Study On Petrogenesis Of Xiayantan Intrusion In Beidashan Area,Southern Margin Of Alax Block,and Its Geological Significance

The Alax block is located in the central part of the Central Asian orogenic belt and is an important site for the study of the Central Asian orogenic belt.Geotectonic location is at the junction of Tianshan orogenic belt and Xingmeng orogenic belt,where multiple plates converge,and it is also the key to solve the problem of structural attributes of Alxa block.The Xiayantan intrusion is located in the Beidashan mafic-utralmafic rock belt,southwestern margin of the Alax block.The rock mass is situated in the inner part of the arc top of the Alashan arc structural belt..It is mainly composed of gabbro,hornblende gabbro,gabbro-diorite,olive gabbro,pyroxenite and diorite,of which hornblende gabbro,gabbro diorite and diorite are Gradual contact.Metallic minerals are mainly composed of ilmenite,magnetite and a small amount of metal sulfide.Intensive magmatic activity in the Beidashan area produce enormous granite,and a little scattered basic-ultrabasic rocks.The magmatism is mainly divided into three phases,namely the Late Carboniferous-Early Permian,Early Permian-Early Permian,and the end of the Permian.However,the geological settings of magmatic activity in the Beidashan area are still controversy,and intracontinental rift,ancient Asian ocean subduction,and post-orogenic extension have been proposed in previous study.In this paper,petrology,mineralogy,petrochemistry and chronology are performed in the Late Carboniferous intrusion in the Xiayantan area,to research magma source area,fractional crystallization,assimilation and mixing,and sulfide segregation.Besides,comparative study with the contemporaneous intrusive rocks in the adjacent area sheds light on the diagenesis,ore-forming processes and tectonic setting of the Xiayantan intrusion.This study mainly achieved the following points:1.The olivine in the Xiayantan intrusion is mainly chrysolite?Fo=78.72⁸0.20?;the clinopyroxene is mainly diopside,and the orthopyroxeneis mainly composed of bronzite.The plagioclase is mainly composed of anorthite(Ab₉An₉₁);the primary hornblende is present in all types of lithofacies,and the content is relatively high?average 3%,more than 10%sometimes?,.indicating.that.the.magma.is.rich.in.water.The.order.of.mineral.crystallization.is.c hrysolite-plagioclase-?orthopyroxene-Clinopyroxene?-hornblende-metal mineral.2.The Xiayantan gabbro have low TiO₂ content?0.74%?,high Al₂O₃ content?20.3%?,and K₂O<Na₂O.Geochemical characteristics indicate it belongs to a sub-alkaline basalt series.The distribution curve of REE is right-dip,and the trace element spider diagram shows enrichment of large ion lithophile elements?Ba,K,Rb,Sr,U?,depletion of high field strength elements?Nb,Ta,Zr,Hf,Th?,especially negative anomaly of Nb,Ta,Ti,positive anomaly of K(average 1590×10^-6),indicating that the Xiayantan intrusion may be generated in the subduction-related island arc environment.The magma comes from the melting of subducted,and is assimilated by the lower crust.3.The zircon LA-ICP-MS U-Pb dating of the omega gabbro in the Xiayantan rock mass is 311.6±1.9 Ma?MSWD=0.51?,which is the product of the Late Carboniferous magmatism activity,and the wilderness of the neighboring area.The formation of basic rock mass is consistent?310Ma?,the sample of the gabbro in the Yejili(⁸⁷Sr/⁸⁶Sr)i is 0.705242⁷08965,and the?Nd?t?is 0.45⁴.75?t=310 Ma?,with an average of 1.81.The characteristics of the magma source area is the continental lithosphere.4.Studies in petrology,geochemistry,chronology,etc.,combined with regional geological comparisons,the tectonic setting of the subsalt intrusive rocks of the Xiayantan is a subduction-related arc environment,possibly Yabulai-Noergong-Honggueryulin.The westward extension of the magmatic rock belt of the nature of the marginal arc suggests that the southward subduction of the ancient Asian ocean in the Late Carboniferous is not over yet.