阿尔泰造山带青河地区早泥盆世和寒武纪末花岗岩的年代学、岩石成因及其地质意义

阿尔泰造山带中广泛发育古生代的花岗质岩浆活动,它们对认识阿尔泰造山带的构造演化具有重要意义。本文对青河县西南方向敖包特花岗岩体中的片麻状黑云二长花岗岩及其捕掳体片麻状英云闪长岩进行LA-ICP-MS定年,分别获得了407±3 Ma和491±3 Ma的~(206)Pb/~(238)U年龄,为早泥盆世和寒武纪末。岩石地球化学特征表明早泥盆世的片麻状黑云二长花岗岩具有钙碱性-碱钙性和镁质花岗岩的特点,其A/CNK比值为0.98~1.08,为准铝质-弱过铝质,CIPW标准刚玉分子数主体小于1,且其具有较高的Y含量(27.2×10~(-6)~40.5×10~(-6))和Th含量(10.2×10~(-6)~27.0×10~(-6)),随着Rb含量的增加均有逐渐增加的趋势,表现出Ⅰ型花岗岩的特征。寒武纪末的片麻状英云闪长岩具有较低的SiO_2含量(60.28%~64.01%),其初始铝饱和指数应为准铝质一弱过铝质,微量元素特征同样表现出Ⅰ型花岗岩的成因特点。通过综合分析认为该早泥盆世的花岗岩与寒武纪末的英云闪长岩均形成于俯冲相关的构造环境。

Geochronology, petrogenesis and geological significance of the early Devonian and terminal Cambrian granites in Qinghe area of China's Altay orogenic belt

There are abundant Paleozoic granites in China''s Altay orogenic belt, which played an important role in the tectonic evolution. In this study, LA-ICP-MS zircon U-Pb dating yielded the ²⁰⁶Pb/²³⁸U ages of 407±3 Ma and 491±3 Ma for the gneissic granite and gneissic tonalite of the Aobaote pluton in Qinghe area respectively, suggesting early Devonian and terminal Cambrian period. The gneissic granite is of calc-alkaline or alkali-calc nature, and belongs to magnesian granite. It is metaluminous-slightly peraluminous with the A/CNK values being 0.98~1.08 and normative corundum values being less than 1. And there are high Y values of 27.2×10^-6~40.5×10^-6 and Th values of 10.2×10^-6~27.0×10^-6, which increase with increasing Rb. The above characteristics indicate that the gneissic granite belongs to I-type granite. There are low SiO₂ values of 60.28%~64.01% for the gneissic tonalite, and its A/CNK values show that its protolith was metaluminous-slightly peraluminous. Together with its trace element characteristics, the authors hold that the gneissic tonalite is characterized by I-type granite. Synthetic analysis shows that the gneissic granite and gneissic tonalite were formed by the subduction.