U-Pb data on granulite facies rocks from fold island, Kemp Coast, East Antarctica

Abstract

The northern tip of Fold Island (67°16′S; 59°20′E) on the Kemp Coast is composed largely of quartzofeldspathic gneisses (charnockitic and enderbitic) and pyroxene granulite. Garnet-biotite-sillimanite gneiss, garnet-clinopyroxene-plagioclase-calc-silicate rock, magnetite-bearing rocks, and ultramafic rocks are very subordinate. The gneisses were deformed by the two phases of isoclinal folding and were subsequently intruded by mafic dikes. A second stage of deformation involving tight, ductile folding post-dates the dikes. This deformation culminated with the emplacement of hypersthene-bearing pegmatites in discordant planar veins and irregular masses, and with the hornblende-granulite facies metamorphism that resulted in the present-day mineral assemblages. U-Pb isotope data on zircon and monazite from two pegmatites define a chord intersecting concordia at 0.94 ± 0.08 Ga and 0.21 ± 0.21 Ga. The 0.94 Ga age dates deformation, granulite facies metamorphism, and the pegmatite emplacement of Stage II. U-Pb data on a total of five fractions of highly rounded zircons from two quartzofeldspathic gneisses define a chord with intersections at 3.08 ± 0.17 Ga and 0.82 ± 0.48 Ga. Our results suggest that the gneiss zircons are xenocrysts and do not provide any direct indication of the age of the host gneisses. The Fold Island gneisses are interpreted to be older (Napier Complex?) rocks that were reworked during the Late Proterozoic (∼ 1000 Ma) Rayner event. A similar sequence of events has been reported for a coastal portion of the Eastern Ghats Province of South India, suggesting that these two areas were part of the same metamorphic complex in Gondwanaland.