The evolution of a layered metaigneous complex in the Rauer Group, East Antarctica: evidence for a distinct Archaean terrane

Abstract

The Rauer Group in Prydz Bay, East Antarctica, contains both Archaean and Proterozoic crustal components. Archaean geological relationships have been defined from the Scherbinina layered complex (SLC), a polydeformed and metamorphosed Fe-tholeiite intrusive cut by several generations of deformed metabasite dykes. The SLC and three of these dyke generations are intruded by tonalitic and trondhjemitic sheets representative of Archaean felsic orthogneisses present outside the complex. UPb shrimp dating of zircons precisely defines the magmatic age of the SLC as 2844 ± 6 Ma. A tonalite sheet that cuts the SLC preserves a complex zircon isotopic array dominated by inherited grains derived from source rocks up to 3470 ± 30 Ma old, but from field evidence was emplaced at ca 2800-2550 Ma. The presence of 3470 Ma source rocks is unique in the East Antarctic Shield, and this feature as well as the prominence of 2840-2800 Ma orthogneisses establishes the Rauer Group as a distinct Archaean terrane. The Archaean of the Rauer Group is not comparable with the nearby Vestfold Hills, where ca 2520-2480 Ma orthogneisses form the dominant rock units. It is likely that the Vestfold Hills and Rauer Group terranes were amalgamated at either 1000 or 500 Ma, rather than at the end of the Archaean. A mid-Proterozoic tectonothermal event documented in the Rauer Group from syn-metamorphic 1030-1000 Ma intrusives (Kinny, P.D., Black, L.P. and Sheraton, J.W., 1993. Zircon ages and the distribution of Archaean and Proterozoic rocks in the Rauer Islands. Antarctic Sci., 5, 193–206) has not been detected in the zircon UPb data obtained on the Archaean gneisses examined here. Instead, the SLC and intrusive tonalite show resetting of zircon UPb systematics at ca 520 Ma. This resetting reflects Pb-loss within irregular compositional zones and healed cracks in older zircon, and is attributed to the effects of fluid infiltration under greenschist to amphibolite facies conditions. These effects correlate in time with ca 530-510 Ma higher-grade deformation and metamorphism evident in Proterozoic paragneisses exposed in areas to the south-west of the Rauer Group, and may record amalgamation of the Rauer Group with these areas during the East Gondwanian Pan-African orogeny.