Characteristics Of Early Permian Intrusive Rocks In Sangejing Area Of Alashang Zuoqi,Inner Mongolia

The study area is located in the Alxa Block,northwestern North China Craton,where diversity of rocks occurs out and experienced multistage tectonic superposition.The tectonic background and evloution of Alxa Block has been a long time debated.Here,we present a combine study of field obseviton and geochemical and geochronological study of the early Permian intrusive rocks in Alxa Block.Integrated with the regional geological background geochemical and geochronological studies,we summarized that the early Permian intrusive rocks in this region formed in an active continental margin,which related to the southward subduction of the Paleo-Asian Ocean.Early Permian intrusive rocks were widely distributed in the study area.dominatly including gabbro,diorite,quartz diorite,granite diorite,monzonitic granite,monzonitic porphyritic granite and biotite porphyritic granite.LA-ICP-MS U-Pb dating is carried on intrusive rocks from the Alxa Block for recognition of granite intrusive age of each stage.Gabbro and diorite sample yeild weighted average²⁰⁶U/²³⁸Pb age of 288.7±2.1 Ma and 286.3±2.7 Ma,respectively.f.U-Pb dating of the medium-fine-grained monzonitic granite gives a weighted average age of 299±2Ma.For the porphyritic monzonitic granite and porphyritic biotite granites yeilds the weighted average ages of 289.77±0.9 Ma and 289.1±1.1 Ma,respectively.The formation time of the pluton is between 286 Ma and 299 Ma.The composition shows an evolutionary trend from basicity to acidity,in which gabbro is formed in the extensional tectonic background.From diorite to quartz diorite to granite diorite,it is typical I-type granites.The acidity of the rocks are obviously enhanced with the magma evolution,the content of SiO₂ and alkalinity increases,especially the K₂O increase significantly,which is consistent with the increase of the content of K-feldspar in the mineral composition.The content of Al₂O₃,TFeO,MgO,TiO₂ and CaO decreased gradually,and the content of K₂O/Na₂O increased.In general,the evolution from early magnesium-rich iron,poor Si,K to late rich Si,K-poor Mg,Fe.The total amount of rare earth elements decreased from early to late,?La/Sm?_N and?Gd/Yb?_N increased,reflecting the ascent of fractional distillation of light and heavy rare earth elements,the main cause of formation is crustal origin,and intrusive rocks are formed on the active continental margin.From medium-fine-grained monzonitic granite to monzonitic porphyritic granite,then to biotite porphyritic granite,it belongs to typical peraluminous calc-alkaline rock and indicated a typical S-type granites.From early to late,SiO₂,K₂O+Na₂O increased slightly,CaO,Al₂O₃,FeO+Fe₂O₃ decreased gradully,K₂O/Na₂O and A/CNK of most rocks are greater than 1.The total amount of rare earth elements in each unit was relatively low,all of which were enrichement on light rare earth elements.The contents of Cs,Rb and Th are relatively high,and the evolution trend is not obvious,which may reflect an island-arc-co-collision tectonic environment.