A structural metamorphic study of Broken Hill Block, NSW, Australia

Abstract

The prograde pressure-temperature (P-T) path for the complexly polydeformed Proterozoic Broken Hill Block (Australia) has been reconstructed through detailed structural analysis in conjunction with calculation of compositionally specific P-T pseudosections of pelitic rock units within a hightemperature shear zone that formed early in the tectonic evolution of the terrane. Whilst the overall P-T path for the Broken Hill Block has been interpreted to be anticlockwise, the prograde portion of this path has been unresolved. Our results have constrained part of this prograde path, showing an early heating event (M1) at P-T conditions of at least c. 600 °C and c. 2.8-4.2 kbar, associated with an elevated geothermal gradient (c. 41-61 °C km-1). This event is interpreted to be the result of rifting at c. 1.69-1.67 Ga, or at c. 1.64-1.61 in the Broken Hill Block. Early rifting was followed by an episode of lithospheric thermal relaxation and burial, during which time sag-phase sediments of the upper Broken Hill stratigraphy (Paragon Group) were deposited. Following sedimentation, a second tectonothermal event (M2/D2) occurred. This event is associated with peak low-pressure granulite facies metamorphism (c. 1.6 Ga) and attained conditions of at least 740 °C at c. 5 kbar. A regionally pervasive, high-temperature fabric (S2) developed during the M2/D2 event, and deformation was accommodated along lithology-parallel, high-temperature shear zones. The larger-scale deformation regime (extensional or shortening) of this event remains unresolved. The M2/D2 event was terminated by intense crustal shortening during the Olarian Orogeny, during which time the first mappable folds within the Broken Hill Block developed.