40Ar-39Ar thermochronology of the southern Gawler Craton, Australia: Implications for mesoproterozoic and neoproterozoic tectonics of East Gondwana and Rodinia

Abstract

40Ar-39Ar thermochronology of the southern Gawler Craton, Australia, reveals a protracted history of slow cooling, with local crustal heating, rapid cooling, and fault reactivation, between ∼1700 and ∼700 Ma. Hornblendes from most of the southern part of the Craton, including the Kalinjala Mylonite Zone, yield ages ∼1580-1610 Ma, with younger more discordant samples showing evidence of retrogression. These results indicate that the region cooled below ∼500°C ∼100 m.y. after the Kimban Orogeny and coincident with the Gawler Range-Hiltaba Suite magmatism, to the north, perhaps associated with postorogenic extension. Biotite and K-feldspar data indicate that regional cooling after this time was more gradual, with a few exceptions. Locally, in the southeastern Eyre Peninsula, rapid cooling at ∼1425 Ma is indicated by nearly concordant hornblende and biotite ages and may record a phase of regional extension. Other exceptions include areas of lower grade Archean rocks that cooled to <350°C by ∼1700-1640 Ma. Greenschist facies reactivation of shear zones occurred at ∼1100 Ma, during the Musgravian Orogeny, on the basis of biotite single-grain data and retrogressed amphiboles. K-feldspar age spectra record cooling at 700-800 Ma following regional intrusion of the Gairdner Dike Swarm and/or denudation during rifting along the Pacific margin of East Gondwana and breakup of Rodinia. Tectonothermal events revealed by these results, previous geochronological, and geological studies and available paleomagnetic data are comparable to other cratons and mobile belts in Australia, East Gondwana, and Laurentia, providing evidence that southeastern Australia and North America were adjacent during much of Proterozoic time.