鄂尔多斯西缘南华山古生代埃达克岩地球化学和年代学

鄂尔多斯西南缘地处北秦岭、北祁连、贺兰山构造带交接部位，构造带西段的南华山弧形构造带中发育有大量早古生代花岗岩体（岩株），侵入于下元古界海原群以及中寒武统香山群、下奥陶统阴沟群中。岩性以花岗闪长岩为主，其次为二长花岗岩和石英闪长岩，并有少量角闪奥长花岗岩和斑状花岗岩等，其中普遍含有暗色闪长质包体。地球化学研究表明，岩石主要为准铝-过铝质、钙碱性系列，具有明显高Al₂O₃、低MgO特征，尤其是低Y和Yb、高Sr和Sr/Y比值特征，类似于典型的埃达克岩地球化学组成。同时，岩石明显亏损Nb、Ta、P 和Ti，富集Cs、Rb、Ba、Th、U、K、Pb、Nd、Zr 等。综合分析认为，早古生代祁连俯冲洋壳板片熔融，熔体与楔形地幔熔体混合后上侵，并与下地壳熔体再次混合形成了南华山花岗岩。利用LA-ICPMS法测得花岗岩锆石U-Pb同位素年龄为416±14 Ma，代表了花岗岩结晶年龄。综合利用区域地质、地层特征和不整合接触关系，限定祁连-秦岭交接区俯冲、碰撞造山和地壳加厚发生在志留纪末期。

Geochemistry and geochronology of Paleozoic adakite in the Nanhuashan,west margin of the Ordos Basin

The southwest margin of Ordos is located at the transition area that connected North Qinling,North Qilian and Helan tectonic zones. The Nanhuashan tectonic belt,located at western part of this area,developed massive Early Paleozoic granitoids that intruded into the Lower Proterozoic Haiyuan Group and Middle Cambrian Xiangshan Group,as well as Lower Ordovician Yingou Group. They are mainly granodiorite,monzogranite and quartz diorite,as well a small amount of hornblende trondhjemite and porphyritic granite containing dioritic enclaves. Geochemical studies show that these rocks are mainly aluminum-peraluminous,calc-alkaline series,with significantly higher Al₂O₃ and lower MgO contents,especially lower Y and Yb,higher Sr and Sr/Y ratios,suggest typical Adakite affinity. Meanwhile,the samples show significant depletion of Nb,Ta,P and Ti,enrichment of Cs,Rb,Ba,Th,U,K,Pb,Nd,and Zr. Comprehensive analysis indicates that the Nanhushan granites were formed by mixing of the melts derived from subducted Qilian oceanic crust and mantle wedge,which were finally affected by lower crust melts. Based on LA-ICPMS isotopic dating,the zircon U-Pb yields a mean age of 416±14 Ma,which represents the crystallization age of the Nanhuashan granite. Combined regional geology,stratigraphy and unconformity,the subduction,collision and crustal thickening at the Qilian and Qinling transition zones should occur in the Late Silurian.