The Discovery of the New UHP Eclogite from the East Kunlun, Northwestern China, and Its Tectonic Significance

Abstract

The East Kunlun Orogenic Belt (EKOB), northwestern China, recording long-term and multiple accretionary and collisional events of the Tethyan Ocean, belongs to a high-pressure to ultra-high-pressure (HP-UHP) metamorphic belt that underwent complex metamorphic overprinting in the early Paleozoic. In this contribution, we carry out an integrated study, including field investigations, petrographic observations, whole-rock analyses, zircon U-Pb dating, and P-T condition modeling using THERMOCALC in the NCKFMASHTO system for the eclogites, especially for the newly discovered UHP eclogite in the eastern part of EKOB. The eclogites exhibit geochemistry ranging from normal mid-ocean ridge basalt (N-MORB) to enriched mid-ocean ridge basalt (E-MORB). Zircons from the eclogites yield metamorphic ages of 416–413 Ma, indicating the eclogite facies metamorphism. Coesite inclusions in garnet and omphacite and quartz exsolution in omphacite and pseudosection calculation suggest that some eclogites experienced UHP eclogite facies metamorphism. The eclogites from the eastern part of EKOB record peak conditions of 29–33 kbar/705–760 °C, first retrograde conditions of 10 kbar at 9.5–12.5 kbar/610–680 °C, and second retrograde conditions at ~6 kbar/<600 °C. New evidence of the early Paleozoic UHP metamorphism in East Kunlun is identified in our study. Thus, we suggest that these eclogites were produced by the oceanic crust subducting to the depth of 100 km and exhumation. The presence of East Gouli and Gazhima eclogites in this study and other eclogites (430–414 Ma) in East Kunlun record the final closure of the local branch ocean of the Proto-Tethys and the evolution from subduction to collision.