Further evidence for ∼8 kbar amphibolite facies metamorphism in the Marymia Inlier, Western Australia

Abstract

Pressure estimates for amphibolite-facies metamorphism at Plutonic Gold Mine (Plutonic), Marymia Inlier, Western Australia, were recently revised significantly upwards from ∼4 ± 2 kbar/550–600°C to ≥8 kbar/∼600°C, based on the calculated stability fields for mineral assemblages in garnet-free mafic rocks. These conditions are anomalous in the context of the Yilgarn Craton. Here, we present new mineral equilibria calculations for rare garnet-bearing rock types from Plutonic that confirm those higher pressure estimates, and provide confidence that the determinations of metamorphic conditions based only on results from metamorphosed mafic rocks are robust and reliable. Taken together, the new estimates (7.3–8.2 kbar/580–590°C) from the garnet-bearing rocks, and the existing results from the mafic rocks, provide evidence that, most probably during the late Archean, rocks now exposed along the northern margin of the Yilgarn Craton underwent substantial increases in pressure, which was likely followed by rapid exhumation.

KEY WORDS:

• mineral equilibria
• Plutonic Gold Mine
• Theriak/Domino
• thermobarometry
• thermocalc

ACKNOWLEDGEMENTS

This paper builds on research conducted during MFG's PhD, which was funded by Barrick Australia Pacific Ltd. We are grateful for the in-depth comments of Alistair White and Steve Hollis on an early draft of this paper. The authors acknowledge the constructive reviews of John Ridley and an anonymous reviewer.

SUPPLEMENTAL DATA

Activity composition models for MG027 and MG327