Petrological And Mineralogical Study Of The Purang Mantle Peridotite In Western Yarlung-Zangbo Suture Zone,Tibet

The Yarlung-Zangbo ophiolite belt,located in southern Tibet,is one of the extensively studied ophiolites in China.Distributed in east-west direction with a distance of ca.2000 km,this ophiolite had been used as an important petrological indicator to constrain the suture zone between India and Asia,and represents the vanished Neo-Tethyan Ocean between them.The Purang ophiolite in the Yarlung-Zangbo suture zone in Tibet is characterized by containing a large mantle peridotite massif of ca.600 km2 in area.The mantle peridotite consists of dominant harzburgite,minor lherzolite and dunite,and contains some dikes or veins of pyroxenite,gabbro and diabase in the massif.Recently,numerous unusual mantle minerals with the possible deep source have been recovered from podiform chromites in the Luobusa ophiolite within the eastern Yarlung-Zangbo suture zone,Tibet.This new finding has attracted high attention in geosciences field and arose many significant issues.This thesis are based on Purang mantle peridotite that are western ophiolite of Yarlung-Zangbo.Basing on detailed field survey,scientific drilling(PLSD-1)and taking a large number of mantle peridotite samples,then carried out systematic mineralogy,geochemical composition and U-Pb iso tope study for study on mantle peridotite formation process,various proceedings and recognitions have been achieved:1)The mantle peridotite(lherzolite,harzburgite,lenticular dunite)is the remnants of partial melted MOR mantle altered by SSZ fluids.The evidences include:field output feature,mineral composition,geochemistry and other characteristics,which are show Purang mantle peridotite had the characteristic of abyssal peridotete and fore-arc peridotete,indicating that it throw a multi-stage evolution and partial melting process.2)The gabbro has a MOR-type composition characterized by having a nearly flat chondrite-normalized REE pattern with only slightly depleted LREE,an N-MORB-normalized spider diagram shows enrichment in LILE and slight depletion in HFSE.U-Pb d ating of zircons from gabbro of the Purang ophiolite yields and age of 130±3 Ma.This age is taken as the crystallization age of the Purang ophiolite.Identified two types dunite:high-magnesium(layered dunite)and low-magnesium dunite(lenticular dunite)found in Purang mantle peridotete.3)After the discovery of diamonds and other UHP minerals in mantle peridotites and chromitites from the Luobusa ophiohte of eastern Neo-Tethyan Yarlung-Zangbo suture.Here,we report the discovery of a similar collectionof minerals in lherzolite of the Purang ophiolite,which lies in the westernmost part of the Yarlung-Zangbo suture,more than 1500 km from the Luobusa massif.The widespread occurrence of UHP minerals in these.ophiolites demonstrates that the Luobusa ophiolite is not a unique diamond-bearing massif,suggesting that UHP and highly reduced minerals may be widely present other ophiolites.