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Journal of the Geological Society, London, Vol. 145, 1988, pp. 999-1007, Printed in Northern Ireland

Middle Jurassic within-plate granites in West and their bearing on the break-up of Gondwanaland

B. C. STOREY,'M. J. HOLE,' R. J. PANKHURST,'I. L. MILLAR' & W. VENNUM2 British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 OET, UK 2Department of Geology, Sonoma State University, Rohnert Park, California 94928, USA

Abstract: Five post-tectonic granitic plutons isolated within the central Ellsworth-Whitmore moun- tains crustal block in form a distinctive geochemical suite. All have some characteris- tics of S-type granites and are atypical of active continental margins. They rangein composition from a within-plate granite (WPG) end member, with the lowest "Sr/'"Sr initial ratio (0.707), to granites with a much more marked crustal signature and high initial ratios (0.722). The granitic suite was emplaced over a restricted Middle Jurassic time interval at the same time as the extensive Ferrar- Karoo-Tasman mafic suite and just prior to the disintegration of the supercontinent Gondwanaland. Petrogeneticmodelling suggests that the WPG end member couldhave been derived entirely by differentiation of the enriched mantle-derived Ferrar magma, and the end member with the highest initial ratio by partialmelting of a crustalsource. Low initial '43Nd/144Nd ratiosand Proterozoic model ages are compatible with a Precambrian crustal component but may alternatively, as in the case of Ferrar Supergroup magmas, reflect partial inheritance from enriched lithospheric mantle geochemi- callycoupled to the lower crust since Precambrian differentiation. Data from these granites are consistent with large-scale underplating of mafic magma and crustal melting in response to a thermal disturbance in the Gondwanaland lithosphere related in some way to break-up of the supercontinent.

Although granitic rocks from a large and important part of thesuite was emplacedjust prior tothe disintegration of thebedrock geology of WestAntarctica, they pre- Gondwanaland itis tectonically importantand mayhave dominantly crop out as a linearbelt along the Pacific margin some bearing on the causes of supercontinent break-up. We and are related toMesozoic subduction processes. However, consider thesource and evolution of thegranite magmas asuite of geographicallyisolated Middle Jurassic granites andapossible petrogenetic relationship tothe contem- (175 f 8 Ma) occurs within the centre of West Antarctica, poraneousFerrar mafic suite. The tectonicsetting of the up to 1500 km from the Pacific margin. These granites have granites is explored by use of adiscrimination diagram previouslybeen interpretedas allochthonous subduction- combined with Sr and Nd isotope data. related granites that moved to their present-day position as a result of the reorganization of the crustal blocks of West Antarctica during Gondwanaland break-up (Dalziel & Elliot Geological setting 1982).Recent palaeomagnetic data reveallimited a Thegranitic suite occurswithin theEllsworth-Whitmore movement of the crustal blocksof West Antarctica (Grunow mountains crustal block (EWM),which is the central one of et al. 1987) andsuggest thegranites may have been five crustal blocks making up the mosaic of West Antarctica emplaced well awayfrom an active margin. Moreover (Fig. 1). The blocks are separated by deep crustal rift zones geochemical and isotopic data from the combined 'British (Storey et al. 1988),and are regarded as having moved AntarcticSurvey-United States Antarctic Research Pro- during thebreak-up of Gondwanalandalthough there is gram West Antarctic tectonics project' (Dalziel & Pankhurst some doubt as to their precise pre-drift positions (Grunowet 1985) indicate that the granites are adistinctive geochemical al. 1987). TheEWM blockhas a deformed Cambrian to suite with some of thecharacteristics of S-typegranites Permian stratigraphy, comparable with the Cape fold belt of (Vennum & Storey1987a) and are atypical of active South Africa and partsof the Transantarctic Mountains. It is continentalmargins. These granites were emplaced at the geologically distinct fromthe adjoining Haag Nunataks sametime as theextensive Ferrar Supergroup mafic suite block, which consists of Precambrian crystalline basement, (179 f 7 Ma) within the Transantarctic Mountains (Kyle et andfrom theAntarctic Peninsula, Thurston Island and al. 1981) and just prior tothe disintegration of the Marie Byrd Land blocks, which for the most part represent supercontinent.The Ferrar tholeiites are noted for their the Mesozoic-Cenozoic proto-Pacific margin of Gondwana- anomalous enriched continental trace element and isotopic land. characteristics and are part of a larger magmatic province The granites are post-tectonic, intruded into the deformed that includes the Karoo and Tasman dolerites. These have sedimentary rocks (Storey & Dalziel 1987), and now form been related to a lithospheric melting event associated with the main exposure within the central partof the EWM block the break-up of Gondwanaland (Dalziel et al. 1987). (Fig. 1). In the Nash and , large granite plutons Thispaper summarizes both new and published are surrounded by thermally altered metasedimentary roof geochemical and isotopic datafrom the unusual suite of pendants,and in theMartin Hills smallleucocratic granites isolated within the centre of West Antarctica. As rhyodacitestocks intrude shallowmarine calcareous 999

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... ..

Fig. 1 (a). The crustal blocks of West Antarctica. AP, ; TI, Thurston Island; MBL, Marie Byrd Land;HN, Haag Nunataks; EWM, Ellsworth-Whitmore mountains; TAM, Trans- antarctic Mountains, WSE, embayment. The solid black areas are the main mountains. (b) The Middle Jurassic granites of the Ellsworth-Whitmore mountains crustal block. J, Jurassic; C, Cambrian; P, Permian.

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sedimentaryrocks of possibleCambrian age (Storey & Geochronology and isotope geology Macdonald 1987). The largest outcrop area, the Whitmore Previousradiometric dating of thesegranites has been Mountains,exhibits two principal granite types; the summarized by Millar & Pankhurst (1987), who presented coarse-grainedMount Seelig granite, which commonly Rb-Srwhole-rock isochron ages of 173 f 3, 175 f 8 and possesses an aligned mica fabric, is cut by the fine-grained 175 f 8 Ma, respectively, for the Pirrit Hills, and leucocratic Linck Nunatak granite. The remaining exposure PaganoNunatak plutons. They proposed that the EWM at Pagano Nunatak is entirely formed of a granitic pluton granites form a coherent and contemporaneous suite. Thisis with no preserved sedimentary envelope. extended by new data which give an age of 176 f 5 Ma for Although thegranitoids are not associated with mafic the LinckNunatak granite of theWhitmore Mountains intrusionsat thepresent erosion level, they are clearly (Pankhurst et al. 1989). Thesefour intrusions are relatedto distinctive low-amplitude,moderate wavelength collectively termed the EWM granitic suite. This does not E-W orientated magnetic anomalies (Garrett et al. 1988). include theMount Seelig granite phase of theWhitmore The only mafic igneous rocks exposed in this area comprise Mountains, which is distinct in terms of its petrography and small gabbro stocksand sills geochemicallysimilar tothe geochemistry,and for which preliminaryRb-Sr and U-Pb Ferrardolerites (Vennum & Storey19876). These occur zircon data suggest an age perhaps as old as Triassic. within deformed metasedimentary rocks at Hart Hills (Fig. The uniformity of initial R7Sr/mSrratios within each 1) and within the same magnetic anomaly as the granite at pluton is strongevidence that anyinternal differentiation Pagano Nunatak, suggesting thatthe granites may be wasby crystal-liquidprocesses such as fractional crystal- underlain by and related to a magnetite-rich mafic batholith. lization and did not involve variable crustal contamination effects. But between the plutons, initial *'Sr/% ratios are extremelyvariable (Table l), rangingfrom 0.707 forthe Petrography Pirrit Hills to 0.722 for the Linck Nunatak granite. These Thegranite outcrops are fairly uniformin appearance, ratios are all higher thanexpected for mantle-derived consistingpredominantly of medium- to coarse-grained magmas, and mostly higher than in subduction-related arc white or palepink leucocratic biotite-(muscovite) granite granites, which suggests a considerableamount of crustal (for details seeVennum & Storey1987a). Plagioclase contribution in theirpetrogenesis. The MiddleJurassic feldspar is An,,,. Some granites are porphyritic, with up to (179 f 7 Ma) basic magmasof the Ferrar Supergroupof East 40% of K-feldsparphenocrysts, which areoften strongly Antarcticashow acomparable range of elevated initial perthitic. Othersare fine-grainedaplogranites, and veins s7Sr/X6Srratios (0.7085-0.7153), in this case ascribed only in andsmall bodies of apliteare common. Alteration and part to crustalcontamination and thought to primarily xenoliths are rare although pegmatitic segregations contain- reflect anomalousmantle (e.g. Kyle 1980). These com- ing tourmaline, beryl and muscovite occur at thePirrit Hills. parisons are supported by Nd-isotope geochemistry of the In theWhitmore Mountains, the coarse-grained Mount EWM granitic suite (Table 1). Initial L43Nd/'"Nd ratios are Seeliggranite differs from therest in that it is very consistentandcomparatively unradiogenic. The hornblende-bearing,although the fine-grained leucocratic depletionin radiogenic L43Nd would normally be taken to LinckNunatak granite is typical of theremainder of the indicate significant incorporation of crustal Nd. The model suite.The onlyaccessory minerals within thesuite are agesfor thegranites are relatively consistent at 1270- apatite, sparse zircon, and small uncommon opaque grains. 1740 Ma,apparently reflecting a Precambrian age for the

Table 1. Summary of Rb-Sr and Sm-Nd isotopic data

Lewis Nunatak PirritHills Nash Hills Pagano Nunatak Linck Nunatak (Ferrar Supergroup)

No. of samples 12 3 9 14 9 Rb (PP4 368-707 292-449 47-57325-425 153-289 Sr (PP) 4-43 26-116 123-1326-89 14-97 87~r/"~r 0.7070 f 16 0.7122 f 8 0.7157 f 14 0.7224 f 8 0.7111 f 5* MSWD 1.8 12.0 5.4 4.5 - Age (Mal 173 f 3 175 f 8 175 f 8 175176 f 5

samples 4 3 3 2 3 3 4 No. of samples 1 Sm (PF4 6-710-13 1 3-14.7 6-7 Nd (PP4 19-4829-32 8-5320 28-32 147Sm/'"Nd 0,156-0.323 0.128-0.191 0.136-0.138 0.137-0.145 0.142 '43Nd/1"Nd 0.51242 f 10 0.51234 f 4 0.51229 f 2 0.512250.51231 f 4 (143Nd/LL4Nd), 0.51209 rt 8 0.51216 f 1 0.512140.51213 * 2 0.51212 &Nd(t) -5.4 -5.0 -5.5 -5.7 -5.3 TDM(Ma) 1740 1270 1410 1541 1453 Data from Millar & Pankhurst (1987) and Pankhurst er a[. (1989). DM, Model age for separation from the Nd-isotope growth curve of depleted mantle. Errors are two-sigma throughout. *Calculated values at 175 Ma.

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crustal component. However, Ferrar Supergroup rocksshow overallsample range of SiO, is 69-77%, withexpected a similar range of Nd-isotope characteristics (Table 1; BAS trends in Harker diagrams: MgO, total Fe oxides, TiO, and unpublished data) suggesting that these featuresmay also be MnO all decrease markedly, Al,03, CaO and P,O, rather inherited in part from ancient enriched lithospheric mantle. less so, whereas Na,O and K,O tendto increase rather This could be the case if the lower crust and immediately erratically.These trends, which may be ascribed to underlying mantle formed a ‘coupled’ system fossilized since fractionation of amphibole,feldspars and apatite, are also an original Proterozoic differentiation event. demonstratedwithin individual plutons. The PirritHills, MartinHills and Linck Nunatak granites are restricted to themore differentiated end of the spectrum,but forthe Geochemistry and magmatic fractionation others there is a clearly defined variation from less evolved The most significant chemicalcharacteristics of the EWM coarse- to medium-grained granites to evolved aplogranites, granitic suite are given in Table 2 (based upon a selection of some of the latter compositions being associated with aplite theleast evolved granites from adata set significantly veins. enlargedfrom that of Vennum & Storey 1987a). All are Compatibility of some of the largeion lithophile highly differentiated,mildly peraluminous, corundum elements (LILE), in particular Ba and Sr, is consistent with normative and subalkaline with high K,O/Na,O ratios. The a crystallizing assemblagedominated by plagioclase and

Table 2. Representative geochemical analysis for the EWM granitic suite and the Ferrar Supergroup

~ PaganoNunatak Martin HillsPirritHillsNash Hills Linck Nunatak Ferrar

V.12 V.26 V.24 V.39 V.48 V.28 V.38V.28 V.48 V.39 V.24 V.26 V.12 V.43’ V.20 A B

Major elements SiO, 71.62 73.74 75.11 74.3676.1770.37 72.35 71.37 71.71 54.67 53.75 TiO, 0.35 0.25 0.13 0.460.17 0.17 0.34 0.37 0.22 1.21 0.70 13.80 13.15 12.41 13.7912.4313.38 13.41 14.19 14.08 13.09 14.33 Fe203 1.13 0.64 0.56 0.471.27 0.35 2.51 0.94 1.52 2.72 9.65 Fe0 0.99 - 0.55 0.86 - 2.32 - 1.35 - 10.27 - MnO 0.05 0.02 0.04 0.060.07 0.06 0.06 0.03 0.03 0.20 0.18 MgO 0.55 0.37 0.45 0.810.14 0.13 0.58 0.66 0.38 4.24 6.64 CaO 1.26 1.00 2.50 1.490.76 0.88 1.39 1.19 0.98 8.43 10.60 Na,O 3.09 2.34 I .06 2.913.30 3.43 2.98 2.89 3.59 2.27 1.83 KZ0 5.14 6.45 4.47 5.65 5.04 5.03 5.05 5.37 5.66 1.27 0.81 p205 0.19 0.14 0.03 0.040.04 0.14 0.11 0.21 0.18 0.13 0.18

H,O + 1.34 1.91 2.97 1.711.10 1.17 1.11 1.27 1.41 1.03 1.31 Total 99.60 99.99 100.28 100.49100.46 99.74 99.88 99.84 99.76 99.53 99.98

Trace elements (ppm) Ba 376 456 324 121 110 512 367 580 278 331 232 Nb 26 27 62 59 60 27 23 24 22 10 - Sr 114 118 148 60 44 132 127 107 73 128 126 Rb 307 333 338 503 600 352 369 290 324 47 30 Th 20 23 82 50 59 38 32 24 17 4 4 Y 42 43 106 123 154 57 71 30 22 39 - Zr 180 143 133 142 144 265 188 210 121 167 83 La 35.8 55.7 51.2 41.5 44.6 37.4 23.9 19.3 Ce 73.5 120.7112.1 88.9 95.3 76.1 51.1 39.4 PI 8.7 14.1 13.3 10.8 11.4 9.3 6.4 5.1 Nd 33.6 54.5 50.8 40.4 43.8 35.2 24.2 20.4 Sm 7.5 13.1 1.2 11.1 9.7 7.6 6.2 4.8 Eu 0.9 0.6 0.6 1.1 1.0 0.8 0.6 1.2 Gd 6.7 12.7 11.5 9.3 8.5 6.3 5.6 5.1 DY 6.3 1 14.18. 14.8 8.6 5.0 4.9 5.5 Ho 1.2 1.5 2.8 3.1 1.7 0.9 0.8 1.1 Er 3.4 4.5 8.4 9.8 5.1 2.3 2.1 3.6 Yb 3.1 3.0 8.1 9.6 5.1 1.6 1.7 3.3 Lu 0.5 1.3 1.5 0.6 0.8 0.3 0.3 0.6

A, sampleV7F from Vennum & Storey (1987~);B, average offive analyses from Kyle (1980). Whole-rock analyses were performed on a Philips PW1400 X-ray fluorescence spectrometer at Bedford College, University of London.Rare-earth element data were obtained on a Philips 65-channel inductively coupled plasma emission spectrometer at Kings College, University of London.

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300 - differentiatedprimarily from precursors with distinctive 200 - isotopicand trace element characteristics although some interplutonictrace element variations may bedue to different crystallizing phase assemblages. Consequently the 100 - individual plutons require different sources and/or differing

W degrees of crustal contamination.

L V C 50 - Pn 7010 5 40- Petrogenesis . MU -W 30- NH 7122 Granite type and tectonic setting i 20- m Graniticrocks are commonly sub-divided into minera- PN 7157 logical/geochemical groups which reflect their petrogenesis, including the composition of theirsource and the mechanisms of bothmelting and crystallization. Whereas 'O1 geochemical and isotopic information do not always lead to unambiguousresolution of thesefactors, widelyisit recognized that differences between granite types can often be empirically related to their tectonic environment. Pearce et al. (1984) havedeveloped the use of traceelement discriminationdiagrams for granites to distinguish, for :F example,subduction-related granites from syn-collisional ' 1La Ce Pr Nd Srn Eu Gd Dy Ho Er Yb Lu granites. The EWM granitic suite shows many of the characteris- Fig. 2. Chondrite-normalized, after Nakamura (1974), Rare Earth tics of S-typegranitoids (Vennum & Storey1987a), element patterns and initial X7Sr/R6Sr ratiosfor the EWM granitic peraluminousleucogranites with no volcanicassociation, suite. PH, Pirrit Hills, MH, ; NH, Nash Hills; PN, high but with a narrow range of SiO,, high K,O/Na,O, low Pagano Nunatak;LN, Linck Nunatak. CaOand Sr, scarcexenoliths (only of metasedimentary rocks) and high but variable initial "7Sr/86Sr ratios. The low K-feldspar,as distribution coefficients for Sr and Ba in initial '43Nd/'44Nd ratios and Proterozoic Nd model ages are K-feldsparand Sr in plagioclase are greater than unity at alsocompatible with an origin by melting of oldcrustal intermediateto acid compositions (Arth 1976). Feldspar material. This would normally be interpreted as indicating crystallization wouldalso produce thelarge negative Eu partial melting of over-thickenedcontinental crust, for anomalies (Fig. 2) in some of theEWM granitoids (e.g. example in a continent-continent collision zone. However, Pirrit Hills Eu/Eu* ~0.1).In addition, Zr, Hf andthe true S-typegranites always containcordierite (which the LREE and possibly Th behave compatibly at high levels of EWM suite donot), and manygranites which otherwise SiO,in themore aluminous EWM granites (e.g. Linck possessonly some S-type affinities, particularlythose in Nunatak, Fig. 2). This is likely to be the result of the cordilleran settings, may be generated by crustal contamina- combinedcrystallization of high field strengthelement tion or by extreme differentiation of I-type melts (White et (HFS)-and REE-bearing minor phases (zircon, sphene, al. 1986). apatite,?allanite); theimportance of minorphase Influence of continental crust in the petrogenesis of the crystallization in corundumnormative granitoids has been EWM graniticsuite is suggested by multi-elementprofiles demonstratedboth with reference to individualintrusions (Fig. 3) normalized to the ocean ridge granite (ORG) values (Tindle & Pearce1981) and on experimentalgrounds of Pearce et al. (1984). These are 'crust-dominated profiles', (Watson 1979). Thus, although interplutonic trace element characterized byhigh lithophileelement abundances with variations may reflect primary source variations, the effects especially high Rb andTh compared to K andBa, and of minorphase crystallization on, in particular REE moderateHFS element abundances (Sm, andY Yb fractionation is important. although Zr is relativelylow). This type of pattern is Betweenplutons there is agood correlation between matched by granites from a variety of crustal environments, initial s7Sr/s6Sr ratios and the degree of REE fractionation notablymany collision-related granites as well asvolcanic forindividual plutons (Fig. 2),[Ce/Yb], ratios increasing arc granites from Chile (Pearce et al. 1984). However, they with initial R7Sr/8hSr ratios.The Pirrit Hills granite has thelow- differ from the collision-generated granites of the Himalayas est initial R7Sr/%rand [Ce/Yb], ratios (0.7070 and 3.1, and the Hercynian belt of Europe in the central portions of respectively); both ratios increase progressively to 0.722 and the profiles, notably in their high Nb,Ce and Nb/Ce. A 12.9, respectively, for the Linck Nunatak granite. Increasing comparison with subduction-related granites of the same age REE fractionation in the order Pirrit < Nash < Pagano < and with similar silica contentsand initial 87Sr/"6Srratio Linck could reflect the increasingimportance of crystal- fromthe Antarctic Peninsula indicates that the EWM lization of an HREE-enriched phase, e.g. amphibole, during granitic suite and in particular the Pirrit Hills granite (Fig. differentiation of the EWM granite suite from a common 4), has higher Nb, Rb, Sm, Y, Th and Yb contents. This is source.However, variations in crystallizing phase as- typical of within-plategranites (WPG), especiallythose semblagesalone cannot explain theinterplutonic initial granites with crust-dominatedprofiles emplaced into crust of 87Sr/@3r ratio variations. normal thickness (e.g. Sabaloka, Sudan, Harris et al. 1983) The coherent trace element-isotope variations exhibited or highly attenuated crust (e.g. the Tertiary granites of the by theEWM suite thus suggest thateach granite Brito-Arcticprovince, Walsh et al. 1979). Onthe

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150 - X 100 - 100 - 50 - 40 - 50 - 30 - 40 - - 30 - 20

20 - 10 -

10 - 0 8 5: a 0, 4 ? 3- - A!Q 0 5- S 4- 2- v) 3- 1- 2- - 0.5 - 1.0 - 0.4 0.3 -

0.2 - 0.5 - R.2620.1 0.4 - L1II,,I,,,,,, K20 Rb Ba Th Ta NbCe Hf Zr Sm Y Yb Fig. 4. Trace element diagram, normalized to Ocean Ridge Granite (ORB) values of Pearce er al. (1984), comparing the Pirrit Hills granite to subduction-related granites from the Antarctic Peninsula (data from Hole 1986). 0.11 I I I I l1 1111 KzO Fib Ba Th Nb CeZr Sm Y Yb Fig. 3. Trace element abundances of the EWM granitic suite, normalized to Ocean Ridge Granite (ORG) values of Pearce et al. fractionation of amantle-derived magma. However the (1984). See Fig. 2 for abbreviations. initial R7Sr/86Srratio of 0.7070 is notcompatible with a depleted mantle source. Rb-Y + Nb discrimination diagram of Pearce et al. (1984), the Pirrit Hills granite falls within the WPG field (Fig. 5). The remainder of the EWM granites have lower Nb and Y Relationship between the Pirrit Hills granite and an abundances than Pirrit Hills, and plotprogressively closer to enriched Ferrar source the syn-collisional granite (synCOLG) field with increasing One of the mostintriguing aspects of theEWM granitic initial 87Sr/86Srratio, the Pagano granite (initial s7Sr/ssSr= suite as a whole is its remarkable similarity, with respect to 0.716) andthe Linck granite (initial 87Sr/86Sr-0.722) some features, to the Ferrar Supergroup. The granites were plottingwithin the synCOLG field. As thePirrit Hills emplacedover a restricted Middle Jurassic time interval granite is most similar to within-plate granites we will now which overlaps with the supposed age range of the Ferrar consider its petrogenesis in terms of it being a within-plate (Kyle et al. 1981). Both have crust-dominated trace element end member of the EWM granitic suite. patterns and an identical range of old continental Sr and Nd Petrogenetic modelling demonstrates that WPG form in isotopic characteristics. These striking similarities raise the acrust-free setting such as those granites from Ascension possibility of a genetic link between the EWM granitic suite Island,fractionated directly from a mafic, mantle-derived andthe Ferrar rocks inthe bordering Transantarctic magma by crystallization of a cpx + plag + mt dominated Mountains, reinforced by the fact that the HartHills gabbro assemblage (Pearce et al. 1984). Dueto lowbulk within theEWM crustalblock itself hasgeochemical distribution coefficients for Rb, Nb and Y the evolution of similarities to the Ferrar suite (Vennum & Storey19876). sucha graniteleads to high Rb,Nb and Y abundances. Furthermorethe EWM granites are underlain by large Furthermore,the plagioclase-dominated assemblage can magneticbatholiths with distinctive magnetic anomalies. result in the flat REE profileswith extremenegative Eu Geophysicalmodelling of theanomalies suggests thatthe anomalies (Eu/Eu* < 0.1)characteristic of ‘crust free’ magnetic source body is mafic in composition and could be WPG . similar to a large Ferrar gabbro exposed in the Dufek Massif The major and trace element characteristics of the Pirrit within theTransantarctic Mountains (Garrett et al. 1988). Hills granite couldconsequently be explained by extreme Consequently it is reasonable to explore the possibility of a

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has an apparently 'crustally dominated' profile on an ORG Rb f rynCOLG / normalizeddiagram (Fig. 3). Weconsider thatthis is an artefact of the generation of the Pirrit Hills granitoid from

h WPG the anomalous Ferrar mantle source region. 0'' Although this model can account for the origin of the ,i Pirrit Hills granite, the most 'within-plate' end member of thesuite, it clearlycannot account forthe apparently Rift ,- ______---- synCOLG trace element affinities and high initial x7Sr/x6Sr / ratios of the other EWM granites. Pearce et al. (1984) demonstrate that where WPG have assimilatedcrustal material (e.g. British Tertiary province and Nigeria), then the high Rb/Nb and [La/Yb], ratios of most crustal rocks result in disproportionate increases in Rb l- ,/' / relative to Nb or Y, as well as causing a relative decrease in absoluteNb and Y abundances compared to crust-free 6 PIRRIT HILLS A NASHHILLS WPG.Furthermore, the addition of crustalmaterial to Forrar PAGAN0 NUNATAK WPGprecursors appears in somecases to changethe LINCKNUNATAK crystallizing assemblage to include amphibole (e.g. Nigeria, .I Bowden & Turner 1974). The high Kd values for the HREE 10 I l I.,'] I I I I I I I I I I Ill reportedfor amphibole in intermediatemagmas will thus 10 100 Nb+Y ppm causerelative depletions in Y(and ?Nb) in WGP which Fig. 5. Rb - (Nb + Y) discrimination diagram for EWM granitic crystallized amphibole. Effectively the addition of a crustal suite. Fields and data for 'crust-free' WPG from Pearceet al. component into WPG causes a shift towards the left of the (1984). Data for WPG emplaced into attenuated continental WPG field in Rb-Y + Nb space, i.e. towards the syn-COLG lithosphere (Mull) from Walsh er al. (1979). Pecked lines: short or VAG fields. dashes, petrogenetic pathway for Ascension Island from Pearce et In the case of the EWM suite, the addition of a high al. (1984); long dashes, petrogenetic pathway for crystallization of LIL/Nb,LREE/HREE and 87Sr/R6Srratio crustal com- Pirrit Hills granite. Assumed mafic precursor (Pb)to the Pirrit Hills ponentto an already anomalously high LIL/Nb,etc., granite (P,) contains 6 ppm Nb, 30 ppm Rb and 25 ppm Y. magma would inevitably result in the shift towards the left Pathways from Pb to an intermediate phase (Pi)and Pi to Pa of the Pirrit Hills field in Rb-Nb + Y space. Furthermore, calculated using the Raleigh fractionation law, mineral assemblages the inclusion of amphibole in the crystallizing assemblages and K, values of Pearce al. (1984, Table 4).Pb-Pi, F (the ef of the high initial s7Sr/"Sr ratio EWM granites could have proportion of residual liquid) = 0.5, Pi-Pa, F = 0.3; WPG, within resulted in their highly fractionated REE profiles and low plate granite; synCOLG, syn-collisional granite; VAG, volcanic arc granite. absolute Yb and Y abundances, as is the case for Nigerian andBritish Tertiary WPG(Pearce et al. 1984).Conse- quently,the high R7Sr/H6Srratio EWM granites exhibit close petrogenetic relationship between the Ferrar magmas syn-COLG trace element affinities. Again, we interpret this and the Pirrit Hills granite. as being an artefact of the addition of a crustal component Figure 5 showsapossible petrogenetic pathway for to analready LILE-enriched anomalous mantle source or differentiation of the Pirrit Hills granite from a Ferrar-type magma. The origin of the Linck Nunatak granite, the most basaltic liquid. The starting point (Pb)is based on data from enriched end member of the EWM suite (87Sr/86Sr= 0.722) high-Cr basaltic rocks from Ferrar sills in the Transantarctic is equivocal.This may haveformed entirely by partial Mountains(Kyle 1980). The pathwayhas been calculated melting of acrustal source without any input from an utilizing the phase assemblage (p1 + mt + cpx), the Rayleigh enriched mantle. All the EWM granites are thus considered fractionation equationand Kd data used by Pearce et al. to havebeen generated in within-platea setting and (1984) for the differentiation of WPG in a continental crust observed interplutonic trace element and isotopic variations free setting. The high proportion of plagioclase in the can be considered as the result of mixing between a mafic crystallizing assemblageresults in the high absoluteYb mantle end member and continental crust. The Pirrit Hills abundances, low [La/Yb],ratios, andlarge negative Eu end member could be produced entirelyby differentiation of anomalies characteristic of the Pirrit Hills granite. Clearly, amantle-derived, enriched Ferrar source magma whereas the calculated Pirrit Hills acid composition (Pa on Fig. 5) the LinckNunatak end member could have formed by lies close theto field forobserved trace element partial melting of a crustal source. It has already been noted characteristics of this granitoid. The isotopicand trace that Nd-isotopecompositions, which would normally be elementcharacteristics of thePirrit Hillsgranite are thus regardedas a crucial test of mixing betweenmantle and consistent with derivation by closed-systemdifferentiation crustal material, are insensitive in this case since the Ferrar from a Ferrar-type basic magma. basaltshave initial 143Nd/144Ndratios which are indistin- Ferrar mafic rocks are generallyconsidered by most guishable from those of both the granites and hypothetical workers to havebeen derived from a mantle source with Proterozoic crust. anomalously high LIL/Nb,LREE/HREE and initial R7Sr/86Srratios, even when the effects of limited upper crustalcontamination and/or fractionalcrystallization are Relationship to Gondwanaland break-up taken into account (Kyle 1980). Although the trace element The possible petrogenetic association of the EWM granitic characteristics of the Pirrit Hills granite have been modelled suite and the Ferrar dolerites suggests for the first time the according toa 'crust-free' fractionationscheme, it clearly presence of a large-scale bimodal Middle Jurassic magmatic

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suitewithin West Antarctica.This forms part of a significant time-lag anda geographicalreorientation of widespread magmatic event in the Antarctic-South Atlantic rifting prior to the final brittle opening of new ocean basins region,linked somein way to thebreak-up of (e.g. the Weddell Sea) in late Jurassic to early Cretaceous Gondwanaland (Kyle et al. 1981; Dalziel et al. 1987). This time. Thethermal perturbation mayhave been sited eventincludes theTasmanian dolerites, theKaroo beneath the EWM, the mainzone of crustal melting, and Supergroup of southern Africa and the Serra Geral basalts notbeneath the zones of maximumextension and crustal of central South America. New ocean floor was formed by rifting likethe Weddell Sea. The massive mafic sills, so the separation of South America and Antarctica (Weddell characteristic of theFerrar Supergroup within the Sea), of and Africa, and by the opening of TransantarcticMountains, may have been emplaced away theMozambique Basin. Onthe Antarctic marginseismic from this ‘central’ EWMheat source. This profound surveyshave identified seaward-dipping reflectors inter- asymmetry is a feature of many extensional zones like the preted as offshore volcanic flows (Kristoffersen & Haugland Basin and Range Province and is attributed to the influence 1986). This rifted margin magmatic province is analogous to of shallow dipping detachment zones (Wernicke 1985). They theoutburst of volcanic activity associatedwith the result in the region of mostpronounced crustal thinning Brito-ArcticTertiary province of thenorth-east Atlantic being displaced from the region where the crustal geotherm (White er al. 1987). Although the age of the earliest known is most perturbed, and minimal thinning of the crust in the sea-floorspreading in theWeddell Sea part of theSouth vicinity of the steepest geotherm (Sandiford& Powell 1986). Atlanticprovince is uncertain,the Ferrar-EWM granitic Along theTransantarctic Mountains lineament the Ferrar suite(175 Ma) preceded sea-floor spreading between East magmasreached high levels in thecrust withonly minor Antarctica and Africa (150 Ma, Bergh 1977)by some 25 Ma. crustalcontamination whereas within the EWMcrustal Contemporaneousbimodal magmatism of theTobifera block the magmas appear to have been ponded within the Formationand ‘Rocas Verdes’ basin (Gust et al. 1985) crust. This led to both extensive crystal fractionation as well occurredalong the Pacificmargin of southernmostSouth as the formation of large amounts of anatectic melt within Americaand South Georgia subduction-relateda in the crust and the emplacement of the EWM granitic suite. back-arcbasin tectonic setting. The relationshipbetween thesetwo magmatic provinces is uncertainand has some bearing onthe precisemechanisms of continental disin- Conclusions tegration. Although the mechanisms are poorly understood We thus believe that there was a Middle Jurassic igneous hypotheses for the break-up of Gondwanaland fall into two event in West Antarctica, of which the EWM granites are groups. One is thatbreak-up wasinitiated as a form of clearlyonly asmall but hitherto unrecognized part.It back-arcspreading related to subduction of oceanic reflects a massive deep-seated thermal disturbance which in lithospherebeneath the edges of the supercontinent (Cox itself mayhave triggered thesubsequent break-up of 1978). The second hypothesis is that break-up was related to Gondwanaland. The granitesare a within-plate suite that a thermala disturbance in themantle underlying the formed by differentiation of previously a enriched continent, perhaps induced by the blanketing effect of the mantle-derivedFerrar-type mafic magmacontaminated by latter during a long preceding intervalof stability (Anderson variable amounts of continental crust. 1982). The geochemistry of theEWM granites has some This work is part of a joint BAS-WARP programme investigating bearingon this problem. It is clearfrom the above the tectonic history of West Antarctica. The US side was supported geochemical comparisons that the Middle Jurassic granites by theDivision of PolarPrograms, National Science Foundation showfew of the chemicalcharacteristics of calc-alkaline throughgrant DPP82-13798 to I. W. D. Dalziel. The full set of granites in general OF more specifically of contemporaneous geochemical analytical data will be stored in the UK-IGBA data file subduction-relatedgranites of theAntarctic Peninsula from which it can be retrieved via the National Geochemical Data magmaticarc. Instead, they show, with the Ferrar Bank of the British Geological Survey. Supergoup,the anomalous long-livedcharacteristics of an ancient crust-lithosphere system. Our interpretation of the References Jurassicbimodal igneous event in Antarcticaseems to favoursome form of thermaldisturbance leading to ANDERSON,D. L. 1982. Hotspots,polar wander, Mesozoic convection, and the geoid. Nature, 297, 391-3. widespreadbut localized melting in the mantle and the ARTH,J. G. 1976. Behaviour of trace elements during magmatic processes-a underplating of massive amounts of basic material (Ferrar summary of theoreticalmodels and their applications. Journal of magma). This resulted in concomitant partial melting of the Research, United States Geological Suruey, 4, 41-7. crust andthe associated contaminated acidmagmatism of BERGH, H. W. 1977. Mesozoic sea floor off Dronning Maud Land, Antarctica. Nature; 269, 6867. the EWM granitic suite. BOWDEN,P. & TURNEQ, D. C. 1974. Peralkalineandassociated Withincontinental rift zones,the relationship between ring-complexes in the Nigeria-Niger Province,West Africa. In: thermalphenomena (magmatism andmetamorphism) and SORENSEN,H. (ed.) The Alkaline Rocks. J. Wiley & Sons, New York, themechanical phenomena of rifting is incompletely 330-51. understood (see Wickham & Oxburgh 1985). In the initial Cox, K. G. 1978. Floodbasalts, subduction and thebreakup of Gondwanaland. Nature, 274, 47-9. stages of rifting the rigid lithosphere may be stretched and DALZIEL,I. W. D. & ELLIOT,D. H. 1982. West Antarctica: Problem child of thinned allowing hot asthenosphere to well up and generate Gondwanaland. Tectonics, 1, 3-19. partial melts; large quantities of melt can be produced by -& PANKHURST,R. J. 1985. Tectonics of West Antarctica and its relation upwelling asthenosphere that is only 100-150 “C hotter than to East Antarctica: USARP-BAS geology/geophysics project. Antarctic Journal of the United States, U), 40-1. normal(White et al. 1987).Although the EWM granites -, STOREY, B.C., GARRET,S. W., GRUNOW, A. M., HERROD,L. D. B. & mayhave been emplaced throughdilated crust in PANKHURST,R. J. 1987. Extensional tectonics and the fragmentation of rift-controlledloci, it is clear thatthere was botha Gondwanaland. In: COWARD,M. P,, DEWEY,J. F. & HANCOCK,P. L.

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Received 25 November 1987; revised typescript accepted 12 May 1988.

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