Oceanic subduction to continental collision in the NE Proto-Tethys revealed by early Paleozoic eclogites with high-temperature granulite-facies overprinting in the East Kunlun orogenic belt, northern Tibet

Abstract

The East Kunlun orogenic belt in the northern Tibetan Plateau records a long-term accretionary and collisional history in the northeastern Proto-Tethys Ocean, which is important for reconstructing the paleogeography of early Paleozoic East Asia. Here, we present an integrated study combining petrology, geochemistry, geochronology, and metamorphic pressure−temperature (P−T) data of newly found eclogites in the middle Nuomuhong segment of the East Kunlun orogenic belt. The eclogites are composed mainly of garnet, omphacite and low-sodium clinopyroxene, amphibole and plagioclase with minor orthopyroxene, biotite, quartz, accessory rutile, ilmenite, titanite, and zircon. Detailed petrographic observations, conventional geothermobarometry, and phase equilibrium modeling point to the presence of five metamorphic mineral assemblages with corresponding P−T conditions related to: (1) prograde M1 stage P−T estimates at >14.0 kbar/∼470−506 °C; (2) Pmax M2 eclogite-facies stage P−T conditions of ∼26 kbar/∼570 °C; (3) early retrograde M3 high-pressure granulite-facies stage; (4) subsequent M4 retrograde medium-pressure granulite facies at Tmax of ∼860−900 °C; and (5) later M5 retrograde amphibolite-facies stage P−T conditions of <6.2 kbar/∼710−730 °C. These P−T estimates define a clockwise P−T path characterized by heating during the Pmax formation of the eclogite facies, to the Tmax exhumation stage of granulite-facies lithologies, the latter of which is identified for the first time in retrograde eclogites from the East Kunlun orogenic belt. Whole-rock geochemical compositions indicate a mid-oceanic-ridge basalt (MORB) affinity for the eclogite protoliths and a fragmented oceanic crust origin. Sensitive high-resolution ion microprobe (SHRIMP) zircon U−Pb isotopic analyses of the eclogite yielded two groups of weighted mean 206U/238Pb ages of 464 ± 8 Ma and 419 ± 4 Ma, which are interpreted as the age of the eclogite protoliths and the lower threshold for peak eclogite-facies metamorphism, respectively. Our new data, together with regional eclogite-facies metamorphism, suggest a ca. 520−460 Ma age for the subduction of the eastern Kunlun oceanic crust, within the northern Proto-Tethys Ocean, to a depth of ∼83 km, with early subduction−accretionary orogenesis at ca. 419 Ma. Overprinting by high-temperature granulite-facies points to exhumation of oceanic crust to the middle to shallow crustal level at this time. Collectively, the preserved eclogite and high-temperature granulite mineral assemblage provide new constraints on the tectonic evolution and the detailed accretionary-to-collisional orogenesis of the Proto-Tethys Ocean. They suggest that the ca. 428−411 Ma subduction-collisional event marked the termination of the Proto-Tethys Ocean and the eventual formation of an ∼500-km-long, high- to ultra-high−pressure metamorphic belt in the East Kunlun orogenic belt.