滇西北羊拉乡贝吾玄武岩形成的大地构造环境探讨

贝吾玄武岩为滇西北金沙江蛇绿混杂岩带中的一个岩块。通过详细的野外调查和岩石地球化学研究认为,贝吾玄武岩为低钾拉斑玄武岩,分异程度较低,大离子亲石元素相对于高场强元素富集,稀土总量低,轻稀土略富集,整体特征介于正常洋脊玄武岩与岛弧玄武岩之间,为似洋中脊玄武岩。其类似于岛弧玄武岩的富集特征表明了成岩过程中俯冲作用的存在,综合岩性组合、构造特征和地质背景分析,认为其属前弧玄武岩,形成于初始俯冲的洋内弧环境。

Tectonic setting of the Beiwu basalts from Yangla,northwestern Yunnan: An approach

The Beiwu basalts constitute a large tectonic block in the Jinshajiang ophiolitic mélange zone in northwestern Yunnan. The integration of detailed field investigation and lithogeochemical study shows that the Beiwu basalts have the features of low-K tholeiitic basalts,low differentiation,remarkedly enrichment of large ion lithophile elements (LILE)relative to the high field strength elements (HFSE),low total REE amount,slightly enrichment of light rare earth elements (LREE). The Beiwu basalts are not assigned to the typical mid-oceanic ridge basalts (MORB)or island-arc basalts (IAB),but to a transitional type between them or to the MORB-like basalts. The similarity of the Beiwu basalts to the island-arc basalts suggests the existence of the subduction during the diagenetic processes. It is concluded from lithologic association,geological background and tectonic setting that the Beiwu basalts may be assigned to the front arc basalts (FAB)which are formed in the initial subduction of intra-oceanic arc environments.