内蒙古北山风雷山地区流纹岩LA-ICP-MS锆石U-Pb年代学及地球化学特征

Geochemical characteristics and age of rhyolite LA-ICP-MS Zircon U-Pb in Fengleishan area of Beishan,Inner Mongolia

在北山地区的红石山-百合山-蓬勃山蛇绿岩带东段以南分布大量的火山岩,通过对北山风雷山地区流纹岩LA-ICP-MS锆石U-Pb年代学研究,表明其形成年代为(318.5±1.2)Ma,即晚石炭世。流纹岩的SiO_2含量为77.05%～77.52%,K_2O+Na_2O含量为6.96%～7.83%;在微量元素原始地幔标准化蛛网图中,显示Rb、K、Th、U、Zr、Hf明显富集,而高场强元素Ta、Nb、Ti明显亏损;岩石地球化学特征表明,流纹岩为高钾钙碱性系列,具火山弧花岗岩属性。结合区域地质特征研究,在早石炭世时期,红石山-百合山-蓬勃山拉张裂解形成初始小洋盆,即类似于"红海型"海槽的环境,在大约早石炭世晚期洋壳发生向南、向北的双向俯冲,到晚石炭世继续裂解的小洋盆南缘向南俯冲,从而形成了大量的弧火山岩。

There are large quantities of volcanic rocks in the south of the eastern section of Beishan area,s Hong- shishan - Baiheshan - Pengboshan ophiolitic belt. The age of rhyolite indicated by LA - ICP - MS zircon U - Pb is （318.5 ±1.2） Ma, Late Carboniferous. The rhyolite has 77. 05% -77.52% of SiO2, and 6. 96% -7.83% of K2O ＋ Na2O. Large ion lithophile （Rb,K,Th,U,Zr,Hf） enrichment and high field strength elements （Ta,Nb, Ti） shortage are displayed in the primitive mantle normalized trace element spider diagram. The petrogeochemical features indicate that the rhyolite shows high potassium calc alkaline series, with the volcanic arc granite attribute. Combined with the study of regional geological characteristics, it indicates that Hongshishan - Baiheshan - Peng-boshan extensional faulting formed the initial small ocean basin. Namely, it,s similar to the Red Sea Type trough environment in Early Carboniferous. The oceanic crust subducted into the south and the north directions in late Early Carboniferous. And the southern edge of the consistently cleaving small ocean basin subducted into the south direction by the Late Carboniferous, so large amounts of arc volcanic rocks formed.