Analysis On Paleozoic Geology And Tectonic Setting Of The Boluokenu Area, West Tianshan

Boluokenu area, located in the joint terrain of Zhungaer microplate and Ilimicroplate with an unique tectonic evolution history, is regarded as a “window” tostudy the evolution of the West Tianshan and even the Central Asian Orogenic Belt.Tectonic movements were active in Paleozoic and Paleozoic formations outcropwidely. Besides, magmatic activities were extensive in Late Paleozoic.The petrological and geochemical analysis of stratigraphy shows that Cambrianstrata deposited in passive continental margin. Middle Ordovician to UpperCarboniferous strata deposited in active continental margin or continental island arc,of which Middle Ordovician Nailenggeledaban Formation tends to active continentalmargin, and Lower Carboniferous Akeshake Formation tends to continental island arc.Lower Permian strata deposited in continental interiors.LA-ICP-MS U-Pb dating of zircon in monzogranite,syenogranite and quartzdiorite from the Husite intrusive body gives the weighted average ages of366.2±4.5Ma,380.2±4.6Ma and372.6±8.4Ma, respectively, indicating its intrusive era oflate Devonian. The age of diorite-porphyrite vein intruded the Husite intrution is372±6Ma. The close ages and similar genesis indicate that this vein and the Husiteintrution have a synchronous emplacment under the same tectonic setting. The age ofthe Husite intrusion gets younger from northwest to southeast and from north to south,corresponding to the “scissor” type closure of North Tianshan Ocean in LatePaleozoic and the migration of magmatic arc to Ili Plate, respectively.The tectonic evolution of Boluokenu area can be proposed as follows. NorthTianshan Ocean opened in Sinian and formed a considerable scale by Cambrian. Thestudy area, located in the north edge of Ili basin, had been a passive continentalmargin until Early Ordovician.North Tianshan Ocean started to subduct beneath Iliplate in Middle Ordovician, and continued into Silurian. The angle of subduction gotsteeper and the subduction got stronger from Middle Devonian. The ocean decreasedconstantly after that. And then, the closure of ocean started from northwest tosoutheast, with the magma arc migrated southward. Zhungaer Plate collided with Ili Plate after the ocean closed during Late Carboniferous, forming an unified continent.In early Permian, the region entered into the intracontinental evolution stage.