ZIRCON AGES AND THE DISTRIBUTION OF ARCHEAN AND PROTEROZOIC ROCKS IN THE RAUER ISLANDS

The application of zircon U-Pb geochronology using the SHRIMP ion micr oprobe to the Precambrian high-grade metamorphic rocks of the Rauer Is lands on the Prydz Bay coast of East Antarctica, has resulted in major revisions to the interpreted geological history. Large tracts of gran itic orthogneisses, previously considered to be mostly Proterozoic in age, are shown here to be Archaean, with crystallization ages of 3270 Ma and 2800 Ma. These rocks and associated granulite-facies mafic rock s and paragneisses account for up to 50% of exposures in the Rauer Isl ands. Unlike the 2500 Ma rocks in the nearby Vestfold Hills which were cratonized soon after formation, the Rauer Islands rocks were reworke d at about 1000 Ma under granulite to amphibolite facies conditions, a nd mixed with newly generated felsic crust. Dating of components of th is felsic intrusive suite indicates that this Proterozoic reworking wa s accomplished in about 30-40 million years. Low-grade retrogression a t 500 Ma was accompanied by brittle shearing, pegmatite injection, par tial resetting of U-Pb geochronometers and growth of new zircons. Mino r undeformed lamprophyre dykes intruded Hop and nearby islands later i n the Phanerozoic. Thus, the geology of the Rauer Islands reflects rew orking and juxtaposition of unrelated rocks in a Proterozoic orogenic belt, and illustrates the important influence of relatively low-grade fluid-rock interaction on zircon U-Pb systematics in high-grade terran es.