内蒙古科尔沁右翼中旗敖兰三队侵入体年龄及地球化学特征

Geochronology and geochemistry of the Aolansandui pluton in Horqin Right Wing Middle Banner of Inner Mongolia

内蒙古科尔沁右翼中旗敖兰三队侵入体的同位素年代学及岩石地球化学研究表明,敖兰三队侵入体巨晶正长花岗岩具有低Sr、高Yb型花岗岩特征,其LA-ICP-MS锆石^(206)Pb/^(238)U年龄加权平均值为220±1Ma,表明该侵入体是晚三叠世早期侵位形成的。岩石高硅、富碱、高铁镁比、贫钙、贫镁和低钛;稀土元素配分曲线呈现"海鸥式"分布特征,显示强烈的Eu负异常。微量元素特征显示较低的Sr、Ba含量,较高的Yb、Zr和Y含量,在微量元素原始地幔标准化蛛网图上显示明显的Sr、Ba和Ti的负异常。岩石具有高的Rb/Sr和Rb/Nb,显示出壳源岩浆的成分特征。综合分析表明,本区巨晶正长花岗岩为低压下地壳部分熔融的产物。根据(Y+Nb)-Rb、(Yb+Ta)-Rb、Nb-Y-Ce三角图解,并结合区域构造演化,认为巨晶正长花岗岩形成于造山后伸展的构造环境。在中—晚三叠世,内蒙古科尔沁右翼中旗地区构造体制经历了重要的转变,由挤压体制转变为造山后软流圈上涌、岩石圈减薄。

Geochronology and geochemistry of the Aolansandui pluton in Horqin Right Wing Middle Banner of Inner Mongolia were studied. Zircon U-Pb dating demonstrates that the weighted mean 206Pb/238U age for the zircons from the syenogranite is 220± 1Ma, suggesting Late Triassic. The syenogranite belongs to low-Sr and high-Yb type granitoids. The syenogranite is characterized by high SiO2, high alkali, high TFeO/MgO ratios, and low CaO, low MgO and lower TiO2 values. The syenogranite is characterized by“sea-gull”REE patterns and significant negative Eu anomaly as well as depletion of Sr and Ba and enrichment of Yb, Zr and Y. The high Rb/Sr and Rb/Nb ratios of the rock indicate a crustal origin. Comprehensive analysis shows that the syenitegranite was formed by low pressure felsic crust partial melting. In （Y＋Nb）-Rb, （Yb＋Ta）-Rb, Nb-Y-Ce diagrams combined with regional tectonic evolution, the authors hold that the syenogranite was formed in a post-orogenic setting. In the Middle Late Triassic period, the regional tectonic regime in Horqin Right Wing Middle Banner of Inner Mongolia underwent significant changes, and experienced asthenosphere upwelling after squeezing into the orogenic lithosphere and thinning .