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

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 p 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,,^ 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.

Late Pan-African granitoids from the Grove Mountains, East Antarctica: Age, origin and tectonic implications

“…Despite of the absence of direct evidence for ocean closure, the compressional and extensional deformation patterns and clear decompressional textures are preserved in metamorphic rocks from Prydz Bay Carson et al, 1995Carson et al, , 1997Fitzsimons, 1996). This is compatible with a collisional orogeny followed by extensional collapse of a thickened crust.…”

Pan-African extension and near-isothermal exhumation of a granulite facies terrain, Dronning Maud Land, Antarctica

“…In the area around Prydz Bay (70 • E, 70 • S) tectonic extension resulted in an uplift of 10-15 km around 514 Ma (Thost, Hensen & Motoyoshi, 1994;Carson et al 1995;Fitzsimons, Kinny & Harley, 1997;Zhao et al 1997). In Lützow Holmbukta (c. 40 • E, 69 • S) the crust was uplifted from c. 11 to 4 kbar under near-isothermal decompression for a period of 20 ± 10 Myr during the late Pan-African event (Fraser et al 2000).…”

Metamorphic P–T paths of metapelitic granulites from the Larsemann Hills, East Antarctica

“…1) Grew et al, 2012). The paragneiss sequence and composite orthogneiss were also referred to as the Brattstrand Paragneiss and Søstrene Orthogneiss respectively by Fitzsimons (1997) for the metasedimentary unit and mafic-felsic metaigneous unit in the region and adjacent areas along the Prydz Bay coast (Carson et al, 1995a;Dirks and Wilson, 1995;Fitzsimons, 1997;Grew et al, 2012;Stüwe et al, 1989;Thost et al, 1991). The paragneisses on Stornes Peninsula, Stornesbukta and Seal Cove of Mirror Peninsula contain borosilicate minerals prismatine and grandidierite (Carson et al, 1995b;Cooper et al, 2009;Grew et al, 2006Grew et al, , 2007Ren and Liu, 1994;Ren and Zhao, 2004), and a sapphirine-bearing assemblage was also reported from Stornes Peninsula (Grew et al, 2006Ren et al, 2008;Tong and Liu, 1997).…”

“…The paragneisses on Stornes Peninsula, Stornesbukta and Seal Cove of Mirror Peninsula contain borosilicate minerals prismatine and grandidierite (Carson et al, 1995b;Cooper et al, 2009;Grew et al, 2006Grew et al, , 2007Ren and Liu, 1994;Ren and Zhao, 2004), and a sapphirine-bearing assemblage was also reported from Stornes Peninsula (Grew et al, 2006Ren et al, 2008;Tong and Liu, 1997). The paragneisses experienced extensive partial melting and migmatization (Dirks et al, 1993), and were considered to represent part of an extensive basinal sedimentary sequence extending about 130 km from the southern Rauer Group to the Bolingen Islands (Carson et al, 1995a(Carson et al, , 1997. The mafic-felsic orthogneisses and paragneisses were generally interpreted as tectonically interleaved Proterozoic basement and sedimentary cover sequence in the region (Carson et al, 1995a;Dirks and Hand, 1995;Dirks et al, 1993;Fitzsimons and Harley, 1991;Sheraton et al, 1984;Stüwe et al, 1989).…”

“…The paragneisses experienced extensive partial melting and migmatization (Dirks et al, 1993), and were considered to represent part of an extensive basinal sedimentary sequence extending about 130 km from the southern Rauer Group to the Bolingen Islands (Carson et al, 1995a(Carson et al, , 1997. The mafic-felsic orthogneisses and paragneisses were generally interpreted as tectonically interleaved Proterozoic basement and sedimentary cover sequence in the region (Carson et al, 1995a;Dirks and Hand, 1995;Dirks et al, 1993;Fitzsimons and Harley, 1991;Sheraton et al, 1984;Stüwe et al, 1989).…”