Study On The Geochronology Of Pivotal Regions Of Gangdese Magmatic And Yarlung Zangpo Ophilite Belts, Tibet

The Gangdese-Nianqing Tanggula tectonic-magmatic belt locating in the north of Yarlung Zangpo suture and the Yarlung Zangpo ophilite suture are an ideal district of studying the formation and development of Tethyan as well as the process of Tibetan plateau uplifting. It is the most widespread magmatic activities within Tibetan plateau during Mesozoic and Cenozoic, and has undergone such accidents as the ocean basin formation, ocean crust subduction, continental collision and post-collision, as well as plateau formation and evolution and all sorts of complicated geological functions. Nevertheless, the geochronology research in this area in the past is inadequate in both quantity and quality. By having conducted 40Ar-39Ar dating of 35 samples and Sm-Nd dating of 19 samples from the Linzizhong volcanic rocks, the post-collision volcanic rocks and a granite dike and the Yarlung Zangpo ophilite, the dissertation provides significant new data and results for optimizing the time framework of tectonic-magmatic events of this region. According to isotopic dating of 9 samples, 12 experiments survey in the main trunk profile, the eruption age of the Linzizhong volcanic rock in the Linzhou basin has been determined as 54-64Ma. among them Pana group 54-56Ma,Nianbo group 57Ma, Dianzhong group 61-64Ma.The 40Ar-39Ar dating of the Sanidine and biotite in the 5 volcanic rock samples from the Yangying Xiang confirmed that it was post-collisionaal volcanism and gave an extrusive age of UMa. Their geochemical characteristics are different from those in Pana fromation of Linzizhong group.Geochronology research on the volcanic rock profile of the Gazha Cun Group in the Wuyii basin indicates that the 40Ar-39Ar age of the post-collision volcanic rock is 12-14Ma. and the granite that cut the volcanic rock is 10.84Ma. Comparing to the data of others, we infer that the age of the post-collision volcanic rock in Gangdese area is younger eastwards.The results of combined internal mineral separates and whole-rock Sm-Nd isochron age for gabbro dikes from: Luobusha, Angren and Xiubugabu in Yarlung Zangpo ophilite zone are 177Ma, 166Ma and 173Ma respectively. The 4lAr-39Ar step heating spectra of Rikangba ophiolite suggest that its formation age is also ISOMa. It indicates that there was a stronger regionally inagmatism during Middle-Early Jurassic, and the time of I75Ma was the main period of the magmatic event of Yarlung Zangpo ophilite. Based on the measurements of Pb. Sr,Nd initial values, we conclude that Tethyan ocean represented by Yarlung Zangpo ophilite possesses the Indian-Ocean-type isotopic affinity.The internal mineral separates and whole-rock Sm-Nd isochron age for gabbro dikes from the Dangqiong ophiolite is 373?8Ma, inferring a magmatic activity during Late Devonian, which eNd(t) = +3.3 suggests that its tectonic setting was probably oceanic island. We deduce that it is a relic old crust that was brought upwards during the formation of Neo-Tethyan ophiolite.The step heating 40Ar-'9Ar spectra show that during 100Ma, Luobusha ophiolite suffered intensely alternation, which affected distinctly the K-Ar system; during 771Ma some basalt formed in the country rock of Xiubugabu ophiolite; during 55Ma, the pillow basalt of Rikangba ophiolite affected by a strong thermal event; All the variety of ages illustrates possible impacts of the subduction-collision between India and Eurasia.Through dissecting mainly the gabbro of Luobusa ophiolite and studying the 40Ar /39Ar. K-Ar and Sm-Nd ages of the mineral separators and rocks of three gabbro rock among them, the method of dating and choosing object for dating on the base rock has been preliminarily evaluated. The author suggests that for those kinds of samples like ophiolite which formed in the underwater environment and had very low potassium content, it should be better to determine their formation age by the Sm-Nd method as it is less sensitive to alteration. However, the step heating 40Ar-39Ar spectra could often reflect the influence of later geological events on isotope...