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J4 ›› 2015, Vol. 21 ›› Issue (3): 508-.

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ZirconU-PbAgesandGeochemicalCompositionofthePolunshan PlutonintheTenchongBlock,WesternYunnan

WU Jiade, WANG Yan, XIE Qinglu, QI Yue, LI Junhui, HE Jianfeng, CHEN Fukun   

  • Online:2015-09-20 Published:2015-10-20

Abstract:

 Mesozoic to Cenozoic magmatic activities, widely distributed in the Tethyan belt of western Yunnan, are induced by the long-time complicated orogenic processes. Zircon U-Pb age data and geochemical characteristics of the Polunshan granite, exposed in the Tenchong block, are reported in this study. The Polunshan granite pluton is composed mainly of porphyritic coarse-grained granodiorites. Geochemically, this granite is characterized by high alkali contents and belongs to the high-potassic calc-alkaline rock type. It enriches in large-ion lithosphile elements and depletes in high field-strength elements (Nb, Ta, Ti and P). Results of the zircon U-Pb dating suggest emplacement ages of 66~63 Ma for the magma(s) of the Polunshan granite, pointing to a magmatic product related to the closure of the Neo-Tethys Ocean. Many analysed zircon grains or crystal domains have high contents of U and Th. They yield U-Pb ages of about 35~30 Ma, significantly younger than emplacement time of the granite, but similar to the age of deformation or metamorphism of the Gaoligongshan metamorphic belt. This phanomenon implies that the U-Pb isotopic system of those zircons of high Th-U-contents was reset during the late thermal event. The Polunshan granite exhibits entrenchment feature in Sr-Nd isotopic composition, with initial 87Sr/86Srratios of 0.7158 to 0.7164 and initial εNd values of -14.5 to -15.3, corresponding to Nd modal ages of 2.04~2.10 Ga, suggesting that Paleoproterozoic crustal material as a major source of the magma(s).

Key words:  Tethyanbelt;Tengchong;granite;Sr-Nd-Pbisotopes,zirconage