TIMING OF PROTEROZOIC DEFORMATION AND MAGMATISM IN A TECTONICALLY REWORKED OROGEN, RAYNER COMPLEX, COLBECK ARCHIPELAGO, EAST ANTARCTICA

The high-grade metamorphic rocks of the Colbeck Archipelago comprise g ranulite facies metasedimentary gneisses and charnockitic gneisses of three ages. A suite of rocks previously mapped as the Colbeck Gneiss i s shown to be composite in age and origin, consisting of metasedimenta ry and orthogneiss that are intruded by granitic gneiss and charnockit ic gneiss. An (?) Archaean granulite facies S1 gneissosity is preserve d in the metasedimentary gneisses and early orthogneiss; the developme nt of S1 was followed by isobaric cooling from peak metamorphic condit ions of T greater-than-or-equal-to 750-degrees-C and P = 5.1 +/- 0.8 k bar. Younger intrusions cut rocks containing S1 structures, but were d eformed by a almost-equal-to 1200 Ma granulite facies D2 event that re sulted in reclined, isoclinal F2 folds oriented parallel to a pervasiv e east-trending L2 mineral and stretching lineation. Rocks bearing S2 Structures are cut by the areally extensive approximately 960 Ma Mawso n Charnockite, which was affected by two upright folding events D3 and D4 at approximately 920 Ma. Events D2-4 comprise the Proterozoic Rayn er Structural Episode, which is characterized regionally by the retrog ression of Archaean assemblages. The two pulses of extensive intermedi ate to felsic magmatism accompanied or immediately preceded the Proter ozoic orogenies evident as D2-4, and it is tempting to infer causal li nks. However, field relationships are consistent with intrusion having been contemporary with anomalously high conductive heat fluxes, which could be due to advective heating from larger intrusions not exposed at the present structural level. The formation of extensional, granuli te facies D5 ultramylonite +/- pseudotachylite zones is the last defor mation event in the area.