -Mtered leucite) and a wyomingi te ; this sui te contains a va r i ed assemblage of accessory minera ls (PRIDE~, 1939, 1965; NORRIS,, 1951) similar to the Leuei te Hil ls volcanics, bu t m a n y are absen t f rom the wr i te r ' s specimens.

Key to Specimen Localities

Leucite Hills, Wyoming, U.S.A. LH. 1 Wyomingite, margin of dyke, Boars Tusk A. 1741 LH.2 Wyomingite, block in agglomerate, Boars Tusk A. 1745 LH.3 Wyomingite, South Table Mountain A. 1759 LH. 4 Wyomingite, vesicular flow A. 1768 LH. 5 Wyomingite, basal flow, Zirkel Mesa A. 1791 LH. 6 Wyomingite, North Table Mountain A. 1771 LH. 7 Wyomingite, partly glassy lava, Steamboat Springs A. 128 LH. 8 Leucite-madupite, partly glassy lava C. 196 LH.9 Orendite, North Table Butte, Cross 1897, p. 130, No. 6

U.S.G.S. Petrographic Collection, No. 570 LH. 10 Olivine-orendite, South Table Mountain A. 1758 LH. 11 Olivine-orendite, South Table Mountain A. 1762 LH. 12 Orendite, North Table Mountain A. 1770 LH. 13 0rendite, basal flow, Orenda Butte A. 1773 LH. 14 Orendite-wyomingite interbanded, North Table Mountain A. 1766 LH. 15 Orendite, North Table Mountain A. 1764 LH. 16 Madupite, west side Pilot Butte, I~oek Springs A. 1805

West Kimberley Australia WK. 1 Fitzroyite, Howe's Hill WK. 2 Cedrieite, top of Maehells pyramid WK. 3 Wolgidite, upper flow, Mr. North WK. 4 Wolgidite, fine-grained, 'P' Hill WK. 5 Wyomingite, 'P' Hill

156-I-2" 156-I-6" 156-I 7* 156-I-9" 156-I-10"

S. 1 Jumillite lava, Mureia, Spain J-14

* These specimens are in the petrographic collections at Berkeley.

Table 1. Modal analyses in volume percent

Phlogopite Olivine Diopside Sanidine Leucite Accessories Groundmass

S-1 9.8 17.0 14.2 22.7 34.0 2.3 - - LH. 3 20.1 trace . . . . 79.9 LH. 10 16.1 3.9 . . . . 80.0 LH. 11 14.3 2.3 . . . . 83.4


The key to the specimen localities is given on page 32; the prefix LH referring to the Leucite Hills, W y o m i n g ; W K to West K imber l ey ; Wes te rn Austraha, and S to the Spanish jumill i te . Modal analyses of a few coarse-grained rocks are given

in Table 1.

C. Analytical Methods Polished thin sections were made of all the specimens and the analyses of the individual minerals were made with an ARL electron microprobe at the University of Chicago and at the University of California at Berkeley. The extensive collection of analysed minerals acquired by Professor J. V. S~XrTH were used to provide standards close to the composition of the unknown feldspars, olivines, and pyroxenes, and so minimized corrections for atomic number, mass absorption, etc. No analyzed leucites were available as standards and the leucite analyses reported here were made using feldspar standards; the sample current was kept below the limit at which volatilization of alkali could be detected. The phlogopite analyses were made using synthetic fluorphlogopite, biotite and olivine standards of similar composition or atomic number. The precision of the determinations of the major elements in the above minerals is approximately ~: 2% of the amount present; the precision will drop for the trace or minor elements to perhaps ~ 10% for Ni and Na. The accessory minerals including amphibole were analyzed by using standards as near as possible in composition and atomic number to the unknowns; in many cases this ideal is just a wishful pipedream. Analyzed pyroxenes, feldspars, biotites, futile, zircon (of unknown Hf content), benitoite, haematite, ilmenite and chromite were used for the analysis of amphibole, perovskite, magnetite, apatite, wadeite and priderite and all correction factors were used in the computations (e.g., mass absorption, atomic number, fluorescence). The determination of the rare earths in apatite and perovskite is very crude; unlike all the other determinations in which the intensity was integrated on sealers, tile samples were scanned and the peak heights were used to estimate concentration by comparison with the peak heights of BaL~ in different concentrations. No minerals with precisely determined concentrations of rare earths (a very difficult job) were available as standards, and so this approximate method was used; the precision is unlikely to be better than :~ 20 %, especially for those elements considerably displaced from :Ba. With the exception of iron in feldspar and leucite, all iron determinations are reported as FeO. Between 10 and 20 grains of each mineral were analyzed whenever possible; for some of the accessory minerals this was not possible, and the values recorded in the tables represent the mean of only 6 or so grains. The rocks were analyzed by the writer using conventional wet methods, except that all the separations were made more lengthy and involved because of the presence of barytes in the specimens. A1203was estimated directly with EDTA after separation of Fe, Ti, Zr, etc., with cupferron. Zr, Sr, Ba, La, u and Rb were determined by X-ray fluorescence by Mr. R. N. JAck: using natural rock standards. Checks by wet techniques of Ba and Zr showed satisfactory correspondence.

D. Mineralogy

1. Leucite

I n the orendites and wyomingi tes from the Leucite Hills, leucite is f requent ly the most a b u n d a n t mineral and is found as small anhedral clear crystals with var ied amount s of fluid inclusions; these are often arranged in a quasi-radial or ienta t ion (Fig. 1). The leucites are u n t w i n n e d and completely isotropic, in contras t to the only fresh leueite crystal found in the Aust ra l ian rocks, which is twinned, weakly anisotropic, and free of fluid inclusions. I t is considered t ha t the isotropie leucites of the Leucite Hills volcanics were quenched above the leucite cubic- tc tragonal inversion tempera ture (FAUST, 1963; circa 600~

8 Contr. Mineral. and Petrol., Vol. 15

34 IA~ S. E. CAR~CHA~L:

whereas the Australian leucite has inverted. Characteristically the Australian rocks and the Spanish jumillite have leucite pseudomorphed by turbid material of siliceous composition (PRI~)Ea and COLE, 1942) and apart from the one crystal noted above, no fresh leucite has been found in these rocks. In the Wyoming orendites, where leucite coexists with sanidine, the relative order of crystallization is variable; sometimes leucite was earlier and sometimes sanidine, and the relative proportions may also vary widely, the extreme being interbanded wyomingite (not entirely feldspar-free) and orendite with perhaps several bands to the inch. Leucite or its pseudomorph may be enclosed by phlogopite phenocrysts and this is well displayed in the Spanish jumillite. No zoning has been detected in the leucites although their size is almost too small for the electron beam to reliably detect this. The microprobe analyses of leueite are given in Table 2 and the totals indicate that the analyses are satisfactory. Compared to the analyses of leueite tabulated by DEER et al. (1963) these leueites contain more Fe and less Ca. However the analyses are striking in another respect, for with the exception of the twinned

Table 2. Analyses o/ leucites. (For lcey to specimens, see page 32)

WK.2 LH. 1 LIt.2 LH.5 LH.7 LH.9 Ltt. 13

SiO 2 54.7 56.2 56.4 57.3 57.4 58.6 56.3 AleO a 22.4 20.3 20.2 19.5 19.7 18.7 20.3 F%03 1.0 2.1 2.3 2.2 2.2 2.2 2.0 CaO 0.01 0.03 0.02 0.03 0.02 0.04 0.03 BaO * * * * * * * Na~0 0.01 0.02 0.04 0.03 0.04 0.06 0.03 K~O 21.5 21.1 21.0 21.5 20.7 20.7 21.5

Total 99.7 99.8 100.0 100.6 100.1 100.3 100.2

* BaO below limit of detection, (0.02 %).

Analyses recalculated in weight percent

Kp 69.5 63.0 62.7 60.5 60.9 58.0 63.0 FeKp 2.4 4.9 5.4 5.2 5.2 5.2 4.7 Ne trace 0.1 0.2 0.1 0.1 0.3 0.1 Qz 27.3 29.3 29.5 29.8 30.9 32.0 28.8 K2S 0.5 2.6 2.4 5.1 2.9 5.1 3.7 Kp, I(~A1Si04; FeKp, KFe:nSi04; ~e, NaA1Si04; Qz, SiO~; K2S, K~Si205

leucite from Australia (Table 2, WK. 2), all the analyzed leucites are non-stoichiometric as they contain a molecular excess of alkali and silica over alumina and ferric iron. In order to demonstrate this, the analyses of the leueites have been recalculated into normative eompoments; the components are kaliophilite (KAISiO4) , iron kaliophilite (KFe'"Si04) , nepheline (NaAISiO4) and the excess potassium has been arbitrarily combined with silica to form potassium disilicate (K2S, Table 2), which SC~AInER and BOWE~ (1955) have shown may coexist with leucite in the system K20-A1203-SiO 2. The excess silica after forming these molecules has been calculated as quartz.


The recalculated analyses in Table 2 show tha t the Wyoming teueites may contain up to 5% of potassium disilicate, and as there is an excess of silica over tha t required to form the leucite-"jadeite" molecules, these leucites tend to approach feldspar composition (Fig. 7). FUDALI (1963) has been able to synthesise leucite with about 8 weight percent of KA1Si30 s in solid solution, presumably at 1,000 bars water vapor pressure, and above the leucite inversion temperature. I t was considered initially in this study that this excess of silica and alkali may have been the result of the electron beam impinging on the small fluid includions in the leucites; however in those leucites where the inclusions are few or absent, the results are identical to those which contain abundant inclusions, and this excess is accounted real. These results substantiate the conclusion of Cross that leucite in the Wyoming rocks contains an excess of silica and may approach feldspar in composition (Cgoss, 1897, p. 132).

"V,, V V , V V V V \

wt% Kp Fig. 7. The recMculatcd analyses of leucite and sanidine (Tables 2 and 3) are plotted in terms of the three components Si02, Kp(KA1SiO4), and KFe"'SiO 4. The feldspars plot on the join Or(KA1Si~Os)-FeOr(KFe'SiaOs) whereas the leucites from the Leucite Hills plot between this join and the join Lc(KA1Si206) and FeLc(KFeH'Si206). Two pairs of co-existing sanidine and leucite are connected by tie-lines. The leucito from West Australia (Table 2, WK. 2) is represented by an open circle

Only one leucite (WK. 2, Table 2; Fig. 7) is stoiehiometric within the limits of analytical error, and this analysis is in accord with those tabulated by D ~ R et al. (1963) for large, probably twinned, leucite phenocrysts. There may be shades of the nepheline story in the composition, inversion and possible reerystalli- sation of leucite. In the lencite analyses (Table 2) Na20 and CaO are only present in very low concentrations; BaO is even more efficiently excluded from the leucite structure, but unlike Na20 and CaO it is relatively abundant in the co-existing sanidine.

2. Sanidine

The only feldspar found in the Wyoming orendites and the Spanish jumillite is sanidine; feldspar is absent from the Wyoming madupites and wyomingites and also from the Australian specimens. Sanidine forms very pale yellow water- clear crystals, is very variable in size and often contains abundant inclusions of diopside, less apati te and sometimes leucite. In one orendite (LH. 9) sanidine is present as square euhedral phenocrysts with little detectable zoning, and in the jumillite it forms extremely large crystals poikilitically enclosing myriads of diopside crystals. Sanidine in all the rocks shows great reluctance to twin, and the cleavage is unusually badly developed; however a determination of the orientation of the

3*

36 IAN S. E. C~mc~A]~L:

opt ic ax ia l p lane in one c rys ta l (LH. 9) b y Professor F. J . T u ~ showed t h a t i t is para l le l to (010) and therefore the sanidine approaches the h igh-sanidine modi f ica t ion (CooMBs, 1954). The microprobe analyses of sanidine are given in Table 3; wi th the except ion of the sanidine f rom the Spanish jumil l i te , all the fe ldspars are soda-poor i ron-r ich sanidines. The analyses have been reca lcu la ted into fe ldspar molecules, and as

Table 3. Analyses ol ]eldspars (]or ]cey to specimens, see page 32)

S. 1 LtL9 LH. 10 LH. 11 LH. 12 LH.13 LIt. 14 LH. 15

SiO 2 63.9 64.1 - - 64.1 64.3 - - - - - - A120 a 16.7 15.6 - - 14.2 16.6 - - - - - - F%O a 2.5 3.0 4.2 4.7 2.7 4.8 3.3 4.0 CaO 0.01 0.07 0.10 0.08 0.06 0.07 0.08 0.11 BaO 0.96 0.60 0.55 0.66 0.79 1.01 0.61 0.66 SrO 0.06 . . . . . . . •aeO 1.04 0.19 �9 0.18 0.14 0.19 0.17 0.32 0.32 K20 14.9 16.2 16.1 15.9 16.3 15.9 16.1 16.0

Total 100.1 99.8 99.8 100.9

Analyses recalculated to weight percent/eldspar molecules

Or 79.2 85.1 80.7 78.6 86.0 77.5 82.7 80.0 Fe0r 9.6 11.5 16.0 18.3 10.3 18.4 12.8 15.2 Ab 8.8 1.6 1.5 1.2 1.6 1.4 2.7 2.7 Cn 2.2 1.4 1.3 1.5 1.8 2.4 1.4 1.6 An - - 0.4 0.5 0.4 0.3 0.3 0.4 0.5 Sf 0.2 . . . . . . . Or, KAlSiaOs; FeOr, KFelllSiaOs; Ab, NaAlSiaOs; Cn, BaAI~Si~Os; An, CaAl~Si~Os; Sf, SrAleSi~Os.

some of t h e m conta in up to 18% of the K F e ' " S i a O s molecule, t h e y are poss ib ly the mos t i ron-r ich fe ldspars found in na tu r e (Fig. 8) ( C o o ~ s , 1954); th is is a surpr is ing paragenesis as orendi tes in common wi th all the o ther potass ic voleanics are i ron-poor magnes ia- r ich rocks (Table 12). I t is pe rhaps unusua l to con t ras t the i ron-enr ichment (Fe'"/A1) of co-exist ing leuci te and sanidine, b u t as m a y be seen in Fig. 7, the sanidine t ends to be enr iched in i ron in each of the two ana lysed pairs. Only small amoun t s of CaO are p resen t in the sanidine analyses , and therefore calc ium is v i r t ua l l y comple te ly exc luded f rom salic minera ls in these h igh ly potass ic voleanics; however BaO is a s ignif icant componen t in all the ana lysed feldspars.

3. Olivine

There are two d is t inc t modes of occurrence of olivine in the volcanics f rom the Leuci te Hil ls ; mos t commonly i t is pa r t i a l l y resorbed, the e m b a y e d crys ta ls being en t i re ly su r rounded b y a reac t ion r im of phlogopi te (Fig. 1), b u t in two rocks, L H . 10 and LH. 11, olivine occurs as small sca t t e red r ed - r immed phenocrys t s


(Table l) with no petrographic evidence of instability. These olivine-orendites are accordingly new to the described assemblages of the Leucite Hills. I n the Spanish jumillite, large fresh phenocrysts of olivine are enclosed by sanidine or phlogopite, bu t in areas of turbid pseudomorphed leucite, olivine is altered to bowlingite. Par t ia l microprobe analyses of the olivines are given in Table 4; the analyses recalculated into the various olivine molecules approximate to 100 and indicate

t ha t the analyses are satisfactory.

Table 4. Analyses o/ olivines (/or key to specimens, see page 32)

S.1 LH.3 LIt. 10 LH. l l LH. 15

TiO~ - - - - - - 0.02 - - FeO 9.7 7.2 7.8 8.5 6.5 MnO 0.21 0.18 0.18 0.18 0.08 MgO 48.7 51.0 50.3 50.0 51.9 NiO 0.44 0.26 0.22 0.20 0.32 CaO 0.18 0.03 0.16 0.18 0.02

Analyses recalculated to olivine molecules (wt. percent) Fo 85.0 89.0 87.8 87.2 90.6 Fa 13.8 10.2 11.1 12.0 9.2 La 0.3 0.04 0.2 0.3 0.03 Tp 0.3 0.2 0.2 0.2 0.11 NiO1 0.6 0.4 0.3 0.3 0.4

Total 100.0

Zoning Fa 12.3 to 14.8

99.8 99.6 100.0 100.4

nil * Fa Fa Fa 10.5 11.1 8.3 to to to 12.7 12.7 10.2"

Fo, Mg2SiO4; Fa, Fe2SiO~; La, Ca2Si04; Tp, Mn2Si04; NiO1, Ni2SiO 4.

* Reaction rim of phlogopite.

Ab

Zoning in all the olivines is l imited in amoun t to perhaps 3 % of fayalite (Table 4); they are all relatively iron- poor nickel-rich olivines. Of the minor elements present in olivine, CaO has been suggested by

FeOr

2O

Or 20 70

Fig. 8. The recalculated analyses of sanidine (Table 3) from the Leucite Hills (filled circles) represented in terms of 0r(KA1Si3Os) , Ab(NaA1SisOs) and FeOr(KFe'"Si~Os). Small open circles are iron-orthoclases taken from CooM]3s (1954); the large open circle is the jumillitie feldspar (Table 3, S. 1)

SIMKIN and S~IT~ (1966) to be indicative of the environment in which the olivine crystallized. They noted tha t a lmost all olivines f rom plutonic rocks contain less t han 0.14 weight percent CaO, whereas olivines f rom volcanic rocks almost always exceed this value. Their suggestion is supported by WmT~ ' s (1966) da ta on the varied ultramafie inclusions found in the Hawai ian lavas ; amongst these, only the lherzolites (ol-cpx-opx) are considered by him to have an unequivocal deep seated origin. The CaO contents of the analysed lherzolitie olivines have been plot ted in Fig. 9 and fall within the limits suggested by SIMKIN and S~IT~. I n the potassie volcanies (Table 4), the two olivines with low CaO (l~ig. 9) have extensive phlogopite reaction rims, while the olivine phenoerysts in the olivine-orendites and in the Spanish jumillite have higher CaO and are assumed to have crystallized in a volcanic regimen (Fig. 9).

38 IA~ S. E. C ~ I C ~ , L :

A similar approach has been taken by FoR~ES and KUNO (1965) and FORBES and BANNO (1966) but their data for the distribution of lqi in olivines in relation to environment seem less convincing.

4. Pyroxene

Small crystals of pyroxene are present in all the Leucite Hills volcanics, in the Spanish jumillite and in many of the Australian rocks. I t is characteristically colorless, although it may have a delicate green tint; zoning is restricted, the crystals being only slightly more iron-rich at their margins. The only occurrence of extensive zoning is in the leucite-madupite (LH. 8) where the pyroxene is zoned towards acmite composition in zeolite filled vesicles.

Ca0

o,2 2z ~ o

0 / �9 I �9 �9

~5 3 . , - o I " * " ; "

O~ ' ' " / 0 ' " I . , 7 8 9 I I 12 13 IZ~ 15 15' Fe W~' % FeO Mfi A/am %

Fig. 9 Fig. 10 Fig. 9. Weight percent CaO of olivines plotted against FeO. Filled circles represent olivines from lherzolites (WroTh, 1966) ; the dashed line represents the upper limit of CaO in plutonie olivines (SI~IN and SMI~, 1966). Open circles are olivines taken from Table 4 Fig. 10. Diopside analyses (Table 5) plotted (filled circles) in terms of Ca, Mg, and Fe + Mn. The magnesian pyroxenes of the Skaergaard intrusion (BRowz% 1957) and their trend are also shown

Only one pyroxene has been found in all the investigated specimens, namely a diopside which in the Leucite Hills specimens is rarely twinned and has an unusual elongate habit. Diopside is rarely found as a phenocryst, except in the madupites and the Australian rocks, and is commonly enclosed by phlogopite or sanidine in the jumillite. The microprobe analyses of the diopsides are given in Table 5, together with a conventional analysis of diopside from the Leucite Hills taken from Cgoss (1897)�9 Both the totals of the analyses and the calculated formulae indicate tha t the microprobe analyses are satifactsory. The diopside analyses have been plotted in Fig. 10 and fall very near the diopside composition corner, an unusual field for igneous groundmass pyroxenes. The data compiled by LE BAs (1962) show that in groundmass pyroxenes which tend to approach these in gross composition (Ca, Mg, Fe ~- Mn), namely those from alkaline and peralkaline environments, there is considerable substitution of A1 for Si in Z-sites amounting to perhaps 10 atomic percent or over, in contrast to the average of 0.4 percent found in these diopsides (Table 5). In practically every diopside analysis (Table 5) there is insufficient A1 to make the tetrahedral sites up to 2, and accordingly small

T~

ble

5. A

naly

ses

and/

o~w

~ula

e (o

n th

e ba

sis

o/6

oxyg

ens)

o/p

yrox

enes

. (F

or k

ey to

spe

cim

ens,

see

pag

e 32

)

S. 1

W

K.2

W

K,3

W

K.4

W

K.5

L

It.

1 L

H,2

L

H.3

L

H,5

L

H,

13

LH

. 16

A

.

SiO

2

54.6

54

.5

54.2

54

.2

54,2

54

.3

55,1

54

.4

54.6

54

.1

54.5

50

.86

~'

TiO

2

0.38

0.

93

0.97

0.

97

0.93

0.

62

0.45

0.

46

0.62

0.

63

0.65

3.

03

7V

AI~

O a

0.36

0.

30

0.11

0.

17

0.13

0.

19

0.20

0.

17

0.19

0.

23

0.76

ni

l o ~

Cr~

O 3

0.67

0.

35

0.13

0.

69

0.56

0.

03

0.04

--

--

0.

02

--

1.1

9(F

%O

3)

FeO

2.

4 2.

0 2.

7 2.

2 2.

2 3.

3 2.

9 3.

2 2.

3 3.

0 2.

1 1.

82

~'Ia

O

0.09

0.

05

0.06

0.

06

0.06

0.

09

0.09

0.

09

0.08

0.

08

0.09

0.

03

)ag

O

17.8

17

.7

17.6

17

.8

17.8

16

.9

17.2

18

.0

18.1

17

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17.8

17

.42

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tr

ace

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06

0.05

0.

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0.02

--

--

0.

03

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--

CaO

23

.7

24.8

23

,6

23.9

23

.8

23,9

24

.2

23.3

23

.9

24.2

24

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23.3

2 N

a~O

0.

20

0.09

0,

18

0,26

0.

22

0,12

0.

15

0.16

0,

12

0,16

0.

24

0.76

To

tal

100,

3 *

100.

8 99

.6

100.

3 10

0.0

99.5

10

0.4

99.8

99

.9

99.6

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6 **

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�9 *

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es 0

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47.0

�9

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49.1

F

e -~

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3.

9 S

i 1.

980

Z

A1

0.01

5 T

i 0.

005

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--

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0.00

5 C

r 0.

019

Fe

0,07

5 W

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n 0

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g 0.

962

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- C

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Y

1.99

9 A

48.6

47

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47.4

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47.8

48

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45.8

47

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48.0

48

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48.2

48

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49.0

49

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46.9

47

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49.1

49

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47.2

48

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3.2

4.3

3.6

3.6

5.3

4.6

5.1

3.5

4.8

3.4

1.96

8 1.

981

1.96

7 1.

973

1.99

1 1,

997

1.98

4 1.

984

1.98

2 1.

965

0.01

2 0.

005

0.00

7 0.

006

0.00

8 0.

003

0.00

7 0.

008

0,01

0 0.

032

0.02

0 0,

014

0,02

6 0,

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0.00

1 --

0.

009

0.00

8 0,

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0.00

3 .

..

..

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..

..

0,

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0.00

4 0.

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0,90

9 0.

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1 --

0,

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0.08

2 0,

067

0.06

7 0.

101

0.08

8 0.

098

0.06

7 0,

092

0.06

3 0.

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0.00

2 0.

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0.00

2 0.

003

0.00

3 0.

003

0,00

2 0.

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3 0.

953

0.95

8 0.

963

0.96

6 0.

923

0,92

9 0.

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0.98

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0.95

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001

0,00

1 0.

001

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--

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1 --

0.

960

0.92

4 0.

930

0.92

8 0.

939

0.94

0 0.

911

0.93

0 0.

950

0.95

4 0.

006

0.01

3 0.

018

0,01

6 0.

008

0.01

0 0.

011

0.00

8 0.

011

0.01

7 2.

000

2.00

0 2.

000

2.00

0 2.

000

2.00

0 2.

000

2.00

0 2.

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2.00

0 1.

997

1.99

5 2.

001

2.00

1 1.

992

1.99

0 2.

005

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6 2.

000

2.00

9 D

iolo

side

fro

m w

yo

min

git

e an

d m

adu

pit

e (C

Ros

s, 1

897)

40 IA~ S. E. C~a~MICHA~L:

amounts of Ti or Fe ' " (undetermined) are likely to be in fourfold coordination. Although the pyn'oxenes from all the investigated rocks have a generally similar composition, those from the Australian specimens have higher Ti, Cr and possibly Ni than those from the Leucite Hills. K is below the limit of detection (0.02) in all the pyroxenes. K v s ~ m o (1960), L~ BAS (1962) and V~HooGv, x (1962a) have all discussed the role o~ minor elements in calcium-rich pyroxenes, particularly Al and Ti, and have shown tha t Ti increases concomittantly with the amount of A1 tha t can substitute for Si in Z-sites, which in turn is favored by a low concentration of silica in the liquid. I t is not surprising therefore tha t despite the high Ca content of the analysed pyroxenes (Table 5), A1 in fourfold coordination is low, for apart from the madupite and the Spanish jumillite, the silica contents of these rocks are all above 50 weight percent (Table 12) and in this way are comparable to silica-rich tholeiites. The diopside phenocrysts in the madupite (Table 5, LI t . 16) have noticeably higher A1 in fourfold coordination (1.6 percent) than those from the orendites and wyomingites (0.36%), presumably a response to the low silica concentration in the madupite (Table 12). The generally very low level of Al in all these pyroxenes is also likely to be a reflection of the low concentration of Al in the rocks. V E g g o o G ~ (1962a) has indicated tha t Ti will enter silicates when the partial pressure of oxygen is relatively high, the silicate phases are magnesium rich and they precipitate at a high temperature. The Ti content in these diopsidic pyroxenes, which is comparable in amount to tha t found in tholeiitic pyroxenes of higher A1 content (LE BAs, 1962, Fig. 5), may accordingly reflect their high temperature of crystallization (p. 58).

5. Phlogopite Possibly apar t from the occurrence of ]eucite or its pseudomorph, phenocrysts of phlogopite are the most characteristic mineral in these potassie volcanics. Phlogopite is apparent ly one of the earliest phases to precipitate in the Leucite Hills volcanics, but is absent in the groundmass. In the Leucite Hills wyomingites and orendites, phlogopite may form large often polysynthetieally twinned crystals with some evidence of resorption at their ragged margins. Phlogopite often shows only weak pleochroism from pale yellow-brown to almost colorless; sometimes however, the core of the phlogopite crystals show more intense pleochroism to shades of red-brown, especially if there are small inclusions of a spinel, or the phlogopite occurs in a reaction rim around olivine or chromite. In the madupites, the anhedra of phlogopite with very uneven ragged margins may show more intense pleochroism than the phlogopites in the wyomingites and orendites, and may also show some evidence of zoning. Phlogopite in the Spanish jumillite is a deep red-brown color, and the large crystals usually enclose leucite or its pseudomorph, apatite and particularly diopside. The phlogopites in the Australian rocks are also dark colored, but may be strongly zoned, the junction between zones in one ease being very abrupt. The microprobe analyses of the phlogopites are given in Table 6, together with a conventional analysis of an Australian phlogopite (PRIDnR, 1939) and a phlogopite from a wyomingite (Czoss, 1897). No determinations of fluorine or water have been made, so the sum of the analyses is of course low; from the data of the conventional analyses and OsA~ (1906)


all these micas are fluorine-bearing, the Wyoming ph]ogopite containing 2.46%, the Spanish phlogopite 2.16% (OsA~N, 1906) and an Australian example 0.66%; this last analysis is deficient however in the fluor-hydroxy] group, as it only has 2.16 atoms in the unit cell (Pgn)~g, 1939). I t is difficult to assess the quality of the microprobe analyses by their totals especially if there are deficiencies in the fluorhydroxyl group but the addition of perhaps 3 to 4% for the fluorhydroxyl components and for the amount of undetermined F%03 indicates that they are reasonable. Moreover if the analyses are recalculated into atoms on the anhydrous basis of 22 oxygens, the distribution of cations between the various sites shows a satisfactory correspondence with the generalised formula of phlogopite.

Phlogopites of twelve orendites and wyomingites from the Leueite Hills were analysed, but as the composition varied throughout the twelve by almost the same extent as the variat ion in the composition of phlogopite in one rock, only the average of the twelve analyses are given, together with the range of values found. In all the analyses, microprobe as well as conventional (P~IDV.R, 1939), there is a deficiency in A1 in tetrahedral sites so tha t either Ti of Fe" ' is likely to be found in fourfold coordination. In the analyses in Table 6, Ti has been added to the Z group as ferric iron has not been determined. Several features of the chemistry of these phlogopites are of interest; Ti is particularly high in the Australian and Spanish phlogopites and is in marked contrast to the average phlogopite of the Leucite Hills, which are richer in Mg, Ni and A1. Ba is an essential constituent in all the phlogopites, unlike Ca, and Cr and Ni are both present in minor amounts. As noted above, one of the Australian phlogopites is strongly zoned, and both the core and the margin of several crystals have been analysed (Table 6, WK. 3). There is a decrease in AI, Cr, Mg, Ni and perhaps K towards the margin which is enriched relatively in Ti, Fe and Ba. The Leucite Hills phlogopitcs are also weakly zoned with the cores of the crystals enriched in Cr and Ni, and mildly in Mg. The average of the twelve phlogopites from the orendites and wyomingites of the Leucite Hills is quite different to the analysis of the madupite phlogopite (Table 6, LH. 16). This phlogopite is more enriched in Fe, Ti and Ba, and impoverished in A1, Mg and K with respect to the average phlogopite.

6. A m phiobole ( Magnophorite ) An amphibole with unusual plcoehroism was identified by C~oss (1897) in an orendite from the Leucite Hills; i t is very common, invariably interstitial and one of the last phases to erystallise, and in one of the orendites (LH. 9) it encloses myriads of little leucite crystals (Fig. 4). OSAN~ (1906) also identified an amphibole in the Spanish jumfllite. In the Australian rocks, P~IDER (1939) has described amphibole as an essential and abundant constituent in many of the rock-types, and as the analysis of this amphibole showed tha t potassium was markedly in excess of sodium, a rare occurrence in an amphibole, he proposed tha t this var iety should be called magnophorite. In essence magnophorite is a potassium richterite (D]~R et al., 1963) which has the generalised formula

Na2CaMghSisO22(OH)2.

Magnophorite is easily identified by its pleoehroism from lemon-yellow to pink or reddish-pink but, unlike the Austral[an paragenesis, it is only found as very small crystals in the Leucite Hills wyomingites and orendites and the Spanish jumillite.

42 IAN S. E. CARMIeHAWL:

Table 6. Analyses and ]ormulae o] phlogopites. (For key to specimens, see page 32)

S. 1 WK.1 W K . 3 W K . 3 W K . 4 W K . 5 A Leuc i t e R a n g e LH. 16 B Core Marg in Hi l l s

average of 12

s i n 2 42.8 42.0 41.9 42.0 41.6 41.1 40.78 41.9 41.1 - - 4 2 . 8 42.3 42.56 TiO~ 5.9 7.8 4.8 7.0 7.1 7.0 8.97 1.95 1.8 - - 2.1 3.5 2.09 A1203 9.9 10.3 10.3 7.4 10.6 9.7 10.95 11.5 11.3 - - 1 1 . 8 8.9 12.18 Cr~O 3 0.80 0.34 0.79 * 0.64 0.20 - - 0.61 0 . 8 2 - - 0.28 0.07 0.73 F % 0 a . . . . . . 2.18 - - - - - - 2.73 FeO 6.6 4.2 4.0 7.8 4.8 6.1 3.73 2.68 3.2 - - 2.5 5.5 0.90 MnO 0.03 0.05 0.04 0.05 0.04 0.03 t r ace 0.03 0 . 0 3 - - 0.05 0.04 - - MgO 18.3 19.3 22.6 19.8 20.0 20.3 19.66 25.2 24.6 - - 2 6 . 0 22.4 22.40 NiO 0.15 0.25 0.15 0.11 0.18 0.14 - - 0.30 0 . 2 3 - - 0.39 0.13 - - CaO 0.04 0.05 0.02 0.02 0.07 0.02 0.11 0.04 0 . 0 3 - - 0.04 0.02 0.20 BaO 1.6 1.2 0.75 1.3 1.2 1.2 0.35 0.82 0 . 4 1 - - 1.13 1.6 1.00 Na20 0.44 0.02 0.01 0.10 0.05 0.03 0.11 0.12 0 . 0 3 - - 0.30 0.30 0.44 K20 9.7 10.6 10.8 10.1 10.5 10.5 10.59 10.6 10.5 - - 1 0 . 8 9.8 10.70 H20 + . . . . . . 1.87 - - - - H~O . . . . . . . 0.19 - - __ ~2"35

F . . . . . . 0.66 - - - - 2.46

Sum 96.3 96.1 96.2 95.7 97.0 96.3 100.15 95.7 94.6 100.80

Less O for F . . . . . . 0.27 - - - - 1.03

To ta l . . . . . . 99.88 - - - - 99.71

.Formulae on Basis o/ 22 oxygens (microprobe results)

Y

Mg Ni

Ba X

Z Y X

Si 6.149 5.994 5.963 6.125 5.917 5.916 5.944 6.163 A1 1.675 1.732 1.728 1.272 1.778 1.647 - - 1.923 - - 1.529 - - Ti 0.176 0.274 0.309 0.603 0.305 0.437 - - 0.133 - - 0.308 - - Ti 0.461 0.563 0.205 0.165 0.455 0.321 - - 0.075 - - 0.076 - - Cr 0.089 0.038 0.089 - - 0.072 0.022 - - 0.068 - - 0.009 - - Fe 1~ 0.792 0.501 0.476 0.952 0.571 0.733 - - 0.318 - - 0.671 - - Mn 0.003 0.005 0.005 0.006 0.005 0.003 - - 0.003 - - 0.005 - -

3.918 4.183 4.793 4.303 4.239 4.355 - - 5.328 - - 4.864 - - 0.017 0.028 0.017 0.013 0.020 0.016 - - 0.034 - - 0.015 - - 0.006 0.008 0.003 0.003 0.010 0.003 - - 0.006 - - 0.003 - - 0.090 0.067 0.042 0.074 0.067 0.067 - - 0.045 - - 0.091 - - 0.122 0.005 0.003 0.028 0.014 0.009 - - 0.032 - - 0.084 - - 1.778 1.931 1.962 1.886 1.906 1.929 - - 1.920 - - 1.823 - - 8.00 8.00 8.00 8.00 8.00 8.00 - - 8.00 - - 8.00 - - 5.28 5.32 5.58 5.44 5.36 5.45 - - 5.83 - - 5.64 - - 2.00 2.01 2.01 1.99 2.00 2.01 - - 2.00 - - 2.00 - -

A = P h l o g o p i t e , ph logopi te - leuc i te l a m p r o i t e (f i tzroyite) , Howes Hil l , W e s t e r n Aus t r a l i a

(PRIDER, 1939). B = Ph logop i t e f rom W y o m i n g i t e of Boars Tusk, CRoss, 1897.

* Be low l i m i t of de tec t ion ; (0.01%)

I n T a b l e 7, t h e m i c r o p r o b e a n a l y s e s of m a g n o p h o r i t e a r e g i v e n t o g e t h e r w i t h t h e

c o n v e n t i o n a l a n a l y s i s of t h e o r i g i n a l m a t e r i a l ( P ~ I o E E , 1939) . A s w i t h t h e p h l o g o -

p i t e s , t h e s u m of t h e m i c r o p r o b e a n a l y s e s c a n n o t b e u s e d a s a n e s t i m a t e of t h e i r

Volcanic Rocks from the Leucite Hills, Wyoming

Table 7. Analyses and/ormulae o/amphiboles (/or key to specimens, see page 32)

43

WK.3 WK.4 A S.1 LH.9 WK.3 WK.4 S. 1 LH.9

SiO~ 54.6 54.6 5 2 . 6 7 53.2 54.7 TiO 2 4.5 4.4 3.53 5.2 6.2 Al20a 0.47 0.45 1.72 1.1 0.55 Cry03 * * n.d. * 0.03 F%03 - - - - 0.58 - - - - FeO 2.6 2.6 2.41 5.7 2.5 MnO 0.06 0.06 0.06 0.13 0.06 MgO 18.9 19.3 21.32 17.3 18.3 NiO 0.06 * - - 0.09 0.06 CaO 6.3 6.6 6.95 5.2 6.1 BaO 0.60 0.59 nil 0.20 0.26 SrO 0.73 0.74 0.15 0.38 - - Na~O 3.2 3.2 3.64 5.1 3.6 K20 6.4 6.5 5.70 4.4 6.0 H~O ~ - - - - 0.46 - - - -

H 2 0 - - - - - - n i l - - - -

F -- -- 1 . 2 9 - - - -

S u m 98.4 99.0 100.48 98.1 98.4 Less 0 for F 0.54

Total 99.94

Z ]Si 7.760 7.724 7.622 7.711 [ A1 0.079 0.075 0.186 0.091

Ti 0 . 4 8 1 0.468 0.560 0.657

J Cr - - - - - - 0.003 y Fe n 0.309 0.308 0.683 0.295

Mn 0.007 0.007 0.015 0.007 Mg 4.004 4.069 3.694 3.845 Ni 0.006 - - 0.009 0.006 Ca 0.959 1.000 0.798 0.922 Ba 0.033 0.032 0.011 0.014

X Sr 0.060 0.060 0.032 - - Na 0 .881 0.877 1.417 0.984 K 1.162 1.173 0.804 1.079 Z 7.839 7.799 7.808 7.802 Y 4.807 4.852 4.961 4.813 X 3.095 3.142 3.062 2.999

A =Analysis of magnophorite from wolgidite, West Kimberley area, WADE and FI~IDER (1940) Table I, No. i. * Below limit of detection; (0.01%). Formulae on the basis of 23 oxygens for the microprobe analyses.

qual i ty because of the unde te rmined ferric i ron and the f luorhydroxyl components . Al though ideally the hydroxyl group will a m o u n t to 2.0 atoms in the un i t cell, th~I~)~g's (1939) analysis of magnophori te showed tha t ig was deficient with only 1.04 a toms in the u n i t cell. The addi t ion of 1.5 to 2.0 weight percent to the microprobe totals to allow for the hydroxyl group suggests tha t there are no gross errors in the analyses. There is a general s imilar i ty between the microprobe analyses of magnophori te from the Aus t ra l ian specimens and the analysis t abu la ted by PI~ID:Etr (1939) (Table 7); the microprobe suggests there is less A1 and Mg, and more Si, Ba and Sr t h a n t ha t found by convent ional analysis. The same general character- istics t ha t were noted in the phlogopite analyses are found in these amphibole analyses and formulae. There is insufficient A1 to make up the te t rahedral sites to 8 with Si, and i t is possible t ha t Ti or F e ' " is present in fourfold coordination. All the amphiboles are rich in Ti and Mg, and for the first t ime Sr as well as Ba is present in significant amounts . Apar t from the magnophori te from the jumill i te , potass ium is in excess of sodium in all the amphibole formulae.

7. Priderite

I n m a n y of the voleanics from the Leucite Hills, prideri te is a conspicuous and common accessory, bu t i t is absent in others. I t is found as small elongate or acicular crystals of high relief and birefringenee which, being v i r tua l ly opaque,

44 IA~ S. E. CARMIC]IAEL:

m a y be identified as rutile (Cl~oss, 1897) (Fig. 4). Priderite is pleochroic in t ints of yellow and brown and in the Austral ian rocks it was also initially identified as rutile (WAD~ and P n m ~ , 1940). NOl~IS~ (1951) identified priderite as a new mineral, the "rutf le" of the Austral ian potassie volcanies, in which it is a constant accessory. Priderite is te tragonal with a s t ructure related to cryptomelane KMns016 (MA- THI~SO~ and WADSL~Y, 1950) and contains 16 oxygens in the uni t cell; the general formula is A2_yBs_~O16 where A is the a toms K and Ba and B is Ti, Fe and A1; y is near un i ty while z is very small. I t is of interest t ha t KuMw et al. (1966) have synthesised germanium orthoclase (KA1G%Os) with a cryptomelane or rutile-like s tructure of high density. The analysis of the original priderite is given in Table 8 (A) together with the formula on a basis of 16 oxygens. The microprobe analysis of a priderite f rom Western Australia (Table 8, W K. 3) is no t too dissimilar to the original analysis,

Table 8. Analyses and ]ormulae (16 oxygens) o/priderites (/or key to specimens, see page 32)

WK. 3 A LH. 9 WK. 3 A LH. 9

Ti02 73.3 70.6 67.8 .AlaO a 0.03 2.3 0.04 Cr20 a 0.6 - - 0.4 FeO 11.4 12.4" 12.6 MgO 0.9 nfl 0.8 CaO 0.04 trace 0.07 BaO 5.8 6.7 13.1 Na20 0.02 0.6 0.07 K~O 7.4 5.6 5.0

Total 99.5 98.2 99.9

Ti 6.84 ~ 6.48 Fe a+ - - ~ 1.14 Fe 2+ 1.18 ~ - -

/

Cr 0.06 [8"25 __ A1 0.004[ 0.33 Mg 0.16 J - - Ca 0.01 ] - - Ba 0.28 ~ 0.32 Na 0.004[ 1.46 0.14 K 1.16 J 0.87

~ 1.37 /

7.95 0.05[ 8.23 /

0.Ol / 0.16J 0.01] 0.67~

1.33 0.02[ 1.52

0.83j

A = Priderite from leucite-lamproite, West Kimberley area. No~I s~ * Fe203. Rb, Sr, Y, Zr, Nb and the rare earths below limit of detection.

(1951).

a l though the microprobe suggests t ha t A1 and Na are no t present except in t race amounts . The priderite f rom the Leucite Hills (Table 8, LIt . 9) contains very much more Ba than the Austral ian examples, and supports IqOR~IS~'S suggestion (1951) tha t there is a solid solution series between K-priderites and Ba-priderites. The presence of iron (and perhaps Ti) of unknown oxidation state in the microprobe analyses makes the calculated formulae at best approximate ; the microprobe totals would suggest t ha t Fe2~- ra ther than Fe3~- is present.

8. Wadeite

Wadeite has been found as a rare accessory mineral in vir tually all of the rocks f rom the Leucite Hills, a l though it is only common in the madupites. I t was first identified by PnID~i~ (1939) in the Austral ian potassie rocks who also suggested t h a t it m a y be present in the Leucite Hills volcanics; wadeite was studied by H ~ s ~ A w (1955) who determined its structure. Wadei te is a hexagonal mineral of high birefrigence with an ideal formula (Zr2K4Si601s) having rings of (Si309) 6- similar to benitoite.

Volcanic t~ocks from the Leucite Hills, Wyoming 45

W a d e i t e is easi ly recognised b y i ts br i l l ian t f luorescence in the e lect ron b e a m ; the smal l wadei te crys ta ls are i n v a r i a b l y moulded onto, or in con tac t with, smal l c rys ta l s of apa t i t e . The original analys is of wadei te ( t ~ I D ~ , 1939) is g iven in TaMe 9, and repor t s considerable amoun t s of P2Oa, CaO and H20. As the micro- p robe analyses (Table 9) of a wadei te f rom Aus t r a l i a and f rom a W y o m i n g

Table 9. Analyses and ]ormulae (18 oxygens) o/wadeites (/or key to specimens, see page 32)

WK.3 A LH.16 WK.3 A LIt. 16

SiO 2 48.8 39.43 47.4 TiO 2 2.8 1.63 0.8 ZrO 2 27.9 21.29 28.5 A1203 0.2 5.98 0.8 F%03 - - trace - - 1%O 0.4 - - 0.6 MgO * 0.28 * CaO 0.1 5.22 0.1 Ba0 0.1 1.20 0.1 SrO * 0.16 * Na~O 0.1 2.82 0.1 K~O 19.7 18.40 21.5 P~O 5 * 3.15 * H~Oq- - - 1.30 - - H~O -- - - nil - -

Total 100.1 100.86 99.9

Si 6.19 5.06 / P - - 0.34~ 6.00 A1 - - 0.60 J

A1 0.03~ 0.31] Fe n 0.05 / - - Ti 0.271 2.07 0.15~

' 0.06! 2.35 Z r . 1 .33 / Na 0.50" 0.03 I Ca } 0.19] 0.02 K 3.02 3.07 0.73[ Ba O.OiJ 3.02[ 4.00

0.06/

6.11

0.12, 0.06 0 .08 2.05

1.7 I 0.03 0.02

3.59 3.53 0.01

A=Wade i t e from wolgidite, West Kimberley area, WADE and I~ID~I~ (1940), Table 1, No. 3; H ~ s m t w (1955). * Mg, P, Nb, St, l~b, Y, HI, and rare earths below limit of detection (0.02%).

m a d u p i t e do no t conta in P~O 5 or s ignif icant CaO, i t seems possible t h a t the or iginal analys is of wadMte ( P ~ w ~ , op. cir.) was made on ma te r i a l c on t a mina t e d wi th a l i t t le apa t i t e , i t s invar iab le associate. The two new analyses and the i r formulae show only general confo rmi ty wi th the ideal formula . I n p a r t this m a y be due to ana ly t i ca l error and the absence of any mic roprobe s t anda rds approach ing wadei te in composi t ion. I t m a y be seen in Table 9 t h a t smal l quant i t i es of Ti, A1 and F e m a y subs t i tu t e for Zr, and there m a y also be considerable deficiencies of a toms in the K sites.

9. Apatite

This minera l is ve ry common as an accessory in all the rocks examined (cf. C I P W norms, Table 12) and is easi ly ident i f ied b y i ts f luorescence in the e lec t ron-beam. D~E~ et M. (1962) and CRypT (1966) have summar ized the var ious subs t i tu t ions t h a t m a y be found in the apa t i t e s t ruc tu re ; among the common rep lacements are Sr and Mn for Ca, ra re ear ths for Ca, and coupled subs t i tu t ions of the t y p e S 6+ for F 5+ toge the r wi th Si 4+ for ps+ or N a + for Ca 2+. I n Table 10, pa r t i a l microprobe analyses of five apa t i t e s are given and t h e y show t h a t over th ree pe rcen t b y weight is rep laced b y var ious ions of which the Ce group of ra re ea r ths p r edomina t e ; the apa t i t e f rom Wes te rn Aus t r a l i a has a h igher concen t ra t ion of ra re ear ths t h a n the o ther apa t i tes . Si is also an a b u n d a n t

46 IA~ S. E. CARMICIIAEL-"

constituent, and S (probably as SOa) has been detected by beam scanning, but has not been determined. The apati te analyses reflect the complex chemistry of these rocks, but within the limits of detection, they are the only phosphorous bearing phases in these assemblages.

10. Perovsl~ite

C~oss (1897) recognised perovskite as madupite from the Leueite Hills, but it

Table 10. Analyses o/ apatites (For key to specimens, see page 32)

WK.3 S. 1 LH.1 LH.9 LH. 16

SiO 2 0.6 0.6 0.6 0.5 1.1 Ti02 0.06 0 .03 0 .04 0 .04 0.06 FeO 0.23 0 .26 0 .23 0 .12 0.31 MgO 0.01 0.31 0.31 0 .33 0.16 BaO 0.47 0 .15 0 .16 0 .35 0.40 SrO 0.31 0 .18 0 .14 0.20 0.46 Na20 0 .02 0 .05 0 .03 0 .05 0.03 KzO 0.11 0 .08 0.09 0.21 0.13 La203 0.6 0.2 0.2 0.2 0.3 Ce208 1.0 0.5 0.6 0.5 0.5 Pr~O s 0.4 0.2 0.3 0.3 0.2 Nd203 0.5 0.4 0.3 0.2 0.3 Gd203 0.2 0.2 0.2 0.2 0.2 Eu~O~ + + + + + S + + + + +

Sum 4.5 3.2 3.2 3.2 4.2

~- ~ present; Y, Zr and Nb below limit of detection (0.02%).

one of the more abundant accessories in is also present in a par t ly glassy madupite (LH. 8). I t is absent in the orendites and wyomingites of the Leucite Hills and also in the jumillite; although WADE and Pgn)~R (1940) report accessory perovskite in some of the Australian rocks, it is not present in the writer's microprobe specimens. Perovskite forms small dark red- brown anhcdral crystals which are almost opaque and have extreme relief. D ~ g et al. (1962) show tha t in many perovskites there is considerable substitution of the rare-earths and alkalis for Ca (vat. knopite) while Nb may substitute for Ti, electrostatic balance being at least partially retained by the substitution of Na for Ca (vat. dysanalyte). The two analyses of perovskite from the Leucite Hills (Table 11) are very rich in the rare earths of which Ce is predominant, but Nb could not be detected. Sr, Na and K are all present in more than insignificant amounts, and the perovskite,

which has not been found to co-exist with magnophorite, reflects the extreme or absurd in these potassie rocks, as it is therefore the most sodic mineral of its co-existing assemblage.

11. Magnetite and Spinels

The madupite from the Leucite Hills is the only specimen from the Leucite Hills, Spain or Australia in which an iron-titanium oxide has been found; ilmenite has never been seen and titaniferous magnetite is the sole iron-titanium oxide in the madupite. The large majori ty of the numerous magnetite crystals are one-phase and show little zoning. The magnetite analysis is given in Table l l, which when recalculated on the ilmenite basis (CARMICI:[AEL, 1967) to obtain a value for the F%O 3 content, gives a respectable total. Vw~OOG~N (1962a) has discussed the occurrence of perovskite with magneti te in rocks, and suggested tha t they will co-exist in strongly undersaturated rocks, of which the madupite is an example (Table 12). He considered the reaction 3FcTiO~ -[- 3CaMgSi206 q- 1/20 ~ = 3CaTiO~ q- F%O~ q- 3MgSiO~ q- 3Si02 the left-hand side of which will exist only at high temperatures, or at low oxygen


Table 11. Analyses o/perovskites and magnetite (/or key to specimens, see page 32)

LH. 8

TiO 2 48.7

A120 a 0.03 REeOa* 11.4 FeO 7.7 M_nO

MgO ZnO CaO 23.5 SrO 5.7 Na20 2.5 K20 0.6

Total 100.2 Ce2Oa* 5.1 La20a 2.4 Nd20 a 2.2 Pr20 a 1.6 Gd~O 3 0.1 Eu~O 3 0.04

LH. 16 LH. 16

48.2 12.8

0.05 0.56 10.6 11.4 76.8 - - 2.26

- - 1 . 3 0

- - 0.32 25.1 trace 3.8 1.0 0.3

100.4 94.0 5.1 2.0 1.9 1.5 0.1

trace

Recalculated Analysis

Ilmenite basis F%03 52.2 ~e0 30.5 Total 99.9

Ulvospinel basis Fe208 43.2 FeO 38.0 Total 98.4

Mol. % ulvospinel 36.7

12. Strontiobarytes

In an attempt to find a sulphur-bearing phase in the rocks of the Leucite Hills, and so to account for the sulphate found in the rock analyses (Table 12), the polished thin-sections were examined by beam-scanning. Sulphur was found in the apatites (see above) and also in combination with Sr and Ba. As no sulphides have been seen in the polished sections, the sulphur is considered to be present as sulphate, and the presence of Sr and Ba suggest a strontiobarytes. Although the crystals have not been analysed, Ba is apparently in excess of Sr. Small crystals of strontiobarytes are present in most of the Leucite Hills volcanics, but they are most abundant in the madnpite; they have also been recognised in the Spanish jumillite. Although none of the analysed specimens contain vesicle infillings, barytes together with zeolites and carbonate are commonly found in vesicles in many of these rocks. I t is of historical interest that the inter- ference due to a sulphate in the Leucite Hills rocks was noted by HILLEB:~AND (I~I1LLEBI~A~CD and LU~DnnL, 1959, p. 87) during the analysis of these rocks, but was not identified as such.

pressures. With a high oxygen pressure and a low activity of silica, the reaction would go to the right as the madupite assemblage indicates. I t is noteworthy that the magnetite, which is presumably in equilibrium with perovskite, contains 12.8 weight percent of Ti02, or 36.7 molecular percent of ulvospinel (Table 11). A chrome-bearing spinel is found in many of the Leueite Hills orendites and wyomingites but invariably has a reaction rim of phlogopite surrounding it. This spinel has not been analysed except that Cr was identified as present by beam scanning. Tiny crystals of a spinel-phase of unknown composition are sometimes found in inclusions in the phlogopite phenocrysts.

Rb, Ba, Y, Zr, and Nb below limit of detection (0.02 %) in perovskite

48 IAN S, ]~. Citt~IOttA~L:

E. Petrology 1. The Absence o/the Iron-Titanium Oxides and Oxygen Fugaeity

The absence of any iron-titanium oxide phase in the orendites and wyomingites of the Leucite Hills indicates that neither magnetite (sensu late) nor ilmenite were stable at liquidus temperatures. I t seems unusual, but nevertheless possible, that this is so solely because of a paucity of the components of the Fe-Ti oxides, for these rocks contain over 2 weight percent Ti02 and over 5 weight percent of total iron (Table 12). Of the possible explanations for the absence of an oxide phase, a combination of two seems most plausible. Firstly, the gross composition of a liquid of wyomingite-orendite composition is such that the Fe-Ti oxide saturation surface does not intersect the crystallisation surface, whereas in a liquid of madupite composition, magnetite is a stable phase. The essential difference between these two liquids is the dominant molecular excess of alkalis (mainly K) over alumina in the wyomingites and orendites, and the virtual absence of this excess in the madupites together with a paucity of silica (Table ]2). This suggests that a reaction of the following general type may occur in the orendites and wyomingites:

K~O + 7SiO 2 + FeO.F%03 --~ 2KFe'"Si30 s + FeSi03

Presumably the iron-metasilicate (or orthosilicate) will become incorporated in the ferromagnesian silicate phases. The second factor contributing to the absence of iron-titanium oxides in the orendites and wyomingites is also illustrated by this reaction, which will obviously be affected by oxygen. Any reduction of the ferric iron component would tend to make the ferromagnesian silicates more iron-rich, and conversely any oxidation of the ferrous component would increase their magnesium content. The two reactions given below, in which the left-hand side represents components in the liquid, are the upper limits of the effect of oxygen, as both reactions would deprive the co-existing ferromagnesian silicates of iron.

5K20 + 24Si0~ + 8FeTiO 3 + 11/202 ---- K2Tis01~ + 8KFe"'Si~O s and 3K~O + 18SiO~ + 2F%04 + 11/~02 = 6KFe'"Si30 s

As both reactions require the complete oxidation of ferrous iron, and hence its absence in the co-existing ferromagnesian minerals, the upper limiting condition of oxygen fugacity in these rocks is the haematite-magnetite buffer and the analogous ilmenite reaction. The lower l~mit is unknown, but obviously above that of the fayalite-magnetite-siliea buffer as all the ferromagnesian phases are relatively magnesian (WoN~s and EvasTnl~, 1965). With respect to the upper limit, the reaction 4FeTiO~ + 02 = 4 T i e 2 + 2F%03 has an equilibrium partial pressure of oxygen (10-s. 1 bars) slightly higher than that for the reaction 4 Fe304 + 03 ---- 6Fe203 (10 -s'25 bars) at 1100~ K (Vm~HOOG~, 1962b); these two reactions at the temperatures of these lavas may be assumed to have the same equilibrium pressures of oxygen. If the orendites and wyomingites of the Leucite Hills erystallised in the range 1250 ~ C--1000 ~ C (p. 58), then the oxygen fugacity required to suppress both magnetite and ilmenite will be in excess of 10 -1"~5 at 1250 ~ C and 10 -5"16 bars at 1000 ~ C (EVCsTm~ and WoN]~s, 1962, Table 2), which


would be unusually high values for natural liquids. That this range is higher than that required to produce the assemblages of the orendites and wyomingites may be indicated by the stability the synthetic compound ferriannite. The equilibria of the composition KFe~+Fe3+SisO10(OI-I)2 (WoNEs, 1962) shows that iron-sanidine is not restricted to the haematite field, but that liquid, iron-sanidine and magnetite can all co-exist. However the iron-sanidine field did not extend to the nickel-nickel oxide buffer curve, to which many natural pyroxene-magnetite- flmenite-liqnid assemblages approximate (CAn~IC~ZAEL, 1967). The composition of the analysed phlogopite (Table 6, B) also indicates by virtue of its F%03 content tha t it may have equilibrated at an oxygen fugacity lower than that of the haematite-magnetite buffer (WO~ES and EUGSTEtr 1965, Fig. 1), but this extrapolation is not too reliable for such a magnesian biotite. Another indication of oxygen pressure is the presence of a sulphate phase (strontiobarytes) rather than a sulphide in the groundmass of many of these rocks. Admittedly it is not clear tha t the strontiobarytes precipitated above the solidus, or in other words is a magmatic phase, but it is possible. The relative abundance of strontiobarytes in the groundmass of the madupite is surprising if it is primary, for the assemblage of magnetite phenocrysts and strontiobarytes in equilibrium without an iron-sulphide or haematite seems unlikely (HoLr.A~D, 1959). With the possible exception of strontiobarytes, there is no mineralogical evidence in any of the Leucite Hills volcanic assemblages that requires an unusually high oxygen fugacity; however the occurrence of magnetite and perovskite in the madupite rather than ilmenite and diopside is favored by oxygen pressure (V]~R- ~OOGWN, 1962a). If the ratio F%Os/FeO q- F%0 s of the rocks (Table 13) is any reflection of the fugacity of oxygen, and this ratio has remained unaltered after quenching of the lavas, then the liquid with the highest value of this ratio is the madupite (LH. 16), certainly suggestive that the gross composition of a liquid may be more effective than oxygen in suppressing the formation of the iron-titanium oxides.

2. The Alumina Deficiency

Some indication has been given above that the presence of a molecular excess of alkali (mainly K) over alumina in the orendites and wyomingites of the Leucite Hills may be the cause of the suppression of magnetite and ilmenite, and hence the appearance of the diagnostic and characteristic minerals of these rocks. Of these, iron-sanidine, leucite, priderite, wadeite and possibly magnophorite are the most conspicuous, and suggest that reactions of the type indicated above may occur. Although oxygen fugacity may influence the extent to which the postulated reactions go, provided that potassium is not consumed, the critical factor is the excess of potassium over aluminum. This excess is illustrated not only in the compositions of the minerals mentioned above, but also in the CIPW norms of the rocks (Table 12). Another possible reaction dependent upon excess potassium but not on oxygen is :

2K20 + 6Si02 + ZrOz ---- K~Zr2Si~01s

to produce the mineral wadeite. As Zr has not been found as a component in any other mineral phase, it appears that Zr has a prior claim to the excess of

4 Contr. Mineral. and Petrol., Vol. 15

50 IAN S. E. CARMICHAEL:

Table 12. Analyses and CIP W norms o/potassic volcanics (For key to specimens, see page 32 and below)

S.1 LH. 1 I I I IV LH.7 LI t .7 LH.9 LH. 10 LH. 12 V LH. 16 VI I glass *

SiO 2 47.02 50.23 50.23 53.70 55.43 58.6 54,17 53.07 55.14 54.08 43.56 42.65 Ti02 1.31 2.30 2.27 1.92 2.64 4.9 2.67 2.41 2,58 2.08 2.31 1.64 Zr02 0.13 0.25 - - - - 0.28 - - 0.22 0.26 0.27 - - 0.27 - - AlcOa 7.55 10.15 11.22 11.16 9.73 7.0 10.16 8.96 10.35 9.49 7.85 9.14 Cr~O s 0.09 0.06 0.10 0.04 0.02 - - 0.05 0.08 0.04 0.07 0.04 0.07 F%Oa 3.32 3.65 3.34 3.10 2.12 - - 3.34 3.86 3.27 3.19 5.57 5.13 FeO 2.93 1.21 1.84 1.21 1.48 4.5 0.65 0.91 0.62 1.03 0.85 1.07 M_nO 0.11 0.09 0.05 0.04 0.08 0.05 0.06 0.08 0.06 0.05 0.15 0.12 MgO 16.43 7.48 7.09 6.44 6.11 3.8 6.62 11.17 6.41 6.74 11.03 10.89 CaO 7.37 6.12 5.99 3.46 2.69 9.6 4.19 3.56 3.45 3.55 11.89 12.36 SrO 0.29 0.32 0.24 0.19 0.27 0.2 0.18 0.27 0.26 0.20 0.40 0.33 BaO 0.39 0.61 1.23 0.62 0.64 1.1 0.59 0.34 0.52 0.67 0.66 0.89 ~a~O 1.02 1.29 1.37 1.67 0.94 0.7 1.21 1.15 1.21 1.39 0.74 0.90 K20 5.10 10.48 9.81 11.16 12.66 8.0 11.91 10.72 11.77 11.76 7.19 7.99 P205 1.90 1.81 1.89 1.75 1.52 0.06 1.59 1.24 1.40 1.35 1.50 1.52 H~O-~ 2.69 2.34 1.72 2.61 2.07 - - 1.01 1.16 1.23 2.71 2.89 2.18 H~O-- 1.52 1.09 0.93 0.80 0.61 - - 0.52 0.40 0.61 0.79 2.09 2.04 SO 8 0.06 0.35 0.74 0.06 0.46 - - 0.16 0.16 0.40 0.29 0.52 0.58 C1 - - - - 0.03 0.03 - - - - 0.06 - - - - 0.04 - - 0.03 F - - - - 0.50 0.44 - - - - 0.36 - - - - 0.49 - - 0.47

Sum 99.79"99.83 100.62 100.40 99.75 98.5 100.21 + 99.80 99.79~= 99.79 99.51 100.11 Less 0 fo rF , Cl 0.22 0.19 - - - - 0.17 - - - - 0.21 - - 0.20

Total - - - - 100.40 100.21 - - - - 100.04 - - - - 99.58 - - 99.91

* Microprobe analysis; * Includes 0.56 COs; + Includes 0.49 C02; ~= Includes 0.20 CO r Analyses S. 1, LH. 1, LH.7, LH. 10, LH. 12 and LH. 16 by I. S. E. C~a~MICgA~. Analyses I I I , IV (wyomingite), L.H. 9, V (orendite) and VI I (madupite) by W. F. HILLEB~AND (C~oss, 1897)

p o t a s s i u m . T h i s is s u b s t a n t i a t e d b y t h e p r e s e n c e of w a d e i t e in t h e m a d u p i t e

( L H . 16), w h i c h c o n t a i n s o n l y a s l i g h t excess of a l k a l i o v e r a l u m i n a , b u t i n w h i c h

p r i d e r i t e (K2TisO~6) is a b s e n t .

A l t h o u g h t h e s u b s t i t u t i o n of F e ' " for Al i n b o t h s a n i d i n e a n d l euc i t e m a y b e

e x t e n s i v e , t h i s A l " d e f i c i e n c y " is n o t c o n f i n e d t o t h e sa l ie m i n e r a l s . I n t h e

p y r o x e n e s a n d m i c a s e i t h e r T i or F e ' " a re r e q u i r e d t o c o m p l e t e t h e o c c u p a n c y of

t h e t e t r a h e d r a l s i tes , a n d a m p h i b o l e w h i c h is c o m m o n l y a r e p o s i t o r y of A1 i n

i g n e o u s rocks , is of r i c h t e r i t e t y p e w i t h n o e s s e n t i a l A1; e v e n so t h e r e a re de f i c i en -

cies of A1 i n t e t r a h e d r a l c o o r d i n a t i o n .

3. Rock Analyses

T h e a n a l y s e s of t h e i n v e s t i g a t e d r o c k s f r o m t h e L e u c i t e H i l l s a n d t h e S p a n i s h

j u m i l l i t e a r e g i v e n i n T a b l e 12, t o g e t h e r w i t h t h o s e a n a l y s e s m a d e b y H ~ L E -

B~A~D fo r C ~ o s s (1897). T h e t w o se t s of a n a l y s e s s h o w a g e n e r a l s i m i l a r i t y w i t h

t h e e x c e p t i o n t h a t t h e n e w a n a l y s e s t e n d t o h a v e l o w e r A12Os, p r e s u m a b l y d u e

t o t h e p r e s e n c e of u n d e t e r m i n e d Z r 0 2 , s t r o n t i o b a r y t e s a n d t h e r a r e e a r t h s , al l

of w h i c h w o u l d t e n d t o i n t e r f e r e i n t h e c lass ica l g r a v i m e t r i c d e t e r m i n a t i o n of

a l u m i n a b y d i f f e rence . T h e t o t a l s of t h e n e w a n a l y s e s a lso t e n d t o b e l o w ; t h i s

Volcanic Rocks from the Leucite Hills, Wyoming

NOTCh8

51

S.I LH.I III IV LH.7 LH.7 LH.9 LH.10 LH.12 V LH.16 VII glass *

Q . . . . . 6.02 15.67 1.20 - - 1.90 2.59 - - - - Z 0.20 0.37 - - - - 0.42 - - 0.33 0.38 0.40 - - 0.40 - - or 30.17 47.76 57.03 60.93 53.10 38.22 55.47 48.93 56.50 51.81 1.67 - - ab 8.18 . . . . . . . . . . . an 1.20 . . . . . . . . . . . . lc - - 6.02 0.74 . . . . . . . 32.00 37.02 ne - - - 1.68 . . . . . . . 0.17 1.37 hl - - - - 0.05 0.05 - - - - 0.10 - - - - 0.07 - - 0.05 th 0.11 0.62 1.31 0.i1 0.81 - - 0.28 0.28 0.71 0.51 0.92 1.03 ae - - 7.58 3.02 8.97 4.39 - - 7.70 7.67 6.70 8.43 2.17 0.94 ns =- - - - - 0.78 - - 1.38 . . . . . . ks - - 1.80 - - 1.39 6.02 2.51 4.13 4.00 3.62 4.90 - - - - wo 10.20 7.26 6.87 1.17 0.88 20.77 0.04 2.60 1.28 1.75 19.70 14.91 en 11.94 6.28 5.93 9.47 15.21 9.46 16.49 16.79 15.96 16.79 17.03 12.89 fs 0.18 . . . . 0.26 . . . . . . . fo 20.64 8.65 8.22 4.60 - - - - - - 7.22 - - - - 7.32 9.98 mt 4.82 . . . . . . . . . . . cm 0.13 0.09 0.15 0.06 0.04 - - 0.07 0.11 0.07 0.10 0.07 0.10 hm - - 1.02 2.30 - - 0.61 - - 0.68 1.21 0.88 0.28 4.82 4.81 il 2.49 2.68 3.89 2.60 3.26 9.31 1.45 2.02 1.38 2.21 2.06 2.45 tn - - - - - - 1.35 2.25 - - 4.68 3.31 4.55 2.24 - - - - pf - - 1.51 0.37 . . . . . . . 2.08 0.60 ap 4.50 4.30 4.48 4.14 3.60 0.14 3.77 2.92 3.33 3.20 3.56 3.60 fr - - - - 0.85 0.74 - - - - 0.59 - - - - 0.88 - - 0.83 Rest 4.77 3.43 2.65 3.41 2.68 - - 1.53 1.56 1.84 3.50 4.98 4.22

Total 99.53 99.37 99.54 99.77 99.29 97.7 99.62*99.00 99.12 99.26 98.95 99.14 + * Includes 1.11 cc. + Includes 4.34 cs.

is u n d o u b t e d l y d u e in p a r t t o t h e p r e s e n c e of u n d e t e r m i n e d t~ w h i c h a v e r a g e s

0.45 p e r c e n t i n HILLEB~AND'S ana ly se s , a n d to t i le u n d e t e r m i n e d c o m p o n e n t s ,

of w h i c h t h e r a r e e a r t h s are one e x a m p l e .

I t was t h e m a r k e d s i m i l a r i t y of t h e a n a l y s e s of t h e w y o m i n g i t e ( lenei te on ly)

(Table 12, L H . 1, I I I , I V a n d L H . 7) (Table 16) t o t h o s e of t h e o r e n d i t e s ( lenci te

p lus s an id ine ) (Table 12, L I t . 9, L H . 10, L I t . 12 a n d V) (Table 16) t o g e t h e r w i t h

t h e a l m o s t c o m p l e t e a b s e n c e of g lass t h a t led C ~ o s s (1897) to t h e i n s p i r e d con-

e lus ion t h a t t h e l eue i t e of t h e w y o m i n g i t e s is n o n s t o i e h i o m e t r i c (Table 2). W i t h

t h e e x c e p t i o n of t h e w y o m i n g i t e (LH. 7), g lass in t h e g r o u n d m a s s of t h e w y o m i n g i t e s

is o n l y p r e s e n t in v e r y sma l l a m o u n t s , so t h a t t h e o c c u r r e n c e of l eue i t e c a n n o t

be a s c r i b e d to t h e i n c o n g r u e n t m e l t i n g of san id ine . T h e a b s e n c e of n o t a b l e

a m o u n t s of f e l d s p a t h o i d in t h e n o r m s of t h e w y o m i n g i t e s a n d o r e n d i t e s i n d i c a t e s

t h a t t h e si l ica b a l a n c e is n o t c r i t i ca l t o t h e a p p e a r a n c e of l eue i te . I t is s t i l l n o t

e v i d e n t in v i e w of t h e ove ra l l s i m i l a r i t y of t h e t w o g r o u p s of a n a l y s e s (Table 16)

w h a t f e a t u r e s of c o m p o s i t i o n a r b i t r a t e t h a t l eue i t e r a t h e r t h a n s a n i d i n e prec i -

p i t a t e , b u t t h e i n t e r b a n d i n g of t h e t w o rock t y p e s m a y s u g g e s t t h a t s h e a r is

one f a c t o r t o be c o n s i d e r e d .

I n t h e w y o m i n g i t e (LH. 7), w h i c h h a s an e x t e n s i v e g l a s sy g r o u n d m a s s , t h e g lass

is m o r e s i l iceous t h a n t h e r o c k (Table 12), a n d t h e i n c o n g r u e n t m e l t i n g of s a n i d i n e

4*

52 IA~ S. E. CARmC~A~L:

may be represented here; nevertheless the leucite is also non-stoichiometrie (Table 2). A similar observation to that of C~oss was made by WA])~ and Plainer (1940) with respect to the holocrystalhne Australian rocks, in which, despite their normative oversaturation of silica, leucite is the only salic mineral found. The leucite in the only fresh example available to the writer is a stoichiometric variety (Table 2). Unless the lcueite of these rocks was originally silica-rich, and on recrystallisation at the inversion point conveniently disposed of this excess perhaps into the turbid alteration product after leucite (P~ID~R and COLE, 1942), it is difficult to understand how or why leucite is the sole salic phase. The general features of the chemistry of these volcanics are self-evident; the combination of high Mg, Cr and undoubtedly Ni with K, Ba, Sr, Zr, P, F and possibly S are also characteristic of igneous rocks which contain markedly more K than Na. Thus the lavas of southwest Uganda (tIoLMES and HAtCWOOD, 1937) (HIGAZu 1954), the plugs of the Navajo area (WILLIAMS, 1936) and the typical kimberlite (DAwsoN, 1962) all share with the volcanics of the Leucite Hills, Australia and the Spanish jumillite these elements in unusual abundance. The glassy wyomingite (LH. 7) (Table 12) must be one of the most potassic rocks recorded in the earth's volcanic history. In addition to the elements reported in Table 12, the results of determinations of Y, La and Rb together with the ratio K/Rb are given in Table ]2. The ratio K/Rb has been shown to vary widely in basaltic rocks depending upon their environment. Thus although GuNN (1965) and H]~IER et al. (1965) found values generally less than 250 for continental basaltic magma, the Karroo dolerites have much higher values of this ratio (ElcLANK and HOrMEY~, 1966). Similarly high values have been found in the Hawaiian basalts (approximately 510) (L~s- sing et al., 1963), but in. the trachytes, this ratio decreases to 260. The values of K/l~b in the five specimens of the Leucite Hills (Table 13) fall within the range of basaltic rocks, continental or oceanic, their high concentration of K being coherent with Rb.

Table 13. Trace elements (in p.p.m.) and iron-ratios

S.1 LH. 1 LH.7 LH.10 LIt. 12 LH.16

Y 40 35 25 25 20 25 La 200 240 260 200 130 360 l~b 170 460 330 290 310 205 K/Rb 250 190 320 310 320 295 Fe~Oa/FeO+F%0 a 53.1 75.0 58.9 80.9 84.1 86.7

Two other aspects of the chemistry of the Leucite Hills volcanics are of interest; unlike many other leucite-bearing potassic lavas such as those from Bufumbira, Uganda (HoLMwS and HA~wooD, 1937), these rocks have a large proportion of their total iron in the ferric state (Table 13), a feature also characteristic of many kimberlites (DAwsoN, 1962) and of the kalsilite-bearing lavas (katungites) lavas of Uganda (COMBE and HOLMES, 1945). Secondly, the lavas of the Leucite Hills, with the exception of the madupites (Table 12, LIt . 16 and VII.) are decidedly


not impoverished in silica, and many contain normative quartz, a feature also shown by many of the rocks of Australia (WAD~ and P~DV, R, 1940; P~IDE~, 1960). Comparisons of the highly potassic volcanies with others are at best superficial, and although their chemical composition and mineral assemblages may show some similarities, these highly potassic lavas are too extreme in their composition to be other than unique. The CIPW norms are also given in Table 12; these were kindly computed by the U.S.G.S. at Menlo Park. As the analysis of the residual glass of the wyomingite (LIt. 7) (Table 12) is a microprobe analysis, the oxidation state of iron is unknown, and the norm is accordingly not comparable with those of the other rocks. As is to be expected in these rocks, there is little correspondence between the norm and the mode, principally because of modal phlogopite together with non- stoichoimetric leucite (Table 2) and iron-sanidine (Table 3). With the exception of the Spanish jumlllite and the two madupites, ks is present in all but one of the rocks. 1 A c is present in all the norms but the jumillite (Table 12), and the principal modal expression of this component is magnophorite or perovskite, the two most sodic phases found.

4. Normative Alkal i Metasilicate

C~Au (1964) has lucidly discussed the logic (or otherwise) underlying the precepts of the CIPW normative calculation when the molecular proportions of Na~O ~- K20 exceed A120 ~ -~ F%03 and an alkaline metasilieate is formed. The authors of the CIPW norm preferentially allotted K20 to A1203, presumably because of the rar i ty or absence of potassium ferromagnesian minerals, and accordingly although the total alkalis may be in excess of A1208 ~ F%03, this excess is always represented to be Na20. Therefore unless the molecular propor- tious of K20 alone exceed those of Al~O 3 in which case ks is produced in the norm (as in these orendites and wyomingites, Table 12), the excess of Na20 ~- K20 over A1203 -~ Fe~O 3 always results in ns. In a discussion of peralkaline rhyolites which contain modal sodic ferromagnesian assemblages, Chayes (op. cir.) states the problem thus " . . . what is the mineralogical (or other) justification for allotting no K20 at all to the metasilicate molecule." For the sake of this discussion, let us disregard the normative convention of forming ac, and therefore in the orendites and wyomingites, where there is a molecular excess of both Na~O and K20 over A1203, we have the conditions to answer CHAu question. All those minerals that contain more alkali than alumina will contribute to this normative excess, namely iron-sanidine, leucite, priderite, wadeite, perovskite, and magnophorite. With the exception of the last two minerals (Tables 8 and 11), the remainder contain only very small amounts of Na20, and therefore do not reflect the Na~O/K~O ratio of the rock. Let the argument be inverted, so that unless minerals such as (sodipotassic) priderite, wadeite and magnophorite are found, and they have yet to be in peralkaline rhyolites (CAI%MICHA~EL, 1962), there is no mineralogical basis for any suggestion that K20 can be considered as a component of normative metasflicate in peralkaline acid rocks. Furthermore,

1 ks is absent in III, which because of undetermined ZrO~ and high SO 3 (p. 50) undoubtedly has lower AlcOa than that reported.

54 IAx S. E. CAR~mHA~L:

those ferromagnesian minerals which do occur in sodic peralkaline rocks generally show very little or no substitution of K for Na (C~MIC~A~L, 1962 ; D~I~ et al., 1963), so that the inclusion of K~O in a normative alkali metasilicate does not apparently have any modal justification. In these potassie voleanies, K has been completely excluded from the diopsidic pyroxenes (Table 5 and p. 40), and the only ferromagnesian minerals which contain both Na and K (and hence ks and ns) are magnophorite and perovskite, which so far have not been found in peralkaline acid assemblages. C~_~YEs' (1964) ensuing argument may now be seen to lack the convincing basis of mineralogy; this evidence is pre-emptive and is based on the observed fact that alkah ferromagnesian minerals in peralkaline acid rocks are sodic, and not sodi- potassie. Sodi-potassic ferromagnesian minerals (e.g. magnophorite, perovskite) with normative alkali metasilicate are not found in these rocks, and therefore their absence precludes the arbitrary juggling of K20/Na~0 ratios in the normative metasilicate molecule.

5. Normative Components and Synthetic Systems If the minor normative minerals are neglected, the majority (68--81%) of the normative component of all these rocks may be expressed in terms of the four components CaMgSi2Oa-Mg2SiO~-KA1Si~O6-SiO ~. This system has yet to be determined, but the constituent joins CaMgSi~O6-KA1Si206-Si Q (Sc~Araw~ and Bow~N, 1938) and Mg2SiO~-KAlSi206-SiO 2 (Scranton, 1954) have been reported. In order to plot the norms (Table 12) in each of these two systems, normative orthoelase and albite have been desilicated to leucite and "jadeite" molecules and nepheline sflicated to "jadeite" molecule; the balance of silica has been added to normative quartz. For the system Mg~SiO4-KA1Si206-SiO~, normative hypersthene beyond that required to form diopside has been reduced to olivine, and the silica added to normative quartz. In Fig. 11, the data scatter parallel to the boundary curve separating the fields of diopside and leucite, and suggest that such a boundary curve could, under natural conditions (hydrothermal), be displaced towards the rock data. A possible sequence of madupite leading to wyomingite and thence to orendite controlled by fractionation of diopside and leucite is accordingly indicated. However only in the madupites is diopside an abundant early erystallising phase, and its later suppression in the wyomingites is clearly shown by the enrichment of diopside in the groundmass and residual glass of LH. 7 (Figs. 3 and 11); leucite is not an early crystallizing phase except in one leueite-madupite (LH. 8). In the system MgeSiOa-KA]Si206-SiO e (Fig. 12) there is some conformity of the normative components of these rocks to the trend of the boundary curve separating the fields of olivine and leucite. This diagram similarly suggests that a madupite liquid could give rise to wyomingite and orendite by ffactionation of olivine and leucite, the former being commonly found as a scarce early crystallizing phase. In both Figs. 11 and 12, the two madupites plot on or close to the joins leucite- olivine and leucite-diopside; in the norm of the madupite (LH. 16) (Table 12), the small amount of orthoclase may be the result of the small amount of secondary chalcedony in the analysed specimen, and it is likely (and hereafter assumed) that it is as silica poor as the madupite VII (Table 12). The plot of the two

Volcanic Rocks ~rom the Leucite Hills, Wyoming 55

DL

31

�9 7. //.Tg / Lo 70. ~" ~ \

I "'1.8 ,7 ~ \ Lc Or St0z

Fig. 11. The normative components of the analysed rocks (Table 12) plotted in a generalised representation of the system CaMgSi206(Di)-KAISi~O6(Lc)-SiO 2 (ScaRIeR and Bow]~N, 1938). The position of 7g is uncertain as F%Oa has not been determined in the microprobe analysis. If there is insufficient FeO to combine with TiO2, then normative sphene (tn) is produced, and hence normative diopside will decrease. The field of potassium feldspar (unlabelled) is the small area between Or and SiO 2. Qz represents the various polymorphs of silica

F0

Lc Or S~02 Fig. 12. The normative components of the analysed rocks (Table 12) plotted in a generalised representation of the system Mg2SiO4-KA1Si~O6-SiO 2 (Sc~AIRER, 1954). The fields of potassium feldspar and rhombic enstatite are very close to the side-line Or-SiO 2. The residual glass 7g is joined to its host rock 7 by a dashed tie-line. En is protoenstatite, and Qz is the various polymorphs of SiO~

56 IAN S. E. CARMICIIAEL:

madupites on these two joins (Figs. 11 and 12) is a consequence of the order of combinations in the CIPW normative calculation. In this calculation for silica- poor rocks, normative diopside is desilicated to larnite (cs) and olivine before any of the leucite is desilicated to form kaliophilite. An alternative procedure for rocks like madupite would be to partially desilicate the leucite to form kalio- phflite and maintain the diopside. In that ease the madupites would not plot on the ortho-sflicate-leucite or diopside-leucite loins in Figs. 11 and 12, but would plot in the composition triangles kaliophilite-leucite-diopside and kalio- phflite-leucite-olivine (cs free). If the joins leucite-diopside and leucite-forsterite are thermal divides (u and TILLEu 1962, Fig. 44) then no process of fractional crystallization can cross them, and hence it may be impossible to derive orendite-wyomingite from a madupite parent by such a process. The system Mg~SiOd-KA1SiOd-SiO2-H20 has been determined at various water vapor pressures by Dr. W. C. LuTI~ who kindly allowed the writer access to his results before publication. In this system, the phlogopite volume is intersected by the liquidus surface at all but relatively low water vapor pressures, and in projection separates the field of olivine from sanidine. Simultaneously the feldspar field is enlarged, the lcueite field constricted, and the olivine-enstatite field boundary (Fig. 12) intersects the top of the sanidine field at low P~2o and then the phlogopite field at higher PH~O. The trend of the points I I I to 7 and 7g in Fig. 12 is similar to that of the boundary curve separating the fields of phlogopite and olivine in the hydrothermal system, indicating, in accord with the petrographic evidence, that fractionation of these two phases from wyomingite could generate an orcndite. However, only in a general way can this system be indicative of the crystallization history of these rocks, as the natural leucite is non-stoiehiometric, diopside is common and the phlogopite is fluorine-bearing (Table 6). By comparison to the high temperature of congruent melting of synthetic fluorphlogopite (1497 ~ C) (K]~LL~.r et al., 1959) these natural phlogopitcs could be assumed to have an intermediate stability between that of hydroxyphlogopite (YoDm~ and EUGSTm~, 1954) and fluorphlogopite. As the phlogopite phenocrusts in the Leucite Hills volcanics almost always show greater or less evidence of resorption, phlogopite presumably becomes unstable at the low volatile pressures of the earth's surface, or under conditions just prior to eruption. This evidence of instability of phlogopite coupled with its rarity as a groundmass constituent in all the rocks examined indicates that one of the invariant points in the synthetic system is not represented here; at this point of fixed water pressure and temperature, phlogopite-leueite-sanidine-olivine and liquid all co-exist. At pressures above this invariant point, leucRe-olivine-phlogopite and liquid may co-exist, whereas at lower water pressures olivine-leucite-sanidine and liquid co-exist. Olivine and spinel were commonly the first phases to precipitate in the orendites and wyomingitcs, possibly in a plutonie environment (cf. low CaO in olivine, p. 37), and as crystallization proceeded and presumably volatile pressure increa- sed, olivine became unstable, and reacted with the liquid to produce phlogopite reaction rims. As this reaction is incomplete, it seems that the orendite and wyomingite liquids passed rapidly through the phlogopite stability range on


their way to the surface, where in a low volatile pressure regime, phlogopite becomes unstable. Depending upon the composition of the liquid, either olivine and diopside (olivine-orendites), or diopside alone, were the ferromagnesian phases to precipitate as microphenocrysts in this regime. The other ferromagnesian minerals, especially magnophorite, were amongst the last minerals to crystallize, indicating tha t under surface conditions of volatile pressure an amphibole is stable, whereas a mica is not.

In both the anhydrous system (Fig. 12) and the hydrothermal system at constant water pressure, the course of crystallization requires leucite to appear before sanidine in a liquid similar to an orendite or wyomingite. The occurrence of sanidine phenocrysts in LIt . 9 would therefore indicate tha t prior to eruption the bulk composition of the liquid was in the sanidine field, but upon eruption, with loss of volatile pressure, and consequent enlargement of the leucite field, leucite became the stable phase. However, due to the non-stoiehoimetrie composition of leueite, the relationship of sanidine to leucite in these rocks compared to a synthetic diagram may be rather tenuous. Only rarely has leucite been seen to crystallize before phlogopite (LH. 8) in the Leucite Hills volcanic, but this order of crystallization is characteristic of the Spanish jumillite, in which the sequence is olivine, leucite, diopside, phlogopite and lastly sanidine.

Unless diopside is considered as an additional component in the system Mg2SiO 4- KA1SiO4-SiO2-H20, the crystallization of the madupites cannot even be super- ficially related. I t is probable tha t the addition of diopside as a component will create a series of bounding surfaces between the volumes of phlogopite, olivine, leueite, kahophilite and sanidine and tha t of diopside. As the madupites are silica-poor (Table 12, LH. 16 and VII) and may in projection plot on the undersaturated side of the join forsterite-leucite (p. 55), fractionation of diopside phenoerysts will only enhance the undersaturation in a residual liquid, and the generation of an oversaturated orendite-wyomingite derivative liquid is impossible. I f the phase relations of phlogopite on the liquidus allow the join forsterite- leucite to be crossed, then an orendite-wyomingite derivative liquid could be generated by crystal fractionation. However madupite is the only rock-type in the Leucite Hills for which there is no evidence of the early crystallization of phlogopite, so tha t any genetic relationship of madupite to orendite-wyomingite may be more complex.

6. Liquidus Temperatures at 1 Atmosphere

Exploratory liquidus determinations of five rocks from the Leucite Hills were made at 1 atmosphere by Professor W. S. FYFE. The length of the runs varied from between 2 and 20 hours in platinum capsules, and the results are given in Table 14. No oxide phase appeared on the liquidus, and the first phase in all cases was either leucite or olivine; this is perhaps surprising as the madupite (LH. 16) contains abundant phenocrysts of diopside. The temperature of beginning of melting was determined by the fritting of the charge; the madupite (LH. 16) had a markedly higher solidus temperature than the others. I t was not possible to completely crystallize the glassy groundmass of LH. 7, and the temperature of the solidus is unknown.

The liquidus temperatures have been plotted in Fig. 13 against the weight ratio Fe20 a ~ FeO/Fe20 s ~ FeO -~ MgO, the values of which are also given in Table 14. Also plotted in Fig. 13 are the liquidus temperatures of the Hawaiian tholeiitic

58

Table 14. Liquidus temperatures ( • 15 ~ at I atmosphere

IA~ S. E. C~nMIc~L:

LH.1 LH.7 LH.10 LH.12 LH.16

Liquidus ~ 1200 1165 1275 1215 1245 1st phase Leueite Leucite Olivine Leucite and olivine Leueite Solidus 1010 ~ < 1000 ~ 1010 ~ 1010 ~ 1040 ~

F%0 s + FeO 0.394 0.371 0.299 0.378 0.368 MgO + F%0 3 + FeO

1500 T%

lz/O0 \~'~,

1200 o /o I

1200

7100

�9 r) zooo

SO~2 013

7G o

o

o 7

\ x

~ o:s o,6 o,7 o,8

Fig. 13. The liquidus temperatures at 1 atmosphere (Table 14) plotted against the weight ratio FeO + 1%203/Mg0 + FeO + F%03 of the rocks. The small filled circles are the temperatures of beginning of melting. T represents the trend of the Hawaiian tholeiitie series (TrnLEu et al., 1963) and A represents the Hawaiian alkaline series (TILLEY et al., 1965). The dashed lines represent the liquidus temperatures of accumulative rocks

and alkaline series (TILLEY, YOD~R and S c ~ A m ~ , 1963, 1965), the dashed line representing liquidus temperatures of picrites or lavas accumulat ive in olivine. The Lcucite Hills specimens have liquidus temperatures similar to those of the more magnesian Hawai ian tholeiites, bu t are of course themselves very much more magnesian. The large melt ing interval of these volcanics and its persistence at 1000 bars water vapor pressure~ suggests tha t with a volatile pressure equivalent to t ha t of the base of the crust (9 k. bars), this melting interval ~dll persist. I f it may be assumed tha t the compositions of the Leucite Hills volcanics are representative of liquids unmodified by exogenous material, then the large melting interval seems to preclude a crustal origin. A n y crustal fusion process would tend to predominant ly produce liquids representing the lower temperature fractions, one of which could reasonably be expected to be the residual glass of LH. 7 (Table 12), and it is the absence of any such voluminous liquid in the Leucite Hills which

The liquidus temperatures of LH. 1 and LH. 12 were lowered approximately 40~ at 1000 bars water vapor pressure, and the solidus temperatures accordingly.


indicates that these volcanics are not the result of crustal fusion. Liquids of the bulk compositions given in Table 12, provided that they are unmodified in their composition subsequent to their generation, can only originate beneath the crust.

F. Petrogenesis

Although over half of the components of the Leucite Hills volcanics may be represented by the system CaMgSi~OG-Mg2SiO4-KAlSi20s-SiO~, the essence of their genesis is how these rocks acquired a composition so that they may, at least partially, be represented in terms of these components. To the writer there are three crucial factors to be considered in any hypothesis of their origin; their composition, their extreme rarity in the igneous record, and the absence of any equivalent plutonic heteromorph. It is on the unusual assemblage or concentrations of both major and trace elements that the variety of hypotheses of the origin of potassium-rich basic magma hinge. These have been summarised by TUl~NE~ and V~HOOGSN (1969), and broadly fall into three categories ; those that require the preferential fusion or incorporation of biotite, which may be concentrated by fraetionation in a liquid or by being the residue of anateetic fusion; those that require the interaction of magma (basic or carbonatitic) with crustal, particularly granitic, material; and lastly those that consider crystal-liquid fractionation. The chemical composition of these lavas, namely the combination of high Mg, Cr and undoubtedly Ni, together with K, l~b, St, Ba, Zr, F, P and S is not a feature solely confined to these voleanics of the Leucite IIflls and Western Austra- lia. It is a characteristic of all potassic basic igneous rocks, particularly those of the Highwood mountains, Montana (IIU~LBUT and G~IGGS, 1939), the lavas of Uganda (HIGAZY, 1954), kimberlites (DAwsoN, 1962) and of certain lampro- phyres, particularly minettes. As these elements are not normally associated in the common igneous rocks, representing in a simple way, a combination of the two opposite extremes of the normal basalt-rhyolite series, it is tempting to examine the role of phlogopite, in which with the exception of Zr, P and S, these elements are conveniently combined (Table 6). Thus we arrive at the first of the three broad hypotheses which seek to explain the origin of this type of igneous product. The fractionation of biotite into hot basaltic magma has been advocated by Bow~N (1928), and a reverse process of fusion of biotite-pyroxenite, biotite-peridotite and amphibolite, concentrated as residues on the expulsion of anatectie granite, has been suggested by WATeRs (1955). In Table 15, the norms of the Leueite Hills volcanics and the Spanish jumillite have been recalculated into anhydrous phlogopite; the amount of "normative" phlogopite is controlled by the amount of Fe, Mn, Mg and Ti in the analyses, so that only about half of the rocks can be expressed in terms of phlogopite. However it is the normative composition of the balance which looks singularly unpromising; this is made up predominantly of quartz and orthoclase, with subordinate wollastonite, alkali metasi]ieate and apatite, an unusual assemblage quite unlil~e that of any igneous rock known to the writer, but nevertheless significantly oversaturated with respect to silica. The amount of normative

60 IA~ S. E. CAnnerY, L:

Table 15. Recalculated norms with phlogopite

S-1 LH. 1 LH.7 LH. 10 LH. 12 LIt. 16

Qtz 10.8 8.6 16.3 13.9 13.4 4.0 Or - - 26.7 29.5 17.8 32.8 5.6 Di 10.6 8.1 2.1 4.2 3.7 20.8 En 7.1 . . . . . ns - - 2.1 1.8 2.2 2.4 1.5 ks - - 2.8 6.6 3.1 4.0 0.4 ap 4.4 4.4 3.4 3.0 3.4 3.7 Zr 0.2 0.4 0.4 0.4 0.4 0.4 Phlogopite 61.8 42.6 36.4 53.4 37.6 57.9 Rest 4.8 * 3.8 3.2 1.7 2.2 5.5

�9 Includes 0.3 anorthite.

alkali metasilicate could be reduced, but not eliminated by assuming that the phlogopite was deficient in Al in the Z-sites (cf. Table 6). Although a granitic liquid is unlikely to have a composition so deficient in soda, so that these rocks could not result from the fusion of phlogopite by an acid liquid, there is the alternative possibility, namely a liquid of phlogopite composition (ultrabasic, silica deficient) incorporating granitic or sialie material. We come therefore to the second of the three hypotheses advocated to explain potassinm-rich basic liquids. Such a mechanism of selective incorporation has been suggested by WILLIAMS (1936) to account for the potassie rocks (minettes) of the Navajo area, Arizona, and by ttOLM~S (1950) and HIcAzr (1954)who sought to derive katungite by the assimilation of granitic material by a carbonatitic magma. An allied proposal has been adumbrated by Moo~E (1962) who related the molecular ratio K20/Na20 ~- K20 in the Cenozoic volcanies of the western United States to the Bouger gravity anomalies. There is a convincing correlation between the location of rocks with high values of this ratio and negative gravity anomalies, or thickness of sialie crust. MOOl~E's conclusion that the "Cenozoic igneous rocks (which includes the Leucite Hills) apparently acquired their alkali ratios from the crust in which they were formed - - probably by melting and contamination" requires a selective incorporation of K (Table 15) (unless Na has been subsequently lost!), which all the experimental data on granitic liquids suggests to be unlikely (TuTTLE and Bowv,~r 1958). Although granitic xenoliths, now in a fused and friable state (p. 29) are found in the Leueite Hills, they are not abundant (CRoss, 1897). If they contributed markedly to the overall composition of these voleanics, the evidence is now occult. The problem with any hypothesis of selective fusion is that the process always achieves that which is desired; any speculations on the content and nature of sialie contamination can only be resolved by isotopic (SrST/Sr s6) examina- tion of these rocks, but on the present data an ultra basic liquid selectively incorporating granitic material does not have great appeal. I t seems unlikely that the lavas of the Leucite Hills could be produced by fusion at the base of the crust ~ as unless their melting interval rapidly diminished with

It is difficult to be certain that such potassic lavas are confined to continental regions; although they have not been found in the oceanic islands, their absence does not necessarily imply a genetic connection to sialie material on account of their scarcity.


pressure (p. 58), their fusion temperatures are comparable to basaltic magma and are therefore above the geothermal gradient. However the rarity of the Leucite Hills and Western Australian volcanies implies an equal rarity of either source material or genetic process or a combination of the two, so that a particular blend of circumstances, especially concentration of heat (V]~R~OOGE~, 1946) could provide the observed result. The third hypothesis is crystal fractionation either of an ultrabasie potassic liquid allied to kimberlite (WADE and PnII)E~, 1940; PRID~R, 1960), or of a liquid produced by partial fusion of kimberlitic material. Such a hypothesis demands as starting material many of the requirements of the result that it is supposed to achieve, and it leaves unanswered the origin of kimberlite itself, and particularly the role of potassium and allied elements in the mantle. The behavior of potassium in rocks commonly ascribed a mantle origin is puzz- ling; tI]~IER (1963) found a poorer correlation between K and Th or U in igneous eclogites than in basalts, indicating that K has some mobility, and FORBES (1965) has shown that there is little correspondence between the K content of eclogites and basalts, particularly if the phlogopite, the sole K-bearing phase, is accounted secondary. The composition of the primary undifferentiated upper mantle has been suggested to be a 1:3 mixture of basalt and peridotite (pyrolite), and the calculated composition is given in Table 16 (RINGWOOD et al., 1964). These authors also give analyses of two heteromorphs, a spinel peridotite (epx @ opx) and a garnet peridotite, representative analyses (op. cir.) of which are also shown in Table 16. In both peridotites, potassium is very low compared to the pyrolite, a distinction further substantiated by the analytical work of HA~ILTO~ and Mou~TJou (1965). Two conclusions may be drawn on the assumption that K is fixed in a mineral phase and is not a mobile component; firstly that periodotiter are either crystal residues or crystal precipitates in a high pressure regime, os

Table 16. Chemical analyses o] ultrabasic rocks together with the average anhydrous analyses o/ the Leueite Hills (recalculated)

1 2 3 4 5 6 7

SiOe 43.15 45.58 44.77 38.98 45.39 54.09 55.59 TiO 2 0.58 0.15 0.19 1.36 2.08 2.35 2.42 A120 S 4.00 2.41 4.16 4.34 8.94 9.94* 9.86 F%03 1.67 0.27 -- 5.73 5.63 3.15 3.53 FeO 6.67 6.41 8.21 4.61 1.01 1.48 0.87 MnO 0.13 0.12 0.Ii 0.07 0.14 0.06 0.06 MgO 39.40 42.60 39.22 34.83 11.53 6.99 8.31 CaO 2.66 2.10 2.42 7.57 12.76 4.71 3.61 Na20 0.61 0.24 0.22 0.38 0.86 1.36 1.28 K20 0.22 nil 0.05 1.17 7.89 11.38 11.73 P205 0.08 0.03 0 01 0.97 1.59 1.79 1.36 Misc. 0.83 0.09 0.64 - - 2.18 2.80 1.38

1 = Pyrolite; 2 = spinel peridotite; 3 ~ garnet peridotite (I%INGWOOD et al., 1964); 4 ~ average kimberlite (Noe~oLI)s, 1954); 5, 6 and 7=average madupite, wyomingite, and orendite respectively (Table 12). * Value of A120 S is the average of LII. 1 and LH. 7 as ZrOe was not determined in III and IV (Table 12).

62 IA~ S. E. C~_a.~Iem~EL:

secondly that K varies considerably in the mantle with local high concentrations. If potassium is present to any extent in the upper mantle, and as pyroxene and garnet almost completely exclude K, it is likely to be found in phlogopite or perhaps even in rutile (cf. priderite, a K-Ba-rutfle), a characteristic mineral of the eclogite facies. Both minerals satisfactorily concentrate K, Ba, Ti and perhaps Sr, and represent in part the essential chemical distinction between peridotite and kimberlite. Kimberlite, which contains abundant xenoliths of garnet- peridotite indicative of a high-pressure (BOY]) and MAcGrEGOr, 1964) and a high-temperature (DAvis and BOY]), 1965) environment is itself generated at considerable depth by a complex interplay of gas-liqnid-solid (crystals and xenoliths) reactions (O'H)mA and MErcY, ] 963). The average analysis of "basaltic" kimberlite (NOCKOL])S, 1954) is given in Table 16, and in comparison to pyrolite it is enriched in Ti, Ca, K and P together with the minor or trace elements Ba and Sr (DAwson, 1962). In order to illustrate the magnitude of the degree of enrichment required in any process of crystal fractionation and so account for the Leucite Hills volcanics (cf. WA])E and P~ID~ , 1940), the average recalculated analyses of madupite, wyomingite and orendite (Table 12) are also given in Table 16. O'tIA~A and u (1963) and O'HAnA (1965) have provided an ingenious hypothesis whereby a kimberlite liquid could be produced by the fractional crystallization of omphacite and garnet (cf, PRIDW~, 1960) from a liquid produced by a partial fusion of garnet-peridotite. Such a process would satisfactorily achieve a liquid with a high K/Na ratio, together with enrichment in Rb, Cr and Ni, but whether it could enrich the liquid in Ca as required by the average analyses of Table 16 is less likely. Much of this Ca may be part of a carbonate which according to their hypothesis is also a product high pressure fractionation, subsequently resulting in a carbonatite. However it is the further fractionation of a kimberlitic liquid, if such exists, that has been proposed as a possible parent of some of the highly potassic volcanics (WADE and P~ID~R, 1940; th~ID~, 1960). The more impoverished the kimberlitic liquid becomes in silica, and hence enriched in K and allied components, the less the likelihood that this trend can be reversed to produce the typical oversaturated wyomingite or orendite, for with the exception of a spinel, all silicate phases will perpetuate or enhance this silica impoverishment (O'HA~A, 1965). I t seems therefore, that madupite is the only possible derivative of the high- pressure fractional crystallization of a liquid produced by fusion of garnet- peridotite (Table 16); the relationship of madupite to a liquid of orendite- wyomingite composition has been considered on p. 57 and may not be the result of crystal-liquid fractionation. The absence of any plutonic rock having the composition of the orendites and wyomingites of the Leucite Hills could suggest that some process only found in a volcanic environment may have been instrumental in their genesis. Moreover, there arc no xenoliths other than gabbro and granite, which could conceivably represent an accumulative fraction of a parent liquid of different composition. I t is not easy to predict the composition of such a crystal accumulate, but as olivine and spinel are the earliest phases to precipitate in the orendites and

Volcanic I%ocks from the Leucite Hills, Wyoming 63

wyomingites, and diopside in the madupites, it could well be peridotific. However, the relationship of the madupites by some process of crystal fractionation to the other lava types is obscure unless phlogopite is also an accumulative phase from a madupitic liquid (p. 57), for which there is no sound evidence petro- graphically. Thus the combination of a hypothetical peridotite with a highly potassic liquid again provokes comparison with kimberlite. The field and petrological evidence of kimberlites indicate that a fluid phase has been dominant in the generation and emplacement of these bodies, and the potassium metasomatic effects of this fluid are characteristic and extensive (O'HARA and MERCY, 1963; DAWSO?r 1962). Moreover there are extensive volcanic breecias surrounding the Western Australian plugs attesting to the partici- pation of gas in their emplacement (PRIDER, 1960). Could it be that such a fluid, having a composition not unlike these highly potassic orendites and wyomingites is an essential part of their genesis ? (PRID~R, 1960). BOWEl'S plea that a"volatile component is exactly like a Maxwell demon; it does just what one may wish it to do" (BowE?r 1928, p. 282) does not endorse an origin with a stipulated role and composition of a high-pressure gas phase. To the writer, it seems strange that with all the vagaries of composition and behavior of a fluid phase, the genetic process involved in the origin of these highly potassic liquids has twice produced (Leueite Hills and Western Australia) an almost identical product, although with little to indicate the path taken. This augurs well for some less random process of erystM-liqnid fractionation, albeit with a gas phase, but not dominantly dependent upon it; the path taken is uncertain and will have to await some further experimental data particularly on the role of potassium in pyrolite partially fused at high-pressure.

C~. Conclusion

The wide melting interval at 1 atmosphere of the volcanics from the Leucite Hills and its' persistance at 1000 bars water vapor pressure, suggests that these liquids could not be produced by partial fusion in the crust. If they are, it is difficult to account for the absence of liquids (or rocks) having a composition similar to the glassy groundmass of LH. 7 (Table 12), which should be close to the dominant eruptive type in a sequence derived by partial fusion of crustal material. There is little evidence of modification of any possible parental liquid by incorporation of crustal material, and the amount and type of this, will, if present, have to await isotopic study. However if contamination or assimilation is a factor contributing to the composition of these lavas, then a madupitic liquid is the most suitable parent requiring less selectivity of added material which could correspond more or less to a potassic granite (Table 12). This would tend to preserve, or even enhance, the K/Na ratio in the contaminated orendite- wyomingite liquids, but may not cause the required increase in the characteristic alumina deficiency of these rocks. Madupites, which are strongly undersaturated with silica (Table 12, LIt . 16 and VII), are unfortunately absent in the volcanic assembleges of Western Australia (WxDE and PnII)ER, 1940; PRIDER, 1960), SO that the hypothesis for the derivation of wyomingite by granitic contamination of madupite lacks universality.

64 IAN S. E. CARMICHAEL-"

I f this suite of volcanics, and part icular ly madupite , is produced by crystal fract ionation f rom a parent liquid (perhaps pyrolite) with a normal K / N a ratio (Table 16), and no alumina deficiency, then only a limited number of options exist. The fract ionat ing crystals in combinat ion will have to be sodie almost to the tota l exclusion of potassium, and a luminum will have to be in excess of alkali. Such a combinat ion could be provided by jadeite and garnet, which implies a high-pressure origin, probably related to kimberlite (O'HA~A, 1965). No phase other t han jadeite separates Na f rom K ; the possibility of fract ionation of a sodic magnesian amphibole does not look promising as a means of deriving a potassic magnesian liquid in the light of the experimental da ta on the high- pressure wet melt ing of basalt (YoDE~ and TILLEr, 1962).

I n summary it would appear tha t a madupit ic liquid could be produced by fract ionation of an eclogitie assemblage f rom a liquid produced by part ial fusion of garnet-peridoti te (O'HAI~A, 1965), bu t thereafter the derivation of an orendite- wyomingite liquid from a madupi te by a process of crystal f ract ionat ion is obscure, unless phlogopite plays an inst rumental role. This would tend to impoverish the resul tant liquids in just those elements with which they are enriched, so tha t contaminat ion m a y indeed be impor tan t as a genetic factor.

Acknowledgements. Professor J. V. SmT~ and the Department of Geophysical Sciences of the University of Chicago generously provided the opportunity and instruction to use the electron- microprobe. Dr. S. 0. AG~E~, Professors F. J. TU~NE~ and 0. F. TUTTLE lent the writer the essential specimens, Dr. W. C. LVTH lent a manuscript before publication, Professor W. S. FYFE made some experimental runs, and Professor B. W. EVANS smoothed the transition from one probe to another. To all these people, and to Professor J. R. GOLDSMITH the writer is greatly indebted. A National Science Foundation fellowship and subsequent research grants (GA. 480 and 338) materially helped this study.

References Bow]~N, N.L.: The evolution of igneous rocks. Dover Publications, Inc. (1956) BOY]), F.R., and I. D.MAcG~Eoo~: Ultramafic rocks. Ann. Rept. Geophys. Lab. Yearbook

6~, 152--156 (1964). BROWN, G. M. : Pyroxenes of the early and middle stages of fractionation of the Skaergaard

intrusion, East Greenland. Mineral. Mag. 31, 511--543 (1957). C~IC~AEL, I. S. E. : Pantelleritic liquids and their phenocrysts. Mineral. Mag. ]3, 86--113

(1962). - - The iron-titanium oxides of salie volcanic rocks and their associated ferromagnesian

silicates. Contr. Mineral. and Petrol. 13, 36--64 (1967). C~AYEs, F. : Descriptive and genetic significance of normative ns. Ann. Rept. Geophys. Lab.

Yearbook 63, 190--193 (1964). COMBE, A.D., and A. HOLMES: The kalsilite-bearing lavas of Kabirenge and Layakauli,

South-west Uganda. Trans. Roy. Soc. Edinburgh 61, 359--379 (1945). COO~BS, D. S. : Ferriferous orthoelase from Madagascar. Mineral Mag. 30, 4 0 9 4 2 7 (1954). C~oss, W.: Igneous rocks of the Leucite Hills and Pilot Butte, Wyoming. Am. J. Sci. 4,

115--141 (1897). C~UrT, E. F. : Minor elements in igneous and metamorphic apatite. Geoehim. et. Cosmochim.

Acta 30, 375--398 (1966). DAvis, B. T. C., and F. R. BoYD: The join Mg~Si2Os-CaMgSi~O 6 at 30 kilobars pressure and

its application to pyroxenes from kimberlites. J. Geophys. Research 71, 3567--3576 (1966). DAwsoN, J. B.: Basutoland kimberlites. Bull. Geol. See. Am. 7], 545--560 (1962).


DEER, W.A. , R. R. HOWIE, and J. ZVSSMAN: Rock-forming minerals, vol 5, Non-silicates. Longmans, London: Green & Co. Ltd. 1962.

- - - - - - Rock-forming minerals, vols. 2 and 4, Chain silicates and framework silicates. London: Longmans, Green & Co. Ltd. 1963.

ERLANK, A. J. , and P. K. HOF~EYR: K/Rb and K/Cs ratios in Karoo dolerites from South Africa. J. Geophys. Research 71, 5439--5445 (1966).

EUGST]~R, H . P . , and D. R. WONES: Stability relations of the ferruginous biotite, annite. J. Petrology. 3, 82--125 (1962).

FAUST, G .T . : Phase transition in synthetic and natural ]eucite. Schweiz. mineral, petrog. Mitt. 43, 165--195 (1963).

FORBES, R. B.: The comparative chemical composition of eclogite and basalt. J. Geophys. Research 70, 1515--1521 (1965).

- - , and S. BANNO: Nickel-iron content of peridotite inclusion and cognate olivine from an alkali-olivine basalt. Am. Mineralogist 51, 130--140 (1966).

- - , and H. KuNo: The regional petrology of peridotite inclusions and basaltic host rocks. Upper Mantle Sympos. New Delhi 1964, Intl. Un. Geol. Sci. p. 161--179 (1965).

FUDALI, R. F. : Experimental studies bearing on the origin of pseudoleucite and associated problems of alkalic rock systems. Bull. Geol. Soc. Am. 74, 1101--1126 (1963).

GuNN, B. M. : K/Rb and K/Ba ratios in Antarctic and New Zealand the]eiites and alkali basalts. J. Gcophys. Research 70, 6241--6247 (1965).

HA~nLTON, W., and W. MOUNTJOY: Alkali content of alpine ultramafic rocks. Geochim. et Cosmochim. Acta 29, 661--672 (1965).

HEIER, K. : Uranium, thorium and potassium in eclogitic rocks. Geochim. et Cosmochim. Acta 27, 849--860 (1963).

- - W. CO~PSTON, and I. M c D o u o ~ L : Thorium and uranium concentration and the isotopic composition of strontium in the differentiated Tasmanian dolerites. Geochim. et Cosmo- chim. Acta 29, 643--659 (1965).

HENS]~AW, D. E. : The structure of wadeite. Mineral. Mag. 30, 585--595 (1955). HmAzY, R. A. : Trace elements of volcanic ultrabasic potassic rocks of southwestern Uganda

and adjoining part of the Belgian Congo. Bull. Geol. Soc. Am. 6~, 39--70 (1954). HILLEBRAND, W. F., and G. E. F. LUNDELL: Applied inorganic analysis. New York: John

Wiley & Sons 1953. HOLLAND, H . D . : Some applications of thermochemical data to problems of ore deposits.

I. Stability relations among the oxides, sulfides, sulfates and carbonates of ore and gangue minerals. Econ. Geol. 54, 184--233 (1959).

HOLMES, A.: Petrogenesis of katungitc and its 8 associates. Am. Mineralogist 35, 772--792 (1950).

- - , and H. F. HARWOOD: The petrology of the volcanic area of Bufumbira. Mere. Geol. Survey Uganda 3 (1937).

HURLBUT, C. S., and D. GRmGS: Igneous rocks of the Highwood Mountains, Part I. Bull. Geol. Soc. Am. ~0, 1043--1112 (1939).

KELLEY, K . K . , 1=~. BARANY, E. G. KING, and A. U. C~RIST~,~SEN: Some thermodynamic properties of fluorphlogopite mica. US. Bureau of Mines. Rept. 5436 (1959).

KEMP, J. F. : The Leucite Hills of Wyoming. Bull. Geol. Soc. Am. 8, 169--182 (1897). - - , and W. C. KNm~T: Leucite Hills of Wyoming. Bull.Geol. Soc. Am. 4, 305--336 (1903). KUME, S., T. MATSUMOTO, and M. KoIzuMI: Dense form of germanatc orthoclase (KA1G%0s).

J . Geophys. Research 71, 4999--5000 (1966). Kvs~gao, I. : Si-AI relations in clinopyroxenes from igneous rocks. Am. J . Sei. 258, 548--554

(1960). LE BAS, M. J. : The role of aluminum in igneous clinopyroxenes with relation to their parentage.

Am. J. Sci. 269, 267--288 (1962). LESSING, P., R. W. DECKER, and R. C. REYNOLDS: Potassium and rubidium distribution in

Hawaiian lavas. J . Gcophys. Research 68, 5851--5855 (1963). MATn~ESON, A. M., and A. D. WADSLEu The crystal structure of eryptomelane. Am. Mineral-

ogist 35, 99--101 (1950). MOORE, J. G." K/Na ratio of Cenozoic igneous rocks of the western United States. Geochim.

et Cosmochim. Acta 26, 101--130 (1962).

5 Contr. !~Iineral. and Petrol., Vol. 15

66 IA~ S. E. C~MIC~EL: Volcanic Rocks from the Leueite Hills, Wyoming

NOOXOLDS, S. R. : Average chemical compositions of some igneous rocks. Bull. Geol. Soc: Am. 6 5 , 1007--1032 (1954).

1NTORRISH, K.: Priderite, a new mineral from the leucite-lamproites of the west Kimberley area, Western Australia. Mineral. Mag. 29, 496--501 (1951).

O'HA~A, M. J. : Primary magmas and the origin of basalts. Scot. J. Geol. 1, 19--40 (1965). - - , and E. L. P. MERCY: Petrology and petrogenesis of some garnetiferous peridotites. Trans.

Roy. Soc. Edinburgh 65, 251--314 (1963). - - , and H. S. YODEl: Partial melting of the mantle. Ann. Rept. Geophys. Lab. Yearbook

6 2 , 66--71 (1963). OsA~, A.: ~Jber einige Alkaligcsteine aus Spanien. Festschr. Rosenbusch. Stuttgart 1906. P~IDER, R. T. : Some minerals from the leucite-rich rocks of the west Kimberley area. Western

Australia. Mineral. Mag. 25, 373--387 (1939). - - The leucite lamproites of the Fitzroy basin. Western Australia. J. Geol. Soe. Australia

6 , 71--118 (1960). - - •oonkanbahite, a potassic batisite from the lamproites of Western Australia. Mineral.

Mag., Tflley Vol. 34, 403-405 (1965). - - , and W. F. COLE: The alteration products of olivine and leucite in the leucite-lamproitcs

from the west Kimberley area, Western Australia. Am. Mineralogist 27, 373--384 (1942). Rr~GwooD, A., E., L D. MAcGI~EGOR, and F. R. BoYD : Petrological constitution of the upper

mantle. Am. Rept. Geophys. Lab. Yearbook 68, 147--152 (1964). SCWAraER, J. R. : The system K20-MgO-A12Os-SiO 2. g. Am. Ceram. Soe. 87, 501--533 (1954). - - , and N. L. BowE~: The system leucite-diopside-silica. Am. J. Sci. 85 A, 289--309 (1938). - - - - The system K20-AI~Oa-SiO 2. Am. J. Sci. 253, 681--746 (1955). S~KI~, T., and J.V. SMITE: Minor element distribution in olivine. Geol. Soc. Am. Abs.

1966, Ann. Mtg. 203 (1966). TILLEY, C.E., H. S. YODER, and J. F. SC~RE~: Melting relations of basalts. Ann. Rept.

Geophys. Lab. Yearbook 62, 77--84 (1963). - - - - - - Melting relation of volcanic tholeiite and alkali rock series. Ann. Rcpt. Geophys.

Lab. Yearbook 64, 69--82 (1965). TVl~ER, F. J., and J. Vv.~HOOGE~: Igneous and metamorphic petrology, 2nd ed. 1--694:

McGraw-Hill Book Co. 1960. TVTTLE, O. F., and N. L. BOWEN: Origin of granite in the light of experimental studies in the

system NaA1SisOs-ICALSiaOs-SiO~. Geol. Soc. Am. Mere. 74, 1--153 (1958). VE~OOGE~, J.: Volcanic heat. Am. J. Sei. 244, 745--771 (1946). - - Distribution of titanium between silicates and oxides in igneous rocks. Am. J. Sci. 260,

211--220 (1962a). - - Oxidation of iron-titanium oxides in igneous rocks. J. Geol. 70, 168--181 (1962b). WADE, A., qnd R. T. P~II)]~: The leucite-bearing rocks of the west Kimberley area, Western

Australia. Quart. J. Geol. Soc. London 96, 39--98 (1940). WATERS, A.C.: Volcanic rocks and the tectonic cycle. Geol. Soe. Am. Spec. Paper 62,

703--722 (1955). WroTE, R.W.: Ultramafic inclusions in basaltic rocks from Hawaii. Contr. Mineral. and

Petrol. 12, 245--314 (1966). WIL~Z~wS, H.: Pliocene volcanoes of the Navajo-Hopi country. Bull. Geol. Soc. Am. 47,

111--172 (1936). WONES, D.R.: Phase equilibria of "Ferriannite" KFea2+Fe3+SiaO10(OH)2. Am. J. Sci. 261,

581--596 (1963). - - , and H. P. EUGSTER: Stability of biotite: experiment, theory and application. Am. Miner-

alogist 50, 1228--1272 (1965). YO1)ER, H. S., and H. P. EVGSTER: Phlogopite synthesis and stability range. Geochim. et

Cosmochim. Acts 6, 157--185 (1954). - - , and C. E. TILLEY: Origin of basalt magmas: an experimental study of natural and syn-

thetic rock systems. J. Petrol. 3, 342--532 (1962).

IA~ S. E. CAR~rC~AEL Dept. of Geology and Geophysics, University of California Berkeley 94720

the mineralogy and petrology of the volcanic rocks from...

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