A two-stage evolution of the Neoproterozoic Rayner Structural Episode: new U-Pb sensitive high resolution ion microprobe constraints from the Oygarden Group, Kemp Land, East Antarctica

U-Pb sensitive high resolution ion microprobe (SHRIMP) zircon analyses have been used to place time constraints on the Proterozoic reworking of Archaean rocks during the Rayner Structural Episode in the Oygarden Group of islands, Kemp Land, East Antarctica. Felsic orthogneiss that preserves evidence for foliations that pre-date the Rayner Structural Episode, was deformed by two events during the Rayner Structural Episode: the first characterised by east-directed sub-horizontal thrusting, and the second by a regional scale south-dipping extensional shear zone. Metamorphic overgrowths on zircon grains from felsic orthogneiss, which have 929 +/- 12 and 924 +/- 17 Ma ages, are interpreted to have formed during granulite facies metamorphism that accompanied the thrusting event. Crystallisation of zircon in migmatite that also formed during this event, occurred in two stages: 904 +/- 16 and 884 +/- 24 Ma. Overgrowths on zircon in a pegmatite that intruded during the second event formed at 931 +/- 14 Ma. Charnockite that was emplaced in the Stillwell Hills prior to a regional thrusting event, was metamorphosed at 913 +/- 8 Ma. However, the distribution of isotopic data suggest the charnockite may have intruded at ca. 960 Ma. The ages for regional thrusting in the Oygarden Group and the Stillwell Hills correlate, suggesting that the Rayner Structural Episode did not affect Kemp Land until ca. 930 Ma. This age for peak metamorphism in Kemp Land is therefore 50 Myr younger than the age inferred for the Rayner Structural Episode further to the east on the Mawson Coast and south in the northern Prince Charles Mountains. The ages from the Oygarden rocks correlate with the second half of the orogenic episode seen on the Mawson Coast and northern Prince Charles Mountains, consistent with the possibility of two phases of collision in a transition from an Andean-style margin to continent-continent collision between 1000 and 900 Ma. Previously unreported ca. 1600-1650 Ma ages were also discovered in orthogneiss and pegmatite samples, indicating a thermal event in Kemp Land at this time. The 1600-1650 and 930-900 Ma ages from the Oygarden Group correlate with metamorphic and magmatic ages from the Eastern Ghats, placing further constraints on the correlation between these metamorphic belts. (C) 2002 Elsevier Science B.V. All rights reserved.