Volcanic rock provenance of Zenong Group in Coqen area of Tibet:geochemistry and Sr-Nd isotopic constraint
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    Abstract:

    Magmatic rocks of Early Cretaceous Zenong Group are extensively developed in the Gangdise belt of Tibet, and the genesis of these rocks has given rise to much controversy. In view of this, the authors sampled and analyzed systematically felsic volcanic rocks of Zenong Group in Coqen area of Gangdise orogenic belt. The results show that magmatic rocks of Zenong Group are characterized by high SiO2 content, relative enrichment of LREE and evident enrichment of LILE, Rb, Ba, Th and U, depletion of such HFS elements as Nb and Ta, and the appearance of Sr negative anomalies, with isotopic features of high Sr (0.70789~0.71017) and low Nd (-3.4~-9.3). They are aluminum-oversaturated (A/CNK=1.09~1.17) and geochemically similar to the crustal magma. In the diagram of εNd(t)-(87Sr/86Sr)i, all the samples fall in the No.4 quadrant and are distributed around the mixture line of the upper crust and the Yarlung Zangbo River MORB-type depleted crust, suggesting the existence of fairly abundant upper crust matters and the participation of small amounts of mantlematerials. Moreover, magmatic rocks of Zenong Group are extensively distributed almost in the whole region. The diagram of La/Sm-La indicates that the felsic magmatic rocks of Zenong Group are probably different partial melting products from the same magma source. The felsic magmatic rocks are greatly enriched in LREE and have a spidergram very similar to that of the upper continental crust. The tectonic-setting discrimination diagram of trace elements implies that magmatic rocks of Zenong Group were formed in a island-arc tectonic setting. Based on previous research results and the latest 1∶250000 regional geological survey, the authors hold that the felsic magmatic rocks of Zenong Group might be products of the dehydration and remelting of the crustal materials caused mainly by the heat of the mantle basic magma with the participation of some mantle materials. The tectonic setting of the magmatic rocks of Zenong Group was probably associated with the southward subduction of the Bangong Lake-Nujiang River Tethys oceanic crust.

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刘 伟,李奋其,袁四化,张万平,卓皆文,王保弟,唐文清,2010,西藏措勤地区则弄群火山岩源区——地球化学及Sr-Nd同位素制约[J].岩石矿物学杂志,29(4):367~376. LIU Wei, LI Fen_qi, YUAN Si_hua, ZHANG Wan_ping, ZHUO Jie_wen, WANG Bao_di, TANG Wen_qing,2010,Volcanic rock provenance of Zenong Group in Coqen area of Tibet:geochemistry and Sr-Nd isotopic constraint[J]. Acta Petrologica et Mineralogica,29(4):367~376.

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