Articles

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

C. J. Carson^a1 c1, P. G. H. M. Dirks^a2 p1, M. Hand^a1 p2, J. P. Sims^a1 and C. J. L. Wilson^a1

^a1 School of Earth Sciences, University of Melbourne, Parkville, 3052, Australia

^a2 Institute of Earth Sciences, University of Utrecht, The Netherlands

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, D₁ thickening event. Subsequent conditions reached low-medium pressure granulite grade, and structures can be divided into two groups, D₂ and D₃, each defined by a unique lineation direction and shear sense. D₂ 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. D₂ comprises a colinear fold sequence that evolved from early intrafolial folds to late upright folds. D₃ structures are associated with a high-strain zone, to the south of the Larsemann Hills, where S₃ is the dominant gneissic layering and folds sequences resemble D₂ folding. Outside the D₃ high-strain zone occurs a low-strain D₃ window, preserving low-strain D₃ structures (minor shear bands and upright folds) that partly re-orient D₂ structures. All structures are truncated by a series of planar pegmatites and parallel D₄ mylonite zones, recording extensional dextral displacements.

D₂ assemblages include coexisting garnet–orthopyroxene pairs recording peak conditions of 7 kbar and 780°C. Subsequent retrograde decompression textures partly evolved during both D₂ and D₃ when conditions of 4–5 kbar and 750°C were attained. This is followed by D₄ 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, D₂ 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 D₃ and D₄ 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.

(Received December 02 1993)

(Revised August 29 1994)

(Accepted November 02 1994)

Correspondence:

^c1 Corresponding author.

^p1 Present address: Dept. of Geology, University of Zimbabwe, P.O. Box MP167, Harare, Zimbabwe.

^p2 Present address: Dept. of Geology and Geophysics, Adelaide University, Adelaide, S.A. 5005, Australia.