内蒙古满都拉泥盆纪基性火山岩特征及其形成环境

Characteristics and tectonic settings of the Devonian basic volcanic rocks in Mandula, Inner Mongolia

满都拉西部出露的蛇绿混杂岩中橄榄辉长岩的单颗粒锆石U-Pb年龄(385.6±1.7)Ma,显示该蛇绿岩形成于早泥盆世晚期。其中的基性火山岩岩块为一套浅变质的枕状玄武岩、橄榄玄武岩,多与超基性岩岩块伴生。玄武岩的SiO2为47.16%～51.93%,TiO2为1.44%～2.13%,Al2O314.08%～17.51%,Na2O3.42%～5.39%,K2O0.22%～0.83%;具低钾特点,显示拉斑玄武岩特征。其富集大离子亲石元素Rb,Ba,Th和U,贫高场强元素Zr,Ti,Y等,似岛弧玄武岩或岛弧-洋中脊过渡型玄武岩或弧后盆地玄武岩。ΣREE为(113.130～189.560)×10—6,LREE/HREE为0.684～1.369,配分曲线呈右缓倾、平坦型,显示岛弧、洋中脊或E-洋中脊玄武岩特征。根据岩石学、岩石地球化学特征,结合区域构造背景,认为该套火山岩或该蛇绿岩的形成环境为弧后盆地。

Ophiolitic complex can be found in the west of Mandula, central-northern Inner Mongolia. Olivine gabbro of the complex has a single zircon U-Pb age of (385.6±1.7) Ma, indicating that the ophiolite was formed in the end of early Devonian or beginning of late Devonian.basic volcanic rocks, usually outcropped with ultrabasic blocks, constitute a part of the complex and consist of slightly metamorphic pillowy basalt and olivine basalt. The basalt is characterized by poor in K(K_2O =0.22%～ 0.83%) and high-field strength elements(Zr, Ti, Y, etc.), rich in large-ion lithophile elements (Rb, Ba, Th, U) with SiO_2=47.16%～51.93%, TiO_2 =1.44%～2.13%, Al_2O_3 = 14.08%～17.51%, Na_2O=3.42%～5.39%, ΣREE=(113.130～189.560)×10^(-6) and LREE/HREE =0.684～1.369. All the pathological and petrochemical features indicate that the basalts were formed in a back-arc basin.