Structure and age of the northern Leeuwin Complex, Western Australia: constraints from field mapping and U–Pb isotopic analysis

ABSTRACT

Rocks in the northern Leeuwin Complex of southwestern Australia preserve evidence of having formed during the breakup of Rodinia and the subsequent amalgamation of Gondwana. Detailed field mapping, structural investigation and U–Pb isotopic zircon analysis, using the Sensitive High-mass Resolution Ion Microprobe (SHRIMP), have revealed that: (i) protoliths of pink granite gneiss and grey granodiorite gneiss crystallised at ca 750 Ma, coeval with breakup of western Rodinia; (ii) granulite/upper amphibolite facies metamorphism occurred at 522 ± 5 Ma, in the Early Cambrian, ∼100 million years later than previous estimates and of identical age to estimates of the final amalgamation of Gondwana; and (iii) three major phases of ductile deformation occurred during or after this metamorphism and represent a progressive strain evolution from subvertical shortening (D1) to subhorizontal east–west (D2) then north-northwest–south-southeast (D3) contraction.