西南印度洋脊原位辉长岩元素地球化学特征及意义

In Situ Gabbro Geochemical Characteristics and Implications from the Southwest Indian Ocean Ridge

通过对位于西南印度洋脊超慢速扩张脊东段的大洋钻探计划(ODP)ODP735B钻孔上部岩心不同位置、不同岩性的样品进行全岩主、微量元素分析,并结合前人研究成果,对旋回内部及旋回之间的全岩地球化学特征差异进行了探讨,对其成因进行了约束.ODP735B岩心全岩主量元素特征主要受控于分离结晶生成的矿物组合及比例.全岩主量元素之间的协变关系对反映玄武质熔体结晶演化过程中矿物生成及化学成分演化具有一定的指示意义.稀土元素的分析表明,除较明显的正Eu异常外,旋回1、2均表现出明显的LREE分馏,而HREE则未出现明显分馏.微量元素的分析表明,西南印度洋超低速洋脊的旋回1和旋回2中均表现出明显的Nb、Ta负异常和Sr、K正异常,但不能依靠其全岩微量元素Nb、Ta负异常特征对其源区地球化学特征成因和地质意义进行判断.

In this paper,geochemical data of the major and trace elements are analyzed for the samples with different depths and lithology of the upper part of ocean drilling program（ODP） ODP 735B hole,Southwest Indian Ocean ridge.Based on these data,together with some other achievements,geochemical comparisions between and within these two cycles are discussed,and their petrogenesis are constrained.The major element characteristics indicate that their variation was well dominated by the mineral assemblage modal which are controlled by fractional crystallization of the rocks.The correlation of the bulk composition of major elements can,to some extent,indicate the formation of minerals and evolution of geochemical characteristics during the crystallization of basaltic melts.Besides the positive Eu anomaly,LREEs are clearly fractionated,while no fractionation of HREEs occurs.The spidergram shows depletion in Nb and Ta,and enrichment in Sr and K;however,this feature cannot be considered as the basis to decide the geochemical characteristics of the source so as to look into the geological significance.