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

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 M1 and 4–7 kbar and 750–900°C for M2. 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 equilibria modelling, the veracity of existing conventional P–T estimates cannot be evaluated. The calculated phase diagrams from this study allow the direct comparison of P–T conditions in the Windmill Islands with phase equilibria models from other regions in the Musgrave–Albany–Fraser–Wilkes Orogen. This shows that the metamorphic evolution of the Wilkes Land region is very similar to that of the eastern Albany–Fraser Orogen and Musgrave Province in Australia, and further demonstrates the remarkable consistency in the timing of metamorphism and the thermal gradients along the ~5,000 km strike length of this system.

Significance of post‐peak metamorphic reaction microstructures in the ultrahigh temperature Eastern Ghats Province, India

“…This provides an example where interpretations of conventional thermobarometry effectually coincide with those from calculated phase equilibria (e.g. Morrissey, Hand, & Kelsey, in press). For M2 pressure decrease to follow an interval of M1 near‐isobaric cooling, renewed convergence (e.g.…”

Changing of the guards: Detrital zircon provenance tracking sedimentological reorganization of a post‐Gondwanan rift margin

“…Given that the geochronological, geochemical and even grain morphological characteristics of zircon minerals from crystalline source regions are becoming well established in southern Australia, refined sediment tracking of primary and recycled components, as well as basin correlations, are becoming increasingly possible (Barham, Kirkland, & Danišík, ; Barham et al, , ; Cawood & Nemchin, ; Cawood, Nemchin, Freeman, & Sircombe, ; Haines, Wingate, & Kirkland, ; Lloyd et al, ; MacDonald et al, ; Makulini, Kirkland, & Barham, ; Olierook et al, ; Sircombe & Freeman, ; Veevers, Belousova, & Saeed, ; Veevers, Saeed, Belousova, & Griffin, ). Although the geology of East Antarctica is largely inaccessible due to ice‐cover, extended Australia‐Antarctica connection prior to rifting means many of the major Australian crystalline basement elements are shared with the pre‐rifted adjacent Antarctic margin (Aitken et al, ; Barham, Kirkland, & Hollis, ; Fitzsimons, ; Morrissey, Hand, & Kelsey, ; Morrissey, Payne et al, ). Recent work on Proterozoic to Cenozoic depocentres around the central southern margin of Australia have increased understanding of clastic sediment dynamics in the region and shown remarkable stability of the detrital zircon fingerprint that supports extensive recycling and sustained source to sink connections for extended periods (Barham et al, , ; Haines et al, ; Hou et al, ; Lloyd et al, ; MacDonald et al, ; Reid, Keeling, Boyd, Belousova, & Hou, ; Spaggiari, Kirkland, Smithies, Wingate, & Belousova, ; Veevers et al, ).…”

The Bunger Hills: 60 years of geological and geophysical research

“…5; Tucker 2018) and the local effects of advective heating due to the syn-metamorphic emplacement of magmas (e.g. Windmill Islands; Morrissey et al 2017a). Peak pressures are low to moderate and differences in metamorphic pressure are smaller than differences in temperature (Fig.…”

“…1; Obruchev Hills, Mount Strathcona, Cape Harrison), also record evidence for Mesoproterozoic metamorphic zircon and monazite growth and recrystallization ( c. 1190–1140 Ma; Table I; Mikhalsky et al 2015a, Daczko et al 2018) and charnockite emplacement (1142 ± 3.6 Ma, Obruchev Hills; Alexeev et al 2011). Evidence for Mesoproterozoic high-temperature metamorphism, deformation and magmatism (between c. 1345–1260 and/or c. 1220–1130 Ma) is also preserved in the Windmill Islands (Post 2000, Morrissey et al 2017a), the Albany–Fraser Orogen in south-west Australia (Clark et al 2000, Smithies et al 2015b), Madura and Coompana provinces in southern Australia (Spaggiari & Smithies 2015, Wise et al 2015, Kirkland et al 2017) and Warumpi Province, Musgrave Inlier and southern parts of the North Australian Craton in central Australia (Figs 4 & 5; Morrissey et al 2011, Smithies et al 2011, Tucker et al 2015, Walsh et al 2015, Wong et al 2015). This shared high-temperature Mesoproterozoic evolution is broadly attributed to convergence between the North and West Australian cratons and the Mawson Continent (South Australian and Mawson cratons; e.g.…”

“…Figure adapted from Tucker (2018). Data are from: 1) Southern Aileron/North Warumpi Province (Wong et al 2015), 2) East Warumpi Province (Morrissey et al 2011), 3) East Musgrave Inlier, Kalamurta Gneiss (Tucker et al 2015) and Mulga Park andalusite-bearing rocks (unpublished), 4) West Musgrave Inlier (Walsh et al 2015), 5) Biranup Zone (Bordorkos & Clark 2004), 6) Bunger Hills (Tucker et al 2017), 7) Highjump Archipelago (Tucker & Hand 2016), 8) Windmill Islands (Morrissey et al 2017a), 9) Madura Province (Salisbury Gneiss; Clark et al 2000), 10) East Nornalup Zone (Ragged Basin; Clark et al 2000), 11) East Nornalup Zone (Malcolm Metamorphics; Clark et al 2000), 12) Fraser Zone (Arid Basin; Clark et al 2014), 13) Fraser Zone (gabbro; Glasson et al 2019) and 14) Barren Basin (Fly Dam Formation; Kirkland et al 2016). AND = andalusite, KY = kyanite, SILL = sillimanite.…”