COMPRESSIONAL AND EXTENSIONAL TECTONICS IN LOW-MEDIUM PRESSURE GRANULITES FROM THE LARSEMANN HILLS, EAST ANTARCTICA

Meta-sediments in the Larsemann Hills that preserve a coherent stratigraphy, form a cover sequence deposited upon basement of mafic-felsic granulite. Their outcrop pattern defines a 10 kilometre wide east-west trending synclinal trough structure in which basement-cover contacts differ in the north and the south, suggesting tectonic interleaving during a prograde, D-1, thickening event. Subsequent conditions reached low-medium pressure granulite grade, and structures can be divided into two groups, D-2 and D-3, each defined by a unique lineation direction and shear sense. D-2 structures which are associated with the dominant gneissic foliation in much of the Larsemann Hills, contain a moderately east-plunging lineation indicative of west-directed thrusting. D-2 comprises a colinear fold sequence that evolved from early intrafolial folds to late upright folds. D-3 structures are associated with a high-strain zone, to the south of the Larsemann Hills, where S-3 is the dominant gneissic layering and folds sequences resemble D-2 folding. Outside the D-3 high-strain zone occurs a low-strain D-3 window, preserving low-strain D-3 structures (minor shear bands and upright folds) that partly re-orient D-2 structures. All structures are truncated by a series of planar pegmatites and parallel D-4 mylonite zones, recording extensional dextral displacements. D-2 assemblages include coexisting garnet-orthopyroxene pairs recording peak conditions of similar to 7 kbar and similar to 780 degrees C. Subsequent retrograde decompression textures partly evolved during both D-2 and D-3 when conditions of similar to 4-5 kbar and similar to 750 degrees C were attained. This is followed by D-4 shear zones which formed around 3 kbar and similar to 550 degrees C. It is tempting to combine D-2-4 structures in one tectonic cycle involving prograde thrusting and thickening followed by retrograde extension and uplift. The available geochronological data, however, present a number of interpretations. For example, D-2 was possibly associated with a clockwise P-T path at medium pressures around similar to 1000 Ma, by correlation with similar structures developed in the Rauer Group, whilst D-3 and D-4 events occurred in response to extension and heating at low pressures at similar to 550 Ma, associated with the emplacement of numerous granitoid bodies. Thus, decompression textures typical for the Larsemann Hills granulites maybe the combined effect of two separate events.