Tracing tectonic processes from Proto- to Paleo-Tethys in the East Kunlun Orogen by detrital zircons

Abstract

The East Kunlun Orogen (EKO), a crucial Tethys composite accretionary belt in the northern Tibetan Plateau, experienced a polyphase tectonic history during an orogenic evolution from the Proto-Tethys to the Paleo-Tethys oceans. It is thus an ideal natural laboratory for tracing the Tethys orogenic cycles. Although numerous studies have been done on the geologic evolution of the East Kunlun Orogen, the timing of initial subduction, final closure, and the transition from the Proto-Tethys to the Paleo-Tethys still remain controversial. This study presents U-Pb-Hf isotopic analyses of 794 detrital zircons from three modern river drainage samples and two sedimentary samples in the EKO. The analysed zircons cluster in five age groups: 2900–2100 Ma, 1900–1300 Ma, 1200–730 Ma, 550–370 Ma, and 290–200 Ma, in which almost 67 % are of Phanerozoic age. The εHf(t) values of the 550–370 Ma detrital zircons have a broad range from −28.8 to + 15.9 and exhibit a trend towards positive values, suggesting that subduction of the Proto-Tethys Ocean initiated at ca. 550 Ma and accretion of the continental arc occurred with increasing addition of mantle material. The εHf(t) values (−8 to + 13) of 290–200 Ma detrital zircons have no obvious evolutionary trend, reflecting the continuous crust-mantle interaction during the evolution of Paleo-Tethys arc. Our results, combined with previous research, reveal two discontinuous and distinct orogenic cycles from the Proto-Tethys to the Paleo-Tethys in the EKO: the Proto-Tethys orogenic cycle from oceanic subduction to continental collision and orogenic collapse at ca. 550–370 Ma, and orogenesis of the Paleo-Tethys Ocean at ca. 290–200 Ma in the EKO, with a gap of ∼ 80 Myr, during which time there was no subduction, and the extensional setting resulted in rare identifiable zircon record.