Regional metamorphism in the Selwyn Range area, northwest Queensland ( Australia).

Abstract

The Selwyn Range, which lies at the E margin of the Mount Isa Inlier, consists of pelitic, psammitic, and calcareous sedimentary and basic igneous rocks, which have been intruded by granite plutons and regionally metamorphosed to amphibolite grade. Three metamorphic zones, based mainly on pelitic assemblages, have been recognised in the main prograde sequence: a lower grade (biotite) zone equivalent to uppermost greenschist/lower amphibolite facies, an andalusite-almandine-staurolite zone of amphibolite grade, and an E upper amphibolite grade zone (sillimanite and sillimanite-K-feldspar). Superimposed on the prograde sequence is a variable retrograde assemblage related to later folding. Regional metamorphic isograds are unrelated to the granitoids, which include both pre-tectonic or syn-tectonic and post-tectonic types. Comparisons with experimentally determined reactions and calculations from various geobarometers for higher grade rocks indicate metamorphic pressures of 3-4kb (300-400MPa). Cation exchange equilibria (mainly garnet-biotite) and experimentally determined dehydration reactions indicate equilibration temperatures of approx 450-680oC for the prograde sequence; the staurolite and almandine isograds are estimated to be at approx 550oC, the sillimanite isograd approx 600oC, and the K-feldspar isograd approx 660oC.-Authors Show more