新疆阿尔泰南缘康布铁堡组钾-钠质流纹岩锆石U-Pb年龄和地球化学

阿尔泰南缘分布着大量的晚古生代康布铁堡组火山岩系,是许多铁矿、铜矿以及铅锌矿的赋矿围岩。阿尔泰南缘麦兹和克朗火山-沉积盆地内的钾-钠质流纹岩的年龄分别为396.7±1.4Ma和394.0±6.0Ma,结合近期研究成果,进一步表明阿尔泰南缘火山岩主要形成于晚古生代早期,锆石U-Pb年龄峰期在400Ma左右。钾-钠质流纹岩具有高硅(SiO₂的含量范围为73%~82%)、高碱(总碱含量介于4%~7%)和过铝质(高A/CNK值>1)的特征,并见有白云母和黑云母的矿物组合,属于高硅高碱过铝质的钙碱性火山岩。此外,它们的Sr和Nd同位素分别为⁸⁷Sr/⁸⁶Sr=0.7074~0.7144,¹⁴³Nd/¹⁴⁴Nd=0.512072~0.512252,具有上地壳来源的特征,说明其岩石成因与初生地壳的部分熔融作用有着密切关系。结合区域地质背景分析,它们都产在与俯冲消减作用有关的陆缘岛弧的地质环境中。因此,我们推断本区钾-钠质流纹岩的原始岩浆为高硅高碱的花岗质岩浆,是由进入陆壳的高侵位玄武岩浆的底侵作用导致其上部地壳近固相线的低程度部分熔融的产物。

Zircon U-Pb ages and geochemistry of the potassic and sodic rhyolites of the Kangbutiebao Formation in the southern margin of Altay, Xinjiang

There are large volumes of the Late Paleozoic volcanic rocks in the Kangbutiebao Formation on the southern margin of Altay, in which host Fe, Cu and Pb-Zn deposits. The potassic and sodic rhyolites in the Mazi and Kelang basins are 396.7±1.4Ma and 394.0±6.0Ma respectively, by using LA-ICP-MS zircon U-Pb dating, which show they were originated from Early Devonian magmatism. Our geochronology results, combining with latest published data, prove that the Altay orogenic volcanic rocks were formed in early Late Paleozoic, with a zircon U-Pb peak age of ca. 400Ma. The potassic-sodic rhyolites are characterized by high-silicon(SiO₂ content ranges from 73%~82%), high-alkali(total alkalis content ranges from 4%~7%)and peraluminous (high A/CNK>1), with occurrence of biotite and muscovite. Additionally, Sr and Nd isotopic characteristics indicate that the two types of rhyolites were generated from partial melting of an juvenile crust, with ⁸⁷Sr/⁸⁶Sr=0.7074~0.7144, ¹⁴³Nd/¹⁴⁴Nd=0.512072~0.512252. Combined with the regional geodynamic evolution in Late Paleozoic, it is plausible that the two types of rhyolites were formed in a continental marginal arc related with subduction. Therefore, we infer that the source magma of the potassic and sodic rhyolites in the Mazi and Kelang basins was a kind of high-silicon, high-alkali granitic magma, which are products of low degree partial melting of juvenile upper crust near-solidus, derived by underplating of high-level basaltic magma in the continental crust.