内蒙古四子王旗乌尔塔高勒庙岩体的侵位时代及岩石地球化学特征

PETROCHEMICAL CHARACTERISTICS AND TIMING OF WUERTAGAOLEMIAO GRANITOIDS,SIZIWANGQI,INNER MONGOLIA

内蒙中部四子王旗乌尔塔高勒庙花岗岩基位于兴蒙造山带的二道井-查干乌拉-红格尔缝合带南缘,主要由角闪正长岩、二长花岗岩及正长花岗岩组成。为精确厘定该岩体形成时代,我们对其三类岩石进行了锆石LA-ICP-MS U-Pb定年。角闪正长岩、二长花岗岩、正长花岗岩分别获得了271Ma±18 Ma(MSWD=4.1),256 Ma±3 Ma(MSWD=2.7)和261Ma±2Ma(MSWD=1.2)的206Pb/238 U年龄。岩石地球化学数据表明二长花岗岩和正长花岗岩属钙碱性岩石系列,而角闪正长岩为碱性岩系列;角闪石正长岩高Ga/Al比值,高Zn,Y,Nb含量等特征与A型花岗岩类一致;二长花岗岩高Sr(407×10-6～626×10-6),低Y(2.2×10-6～4.8×10-6)和Yb(0.27×10-6～0.45×10-6)以及高Sr/Y(84～235)和LaN/YbN(8.9～49.6)比值,δEu正异常(1.3～3.3)等特征,暗示源区残留相以石榴石+角闪石+斜长石为特征,具加厚地壳特征;该类型岩石通过斜长石的分离结晶形成具低锶花岗岩特征的正长花岗岩。同位素定年和岩石地球化学特征表明乌尔塔高勒庙岩体形成于碰撞后构造环境,是大规模岩浆底侵作用导致地壳熔融的结果。

Wuertagaolemiao granitoid in the Siziwangqi area of Inner Mongolia tectonically belongs to the Xing-Meng orogenic belt and south of the Erdaojing-Chaganwula-Honggeer suture zone.The granitoid is predominantly composed of hornblende-syenite,monzogranite,and syenogranite.LA-ICP-MS U-Pb zircon dating and petrochemical analysis are used to study and understand its formation age and nature.As a result,zircons from hornblende-syenite,monzogranite,syenogranite,yield U-Pb ages of 271 Ma±18 Ma（MSWD=4.1）,256 Ma±3 Ma（MSWD=2.7）,261 Ma±2 Ma（MSWD=1.2）.Petrochemical data indicate that monzogranite and syenogranite belong to cala-alkaline,while hornblende-syenite to alkaline series.Geochemistry of the Hornblende-syenite with high Ga/Al ratio and high abundance of Zn,Y,Nb,indicates an affinity to A-type syenitic magma.The monzogranite with high Sr content（407×10-6~626×10-6）,low Y and Yb content（Y=2.2×10-6~4.8×10-6,Yb=0.27×10-6~0.45×10-6）,and high Sr/Y（84~235）,LaN/YbN（8.9~49.6） ratios,without δEu anomaly,suggest that it was resulted from partial melting from post-collisional over-thickened lower crust with garnet＋hornblende＋feldspar as dominant residual mineral assemblages in the source.The syenogranite with low Sr abundance,relatively high SiO2 and K2O,was generated by crystal fractionation of feldspar from the monzogranite.Geochronological and petrochemical data imply that the Wuertagaolemiao granitoid is related to a post-collisional regime,triggered by an extensive underplating event in the Siziwangqi area,Inner Mongolia.