Detrital rutile tracks the first appearance of subduction zone low T/P paired metamorphism in the Palaeoproterozoic

Abstract

Earth is unique in the Solar System as the only planet characterised by an active plate tectonic regime. However, there is still no consensus on whether this was fully operational in the Archaean or if it did not develop until the Proterozoic. The metamorphic record offers valuable insight into this debate, since paired, high and low T/P metamorphic conditions are a hallmark of modern plate tectonics. However, much of the metamorphic rock record has either been eroded or overprinted by subsequent tectonothermal events. To address this issue, we examine the detrital rock record and evaluate the potential of rutile, a mineral that forms during subduction and deep-crustal metamorphic processes, as a proxy to track the existence of low T/P and paired metamorphism. At temperatures of 550°C, rutile is mostly stable at pressures higher than ca. 13 kbar. We use this relationship and the Zr-in-rutile thermometer to determine peak metamorphic temperatures of detrital rutile grains to track the temporal distribution of low T/P metamorphism. We have compiled a dataset of detrital metamorphic rutile from Archaean to Permian age to trace the occurrence of low and high T/P metamorphism through time. The earliest evidence of low T/P from detrital rutile is at about 2.1 Ga. This agrees with the earliest evidence of eclogite facies conditions from the rock record and it also implies that these conditions must have been more prevalent than the present rock record seems to indicate. Together, our study confirms that subduction zone low T/P metamorphism has been fully operating since at least the late Palaeoproterozoic.