The Geochemical Characteristics And Tectonic Setting About Ophiolite In Yongzhu-Namucuo, Tibet Plateau

This paper study the ophiolite which lies in the middle of Shiquanhe-yongzhu-jiali suturezone between the Yaluzangbu suture zone and Banggong-Nujiang suture zone.The ophiolite isclose to the Banggong-Nujiang suture zone and assume a belt extending in a NW-EW.Theophiolite is mainly in fault contact with Paleozoic strata and Mesozic strata,but there are fewintrusive contacts which overlay by the Neogene. The rock association of Yongzhu-Namucuo ophiolite are relatively complete,which includefive unit ,namely ultramafic rocks unit ,cumulate rocks unit ,sheeted dike complex unit,pillowbasalt unit and radiolarite unit.and ultramafic rocks can be divided into two periods:sphericityultramafic and (without sphericity)ultramafic.According rock association,spatial orientation andthe relation of in the field,Yongzhu-Namucuo ophiolite is divided into six mapping units. According to analysis datas of the samples, in order of ultramafic rocks－cumulate rocks－sheeted dike complex－pillow basalt(1)the content of oxide has clear change law:MgO isobvious declined ； the change of FeO,Fe2O3,MnO is not clear;the change ofAl2O3,SiO2,CaO,K2O,Na2O,TiO,P2O5 is obvious increased by degree.(2)Along with the declineof SI,the differentiation of ΣREE increases,and the sum of ΣREE gradually increase.(3)traceelements are gradually enriched in large-ion lithophile elements(Rb,Sr,Ba,Zr) and high fieldstrength elements(Th,Hf).The evolvement character reflects that the units of Yongzhu-Namucuoophiolite have the feature of the same source evolution coming from the same mantle source. By tectonic setting distinguish diagram,such as FeOt－MgO－Al2O3,FeOt/MgO－TiO2,Ti/Cr － Ni,sheeted dike complex unit and pillow basalt unit mainly fall intoMORB.Secondly fall into the boundary of MORB and IAT.These inclined that the ophiolite hastransitional feature between volcanic arc and oceanic ridge. The ultramafic rocks,cumulate rocks,sheeted dike complex ,pillow basalt are relativelyenriched in LREE,which are distinguished from normal oceanic ophiolite. The sheeted dike 54吉林大学硕士学位论文complex belongs to calc-alkaline series rocks which is consistent with island arc rocks.REE andtrace element are relatively enriched,which is consistent with enriched MORB.The pillow basaltbelongs to alkali rocks,and has the character of high K2O,P2O5,middle TiO2.The LREE andlarge-ion lithophile elements are relatively enriched.These characters reflect the thickness ofcrust is relatively thick during that time. The tectonic setting is a short-termspreading,small-scale,deep and not complete micro-ocean.Integrated the regional tectonicsetting,the author consider that the ophiolite developed on the interarc rift basin. Through U-Pb isotope age study in sheeted dike complex, has gained the isochrone age is114-133Ma.The age is early cretaceous,which indicates the time of formation of sheeted dikecomplex.Through K-Ar isotope age study in pillow basalt,has gained the isochrone age is139Ma.The age is the formation age of pillow basalt.Compare with radiolarite, the age of theophiolite is late Jurassic-early Cretaceous.This age is later than Bangonghu-Nujiang suture zoneand earlier thanYaluzangbujiang suture zone.