Tectonic Affinity Of The Shiquanhe-Namco Ophiolitic Mélange Zone In Tibet

The Shiquanhe-Namco ophiolitic mélange zone(SNMZ),together with the Bangong-Nujiang suture zone,records the evolutionary history of the Meso-Tethys Ocean.However,the tectonic affinity of the SNMZ remains highly controversial,and there is no unified understanding of whether it belongs to a in-situ ophiolitic mélange belt or the tectonic nappe of the Bangong-Nujiang suture zone.The unclear tectonic affinity of the SNMZ strongly restricts the in-depth understanding of the evolutionary history of the Meso-Tethys Ocean.To clarify the tectonic affinity of the Shiquanhe-Namco ophiolitic mélange belt,it is necessary to determine the spatiotemporal framework of the ophiolite mélange within the SNMZ.Considering the distribution and research degree of the multiphase ophiolite in SNMZ,this paper selects the ophiolite mélange in Shiquanhe,Lhakorco,Zhongcang and Asa areas for the detailed field geological survey and detailed petrological,petrographic,whole-rock geochemistry and zircon U-Pb chronology studies were carried out on the multiphase ophiolites identified in the field geological survey.In addition,the sedimentology,petrology and detrital zircon U-Pb age of the Lower Cretaceous turbidites exposed in the ophiolitic mélange belt in the Asa,Zhongcang and Lhakorco areas are studied in this paper.On the basis of the above studies,this paper systematically summarizes the spatiotemporal distribution characteristics of ophiolitic mélange in the SNMZ,discusses the tectonic affinity of the SNMZ,and further analyzes the specific indicative significance of the Shiquanhe-Namco ophiolitic mélange belt to the evolution of the Meso-Tethys Ocean.Here we carried a detailed field survey for the Early Jurassic ophiolite in the Shiquanhe and Lhakorco areas where previous research was weak,and we also found the Early Jurassic gabbro in the Asa ophiolitic mélange.The results of the field survey show that the Early Jurassic ophiolite in Shiquanhe area comprises metamorphic peridotite,cumulate gabbro,isrtropic gabbro and basalt,whereas the Lhakorco Early Jurassic ophiolites is mainly composed of metamorphic peridotite,cumulate gabbro,isotropic gabbro,diabase dyke,plagiogranite and minor maeta-basalt.zircon U-Pb dating shows that the formation ages of the Early Jurassic ophiolite in the Shiquanhe and Lhakorco areas are 191-181 Ma and 184-183 Ma,respectively,while that of Early Jurassic gabbro in the Asa area is ?189Ma.The basic rocks of the Early Jurassic ophiolite in these study areas are depleted in high-field-strength elements(HFSEs),weakly enriched in large-ion-lithophile elements(LILEs)but not in light rare earth elements(LREEs),originating from the refractory depleted mantle after MORB melting in the early stage,which was only added by very limited subduction flui materials.The Early Jurassic ophiolites and ophiolitic fragments in Shiquanhe,Lhakorco,and Asa areas all were formed in the fore-arc ocean basin environment of the intra-oceanic subduction system.The Mid-Late Jurassic ophiolite newly discovered in Asa area mainly consists of peridotites,cumulate gabbros,isotropic gabbros,plagiogranites,basalts and greenschists.Zircon U-Pb dating results of cumulate gabbro,isotropic gabbro and plagiogranite show that the formation age of ophiolite in Asa area is around 165-158 Ma.The isotropic gabbro,basalt and greenschist are characterized by varying degrees of enrichment of LREEs and LILEs,strong depleted of HFSEs,suggesting a spinel peridotites mantle source modified by subduction fluids and sediments.Combined with previous research,we suggest that the Mid-Late Jurassic Shiquanhe-Lhakorco-Asa ophiolites have SSZ-type ophiolites sequences,which were formed in the intra-oceanic back-arc basin caused by the further development of the Early Jurassic intra-oceanic subduction zone.However,the Mid-Late Jurassic ophiolites in Yunzhug and Renco area have Penrose-type ophiolite rock assemblages,together with the Early Jurassic ophiolite in Namco area,were both formed in mid-ocean ridge setting.Early Cretaceous ophiolite in Asa area is composed of cumulate gabbro,isotropic gabbro,and pillow basalt,while the Early Cretaceous Zhongcang Ophiolite comprises cumulate gabbro,isotropic gabbro,sheeted dike,massive and pillow lava.Zircon U-Pb dating for the mafic rocks in Asa and Zhongcang ophilities yielded age of 126-114 Ma and126-114 Ma,respectively.Whole-rock geochemical,Sr-Nd isotopic,and zircon Hf isotopic data show that the Asa opholites and pillow basalt in Zhongchang Ophiolite are depleted in HFSEs such as Nb,Ta),which is originated from partial melting of spinel peridotite affected by subduction sediments.The gabbro,diabase-porphyrite dyke and dolerite in Zhongcang Ophiolite,however,have similar geochemical characteristics with MORB,which was formed in the partial melting of spinel mantle peridotite without subduction materials additions.The Early Cretaceous Ophiolite in Asa and Zhongcang may form in the fore-arc oceanic basin formed by the ocean-continental subduction of the Meso-Tethys Ocean.In addition,this paper carried out detailed petrological,sedimentological and provenance analysis studies on the turbidites associated with ophiolite in the Asa,Zhongcang and Lhakorco areas.The results show that the turbidites in the Asa,Zhongcang and Lhakorco areas are all deposited in the bathyal to abyssal basin.The youngest ages of detrital zircon from the turbiditic sandstone samples from the Asa,Zhongcang,and Lhakorco turbidties,together with ?115 Ma U-Pb concordant age from the tuff intercalation within the Asa turbidties,indicate an Aptian age of the Asa,Zhongcang,and Lhakorco turbidties.Sandstone petrographic and detrital zircon analysis combine to show that the Zhongcang and Asa turbidites provide records of the recycled from the accretionary complex and Triassic-Jurassic strata in the southern Qiangtang margin,with the minor addition of Qiangtang magmatic arc rocks,while sediments in the Lhakorco turbidites were derived mainly from recycled orogen source and Late Jurassic-Early Cretaceous magmatic arc in the Lhasa Terrane.Further analysis suggests that the Aptian turbidites were deposited in a deep-marine oceanic basin of Meso-Tethys Ocean and this oceanic basin prevented Qiangtang-derived sediments transported onto the Central Lhasa terrane.Based on our research,we suggest that the SNMZ contains the Early Jurassic,Mid-Late Jurassic,and Early Cretaceous ophiolites,and retains the Lower Cretaceous turbiditic depsoits.Combined with previous research results,we found that after the Jurassic ophiolite within SNMZ was emplaced in the late Jurassic-early Cretaceous,but the Early Cretaceous oceanic crust still existed in Meso-Tethys Ocean,indicating that the Jurassic ophiolite was emplaced before the final closure of Meso-Tethys Ocean.Regional geology comprehensive comparison also shows that the rock assemblage,chronological sequence,tectonic setting and matrix rocks composition of the ophiolite in the SNMZ and the Bangong-Nujiang suture zone are generally not similar.Based on the above evidence,we suggest that the SNMZ is not a tectonic nappe formed during or after the closure of the Meso-Tethys Ocean,but is an in-situ ophiolitic mélange belt records the closure location of several oceanic basins which belongs to the Meso-Tethys Ocean.The rock assemblage,geochronological sequence and geochemical characteristics of the ophiolites in the SNMZ show that the SNMZ is more likely to be part of the Meso-Tethys main suture zone.The ophiolite in the SNMZ belongs to the material remnants of multiple ocean basins formed by the Jurassic Mid-ocean ridge spreading,Jurassic intra-oceanic subduction,and Early Ceataceous ocean-contiental subduction of Meso-Tethys Ocean.In summary,we suggest the Meso-Tethys Ocean developed a northward intra-oceanic subduction arc-basin system in the Jurassic,and there was an expanding mid-ocean ridge in the same period.The initial intra-oceanic subduction occurred in Early Jurassic and formed a fore-arc ocean basin.During the Mid-Late Jurassic,the intra-oceanic subduction zone further developed and formed an arc-back-arc system.The intra-oceanic island arc collided with the Lhasa terrane during the Late Jurassic-Early Cretaceous,resulting in the emplacement of the Jurassic ophiolite in SNMZ.The Mesotethys experienced diachronic closure from east to west during the Early Cretaceous.The diachronous closure resulted in a strong slab roll-back and fore-arc extension in the northward ocean-continental subduction zone,forming the Early Cretaceous ophiolite in the Zhongcang and Asa area.After the final closure of the Meso-Tethys Ocean,the Bangong-Nujiang suture zone and the SNMZ,together constitute the Meso-Tethys suture zone.