鄂尔多斯盆地西南部晚三叠世辉绿岩年代学、地球化学及其构造意义

Late Triassic diabase in the southwestern Ordos Block:Chronology,geochemistry and structural significance

鄂尔多斯盆地西南部中奥陶统马家沟组灰岩内部发育多条辉绿岩脉。辉绿岩富集高场强元素(HFSE)Nb、Ta、Ti、Zr、Hf等,具有板内拉斑玄武岩的地球化学特征,其地球化学数据显示原始岩浆经历过比较强烈的结晶分异演化。对辉绿岩的锆石进行U-Pb LA-ICP-MS测年获得的^(206) Pb/^(238) U年龄为750~195 Ma。根据峰值年龄并结合区域地质资料,本文把辉绿岩的形成年龄确定为224 Ma(晚三叠世)。辉绿岩反映鄂尔多斯盆地西南部在晚三叠世处于类似大陆裂谷的伸展构造环境,晚三叠世西邻陇山地区发生的强烈碰撞后伸展作用也可能有助于进一步强化鄂尔多斯盆地西南部的伸展构造环境。当时该地区岩石圈的稳定性相对较弱,为该地区后续的早侏罗世和早白垩世岩浆活动提供了物质基础。晚三叠世及后期的岩浆活动提高了鄂尔多斯盆地西南部的地温梯度并加快了该地区烃源岩的热演化进程,可能对该地区的油气成藏条件造成潜在影响。

A few of diabasic dikes intrude into limestone of the Middle Ordovician Majiagou Formation,Leijiagou Village,southwestern Ordos Basin.The diabase is enriched in HFSE(Nb,Ta,Ti,Zr,Hf,etc.)and geochemically similar to intraplate tholeiite,while geochemical evidence indicates the primitive magma experienced relatively strong crystallization differentiation.Zircon U-Pb dating by LA-ICP-MS yielded ^(206) Pb/^(238) U ages of 750—195 Ma for the diabase.According to the peak age combined with regional geological data,the formation age of the diabase is determined to be 224 Ma(Late Triassic).The diabase suggests an extensional structural setting like continental rift existed in the southwestern Ordos Block in the Late Triassic,while strong post-collisional extension of its western part probably strengthened the extensional structural setting.And relatively weak lithospheric stability—implied by the Late Triassic diabase—perhaps provided material basis for the subsequent igneous activities of the Early Jurassic—Early Cretaceous in the southwestern Ordos Block.The igneous activities of the Late Triassic and beyond increased the regional geothermal gradient and thermal evolution of hydrocarbon source rocks,potentially affecting the reservoir-forming environment of the southwestern Ordos Block.