The Geological Characteristics And Significance Of Late Ordovician To Early Silurian Intrusive Rocks In Beishanarea,Nei Monggol

The study area is located in the beishan area where is the crossing of Gansu,Xinjiang and Mongolia,north of the hongliuhe-baiyunshan-xichangjing ophiolite melange belt.The study area are mainly distributed in XingXingXia-MingShui-Han mountain block(?-3)and spring-Dongqiyi mountain early Paleozoic magmatic arc(?-4).Based on t he regional geological and mineral survey projects of 1:50000 Biao mountain(K47E013011),Xianwei well(K47E013012),bone well(K47E014011)and fossil mountain(K47E014012)in Inner Mongolia,this paper selects the late Ordovician period and early Silurian period intrusive rocks that are the most widely distributed in the research area as the research objects on the basis of fully collecting relevant data of predecessors.Based on the field survey of 1:50000 geological units and the collection of various supporting samples,the authors studied the petrology,petrochemistry,geochemistry,isotope chronology,petrogenetic types and tectonic environment of the intrusive rocks in this period.The petrological and petrochemical characteristics show that the continuous evolution traits of late Ordovician gneissic quartz diorite,early Silurian gneissic granodiorite,gneissic monzogranite and gneissic syenogranite in the study area are obvious,which showing the characteristics of homologous magma sequence.The rock shows the characteristics of slightly rich silicon,poor aluminum and magnesium iron,belonging to quasi-aluminum,medium-high potassium calc-alkali series,and highly differentiated type I granite.The total amount of REE showed an overall growth trend,which belonging to the REE enrichment type(LR/HR = 5.86 ~ 21.75),the ?Eu showed an overall increase trend,and the REE distribution curve showed a right-leaning trait.High field intensity elements Nb,Ta,P and Ti are relatively deficient,and incompatible elements La,Ce,Hf,Rb,Th and U are relatively enriched.The spidergram of trace elements shows the traits of Nb,Ta,P,Ti relatively deficient,and Th,La and Hf relatively enriched,which have typical island arc magmatic geochemical characteristics.The petrochemical and geochemical characteristics show that the late Ordovician and early Silurian intrusive rocks in the study area are island arc granite,which is a geological record of the strong subduction of the ancient Beishan-TianShan ocean crust along the line of Hongliuhe-BaiYunshan-Xichang well in beishan area,indicating that the Beishan-TianShan ocean in beishan area was closed in the early Silurian.