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Geochemistry of mafic igneousrocks of the northern Prince CharlesMountains, AntarcticaJ. W. Sheraton aa Bureau of Mineral Resources , P.O. Box 378, Canberra City ,A.C.T. , 2601Published online: 01 Aug 2007.

To cite this article: J. W. Sheraton (1983) Geochemistry of mafic igneous rocks of the northernPrince Charles Mountains, Antarctica, Journal of the Geological Society of Australia, 30:3-4,295-304, DOI: 10.1080/00167618308729257

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Journal of the Geological Society of Australia (1983) 30, 295-304© Geological Society of Australia, inc., 1983

Geochemistry of mafic igneous rocks of the northernPrince Charles Mountains, AntarcticaJ. W. Sheraton

Bureau of Mineral Resources, P.O. Box 378, Canberra City, A.C.T., 2601

ABSTRACTRare mafic dykes, which intrude 1000 Ma high-grade metamorphic rocks of the

northern Prince Charles Mountains-Mawson Coast area, are compositionally distinctfrom abundant early to middle Proterozoic tholeiite dykes, which are confined toArchaean or early Proterozoic terrains in the southern Prince Charles Mountains andelsewhere in East Antarctica, and which have therefore proved useful asstratigraphic markers. The younger dykes (and extrusive rocks) are a composition-ally heterogeneous group with a wide range of ages (at least Cambrian to Eocene),although most are of K-rich alkaline composition or have alkaline affinites. Theirstrong enrichment in highly incompatible elements (Rb, Ba, Th, Nb, K, Pb, Th andU) relative to less incompatible elements (La, Ce and P) suggests derivation bypartial melting of more enriched mantle source regions than those of most of theProterozoic tholeiite suites. However, unlike the latter, many incompatible elementratios have been significantly affected by fractional crystallisation and possibly alsoby the presence of residual minor phases during low degrees of melting.

KEY WORDS: East Antarctica, alkaline mafic dykes, geochemistry, mantlesource composition.

INTRODUCTIONMafic dykes of the East Antarctic Precambrian

Shield are largely confined to isolated Archaean toearly Proterozoic cratonic blocks, in which theycommonly form dense swarms; hence, they areuseful as stratigraphic markers (Sheraton et al.,1980; Tingey, 1982; James & Tingey, in press).These dykes, of early to middle Proterozoic age,comprise several suites of continental tholeiites(dolerites), as well as a group of high-Mg tholeiites;they have been described by Sheraton & Black(1981). Adjacent late Proterozoic (~ 1000 Ma)high-grade gneiss terrains are cut by very fewunmetamorphosed dykes, and most of these havealkaline affinities, ranging from alkali olivine basaltto alkali melasyenite and alnoite. Whereas Jurassictholeiites appear to be confined to parts of thePrecambrian Shield adjacent to the TransantarcticMountains, such as Coats and western DronningMaud Lands (Juckes, 1972; Clarkson, 1981) andGeorge V Land (Ravich et al., 1968), alkaline dykes(and rare extrusives) are much more widespread.They have been reported from the Vestfold Hills ofPrincess Elizabeth Land (Fig. 1; Sheraton & Coller-son, 1983), the Prince Charles Mountains ofMacRobertson Land (Fig. 1; Tingey, 1981, 1982),Enderby Land (Sheraton et al., 1980), and DronningMaud Land (Ravich & Solov'ev, 1969). Althoughthey intrude Archaean, as well as Proterozoic,metamorphics, all those dated have givenPhanerozoic ages (Walker & Mond, 1971; Sheraton& England, 1980; Table 1). Extremely K-rich

olivine leucitites of the Pleistocene volcano ofGaussberg in Wilhelm II Land have been describedby Sheraton & Cundari (1980).

In MacRobertson Land (Fig. 1), mostly am-phibolitised tholeiitic dolerites intrude Archaeangranitic basement gneisses, as well as overlying lateArchaean to early Proterozoic metasediments of thesouthern Prince Charles Mountains (SPCM), but areabsent from late Proterozoic high-grade metamor-phics to the north (Sheraton & Black, 1981; Tingey,1982). However, a few mafic dykes (as well as sillsand a leucite tristanite lava flow) crop out in the lateProterozoic terrain of the northern Prince CharlesMountains (NPCM) — Mawson Coast area, and thispaper deals with the geochemistry of these rocks;comparisons are made with the Proterozoictholeiites. Available K-Ar dates are given in Table1. The Silurian alkali melasyenite dyke at MountBayliss in the SPCM cuts Archaean granitic gneissand has been described by Sheraton & England(1980). A chemically similar dyke from PriestleyPeak in Enderby Land has given a Rb-Sr isochronage of 482 ± 3 Ma (Sheraton & Black, 1981). TheCretaceous alnoite sills of Radok Lake have beendescribed by Walker & Mond (1971).

GEOCHEMISTRYSix mafic dyke samples, as well as 2 specimens of

the leucite tristanite flow, which covers an area ofabout 8 sq km at Manning Massif, were analysed formajor and a range of trace elements (Table 2). Afresh trachybasalt dyke from the Bunger Hills,

MS. received 25 November, 1982; revised MS. received 17 March 1983.

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296 J. W. SHERATON

Vestfold¥Hills

MAC ROBERTSON LAND

^MM*/iiiliiil

gnl Permian sediments+

++ | Cambrian acid intrusive s

:::::| Late Proterozoic(~IOOOMa)gneisses

Late Proterozoic metasediments

Middle Proterozoic tholeiite dykes

Early Proterozoic metasediments

%%£| Archean metasediments

W\ Archean (~2800Ma)basement gneisses

16/9/102

Fig. 1. Geological map of the Prince Charles Mountains area (based on Tingey, 1982) showing locations of analysedsamples and of the Mount Bayliss alkali melasyenite and Radok Lake alnoites. Outcrops are shown in black.

Queen Mary Land was included for comparison.Petrographic details are given in an Appendix.

Many of the analysed samples show extensivedeuteric alteration, although none appear to havebeen metamorphosed. The youngest documentedmetamorphic event in the NPCM was associatedwith granitoid and pegmatite emplacement and re-sulted in the resetting of mineral isotopic systemsabout 500 Ma ago (Tingey, 1982; James & Tingey,

in press). Because of this alteration, a chemicalclassification, that of Irvine & Baragar (1971) whichincludes both alkaline and subalkaline volcanicrocks, is used in this paper. The nomenclature issimilar to that used by Coombs & Wilkinson (1969)for alkaline volcanics, except that in their schemethe alkali olivine basalts would be termedtrachybasalts. Chemical changes during alterationwere probably not very great (see below), but cannot

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MAFIC ROCKS OF PRINCE CHARLES MTS, ANTARCTICA 297

TABLE 1. K-Ar ages of mafic igneous rocks from the Prince Charles Mountains.

Sample No. Rock Type Locality Mineral Age (Ma)

69280225

73281545

71280126

69280152

69280153

69280334

73281594

Alkali olivine basalt

Alkali melasyenite

Calc-alkali basalt

Alnoite

Leucite tristanite

Fox Ridge

Mount Bayliss,

SPCM

Taylor Platform

Radok Lake

Manning Massif-

Cl inopyroxene

K-richterite

K-arfvedsonite

Plagioclase

Phlogopite

Whole Rock

504+20

414+10

413+10

430+12

246+ 6

110+ 3

110+ 3

108+ 3

51.8+ 2.0

49.1+ 2.0

Constants used:4°K = 0.0119 atom 7.; Xp = 4.72 x 10 °/yr;Ae= 0.584 x 10 10/yr

Dating by Australian Mineral Development Laboratories, Adelaide

IZ

10

8

OCM

+ 6O

CMDZ

4

2

n

1 1

Alkaline

o

I i

I

A

O

•

•x '

1

A

Aa A

^ <

nj*" ~" i • ~

X

JV1 r ^Subalkaline

i 1 1

I

—

—

i48

Si •56

16/09/103

Fig. 2. Alkalies — SiCh plot for mafic igneous rocks fromthe NPCM and adjacent areas of East Antarctica.Alkaline and subalkaline fields of Irvine & Baragar(1971) are shown; other fields and symbols as inFigure 3.

be discounted in every case. In view of the presenceof both primary (e.g., in ocelli) and secondarycarbonate, all C.I.P.W. norms were calculated CO2-free.

Major elementsMafic igneous rocks from the NPCM have a wide

range of compositions (Table 2). Although most arealkali basalts of various types (Fig. 2), 3 of theanalysed dykes are subalkaline. Nevertheless, eventhe latter are chemically distinct from the abundantpre-1000 Ma tholeiites of the SPCM and elsewhere.702802961 plots just within the alkaline field on analkalies-SiO2 diagram (Fig. 2), but is subalkaline interms of the Ol'-Ne'-Q' and Cpx-Ol-Opx diagrams

NORTHERN PRINCECHARLES MTS+ Tholeiitic basalt

* Calc-alkali basalt

0 Olivine basalt

O Alkali olivine basalt

• Leucite tristanite

x SPCM tholeiitesA Mt Bayliss

melasyeniteA Priestley Peok

melasyenite

• Bunger Hillstrachybasalt

— Field of EnderbyLand thoieiites

• Field of VestfoldHills tholeiites

M

Fig. 3. F (total FeO) — M (MgO) — A (Na2O + K2O)diagram for mafic igneous rocks from the NPCMand adjacent areas of East Antarctica, showingfields of Proterozoic tholeiitic dykes from EnderbyLand (Sheraton & Black, 1981) and the VestfoldHills and SPCM (unpublished data). Dot-dashedline divides the tholeiitic (upper) and calc-alkalinefields of Irvine & Baragar (1971), and the dottedlines indicate the immiscibility field of Philpotts(1976).

of Irvine & Baragar (1971). 71280126 and 70280131are just calc-alkaline on AFM (Fig. 3) and AI2O3 -normative plagioclase composition diagrams (Irvine& Baragar, 1971), whereas 70280296 is tholeiitic inboth cases. 69280217 is slightly hypersthene-normative and is therefore termed olivine basalt, incontrast to 69280225 and 71280007, which arenepheline-normative alkali olivine basalts. Allanalysed rocks except calc-alkali basalt 71280126belong to the potassic series of Irvine & Baragar

1Bureau of Mineral Resources registered sample number

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298 J. W. SHERATON

TABLE 2. Chemical analyses and C.I.P.W. norms of mafic igneous rocks from the NPCM-Mawson Coast area (A-H) andBunger Hills.

sio2

TiO2

A12°3Fe2°3FeO

MnO

MgO

CaQ

Na2O

K20

P2°5H20+

H 20-

co2

Total

qorabannedihyolmtilap

VCrNiCuZnGaRbSrYZrNbBaLaCePbThUAs

kmg

A

45.7

2.29

13.75

3.20

7.55

0.36

7.80

7.61

1.69

1.63

0.45

4.94

2.45

99.42

C.I.P

1.33

9.6314.3025.12_7.77

23.79-

4.64

4.351.07

Trace

199238175561021532

4602117241672366144

<0.50.5

0.390.56

B

53. 1

1.64

15.15

2.04

6.70

0.12

4.57

7.43

3.50

1.62

0.43

0.51

0.26

3.22

100.29

.W. Norms

4. 179.57

29.6220.84_

10.8114.22-2.963.111.02

elements in

1131 16862011920445352015827907315954

1 .0

-

0.230.48

C

53.9

2.05

16.40

1.38

7.03

0.18

2.48

5.54

2.59

3.28

0.50

2.66

2.20

100.19

8.3219.3821.92

23.44-

0.6514.57-2.00

3.891. 18

parts per

851411168615826012421849

13446110011

130.51.5

0.450.34

D

48.3

2.45

13.87

2.60

6.68

0.13

6.65

8.26

2.99

2.67

0.83

3.04

0.60

99.07

15.7825.30

16.54-

15.29

3.538.633.77

4.651.97

million

1389184329217337402024062

1378

558926

0.51.0

0.370.56

E

44.9

2.32

14.09

3.45

6.35

0.16

7.40

9.64

2.99

3.56

0.78

2.80

0.29

1.98

100.71

_

21.04

7.31

14.519.75

22.54-

9.285.00

4.411.85

153218149458115126

10222623887

1232

6011448

1.5-

0.440.58

F

48.8

1.64

14.84

2.07

6.55

0. 14

7.21

7.31

3.22

3.88

0.51

2.35

0.24

1.98

100.74

22.9318. 1014.584.9514.81-

13.503.003. 1 11.21

1241791323192171316602322857

1 144488368

1.0-

0.440.60

G

51.4

1.21

16.74

2.48

3.25

0.10

3.88

4.26

4.67

6.75

1.61

2.06

1.04

0.08

99.53

_

39.8920.044.7810.554.64

6.833.602.303.81

456061278326132

115327

1 161. 155139415124238375.0-

0.490.55

H

51.1

1.17

16.86

2.70

3.25

0.09

4.00

4.11

5.75

5.59

1.42

2.53

0.73

0.10

99.40

_

33.0423.813.9213.226.23-

6.403.912.223.36

486866309023130

120826

11 18162

142015624336364.01.5

0.390.55

I

. 50.5

1.46

17.67

2.59

4.79

0. 13

4.04

6.14

3.06

5.88

1.15

1.04

0.03

0.25

98.73

_

34.7519.0817.113.694.71-

8.863.762.772.72

1278247588318

213206030

46825

4660259433104961 I

4.5

0.560.50

H O , H_O~, and C0 9 were determined by the Australian Mineral Development Laboratories, Adelaide

(where no H,O~ figure is given, H O is total water); remaining elements by XRF and AA in Bureau of

Mineral Resources laboratory (see Sheraton & Labonne, 1978, for details of methods).

k = atomic K/(K+Na); mg = atomic Mg/(Mg+total Fe)

A. Tholeiitic basalt (70280296), Amery Peaks F. Alkali olivine basalt (7 1280007), Mount KirkbyB. Calc-alkali basalt (71280126), Taylor PlatformC. Calc-alkali basalt (70280131), Mount RivettD. Olivine basalt (69280217), New Y.eak Nunatak.E. Alkali olivine basalt (69280225), Fox Ridge

G. Tristanite lava (71280140), Manning MassifH. Leucite tristanite lava (73281594), Manning MassifI. Trachybasalt (77284730), Bunger Hills

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MAFIC ROCKS OF PRINCE CHARLES MTS, ANTARCTICA 299

TABLE 3. Incompatible element ratios of mafic igneous rocks from the NPCM — Mawson Coast area compared withthose of tholeiite and K-rich dykes.

K M

Ti/P

P/Ce

Zr/P

K/Rb

K/Ba

K/Ce

K/Nb

K/Zr

Ce/Ba

Ti/Zr

Ti/Nb

Zr/Nb

Ce/Y

10.3 14.3

14 36

0.23 0.151

179

33

264

1500

82

0.13

45

820

18

1.8

263

25

186

790

34

0.14

95

2200

23

0.57

11.0

27

0.124

296

21

139

450

41

7.0

32

0.088

423

20

221

330

78

89

990

11

1.8

5.2 5.6

32 22

0.084 0.100

306

0.15 0.091

80

330

4.2

2.9

15

228

500

85

332

2ff

272

560

125

4.1 4.1

41 30

0.066 0.070

672 235

0.065 0.074

62 56

360 250

5.9 4.4

3.0 4.2

16

249

360

93

61

240

3.9

4.5

24

259

340

124

4.4 1.07 1.74 3.78 1.44

27 27 12 29 46 33

0.103 0.173 0.093 0.180 0.123

246 391 229 421 280 400

36 10.5 87 '•' 6.0 28

211 113 227 248 140

320 1950 610 1360

45 104 54 44

0.065 0.093

58

160

2.7

4.4

28

388

560

141

0.073

43

170

4.0

3.6

0.17 0.093

6.3

45

7.2

9.0

19

350

19

14

0.32 0.024 0.17

21 12

240 370

II 31 6.6

7.9 8.6

D.E.F.G.

Average early Proterozoic high-Mg tholeiite from 11.Enderby Land (A-C from Sheraton & Black, 1981) I.Average middle Proterozoic group II tholeiitefrom Enderby Land K.Average middle Proterozoic group I tholeiite L.from Enderby LandTholeiitic basalt (70280296) M.Calc-alkali basalt (71280126) N.Calc-alkali basalt (70280131)Olivine basalt (69280217)

Alkali olivine basalt (69280225)Alkali olivine basalt (71280007)

J. Average tristanite (71280140, 73281594)Trachybasalt (77284730)Average Mount Bayliss alkali melasyenite (L and Mfrom Sheraton & England, 1980)Average Priestley Peak alkali melasyeniteAlkali basalt source (Sun 4 Hanson, 1975a; Zr/Nbratio is for ocean island basalt from Erlank &Kable, 1976)

I5O0

000

500

-

-

•

A

|

AA

OO

1

A

1

1

A

-

150

Nb100 150

16/09/109

Fig. 5. Plot of Zr against Nb. Fields and symbols as inFigure 3.

Ti-rich phase, such as ilmenite, could explain theparticularly low Ti contents (relative to other incom-patible elements) of the tristanites and trachybasalt(Fig. 4B), as well as the low V of the tristanites(Table 2).

The abundance patterns of Figure 4, and thesystematically lower Ti/Zr and Ti/P ratios of thealkaline rocks (Table 3) are consistent with Ti beingconsiderably more compatible than Zr or P. Zr/P

4 0 0

300

200

100

1

-

-

-

A

• •

]

AA

1

A

AA

^ m m ^

1—

-

-

10 20 4 0 50 60Y

I6/O9/IOFig. 6. Plot of Ce against Y. Fields and symbols as in

Figure 3.

shows a relatively small decrease with increasingalkalinity, indicating that Zr is only slightly morecompatible than P, in agreement with mineral-meltdistribution coefficients for probable major residualand fractionating phases (Pearce & Norry, 1979).However, mantle melting calculations, assumingpercentage melting and residual compositions similarto those of Frey et al. (1978) for comparable rocks,suggest that a residual Ti-rich mineral (cf. Sun &

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300 J. W. SHERATON

2000

1000

800

600

400

200

10080

60

40

20

61-

O Leucite Iristonite• Bunger Hills trachybasalt -j• Mt Bayliss melasyeniteA Priestley Peak melasyenite

4 0 0

2 0 0

100

8 0

6 0

4 0

20

10

Rb

- •

J

--

-

- aA

. A••o

- 1

Ba Th Nb K La Ce Sr

i i i i i t i

70280296-Tholeiite71 280126 1 Calc-alkali70280131/ basalt69280217-01. basalt71 2800071 Alkali69280225J 01.basalt

1 I i i i i i

P Zr Ti Y

-

A

_

i i i i "

Rb Ba Th Nb K La Ce Sr P Zr Ti Y16/09/106

Fig. 4. Normalised trace element abundance patterns forA, dykes from the NPCM, and B, K-rich dykesand tristanite lava. Stippled area indicates range ofpatterns of middle Proterozoic tholeiites fromEnderby Land (Sheraton & Black, 1981). Nor-malising values used are estimated primodial man-tle concentrations given by Sun (1980).

(1971) and (for the alkaline compositions) Coombs& Wilkinson (1969); this is in marked contrast to theProterozoic tholeiites, which are mostly K-poor to'average'.

Several dykes (including both calc-alkali basalts,as well as one of the Mount Bayliss melasyenites)contain carbonate-rich ocelli that may reflect liquidimmiscibility in the magma (Ferguson & Currie,1972; Freestone, 1978). These plot within the

immiscibility field of Philpotts (1976) in Figure 3,although other rocks that plot in this field do nothave ocellar textures.

Minor elementsNormalised abundance patterns, in which the

order of elements is considered to be one ofincreasing incompatibility (Sun et al., 1979; Woodet al., 1979) show the relatively strong enrichment inelements from Rb to Zr of the NPCM rocks, relativeto typical Proterozoic tholeiites of Enderby Land(Fig. 4A). However, a few olivine tholeiites fromEnderby Land have generally similar patterns to theleast enriched dykes (70280296, 71280126) (cf.Sheraton & Black, 1981). The tristanites, K-richalkali melasyenites, and Bunger Hills trachybasaltshow extreme enrichment in most incompatibleelements (Fig. 4B). The systematic variations ofincompatible element abundances in individualrocks, both fresh and deuterically altered, suggeststhat any chemical changes during alteration wereminor. Although there is relatively more scatter forhighly incompatible ( i . e . , mostly large-ionlithophile, LIL) elements (Rb to K), which, withrare-earth elements and Sr, are those most likely tobe affected by alteration processes (Smith & Smith,1976; Hellman et al., 1979), the greatest variation isshown by the melasyenites and trachy-basalt whichare unaltered. Hence, incompatible element abun-dances are considered to be essentially primary.

Incompatible element ratios of tholeiitic suites arethought to largely reflect those of the mantle source(e.g., Erlank & Kable, 1976; Sun et al., 1979). Forthe Enderby Land tholeiites only ratios involving Sr,Y and, in a few cases, Ti have been significantlymodified by low-pressure fractional crystallisation(Sheraton & Black, 1981). In contrast, those of thealkaline rocks tend to be much more varied (Table 3,Figs 5-8) and, particularly for the more extremecompositions, may depend on several factors otherthan source composition. Petrogenetic modelling fora wide range of primary tholeiitic to alkaline basalts(Frey et al., 1978) has shown that much of theirminor element variation can be explained by differ-ences in percentage melting and residual mineralogy(allowances being made for fractional crystallisa-tion). Nevertheless, systematic differences in manyincompatible element ratios imply a mantle sourcethat is heterogeneous with respect to LIL elements inparticular (Sun & Hanson, 1975a; Frey et al., 1978;Wood et al., 1979; Sanders et al., 1980; Sheraton &Black, 1981).

The relatively low Ni, Cr and mg (atomic Mg/(Mg+ total Fe)) values of the NPCM rocks (Table 2)imply extensive crystal fractionation (Ringwood,1975; Frey et al., 1978). Only the Priestley Peakmelasyenite is close to a primary magma in composi-tion, but even this shows evidence for phlogopitefractionation (Sheraton & England, 1980). The widerange of age and composition, and the lack of clearlydefined fractionation trends makes quantitative pet-rogenetic modelling difficult, but likely combina-tions of plagioclase, clinopyroxene, and olivinefractionation cannot account for the large differencesin many incompatible element ratios between tholeii-tic and alkaline rocks (Table 3). Fractionation of a

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MAFIC ROCKS OF PRINCE CHARLES MTS, ANTARCTICA 301

3 -

2 -

O _

1 H

1

_

-

&*^ 1

1 A 'A

A 1

s •

1 1

A i >

—

q

-i

100 200Ce

300 40016/09/107

Fig. 7. Plot of P2O5 against Ce. PzOs/Ce ratio of 75 is thatestimated by Sun & Hanson (1975a) for the mantlesource of alkali basalts. Fields and symbols as inFigure 3.

8 -

6 -

1

Alkali- Basalt

- ° 0

1 A '

•AA A

: /

^f_.—-. Nephelinite)

1 1

1

•

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1100 200

Ce300 400

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Fig. 8. Plot of K against Ce. K/Ce ratio of 140 is thatestimated by Sun & Hanson (1975a) for the mantlesource of alkali basalts. Fields of primary alkalibasalt and nephelinites from ocean islands (Sun &Hanson, 1975fc) are shown (note that on thediagram given in Sun & Hanson (1975a) the Kscale is incorrect). Other fields and symbols as inFigure 3.

Hanson, 1975a) may be required to explain theanomalously low Ti contents of the more alkalinerocks (Fig. 4B). The results of such modellingcritically depend on the chosen distribution coeffi-cients, which are subject to large uncertainties, andthe importance of fractionation of a Ti-rich phase isdifficult to assess, but experimental studies haveshown that rutile and sphene, at least, are stable wellinto the partial melting field of hydrous maficcompositions (Green, 1981). The stability of suchphases may well be enhanced at high PH2O (Saun-

ders et al., 1980; Sun, 1980). Residual rutile orsphene, rather than ilmenite (McCallum & Charette,1978), could account for the marked negative Nbanomalies shown by some rocks; ratios involving Nbare particularly variable (Table 3; Fig. 5). Residualaccessory minerals are likely to be of most impor-tance in controlling the minor element compositionsof strongly alkaline melts (Sun & Hanson, 1975a;Campbell & Gorton, 1980; Green, 1981), whichrequire very small degrees of melting (Green, 1973;Ringwood, 1975). Y is even more compatible thanTi during partial melting, and the large increase inCe/Y with alkalinity (Fig. 6) reflects the increasingimportance of residual clinopyroxene and, in particu-lar, garnet with decreasing degree of melting (Freyet al., 1978; Hanson, 1980). Clinopyroxene fractio-nation cannot account for such large variations.

In contrast, P/Ce and K/Ce are not stronglycorrelated with bulk composition (Figs 7 ,8 ) . Sun &Hanson (1975a) considered the relative constancy ofP/Ce to indicate that apatite is not a residual mineral,an interpretation since supported by experimentalstudies (Watson, 1980). It is thus probable that muchof the variation in this ratio is due to differences insource composition. K/Ce ratios of the alkaline rocksare higher than those of the tholeiites (with theexception of the rare high-Mg tholeiites, which haveunusually high LIL elements contents), consistentwith a more strongly enriched source. This contrastswith the much lower K/Ce of ocean island nepheli-nites compared to related alkali basalts (Fig. 8),attributed by Sun & Hanson (1975a) to the possiblepresence of residual pholgopite during generation ofthe nephelinite magmas. If phlogopite was a residualphase for the NPCM rocks (Green, 1981), enrich-ment of the mantle source would have been propor-tionally greater. The low K/Ce of the trachybasaltmay partly be due to fractionation of K-feldspar.Ratios of other highly incompatible elements, suchas Ce/Ba and K/Ba, show considerable variationsand may also reflect differences in source composi-tion. The extreme K contents of rocks such as thealkali melasyenites and Gaussberg leucitites suggestpartial melting of unusually phlogopite-rich mantle(Flower, 1971; Lloyd & Bailey, 1975; Wendlandt &Eggler, 1980).

DISCUSSIONThe validity of using tholeiitic dyke swarms as

stratigraphic markers in subdividing metamorphiccomplexes of the East Antarctic Shield has beenconfirmed by geochemical and geochronologicaldata. Only one analysed dyke from the late Pro-terozoic terrain of the NPCM is of tholeiitic compos-ition, and this has minor element characteristicsquite different to those of the typical Proterozoictholeiites. Almost all other post-1000 Ma maficrocks are of alkaline composition and have highK/Na ratios, in contrast to the generally much lowerK/Na of the tholeiitic suites. No comparable alkalinedykes of known Precambrian age have been found.

The Phanerozoic rocks form a compositionallyheterogeneous group of a wide range of ages, fromCambrian to Eocene, and hence cannot be geneti-cally related. However, their enrichment in highlyincompatible elements (Rb, Ba, Th, K, Pb, Th, U,

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302 J. W. SHERATON

and in most cases, Nb), relative to La, Ce and P,suggests derivation by partial melting of moreenriched mantle source regions than those of most ofthe Proterozoic tholeiitic suites. Some Jurassictholeiites from Antarctica, including the FerrarDolerite (Gunn, 1966), have higher K/P and K/Zrthan the Proterozoic dykes, also consistent with arelatively enriched source, but others (Juckes, 1972;Clarkson, 1981) appear to have been derived from asource more like that of the Proterozoic tholeiites.Such enrichment may be a consequence of LILelement metasomatism, either prior to, or during,magma generation (Sun & Hanson, 1975a; Wood etal., 1979; Tarney et al., 1980; Sheraton & Black,1981). Therefore, East Antarctic mafic rocks havebeen derived from a variety of mantle sourcesthroughout geological time.

At least some of the Phanerozoic mafic rocks maybe related to rifting processes before, or during, thebreak-up of Gondwanaland (Tingey, 1982). Unfor-tunately, it is difficult to relate the distribution ofthese rocks with respect to possible rift structures, asessentially all exposures of the East Antarctic Shieldare either near the coast (including Priestley Peakand the Bunger Hills) or adjacent to the major,apparently pre-Permian, rift structure of the LambertGlacier-Amery Ice Shelf (Wellman & Tingey, 1976;

Federov et al., 1982; Grew, 1982; Fig. 1). In otherwords, it is not known if such rocks occur in areasremote from major crustal fractures. However, itmay be significant that Cretaceous lamprophyres ofvirtually identical age and geological setting to thoseof Radok Lake have been reported by Sarkar et al.(1980) from the Gondwana basins of eastern India.This part of India is adjacent to MacRobertson Landin most Gondwanaland reconstructions (e.g., Smith& Hallam, 1970), and separation of the two conti-nents had probably not progressed very far at thattime (Norton, 1982).

ACKNOWLEDGMENTSThanks are due to B. I. Cruikshank, K. H.

Ellingsen, J. Fitzsimmons, C. Madden, J. G. Pykeand T. I. Slezak for chemical analyses; N. G. Warefor assistance with microprobe analyses; the Cartog-raphy Section, BMR, for drafting the figures; and J.Ferguson, A. Y. Glikson, R. J. Tingey, and twoanonymous reviewers for invaluable comments onthe manuscript. Logistic support in the field wasprovided by the Antarctic Division, Department ofScience and Technology, Hobart. This paper ispublished with the permission of the Director,Bureau of Mineral Resources.

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FEDEROV, L. V., GRIKUROV, G. E., KURININ, R. G. &MASOLOV, V. N., 1982: Crustal structure of theLambert Glacier area from geophysical data; inCraddock, C. (ed.) Antarctic geoscience. Universityof Wisconsin Press, Madison, 931-6.

FERGUSON, J. & CURRIE, K. L., 1972: The geology andpetrology of the alkaline carbonatite complex atCallander Bay, Ontario. Can., Geol. Surv., Bull.,217.

FLOWER, M. F. J., 1971: Evidence for the role ofphlogopite in the genesis of alkali basalts. Contrib.Mineral. Petrol., 32, 126-37.

FREESTONE, I. C., 1978: Liquid immiscibility in alkali-richmagmas. Chem. Geol., 23, 115-23.

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GREEN, T. H., 1981: Experimental evidence for the role ofaccessory phases in magma genesis. J. Volcanol.Geotherm. Res., 10, 405-22.

GREW, E. S., 1982: The Antarctic margin, in Nairn, A. E.M. & Stehli, F. G. (eds) The ocean basins andmargins. Plenum, New York, 697-755.

GUNN, B. M., 1966: Modal and element variation inAntarctic tholeiites. Geochim. Cosmochin. Acta, 30,881-920.

HANSON, G. N., 1980: Rare earth elements in petrogeneticstudies of igneous systems. Annu. Rev. Earth PlanetSci., 8, 371-406.

HELLMAN, P. L., SMITH, R. E. & HENDERSON, P., 1979:The mobility of rare earth elements: evidence andimplications from selected terrains affected by burialmetamorphism. Contrib. Mineral. Petrol., 71, 23-44.

IRVINE, T. N. & BARAGAR, W. R., 1971: A guide to thechemical classification of the common volcanic rocks.Can. J. Earth Sci., 8, 523-48.

JAMES, P. R. & TINGEY, R. J., in press: The geologicalevolution of the East Antarctic metamorphic shield —a review. Proc. 4th Int. Symp. on Antarct. Earth Sci.,Adelaide, August, 1982.

JUKES, L. M., 1972: The geology of north-eastern Heimef-rontfjella, Dronning Maud Land. Br. Antarct. Surv.,Sci. Rep., 65.

LLOYD, F. E. & BAILEY, D. K., 1975: Light elementmetasomatism of the continental mantle: the evidenceand the consequences. Phys. Chem. Earth, 9, 389-416.

MCCALLUM, I. S. & CHARETTE, M. P., 1978: Zr and Nbpartition coefficients: implications for the genesis ofmare basalts, KREEP, and sea floor basalts. Geochim.Cosmochim. Acta, 42, 859-69.

NORTON, I. O., 1982: Paleomotion between Africa, SouthAmerica and Antarctica, and implications for theAntarctic Peninsula; in Craddock, C. (ed.) Antarcticgeoscience. University of Wisconsin Press, Madison,99-106.

PEARCE, J. A. & NORRY, M. J. 1979: Petrogeneticimplications of Ti, Zr, Y, and Nb variations involcanic rocks. Contrib. Mineral. Petrol., 69, 33-47.

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PHILPOTTS, A. R., 1976: Silicate liquid immiscibility: itsprobable extent and petrogenetic significance. Am. J.Sci., 276, 1147-77.

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RINGWOOD, A. E., 1975: Composition and petrology of theEarth's mantle. McGraw-Hill, New York.

SARKAR, A., PAUL, D. K., BALASUBRAHMANYAN, M. N. &SENGUPTA, N. R., 1980: Lamprophyres from IndianGondwanas — K-Ar ages and chemistry. Geol. Soc.India, J., 21, 188-93.

SAUNDERS, A. D., TARNEY, J. & WEAVER, S. D.. , 1980:Transverse geochemical variations across the Antarc-tic Peninsula: implications for the genesis of calc-alkaline magmas. Earth Planet. Sci. Lett., 46, 344-60.

SHERATON, J. W. & BLACK, L. P., 1981: Geochemistry andgeochronology of Proterozoic tholeiite dykes of EastAntarctica: evidence for mantle metasomatism. Con-trib. Mineral. Petrol., 78, 305-17.

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APPENDIX:

J. W. SHERATON

Petrography of analysed samplesTholeiitic basalt (70280296) from Amery Peaks contains

salite (Ca47Mg4oFei3, 5%) and altered ferromagnesian(15%) phenocrysts. The latter pseudomorphs (calcite +chlorite +?talc) may be altered orthopyroxene, as the rockis strongly hypersthene and slightly quartz-normative.Alternatively, if the dyke has been significantly altered incomposition, they may be altered olivine. The groundmassconsists of plagioclase (An4i-4!>), clinopyroxene, altered?orthopyroxene, and ilmenite; chlorite + anthophyllite +calcite ocelli are present. A similar dyke crops out at NeillPeak near Mawson.

Two calc-alkali basalts were analysed. 71280126 fromTaylor Platform is petrographically similar to the Pro-terozoic dolerites except for the presence of carbonate-richocelli. Andesine-labradorite (11%) and clinopyroxene(with cores of orthopyroxene, 4%) phenocrysts are en-closed in a relatively fresh, well crystallised groundmass ofclinopyroxene (30%) plagioclase (44%), Fe-Ti oxides(3%), and carbonate (3%). 70280131 from Mount Rivetton the Mawson Coast contains labradorite (18%) andaltered pyroxene (carbonate + ?serpentine, 7%) pheno-crysts in an altered groundmass of feldspar, carbonate,reddish-brown biotite, chlorite, Fe-Ti oxides (2-3%), andquartz. A few quartz + carbonate ocelli are present.

An olivine basalt dyke (69280217) from New YearNunatak is petrographically similar to the alkali olivinebasalts but is slightly hypersthene-normative. It containszoned phenocrysts of clinopyroxene (15%) and altered

olivine (5%) in a groundmass of altered feldspar,clinopyroxene, Fe-Ti oxides (3-4%), and minor reddish-brown amphibole and carbonate. Olivine phenocrysts arepseudomorphed by chlorite.

Alkali olivine basalt dykes crop out at Fox Ridge(69280225) and Mount Kirkby (71280007). Both containaltered olivine (~ 10%) and zoned augite/salite (nearCa45Mg43Fei2, 5-15%) phenocrysts in a partly alteredgroundmass of clinopyroxene, plagioclase (An47 in69280225), alkali feldspar, reddish-brown amphibole (10%kaersutite in 69280225), and minor ilmenite (2-3%).Olivine in 71280007 is about Fos2, but that in 69280225 isentirely pseudomorphed by serpentine, carbonate, chlorite,and talc. A petrographically similar dyke was found atScullin Monolith on the Mawson Coast.

Two samples of the leucite tristanite lava at ManningMassif were analysed. 73281594 contains sparse olivinemicrophenocrysts (F077-80, 3%) in a very fine-grainedgroundmass consisting of altered leucite, alkali feldspar,clinopyroxene, ilmenite, and altered glass. A second,coarser grained sample (71280140) contains olivine mic-rophenocrysts, clinopyroxene, alkali feldspar, reddish-brown biotite, and Fe-Ti oxides, but no leucite. Amygdalescontaining phillipsite are present in some of the lavas.

Trachybasalt 77284730 from the Bunger Hills, QueenMary Land contains phenocrysts of zoned plagioclase(Anss cores) with alkali feldspar rims (10%), and olivine(F071-73, 4%). The groundmass consists of clinopyroxene(Ca47Mg38Feis, 10%), dark brown biotite (mgi\, 5%),pale yellow to reddish-brown amphibole (4% ferroanpargasite), plagioclase, alkali feldspar, Fe-Ti oxides (3-4%), and minor apatite.

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