American Mineralogist, Volume 67, pages 907416, l9E2

The Marathon Dikes: ultrabasic lamprophyres from the vicinityof McKellar Harbour. N.W. Ontario

R. Genrs Plerr euo RocBR H. MrrcHBI-I-

Department of Geology, Lakehead UniversityThunder Bay, Ontario P7B 5El, Canada

Abstract

Lamprophyre dikes from the vicinity of McKellar Harbour, Lake Superior, consist ofphenocrysts of olivine (mg : 0.81-0.87) and titanian phlogopite set in a groundmass oftitanian phlogopite, spinel, apatite, perovskite and carbonate. Spinels range in compositionfrom titanian-aluminous-chromites to ulvcispinel-magnetites and a compositional trendthat indicates affinities with alkaline rock-carbonatite magmatism. Rare groundmassilmenites are MnO-rich (2-8 wt.%). Minerals of a lath-shaped habit, now composed of anintimate mixture of andraditic garnet and calcite, are thought to be melilite pseudomorphs.Rare small xenoliths consisting of granular intergrowths of picroilmenite (MgO : 7-10wt.Vo) and carbonate-talc-quartz pseudomorphs after olivine(?) occur. The dikes areextremely silica-undersaturated, alkaline, ultrabasic CO2-rich rocks of alnciitic affinity.They represent a previously-unrecognized period of alkaline magmatism (1653 m.y.) in theLake Superior Basin.

Introduction

Platt and Mitchell (1979) introduced the name"Marathon Dikes" as a reference term of conveni-ence for the numerous dikes, of diverse mineralogyand age, found in the vicinity of Marathon, N.W.Ontario (48"43'N. 86"22"W). This second contribu-tion to the petrology of these dikes describes apreviously unrecognized group of lamprophyres,with alnoitic affinities. from the McKellar Harbourshoreline of Lake Superior. Additional exposuresoccur in road and rail cuts in the same general area.Although the dikes described have a relativelyrestricted distribution, we believe that their trueextent is camouflaged by the poor outcrop withinthe dense boreal forest cover of this region. Walker(1957) initially mentioned the occurrence of lampro-phyres in the McKellar Harbour area but did notelaborate on their mineralogy.

The lamprophyres intrude the metavolcanic-metasedimentary sequence of the east-west trend-ing Archean Schreiber-White River greenstone belt.The predominant strike of the thin (generally lessthan I m) vertical dikes is north-south. However,for short distances, they often follow the dominanteast-west regional trend of the country rocks.

As a group, the lamprophyres are essentially fine-to medium-grained, and when fresh they are charac-

teristically dark grey-green in color. In contrast, theweathered surfaces are typically dun-brown. Inmany dikes, small mica phenocrysts, and in someexamples, large mica macrocrysts a centimeter orso in diameter are observed. Olivine phenocrysts,commonly pseudomorphed, are readily apparent.In general the dikes show no evidence of chilledmargins, and only one composite dike has beenobserved. Recent whole rock Rb-Sr isotope studies(Platt and Mitchell, in preparation) indicate an ageof 1653t122 (2o) m.y. for the McKellar Harbourdike suite. Such an age is considerably older thanthe Keweenawan igneous activity associated withthe Mid-continent rift system in the Lake Superiorbasin (l.e., 950-1170 m.y.; Douglas, 1980).

We believed initially that the dikes were in someway related to the carbonatitic activity of the PrairieLake complex (1023-174 m.y.; Bell and Blenkinsop,1980) or the alkaline activity of the Coldwell com-plex (1M4.5-+6.2 (2o) m.y.; Platt and Mitchell,1982), both of which lie in close proximity toMcKellar Harbour (30 km and 5 km respectively).These two complexes, together with that of KillalaLake (1098l-48 m.y.; Bell and Blenkinsop, 1980)may have formed within an aulacogen of a plume-generated triple junction associated with the Superi-or Mid-continent rift (Mitchell and Platt, 1978).However, petrological studies (Currie, 1980 and

0003-fix)v82l09 l0-0907$02.00 w

PLATT AND MITCHELL: ULTRABASIC LAMPROPHYRES

Mitchell and Platt, 1982) of the Coldwell complexhave shown that no dikes ofthe type described hereoccur within this complex.

These observations and the Rb-Sr age determina-tion clearly demonstrate that the dikes are notconnected with the Neohelikian (950-1170 m.y.,Douglas, 1980) magmatism but are examples of apreviously unreported Paleohelikian/Aphebian peri-od (1653 m.y., Platt and Mitchell, in preparation) ofalkaline magmatism in the Lake Superior Basin.Alkaline rocks and carbonatites of a similar agehave been reported, however, from the Kapuskas-ing High 200 miles to the east of the Marathon area(Currie, 1976).

Petrography

Mineralogically, all the dikes studied consist ofvarying proportions of olivine, phlogopite, primarycarbonate, spinel and apatite. Olivine is strictly aphenocrystic phase and shows varying degrees ofalteration to either a talc-dolomite-quartz mixtureor to serpentinelike minerals; complete pseudo-morphing is common. Phlogopite occurs as pheno-crysts and as a groundmass phase. Apatite andspinel form small acicular and subhedral grains.They are minor but significant phases. Smallopaque to deep red perovskite crystals have devel-oped in some of the dikes. The above minerals areset in a mesostasis of anhedral primary carbonate.

Many of the dikes are characterized by the occur-rence of small lath-shaped crystals. These crystalsare now completely pseudomorphed by an intimatemixture of andraditic garnet and carbonate. Similarpseudomorphs described by von Eckermann (1948),from rocks of the Alno complex, were considered tobe pseudomorphs after melilite. Accordingly, webelieve that melilite was a common mineral in thelamprophyres. Pyroxene is absent in all the dikesstudied. In one particular dike, we observed therare assemblage calcite, phlogopite, tetraferriphlo-gopite, Zr-Ti-rich andradite, monticellite, perov-skite, and spinel, together with pseudomorphs aftermelilite. This dike has in part been described byPlatt and Mitchell (1979). The absence of pyroxeneand the presence of perovskite, monticellite, andmelilite (?) indicates the extremely silica-undersatu-rated nature of the magmas of these dikes.

Xenoliths are rare. They have been identifiedfrom a single dike (ClO). These very small xenoliths(< 5 mm) consist of a mixture of olivine pseudo-morphs (carbonate-talc-qtartz mixtures) inter-

grown with picreilmenite. This dike also containslarge corroded ilmenite macrocrysts.

Petrochemistry

Whole rock analyses of representative dikes aregiven in Table 1. The general characteristics of thedikes are their extreme ultrabasic nature, moderateto high total iron and CaO contents, high MgO, lowAl2O3 and Na2O contents, and high K2O/Na2Oratios. Contents of COz are variable but essentiallyhigh.

The dikes contain abundant Rb and Ba; a strongpositive correlation between K, Rb and Ba (rsulK :0.94; rp6711 : 0.87; rRb/Ba : 0.81) indicates that Rband Ba are hosted by phlogopite. The high Nicontents reflect the Ni-rich nature of olivines (Ta-ble 2). Significant negative correlations between Niand Ba (rNveu : -0.76) and Ni and K (rNvr :-0.70) reflect varying modal proportions of phlogo-pite and olivine.

The Ni content of the dikes (247-644 ppm) falls inthe upper range of values accepted for primaryupper mantle liquids in equilibrium with lherzolite(Frcy et al.,1978). However, these Ni contents mayhave been enhanced by olivine accumulation. Scan-dium contents (16-33 ppm) also cover the range ofprimary upper mantle liquids (15-28 ppm; Frey eral., 1978).

Chemically, the McKellar Harbour dikes are sim-ilar to alnoites (von Eckermann, 1948), kimberlites(Gurney and Ebrahim, 1973), and lamprophyres(Ferguson and Currie, 1972).

Mineral chemistrv

Olivine

Representative analyses of olivine phenocrystsare given in Table 2 (Analyses 1-3). These olivinesare moderately rich in NiO and contain minoramounts of CaO and MnO. Ratios of Mg/Mg +Fe2+(mg) (0.81-0.87) correspond closely to those ofolivines from a variety of mafic and ultramaficrocks (e.9., peridotites, olivine nephelinites, xeno-liths and megacrysts from alkali basalts; Meyer,1977).They are, however, more evolved than thosegenerally associated with kimberlites (mg : 0.86-0.94) (e.g., Mitchell, 1973; Boyd and Clement,1977), or those in equilibrium with primitive uppermantle liquids (mg : 87-0.90) (e.9., Frey et al.,1978; Ringwood, 1975).

In general, the olivine phenocrysts are eitherpartially or wholly serpentinized or completely

PLATT AND MITCHELL: ULTRABASIC LAMPROPHYRES

Table l. Representative whole rock analyses

909

S a m p l e N o . C l 0 c69o c593 c695 c696 c599 C800 c 844 c 8 4 7 cE5r cE 53

s i 0 2 2 8 . 1 0

r i 0 2 3 . 2 6A l 2 0 3 3 . 3 7FerO, 2 .69

F e O 9 . 2 8H n O 0 . 1 7

l '1s0 19.72

C a O 9 . 4 I

N a r 0 0 . 2 8

K z O 1 . 7 3

P z 0 5 0 . 3 5

HzO 2 ,25

coz 20 .02

T o t a l 1 0 0 . 5 3

T r a c e E I e m e n t s

Ba nd

Rb nd

S r n d

Sc nd

N i n d

3 0 . 9 0 2 7 . 8 0

4 . 2 8 3 . 9 3

3 . 6 6 3 . \ 21 0 . 2 9 \ . \ 3

5 . 8 0 8 . 9 2

o . z 7 0 . 2 2

t9 .25 19 .3 \

1 4 . 3 7 1 2 . 2 8

0 . 3 r 0 . 3 0

o . 9 5 | . 0 9

0 . 7 8 0 . 5 2

5 . 0 4 \ . 3 2

4 . 6 2 1 4 . 4 1

r 0 0 . 5 2 1 0 0 . 9 8

(ppr )

738 662

37 66

1 1 2 2 I 1 3 0

r 8 2 8

644 526

3 3 . 1 0 2 5 . 8 0 2 5 . 2 0

3 . 9 5 3 . t 2 3 . 3 74 . 5 3 3 . 0 7 3 . 4 64 . s 0 5 . 7 0 8 . 5 0

6 . 9 2 6 . 6 0 5 . 8 0

0 . 2 6 0 . 2 7 o . 2 2

1 7 . 8 4 1 6 . 0 5 2 2 . 1 8

t 3 . \ 7 1 8 . 4 0 t 5 . 6 3

0 . 3 7 O . 3 z o . 4 l

l . 4 4 r . 6 0 t . 6 3

0 . 8 2 2 . 2 8 0 . 9 I

4 . 4 1 4 . 3 2 3 . 7 8

9 . 2 4 r 3 . 3 r 8 . 4 7

| 00 .92 | 00 .84 99 .66

30.20 29.20

3 . 3 3 3 . t 25 . 5 ' J . o o

3 . 5 2 3 . 8 4

9 . 2 0 6 . 8 8

0 . 2 3 0 . 2 6

t 8 . 7 6 1 9 . 8 0

1 3 . 1 0 t 2 . 4 5

0 . 4 0 0 . 3 7

l . l 8 2 . t 0

0 . 6 3 0 . 9 8

3 . 4 2 2 . 0 7

t 4 . 0 8 1 5 . 9 5

r o r . 3 9 l o o . 5 8

7\o r 433

5 4 r 0 o

7\2 1236

l 8 2 7

56\ 506

2 7 . 2 0 3 1 . r 3\ . g t 4 . 1 6

\ . 3 6 \ . 2 8

6 . 8 4 5 . 4 5

9 . 4 8 t 0 . 7 2

0 . 2 6 0 . 2 4

1 3 . 1 8 t 7 . 5 2

1 5 . 1 2 1 0 . 4 3

0 . 2 9 0 . 3 5

2 . 3 6 2 . 2 1

t . 3 2 0 . 7 0

1 . 9 8 9 . 9 01 3 . 4 2 3 . 4 2

| 0 0 . 7 8 | 0 0 . 5 |

2 9 . 8 0

3 . 7 8( R (

5 . 7 |

9 . 2 80 . 3 5

t 4 . 7 9

1 5 . 5 0

0 . 3 5

I ' 9 0

r . 6 4

3 . 5 0

9 . 2 \1 0 1 . 8 0

872 903

73 50

876 1 495

2 \ r 5

563 \\6

t262 1572 1259

75 129 I 45

l l 5 5 1 3 5 7 l l 0 0

2 7 3 3 2 8

337 2\7 485

828

5 b

I z o o

n d

o 5 t

n d = n o t d e t e r m i n e d .

transformed into a fine-grained mixture of dolomite+ talc + quartz. These alterations occur to themutual exclusion ofeach other and have never beenobserved in the same dike.

No in-depth studies have been made on thenature of the serpentinization of the olivines; how-ever, some chemical information is given in Table 2.Analyses 4-5 are of pale green to colorless serpen-tines immediately surrounding patches of remnantolivine. They commonly appear dusty due to myri-ads of minute globular opaque grains. Impossible toznalyze directly, the presence of S and Ni in someof the serpentine analyses suggests that theseopaques may be Fe-Ni sulphides of the type report-ed by Wicks and Plant (1979 , p. 793) . These serpen-tines are chemically similar to the pale green, high-relief lT-lizardites with anomalously high SiO2associated with the Type 3 retrograde serpentiniza-tion of Wicks and Plant (1979, p. 797).

Analyses 6 and 7 (Table 2) are green serpentinesagain associated with patches of remnant olivine.They are chemically akin to lizardites but containslightly anomalous Al2O3 (Wicks and Plant, 1979).Orange-brown serpentines, commonly occurring asislands within the greener varieties, are represented

by analyses 8 and 9 (Table 2). These are SiO2-poorand comparatively iron-rich. The occurrence ofiron-rich serpentine is common in alpine perido-tites, layered ultrabasic intrusions, and kimberlites(Mitchell, 1978; Smith et al.,1978; Wicks and Plant,1979), where it forms by metamorphic reactions.

In general, olivine serpentinization appears toconform to the Type 3-retrograde event of Wicksand Plant (1979, p. 796).

Olivine commonly alters to an intimate mixture oftalc and dolomite. Minor quartz is frequently ob-served, and opaque grains (magnetite?) commonlyoutline the pseudomorphed olivines. These grainsalso form within the body of the pseudomorph.Alteration of the olivine is always complete. Ifintermediate products (e.9., serpentine) wereformed during the reactions, they have not beenpreserved, although the opaque grains may be rem-nants of a prior serpentinization (Coats and Buchan1979, p. 853).

Carbonation by CO2-charged aqueous fluids isthe logical cause of the talc * dolomite (+ quartz)alteration. For the formation of these three phases,moderate to high CO2 contents of the fluid phasewere required at moderate temperatures. The actual

910 PLATT AND MITCHELL: ULTRABASIC LAMPROPHYRES

Table 2. Representative olivine and serpentine analyses

S a m p l e N o . C 3

A n a l y s e s N o . I

c 59oc2

,5'90 n?

c 590

4c 698 c 590

6

c846

7

c846

8

c846q

s i 0 ^

A l - 0 -z 5Fe0't

Mn0

M9o

Ca0

N i 0

Tota I

3 9 . 3 8

t 8 . 0 5n q (

\ 2 . t 2

n , o

l 0 0 . 3 9

\ 2 . 7 \

0 . 7 5

r . 9 5

2 0 I t

n n a

8 4 . 5 5

l r ? q l

0 . 8 5

J . O l

3 8 . 4 4

85.82

2 . t 2

5 . 3 7

38.6t1

8 5 . r 8

3 . 8 0 0

0 . 2 4 3

0 . 4 3 6

5 . 6 0 0

4 0 . 3 9t l !7

b . 4 5

J O . ) U

8 5 . 8 r

3 .9030 . 2 8 1

0 . 5 2 0

4 0 . 5 9 4 0 . 5 1

I I . 5 9 1 3 . 9 7

0 . l 5 0 . 2 3\ 6 . 7 4 4 S . l e0 . I 9 0 . 2 5

0 . 3 8 0 . 4 5

9 9 . 7 4 r 0 0 . 7 9

36.76 36 .93

3 . 3 7 I . 0 5

8 . 8 9 I 1 . 8 4

3 6 . 8 6 3 6 . 2 60 . 1 2

86 . 00 85 . 09

3 . 6 2 6 3 . 7 0 3

0 . 3 9 1 0 . 1 2 5

0 . 7 3 2 0 . 9 9 r

5 . 4 1 6 5 . 4 1 5

0 . 0 1 3

C a t i o n C o n t e n t ( 0 1 i v i n e = 4 o x y g e n s ; S e r p e n t i n e s = 1 4 o x y g e n s )

S i

A I)+

t e

M n

Mg

C a

N i

r . 0 0 t

0 . 3 8 40 . 0 1 2

t . ) y o

0 . 0 0 8

4 . 0 7 6

0. 084

0 . 1 5 5

5 .557

0. 009

\ . 0 7 0

0 . 0 9 6

0 . 2 8 6

5 . 4 3 0

I . 0 0 9 I . 0 0 5

0 . 2 \ t 0 . 2 9 0

0 . 0 0 4 0 . 0 0 5

| . 7 2 7 1 . 6 7 9

0 . 006 0 . 007

0 . 0 c 8 0 . 0 0 9

' t t o t a l i r o n a s F e 0 , C = c o r e R = r i m

A n a l y s e s l - 3 = o l i v i n e ; A n a l y s e s 4 - 9 = s e r p e n t i n e ( s e e t e x t )

values depend on the fluid pressures operativeduring the reactions (Slaughter et al., 1975). At 1kbar Ps, and assuming the formation of tremolite(now completely destroyed) as an intermediateproduct in the carbonation process, maximum tem-peratures on the order of 420-440"C, and an aque-ous fluid composition of 70-95 mole percent CO2,could well have existed (Slaughter et al., 1975).

Mica

Mica is a ubiquitous phase in all the dikes; itoccurs both as a groundmass phase and as smallphenocrysts. Large macrocrysts were observed inone dike (C10, Figure 1), and further mention ofthese is made below. Compositionally (Table 3) allthe micas analysed are phlogopites with Al inexcess of two atoms in the tetrahedral sites. TiO2contents are variable but significant. For the mostpart CaO, Na2O, Cr2O3 and MnO are either unde-tectable or insignificant. Inter-dike variations seemno more significant than intra-dike variations.

The large megacrysts or macrocrysts in dike C10(Fig. l) reach a size of about a centimeter acrossand they are commonly bent and distorted. Theyare distinctively composite in nature with largecores and narrow rims. The cores have oranse to

Fig. l. Photomicrograph (plane polarized light) of McKellarHarbour dike Cl0 showing phenocrysts of ilmenite, phlogopiteand olivine (pseudomorphed) in fine grained groundmass ofspinel, carbonate and apatite.

PLATT AND MITCHELL: ULTRABASIC LAMPROPHYRES 9 l l

S a m p l e N o . C 6 8 9 c 6 9 0

A n a l y s i s N o . l 2

c t o ^ c t o ^

Table 3. Representative phlogopite analyses available, but optically similar micas have beendescribed from kimberlites, carbonatites, and ultra-mafic alkaline rocks (e.g., Farmer and Boettcher,r981) .

Spinel

The main compositional variations found in thespinels are shown in Figure 2, and representativeanalyses are given in Table 4. The rnost primitivespinels, in terms of Ti-Fe-Cr content, are presentas inclusions in olivines, as corroded crystals and asrare large euhedra. These early spinels (Table 4,analyses l-2) are titaniferous-aluminous-magne-sian chromites with Fe/(Fe + Mg) : 0.6-0.7, Cr/(Cr+ AD : 0.4-0.8 and Ti/(Ti * Cr * AD : 0.1-0.4.

Intermediate groundmass type spinels (Table 4,analyses 3-4) are richer in Ti and Fe and aretitaniferous aluminous chromites with Fe/(Fe +Mg) : 0.8-1.0; Crl(Cr + Al) : 0.4-0.6 and Ti/(Ti +Cr + Al) : 0.3-0.8. The most evolved spinels areulvrispinel magnetites (Table 4, analyses 5-6) withFe/ (Fe + Mg) :0 .8 -1 .0 , Cr i (Cr + A l ) :0 -0 .1 , andTil(Ti + Cr + Al) : 0.6-0.9. As the spinels becomericher in total Fe, they become poorer in Cr andMg, and richer in Mn. The spinel trend, establishedby the analysis of zoned crystals, is alongthe axis ofthe spinel prism rising from a point well above theTi-free base towards the Fe-rich ternary face, andultimately, with further increase in Ti and Fe,towards the FezTiO+ aPex (Fig. 2).

This trend is markedly different to that observedin kimberlites (Mitchell, 1979), and the spinels inthe McKellar dikes are characteristically poorer inthe MgzTiOa molecule than are kimberlite spinels.The McKellar trend has some similarities withtrends found in lamprophyres from the Fen com-plex and the W. Kentucky dikes (Mitchell 1979),but the McKellar spinels are markedly poorer inAl2o3.

A subsidiary trend to the main McKellar Harbourtrend of spinel compositions has been documentedfrom zoned spinels in a single dike, C10. This trendand representative chemical analyses of the spinelsare shown in Figure 3 and Table 4 (Analyses 7-9)respectively. The most primitive (Cr-rich), andpresumably earliest, spinels in this subsidiary trendare titaniferous aluminous magnesian chromiteswith Fe/(Fe + Mg) : 0.5-0.7, Crl(Cr + Al) : 9.5-0.7, and Til(Ti + Cr + Al) : 0.05-0'15. Thesespinels are similar in composition to early spinels inthe main trend, although they are distinctly poorerin TiO2. The spinels appear to evolve parallel to the

c8464

c 5 9 l

3

s i 0 ^

T i 0 -

A t ̂ 0 ^2 5

C r ^ 0 "

Fe0,!

l4n 0

35. 89 37 .52 39 26

5 7 7 2 . 6 \ 3 0 6

1 4 8 7 1 4 . r o r . 9 8

9 . 9 8 7 . 1 6 8 . 4 6- 0 . 2 1

H s o t 7 . \ 6 2 2 . 3 1 2 1 . 8 1

* 2 0 9 . 5 6 9 . 7 5 9 7 9

Tota l 9 \ .62 93 .93 9 \ 36

cat ion conten t (22 oxygens)

5 . 3 2 0 5 . 5 0 3 5 . 7 4 3

0 . 6 3 9 0 . 2 9 1 0 . 3 3 7

2 . 5 8 t 2 . \ 5 7 2 0 6 7

1 . 2 2 9 0 . 8 7 9 l . 0 1 6- 0 . 0 2 6

3 . 8 3 4 4 8 8 r 4 . 7 6 0

| .796 | 826 | .828

38.79 39 .29

l . 5 r 2 . 3 \

1 3 . 9 2 1 2 . 2 0- 0 . 3 5

6 . r r 0 7 . 6 2

0 l 0

2 3 9 0 2 2 . 3 7

8 . 9 2 9 . 5 1

93.5 \ 93 69

5 634 5 .758

0 1 6 5 0 2 5 8

2 3 8 \ 2 . 1 0 8- 0 . 0 4 2

o . 7 7 8 o . 9 3 4

0 . 0 1 2

5 t79 4 a9O

1 6 5 \ 1 . 7 7 9

3 7 . 0 5

5 . 2 6

r 4 . 8 6

0 . 3 2

7 . 8 0

r 9 . 8 2

9 2 6

94 -66 : , *

5 . 3 9 6

0 576

2 . 5 5 2

0 . o 3 7

0 . 9 5 1

q 306

1 . 7 2 2

T i

A I

C r

ilh

6g

K

r . t o t 3 l i r o n a s F e o . : k , r t o t a l c o n t a i n s 0 l 5 2 C a 0C = c o r e , R = r i m

light orange pleochroism, whereas rims have red-brown to pale orange pleochroism. The core-rimboundary is invariably sharp, suggesting two dis-tinct generations of mica. The rim mica opticallyresembles some smaller phenocrysts and thegroundmass micas of the dike, a similarity con-firmed by the chemical data. From Table 3, it isseen that the rim/groundmass micas are enriched inTiO2, Al2O3 and Fe/Mg and depleted in SiOz andK2O (Analysis 6). In comparison to the phlogopitesfrom other dikes (Analyses I to 4, Table 3), the corephlogopites are generally richer in SiO2 and Cr2O3and have a lower Fe/Mg ratio. The rim phlogopitesare also different in having lower K2O, often lowerSiO2, and higher Cr2O3 values.

The phlogopite cores could either be xenocrystsor early-formed phenocrysts. The distorted natureof the crystals suggest some degree of stress, per-haps induced by transportation. The relatively highTiO2 contents of the crystal cores precludes aprimary upper mantle lherzolite origin (Carswell,1975). However, they may indeed be high pressurephenocrysts. It is now generally accepted that thestability field of phlogopite extends well into theupper mantle. The crystal rims and smaller pheno-cryst/groundmass micas probably represent lowpressure shallow level phases.

Reversely pleochroic mica (tetraferriphlogopite)was observed in a dike previously described byPlatt and Mitchell (1979). No chemical data are

Fe2TiO4

Mg.TiO.

Y-:(o'

* ^A U\ o a

McKELLAR HARBOUR

FeCr.Oo

A^ 1

MgAl.O.

{ : , ; : r* '

MgC12OaFig. 2. Spinel compositions and trend from the McKellar Harbour dikes plotted in a reduced iron spinel prism. Kimberlite and Fen-

West Kentucky lamprophyre trends plotted after Mitchell (1979).

Table 4. Representative spinel analyses

S a m p l e N o . C 5 9 0

A n a l y s e s N c . I

c 5902

c 694

3

c 5904

c59 lq

c 846o

c 1 0

7

c l 0

8

c t 0

9

r i o z 5 . 1

A l 2 0 3 9 . 4

C , 2 0 3 3 5 . 9

FeO't 37 .9

l '1n0 0.0

M g O I 0 . 4

To ta l 99 .7

Reca l cu l a ted ana l yses ; ! r ,

F " 2 0 3 | 5 . 3

t e u

7 . 5 3 8 . 1 9

2 . 8 2 2 . | 33 2 . 6 5 3 0 . 7 94 7 . 6 3 S 5 . r 80 . 2 8 0 . 3 1

6 . 1 9 t . 5 7

9 6 . 8 9 9 8 . 1 7

zo.s8 20 .38

28.90 36 .8 \

9 8 . 9 5 I 0 0 . 2 l

7 . 8

| l . 4 1

28 .7

0 . 2

I 0 . l l

9 7 . 5

t o . J

24 .3

9 9 . I

t 8 . 6

1 8 . r

0 . 4

1 3 . 7

o . z

6 t . 7

z . l

0 . 5

J O . T

2 \ . 6

? q q

q a ,

2 . 3

3 . 84 0 . 3

t 2 . 2

l ? n

5 8 . 02 . 8L q

q 7 I

99.9

4 . 5) L ?

\ 7 . 3

5 . 40 . 0

69.\

0 . 7

3 . 2

9 5 . 0

? n ?

o o n

8 . 1

t q

1 . 0

L ( n

7 . 6

3 . 50 . 0

j 8 . 5

u . )

4 . 8

9 4 . 6

2 n o

i 0 0 . 0

5 . 1

9 . 4

0 . 8

n . l

4 . 6 5

8 . 8 1

4 0 . r 2

35.030 . 0 0

1 0 . 7 4

99.35

1 5 . 2 7

2 t . 2 7

1 0 0 . 8 8

t l l o

1 5 . 0

t 0 . 9

4 . 8

t 6 . 4

u . )

7 . 5

3 . 72 . 4

0 . 5? 4 . 0

m o l . Z e n d m e m b e r m o l e c u l e s

T o t a l l 0 l . 3

M s A l r o . 1 5 . 5

MsT i 04 19 .2

M n T i 0 1 ,

t e ^ t l u ,z q

M S C r r 0 4 2 . \

M n C r ^ 0 ' .

FeCr204 37 .3

Fe r04 25 .7

l 4 . l

39.6

t 3 . 5

4 0 . 7 79.8 2 L a

3 7 . 2 J ) . O

? ? 74 3 . 0

x t o t a l i r o n a s F e o : k ; t R e c a l c u l a t e d b y C a r m i c h a e l , s ( 1 9 5 7 ) m e t h o d .

PLATT AND MITCHELL: ULT'RABASIC LAMPROPHYRES

6

913

Mg.TiOo

g 2

o \

MgAl.Oo

base of the spinel prism (Fig. 3) towards FeCr2Oa.This trend is one of increasing Fe and Cr at theexpense of Mg and Al with approximately constantTi. The most evolved spinels are titaniferous chro-mites with Fe/(Fe + Mg) = 0.9, CrlCr + Al) : 0.9and Tii(Ti * Cr * Al) : 0.2. As seen from Figures Iand 2, this subsidiary trend is also different fromthat shown by kimberlites.

In general, the spinels from the McKellar Har-bour lamprophyres have evolutionary trends thatindicate affinities with alkaline rock-carbonatitemagmatism (Mitchell, 1979).

Ilmenite

Ilmenite grains are found in three parageneses.They occur as small groundmass crystals, as large,corroded, phenocrysts, and as constituents of smallultramafic xenoliths. The latter paragenesis is dis-cussed below. Representative analyses are given inTable 5.

The small groundmass crystals, which occur spo-radically in the majority of dikes, are rich in MnO(2-8wt.%). The chemical characters of these ilmen-

FeCr"Oo

- " , g " t $ q( e ,

MgCr2OaFig. 3. Subsidiary spinel compositions and trend from McKellar Harbour dike ClO plotted in a reduced iron spinel prism.

ite show similarities to ilmenites associated withcarbonatitic magmatism, particularly with respectto their Mn content (Mitchell, 1979).

Large (up to .5 cm) ilmenite phenocrysts have

Table 5. Representative i lmenite analyses

S a m p l e N o . C 4

A n a l y s i s N o . I

c t o c t o c l o c l o c l o

2 3 \ 5 6

C r . 0 .

T i 0 -

Fe.0. ' r

F e 0

H n 0

l'190

D O t r u ^' l

F e T i 0 3

Cr 203' " r03

ro ta I 98 -72 99 .32

Hol Z end member molecu les

rnT i 03 6 . 85

- 0 . 5 0

5 2 . t t 4 9 4 4 5 r . 3 8- 5 . l o 2 . 6 7

t 4 j . \ 5 \ 3 8 2 \ 5 . 7 5

3 . 1 6 - 0 . 4 5_ o . J 6

2 . 5 0 2 . \ 9 3 . 4 2

52.73 50 .92 50 27

5 . 7 2 6 . t 3 7 . 4 8

2 7 . 9 6 3 3 3 3 3 1 8 0

0 . 4 5 0 . 3 6 0 3 0

1 0 . 1 2 6 . 7 9 7 . 3 5

9 9 . 9 8 l 0 0 . 0 r r 0 0 . 6 2

o . 9 o 0 7 \ 0 . 6 2

3 7 . 7 8 2 \ 6 \ 2 6 . 5 5

55.1 67 86 6 \ \6

t . t 2 t . r 5 1 . 5 7

S . o 9 5 . 6 1 6 . 8 2

1 0 0 2 5

0 . 9 6

9 5 . 5 0

2 . 5 3

_ t 3 7

9 3 t 5 9 3 . 4 5- n t o

- 4 . 9 0

, ! F e 2 c 3 c a l c u i a t e d f r o m t o t a l i r o n b y c a r m i c h a e l ' s ( 1 9 6 7 ) m e t h o d

A n a l y s i s N o I = g r o u n d m a s s i l m e n i t e ; 2 6 3 = r a g g e d i l m e n i t ep h e n o c r y s t s ; 4 , 5 E 6 = p i c r o i l m e n i t e s f r o m u l t r a m a f i c x e n o l i t h

FezTiO.+

914 PLATT AND MITCHELL: ULTRABASIC LAMPROPHYRES

been observed in one dike (C10) (Fig. 1). Typicalanalyses are given in Table 5, and compositionalvariations are plotted in Figure 4. Figure 4 alsoserves to delineate the compositional range of thepicroilmenites occurring in small ultramafic xeno-liths within the same dike, C10.

Cqrbonate

The essential groundmass carbonate in most ofthe dikes studies is calcite, which commonly con-tains minor amounts of FeO, MnO, or MgO. Theexception is dike C10, in which ankeritic dolomiteor perankerite (Analysis 4, Table 6) occurs in closeassociation with the Mg/Fe carbonate breunnerite(Analysis 5, Table 6) in the groundmass. Secondarydolomitic-carbonate (Analysis 6, Table 6) occurswith talc and quartz as a replacement of olivine.Calcite forms an intimate intergrowth with andradi-tic garnet as a probable replacement of melilite.

Perovskite

Perovskite is a minor phase in a number of dikes,where it occurs as small subhedral to euhedral deepbrown to opaque grains. The majority of the perov-skites analyzed contain moderate but significant

Table 6. Representative perovskite and carbonate analyses

sample No, c69 l c698

A n a l y s i s N o I 2

c598

3

c t 0

!

c l o c 6 9 8

s i 0 2

T i 0 2

A t ̂ 0 -

Fe0't

liln 0

l'l9o

C a 0

N b " 0 .

t o t a l

\ . 2 8

o . 7 5

1 8 . 3 7

28.29

o . 2 9

55 7 t 53 .59o . 30 0 .49I 5 t 2 . 1 7

3 9 . 1 4 3 8 . r 1o 3 2 r . 0 3

97 .08 95 .58

n ( q

\ ? 7 ?

8 . 0 3 6 9 4 2 3 . 7 0

t . 7 7 0 . 2 8 0 6 2- t 7 . 6 3 2 9 . 0 6

28.2 \ 28 .65 0 33

9 4 . 0 2

' ! t o t a l i r o n a s F e o

A n a l y s e s l - 3 p e r o v s k i t e s ; A n a l y s e s 4 - 5 c a r b o n a t e s

amounts of Nb2O5 and small amounts of FeO,(Table 6, Analyses 1-2). Low totals suggest thepresence of unanalysed elements (e.g., REE or Sr).These perovskites are similar to those describedfrom kimberlites (Mitchell,19721' Boctor and Boyd,1980).

Markedly different perovskites (Table 6, Analysis3) have been analyzed from an olivine-free, phlogo-pite- calcite- zirconium-rich, titanian garnet-meli-lite (?)-spinel unit with the composite dike de-

Xenolith IlmenitesPhenocryst/ Xenocryr

IlmenSeporofed Ilmenites

( porogenesis nol oi

MnTiO3

LAMPROPHYRESGRANITESBASALTSCARBONATITES

Fe203

20

Fe2O3

FeTiO3

oI

Il t

a

MgTiO3

MgTiOg

Xenocryst IIlmeniles i

lmenites Iis nol observed) I

MnTi03

Fig. 4. Ilmenite compositions from the McKellar Harbour dikes.FeTiOg

PLATT AND MITCHELL: ULTRABASIC LAMPROPHYRES 915

scribed by Platt and Mitchell (1979). These perovs-kites are much richer in FeO, MnO and Nb2O5 thanthose generally observed in the present dike suite.

Xenoliths

Xenoliths have been found only in dike ClO.They occur as small (2 mm or less) fragmentssparsely scattered through the dike. They consist ofcarbonate-talc-quartz intergrowths (olivine pseu-domorphs) and picroilmenite. Analyses of the il-menite (Table 5) show significant MgO and Cr2O3contents, and are quite different in compositionfrom the ragged ilmenite phenocrysts found in thehost dike (Table 5). The xenoliths, although small,display a distinctive granular texture.

The textural and mineralogical nature of thesexenoliths suggests a high pressure origin. None ofthe low-temperature breakdown stages of Cr-ilmen-ite (Wyatt 1979, p. 26112) has been observed, sug-gesting high temperature origin for the xenoliths.Figure 4 shows the close similarity between magne-sian ilmenites from kimberlites and those reportedhere.

Conclusions

The McKellar Harbour dikes are characterizedby the mineral assemblage phlogopite-olivine-spinel-perovskite-apatite-carbonate (calcite). Pseudo-morphs considered to be after melilite are common.In North America, such calcite-rich ultrabasic dikesare commonly referred to as "kimberlitic dikes"(e.9., Watson et al., 1978). The mineralogy of thedikes, however, indicates no affinites with kimber-lites; i.e., they lack Cr-pyrope or picro-ilmenitemegacrysts, they have a lamprophyric trend ofspinel compositions, and they lack magnesian ulvo-spinels (Mitchell, 1979). Andradite-calcite replace-ments of melilite, rare occurrence of monticellite,and the presence of Zr-Ti-rich garnets indicatesthat the dikes have afrnities with alnriites. Nototally satisfactory petrographic term exists to de-fine these lamprophyres (e.9., Streckeisen 1978;Kresten 1979). We consider the McKellar Harbourdikes to be ultrabasic lamprophyres with aln<iiticaffinities.

The alkaline ultrabasic magmatism generating thelamprophyres has not been previously recorded inthe Lake Superior Basin. They are not related to thelate Neohelikian (1010-1050 m.y.) alkaline magma-tism expressed as the Coldwell, Prairie Lake andKillala Lake complexes. Their characteristic miner-alogy and chemistry suggest derivation from upper

mantle liquids. We are unfortunately unable tospeculate further on the petrogenesis of these dikesas we lack information regarding any contempora-neous or consanguineous magmatism.

AcknowledgmentsThe authors wish to acknowledge the Natural Sciences and

Engineering Research Council of Canada for their support of thiswork. We would also thank Dr. P. Hill and the microprobe unitof the University of Edinburgh, Scotland for their valuableassistance in this study.

ReferencesBell, K. and Blenkinsop, J. (1980) Ages and initial 875r-865r

ratios from alkalic complexes of Ontario. Ontario GeologicalSurvey, Miscellaneous Paper 93, 16-23.

Boctor, N. Z. and Boyd, R. F. (1980) Oxide minerals in theLiqhobong kimberlite, Lesotho. American Mineralogist, 65,63 l-638.

Boyd, F. R. and Clement, C. R. (1977) Compositional zoning ofolivines in kimberlite from the De Beers mine, Kimberley,South Africa. Carnegie Institution of Washington Year Book,76,485-493.

Carmichael, I. S. E. (1967) The iron-titanian oxides of salicvolcanic rocks and their associated ferromagnesian silicates.Contributions to Mineralogy and Petrology, 14,36-74.

Carswell, D. A. (1975) Primary and secondary phlogopites andclinopyroxenes in garnet lherzolite xenoliths. Physics andChemistry of the Earth, 9,417-429.

Coats, C. J. A. and Buchan, R. (1979) Petrology of serpentinizedmetamorphic olivine, Bird River sill, Manitoba. CanadianMineralogist, 17, 847 -856.

Currie, K. L. (1976) The alkaline rocks of Canada. GeologicalSurvey of Canada, Bulletin 239.

Currie, K. L. (1980) A contribution to the petrology of theColdwell alkaline complex. Geological Survey of Canada,Bulletin 287.

Douglas, R. J. W. (1980) Proposals for time classification andcorrelation of Precambrian rocks and events in Canada andadjacent areas of the Canadian shield. Part 2: A provisionalstandard for correlating Precambrian rocks. Geological Surveyof Canada Papers 80-24.

Farmer, G. L. and Boettcher, A. L. (1981) Petrologic and crystalchemical significance of some deep seated phlogopites. Ameri-can Mineralogist, 66, 1154-1163.

Ferguson, J. and Currie, K. L. (1972) The geology and petrologyof the Callander Bay alkaline complex. Geological Survey ofCanada, Bulletin 217.

Frey, F. A., Green, D. H. and Roy, S. D. (1978) Integratedmodels of basalt petrogenesis: A study of quartz tholeiites toolivine melilitites from South Eastern Australia utilizing geo-chemical and experimental petrological data. Journal ofPetrol-ogy, 19, 46f-513.

Gurney, J. J., and Ebrahim, S. (1973) Chemical composition ofLesotho kimberlites. Lesotho National Development Corpora-tion. Maseru. 280-284.

Kresten, P. (1979) The Alnci complex: Discussion of the mainfeatures, bibliography, excursion guide. Nordic CarbonatiteSymposium/Alnti.

Meyer, H. O. A. (1977) Mineralogy of the upper mantle: A

9r6 PLATT AND MITCHELL: ULTRABASIC LAMPROPHYRES

review of minerals in mantle xenoliths from kimberlite. EarthScience Reviews. 13. 251-281.

Mitchell, R. H. (1972) Composition of perovskite in kimberlite.American Mineralogist, 57, 17 48-17 53.

Mitchell, R. H. (1973) Composition of olivine, silica activity andoxygen fugacity in kimberlite. Lithos, 6, 65-82.

Mitchell, R. H. (1978) Mineralogy of the Elwin Bay kimberlite.American Mineralogist, 63, 47-57.

Mitchell, R. H. (1979) The alleged kimberlite-carbonatite rela-tionship: Additional contrary mineralogical evidence. Ameri-can Journal of Science, 279,570-589.

Mitchell, R. H. and Platt, R. G. (1978) Mafic mineralogy offerroaugite syenite from the Coldwell alkaline complex, Ontar-io, Canada. Journal of Petrology, 19,627-651.

Mitchell, R. H. and Platt, R. G. (t982) Mineralogy and petrologyof nepheline syenites from the Coldwell alkaline complex,Ontario, Canada. Journal of Petrology, 23, 186-214.

Platt, R. G. and Mitchell, R. H. (1979) The Marathon Dikes. t:Zirconium-rich titanian garnets and manganoan magnesianulvdspinel-magnetite spinels. American Mineralogist, &, 546-550.

Platt, R. G. and Mitchell, R. H. (1982) Rb-Sr geochronology ofthe Coldwell complex, N.W. Ontario, Canada. Canadian Jour-nal of Earth Sciences, in press.

Ringwood, A. E. (1975) Composition and petrology of theearth's mantle. McGraw-Hill International Series in the Earthand Planetary Sciences. McGraw-Hill Incorporated, U.S.A.

Slaughter, J., Kerrick, D. M. and Will, V. J. (1975) Experimen-

tal and thermodynamic study of equilibria in the system CaO-MgO-SiO2-H2O-CO2. American Journal of Science, 275,143-162.

Smith, J. V., Brennesholtz, R. and Dawson, J. B. (1978) Chem-istry of micas from kimberlites and xenoliths-|. Micaceouskimberlites. Geochemica et Cosmochimica Acta, 42, 959-97 l .

Streckeisen, A. (1978) Classification and nomenclature of vol-canic rocks, lamprophyres, carbonatites and melilitic rocks.Neues Jahrbuch fuer Mineralogie, Abhandlungen, 134, l-14.

von Eckermann, H. (1948) The alkaline district of Alnri Island.Sveriges Geologiska Undersoekning, Serie Ca, 36.

Walker, J. W. R. (1957) Geology of the Jackfish-Middleton area.Ontario Department of Mines, Geological Report 50.

Watson, K. D., Bruce, G. S. W. and Holiday, L. B. (1978)Kimberlitic dike in Keith Township, Ontario. Canadian Miner-afogist, 16, 97, 102.

Wicks, F. J. and Plant, A. G. (1979) Electron microprobe and X-ray-microbeam studies of serpentine textures. Canadian Min-eralogist, 17, 785-830.

Wyatt, B. (1979) Kimberlitic chromian picroilmenites with inter-growths of titanian chromite and rutile. In F. R. Boyd andH. O. A. Meyer, Eds., Proceedings of the Second Intemation-al Kimberlite Conference. Yol. 2, 257-264. American Geo-physical Union, Washington D.C.

Manuscript received, September 29, I98I;acceptedfor publication, May 10, 1982