Timing of Proterozoic deformation and magmatism in a tectonically reworked orogen, Rayner Complex, Colbeck Archipelago, east Antarctica

Abstract

The high-grade metamorphic rocks of the Colbeck Archipelago comprise granulite facies metasedimentary gneisses and charnockitic 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 charnockitic gneiss. An (?) Archaean granulite facies S 1 gneissosity is preserved in the metasedimentary gneisses and early orthogneiss; the development of S 1 was followed by isobaric cooling from peak metamorphic conditions of T⩾750°C and P=5.1±0.8 kbar. Younger intrusions cut rocks containing S 1 structures, but were deformed by a ≈ 1200 Ma granulite facies D 2 event that resulted in reclined, isoclinal F 2 folds oriented parallel to a pervasive east-trending L 2 mineral and stretching lineation. Rocks bearing S 2 structures are cut by the areally extensive ∼ 960 Ma Mawson Charnockite, which was affected by two upright folding events D 3 and D 4 at ∼ 920 Ma. Events D 2–4 comprise the Proterozoic Rayner Structural Episode, which is characterized regionally by the retrogression of Archaean assemblages. The two pulses of extensive intermediate to felsic magmatism accompanied or immediately preceded the Proterozoic orogenies evident as D 2–4, and it is tempting to 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 formation of extensional, granulite facies D 5 ultramylonite±pseudotachylite zones is the last deformation event in the area.