阿尔泰南缘麦兹盆地康布铁堡组变质酸性火山岩年龄及岩石成因

Geochronology and genesis of the meta-felsic volcanic rocks in the Kangbutiebao Formation from the Maizi Basin at the southern margin of the Altay,Xinjiang

阿尔泰南缘麦兹盆地康布铁堡组赋存有铁铅锌大、中型矿床,但该组火山岩的形成时代和岩石成因以及构造背景还没有形成一致的认识。本文对该组两个变质流纹岩样品进行了LA-ICP-MS锆石U-Pb年龄测定,获得其年龄分别为410.5±1.3 Ma和400.4±2.1 Ma。岩石地球化学分析结果表明:所有酸性火山岩具有高硅(72.15%～81.26%),低铝(10.56%～14.87%)和钛(0.16%～0.27%),极低的MgO(0.04%～0.14%)、CaO(0.04%～2.42%)和TFeO(0.11%～1.64%)含量;在原始地幔标准化图上显示出Ti、P、Sr、Ba、Nb和Ta明显负异常,Th、U、Pb、Zr和Hf的正异常,LREE相对HREE略富集的特征,除个别样品外,大多数样品显示明显的Eu负异常(δEu=0.48～0.91)。结合其区域地质特征,认为本区变质酸性火山岩形成于活动大陆边缘环境,是玄武质岩浆底侵作用使下地壳发生了部分熔融后演化的产物。

LA-ICP-MS U-Pb zircon ages and geochemical data of whole rocks are reported for meta-rhyolites from the Kangbutiebao Formation in the Maizi Basin at the southern margin of Altay orogenic belt.The zircon U-Pb isotopic data constrain their magmatic emplacement at 410.5±1.3 Ma and 400.4±2.1 Ma,respectively.The results presented in this study indicate that meta-rhyolites are characterized by high concentrations in SiO2（72.15% to 81.26%）,total alkali（Na2O＋K2O）（7.23% to 11.55%）,and low concentrations in Al2O3（10.56% to 14.87%）,TiO2（0.16% to 0.27%）,MgO（0.04% to 0.14%）,CaO（0.04% to 2.42%）and TFe2O3（0.11% to 1.64%）.They display obvious negative anomaly of Ti,P,Sr,Ba,Nb and Ta,obvious enrichment of Th,U,Pb,Zr,Hf and LREE.Most samples show noticeable negative Eu anomalies（δEu=0.48～0.91）.These features,together with the regional geology characteristics,indicate that these felsic volcanic rocks were generated by partial melting of lower crust in an active continental margin.The primitive magma was likely resulted from the underplating,and underwent fractional crystallization and minor wall-rocks assimilation during the ascending of magma.