内蒙古宾巴勒查干粗中粒正长花岗岩年代学及岩石地球化学

Geochronology and Geochemistry of the Coarse-medium Grained Syenogranites in the Binbalechagan Area, Inner Mongolia

内蒙古东乌旗宾巴勒查干分布有大面积的侵入岩,本文选取其中的粗中粒正长花岗岩进行了同位素年代学及岩石地球化学研究,采用锆石SHRIMP U-Pb法得到加权平均年龄为220.4±2.5 Ma(MSWD=0.83),所测锆石震荡环带发育,具有岩浆锆石特征,表明该年龄为花岗岩成岩年龄。岩体属富钾的过铝质岩石,稀土配分曲线以Eu负异常明显、轻稀土相对富集、重稀土具微弱的负斜率为特征。微量元素显示岩体的Ba具明显的负异常,Rb、Th相对富集,其它不相容元素相对亏损,Y、Yb均低于洋脊花岗岩标准化值。笔者认为岩体形成于西伯利亚板块和中朝板块拼合之后的后造山伸展背景下。

Intrusive rocks are widespread in the Binbalechagan, Dongwuqi, Inner Mongolia Autonomous Region. This paper selectes coarse-medium grained syenogranites （CMGS） in the region for isotope chronological and geochemical studies. By zircon SHRIMP U-Pb dating, weighted mean age of 220.4 ± 2.5 Ma（MSWD = 0.83） is obtained. The zircon oscillatory rings are well developed with typical pattern of magmatic zircon, so the result may represent the diagenetic age of the granite. CMGS is characterized by K enrichment, peraluminous, LREE relatively enriched, substantial negative Eu anomalies, HREE with weak negative gradient, Ba with obvious nega- tive anomalies, Rb and Th relatively enriched, other incompatible elements relatively depleted, both Y and Yb lower than the normalized values of ocean ridge granite. CMGS probably formed under post-orogenic extension setting after the convergence between the Siberian plate and Sino-Korean plaeo-plate.