Geochemical Study On Petrogenesis Of Aral Granite And The Keketuohai No.3Pegmatite Vein, Altay Xinjiag

The Altay mountains is located in the north of Xinjiang, which is an important part of the Central Asian Orogenic Belt, have numerous of granites and pegmatite veins, and famous for rare-element pegmatite deposits. The origin of the Altay pegmatite veins have been controversial. In addition, it has obviously evolutionary relationship between pegmatite and granite in time and space at Archean in Canada Craton. In Altay, the existence of similar evolution is a valuable research direction. According to some researches, pegmatites are mainly formed in the Mesozoic Altai Orogenic Belt, the Keketuohai No.3pegmatite vein is the most typical, so study of Mesozoic granite is particularly important for revealing the origin of rare metal pegmatites. This paper selects the Aral granite batholith as the main research object, which has closely relationship with the Keketuohai No.3pegmatite vein in time and space. Through research the petrology, major elements, trace elements, zircon U-Pb dating, Hf isotope and K-feldspar of the Aral granite determine the petrology classification, genetic types, formation age, material source and the tectonic background, and explore its relationship with the No.3pegmatite vein.Zircon U-Pb dating results show that Aral granite formation in the Triassic(218.1～219.0Ma). Rock chemical analysis of the Aral granite showed that, the content of SiO2in the66.11%～76.51%, the content of Al2O3in the16.03%～12.05%, and with relatively high content of Fe, Mg, Ca (Fe2O3+MgO+CaO=1.99%～8.33%) and alkali (K2O+Na2O=7.37%-10.29%), and the content of K2O>Na2O, and metaluminous-peraluminous characteristic (ASI=0.99-1.16). Enriched in LILEs, such as Rb, Th, K, Ce and Nd, and so on. Lost of HFSEs, such a sBa, Sr, Ta, Nb, P and Ti, etc. Chondrite normalized REE distribution patterns of Aral granite shows LREE enrichment, Eu medium loss (8Eu=0.35～0.77) and the rightward feature. On the tectonic discrimination diagram, almost all of the sample points fall in the VAG and syn-COLG granite area, it can be seen that the Aral granite formed in the volcano to syn-collisional arctectonic background. The Aral granite is crust remelting type granite. The Aral granite is formed by psammitic rocks occur anatexis in the structure pattern from compression to extension under the background of the thickened crust denudation decompression in the collision orogenic environment. Because the Aral granite formation age after the closure of the paleo Asian Ocean(-250Ma), therefore, can speculate the causes Aral granite is related to psammitic rocks occur partial decompression melting under the background of Junggar plate and Siberia plate continental collision orogenic from compression to extension during the Triassic.The K-feldspar of Aral granite containing low Rb and Cs content, and high K/Rb ratio, belonging to the barren granite, significantly different from the K-feldspar in fertile granite and rare-element pegmatite field. Rayleigh fraction law calculations show, it may not be parent fertile granite of the rare-element pegmatite veins in Keketuohai pegmatite field.