Refining the physical and temporal record of metamorphism during the Delamerian Orogeny

Abstract

The Delamerian Orogen formed via convergence against the southeastern margin of the Gawler Craton during the Late Cambrian and is characterised by low-P, high-T metamorphism. The evolution of the Delamerian Orogen has been extensively studied by several authors through U-Pb zircon and Sm/Nd geochronology, but lacks fundamental characterisation of its thermal history. Metamorphic studies, of where there are few, suggest an ‘anticlockwise’ P-T evolution for the Delamerian Orogen. Garnet-staurolite-andalusite-bearing and sillimanite-bearing metapelites in this study were investigated using U-Pb LA-ICP-MS and have metamorphic monazite ages between 514-480Ma. Calculated phase equilibria modelling constrains metamorphism in the andalusite-staurolite-bearing schists to ~4.6kbar and 570°C, to 3.7kbar and 635°C in the sillimanite-bearing schists, and 1.0-4.4kbar and 790-860°C in the highest-grade parts of the orogen. The coupled 206Pb/238U monazite ages and phase equilibria modelling suggest that the crust deformed in temporal association with the repeated intrusions of granitic material that weakened the comparatively rigid mid-upper crust, leading to the formation of temporally discrete ‘anticlockwise’ P-T paths. Older metamorphic ages on Kangaroo Island and younger ages on mainland South Australia indicate that the thermal locus moved northwards over time, resulting in a metamorphic terrain that gives the appearance of a patchwork orogen of discrete metamorphic subdomains, with the highest-grades surrounding granitic intrusions which occur late in the evolution of the Delamerian Orogen.