| Summary: | The high-grade metamorphic rocks of the Colbeck Archipelago comprise granulite facies metasedimentary gneisses and chamockitic gneisses of three ages. A suite of rocks previously mapped as the Colbeck Gneiss is shown to be composite in age and origin, consisting of metasedimentary and orthogneiss that are intruded by granitic gneiss and a biotite :t orthopyroxene chamockitic gneiss. An (?) Archaean granulite facies 81 gneissosity is preserved in the metasedimentary gneisses and early orthogneiss; the development of 81 was followed by isobaric cooling from peak metamorphic conditions of T2750°C and P=5.1i0.8 kbar. The granitic gneiss and biotite i orthopyroxene charnockitic gneiss intrusions cut rocks containing 51 structures, but were deformed by a 21200 Ma granulite facies D2 event that resulted in reclined, isoclinal F2 folds oriented parallel to a pervasive east-trending L2 mineral and stretching lineation. Rocks bearing 82 structures are cut by the areally extensive z960 Ma Mawson Charnockite, which was affected by two upright folding events D3 and D4 at =920 Ma. Events D2_4 form the Proterozoic Rayner Structural Episode, which is characterised regionally by the retrogression of peak M1 assemblages. The two pulses of extensive intermediate to felsic magmatism accompanied or immediately preceded the Proterozoic orogenies evident as D24, and I infer causal links. 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 Proterozoic granulite facies gneisses in the region are cut by numerous myloniteultrarnylonite zones of probable Cambrian age, which comprise the last deformation event (D5) seen in the area. In gamet-absent mafic two pyroxene gneisses and gamet-bearing chamockitic orthogneisses, the mylonite-ultramylonite zones are characterised by the growth of garnet at the expense of ilmenite, pyroxene and plagioclase. ...
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