Thermal regime of the lower crust in the eastern Khondalite Belt, North China Craton, constrained by Zr-in-rutile thermometry mapping

Abstract

Ultrahigh temperature (UHT) metamorphism represents the thermally extreme metamorphic conditions in the mid- to lower crust, and its generation is significant for our understanding of crustal differentiation and crust-mantle interaction. Several Paleoproterozoic UHT metamorphic localities have been reported from the eastern Khondalite Belt, North China Craton. However, the precise spatial scale that was affected by this UHT metamorphism is still unknown, and therefore the mechanism to generate UHT metamorphism is broadly debated. Zr-in-rutile thermometry is regarded as a valid tool to estimate the near-peak metamorphic temperature of UHT granulites. Here we establish the thermal regime of the lower crust in the eastern Khondalite Belt using large-scale (∼10,000 km2) Zr-in-rutile thermometry mapping. Thirty-eight rutile-bearing garnet-sillimanite gneisses were collected, and rutile that is far from zircon and lacks ilmenite reaction rims was selected for electron probe microanalysis. Results show that the size of UHT metamorphism in the eastern Khondalite Belt is at least 3000 km2. UHT rocks closely correspond with large-scale charnockites, and small exposed mafic rocks imply that UHT metamorphism is not caused by intrusion of mafic magma. UHT metamorphism likely developed during post-collisional extension in which anomalous heat reflects both the radiogenic heating of an overthickened orogen and additionally elevated mantle flux due to subsequent lithosphere extension and asthenosphere upwelling. Nearly all large-scale UHT metamorphic terranes formed prior to ∼550 Ma, but their drivers are probably not simply the same.