Thermal modelling of very long-lived (>140 Myr) high thermal gradient metamorphism as a result of radiogenic heating in the Reynolds Range, central Australia

Abstract

The Reynolds Range in the Paleo-Mesoproterozoic Aileron Province of central Australia exposes a differentially exhumed metamorphic rock system varying from greenschist-to granulite-facies. Calculated P–T pseudosections indicate that peak metamorphic conditions in greenschist-facies areas reached temperatures of ∼480 °C and pressures of 3.2–5 kbar. Peak conditions at amphibolite-facies are constrained to 500–620 °C and 1.3–3.8 kbar. P–T modelling of two petrologically different granulite-facies samples indicate that peak conditions in the exhumed granulite-facies rocks reached 780–920 °C and 3.5–5.4 kbar, corresponding to high apparent thermal gradients of 152–218 °C/kbar. In the amphibolite-facies rocks, in situ LA–ICP–MS U–Pb monazite geochronology records ages that range between 1680 and 1550 Ma. In contrast, in the granulite-facies rocks, monazite records ages of ca. 1560 Ma. The terrane contains significantly elevated rates of crustal heat production compared to global norms, and when incorporated into lithospheric thermal models with reasonable boundary conditions and average thermal properties, can plausibly account for the long-lasting metamorphic conditions. The thermal modelling shows that the long lasting (>100 Myr) record of monazite U–Pb ages, supported by previous studies, reflects a continuous duration of elevated crustal temperatures, an interpretation consistent with texturally simple mineral assemblages that lack any evidence for polymetamorphism. In high-crustal heat production terranes, high thermal gradients and high temperatures will persist until exhumation removes the upper crust. This physical reality provides a basis for the existence of arbitrarily long-lived metamorphic systems, even under high thermal gradients that are generally considered to be thermally anomalous. However, such thermal anomalism is not remarkable, as there is abundant evidence for elevated rates of crustal heat production in the Reynolds Range and worldwide. In terranes where accessory minerals record large continuous ranges of age, robust volumetric measurements of crustal heat production rates are a necessary step in evaluating the potential thermal significance of such geochronological records. Failure to document rates of crustal heat generation invalidates many interpretations for drivers of metamorphism.