Reinterpretation of the tectonic context of high-temperature metamorphism in the Broken Hill Block, NSW, and implications on the Palaeo- to Meso-Proterozoic evolution

Abstract

The origin of high temperatures during regional low-pressure granulite facies metamorphism within the Proterozoic Broken Hill Block, Australia, has been reinterpreted to be the result of burial of anomalously hot rock packages for which the lithospheric geothermal gradient was initially elevated during early rifting ca. 1.71–1.67 Ga, and then maintained during a ca. 1.62 Ga short-lived mid-crustal extensional event. Mid-crustal extension at ca. 1.62 Ga was associated with amphibolite facies metamorphism and elevated lithospheric geothermal gradients, and occurred prior to activity along D2 high-temperature shear zones, peak low-pressure granulite facies metamorphism (ca. 1.60 Ga) and crustal shortening during the Olarian Orogeny (ca. 1.60–1.59 Ga). This reinterpretation places the Broken Hill Block within an environment in which multiple episodes of transient extension (ca. 1.71–1.62 Ga) were followed by a switch to shortening at ca. 1.60 Ga. The interpreted tectonic environment is a continental back-arc setting located in the over-riding plate of a subduction zone along the southern margin of Palaeoproterozoic Australia.