A duality of timescales: Short-lived ultrahigh temperature metamorphism preserving a long-lived monazite growth history in the Grenvillian Musgrave–Albany–Fraser Orogen

Abstract

In situ LA–ICP–MS U–Pb monazite geochronology from the eastern Musgrave Inlier, central Australia, suggests that high-T, suprasolidus conditions were maintained for >70Myr during the Grenvillian (c. 1220–1140Ma). Calculated phase equilibria modelling constrain peak metamorphic conditions to ∼900°C at 6.0–6.5kbar. Collectively, the P–T–t conditions reflect that high-T, low–P metamorphism was contemporaneous with similarly thermally extreme metamorphism in the west and central Musgrave Inlier, and therefore that regionally high geothermal gradients (140–150°C/kbar) prevailed across the entire terrane. However, despite an apparently long-lived history of monazite growth, the results of this study also find evidence for arguably shorter-lived, high-T to ultrahigh temperature metamorphism. Conceivably, the preservation of high-T major element compositions and relict Y+REE growth profiles in small (<500μm) garnet grains suggest that the rocks cooled relatively rapidly to sub-diffusive temperatures. At least initially the peak to retrograde P–T path was also isobaric, or experienced a slight increase in pressure. Grenvillian-aged metamorphism in the eastern Musgrave Inlier accordingly appears to also record the effects of a transient thermal anomaly. This likely developed in response to magmatic-loading and the advection of heat from syn-metamorphic magmatic rocks of the Pitjantjatjara Supersuite, which were emplaced at mid-lower crustal depths, within a longer-lived system characterised by deep-seated, mantle-driven regional metamorphism.