Geochronology, structural geology and petrology of the northwestern Lagrange Nunataks, Shackleton Range, Antarctica
The metamorphic rocks of the La Grange Nunataks, northwestern Shackleton Range, have a complex structural and metamorphic history, involving Mid-Proterozoic and Ross- (Cambro-Ordovician) aged events. The protolith of the Stratton Group Mathys Gneiss was emplaced at 2328 ± 47 Ma (U-Pb zircon), and underwent migmatization at 1715 ± 6 Ma. Migmatization took place at P-T conditions of > 640 ºC / > 4-6 kbar and was accompanied by the formation of multiple generations of ductile structures (D1) and the development of local shear zones (D2). Three distinct deformation events (D3 – D5) took place during the Ross Orogeny, affecting the supracrustal Schimper Group, and to a lesser extent, the Mathys Gneiss. The Ross events, which disturbed the U-Pb systems of zircons in the Proterozoic gneisses, are dated by an Sm-Nd garnet age of 535 ± 22 Ma for Schimper Group kyanite-staurolite-garnet-gneiss. The penetrative event D3, which is characterized by west-directed shearing, took place under Barrovian-type peak P-T conditions of 645 ºC / 7.05 kbar, reflecting crustal thickening during the Ross Orogeny. Subsequent polyphase folding (D4) formed the dominant E-W trending fold axes, and can be correlated with a NW-SE to N-S crustal shortening during initial exhumation. The final deformation episode (D5) is characterized by sinistral strike-slip displacement related to a final transpressional overprint. The D5 event took place under low grade metamorphic conditions, in the stability field of chlorite.