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 stratig raphy, form a cover sequence deposited upon basement of mafic-felsic g ranulite. Their outcrop pattern defines a 10 kilometre wide east-west trending synclinal trough structure in which basement-cover contacts d iffer in the north and the south, suggesting tectonic interleaving dur ing a prograde, D-1, thickening event. Subsequent conditions reached l ow-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 dominan t gneissic foliation in much of the Larsemann Hills, contain a moderat ely east-plunging lineation indicative of west-directed thrusting. D-2 comprises a colinear fold sequence that evolved from early intrafolia l folds to late upright folds. D-3 structures are associated with a hi gh-strain zone, to the south of the Larsemann Hills, where S-3 is the dominant gneissic layering and folds sequences resemble D-2 folding. O utside the D-3 high-strain zone occurs a low-strain D-3 window, preser ving low-strain D-3 structures (minor shear bands and upright folds) t hat partly re-orient D-2 structures. All structures are truncated by a series of planar pegmatites and parallel D-4 mylonite zones, recordin g extensional dextral displacements. D-2 assemblages include coexistin g garnet-orthopyroxene pairs recording peak conditions of similar to 7 kbar and similar to 780 degrees C. Subsequent retrograde decompressio n textures partly evolved during both D-2 and D-3 when conditions of s imilar 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 tect onic cycle involving prograde thrusting and thickening followed by ret rograde extension and uplift. The available geochronological data, how ever, present a number of interpretations. For example, D-2 was possib ly associated with a clockwise P-T path at medium pressures around sim ilar to 1000 Ma, by correlation with similar structures developed in t he Rauer Group, whilst D-3 and D-4 events occurred in response to exte nsion and heating at low pressures at similar to 550 Ma, associated wi th the emplacement of numerous granitoid bodies. Thus, decompression t extures typical for the Larsemann Hills granulites maybe the combined effect of two separate events.