Geological And Geochemical Characteristics And Tectonic Evolution Of Tanjianshan Group In The Saishiteng Mountain,northern Margin Of Qaidam Basin

The Tanjianshan Group is an important material record of Early Paleozoic ocean-continent evolution in the Northern Margin of Qaidam Basin(NMQB).Based on the systematic geological survey and indoor analysis about dacite,metabasalts and clastic rocks of Tanjianshan Group in the Saishiteng Mountain,including whole-rock chemical analysis,zircon U-Pb dating and Lu-Hf isotopic analysis.We discussed the petrogenesis,formation age and tectonic setting of the rocks.Meanwhile,based on previous research results,we simulated the evolutionary history during Early Paleozoic of the Northern Margin of Qaidam Basin(NMQB).The results are as follows:1.The Tanjianshan Group in the Saishiteng Mountains mainly composed of volcanic rocks,low-grade metamorphic clastic rocks and low-grade metamorphic carbonatite.The rock assemblages suggest that they formed in the island arc and back?arc basin tectonic setting.2.LA-ICP-MS zircon U-Pb geochronology results show that the youngest zircon of metamorphic greywacke is 436 Ma,the formation age of dacite and metabasalts is 442.8±1.8Ma and 443.6±3.4 Ma,respectively.Combined with regional research results,the Tanjianshan group was formed from Cambrian II to Early Silurian(520?436 Ma).3.The dacite is characterized by high Si,alkali-rich,low Mg and Ti,more enriched in LREE relative to HREE,LILEs enrichment of Rb,Th,U and Pb,while depled HFSEs such as Nb?P?Ti and Sr.The protolith of metabasalts is basalt with low Si,rich in Nb(13.3?13.8ppm),Ti O₂(1.39%?1.49%),Na₂O(2.43%?3.02%)and poor in K₂O(0.39%?0.52%).The ratio of(Nb/Th)_PM(1.08?1.63)is greater than 0.6,and the ratio of(La/Nb)_PM(0.84?0.92)is low,relatively rich in HFSEs(such as Th,Pb,Zr and Ti)and LREE.These characteristics can be compared with typical Nb-enriched basalts.To summarise,dacite and metabasalts all have the element distribution characteristics of island arc volcanic rocks.4.The zircon?Hf(t)of dacite is 9.58?15.11 and T_DM2=462?812 Ma.Combined with the geochemical characteristics,it indicates that the dacite is formed by partial melting of crustal materials and underwent different degrees of crystal fractionation during ascent.The metabasalts(protolith is Nb-enriched basalts)is formed by partial melting of depleted mantle wedge,then replaced by adakitic melt produced by subducting oceanic crust.The clastic materials of gravelly metamorphic greywacke are mainly derived from volcanic rocks and magmatic rocks of 436?488 Ma.Meanwhile,the basement rock series of the Dakendaban Group also provides part of the provenance.In addition,the zircon chronological distribution indicates the existence of Archean basement in the NMQB.5.Saishiteng dacite and metabasalts have the characteristics of island arc volcanic rocks,suggesting the western part of NMQB was in the end of subduction in Late Ordovician.Combined with regional magmatic events,it has indicative significance for the tectonic evolution of the NMQB:the initial subduction of the Tanjianshan Ocean was earlier than 520 Ma,and closed during the late Ordovician(?442 Ma),the Tanjianshan island arc magmatic rocks and back arc basin clastic rocks were formed during 520?442Ma.During 441?420 Ma,the Qaidam block was deeply subducted to the Oulongbuluke Block,and about 420 Ma,the subducted High Pressure-Ultra-High Pressure(HP-UHP)metamorphic block was exhumated,forming a HP-UHP metamorphic belt in the NMQB.