Antarctic Science 4 (7): 71-87 (7992)

Suite subdivision and petrological evolution of granitoids from the Taylor Valley and Ferrar Glacier region, south Victoria Land

ROBERT W. SMlLLlE Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand

Abstract: Detailed geological mapping and geochemical analysis of early Palaeozoic granitoid plutons and dykes from the Taylor Valley and Ferrar Glacier region in south Victoria Land reveal two distinct suites. This suite subdivision-approach is a departure from previous lithology-based schemes and can be applied elsewhere in south VictoriaLand. Theoldercalc-alkaline Dry Valleys 1 suite is dominated by the compositionally variable Bonney Pluton, a flow-foliated concordant pluton with an inferred length of over 100 km. Plutons of this suite are elongate in a NW-SE direction and appear to have been subjected to major structural control during their emplacement. The younger alkali-calcic Dry Valleys 2 suite comprises discordant plutons and numerous dyke swarms with complex age relationships. Field characteristics of this suite indicate that it was passively emplaced into fractures at higher levels in the crust than the Dry Valleys 1 suite. Whole-rock geochemistry confirms this suite subdivision based on field relationships and indicates that the two suites were derived from different parent magmas by fractional crystallization. The Dry Valleys 1 suite resembles Cordilleran I-type granitoids and is inferred to be derived from partial melting of the upper mantle and/or lower crust above an ancient subduction zone. The Dry Valleys 2 suite resembles Caledonian I-type granitoids andmay have resulted from alater episode of crustal extension.

Received 18 December 1990, accepted 24 July 1991

Key words: Antarctica, Dry Valleys, granitoids, suites, south Victoria Land, petrogenesis, plutons

Introduction

In south Victoria Land, numerous early Palaeozoic granitoid plutons of varying compositions and intrusive styles intrude multiply-deformed Koettlitz Group metasediments (Grindley & Warren 1964). Together these rocks form part of the pre- Devonian basement of the Ross Orogen which is considered to have formed along the margin of the East Antarctic craton during the late Precambrian to early Palaeozoic (Adams et al. 1982, Borg et al . 1987). These basement rocks are overlain by flat-lying Beacon Supergroup sedimentary rocks of Devonian to Triassic age and are intruded by Jurassic diabase sills and basalt flows of the Ferrar Group, all of which have been uplifted to form the Transantarctic Mountains.

This paper is concerned with granitoid plutons and dykes exposed in the vicinity of Taylor Valley and Ferrar Glacier, in the Dry Valleys region of south Victoria Land (Fig. 1). These rocks were first studied at the turn of the century by geologists of the British Antarctic expeditions (Ferrar 1907, Priestley 1909, Mawson 1916, Smith 1924). Nearly half a century was to pass before these rocks were the subject of further investigation, initially as part of regional geological mapping programmes (e.g. Allen & Gibson 1962, Gunn & Warren 1962, McKelvey & Webb 1962, Angino et al. 1962, Blank et al. 1963, Haskell et al. 1965), and later as the main subject of field research (e.g. Fikkan 1968, Murphy 1971, Lopatin 1972, Smithson et al. 1972, Findlay 1983, 1985, 1990, in press, Skinner 1983).

Many of the early workers in southVictoria Land recognized the presence of older and younger groups ofgranitoids. Later recognition of lithologic complexity within widely mapped granitic lithologies has resulted in a plethora of lithology- based classification schemes. These schemes are difficult and in many instances impossible to apply, due mainly to the lithological diversity commonly found within individual plutons (see Allibone et al. 1991). The usefulness of these schemes is also limited by the lack of integrated geochemical data as only a handful of workers (Ghent 1970, Lopatin 1972, Palmer 1987) have undertaken geochemical work on these rocks.

In this paper detailed field observationsof granitoids, which have well-constrained positions in the intrusive sequence, are integrated with whole-rock geochemistry in an attempt to devise a practical and meaningful classification scheme. This scheme is sufficiently general to allow correlation with other areas.

Due to the paucity of petrological and geochemical data for granitoids from south Victoria Land, hypotheses relating to their origin are rare and based almost exclusively on field observations. Smithson et al. (1972) and Skinner (1983) have suggested an anatectic metasedimentary origin for these rocks and Findlay (1985) suggested that his younger Victoria Group and older Larsen Group are related by differentiation. These hypotheses were testedusing new petrological andgeochemical data to gain greater insight into the origin and evolution of these rocks.

71

72 ROBERT W. SMlLLlE

Granitoids in south Victoria land have been collectively referred to as the Granite Harbour Intrusive Complex (Gunn &Warren 1962), and granitoids which crop out throughout the Transantarctic Mountains have since been assigned to this unit. However, from their work in the central Transantarctic Mountains,Borgeta1.(1990)suggestedthat theuseofthe term Granite Harbour Intrusive Complex should be re-evaluated, and they avoided the use of this name. This paper adopts the same approach for similar reasons.

Multiply deformed orthogneisses which predate intrusion of the granitoids are not included in this study. Those rocks are the subject of recent work by Cox & Allibone (1991) who have shown that the orthogneisses have similar mineralogies and chemistries to many of the granitoids, suggesting that the two rock types are genetically related.

Fig. 1. General- ized geological sketch map of Taylor Valley and Ferrar Glacier region (includes areas mapped by Findlay 1983 and Allibone et al. 1991).

Table I. Intrusive sequence for granitoids from the Taylor Valley - Ferrar Glacier region. Pluton names taken from Allibone et al. (1991)

“younger” biotite granite dykes

Rhone Pluton

K-Porphyry dykes

micromonzonitc dykes

biotite granite dykes

Hedley Pluton I

Catspaw Pluton

Bonney Pluton

I I I

I 1

I

Field relations and petrography

Several different plutons were sampled and mapped in the study area, and are described below in order of intrusion. This intrusive sequence is based partly on the recent 1:50 000 mapping work of Allibone et al. (1991), and a summary of this sequence is shown inTable I. Pluton names used in this paper are taken from Allibone et al. (1991).

Bonney Pluton

The Bonney Pluton is a large, heterogeneous body trending NW throughout the study area, and has previously been referred to as the Larsen Granodiorite (Haskell et al. 1965). The exact extent of this pluton is uncertain. It crops out extensively in Wright Valley some 40 km to the north (Cox 1989, Allibone et al. 1991), and recent work has revealed

identical granitoid 50 km farther south of the study area in Miers Valley (R.W.Smillie, unpublished), which may mark its southern limit. It is therefore approximately 100 km in length and of batholithic proportions. The pluton has a maximum width of 25 km with the western and eastern marginsofthepluton intruding KoettlitzGroupmetasediments and intercalated orthogneisses.

The dominant minerals in the pluton are hornblende, dark brown biotite, plagioclase, alkali feldspar, and interstitial quartz, Proportions of plagioclase, alkali feldspar and quartz vary between monzodiorite, quartz monzodiorite, granodiorite and monzogranite compositions. Accessory minerals include clinopyroxene, apatite, allanite, titanite, zircon, ilmenite and magnetite. Chlorite, green biotite and sericite crystallized in variable amounts during subsolidus alteration. Textural relationships indicate that clinopyroxene, amphibole,

GRANlTOlDS FROM SOUTH VICTORIA LAND 73

plagioclase and biotite crystallized first, followed by alkali feldspar megacrysts, (up to 8 cm long), and quartz.

There is considerable textural variation within the pluton, due to varying degrees of alignment and segregation of alkali feldspar megacrysts, hornblende grains, mafic enclaves and xenoliths. At the margins of the pluton, augen gneiss textures and prominent bands of mafic and felsic minerals several metres long define a foliation parallel to xenolith lineation (Fig. 2). Toward the centre of the pluton such banding is less common, and minerals and enclaves are distributed in a more random fashion.

Extreme concentration of alkali-feldspar megacrysts is occasionally found in isolated pods and swarms, up to a metre across, which also contain abundant coarse-grained hornblende. Megacrysts within the more elongate pods have their long axes aligned sub-parallel to the long axis of each pod.

The Bonney Pluton also contains abundant mafic enclaves. These crop out as single isolated enclaves, in localized swarms

covering several square metres, and in zones several metres in length (Fig. 3). Most are well-formed ellipsoids whereas others have a more angular shape. They are typically less than 50 cm in maximum exposed dimension and have sharp cuspate margins, usually convex toward the host. Most of the enclaves have very fine-grained margins grading inward to fine- to medium-grained interiors and in many instances granitoid host minerals transgress enclave boundaries. Although primarily allotriomorphic or hypidiomorphic granular, a variation of enclave textures may be observed at a single outcrop, including porphyritic and porphyritic-cored varieties.

Within each enclave a fine-grainedgroundmass of subhedral plagioclase, biotite and hornblende defines a weak to moderate foliation parallel to the long axis. Plagioclase and hornblende and less commonly clinopyroxene, may also be present as phenocrysts. Alkali feldspar is rare, and but for the absence of quartz, the mineralogy of the enclaves is essentially the

Fig. 2. Strongly foliated western margin of the Bonney Pluton, north side of the Taylor Valley. The foliation is deflected around undeformed amphibolite xenoliths, as a result of magmatic flow.

Fig. 3. Flow-aligned mafic enclaves within the Bonney Pluton, south side of Taylor Valley. Hammer handle is 80 cm long.

74 ROBERT W. SMlLLlE

same as that of the host pluton. Acicular apatite crystals with axial ratios up to 40:l are widespread throughout the enclave groundmass, a feature typical of many other granitoid enclaves (Vernon 1983).

Of particular interest is the occurrence of interdigitating compositional zones of granitoid within the pluton, up to 5 m wide and 50 m long, delineated by internal contacts. These zones are typically moderately foliated with inclusions of each granitoid occuring within the other (Fig. 4). Some inclusions have roughly linear outlines parallel to the contact between the granitoids and others have and more wispy, irregular shapes. Several inclusions are deformed and form discontinuousbands within their host granitoids. These bands bear a strong resemblance to schlieren which crop out throughout the Bonney Pluton in parallel and sub-parallel groups. The schlieren are commonly folded and offset by small shears and are graded from fine-grained mafic mineral phases to coarser-grained feldspars and quartz. Generally, the contact between themaficlayerandgranitoid host is sharp and the contact between the felsic layer and the granitoid host is gradational and diffuse.

Orbicular granitoid occurs at several localities within the Bonney Pluton, for the large part confined to zones or pods within a few hundred metres of the Koettlitz Group contact. Present in a variety of shapes and sizes, the orbicules are composedchiefly ofradialand tangential shellsofplagioclase and radial shells of hornblende surrounding varying core material such as xenoliths and enclaves. Dahl & Palmer (1983) considered them to be the result of metasomatic processes. However more recent work has shown them to be the product of magmatic crystallization (Smillie 1989).

Catspaw PZuton

This is a narrow, elongate monzogranite pluton approximately

Fig. 4. Interdigitating contact between quartz monzodiorite and granite within the Bonney Pluton, north side of Taylor Valley. Inclusions of each granitoid type with varying shapes occur within the other. Hammer handle is 80 cm long.

5 km across, trending "W from the Kukri Hills west of Hedley and Borns glaciers, to Catspaw Glacier on the north side of upper Taylor Valley (Allibone et al. 1991). The pluton is a coarse-grained monzogranite comprising equigranular quartz, alkali feldspar and plagioclase with accessory amounts of biotite, hornblende, allanite, titanite, zircon and traces of opaques. Occasionally, alkali feldspar ismantled by plagioclase forming classic Rapakivi texture, Post-crystallization alteration is quite marked in places, especially in close vicinity to the Ferrar Dolerite sills (Allibone etaZ.1991). Unlike the Bonney Pluton, the Catspaw Pluton is a homogeneous granitoid lacking a flow-foliation or mineral segregation. It does, however, contain irregularly shaped mafic enclaves, up to 10 cm long, with margins that interdigitate with the granite host. The enclaves comprise large (up to 0.5 cm) subhedral grains of quartz, plagioclase, alkali feldspar and hornblende in a groundmass of similar composition.

Hedley Pluton

The Hedley Pluton is another narrow elongate pluton trending "W from Cathedral Rocks on the south side of Ferrar Glacier to Hedley Glacier in the Kukri Hills. The northern limit of the Hedley Plutonwasmappedby Alliboneet aL(1991) imediately west of Borns Glacier on the northern face of the Kukri Hills. The pluton is a fine- to medium-grained granodiorite containing minor biotite and does not contain hornblende. Alkali feldspar is interstitial to quartz and plagioclase, although rare phenocrysts, up to 1 cm in length, occur randomly throughout the pluton. Accessory minerals include apatite, allanite, ilmenite, magnetite and zircon. Prehnite has grown along biotite cleavage planes andvariable amounts of cholrite and sericite also occur as secondary mineral phases.

The pluton is a relatively homogenous body and Iacks mafic enclaves. Flow alignment and segregation of biotite is restricted to the pluton margin and to narrow dykes which radiate from the pluton. At Cathedral Rocks the eastern margin of the pluton forms a spectacular intrusion breccia up to 1 km across where it intrudes the Bonney Pluton. Asimilar intrusive style is found beteween the pluton and Koettlitz Group rocks in the western Kukri Hills (Allibone et at. 1991).

Leucocratic biotite granite dykes

Swarms offine- to medium-grained leucocratic biotite granite dykes and occasional stocks, up to 200 m wide, cross-cut the Bonney and Hedley plutons. They are similar in mineralogy to the Hedley Pluton, but contain less biotite. The dykes generally lack flow foliation or segregation, with some displaying chilled margins.

Mafic dykes

A wide range of mafic dyke lithologies form prominent

GRANlTOlDS FROM SOUTH VICTORIA LAND 75

swarms on the south side of Taylor Valley. The dykes vary in width from 2-10 m, have chilled margins and are continuous over several kilometres. Fine-grained equigranular micromonzonite dykes withintergranular textures are common, composed of varying amounts of biotite and clinopyroxene, amphibole,zonedcalcicplagioclase (Anw-Anss) andinterstitial K-feldspar. Accessory phases include apatite (with axial ratios of up to 40: l), titanite and zircon. Many of the dykes are pervasively altered with green hornblende rimming brown hornblende, feldspar being altered to sericite and kaolinite, and chlorite replacing biotite.

Porphyry dykes

Grey monzonite to monzogranite porphyry dykes form prominent swarms approximately 4 km west of the snout of Taylor Glacier on the southern side of Taylor Valley and on the northern side of Taylor Valley between Stocking and Rhone glaciers. These dykes are usually closely associated with the mafic dykes described above.

The dominant minerals of the dykes are alkali feldspar, plagioclase and hornblende phenocrysts lying in a groundmass of biotite, quartz, alkali feldspar, hornblende and plagioclase. The monzonite dykes have a felty pilotaxitic texture which is particularly well developed where the groundmass follows t he outline of the larger grains. Many of the phenocrysts are rounded, with their long axes commonly aligned parallel to the strike of the dykes. Composite dykes of felsic porphyry and alkali-feldspar monzonite are common on the south side of Taylor Valley.

Rhone Pluton

The Rhone Pluton (Smillie 1989) is a small tabularbody which intrudes the Bonney Pluton along the cliffs either side of Rhone Glacier on the northern side of Taylor Valley. The pluton contains pink to brick-red to orange euhedral alkali feldspar phenocrysts, randomly oriented within a medium- to coarse-grainedgroundmass of plagioclase, biotite, hornblende and quartz. Accessory phases include apatite, allanite, tit anite, zircon, monazite and opaques. The Rhone Pluton is variable in composition, ranging from monzonite through quartz- monzonite to monzogranite. It cross-cuts the host rock at a sharp angle, and lacks a well-defined foliation. Plutons with identical field relations, mineralogy, texture, and geochemical characteristics include the Mount FaIconer Pluton in lower Taylor Valley (Ghent & Henderson 1968, Ghent 1970), and the Pearse Pluton in upper Taylor Valley (Allibone et al. 1991).

The Rhone Pluton contains a large number of porphyritic enclaves, identical in mineralogy and texture to the porphyry dykes described above. Where the Rhone Pluton cross-cuts the Bonney Pluton as dykes, the enclaves are aligned parallel to the strike of the dyke. The K-feldspar phenocrysts within the enclaves are also aligned parallel to the long axis of the

Fig. 5. Porphyritic enclave types within the Rhone Pluton. a. melanocratic enclave with chilled margin. b. mesocratic enclave exhibiting gradational contact with host granitoid. Lens cap is 50 mm in diameter.

enclave. Where the enclaves are up to half a metre in length, smaller pieces of the enclave appear to have broken off into the host granitoid. Close inspection of the enclaves reveals two distinct types: a melanocratic type with,a dark, chilled margin and and mesocratic type with a more diffuse margin (Figs 5a, b). In the latter type, host granitoid phenocrysts commonly transect the boundary between the enclave and the host. One hornblende crystal partially engulfed by an enclave has disaggregated into small grains.

Younger biotite granite dykes

These dykes, which are up to a metre wide and cross-cut the Rhone Pluton and the porphyry dykes, are the youngest granitoids in the study area. They are fine-grained, lack amphibole and are somewhat similar to the leucocratic biotite granite dykes described above, although they tend to be finer- grained and darker grey.

76 ROBERT W. SMlLLlE

Table 11. Selected major element (wt% oxide) and trace element (ppm) analyses of Rranitoid samples.

60466 60517 60527 60542 60489 60543 60538 60539 60546 60530 Sample 60511 BP BP BP BP BP BP HP HP HP CP CP

SiO, TiO, 4 0 3

FeO MnO

CaO Na,O

MgO

$0 p205 L.O.I. Total

Rb Ba Th U Pb Nb Sr Zr La Ce Nd Y sc V Ga c u Zn Ni Cr

53.77 1.18

18.99 2.16 6.02 0.11 2.90 6.80 3.63 2.76 0.32 0.75

99.39

108 1128

6 1

15 16

823 328 29 85 52 49 24

135 26 2

125 7

24

54.61 1.07

19.61 1.60 5.21 0.10 3.17 6.70 4.38 2.98 0.31 0.55

100.29

144 733

6 1

15 14

798 301 22 53 22 31 15

112 27 14

133 9

20

57.56 60.97 0.98 0.97

18.68 16.62 2.19 1.61 4.46 5.48 0.09 0.11 2.37 2.47 5.72 4.60 4.13 4.12 3.07 2.34 0.28 0.22 0.68 0.71

100.21 100.22

121 131 810 665

4 20 1 2

16 12 15 26

655 467 276 291

17 72 44 146 19 59 30 76 13 18 91 91 24 23 4 8

91 120 7 9

17 20

65.05 0.74

16.82 1.31 3.74 0.07 1.58 4.19 3.53 3.19 0.18 0.48

100.88

116 699

13 2

20 15

489 204 45 88 29 33 11 64 21 5

78 6

12

70.92 00.37 14.76 1.36 1.10 0.03 0.79 2.55 2.94 4.65 0.08 0.49

100.04

132 992 24

1 22 7

448 I42 55

101 25 7 4

34 16 2

39 3 8

69.32 0.32

15.94 0.54 1.84 0.04 0.54 2.45 3.82 3.94 0.08 0.76

99.59

110 1445

13 4

43 9

708 208 51 85 27 7 4

17 18 1

92 3 6

70.07 0.26

16.12 0.54 1.29 0.03 0.42 2.79 3.63 3.96 0.05 1.51

100.67

115 1483

11 1

22 7

648 138 34 55 11 6 4

11 18 1

32 3 5

70.16 0.29

15.82 0.14 2.06 0.03 0.52 2.62 3.76 4.03 0.07 0.46

99.96

134 1415

11 1

23 7

636 192 47 80 21 8 2

14 19 1

45 2 5

71.86 0.26

14.27 1.21 1.42 0.05 0.42 1.70 3.69 4.79 0.07 0.37

100.11

20 1 702

19 2

22 13

207 173 55 99 31 30 5

16 20 1

53 3 6

71.99 0.27

13.87 2.11 0.35 0.04 0.44 1.75 3.68 4.27 0.07 1.18

100.02

157 690

19 2

22 14

269 192 48 88 29 27 5

17 20 1

58 3

~ ~~~

(BP=Bonney Pluton, CP=Catspaw Pluton, HP-Hedley Pluton, BG=biotite granite dykes, MZ=micromonzonite dykes, RP=Rhone Pluton, Por=porphyry dykes, yBG=younger biotite granite dykes).

Geochemistry and classification

Major and trace element chemistry

Seventy eight samples were analysed by X-ray fluorescence spectrometry for 10 major and 21 trace elements. Major elements were analysed using fused glass discs (Norrish & Hutton 1969) and trace elements were determined using pressed powder pellets (Norrish & Chappell 1967). Ferrous ironcontent wasdeterminedby titration following the procedure of Wilson (1955). Representative whole-rock analyses of the different plutons and dykes are presented in Table 11. A full set of analyses is available from the author on request.

The SiO, content of the granitoids ranges from 53-73% (Fig. 6). The Bonney Pluton has the widest SiO, range of all the plutons, from 53-71%. Na,O and &O contents correlate positively and all other major elements correlate negatively with SO, content. Increasing &O content of the pluton is reflected in the mode by increasing amounts of alkali feldspar with host rock evolution. Decreasing TiO,, MgO, FeO (total),

Al,O,, CaO, Ni and Cr contents within the pluton can be attributed to fractionation of mafic mineral phases with host rock evolution. Decreasing CaO can also be attributed to fractionation of calcic plagioclase. Sc and V behave similarly to Ni and Cr which are preferentially partitioned into biotite and amphibole (Fourcade & Allegre 1981) and so also have a negative correlation with SO,. Decreasing TiO, content also results from fractionation of titanite and opaque phases and decreasing P,O, is most likely due to removal of apatite. The Zr decrease within theBonney Pluton reflects fractionation of zircon, which is incorporated as inclusions within biotite.

The only other intrusion with a wide range of Si0,content is the Rhone Pluton (6149%). Apart from a small positive Y O to SiO, variation, the Rhone Pluton exhibits many of the geochemical trends of the Bonney Pluton, and probably evolved along a similar path. The porphyry dykes also exhibit a wide SO, range (57% -70%), and have very similar compositions to the Rhone Pluton.

GRANlTOlDS FROM SOUTH VICTORIA LAND 77

Table I1 continued. Selected major element (wt% oxide) and trace element (ppm) analyses of granitoid samples. ~~

Sample 60504 60506 60479 60468 60462 60520 60474 60509 60529 60463 60528 BG BG MZ RP RP RP Po1 Por Por YBg YBg

SiO, TiO, 4 0 3

Fez03 FeO MnO MgO CaO Na,O q o p*o, L.O.I. Total

Rb Ba Th U Pb Nb Sr Zr La Ce Nd Y sc V Ga cu Zn Ni Cr

72.66 0.20 14.40 0.51 1.43 0.03 0.37 2.02 3.17 4.68 0.05 0.43 99.95

146 1264 20 1 28 8

435 156 61 103 29 12 4 10 16 1 35 1 4

75.97 0.07 13.13 0.13 0.28 0.00 0.11 1.46 2.26 5.83 0.01 0.51 99.76

147 2135

1 1 24 1

546 19 6 0 0 0 2 8 11 1 6 2 6

52.52 0.93 14.70 2.61 5.33 0.13 7.38 7.68 3.22 3.92 0.37 1.80

100.59

242 1044 10 3 15 13 647 21 1 38 86 39 39 20 147 18 10 117 141 523

61.11 68.96 0.65 0.33 15.71 15.08 1.20 0.35 4.49 2.76 0.00 0.06 1.25 0.56 3.79 2.35 5.01 3.70 5.53 5.30 0.25 0.12 1.19 0.88

100.24 100.44

234 288 845 692 17 21 5 3 25 25 33 19 568 378 397 220 74 58 145 109 66 31 44 33 5 4 27 16 22 18 10 1 85 59 4 3 10 6

74.79 0.02 13.61 0.23 0.58 0.11 0.07 1.39 3.11 5.18 0.01 0.45 99.55

220 129 4 2 39 8

131 70 6 1 0 31 2 5 18 3 8 2 -1

57.24 0.79 17.92 1.62 5.70 0.12 1.58 4.69 3.45 5.59 0.35 0.5 99.93

238 1223 13 3 20 24 867 256 66 134 52 29 6 22 21 3

108 5 4

64.02 0.55 16.17 1.30 3.73 0.09 1.24 3.53 3.59 5.06 0.22 0.62

100.12

268 804 20 3 22 19 536 272 63 120 44 40 6 26 19 7 95 5 10

70.80 0.27 14.31 0.93 1.62 0.04 0.43 1.76 3.48 5.26 0.08 0.5 99.48

263 667 24 5 29 18 327 213 58 113 45 37 3 14 20 8 62 4 3

68.41 0.17 15.99 0.96 1.66 0.04 0.36 1.53 4.32 5.39 0.03 0.69 99.56

407 465 35 6 43 22 234 394 62 91 36 41 2 7 21 18 40 3 5

75.36 0.13 12.75 0.23 1.17 0.02 0.18 0.94 2.49 5.42 0.01 0.45 99.16

280 226 23 5 44 10 194 85 *42 84 18 12 1 9 18 4

so 4 7

Granitoid classification

The vast majority of the granitoids plot well within the field of the Australian I-types in the ACF diagram of White & Chappell(l977) (Fig. 7). The strong I-type character of the granitoids is also evident on a Na,O versus YO projection (Fig. 8a), and on an Alumina Saturation Index (ASI) versus SiO, plot (Fig. 8b) in which their AS1 values correspond closely to those of other I-type granitoids from Australia (White & Chappell 1983). Most of the granitoids have an AS1 <1 and are therefore metaluminous with diopside in the norm. The more silica-rich granitoids have an ASb1 and are either peraluminous or subaluminous, respectively.

The I-type chemistry of the granitoids is reflected by their I-type mineralogies. Most of the granitoids contain hornblende and relict clinopyroxene and lack diagnostic S-type minerals such as cordierite, muscovite and garnet. The occurrence of abundant mafic enclaves in many of the plutons is also typical of I-typegranitoids (e.g. Vernon 1984, Frost & Mahood 1987, Ague & Brimhall 1988).

Suite subdivision

An important part of any regional granitoid study is the recognition of possible suites. Detailed chemical work on Australian granitoids within the Lachlan Fold Belt (e.g. White & Chappelll983) focussed interests on suites within granitoid batholiths. Pitcher (1984) assigned Peruvian CoastalBatholith rocks to “...distinct plutonic units, consanguineous sequences of which form super-units (suites)”(Pitcher 1984, p.160). Each superunit or suite is characterized by auniquegeochemical signature.

The possibility of a suite subdivision for the granitoids from the Taylor Valley and Ferrar Glacier region is hinted at in the major and trace element variation diagrams, with the Bonney Pluton forming one well-defined group, the Rhone Pluton and the porphyry dykes another. The Bonney Pluton differs from the Rhone Pluton and the porphyry dykes in its higher TiO,, MgO, CaO, V, Sc and Cr contents and lower q0, Rb and, to a lesser extent, Pb, and Zr contents. This separation into two groups is best illustrated in the $0 and Rb variation

78

0.5

0.4

0.3

0.2

0.1

0

p2°5

ROBERT W. SMlLLlE

- 400 - 0

a 350 . AA 300 .

A X 0 .

. * * A

* 5'.

: x A e m A O A A *.?*** .* ***& A Zr 250 200. 'b$tt '%:A .

** e A =^x : 150. :?J

*A 66, ...a+# 1 0 0 . 'h 00. 50

'

400 350 1 A

1 3 0

12 50 55 60 65 70 75 80

Si02

1.2

5 0 I 50 55 6 0 65 70 75 80

Si02

35

3.1 A

1 0 50 55 80 85 70 75 80

Si02

160

1401

50 55 80 85 70 75 80 50 5 5 6 0 6 5 70 75 80

Si02 Si02

7 , 30 r

50 55 6 0 85 70 75 80 50 55 80 65 70 75 80

Si02 Si02

Fig. 6. Harker variation diagrams of selected major elements (wt% oxide) and trace elements (ppm). 0 = Bonney Pluton; A= Catspaw Pluton; 1 = Hedley Pluton; x= biotite granite dykes; *= micrornonzonite dykes; o = Rhone Pluton; A = porphyry dykes; J = "younger" biotite granite dykes.

79 GRANITOIDS FROM SOUTH VICTORIA LAND

diagrams where at 61% SiO, the Bonney Pluton has approximately 3% q0 and 150ppm Rb compared with 5.5% Y O and 250 ppm Rb for the Rhone Pluton.

The high SiO, contents of the other granitoids makes their association with one group or the other somewhat difficult to ascertain, as the two groups tend to converge at high values of SiO, on many of the variation diagrams. However, the Catspaw Pluton, Hedley Pluton and biotite granite dykes have significantly lower Rb contents than the Rhone Pluton and porphyry dykes (approximately 120 ppm at 70% SiO,), similar to the Bonney Pluton. In contrast, the high Rb contents of the mafic dykes and younger biotite granite dykes suggest affinities with the Rhone Pluton and porphyry dykes.

Thus, the variation diagrams indicate a possible two-fold subdivision of the granitoids; an older suite comprising the Bonney Pluton, Catspaw Pluton, Hedley Pluton and leucocratic biotite granite dykes, and a younger suite composed of the mafic dykes, Rhone Pluton, porphyry dykes and younger biotite granite dykes. Significantly, a QAP diagram of granitoid normative mineral contents shows an similar two- fold subdivision (Fig. 9), with the older suite defining a trend from monzodiorite through quartz monzodiorite, granodiorite to monzogranite, whereas the younger suite lies closer to the A apex of the diagram and defines a trend from monzonite through quartzmonzonite tomonzogranite. The oldergranitoid

A

hornblende --I C< 'F

Fig. 7. Granitoids from the Taylor Valley and Ferrar Glacier region plotted on an (Al-Na-K)-Ca-(Fe2+ t Mg) diagram after White & Chappell(l977). I-type and S-type fields from the Kosciusko Batholith, Australia (Hine et al. 1978).

suite defines a trend typical of calc-alkaline rocks whereas the younger granitoid suite lies along the trend commonly defined by alkali-calcic rocks. A similar association of calc-alkaline granitoids post-dated by younger alkali-calcicgranitoids occurs in other granitoid provinces, including the French Massif Central (Pagel & Leterrier, 1980) and western United States (Bateman ef al. 1963).

This geochemical subdivision supports field evidence of a two-fold subdivision, whereby each of the two granitoid suites has a less felsic precursor and becomes progressively enriched in silica with time. The older suite has the early monzodioritic intrusive phase of the Bonney Pluton and culminates with intrusion of leucocratic biotite granite dykes. The younger granitoid suite begins with intrusion of swarms of mafic dykes and ends with the younger biotite granite dykes. Pitcher & Cobbing (1985) have recognized similar silica enrichment trends within granitoids in the Peruvian Andes, which they refered to as basic to acid rhythms, or suites.

From the foregoing it is possible to conclude that the granitoids from the Taylor Valley and Ferrar Glacier region comprise two distinct suites, with different compositions and relative ages. These suites appear to be broadly similar to the metaluminous calc-alkaline I-type suite and part ofthe potassic syenogranite-monzonite suite described by Armienti et al. (1990) from North Victoria Land.

0

- 0 - - - - ,--- - a :[,// I-TYPE - - - - -

I - _ - - - - \

2 - _ _ _ _ _ - - - /- t I S-TYPE ,

b 1.20

1.10

1 .oo

A.S.I. 0.90

0.80

0.70

X 0.60

50 55 60 65 7 0 75 80

SiO;!

Fig. 8. Diagrams demonstrating the strong I-type character of granitoids from the Taylor Valley and Ferrar Glacier region. a. Na,O v. q0 diagram (fields of I- and S-type granitoids from White & Chappell 1983). b. A1,O3/CaOtNa,O+~O v. S O , diagram (boundary between I- and S-type granitoids from White & Chappell 1983).

80

Q

ROBERT W.

Fig. 9. CIPW normative mineral contents plotted on the QAP Streckeisen diagram (from Le Maitre 1989) showing the division of the granitoids into two suites.

Granitoid subdivision and nomenclature in south Victoria Land A proliferation of different nomenclature and subdivision schemes have been devised for thegranitoidsof south Victoria Land by numerous workers (see Table 111), and is a source of some confusion amongst present workers in this region. Gunn & Warren (1962) divided granitoids into pre-tectonic, syn- tectonic and post-tectonic groups, considering the granitoids to be emplaced before, during and after the Ross Orogeny. These groups were based primarily on the amount of granitoid deformation, with the most gneissose comprising the pre- tectonic group, and the least deformed the post-tectonic group. McKelvey &Webb (1962) divided granitoids from the Wright Valley into the Wright Intrusives and Victoria Intrusives, and Lopatin (1972) sub-divided granitoids in a similar manner to that of Gunn & Warren. More recently, Findlay (1985, in press) has devised a subdivision scheme with sub-groups orphases occuring within some of his groups.

The granitoid groups of these previous workers were determinded primarily by lithology and inferred relative age. In most instances pluton boundaries were not mapped, and granitoids were given formational status, e.g. Larsen Granodiorite, Irizar Granite. Thus any granitoid which appeared lithologically similar to another many kilometres away was assigned to the same formation. However, the work of Allibone etal. (1991) has shown that lithologically similar granitoids were commonly emplaced at different times, and the textural and mineralogical variations within bodies are sufficient for different areas of individual plutons to correlate with many of the lithology-based groups of different workers.

SMl LLlE

Furthermore, granitoids with similar gross lithologies in some instances have contrasting chemistries.

These poorly-defined subdivision schemes are further complicated by their nomenclature. In many instances different workers have given the same granitoid different names, and different granitoids the Same name. For example, the Larsen Granodiorite of Gunn & Warren (1962) was correlated with the Dais Granite of McKelvey & Webb (1962) by Haskell et al. (1965), despite the marked petrographic differences apparent from Gunn & Warren’s (1962) and McKelvey & Webb’s (1962) descriptions. This lack of consistency among early workers resulted in later workers using different names from various nomenclature schemes for newly discovered granitoids.

Clearly, such lithology-based subdivision and nomenclature schemes do not work for granitoids in this region, and explain why some workers were unable to assign granitoids to a particular group (e.g. Findlay 1985, p. 13). For this reason, recent workers (e.g.Graham & Palmer 1987, Allibone et al. 1991) have avoided using previous subdivision and nomenclature schemes.

Skinner (1983), considered the subdivision scheme of Gunn & Warren (1962) invalid due to the multiple deformational history of south Victoria Land, and instead divides the granitoids with respect to particular deformation events. Skinner’s scheme, is based on the premise that foliation development within plutons records a discrete deformation phase. Such a scheme, however, may not be a sound basis for granitoid sub- division in this regioneither as foliationswithingranitoidscan also occur during pluton emplacement and from magmatic flow (Cox & Allibone 1991). In addition, inhomogenous regional deformation would result in plutons of similar ages exhibiting different foliations.

It is now generally accepted that different granitoids arebest subdivided by referring to geographically contiguous bodies as batholiths or plutons, and by reference to granitoid suites which have similar chemistry, mineralogy, and age (e.g. Chappelll978, Pitcher & Cobbing 1985, White & Chappell 1983, 1988). The suite subdivision suggested above for granitoids from the Taylor Valley and Ferrar Glacier regions is practical and meaningful, and can be easily applied to granitoids elsewhere in south Victoria Land with further mapping and geochemical analysis. It is acknowledged that this two-fold suite subdivision is based on a relatively small number of granitoids from a comparatively small region, and it is quite possible that further suites may exist elsewhere in south Victoria Land.

The two suites described here are hereafter informally referred to as the Dry Valleys 1 suite (the Bonney, Catspaw, and Hedley plutons and leucocratic biotite granite dykes), and the Dry Valleys 2 suite (the mafk dykes, porphyry dykes, Rhone Pluton, and the younger biotite granite dykes).

Radiometric dating of granitoids in south Victoria Land by Faure & Jones (1974) indicated that granitoids with Dry Valleys 1 suite affinities have ages of approximately 500 Ma

GRANlTOlDS FROM SOUTH VICTORIA LAND 81

Table III. Previous classification and nomenclature schemes for granitoids from south Victoria Land.

Gunn & Warren (1962) McKelvey &Webb (1962) Looatin (1972’1 , I . , , . .

POST-TECTONIC Delta Diorite Irizar Granite Gauss Granodiorite Skelton Granodiorite

SYN-TECTONIC

Larsen Granodiorite

PRE-TECTONIC Granodiorite Gneiss

VICTORIA INTRUSIVES POST-TECTONIC Vanda Lamprophyre porphyrys & lamprophyre Vanda Porpyhry quartz diorite-monzonite Vida Granite amphiboletbiot. granite

WRIGHT INTRUSIVES Theseus Granodiorite Loke Microdiorite Dais Granite diorite-granite Olympus Granite Gneiss

SYN-TECTONIC late biot. granite

PRE-TECTONIC Olympus Granite Gneiss

Skinner (1983) Findlay (1985) Smillie (this paper)

POST F3 Vanda lamp, poryh Delta Diorite Murray Granite Crags Granite Vida Granite

LATE SYN F2-F3 Skelton Granodiorite Theseus Granodiorite Larsen Granodiorite

VICTORIA INTRUSIVE GROUP Vanda lamp, poryh Vida Granite Grey Biotite Granite Irizar Granite ?

LARSEN INTRUSIVE GROUP Catacomb Member Briggs Hill Granodiorite Dais Granite Olympus Granite Gneiss

DRY VALLEYS 2 SUITE “Young” biotite granite dykes Rhone Pluton Porphyry dykes Micromonzonite dykes

DRY VALLEYS 1 SUITE Biotite granite dykes Hedley Pluton Catspaw Pluton Bonney Pluton

SYN-F2 KUKRI HILLS GROUP Renegar Mafic Gneisses Chancellor Orthogneiss ROCKS NOT INCLUDED Chancellor Orthogneiss Portal Augen Gneiss IN THIS STUDY POST Fl-PRE-F2 (see Cox & Allibone 1991) Portal Augen Gneiss Orthogneisses

Plutons within the suites defined by Smillie (this paper) do not correlate directly with the “intrusive Groups” defined by earlier workers.

and those with Dry Valleys 2 affinities, 480 Ma. However, the significance of these data is uncertain, as many of the samples studied by Faure & Jones are from rocks in moraine deposits (Findlay et al. 1984). Preliminary work of Graham & Palmer (1987) suggests that granitoids similar to those of the Dry Valleys 2 suite may be at least 12 m.y. younger than the Dry Valleys 1 suite.

Granitoid petrogenesis

Evolution of magma

The evolution of granitoid magmas remains a matter of some controversy. Different hypotheses include magma mixing and mingling (Wall etal.1987 and references therein), restite unmixing (White & Chappelll977, Chappell et al. 1987) and fractional crystallization (Sparks et al. 1984, Wall etal. 1987).

All these ideas have been invoked to explain linear trends on Harker Variation diagrams, and indeed there is evidence to suggest that all of these processes may be involved to different degrees in the evolution of different granitoid magmas.

Magma mingling and mixing have occurred to some degree in both suites. Magma mingling occurs when one magma is effectively quenched by another and is likely when the viscosity contrasts are large, whereas magma mixing involves the physical interaction of magmas to form a hybrid when the viscosity contrast betweenend members is low (Huppert et al. 1984). The diffuse, internal contact between quartz- monzodiorite and granite within the Bonney Pluton is an example of magma mingling (see Fig. 4), where immiscible blobs of each granitoid occur within the other where they meet. Some of these blobs have been distorted by magmatic flow to form schlieren. The occurrence of schlieren in hybrid zones between magmas of contrasting compositions has been

ROBERT W. SMlLLlE 82

reported elsewhere (e.g. Frost & Mahood 1987, Ayrton 1988). Magma mixing has occurred between enclaves and host

granitoid of the Catspaw Pluton and the Rhone Pluton. Large K-feldspars within the enclaves of the Rhone Pluton are identical in size and composition to the host, but are rounded and have partially resorbed and altered rims. The K-feldspars within the enclaves are interpreted to be xenocrysts, resulting from bilaterial mechanical interchange of enclave magma with host magma (cf. Cantagrel et al. 1984).

While magma mingling and mixing have occurred in both suites, it is unlikely that these are the dominant processes in their chemical evolution. Modelling by Frost & Mahood (1987) indicated that mixing of basalt and granite minimum melts is not capable of yielding large volumes of magma with compositions as silicic as dacite or rhyodacite. The maximum predicted silica content of their hybridmagma under reasonable crustal conditions does not exceed 63%. At least half of the samples of each of the suites exceed this, reaching values up to 73% SO,. Further evidence against a magma-mixing origin is shown in an Al,O, versus SiO, variation diagram (Fig. lo), where the mafic enclaves of the Bonney Pluton lie off the trend defined by their host pluton. If enclaves are interpreted to be globules of cognate mafic magma which have escaped the hybridization process of magma mixing (e.g Frost & Mahood 1987) they should lie at the SO,-poor end of a linear trend on variation diagrams.

Geochemical evidence tends to indicate that each suite evolved by fractional crystallization from a basic source. In Fig. 11, the covariance of Rb v Sr is shown for both suites, along with vectors indicating the effects on melt composition of crystallizing single mineral phases assuming Rayleigh fractionation (cf. Tindle & Pearce 1981). Both suites display separate, roughly parallel trends, the less evolved samples of the Bonney Pluton, Rhone Pluton and porphyry dykes having low Sr:Rb ratios. This trend of decreasing Sr:Rb with increasing S O , content is considered to result from fractional crystallization (cf. Arth 1976, Tindle & Pearce 1981). The similarity of the trends defined by the two suites and the plagioclasevector demonstrates the importance of plagioclase fractionation for both suites. The Catspaw Pluton lies well below the most evolved Bonney Pluton samples suggesting it may well result from fractionation of Bonney Pluton-type magma. The Hedley Pluton has similar Rb:Sr values to the Bonney Pluton and therefore cannot be derived from direct fractional crystallization of Bonney Pluton-type magma. The leucocratic biotite granite dykes, however, do appear to have evolved by this process from Hedley Pluton-type magma. A plot of CaO against Y (Fig. 12) shows a broad positive correlation of CaO with Y for each suite considered to result from fractionation of hornblende and clinopyroxene (cf. Atherton & Sanderson 1989, bothofwhichare phases present in the less evolved members of each suite.

Findlay (1985) has suggested that granitoids from his younger Victoria Intrusive Group may be differentiates of those from his older Larsen Intrusive Group (broadly similar

to the Dry Valleys 2 and 1 suites respectively). However, because each suite forms a separate trend on the Rb:Sr diagram they cannot have fractionated from the same parent magma and are thereforeunrelated. This is in accordance with the suite definition, whereby each suite is inferred to have a distinct source.

Intrusive styles and pluton emplacement

There is a strong contrast in internal structure and intrusive style between the Bonney Pluton of the Dry Valleys 1 (DV1) suite and plutons of the Dry Valleys 2 (DV2) suite. The preferred orientation of undeformed euhedral minerals, enclaves, xenoliths, and the presence of schlieren layering within the Bonney Pluton all indicate magma flow during pluton emplacement (cf. Paterson et al. 1989). The increasing intensity of development of foliations toward the external margin of the pluton is also an indicator of magmatic flow. However, Cox (1989) has demonstrated foliation at the extreme margin of the pluton to be a result of solid-state flow. Paterson etal. (1989) have suggested that foliations developed by solid-state processes at themargins ofplutonsmay overprint an earlier magmatic foliation and are likely to have resulted from expansion or ballooning of plutons during final emplacement.

The fluidal orientation of minerals and xenoliths within the Bonney Pluton is a typical feature of concordant plutons (cf. Castro 1987). Other features of concordant plutons which the Bonney Pluton exhibits include an ovoid-shaped geometry in horizontal section and a planar foliation parallel to contacts. The regional NW structural trend of the Koettlitz Group, parallel to the major axis of the pluton, and the development of deformation structures within the Koettlitz Group, determinded by Cox (1989) to have resulted from pluton emplacement, are are also features which define concordant plutons. Classically, concordant plutons have been interpreted as diapirs or ballooning diapirs (Bateman 1984), and many are interpreted as synkinematic with a regional deformation phase (Castro 1987).

The Hedley and Catspaw plutons also have characteristics typical of concordant plutons, including ovoid-shaped geometries and major axes parallel to trend of the host rocks. The similar N W trend of the major-axes of these plutons to that of the Bonney Pluton indicated by the mapping of Allibone et al. (1991) suggests that the intrusion and emplacement of the DV1 suite was subjected to major structural control.

In complete contrast to the Bonney Pluton, plutons of the DV2 suite are generally lacking in internal structures, have irregular shapes in horizontal sections, and have sinuous contacts that are commonly interpenetrated with the host rock. DV2 plutons also cross-cut the host-rock foliation and have not disturbed the host during emplacement. These features are typical of discordant plutons (Castro 1987) which, unlike concordant plutons, are passively emplaced into the host rock. Discordant plutons are considered to be late-kinematic

GRANITOIDS FROM SOUTH VICTORIA LAND 83

2 0

1 9

1 8

A1203 1 7

1 6

1 5

1 4 4 5 5 0 5 5 6 0 6 5 7 0 7 5

Si02

Fig. 10. Al,O, v. SiO, variation diagram of the Bonney Pluton (closed circles) and its mafic enclaves (bars). The enclaves lie off the trend defined by their host pluton, suggesting that magma-mixing was not the dominant evolutionary process for the pluton.

intrusions emplaced in the upper part of the crust, where magma ascends via narrow fractures and dykes which progressively widen into an irregularly-fractured upper reservoir (Castro 1987). The abundant dyke swarms associated with this suite, many with compositions similar to other DV2 intrusives, indicates that plutons of the DV2 suite may have been emplaced by such a mechanism. Chilling of many DV2 pluton and dyke margins may also indicate intrusion at higher levels in the crust compared with plutons of the DV1 suite.

Relationship to the Koettlitz Group

Previous workers have suggested an anatectic metasedimentary origin for some of the granitoids of south Victoria Land (Smithson et al. 1972, Skinner 1983), based primarily on the observations of migmatites within the Koettlitz Group. However, theoretical considerations by Wickham (1987) have demonstrated that migmatites observed by him result from small-scale partial melting processes that occur at relatively high structural levels in the crust, and can seldom be genetically linked to large granitoid plutons. Allibone (1988) and Allibone & Norris (in press) have shown that migmatites within the Koettlitz Group were produced by partial melting only locally at low melt fractions. Consequently, only volumetrically minor microplutons, up to 50 m across, were able to segregate from their residue.

Large-degree partial melting at greater depths of the Koettlitz Group would most likely produce a spread ofmelt compositions from end member S - and I-type granitoids, as it comprises a mixture of marbles, quartzo-feldspathic, psammitic, pelitic and semi-pelitic schists, amphibolites and calc-silicate rocks (Haskell etal. 1965). However, the strong I-type characteristics of the DV1 and DV2 suites indicates derivation from fairly homogenous igneous sources which is incompatible with a Koettlitz Group origin.

1000

Sr

t o o 1 0

0 ="x

A

I

1 0 0

Rb 1 0 0 0

Fig. 11. Rb v. Sr diagram of the Dry Valleys 1 suite (left trend) and Dry Valleys 2 suite (right trend). Vectors indicate the theoretical effects on melt composition of crystallising single mineral phases assuming Rayleigh fractionation (after Tindle & Pearce 1981).

6 7 1

5

c a o 4

3

2

1

=. hJ X x x

X 0

0

0

" 1 0 20 30 40 50 60 70 0

Y

Fig. 12. CaO v. Y diagram showing the broad positive correlation of CaO with Y for the Dry Valleys 1 and Dry Valleys 2 suites, indicating a fractional crystallization origin.

Tectonic setting and source

Dry Valleys 1 suite

Studies of late Palaeozoic and younger granitoid provinces have resulted in the development of schemes relating the chemical composition of granitoids to tectonic setting (e.g. Pitcher 1987, Pearce et al. 1984). On the Rb vs. YtNb discrimination diagram of Pearce et al. (1984), the DV1 suite lies within the field occupied by granitoids formed in volcanic arcs (Fig. 13). Analyses of enclaves from the Bonney Pluton plot well within the field occupied by calc-alkaline basalts on the discrimination diagram of Pearce & Cann (1973) (Fig. 14), rocks also considered to be produced at volcanic arcs. A

a4 ROBERT W. SMlLLlE

I o o o F syn-COLG /

.in t VAG V I ORG

I 1

1 1 0 1 0 0 1000

Y+Nb

Fig. 13. Rb v. Y+Nb tectonic discrimination plot (after Pearce et al. 1984) of the Dry Valleys 1 (closed symbols and crosses) and Dry Valleys 2 suite (open symbols). ORG= ocean ridge granitoids, WPG=within plate granitoids, syn-COLG=collisional granitoids, VAG=volcanic arc granitoids.

TI/ 100

/ \ Zr 3 Y

Fig. 14. Ti-Y-Zr plot of Bonney Pluton enclaves (a =within plate basalts, b =low K tholeiites, c =ocean floor basalts, d =calc-alkaline basalts (after Pearce & Cann 1973).

volcanic arc tectonic setting for this suite is also suggested by the calc-alkaline, I-type chemistry, typical of Cordilleran granitoidsformed incontinental arcs(e.g. Athertonerd. 1979, Pitcher 1983, 1987). The fractional crystallization origin inferred for this suite is also consistent with a continental arc tectonic setting, as this process is considered the main mechanism for the theevolution ofmagmaformed incontinental arcs (e.g Atherton & Sanderson 1985, Frost & Mahood 1987, Pitcher 1987).

The batholithic size of the Bonney Pluton is a another characteristic feature of continental arc granitoids, as the production of such a large body of magma reflects the very high energetics associated with subduction zones (cf. Huppert &Sparks 1988). Inaddition,thesimilarNWtrendofthemajor axes of the plutons of this suite, indicating a major structural control on their intrusion, may have resulted from up-welling of magma through deep-seated lithospheric lineaments which are postulated to exist above subduction zones (Pitcher 1987).

It is therefore suggested that the granitoids of the DV1 suite were derived from partial melting of upper mantle and/or deep continental I-type crust above an ancient continental arc in Palaeozoic times. Granitic rocks of similar age and composition from North Victoria Land and the central Transantarctic Mountains are also considered to have formed in a Cordilleran- type margin setting, above an active W-dipping subduction zone during Cambro-Ordovician times (Borg et al. 1987, 1990, Vetter & Tessensohn 1987).

Dry Valleys 2 suite

The different geochemical signature of this suite compared to that of the DV1 suite implies that it is derived from partial melting of a different source. Radiometric dating by Graham & Palmer (1987) suggests that the DV2 suite may at least 12 Ma years younger than the DV1 suite. It is possible therefore that its different chemistry and implied different source may reflect a change in tectonic environment.

In the Rb v Y+Nb discrimination diagram (Fig. 13) the DV2 suite straddles the volcanic arc and syn-collisional granitoid fields. This is typical of granitoids which post-date major tectonic events (Pearce er al. 1984), such as the Caledonian granitoids as defined by Pitcher (1983). The discordant nature of the plutons of the DV2 suite is characteristic of Caledonian granitoids, and their alkali-calcic compositions, low CaO contents, and high YO, Rb, U, Pb and Th contents are also typical of this granitoid type (cf. Pitcher 1983, 1987). Caledonian granitoids occur as later intrusions in the tectonomagmaticsequence in orogenicbelts, and may therefore be considered post-tectonic, although Paterson er al. (1989) pointed out that such a term is best used when referring to a particular stage of plutonemplacement. CaIedoniangranitoids areconsidered to beunrelated to subduction processes, forming in extensional tectonic settings. The abundant dyke swarms of the DV2 suite, many similar in composition to the plutons of this suite, indicate emplacement in such a setting.

The high Cr and Ni contents of the micromonzonite dykes, which predate the intrusion of the granitoids of the DV2 suite (Table 11), indicate a mantle origin for the dykes. These dykes may represent the mantle material which promoted the large- scale melting of I-type lower crust resulting in the formation of the DV2 suite. Compared to the DV1 suite, the DV2 suite has fewer I-type characteristics, possibly a result of a higher crustal component arising from a relatively shallower emplacement level in the crust. Strontium isotope analysis of

85 GRANlTOlDS FROM SOUTH VICTORIA LAND

granitoids, 60 km north of the study area at Granite Harbour, with similar compositions and field relations to the DV2 suite, yielded initial 87SrP6Sr ratios of w.70890 (Graham & Palmer 1987). These values are higher than those of moderately crustal-contaminated Cordilleran I-type granitoids from California (Ague & Brimhall 1988) and indicates that the DV2 suite may contain a high crustal component. Variable mixtures of mantle and crustal- derived magmas have been invoked to explain the variations of late-kinematic granitoids elsewhere (e.g. Fourcade & Allegre 1981, Oliver et al. 1983) and it is suggested here that the granitoids of the DV2 suite may have formed from such a magma.

Conclusions

Older and younger groups of early Palaeozoic granitoids have been known to exist in south Victoria Land since the turn of the century. A major problem that has emerged over the last three decades, and one which this paper addresses, is how best to subdivide these groups of granitoids in a manner which is simple yet sufficiently informative, readily applicable, easily modified, and (hopefully) geologically meaningful.

The two-fold suite subdivision of granitoids from the Taylor Valley and Ferrar Glacier region presented serves to illustrate a method which future workers can adopt, i.e. systematic pluton mapping combined with petrography and geochemical analysis. Geochemistry in this study serves only to support what can be seen in the field: a broad, two-fold suite subdivision. The fact that the geochemistry reinforces this field-based subdivision illustrates the usefulness of such an approach.

Both the suites identified are metaluminous I-type granitoids that have evolved by fractional crystallization of basicmagma. The older DV1 suite is dominated by the compositionally variable Bonney Pluton, a concordant calc-alkaline intrusion of batholithic proportions. The plutons of this suite have chemistries typical of Cordilleran I-type granitoids formed at continental margins, and are inferred to be the product of partial melting of the upper mantle and/or lower crust above a subduction zone at the margin of the East Antarctic craton. The youngerDV2 suite comprises numerous dyke swarms and alkali-calcic discordant plutons which have many similarities with Caledonian I-type granitoids. Such granitoids are considered to post-date the subduction process, and typically occur as later intrusions in the tectono-magmatic sequence in orogenic belts in extensional tectonic settings. Sources for this suite may include mixtures of variable amounts of mantle and crustally-derived magma.

These conclusions are drawn from granitoids from a comparatively small region in south Victoria Land. Further geological mapping and geochemical analysis of plutons whose positions are well-constrained within the intrusive sequence, together with radiometric dating and isotopic analysis, are now needed to obtain a more comprehensive understanding of the plutonic and tectonic history of this region.

Acknowledgements

This paper is the result of M.Sc. research carried out at the University of Otago, New Zealand. I am grateful to the Univeristy of Otago Research Committee who provided funds for this research, and to the staff of Antarctic Division (DSIR) in Christchurch and at Scott Base who provided logistic support. Max Wendon and Adrian Daly are thanked for their untiring assistance in the field. Helicopter support was provided by VXE-6 squadron, U.S. Navy. My thanks go to Dave Craw and Alan Cooper who supervised this study, and to Andrew Allibone, Simon Cox and Hyram Ballard who provided constructive criticism of the manuscript. Discussions with Ian Turnbull (New Zealand Geological Survey) and Tony Reay helped to clarify some of the ideas in this paper. I also thank Steve Weaver (Canterbury University) and Ed Stump for their reviews of the final manuscript. The technical assistance of Roy Johnstone, John Pillidge and Don Weston is gratefully acknowledged.

References

ADAMS, C.J.D., GAEITES, J.E. & GRINDLEY, G.W. 1982. Orogenic history of the centralTransantarctic Mountains: new K-Ax age data on the Precambrian- Lower Paleozoic basement. In CRADDOCK, C., ed. Antarctic geoscience. Madison: University of Wisconsin Press, 817-826.

AGUE, J. J. & BRIMHALL, G. H. 1988. Regional variations in bulk chemistry, mineralogy and the compositions of mafic and accessory minerals in the batholiths of California. Geological Society of America Bulletin, 100, 891-911.

ALLEN, A.D. & GIBSON, G. W. 1962. Geological investigations in southern Victoria Land, Antarctica. Part 6: Outline of the geology of the Victoria Valley region. New Zealand Journal of Geology and Geophysics, 5,234- 242.

ALLIBONE, A. H. 1988. Koettlitz Group meta-sediments and intercalated orthogneisses from the mid Taylor Valley and Ferrar Glacier regions. M.Sc thesis, University of Otago, 233pp. [Unpublished.]

ALLIBONE, A. H., FORSYTH, P. J., SEWELL, R. J., TURNBULL, I. M. & BRADSHAW, M.A. 1991. Geology of the Thundergut area, southern Victoria Land Antarctica. 1: 50.000 Miscellaneous Series Map 21, Geology and Geophysics Division. Wellington: New Zealand Department of Scientific and Industrial Research [With supplementary text]

ALLIBONE A. H. & NORRIS, R.J. In press. Segregation of leucogranite microplutons during syn-anatectic deformation: an example from the Taylor Valley, Antarctica. Journal of metamorphic geology.

ANGINO, E. E., TURNER, M. D. & ZELLER, E. J. 1962. Reconnaissance geology of the lower Taylor Valley, Victoria Land, Antarctica. Geological Society ofAmerica Bulletin, 73, 1553-1562.

ARMIENTI, P., GHEZZO, C., INNOCENTI, F., MANEIT, P., ROCCHI, S. & TONARINI, S. 1990. Palaeozoic and Cainozoic intrusives of Wilson Terrane: geochemical and isotopic data. Memoriedelfa Societa Geologicaltaliana,

ARTH, J. G. 1976. Behaviour of trace elements during magmatic processes: a summary of theoretical models and their applications. Journal of Research of the US. Geological Survey, 4,41-47.

ATHERTON, M. P. & Sanderson, L. M. 1985. The chemical variation and evolution of the super-units of the segmented Coastal Batholith. In PITCHER, W. S., ATHERTON, M. P., COBBING, E. J. & BECKINSALE, R. D., eds. Magmatism at a plate edge: The Peruvian Andes. Glasgow: Blackie & Son Ltd., 208-227.

53,67-76.

ROBERT W. SMlLLlE 86

ATHERTON, M.P., MCCOURT, W.J., SANDERSON, L.M. & TAYLOR, W.P. 1979: The geochemical character of the segmented Peruvian Coastal Batholith and associated volcanics. In ATHERTON, M.P. & TARNEY, J., eds. Origin of granite batholiths: geochemical evidence. Orpington: Shiva, 45-64.

AYRTON, S. 1988. The zonation of granitic plutons: the failed ring-dyke hypothesis. Schweizerische mineralogische und petrographische Mitteilungen, 68,l-19.

BATEMAN, P.C., CLARKE, L.D., HUEER, N.K, MOORE, J.G. & RINUIART, C.D. 1963. The Sierra Nevada Batholith - a synthesis of recent work across the central part. Professional Paper US. Geological Survey, NO. 414-D, 1- 46.

BATEMAN, R. 1984. On the role of diaprism in the segregation, ascent, and final emplacement of granitoid magmas. Tectonophysics, 110,211-231.

BLANK, H.R., COOPER, R.A., WHEELER, R.H. & WIUIS, I.A.G. 1963. Geology of the Koettlitz-Blue Glacier region, southern Victoria Land. Transactions of the Royal Society ofNew Zealand, 2,79-100.

BORG, S.G., DEPAOLO, D.J. & Smm, B.M. 1990. Isotopic structure and tectonics of the central Transantarctic Mountains. Journal of Geophysical Research, 95, 6647-6667.

BORG, S.G., STUMP, E., CHAPPELL, B.W., MCCULLOCH, M.T., WYBORN, D., ARMSTRONG, R.L. & HOLLOWAY, J.R. 1987. Granitoids of northern Victoria Land, Antarctica: implications of chemical and isotopic variations to regional crustal structure and tectonics. American Journal of Science,

CANTAGREL, J., DIDIER, J. & GOURGAUD, A. 1984. Magma mixing: origin of intermediate rocks and enclaves from volcanism to plutonism. Physics of the Earth and Planetary Interiors, 35, 63-76.

CASTRO, A. 1987. On granitoid emplacement and related structures. A review. Geologische Rundschau, 76, 101-124.

CWPELL, B.W. 1978. Granitoids from the Moonbi district, New England Batholith, Eastern Australia. Journal of the Geological Society ofAustralia,

CHAPPELL, B.W., WHITE, A.J.R. & WYBORN, D. 1987. The importance of residual source material (restite) in granite petrogenesis. Journal of Petrology, 28, 1111-1138.

Cox, S.C. 1989. Gneiss geology: a structural perspective of foliated granitoids and their host rocks in the Wright Valley, south Victoria Land, Antarctica. M.Sc thesis, University of Otago, 185pp. [Unpublished.]

Cox, S.C. & ALLIBONE, A.H. 1991. Petrogenesis of granitoid orthogneisses from the Dry Valleys region, south Victoria Land, Antarctica. Antarctic Science, 3,405-417.

DAHL, P.S. & PALMER, D.F. 1983. The petrology and origin of orbicular tonalite from western Taylor Valley, Southern Victoria Land, Antarctica. In OLIVER, R.L., JAMES, P.R. & JAGO, J.B., eds. Antarctic earth science. Canberra: Australian Academy of Science & Cambridge: Cambridge University Press, 156-159.

FAURE, G. & JONES, L.M. 1974. Isotopic composition of Sr and geologic history of the basement rocks of Wright Valley, southern Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics, 17, 611- 627.

FERRAR, H.T. 1907. Report on the field geology of the region explored during the Discovery Antarctic expedition. NationalAntarcticExpedition I901 - 1904. Natural History Reports, 1, 1-100.

FIKKAN, P.R. 1968. Granite rocks in the Dry Valley trgion of southern Victoria Land, Antarctica. M.Sc. thesis, University of Wyoming, 119pp.[Unpublished.]

FINDLAY, R.H. 1983. A re-interpretation of the basement granites, McMurdo Sound, Antarctica. [Abstract.] In OLIVER, R.L., JAMES, P.R. & JAGO, J.B., eds. Antarctic earth science. Canberra: Australian Academy of Science & Cambridge: Cambridge University Press, 164.

FINDLAY, R. H. 1985. The Granite Harbour Intrusive Complex in McMurdo Sound: progress and problems. New ZealandAntarcticRecord, 6 (3), 10- 22.

FINDLAY, R.H. 1990. Is the Larsen Granodiorite of the Transantarctic Mountains Vendian? A discussion. New Zealand Antarctic Record, 10

287, 127-169.

25, 267-283.

(l), 49-55.

FINDLAY, R. H. In press. Antarctica. In, NAIRN, A.E., ed. The Palaeozoic of the World. Amsterdam: Elsevier Publishing Co.

FINDLAY, R.H., SKINNER, D.N.B. & CRAW, D. 1984. Lithostratigraphy and structure of the Koettlitz Group, McMurdo Sound, Antarctica. New Zealand Journal of Geology and Geophysics, 27,513-536

FOURCADE, S. & ALLEGRE, C.J. 1981. Trace element behaviour in granite genesis: a case study. The calc-alkaline plutonic association from the Quierigut Complex (Pyrenees, France). Contributions to Mineralogy and Petrology, 16, 177-195.

FROST, T.P. & MAHOOD, G.A. 1987. Field, chemical and physical constraints on mafic-felsic magma interaction in the Lamarck Granodiorite, Sierra Nevada, California. GeologicalSociety ofAmerica Bulletin, 99,272-291.

GHENT, E.D.1970. Chemistry and mineralogy of the Mount Falconer pluton and associatedrocks. Lower Taylor Valley, south Victoria Land, Antarctica. Transactions of the Royal Society ofNew Zealand, 8, 117-132.

GHENT, E.D. & HENDERSON, R.A. 1968. Geology of the Mount Falconer Pluton, Lower Taylor Valley, southern Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics, 11,851-880.

GRAHAM, I.J. &PALMER, K. 1987. New precise Rb-Sr mineral and whole-rock dates for I-type granitoids from Granite Harbour, southernVictoria Land, Antarctica. New Zealand Antarctic Record, 8 (I), 72-80.

GRINDLEY, G.W. & WARREN, G. 1964. Stratigraphic nomenclature and correlation in the western part of the Ross Sea. h h I E , R.J., ed.Antarctic Geology. Amsterdam: North Holland Publishing Co., 314-333.

GUNN, B.M. & WARREN, G. 1962. Geology of Victoria Land between the Mawson and Mulock Glaciers, Antarctica. New Zealand Geological Survey Bulletin, 71, 157pp.

HASKELL, T.R., KENNETT, J.P., PEBBLE, W.M., SMITH, G. & WILLTS, I.A.G. 1965. The geology of the middle and lower Taylor Valley of southern Victoria Land, Antarctica. Transactions of the Royal Society of New Zealand, 12,169-186.

HINE,R.H., WILLIAMS,I.S.,CHAPPELL,B.W.&WHITE,A.J.R. 1978. Geochemical contrasts between I and S-type granitoids of the Kosciosko Batholith. Geological Society ofAustralia Journal, 25,219-234.

HUPPERT, H.E. & SPARKS, S.J. 1988. The generation of granitic magmas by injection of basalt into the continental crust. Journal of Petrology, 29,

HUBBERT, H.E., SPARKS, S.J. &TURNER, 1984. Some effects of viscosity on the dynamics of replenished magma chambers. Journal of geophysical research, 89B: 6857-6877.

LE MTRE, R.W. ed. 1989. A classification of igneous rocks and glossary of terms: recommendations of thelnternational Union of GeologicalSciences Subcommission on the systematics of igneous rocks. Trowbridge: Blackwell Scientific Publications, 192pp.

LOPATIN, B. G. 1972: Basement Complex of the McMurdo oasis, southern Victoria Land. In ADIE, R.J., ed. Antarctic geology and geophysics. Olso: Universitetsforlaget, 287-292.

MCKELVEY, B.C. &WEBB, P.N. 1962. Geological investigations in Southern Victoria Land, Antarctica Part 3: Geology of Wright Valley. New Zealand Journal of Geology and Geophysics, 5,143-162.

MAWSON, D. 1916. Petrology of rock collections from the mainland of south Victoria Land. In British Antarctic Expedition 1907-9 Reports on the Scientific Investigations, Geology, 2,201-234.

MURPHY, D.J. 1971. Thepetrology anddeformational historyoftheBasement Complex, Wright Valley,Antarctica. Ph.D thesis, University of Wyoming, 114 pp. [Unpublished.]

NORRISH, K. & CWPELL, B.W. 1967. X-ray fluorescence spectrography. In ZUSSMAN, J., ed. Physical methods in determinative mineralogy. London: Academic Press, 514pp.

NORRISH, K. & HUTTON, T.J. 1969. An accurate X-ray spectrograph method for the analysis of a wide range of geological samples. Geochemica et Cosmochimica Acta, 33,431-453.

OLIVER, R.A., Vlrroz, P., DE BOISSET, T., VIVIER, G. & KERR, S.A. 1983. Bimodal magmatism in the Grandes French Massif, western French Alps. Journal of the Open Universi& Geological Sociefy, 4,228-233.

599-624.

GRANlTOlDS FROM SOUTH VICTORIA LAND 07

PAGEL, M. & LETERRIER, J. 1980. The subalkaline potassic magmatism of the Ballons massif (Southern Vosges, France): shoshonitic affinity. Lithos,

PALMER, K. 1987. XRF analyses of granitoids and associated rocks from southern Victoria Land, Antarctica. Research School of Earth Sciences Geology Board of Studies Publication No. 3, Victoria University of Wellington, 26pp. [Unpublished.]

PATERSON, S.R., VERNON R.H. & TOBISCH, O.T. 1989. A review of the criteria for the identification of magmatic and tectonic foliations in granitoids. Journal of Structural GeoIogy, 11,349-363.

PEARCE, J.A. & Cann, R.J. 1973. Tectonic setting of basic volcanic rocks determined using trace element analysis. Earth and Planetary Science Letters, 19, 290-300.

PEARCE, J.A., HARRIS, N.B.W. & TINDLE, A.G. 1984. Trace element discrimination diagrams for the interpretation of granitic rocks. Journal of Petrology, 25, 956-983.

PITCHER, W.S. 1979. The nature, ascent and emplacement of granitic magmas. Journal of the Geological Society of London, 136,627-662.

PITCHER, W.S. 1983. Granite type and tectonic environment. In Hsu, K. ed. Mountain buildingprocesses. San Diego: Academic Press, 19-40.

PITCHER, W.S. 1984. Phanerozoic plutonism in the Peruvian Andes. In HARMON, R.S. & BARREIRO, B.A., eds. Andean magmatism: chemical and isotopic constraints. Nantwich: Shiva, 152-167.

PITCHER, W.S. 1987. Granites and yet more granites forty years on. Geologische Rundshau, 76,51-79.

PITCHER, W.S. & COBBING, E.J. 1985. Phanerozoicplutonism in the Peruvian Andes. In PITCHER, W.S., ATHERTON, M.P., COBBING, E.J. & BECKINSALE, R.D., eds. Magmatism at a plate edge: the Peruvian Andes, New York John Wiley and Sons, 19-25.

PRIESTLEY, R.E. 1909. Scientific Results of the Western Journey. In SHACKELTON, E.H. The heart of the Antarctic. Appendix 111, London: Heinemann, 315-333.

SKINNER, D.N.B. 1983. The granites and two orogenies of Southern Victoria Land, Antarctica. In OLIVER, R.L., JAMES, P.R. & JAGO, J.B.,eds. Antarctic earth science. Canberra: Australian Academy of Science & Cambridge: Cambridge University Press, 160-163.

13,l-10.

SMnm, R.W. 1989. Granite Harbour Intrusivesfrom the Taylor Valley and Ferrar Glacier region, southern Victoria Land, Antarctica. M.Sc thesis, University of Otago, 247 pp. [Unpublished.]

SMITH, W.C. 1924.Theplutonicand hypabyssalrocksofsouthVictoria Land. British Antarctic (“Terra Nova”) Expedition 1910. Natural History Report Geology, 1,167-227.

SMITHSON, S.B., FIKKAN, P.R., MURPHY, D.J. & HOUSTON, R.S.C. 1972. Development of augen gneiss in the ice-free valley area, southern Victoria Land, Antarctica. In ADIE, R.J., ed. Antarctic geology and geophysics. Olso: Universitetsforlaget, 293-298.

SPARKS, R.S.J., HUPPERT, H.E. & TURNER, J.S. 1984. The fluid dynamics of evolving magma chambers. Philosophical Transactions of the Royal Society of London, A310,511-534.

TINDLE, A.G. & PEARCE, J. A. 1981. Petrogenetic modelling of in situ fractional crystallization in the zoned Loch Doon Pluton, Scotland. Contributions to Mineralogy and Petrology, 78, 196-207.

VERNON, R.H. 1983. Restite, xenoliths, and microgranitoid enclaves in granites. Journal and Proceedings of the Royal Society of New South Wales, 116, 77-103.

VERNON, R.H. 1984. Microgranitoid enclaves in granites - globules of hybrid magma quenched in a plutonic environment. Nature, 309,438-439

VETTER, U. & TESSENSOHN, F. 1987. S- and I-type granitoids from Northern Victoria Land, Antarctica, and their inferred tectonic setting. Geologische Rundshau, 76,233-243.

WALL, V.J., CLEMENS, J.D. & CLARKE, D. B. 1987. Models for granitoid evolution and source compositions. Journal of Geology, 95,731-749.

WHITE, A.J.R. & CHAPPELL, B.W. 1977. Ultrametamorphism and granitoid genesis. Tectonophysics, 43, 7-22.

WHITE, A.J.R. & CHAPPELL, B.W. 1983. Granitoid types and their distribution in the Lachlan Fold Belt, southeastern Australia. In RODDICK J.A. ed. Circum Pacific plutonic terranes. Geological Society ofAmerica Memoir

WICKHAM, S. M. 1987. The segregation and emplacement of granite magmas. Journal of the Geological Society of London, 144,281-298.

WILSON, A. D. 1955. A new method for the determination of ferrous iron in rocks and minerals. Bulletin of the Geological Survey of Great Britain, 9,

NO. 159,21-34.

56-58.