Heating–cooling–heating cycles within ca. 70 Myr recorded in UHT granulites in the Khondalite Belt, North China Craton

Abstract

Revealing the thermal evolution history of ultrahigh-temperature metamorphism (UHT) could help shed light on the genesis and evolution of the orogenic crust. However, it is generally difficult to constrain the duration of metamorphism, especially the heating stage due to the complex behavior of the datable accessory minerals (e.g., zircon and monazite). The Khondalite Belt of the North China Craton records Paleoproterozoic UHT metamorphic event which was previously constrained to be ca. 1920 Ma by using the weighted mean age of zircon U–Pb dating results, however, zircon could grow during both prograde and retrograde periods. Thus, the age of ca. 1920 Ma may be an oversimplified explanation and there could be a complex thermal evolution. In this study, combined with zircon U–Pb dating and Ti-in-zircon thermometry, the duration of the UHT metamorphism in the eastern Khondalite Belt was constrained to be 60–70 Myr with two short periods of decompression-heating (both lasting for ∼20 Myr) intervened by a period of cooling process (lasting for ∼30 Myr). This finding further expands our knowledge that there was a heating–cooling–heating cycle rather than a continuously prolonged cooling process in a long-lived UHT metamorphism. Our results show that the UHT metamorphism in the eastern Khondalite Belt requires two stages of lithosphere extension, which were possibly related to shallow slab breakoff and post-collisional lithospheric delamination, respectively. It further indicates that Paleoproterozoic orogenesis, although dominated by subduction of rheologically weak slab, is comparable to the formation of modern Himalaya orogens.