Reworking of Archaean and Early Proterozoic components during a progressive, Middle Proterozoic tectonothermal event in the Albany Mobile Belt, Western Australia

Abstract

Isotopic data, derived mainly from UPb zircon ion-microprobe analyses, are presented for each of the three tectonic domains of the Albany Mobile Belt, Western Australia. They show that not only the Northern Domain, but also parts of the Central domain originally crystallised about 3 100 Ma ago; they represent reworked Archaean rocks similar to those of the adjoining Yilgarn Block. On the basis of SmNd model ages, a somewhat younger, less precisely defined crustal formation event of probable Early Proterozoic (∼ 2100 Ma) age can be inferred for the southern Domain and the remainder of the Central Domain. The younger component of the Albany Belt is older than some previous estimates, being of similar initial age to most of the Early Proterozoic orogenic provinces of northern and central Australia. Interpretation of the SmNd data is not strictly based on conventional model ages, for it appears that significant fractionation of the rare-earth elements occurred during Middle Proterozoic crustal melting. Pegmatites and granites dated by UPb on zircon provide a temporal framework for the Middle Proterozoic evolution of the Albany Mobile Belt. Progressive deformation, incorporating foliation development, up to four superimposed fold generations, late-tectonic granite intrusion and conjugate shear zone arrays took place under a consistent orientation of maximum compressive stress over a geologically brief time span, about 1190 Ma ago. Dextral transpression and thrusting at 1190 Ma postdates granulite facies metamorphism and major deformation events in the Fraser Mobile Belt. The widespread 1190 Ma rocks are of comparable age to intrusives rocks emplaced during a similar high-grade metamorphic event in the Bunger Hills of Antarctica (a region that is commonly juxtaposed with the Albany Mobile Belt in Gondwana reconstructions).