Compressional and extensional tectonics in low-medium pressure granulites from the Larsemann Hills, East Antarctica

Abstract

Abstract 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 ∼ 7 kbar and ∼ 780°C. Subsequent retrograde decompression textures partly evolved during both D 2 and D 3 when conditions of ∼ 4–5 kbar and ∼ 750°C were attained. This is followed by D 4 shear zones which formed around 3 kbar and ∼ 550°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 ∼ 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 ∼ 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.