Metamorphic events in the Eastern Arunta Inlier, Part 1. Metamorphic petrology

Abstract

In the Entia Dome of the eastern Arunta Inlier kyanite-facies metamorphism is recorded over a substantial area (at least 160 km2) and thus contrasts with the regionally extensive belts of high-temperature-low-pressure andalusite-sillimanite facies metamorphism documented elsewhere in the Arunta Inlier. This amphibolite facies event appears to postdate regional granulite facies metamorphism, evidence for which is almost totally obliterated. In the Entia Dome prograde kyanite occurs in meta-basites in comparatively rare associations involving hornblende, staurolite and anorthite, and in other rocks in association with gedrite and hornblende or garnet and biotite. These kyanite-bearing assemblages equilibrated at intermediate temperatures (570–710°C) at peak pressures of ~7 kbar and are overprinted by lower-pressure assemblages including hornblende-gedrite, cordierite and sillimanite. While this sequence of reactions may be simply interpreted in terms of a single “clockwise” orogenic cycle, textural evidence and the available mineral isotopic data suggest a more complex history involving multiple heating and cooling episodes during the Mesoproterozoic, and, possibly, the Palaeozoic. The occurrence of regional retrogressive kyanite-facies assemblages in the eastern Arunta Inlier provides evidence that the widely held perception that Proterozoic crustal evolution in Australia involved a distinct ensialic orogenic style characterised by high geothermal gradients (Etheridge et al., 1987; Loosveld and Etheridge, 1990; Sandiford and Powell, 1991) is not universally true. Rather, petrological evidence, presented here with isotopic constraints described by Foden et al. (1995), suggests that the Harts Range region has experienced substantial crustal reworking at elevated temperatures, during both the Proterozoic and Palaeozoic, under thermal regimes more familiar to modern continental collision zones.