The Characteristics And Tectonic Significances Of Indian Granites In Mayigangri Area, Central Qiangtang, Tibet

Qiangtang Area, whose southern boundary is Bangong Lake-Nu River suture zone and northern boundary is Xijir Ulan Lake-Yushu plate suture zone, is located in Qinghai-Tibet Plateau Hinterland, and the Longmucuo-Shuanghu plate suture zone go through the centre of it from West to East. The constitution and evolution of the Qiangtang Area is the key problem of the geological study of this area, and remain many questions to resolve. There are two typical views of the constitution and evolution of this area, they are distinctly different from each other. One thinks that there is a plate suture zone (such as, Longmucuo-Shuanghu-Jitang suture zone) which stands for the ocean between Gondwanaland and Eurasia in Paleo-Tethyan period, located in the cntral Qiangtang Area. The other point deny the existence of suture zone in central Qiangtang Area, and thinks that there is only a Late-Paleozoic Rift ( Chasang-Chabu Rift system).We discover a large volume of granitiods in Mayigangri Area (central Qingtang, North Tibet) during the process of"1:250000 Geological Survey of Qinghai-Tibet Plateau Project (Mayigangri sheet)". The granitiods who has a area of 1818 Km2 is oriented EW. Granites can be divided into four categories according to ages, from early to late: Early Carboniferous, Late Carboniferous, Late Triassic, Late Jurassic. Among them, Indosinian granites dominate, accounting for 76% of the whole area of granites. This paper conducted a detailed study on Indosinian granites of this area, bearing the target of settling the dispute mentioned before.The results show that the Indosinian granites is mainly consisted of (porphyritic-like) two - mica granites and (giant porphyritic) biotite granites, and we can observe a small volume of orthophyre and granodiorite. Indosinian granites intrude into the surrounding rock mass, such as Cameng Formation (C2ch) and Qudi Formation (P1q), but has a sedimentary contact with Kangtuo Formatio(E3k). And Indosinian granites contain large amounts of quartz schist xenoliths. The Indosinian granites contain large amounts of granodiorite xenolithsand fine-grained diorite enclaves, and the general diameter of them is 3 to 5 cm, and generally not more than 30 cm.A / CNK = 0.9～1.99, only a sample is less than 1.1, with an average of 1.47. The Mass Fraction of Corundum standard molecular is 1.21%-8.64%, all of them are more than 1%, with an average of 5.05%. This means that Indosinian granites are strong peraluminous (SP), and also is S-type granite.They are classified into CPGs in Barbarin B. Classification. The peraluminous minerals in Indosinian granites are tourmalines, biotites, muscovites and a few of garnets, but no cordierites. The absence of cordierites maybe implys that the water-content in magmatic source is richer than other general CPGs. Indosinian granites are calc-alkaline.REE content is lower than general granites,∑REE = (31.59 - 262.10)μg / g, with a average of 127.63μg/g, LREE / HREE = 3.87~9.31. REE fractionation is not very significant ((La / Yb) N = 4.37~12.90). And patterns of REE distribution standardized by chondrite inclining right in "V"-shape, being rich in LREE and relatively depleted in HREE with a severe depletion in Eu(δEu=0.26～0.53, the average is 0.36), which maybe implys that there are a large amount of feldspars or hornblendes residual in source area. Trace elements Ba, Sr, Ti are severe depletable, which maybe implys that source area of Indosinian granites is residual of feldspars. Nb, Zr are slightly depletable.w(Al2O3) / w (TiO2) = 16.36%-26.49%, 21.02% on average.They are all are less than 100, and the results suggest that Indosinian granites are generated in the "high temperature" environment (Figure 20). Therefore, the Indosinian (210-225Ma) SP granites of Mayigangri Area are classified into"high temperature"type. And more than 90% of the SP granites in Southern Tibet didn't intrude until late than Early Cretaceous (K1), and they are classified into MPGs, and most of them are "low-temperature" type (Zhong-Li Liao, 2003a).According Rb-(Y + Nb) diagram (Figure 22), Y-Nb diagram (Figure 24), (FeOt+ MgO) -CaO discrimination diagram (Figure 26), R1-R2 diagram (Figure 23) and the the geochronology of Indosinian granites, we conclude that Indosinian SP granites generate in the"syn-collision"tectonic environments. I propose a hypothesis of the genearation and the evolution of Indosinian granites as follows: during the Indosinian collision between Gondwanaland and Eurasia, the underplating or inject of hot magmas derived from mantle leading to thedehydration-melting of pelitic resources, and the Indosinian"high temperature"strongly peraluminous calc-alkaline granites generated in the end.Indosinian SP granites support the existence of plate suture zone in the study area. Indosinian SP granites in Mayigangri area describe the early evolution history of Qinghai-Tibet Plateau, and make sure that the closing time of Paleo-Tethys is Late Triassic.