A metamorphic, geochronological and structural framework for the evolution of the Leeuwin Complex, south west Western Australia

Abstract

The Leeuwin Complex of southwestern Western Australia forms the best exposed component of the Pinjarra Orogen, where early Cambrian deformation and metamorphism represents one of the final events in Gondwana amalgamation. Three structurally different domains within the Leeuwin Complex are identified and a structural model for pure shear (coaxial strain) is preferred for the subhorizontal structures of the central structural domain. This is based on the identification of four phases of progressive deformation within the central structural domain that are interpreted to have occurred during or after peak metamorphism. Subvertical compression accounts for D1 and D2 deformation and D3 represents a transition from subvertical compression to pure shear east-west shortening. D4 structures are interpreted as post peak metamorphism east-west extension.

U-Pb SHRIMP and LA-ICPMS analysis of zircon and monazite constrain peak metamorphism to c522 Ma. Thermobarometric calculations for peak metamorphic mineral assemblages yield average P-T estimations of 7.24 ± 0.61 kbar and 687 ± 28°C for the Leeuwin Complex. P-T calculations based on mineral rim compositions reflect retrograde conditions ~0.5 - 1 kbar less and ~50 - 100°C cooler than peak metamorphic conditions, implying that the Leeuwin Complex remained at mid-crustal levels after peak metamorphism. Analysis of chemical zoning in garnet suggests that after a prolonged period at mid-crustal depths, the Leeuwin Complex was exhumed rapidly.

Sm - Nd isotopic data are presented for selected samples that reflect the two apparent protoliths of the felsic orthogneisses. Epsilon-Nd values are comparable to Sm - Nd isotope data from eastern Antarctica. Evolved Epsilon Nd is interpreted to indicate that the petrogenesis of the Leeuwin Complex protoliths involved partial melting of an enriched crustal source, is interpreted to be the Mesoproterozoic Naturaliste Plateau, which is considered to be a western continuation of the Albany-Fraser crust.