香山群辉绿岩地球化学特征及其构造背景

GEOCHEMICAL CHARACTERISTICS AND TECTONIC SETTING OF DIABASE IN XIANGSHAN GROUP,NINGXIA

现有地质资料分析,把发育于香山群第三亚群辉绿岩时代下限定为早古生代。辉绿岩主元素以高钠、镁,低钾、钛为特征。稀土总量（∑REE）较低,轻、重稀土分馏作用不明显;基本不显示Eu异常,球粒陨石标准化配分曲线具有平缓的特征。与原始地幔相比,微量元素值约是原始地幔标准值的2~10倍,大离子亲石元素（LILE）相对亏损,高场强元素（HFSE）分布平坦且不分异;N-MORB标准化蛛网图上,呈现大离子亲石元素（LILE）富集特征。地球化学特征表明：辉绿岩属于大洋拉斑玄武岩系列,显示了伸展作用的构造背景。另外,结合岩石学、沉积背景和同构造期岩套特征分析,再次验证了在中晚寒武世,阿拉善地块南缘与鄂尔多斯地块西南缘具离散型大陆边缘性质。

Analyses of geological data indicate that the diabase in the third member of Xiangshan Group is of Mid - Late Cambrian in age. The diabase is characterized by high Na, Mg and low K, Ti contents, low total REE contents, and absence of fractionation between LREEs and HREEs, in addition to no pronounced Eu anomaly. The REE patterns normalized by chondrite are lowly and slightly inclined, while the value of trace elements is about 2 - 10 times higher than that for primitive mantle. In comparison with the case of primitive mantle, the trace element data for the diabase show relative depletion in LILEs and no fractionation in HFSEs, while N-MORB normalized trace element spidergrams show enrichment in LILEs. These geochemical characteristics suggest that the diabase belongs to the series of oceanic tholeiite derived from extensional setting. In addition, results of chemical composition, sedimentary environment and rock assemblage demonstrate that a continental margin in spreading tectonic setting have existed between the southwestern margin of Ordos plate and the southern Alashan plate.