Geology of the Bunger Hills-Denman Glacier region, East Antarctica

The Bunger Hills area, which forms part of the East Antarctic Shield, consists predominantly of 
granulite facies orthogneiss (pyroxene-quartz-feldspar gneiss), with subordinate maficgranulite and garnet, sillimanite, and 
cordierite-bearing paragneiss. The igneous precursors of granodioritic orthogneiss crystallised about 1500 - 1700 Ma ago, 
whereas late Archaean (2640 Ma) tonalitic orthogneiss occurs in the Obruchev Hills, in the southwest of the area. Metamorphism 
reached a peak of about 750 - 800 Â° C and 5 - 6 kb (Mj) 1190Â±15 Ma ago (U-Pb zircon age) and was accompanied by the first of 
three ductile deformation events (Dj). Voluminous, mainly mantle-derived plutonic rocks were emplaced between 1170 (during D 3 ) 
and 1150 Ma. They range in composition from gabbro, through quartz gabbro, quartz monzogabbro, and quartz monzodiorite, to granite.
 Abundant dolerite dykes, of at least four chemically distinct groups, were intruded at about 1140 Ma. Their intrusion was associated 
 with the formation of shear zones, indicating at least limited uplift; all subsequent deformation was of brittle-ductile or brittle 
 type. Alkaline mafic dykes were emplaced 500 Ma ago. Marked geochronological similarities with the Albany Mobile Belt of Western 
 Australia suggest that high-grade metamorphism in both areas was the result of continental collision between the Archaean Yilgarn 
 Craton of Australia and the East Antarctic Shield. However, Gondwana reconstructions and the composition of the plutonic rocks 
 suggest that the Bunger Hills metamorphics may have formed in an Andean-type continental arc, with the actual collision zone 
 having been to the east of the present Bunger Hills. Exposures west of the Denman Glacier are also mainly granulite-facies 
 gneiss, intruded by a variety of mafic to felsic plutonic rocks. They differ from the Bunger Hills in being partly derived 
 from Archaean protoliths (- 3000 Ma), in lacking isotopic evidence for a Mesoproterozoic high-grade event, and in not being 
 intruded by dolerite dyke swarms. They also show evidence of much more extensive 500 - 600 M a (Pan-African) metamorphism 
 and plutonism (syenite to granite), and in this regard they are comparable with the Leeuwin Block metamorphics of southwestern 
 Australia, although these were derived from significantly younger protoliths (T^D  model ages: 1100 - 1500 Ma). If this early 
 Palaeozoic activity was also a consequence of continental collision, it would explain the markedly different geological history 
 of the terranes on either side of the Denman Glacier and could account for the final uplift of the Bunger Hills. However, the 
 compressional tectonic regime implicit in the collision hypothesis was followed by an extensional regime, which, in southwestern 
 Australia, eventually resulted in the formation of the Perth Basin rift zone. This structure is aligned with the Denman Glacier 
 trough on our preferred Gondwana reconstruction, suggesting that it may have extended well to the south before the breakup of 
 Gondwana.

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