Geological Characteristics And Tectonic Significance Of The Kekesentao Ophiolite Melange In Xinjian

On the foundation of field geological characteristics,rock assemblage characteristics,petrological characteristics,geochemical characteristics and isotope chronology of the Kekesentao ophiolite melange,which is located in the Ertiz fault zone,and former research related with predecessors,this thesis preliminarily draw several conclusions as following:1.According to the outcrop situation,contact relationship,rock association and petrological characteristics of each lithologic unit in the Kekesentao ophiolite melange belt,the sequence of Kekesentao ophiolite was re-divided,and the sequence profile and rock association of Kekesentao ophiolite from bottom to top were restored as follows:ultramafic complex?augite peridotite,olivine pyroxenolite,serpentine??gabbro?basalt?Siliceous rock and radiolarian siliceous slate.In addition,the ophiolitic melange belt also includes schist,gneiss,diorite and biotite plagioclase granite,but there is no sheet dike swarms formed in tensional environment.2.The four groups of LA-ICP-MS zircon U-Pb ages in this paper are as follows:gabbro is405±4Ma,biotite plagioclase gneiss is 365±1.8 Ma,gneissic biotite quartz diorite is 361±2Ma and altered biotite plagioclase granite is 367±3 Ma.Based on previous data,the Kekesenstao ophiolite should have formed between the Early Devonian and the Early Carboniferous.3.Serpentinization is common in ultramafic complexes.The major elements are characterized by poor Al₂O₃,TiO₂,CaO,total alkali and high MgO which similar to depleted mantle.The trace elements are rich in Rb,Th,U and Nb,and deficient in Ta and Ti,showing both the characteristics of the depleted mantle source area and the characteristics of crust-derived fluids in some subduction zones.4.The major elements of gabbro are characterized by high of Al₂O₃,CaO,MgO,TFeO and poor total alkali?especially poor of K₂O?and TiO₂,which similar to ocean tholeiite and island arc basalt.The REE characteristics of gabbro are basically consistent with those of E-MORB,and the magma originates from the enriched mantle.Trace elements are characterized by relative enrichment of large ion lithophile elements of Rb,Th,K and Sr,and relative depletion of high field strength elements of Nb,Ta and Ti,related to fluid metasomatism in subduction zone.In addition,the Lu/Yb ratio indicates that some gabbro rocks are contaminated by crustal materials.5.The major elements of basalts show high of Al₂O₃,MgO,CaO and poor of TiO₂,MnO,P₂O₅ and total alkali?Na₂O>K₂O?.Rare earth elements show that there are three types of basalts:normal mid-oceanic ridge basalt?N-MORB?derived from depleted mantle,enriched mid-oceanic ridge?E-MORB?derived from enriched mantle and island arc basalt contaminated by crust.6.The major elements of biotite plagioclase schist,biotite plagioclase gneiss and biotite quartz diorite are rich in Al₂O₃,TFeO,poor in TiO₂,MgO,CaO and total alkali,especially poor in MnO and P₂O₅.Trace elements show enrichment of large ion lithophile elements of Rb,Ba,Th and high field strength elements Zr,Hf,and obvious depletion of Nb,Ta and Ti,which are consistent with the characteristics of island arc basalts.7.The Kekesentao ophiolitic melange shows not only the characteristics of mid-oceanic ridge,but also the characteristics of island arc,which is consistent with the characteristics of back-arc basin.Therefore,the Kekesentao ophiolitic melange should be formed in the back-arc basin environment formed by the subduction of the Paleo-Asian Ocean to the Siberian Plate from the Early Devonian to the Early Carboniferous.8.In terms of time,spatial distribution and tectonic environment,the Kekesentao ophiolite is similar to the Kuerti ophiolite,the Shaerbulake ophiolite and the Buergen ophiolite,they are all the back-arc systems formed by the subduction of the Devonian Paleo-Asian Ocean to the Siberian plate in the northern margin of Junggar.These ophiolite belts together with the Zhaisang ophiolite belt in Kazakhstan and the Biji ophiolite belt in Mongolia,constitute the suture zone between the Siberian plate and the Junggar-Kazakhstan plate—Ertiz suture zone.