Local Rapid Exhumation and Fast Cooling in a Long-lived Paleoproterozoic Orogeny

Abstract

Abstract The exhumation and cooling rates of high-grade metamorphic rocks are crucial for inferring orogenic processes and understanding the regimes of heat transport in the Earth’s crust. Quantification of these rates remains challenging for Precambrian terranes, because the temporal resolution of geochronology becomes coarser in deeper geological time. This limitation is partly reflected by a striking lack of Proterozoic or older short-duration events (<10 Myr); most documented cases of fast metamorphism are confined to the Phanerozoic. In this study, we use garnet geospeedometry to explore the metamorphic rates of Paleoproterozoic high-grade rocks from two representative areas within the long-lived (1·95–1·80 Ga) Jiao-Liao-Ji orogenic belt, North China Craton. The pelitic granulites in the Taipingzhuang area record high-pressure granulite-facies (HPG) metamorphism of ∼12 kbar and ∼800 °C, followed by a fast decompression-cooling to ∼5 kbar and ∼600 °C within ∼5 Myr, at c. 1·87 Ga. The pelitic granulites in the Rizhuang area document a brief (<1 Myr) thermal excursion to ultrahigh-temperature (UHT) metamorphism of ∼8 kbar and ∼940 °C at c. 1·85 Ga, followed by a fast cooling to ∼600 °C within 1–5 Myr. In light of available geological data, the fast decompression-cooling of HPG granulites is interpreted as the syn-collisional exhumation of thickened lower crustal segments at c. 1·87 Ga, most probably through tectonic extrusion. The thermal excursion transiently reaching UHT conditions is inferred to be triggered by localized syn-metamorphic mafic intrusions in association with magmatic underplating during post-collisional extension at c. 1·85 Ga. These metamorphic pulses were interspersed within the protracted Paleoproterozoic orogenesis and require geodynamic processes resembling modern plate tectonics. Notably, these ancient rapid events are beyond the temporal resolution of commonly used in situ geochronology, which tends to yield apparent longer durations given errors and uncertainties. We therefore note that most ancient metamorphic rates might be underestimated using geochronological data, and recommend garnet geospeedometry as a promising alternative approach. The largely similar rates recorded by Paleoproterozoic and Phanerozoic orogens, as well as high-pressure metamorphism at 1·9–1·8 Ga, support the operation of modern plate tectonics in Paleoproterozoic time.