The Zircon U-Pb Isotopic Geochronology, Geochemistry And Implications Of The Mafic Rocks In Northern Xinjiang, NW China

The research on Precambrian tectonic evolutions of Tarim block and Yili block is important to understand the geodynamic process of Tianshan orogen. The thesis take research on geochronology and geochemistry to basalt interlayers in Sugetbrak Formation in Aksu, NW Tarim Block and basic dyke swarm in Kusumchik Valley to the east of Lake Sairim at the north margin of Yili Block, to give some implications on the Precambrian evolution of the continents in Xinjiang area.The LA-ICP-MS U-Pb isotopic dating on the zircons of basalt samples from Aksu yielded three groups of ages at ～800Ma,～1400Ma and ～1900Ma. They represent three magmatic and metamorphie events of the basement of northern Tarim at Neoproterozoic, Mid-Proterozoic and Paleoproterozoic, respectively. Because the author did not get any zircon originated from the magma crystallization, it is unable to get the age when the basalt layers formed. However, a youngest age of 755±10Ma is obtained and it might constrain the age of the fonnation of the basalts as to later than-755Ma.11 basaltic samples are analyzed for whole-rock geochemistry, and the results show that these samples are very similar to typical continental flood basalts. Tectonic discrimination diagrams suggest that the basalts formed in an intracontinental extensional environment, the origin of the magma is likely from OIB-type enriched mantle, and they variably suffered crustal contamination.The LA-ICP-MS U-Pb isotopic dating is also taken on zircons from 3 gabbro samples and 1 granitic vein sample. Four similar concordant ages are obtained, namely 783.8±2.4Ma,782.6±4.7Ma,782.4±4.3Ma and 782.7±1.7Ma. It can be confirmed that the four ages represent the formation time of the dyke swarms. In addition, a much older age of～2600Ma is got from the sample 10w10b and it implies the existence of a possilble Neoarchean-Paleoproterozoic basement under the norther Yili block. Whole-rock geochemical analyses are taken on 12 gabbro samples and 1 granitic vein sample, and the results show the mafic dyke swarms is also similar to typical continental flood basalts. It could be recognized as to be formed at an intracontinental extensional environment from the geochemical characteristics, and the origin of the magma is OIB type enriched mantle that suffered heavy crustal contamination; some of it approach to crustal component because the magma stay a long time in upper crust.On the basis of above results and comparison with other basic rocks in the northern part of Xinjiang, the basic rocks in northern Yili and in several places of northern Tarim have similar geochemical characteristics and tectonic background. We can infer that Tarim Block and Yili Block were all in an intracontinental extensional environment in late Neoproterozoic period. Compared with the coeval breakup event of Rodinia supercontinent, it could be proposed that the Tarim Block and Yili Block were all parts of the Rodinia with tight relationship and break off during 790-750Ma, and many continental flood basalts were produced at the margins of these two blocks. The basalt interlayers in Sugetbrak Formation in Aksu and dyke swarm in Kusumchik Valley in northern Yili are remained parts of these continental flood basalts after long times of geological evolution.