Geochemistry And Significances Of The Vocanic Rocks From Nianbo Formation Of Linzizong Group In Cuomai, Tibet

The Tertiary Linzizong volcanic rocks, widely distributed in southern Gangdese belt in Tibetan plateau, show strong petrological and geochemical relationship with the collsional prcesses between India and Asia. It contains the information from syn-collisioal to post-collisional affinities in the whole group. The study area Cuomai is satuated in the central southern Gangdese belt. It belongs to the southern Gangdese magma arc tectonic unit. The detailed petrological, element and isotopic geochemistry of the volcanic rock of Nianbo formation of Linzizong group in Cuomai area were studied. The petrogenesis, evolution and source region of the rocks were also discussed. On the base of previous studies, the results of this thesis were also compared to the Nianbo formation in Linzhou basin near Lhasa city, and to that distributed in other different regions of southern Gandese belt, in order to find the spatial and temporal change among these volcanic rocks.Most of Nianbo volcanic rocks in Cuomai are belong to prealuminous high-potassium calc-alkaline and calc-alkaline series rhyolite, and some of them are shoshonites. They show LREE enrichment with a significant Eu negative anamaly (the averageδEu is 0.59). The rocks are depleted in Nb,Ta,Ti and other HFSE, and enriched in Rb, Th, U, K and other LILE, showing typical arc-related magmatism affinities. The rocks are enriched in Sr-Nd-Pb (⁸⁷Sr/⁸⁶Sr = 0.709100-0.726661,εNd = -6.9 -9.7, ²⁰⁶Pb/²⁰⁴Pb = 18.626-18.870,²⁰⁷Pb/²⁰⁴Pb = 15.683-15.702, ²⁰⁸Pb/²⁰⁴Pb = 39.169-39.589), exhibiting a mixture source region controled by depleted mantle and the crustal basement materials in Gangdese belt. The isotopes also show a much greater input of continental component in Cuomai, in comparison to the Nianbo formation in Linzhou basin. Reviewing the dating data on the Nianbo formation in diferent region of southern Gangdese belt, the Nianbo volcanic rocks in western part (58 Ma) and the eastern part (57 Ma), are earlier than that in the central part (50 Ma), which indicate that the Linzizong volcanic rocks in the west-east trending Gangdese are possibly the diachronous.