The timing and duration of high-temperature to ultrahigh-temperature metamorphism constrained by zircon U–Pb–Hf and trace element signatures in the Khondalite Belt, North China Craton

Abstract

Understanding the timing and duration of metamorphism is fundamental to correctly interpreting the geodynamic evolution of orogenic events. Metamorphic constraints are still ambiguous in the Khondalite Belt, North China Craton, so Paleoproterozoic orogenic processes remain unclear. Here, we investigate a suite of high-grade rocks from the Dongpo locality in the Daqingshan Terrane by integrating zircon trace elements and Hf isotopic signatures with U–Pb ages to track zircon formation events and confidently constrain the timing and duration of metamorphism. The application of the Ti-in-zircon thermometer confirms the occurrence of regional HT-UHT metamorphism. Linking zircon HREE patterns and U–Pb ages, in combination with HREE partitioning between zircon and garnet, reveals zircon formation during partial melting to thermal peak stage, and during the post-peak cooling stage. Local reaction control by garnet breakdown has affected some zircons, which show a more radiogenic 176Hf/177Hf ratio (0.28164–0.28174) relative to unaffected grains (0.28128–0.28156). Metamorphic zircon U–Pb ages, combined with results from previous studies, constrain partial melting to UHT conditions with decompression-heating at 1.94–1.90 Ga, post-peak near isobaric cooling at 1.90–1.86 Ga, and the thermal peak very close to ca. 1.90 Ga. Local garnet breakdown occurred during another decompression-heating stage to HT-UHT conditions at 1.86–1.84 Ga, and further cooling occurred until ca. 1.80 Ga. Our work reveals relatively short-lived partial melting (less than 40 million years) in comparison to long-lived cooling stages (nearly 100 million years) in the eastern part of the Khondalite Belt. Two pulses of decompression-heating to HT-UHT conditions in the deep crust could result from repeated asthenospheric upwelling driven by lithospheric extension after subduction-collision. Our study sheds new light on zircon behaviour during HT-UHT metamorphism and elucidates the underlying driver of UHT metamorphism in this part of the Khondalite Belt.