The petrology and geochemistry of granitic gneisses from the East Arunta inlier, central Australia: implications for Proterozoic crustal development

Abstract

The Entia Gneiss Complex is an Early to Middle Proterozoic inlier composed of upper amphibolite facies orthogneisses and paragneisses in the eastern end of the Arunta Block in central Australia. It is separated from overlying, structurally distinct, compositionally different and probably younger, gneisses of the Harts Range Complex, by a subhorizontal detachment zone. This paper discusses the petrology of four major granitic gneiss bodies. One of these (the Bruna Gneiss) has intruded this detachment zone, the other three are components of the structurally underlying Entia Gneiss Complex. The Entia Gneiss Complex has a distinctive subhorizontally layered character with recumbent folds and flat-lying schistosity. The granitic gneisses that are components of this complex appear to have intruded an actively deforming terrain and are folded sheet-like bodies. The orthogneisses of the Entia Complex range from gabbro to true granite in composition and the granites are I-type on both geochemical and petrographic criteria. They are significantly different from the relatively alkali-rich granites of equivalent age which occur in some of the other northern Australian Proterozoic Blocks and are most like the granites of the modern cordilleran belts, particularly in the association of diorite and gabbro with granodiorite. It is suggested they are formed in narrow collisional belts formed as a result of closure of intracratonic rift basins. The high surface-area to volume ratio of these granites, which is a consequence of the deformational environment into which they were intruded, has resulted in considerable wall rock exchange. In the more differentiated end members of the suites this has led to contamination by a net gain of K 2O, Rb and Ba and loss of relatively compatible elements including the REE, Y, Nb and TiO 2. Compared with the syn-tectonic Entia granites, the Bruna Granite Gneiss is post-tectonic and shows geochemical affinities with A-type or anorogenic Palaeozoic granites. Most of this granite is confined to the detachment zone, but the very latest phases also intrude the Harts Range Cover sequence. An important conclusion of this study is that detachment zones of the type described from this area of the eastern Arunta Block, exercise strong control on plutonism into the Early to Middle Proterozoic crust. As long as the very mobile style of Early Proterozoic tectonics continues, the intracrustal dislocation zone confines syn-tectonic granitic magmatism to deeper levels.