Metamorphic petrology of the Winneke domain, Central Australia: P-T-t constraints on the granulite to lower amphibole facies terrain

Abstract

The Winnecke Gorge-Two Mile Bore region (Winnecke Domain) of the Arunta Block, central Australia, contains a highly condensed geological section from greenschist-facies Amadeus Basin cover sequences (Heavitree Quartzite and Bitter Springs Formation), through amphibolite-grade assemblages (Ankala gneiss), to granulite-facies mafic, felsic and calc-silicate lithologies (Strangways Metamorphic Complex). Juxtaposition of these blocks of widely varying metamorphic grade has been facilitated via the activation (and probable reactivation in many cases) of several major sub-parallel, E-W trending shear zones. Greenschist-facies shear zones to the immediate north of the Heavitree Quartzite have previously been demonstrated to have been associated with the Palaeozoic Alice Springs Orogeny. In this study, the petrological character of assemblages from across the Winnecke Domain have been investigated. Thermobarometric estimates (using THERMOCALC and other pressure-temperature estimators) have been used to constrain the change in pressure-temperature conditions across the Winnecke Domain. Pressures obtained from gneissic assemblages increase from around 4.0 kbars in the Winnecke South gneiss (the southern extent of the transect) to around 8.5 kbars in the Cadney metamorphics (the northern extent), and step significantly at major lithological boundaries (i.e. Erontonga metamorphics - Two Mile Bore shear zone contact). The range of pressures obtained from schistose assemblages is quite variable (between 3.3 and 6.8 kbars), and does not increase consistently towards the north. This possibly indicates several phases of activation of shear zones in the region, or it may reflect the presence of variable, but significant, amounts of non-KFMASH components (e.g. Mn) in phases such as garnet. Significant, but variable, potassium and iron metasomatism was typically associated with the development of schists throughout the Winnecke Domain, and was often accompanied by coarse grained biotite, muscovite and magnetite growth. The source of such large quantities of potassium in the potassium-poor granulite terrain is unknown at present. A significant occurrence of a whiteschist (kyanite/talc-bearing) assemblage, the first of its type documented from mainland Australia, is described from the southern margin of the Erontonga metamorphics. The first reported occurrence of a kyanite-bearing schist from the Cadney metamorphics (in the Marbles Bore region) is documented in this study.