Th‐U powered metamorphism: thermal consequences of a chemical hotspot

Abstract

The Arkaroola region of the northern Flinders Ranges, South Australia, records high geothermal gradient mineral assemblages that are not spatially or temporally associated with intrusive magmatism. Cordierite-bearing schists from the base of a ~12 km thick Neoproterozoic sedimentary sequence known as the Adelaide Rift Complex directly overlie Mesoproterozoic metasedimentary and granitic rocks with regional heat production values of ~7.9 µW/m3 at 580 Ma, two to three times greater than global average values for granitic rocks. We integrate in-situ U–Pb monazite geochronology, Y+HREE-in-monazite thermometry and mineral equilibria modelling to show that rocks at the base of the sedimentary succession record amphibolite facies metamorphism at c. 580 Ma while the overlying sediments were still accumulating. Metamorphism took place under average geothermal gradient conditions in excess of 180°C/kbar (>60°C/km) that propagated to depths of at least 12 km. These thermal gradients persisted for upwards of 150 Ma, maintained by a lack of crustal erosion, and are documented by long-lived crustal anatexis. This system may be the archetypal example of Th–U powered metamorphism, recording the interplay between chemically extreme basement and thermally insulating sedimentary cover.