甘肃昌马地区早寒武世地层中辉绿质次火山岩地球化学特征及构造背景

GEOCHEMICAL CHARACTERISTICS AND TECTONIC SETTING OF DIABASIC SUB-VOLCANIC ROCK IN LOWER CAMBRIAN STRATA IN CHANGMA AREA,GANSU

利用已有的资料,将昌马平头山一带早寒武世地层中的辉绿质次火山岩时代限定为早寒武世,辉绿质次火山岩主量元素以高钠、镁、钛,低钾为特征,属钠质碱性系列,稀土总量(∑REE)较低,轻、重稀土分馏作用明显,无铕异常,球粒陨石标准化配分曲线具平滑的特征,N-MORB标准化蛛网图上,呈现大离子亲石元素(LILE)富集,微量元素值约是原始地幔标准值的10～100倍。结合区域构造背景分析,认为辉绿质次火山岩形成于板内裂谷环境。

The present geological data indicate that the age of the diabasic subvolcanic rock in Changma area is Lower Cambrian.The rock is characterized by high Na,Mg and Ti,but low K contents,it belongs to the series of alkaline with low total REE contents,and obvious fractionation between LREEs and HREEs.REE pattern normalized by chondrite are slightly inclined,while N-MORB normalized trace element spider diagram show enrichment in LILEs.In comparison with the case of primitive mantle,the value of trace elements is about 10~100 times higher than that for primitive mantle,and the trace element data for the diabase show relative depletion in K,Sr and enrichment in Nb,Ta.All above-mentioned data indicate that the diabase was formed under intraplate rift setting.