Some isotopic constraints on the evolution of the granulite and upper amphibolite facies terranes in the eastern Musgrave Block, central Australia

Abstract

The central to eastern Musgrave Block, central Australia, consists of the Meso- to Neoproterozoic Fregon and Mulga Park subdomains that are separated by a zone of mylonites and pseudotachylytes known as the Woodroffe Thrust. The subdomains were previously considered to have undergone different geological histories, but the differences relate mainly to metamorphic grade. The Fregon Subdomain attained at least transitional granulite facies metamorphic conditions and the Mulga Park Subdomain reached upper amphibolite facies. Felsic gneisses from both the Fregon and Mulga Park subdomains contain zircons that are structured with euhedral centres overgrown by round elongate rims. Ion-microprobe UPb analyses of the cores yield ages of 1557±24 and 1554±28 Ma, respectively, and are interpreted to represent the time of igneous crystallization. These ages place a maximum age constraint on the deposition or emplacement of the gneiss precursor. High-grade metamorphism, grading from upper amphibolite to granulite facies, took place between 1200 and 1150 Ma and prior to the intrusion of several granitoid suites at around 1150 Ma. The Kulgera Adamellite and Ayers Ranges Adamellite granitoid suites of the Fregon Subdomain yield ion-microprobe UPb crystallization ages of 1150±12 and 1152±11 Ma, respectively. A concordant UPb isotope dilution analysis on the Ayers Ranges Adamellite yields a more precise age of 1152±3 Ma. The porphyritic suite intruding the Mulga Park Subdomain yield an ion-microprobe UPb magmatic age of 1159±20 Ma. These emplacement ages indicate extensive Grenvillian felsic magmatism not previously recognized in Australia. The Woodroffe Thrust formed during a period of substantial crustal shortening, the Petermann Orogeny (∼550 Ma), when the Fregon Subdomain was thrust onto the Mulga Park Subdomain. Synkinematic muscovite and biotite record KAr and RbSr ages between 560 and 520 Ma. This Petermann event is not recorded in the Fregon Subdomain due to confinement of the strain and associated mineral recrystallization to the underlying Mulga Park Subdomain. We suggest that the granulites were not exposed prior to the Petermann Orogeny, as the basal quartzites of the Amadeus Basin occur only on the amphibolite facies Mulga Park Subdomain.