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 ° C and 5 - 6 kb (Mj) 1190±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.
