峨眉山大火成岩省中苦橄岩与其共生岩石的地球化学特征及其对源区的约束

Geochemistry of the Picrites and Associated Basalts from the Emeishan Large Igneous Basalt Province and Constraints on Their Source Region

本文对在峨眉山玄武岩省中新发现的苦橄质岩石及其共生玄武岩的地球化学特征进行了研究,结果表明,除苦橄质岩石外,与其共生的玄武质岩石均属高Ti玄武岩。其主要元素特征与大多数大陆溢流玄武岩省相似,表现为高Fe_8、(CaO/Al_2O_3)_8和低Na_8,指示其形成压力高;其稀土和微量元素配分曲线相似,表现为轻稀土富集、高场强元素(HFSE)相对亏损,并且不存在Nb、Ta的负异常,而存在P和K的相对亏损。一些反映源区特征的比值,如La/Ta、La/Sm、(La/Nb)_(PM)、(Th/Ta)_(PM),Ta/Hf,Nb/Zr等,变化范围小,均指示了其地幔柱成因,且上升过程中很少或没有受到岩石圈地幔或地壳物质的混染,是石榴子石二辉橄榄岩在>75km时经大约7％的部分熔融的产物。而地幔柱的轴部位置可能位于现今云南丽江县城一带。

The geochemical characteristics of the picritic rocks that we discovered recently and their associated basalts show that except the picrites, all the associated basaltic rocks belong to the high-Ti type. Like many other continental flood basaltic provinces, they are characterized by high Fe8 and (CaO/Al2O3)8 and low Na8, indicating high pressure. Their chondrite-normalized REE patterns and primitive mantle-normalized trace element patterns are similar, all of which display an enrichment of LREE and relative depletion of high field strength elements (HFSE) associated with an absence of Nb and Ta negative anomalies but a presence of P and K negative anomalies, as characterized by most flood basalts. Some trace element ratios characterizing the source regions, such as La/Ta, La/Sm, (La/Nb)pM, (Th/Ta)PM, Ta/Hf and Nb/Zr are within a narrow range. All these observations suggest the origin of mantle plume, and no or little crustal or lithospheric mantle contamination. They are generated by about 7% of partial melting of garnet Iherzolite at more than 75 km. Thus, the axis of the Emeishan mantle plume should be located beneath the Lijiang County Town, Yunnan Province.