Geochemistry and Geological Significance of Intrusive Rocks of Early Paleozoic in Western Kunlun, Xinjiang

Nanhua-Early Paleozoic era is the ocean-continent conversion phase in Kunlun Mountains, when the ancient ocean basin was formed by regional expansion along Kegang-Kudi-Qimanyute area. Massive subduction occurred in Ordovician (481-440Ma, subduction-type intrusive rocks developed), and Silurian orogeny ended the oceanic evolution process. Tectonic evolution process of West Kunlun Mountains is reconstructed by studies on the geochemical characteristics of Akazi intrusive rocks as well as its zircon U-Pb dating of LA-ICP-MS research. The Early Paleozoic intrusive rocks can be subdivided into crust-mantle mixed source sequence in late Cambrian, in late Ordovician and in late Silurian. From late Cambrian to late Silurian, intrusive rocks show the following features: firstly, the rock aluminum saturation index(A/CNK) values (0.86→1.23) evolved from more basic to weak acidity; Secondly, chondrite-normalized REE curves show a series of parallel curve clusters, with the amount of REE(228.96→379.39) significantly increasing while δEu (0.79→0.16) decreasing, indicating that the magmatic differentiation grows larger, the negative Eu anomalies increases apparently, and the crustal maturity improve. Besides, trace elements analysis shows enrichment in large ion lithophile elements (LILE), and loss of high field strength elements (HFSE). Combined with geotectonic environment of intrusive rocks formation, the early Paleozoic intrusive rocks were formed in the subduction-extinction-close process of ancient Kunlun Ocean Basin, and in the orogenic stage of collision between Kunlun block and Tarim block. New geological data is thus provided for the study of the subduction-extinction tectonic evolution of ancient Kunlun Ocean in this area.