Geochronology And Geochemistry Of Magmatic Rock In Yongzhu At North Lhasa Block, Tibet

Lhasa block was bounded by the Indus-Yarlung Zangpo suture in the south and the Bangong-Nujiang suture in the north. There is a large-scale magmatic activity in the North and Central Lhasa block during the Early Cretaceous. The petrogenesis and tectonic setting of these magmatic rocks are controversial. This dissertation investigated Duoni Formation acid vocanic rocks and Xiongmei adamellite in the Yongzhu area of North and Central Lhasa block, Tibet. Zircon U-Pb dating, bulk-rock major element and trace element analysis are main study methods. 24 representative samples, acid vocanic rocks including rhyolite from three locations, Daguo, Shibucuo, of the Duoni Formation and dacite from Shibucuo, were selected for geochemistry test and 3 of them for U-Pb age dating.The zircon U-Pb dating results show that the age for Daguo rhyolite is 116 Ma, the age of Shibuco rhyolite and Xiongmei adamellite is 115 Ma, which may indicate the same magmatic event.Acid vocanic rocks from Duoni Formation and Xiongmei adamellite are poltted in the subalkalic series.The average SiO₂ contents for rhyolite, dacite and adamellite are 75.47%, 68.57%, 73.23%, respectively, Na₂O+K₂O average contents are 8.06%, 8.17%, 7.49%, respectively. In K₂O-SiO₂ diagram, most samples are plotted in the high-K calc-alkaline series while rhyolite samples from west bank of Shibuco are plotted in the shoshonite series. A/NK-A/CNK diagram shows that dacite is meta-aluminous to prealuminous, rhyolite is strongly prealuminous, adamellite is prealuminous. Averge Mg# of the vocanic rocks and the adamellite is 13.33, 53.09, respectively. Normalized trace element patterns shows Rb, Th, U, K, Pb, Zr, Hf positive anormalies and Ba, Nb, Ta, Sr, Ti negative anormalies. Acid vocanic rocks and adamellite have similar patterns, but the acid vocanic rocks are more enriched for positive anormaly and more depleted for negative anormaly than adamellite. In the REE（rare earth element） diagram, all samples are enriched in LREE and depleted in HREE with obvious negative Eu anormalies. Zr/Hf and Nd/Ta values may indicate crust origin. Acid vocanic samples have high SiO₂,K₂O+Na₂O, low CaO, MgO contents, with Zr, Nb, Ce, Y, Ga enrichment, showing A-type granite affinity. Prealuminous geochemistry feature（A/CNK=1.00¹.03）, containing C member in CIPW composition（less than 1%） and hornblende occurrence all show that adamellite is I-type.The acid vocanic rocks in Duoni Formation were derived from upper to central crust, while the adamellite were the melting products of central to lower crust. All samples were formed in a post-collisional extensional stage, when the breaking subducted slab caused the rise of asthenosphere mantle. The samples are mixtures of mantle derived magma and crustal derived magma. Our data show that the Bangonghu-Nujiang Tethys may be closed before 115 Ma.