Geochemistry Of The Late Carboniferous Volcanic Rocks Of Benbatu Formation In Sonid Youqi, Inner Mongolia And Its Tectonic Significance

IntrodcuctionThe Late Carboniferous volcanic rocks of Benbatu Formation are poorly outcroppedin Inner Mongolia. The research of rock associations and geochemical characteristics ofthe volcanic rocks will provide an important evidence for the tectonic evolution ofPaleo-Asian Ocean in Late Paleozoic.In this paper，the volcanic rocks which outcrop in Sonid Youqi area are the mainresearch object. Fist，the rock associations and geochemical characteristics of the volcanicrocks are solved. Then，by the detailed analysis of the petrologic and geochemical data ofthe volcanic rocks，the characteristics of magma source and genetic process are clearlyillustrated. And，the background of the volcanic rocks will be cleared. Finally，combinedwith the early-late Carboniferous magmatic arc，Heggenshan ophiolite and the stratigraphymaterial，the tectonic evolution of Paleo-Asian Ocean in Late Paleozoic will beelucidated.1． The spatial and temporal distribution of the volcanic rocks of BenbatuFormationThe volcanic rocks of Benbatu Formation mostly outcrop in the northeast part ofSonid Youqi，Inner Mongolia. The volcanic rocks are mainly composed of basalticandesite and a few dacite and andesite. The U-Pb isotopic LA-ICP-MS dating of theandesite（ZQ50-112）shows an age of300.9±1.6Ma. So，the volcanic rocks are formed inLate Carboniferous. 2． The rock associations and geochemical characteristics of the volcanic rocks ofBenbatu Formation in Sonid Youqi areaThe volcanic rocks are mainly composed of basaltic andesite and a few dacite andandesite which belong to low-K tholeiite series.The contents of SiO₂of the volcanic rocks are between52.30%⁶8.35%（Remove theLOI）. They arc characterized by relatively high content of Al₂O₃（12.44%¹5.09%）、TiO2（0.61%².52%）and Na₂O（2.82%⁵.48%），but relatively low content of MgO（Mg#=16⁵3） and K₂O（0.08%⁰.47%）. With the increase of the content of SiO₂，the content ofFeOT、MgO、TiO2、CaO、P2O5、Al₂O₃、K₂O presents a trend of decrease，but the contentof Na₂O shows a trend of increase.The rare earth elements of Benbatu volcanic rocks have similar characteristics，whichare the similar total rare earth element contents （∑REE=46.48μg/g²03.04μg/g） and thesimilar anomalies of Eu （δEu=0.68⁰.90）. TiO₂the chondrite-normalized REE diagram，therocks are characterized by LREE and HREE without significant fractionation and slight orweak depletion of Eu. TiO₂the Primitive mantle-normalized spider diagram，the rocks areenriched in incompatible elements （Such as Th、U、La、Ce、Zr、Hf），but depleted ofhigh filed strength elements（HFSE，such as Nb、Ta、Ti）. The characteristics both on thechondrite-normalized REE diagram and on the Primitive mantle-normalized spiderdiagram are similar to the St.Kitts island arc volcanic rocks. Combined with thecharacteristics of REE and the major elements，the contents change of Benbatu volcanicrocks is considered to be the result of the comagmatic fractional crystallization.3. Nature of magmatic source and Magmatic evolution of Benbatu volcanic rocksCompared with the geochemical characteristics of the mantle-derived primarymagma，the magma of the volcanic rocks of Benbatu Formation are not primary magma，which is not only because the rocks are lack of the mantle-derived inclusions，but alsohave the low Mg#values and low contents of Sc，Co，and Ni.Compared with the petrologic and geochemical characteristics of the ordinaryvolcanic rocks and adakite which both develop in subduction zones，the volcanic rocks are considered to be originated from partial melting of the overlying mantle wedge which ismetasomatized by the fluid which is dehydrated from subducted oceanic crust. And，thefractional crystallization mainly relies on plagioclase in the process of magma evolution.4.The tectonic setting of the volcanic rocksThe association of basaltic andesite，andesite and dacite of the Late Carboniferousvolcanic rocks of Benbatu formation in Sonid Youqi is similar to the island arc volcanicrocks. They have the similar geochemical characteristics with the St. Kitts island arcvolcanic rocks both on the chondrite-normalized REE diagram and on the Primitivemantle-normalized spider diagram. TiO₂the La-Y-Nb diagram，all of the samples fall intothe island arc volcanic rocks area. TiO₂the La/Nb-La/Ti diagram，the rocks have a similarcomposition of the St. Kitts island arc volcanic rocks，which suggest the rocks wereformed in an island-arc environment.5. Regional main magmatic activity from Carboniferous to Early PermianRegionally，An active continental margin magmatic arc which was formed at the lateEarly Carboniferous-the early Late Carboniferous lies from Sonid Zuoqi to Xiwuqi. In thesouth of the magmatic arc，there is the existence of the volcanic rocks of Benbatuformation which were formed at the late Late Carboniferous-early Early Permian，and inthe north of the magmatic arc，there is the existence of the Hegenshan ophiolite which wasformed at early Early Permian.Finally，combined with the regional stratigraphic data，a few basic understandings areobtained：（1）The volcanic rocks of Benbatu formation in Sonid Youqi area，Inner Mongolia，mainly consist of basaltic-andesite， andesite and a little dacite. The geochemicalcharacteristics of volcanic rocks generally belong to the tholeiitic series. Zircon U-Pbisotopic dating revealed that the andesite was formed at300.9±1.6Ma.（2） They are characterized by enrichment in LILE but depletion in HFSE（Nb、Ta、Ti），which is similar to the St. Kitts island arc volcanic rocks.（3）The Paleo-Asian Ocean has subducted northward along the south edge of the Jiamushi-Mongolia block since Early Carboniferous. In Late Carboniferous， thesubduction slab became drop back，which made the Carboniferous magmatic arc movefrom north to south and the formation of Hegenshan ophiolite. So，the Paleo-Asian Oceanis considered to close along the Xar Moron River，and the continental collision couldhappen at Late Permian-Early Triassic.