A curious case of agreement between conventional thermobarometry and phase equilibria modelling in granulites: New constraints on P–T estimates in the Antarctica segment of the Musgrave–Albany–Fraser–Wilkes Orogen

• Published in: Journal of Metamorphic Geology
• Main Authors: Morrissey, Laura J., Hand, Martin, Kelsey, David E.
• Other Authors: Australian Antarctic Science
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2017
• Subjects: East Antarctica; Wilkes Land; Windmill Islands; Antarc*; Antarctica
• Online Access: http://dx.doi.org/10.1111/jmg.12266; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fjmg.12266; https://onlinelibrary.wiley.com/doi/pdf/10.1111/jmg.12266

Abstract The Windmill Islands region in Wilkes Land, east Antarctica, preserves granulite facies metamorphic mineral assemblages that yield seemingly comparable P–T estimates from conventional thermobarometry and mineral equilibria modelling. This is uncommon in granulite facies terranes, where conventional thermobarometry and phase equilibria modelling generally produce conflicting P–T estimates because peak mineral compositions tend to be modified by retrograde diffusion processes. In situ U–Pb monazite geochronology and calculated metamorphic phase diagrams show that the Windmill Islands experienced two phases of high thermal gradient metamorphism during the Mesoproterozoic. The first phase of metamorphism is recorded by monazite ages in two widely separated samples and occurred at c . 1,305 Ma. This event was regional in extent, involved crustally derived magmatism and reached conditions of ~3.2–5 kbar and 690–770°C corresponding to very high thermal gradients of >150°C/kbar. The elevated thermal regime is interpreted to reflect a period of extension or increased extension in a back‐arc setting that existed prior to c . 1,330 Ma. The first metamorphic event was overprinted by granulite facies metamorphism at c . 1,180 Ma that was coeval with the intrusion of charnockite. This event involved peak temperatures of ~840–850°C and pressures of ~4–5 kbar. A phase of granitic magmatism at c . 1,250–1,210 Ma, prior to the intrusion of the charnockite, is interpreted to reflect a phase of compression within an overall back‐arc setting. Existing conventional thermobarometry suggests conditions of ~4 kbar and 750°C for M 1 and 4–7 kbar and 750–900°C for M 2 . The apparent similarities between the phase equilibria modelling and existing conventional thermobarometry may suggest either that the terrane cooled relatively quickly, or that the P–T ranges obtained from conventional thermobarometry are sufficiently imprecise that they cover the range of P–T conditions obtained in this study. However, without phase ...