Study On The Tectonic Setting For Mafic Rocks In The Xigaze Ophiolites,Tibet

Yarlung Zangbo ophiolites zone in southern Tibet discontinuously exposed along the Yarlung Zangbo river,considered as final structural suture zone between Indian and Eurasian plate,is represented the relics of the Neo-Tethyan oceanic lithosphere.The Xigaze ophiolites located in the middle section of the Yarlung Zangbo Suture Zone(YZSZ),is one of the best-preserved ophiolites.Having been researched decades,the tectonic setting and evolution history of the Xigaze ophiolite,however,are still controversial.One of the important reasons is divergent views still exist about the occurrences of oceanic crust rocks assemblage.This article research for the whole rock geochemical characteristics,zircons age and Lu-Hf isotope of the mafic rocks and the Chongdui Formations in Xigaze,base on the special mapping of ophiolites.Here,we discuss the tectonic setting and timescale of the Xigaze ophiolites.The main conclusions and understanding were as follows:(1)The Xigaze ophiolites are scarcity of cumulates,mainly composed of mantle peridotites,mafic intrusive rocks and basalts,which are similar to the lithological assemblages in slow spreading ridges.The core section of peridotite is composed of harzburgites,lherzolites and dunites,and the edge is tectonite zone.The ophiolites oceanic crust consists mainly of sills and basalts,as well as a few diabasic dikes and/or veins.The diabases downwards intruded into the peridotites and gradually develop upwards into basalts.Diabasic dikes and veins are distributed throughout the ophiolite sequence,constituting a special pattern different from the typical Penrose ophiolites(such as the Oman ophiolites).(2)Mafic rocks in the ophiolites and the Chongdui Formations are continuous in time and space.Basalts are conformably overlain by the cherts and sandstone of the Chongdui Formations on the top section of ophiolites,where typical Oceanic Plate Stratigraphy(OPS)occurred;Statistical peaks of zircon ages from ophiolites crust are127±0.77 Ma,arranging from130 Ma to 120 Ma,which are continuous with radiolarian cherts'(123?112Ma).It suggest the Xigaze ophiolites are probably oceanic lithosphere relics in situ,which origin from the Asian continental margin during 130 Ma to 120 Ma,represented a slow spreading and small ocean basin close to the Asian continental margin.(3)Mafic rocks with different occurrences are similar in geochemistry.Their whole rock Sr-Nd-Pb isotopic and zircon Lu-Hf isotopic data shows a mantle source characterized by depleted mantle(DM);The negative anomalies of Nb and Ta in the oceanic crust might be an inherent attribute of the depleted mantle source rather than the contribution of fluid metasomatism or plate melting in the subduction zone.We suggest that the mantle source have experienced an earlier melting event.