Characteristics Of Late Paleozoic Granitearound The Ya Gan Fault Zone In Northernalxa Block

The northern part of the Alxa block is a combination of multi-tectonic units such as the North China plate,the Tarim plate and the Kazakhstan plate andalso a important area for studying the closure process of the Ancient Asian ocean,which located inthe west of Inner Mongolia and in the central part of Southern Central AsianOrogenic Belt(CAOB).The Yagan fault zone is an important structural dividing line in the northernmost part of the Alxa block.In this paper,the formation age,type of source rock,genesis mechanism and tectonic evolution of the granite around the Yagan fault zone are studied from the aspects of petrography,petrography,chronology and volcanic-sedimentary facies.The Chulunanggaoci rock located in the north of the Yagan fault,that has a high Mg index and the mixed source characteristics of the crust and mantle,diferes from the Ganjianggashun rock and the eastern Chagantaolegai rockthat in the south of the Yagan fault.The Late Triassic(226.7±1.8Maand220.5 ± 1.9Ma)monzonitic granites in the Chulunanggaoci rock are of high potassium and partial alumina calc-alkaline series,the type of source rockisI-type granite,which is formed in the post collision.The early Permian(288.5 ± 1.9Ma)diorite and late Permian granite in the Ganjianggashun rock are ofhigh potassium metaluminous calcium alkaline-alkaline series,the source rock for I-type but both characteristics of post-orogenic A-type granite,formed in the collision and post collision environment.The early Permian(284.2 ± 1.7Ma and 269.3 ± 2.4Ma)granites of the eastern Chagantaolegai rock are of high silicon-potassium and littleperaluminous calc-alkaline series,thetype of source rock are I-type granite,formed in the collision and post collision environment.The oceanic tectonic evolution represented by Mingshui-Xiaohuangshan-Yagan fault zone(ophiolite zone)can be summarized as follows: Before Ordovician,the oceanic basin on the north side of the Yagan fault zone occurred southward subduction formed Middle Ordovician volcanic rocks in Huhetaoergai,The south side of the Zhusileng-Hangwula tectonic belt is in the post-collisional stretching environment,may form the early Paleozoic volcanic-sedimentary construction;Late Silurian-Middle Devonian,Mingshui-Xiaohuangshan-Yagan oceanic basin dived southward;late Devonian-early Carboniferous,oceanic basin turned northward subduction and formed Beishan Mingshui-Ejinaqi Huhetaoergai-Mongolian Harshatewula early Carboniferous volcanic arc;the early Permian-Late Triassic post-collision granite with this paper,indicating that the south branch of Ancient Asian ocean closed in early Permian(288.5±1.9Ma)and ended the evolution of the ocean.