The Feature And Tectonic Setting Of Chromitite From The Yilashan Ophiolite In Middle Section Of Bangong-Nujiang Suture Zone,Tibet

The Bangong-Nujiang ophiolite belt and the Yarlung Zangbo ophiolite belt are important components of the Qinghai-Tibet Plateau and are of great significance for studying the evolutionary history of the ancient ocean and crust-mantle dynamics.The Yilashan area is located in the southern part of the middle section of the Bangong-Nujiang suture zone in Tibet,where exposes ophiolites and acid magmatic rocks.The Yilashan ophiolite is mainly composed of strongly altered harzburgite and dunite,gabbro and chromite.Base on detailed field surveys and laboratory study,this paper carried out mineralogy,geochemistry,chronology,and mineral inclusions of chromite and peridotite from Yilashan ophiolite,and the zircon Hf isotopes of the Yilashan granite also have been studied.Besides,this paper make Yilashan ophiolite compare with the previous research achievements of the Bangong-Nujiang ophiolite belt,the Shiquanhe-Yongzhu-Namcu ophiolite belt,and the Robsha ophiolite in the Yarlung Zangbo suture zone,and have achieved the following progress and recognition:?1?Electron microprobe analysis shows that the chromian spinel in the Yilashan chromite has Cr^#values of 64.2⁷3.9,46.9⁷1.6 of Mg^#,TiO₂ of 0.03wt%?0.31wt%,Al₂O₃ is 4.5wt%¹8.7wt%,indicating that the Yilashan chromite is high-Cr type chromite.The platinum group elements of chromite are characterized by IPGE enrichment and PPGE loss with right-dipping distribution patterns and no clear correlation between Pd/Ir and Pt/Pt*,reflecting that the Yilashan ophiolite had experienced the rock-melt reaction.According to the calculation of the oxygen fugacity of chromite,harzburgite and dunite from mantle peridotite,this paper found all of them exhibit the characteristics of the transition from the mid-ocean ridge?MOR?to suprasubduction zone?SSZ?.Combined with the comparative study of other chromite ores,it is believed that the Yilashan chromite may be formed by the reaction of boninte melt with rocks in the subduction environment and undergo a multi-stage evolution process.?2?The weighted average zircon ²⁰⁶Pb/²³⁸U age of the Yilashan gabbro is 162.8±3.0Ma,indicating that the gabbro was formed in the Middle Jurassic.Geochemical studies show that gabbro is relatively rich in light rare earth elements?LREE?,depleted in heavy rare earth elements?HREE?,and strongly enriched in large ion lithophile elements like Rb,K,Th,while high field strength elements such as Ta,Nb,Zr,HF,Ti are deficient.The rare earth and trace elements show the characteristic of island arc volcanic rocks.In the basalt-type tectonic environment discrimination diagram,gabbro not only shows the characteristics of volcanic arc basalt,but also shows the characteristics of mid-ocean ridge basalt.In addition,the heterogeneity of the zircon Hf isotopic composition of gabbro,and the Ce/Pb value which is smaller than the typical mantle,further indicates that gabbro may have undergone multi-stage evolution.?3?The zircon U-Pb dating shows that the age of Early Cretaceous granite in Yilashan is 116.1±1.6 Ma;Geochemistry shows that the granite are relatively enriched in LREE and depleted in HREE.In primitive mantle-normalized spider diagrams,the large ion elements such as K and Rb and the high field strength elements such as Th,Ce and Zr are relatively enriched,and the large ion lithophile elements such as Sr and Ba and the high field strength elements such as Nb,Ta,P and Ti are relatively depleted,which shows the geochemical characteristics of the continental margin arc magmatic rocks.The Yilashan granite is a typical type I granite,which formed by the magma mixing between the melt from remelting of ancient crustal materials and bases whose heat supplied by the mantle-derived magma above the subduction zone,and the mantle-derived magma from the lithosphere under the geodynamic background of bi-directional subduction of Bangong-nujiang ocean.?4?Based on the comparison of the rock assemblage,geochemical characteristics and geochronology between Yilashan ophiolite and Bangong-Nujiang ophiolite and Shiquanhe Namucuo ophiolite,it is considered that the Yilashan ophiolite is mainly composed of strongly altered harzburgite and dunite,gabbro,and podiform chromite.The Yilashan ophiolite was formed in the mid-ocean ridge environment and was transformed by the subduction zone environment.It is believed that there are basic-ultrabasic diagenesis events at Yilashan from 183 to 162 Ma,which indicates that the Bangong-Nujiang ocean basin was converted from expansion to northward subduction of the oceanic crust in the Early Jurassic.It is speculated that the tectonic subdivision of the Yilashan ophiolite is Bangong-Nujiang suture zone.