The Origin Of The Late Carboniferous Alka-Feldspar Granites In The Region Of Sonid Zuoqi,Inner Mongolia

The study area is located in the Sonidzuoqi of the Inner Mongolia Autonomous Region,and the tectonic position is in the east of Central Asian orogenic belt which between the North China plate and the Siberia plate,the east of the CAOB is also callede Xinmeng orogenic belt.Suo Lun Suture Zone divides Xinmeng orogenic belt into southern orogenic belt and the northern orogenic belt,the study area is located in the northern orogenic belt,and across the Uliyasitai active continental margin and Erlianhot–Hegenshan ophiolite arc accretion zone.The study area exposed two alka-feldspar granites,which are located in the area of Zunheremuyinsumu in the north and Taogaotingbuqi in the south.The Cretaceous Baiyin gaolao formation volcano rocks erupted unconformably aroud the Zunheremuyinsumu alka-feldspar granites,in the south the granite intruded into the Carboniferous first section of Baoligaomiao group metamorphic sandstones.The Taogaotingbuqi alka-feldspar granite intuded into Baoligaomiao formation volcano rocks and the earlier Carboniferous fine grained biotite monzogranite,hornblende monzogranite medium grained granodiorite.The minerals of the rock are quartz with two colors including black and colorless,biotite in rich of Fe,and the accessory minerals are Magnetite,limonite,monazite、zircon and so on.The miarolitic structure can also be finded in the rocks.Using the zircon U-Pb isotopic dating analysis obtaind the zircon ²⁰⁶Pb/²³⁸U weighted average age of the Zunheremuyinsumu alka-feldspar granite is 312.2±7.4 Ma,and the weighted average age of Taogaotingbuqi alka-feldspar granite is 304.2±4.4 Ma,two granites are both the products of the late Carboniferous,but the formation time are diffirent.The results of geochemical elements show that rocks are high silica and alkali-rich（Na₂O+K₂O）,but MgO and CaO contents are low,the rocks belong to the high potassium calcium alkaline series.REE distribution curve show a light right trend with obvious negative Eu anomaly,being typical of the"Flying Seagull".In the incompatible trace element spider diagram,the Carboniferous alkali feldspar granites have Ba,Sr,P,Nb（Ta）and Ti negative anomalies,Rb are higher,displaying geochemical affinity of typical A-granite.Geochemical data also show that the two alkali feldspar granite magma is derived from partial melting of basaltic crust.Zunheremuyinsumu alkali feldspar granite has higher radiogenic Pb isotopic composition,means that the crustal basement material run into the magma system.In the A type granite classification diagram,the majority of the north and South alka-feldspar granite samples fall into the A2 region.A large number of calcium alkaline magmatic rocks are distributed in the study area:Carboniferous second section of Baoligaomiao formation（310.9±8.1Ma）and granodiorite-syenite granite intrusive body（313±16.9³15±3.1Ma）are confirmed to be the products of the continental arc environment,the northern alka-feldspar granite is located in the 50 km north of them and formed in the same time.According to the dynamic background of the northward subduction of the paleo Asian Ocean,the alkali feldspar granite is formed in the back arc environment;The formation of the Late Carboniferous alkali feldspar granite with A granite geochemical affinity in the south marked the extensional dynamic background,they are consistent with the bimodal magmatic rocks including gabbro diorite-quartz syenite rock sequences（310.9±2.4³03±2.2Ma）,third section of Baoligaomiao alkaline rhyolite（303.8±3.6Ma）which formed in the same time in the study area,they proved the establition of the extensional tectonic framework after collision,thus the southern alkali feldspar granite body formed in the extensional environment after collision.