The Yongzhu-Ren Co Ophiolite From The Central Tibetan Plateau-New Constraints For The Evolution Of The Meso-Tethys Ocean

The Bangong-Nujiang suture zone is the relic of the Meso-Tethys Ocean,and it plays a key role in understanding the evolution history of the Meso-Tethys Ocean and subsequent collision between the Lhasa and South Qiangtang terranes.It is still unclear for the tectonic affinity,evolution history and even the location of the mid-ocean ridge of this ancient ocean basin.In this study,we present a detailed mapping,petrologic,mineralogical,zircon U-Pb geochronological,geochemical and Sr-Nd-Hf-O isotopic investigations for the Yongzhu and Ren Co ophiolites of the middle part of the Bangong-Nujiang suture zone,as well as arcvolcanic rocks in the north margin of the Lhasa terrane.These data will provide new constraints for the geodynamic evolution of the Bangong-Nujiang Meso-Tethys Ocean.In this study,we undertook detailed geological mapping for the Yongzhu and Ren Co ophiolites in the central Tibetan plateau.A typical Penrose-type ophiolite sequences was newly documented in the Ren Co area.The Ren Co ophiolite comprises serpentinized peridotite,cumulate rock,gabbro/diabase,sheeted dike,massive and pillow lavas,and minor red chert.Whereas the Yongzhu ophiolite is mainly composed by mantle peridotite and sheeted dike,with minor gabbro and basalt,showing a dismembered ophiolite suite.Zircon U-Pb dating for gabbro and plagiogranite yielded ages of 169-147 Ma,which suggest that the Yongzhu and Ren Co ophiolites were formed during the Middle to Late Jurassic.Combining with published data,the ages of ophiolites from the Bangong-Nujiang suture zone focused on the Jurassic(?190-147 Ma)with an time span of?40 Ma.Harzburgite in the Ren Co area shows similar features to those of abyssal peridotite indicating the residues of the oceanic mantle,whereas that in the Yongzhu area mainly exhibits refractory compositions,suggesting multiple melt extraction and melt-rock reaction.Mafic rocks(basalt,diabase and gabbro)of the Yongzhu and Ren Co ophiolites show geochemical features similar to those of typical MORB.Moreover,all samples have positive whole-rock ?Nd(t),and zircon ?Hf(t)and mantle-like ?18O values.These features suggest that these rocks were derived from a long-term depleted mantle source,thus the Yongzhu and Ren Co ophiolites were typical MOR-type ophiolite.Hence,the Yongzhu and Ren Co ophiolites were formed in a mid-ocean-ridge(MOR)setting,and they most likely represented the main oceanic basin of the Bangong-Nujiang Ocean.Further comparing with other ophiolites in the Bangong-Nujiang suture zone,we suggest that the Shiquanhe-Yongzhu tectonic belt probably belongs to or is part of the Bangong-Nujiang suture zone and no isolated Shiquanhe-Yongzhu oceanic basin.In addition,we found a series of intermediate-acid volcanic rocks in the south of the Ren Co,Xinj i area.They lie on the north margin of the Lhasa terrane.These rocks are composed of andesite,dacite,rhyolite and volcaniclastic rocks.Zircon U-Pb dating indicate these volcanic rocks were formed in the Late Jurassic to Early Cretaceous(161-130 Ma).Andesite,dacite and rhyolite had typical arc calc-alkaline signatures,and they were formed in an Andean-type magmatic arc setting.These volcanic rocks were most likely produced by southward subduction of the Bangong-Nujiang Ocean.Therefore,a bidirectional subduction model was suggested for the Bangong-Nujiang Ocean during the Jurassic to Early Cretaceous.In a summary,we suggest that the middle part of the Bangong-Nujiang Ocean were composed of one main ocean and two small ocean basins with the Amdo microcontinent in it.The Bangong-Nujiang Ocean opened and formed a mature ocean basin before the Late Triassic.This ocean started northward subduct beneath the South Qiangtang terrane in the Early Jurassic.Then continental collision between the Amdo microcontinent and South Qiangtang terrane happened in the late Early Jurassic to the early Middle Jurassic.A double-sided subduction of the Bangong-Nujiang Meso-Tethys Ocean could have existed since the Middle Jurassic until finally closed in the Late Jurassic to Early Cretaceous.