内蒙古察右中旗地区万隆昌-大脑包花岗岩成因研究

摘要

内蒙古察右中旗地区位于华北板块北缘中段,紧邻中亚造山带.此次研究对区内大东山复式岩基内的万隆昌-大脑包花岗岩体进行了年代学和地球化学研究,锆石LA-ICP-MS定年结果表明万隆昌-大脑包花岗岩形成于244～247Ma,属中三叠世岩浆活动产物.岩石地球化学研究显示,万隆昌-大脑包花岗岩的SiO2含量变化大(65.9％～76.7％),富碱(K2O+Na2O=8.29％～9.46％),属于碱钙性、弱过铝质Ⅰ型花岗岩;显示了Rb、Th、K、Nd、Hf、Pb和LREE相对富集,Ba、Nb、Ta、P、Ti和HREE相对亏损的特征.万隆昌-大脑包花岗岩在成岩过程中经历了以长石矿物为主的分离结晶作用.其中低硅花岗岩相对于其他高硅样品具有较高的锆石饱和温度TZr(℃)和Mg#值,并显示了富Sr、贫Y和Yb、高的Sr/Y和(La/Yb)N比值的埃达克岩的地球化学特征,表明它们源于增厚的下地壳部分熔融并可能有幔源物质的参与.万隆昌-大脑包花岗岩形成于同碰撞向碰撞后伸展转换的构造背景下,这也暗示在中三叠世华北板块北缘中部地区已与中亚造山带拼贴,并且进入到碰撞后岩石圈伸展减薄的构造环境.%The studied area is located in the middle part of the north margin of the North China Craton (NCC) and adjacent to the Central Asian Orogenic Belt (CAOB).LA-ICP-MS zircon U-Pb dating indicates that the Wanlongchang-Danaobao (WD) pluton was emplaced at about 244～247Ma,and thus was the product of the Middle Triassic magmatism.The SiO2 WD granites show variable SiO2 contents(65.9％～76.7％)and high contents of alkali (K2O + Na2O =8.29％～9.46％),and belong to alkali-calcic and weakly peraluminous Ⅰ-type granites.They are enriched in Rb,Th,K,Nd,Hf,Pb,LREEs and depleted in Ba,Nb,Ta,Sr,P,Ti,HREEs.The fractional crystallization of feldspar,apatite and zircon played an important role in the formation of the WD granites.The WD granites with low SiO2 contents are characterized by low Y,Yb contents,but high zircon saturation temperatures,Mg# value,Sr content,Sr/Y and (La/Yb) N ratios,which indicate that their parental magma might be from the thickened lower crust with some involvement of mantle-derived materials.The WD pluton is likely to be formed in a syn-collisional to post-collisional/post-orogenic tectonic setting,which indicates that the final amalgamation of the CAOB and the margin of NCC had finished prior to middle Triassic and then the margin of NCC turned into a post-collisional/ post-orogenic extensional setting.