LITHOLOGY AND STRUCTURE OF THE GRENVILLE-AGED (SIMILAR-TO-1.1 GA) BASEMENT OF HEIMEFRONTFJELLA (EAST ANTARCTICA)

The Heimefrontfjella mountains, Western Dronning Maud Land (East Antar ctica), are dominantly composed of Grenville-aged (similar to 1.,1 Ga) rocks, which were reworked during the Pan -African orogeny at similar to 500 Ma. Three discontinuity-bounded Grenville-aged terranes have b een recognized namely (from north to south) the Kottas, Sivorg and Var deklettane terranes. The terranes contain their own characteristic lit hological assemblages, although each is made up of an early supracrust al sequence of metavolcanic and/or metasedimentary gneisses, intruded by various (predominantly granitoid) suites. No older basement upon wh ich the protoliths of these older gneisses were deposited has been rec ognized. In each terrane the older layered gneisses were intruded by v arious plutonic suites ranging in age from similar to 1150 to similar to 1000 Ma. The Vardeklettane terrane is characterized by abundant cha rnockites and two-pyroxene granulite facies parageneses in metabasites , whereas the Sivorg and Kottas terranes were metamorphosed to amphibo lite facies grade. P-T estimates show that peak metamorphic conditions changed from similar to 600 degrees C at 8 kbar in the south, to simi lar to 700 degrees C at 4 kbar in the northern Sivorg terrane, Regiona l greenschist retrogression of high-grade assemblages may be of Pan-Af rican age. The Heimefrontfjella terranes were juxtaposed and pervasive ly deformed during a complex and protracted period of E-W collision or ogenesis in a transpressive regime at similar to 1.1 Ga. This is manif est as early, gently dipping thrust-related shear fabrics (D-1), succe eded by the initiation of an important (D-2) steep dextral shear zone (Heimefront shear zone, HSZ), during which the early fabrics and struc tures were steepened and rotated in an anticlockwise sense. The HSZ is a curvilinear structure which changes from a dextral oblique strike-s lip lateral ramp in the north to a steep dip-slip frontal ramp in the south, where it forms the boundary between the Sivorg and Vardeklettan e terranes. The Pan-African event is manifested as discrete, low- to m edium-temperature ductile to brittle shears (D-3) and numerous K/Ar co oling ages.