Late Cambrian adakitic granite at Kinchina Quarry: implications for the evolution of the Delamerian Orogen

Abstract

The Monarto Granite is exposed as 0.2-3 metre sills at the Kinchina Quarry west of Murray Bridge. These medium-fine grained granite sheets intrude migmatite formed from Early Cambrian Kanmantoo Group turbidites and are interleaved with meta-dolerite sills of similar thickness. The granites and mafic sills appear to be synchronous with the late stages of Delamerian folding and metamorphism and the granite was dated at 495.4 ± 0.6 Ma using U-Pb on monazite. Kinchina granite sills have geochemical features that make them unique amongst other Delamerian granites. They are peralumious, with very low Y (<10ppm) and with high Sr/Y ratios and steep, LREEenriched and HREE-depleted rare earth patterns without any Eu anomalies. Their geochemical characteristics clearly define them as silica-rich adakites. With initial εNd values of +1 -+2, similar to the interleaved mafic sills (εNd +2-+3), these granites have the most primitive (mantle-like) isotopic compositions of all the Delamerian granites. The composition of the Kinchina metadolerites is consistent with a MORB-like parent melt contaminated by about 5-8% continental crust.

Geochemical modelling demonstrates that these adakite melts can form by partial melting of mafic rocks with compositions exactly the same as those of their near contemporary Kinchina sills (including their crustal contamination). Modelling is best matched by ~20% melting in the plagioclase-absent, garnet –CPX-rutile stable field. This eclogite facies melting must occur at pressures >1.4GPa (>50 Km depth) and ~980oC. Such upper mantle pressures suggest that the crustal-contaminated mafic parents were partially melted following delamination from the lower crust.

There is good evidence that the Delamerian Orogeny was terminated by a latest Cambrian phase of rapid uplift, exhumation and erosion, prior to the post-tectonic (480± 5 Ma) phase of voluminous type magmatism. The evidence provided by the timing and petrogenesis of the Kinchina adakites strongly support the suspicion that this terminal Delamerian exhumation was driven by delamination of dense, mafic crustal underplate.