峨眉山大火成岩省西部苦橄岩及其共生玄武岩的地球化学:地幔柱头部熔融的证据

Geochemistry of picritic and associated basalt flows of the western Emeishan flood basalt province,China:evidence for a plume-head origin.

丽江地区的苦橄岩位于峨眉山大火成岩省的西部，其与辉斑玄武岩、无斑玄武岩和玄武质火山碎屑岩共生。苦橄岩中的斑晶主要为富镁橄榄石，其F0含量最高达91．6％，CaO含量最高达0．42％，其内含有少量玻璃包裹体，指示了橄榄石是在熔体中结晶形成的。苦橄岩中的铬尖晶石具有高的Cr#值（73-75）。计算的初始岩浆的MgO含量大约为22wt％，初始熔融的温度为1630-1680℃。研究结果表明，玄武质岩石是苦橄质岩浆通过橄榄石和单斜辉石分离结晶形成的。苦橄岩和玄武岩的Nd-Sr-Pb同位素比值差别不大，只落在一个很小的范围内（如εNd（t）=-1．3 to＋4．0）。高的εNd（t）值以及抗蚀变不相容元素的原始地幔标准化图解与洋岛玄武岩相似，并且其重稀土元素特征指示了源区有石榴子石的残余，而且是低部分熔融的产物。同位素比值与抗蚀变不相容元素比值（如Nb／La）的相关性表明，岩浆形成过程中有少量的大陆地壳物质或者相对低εNd（t）组分的大陆岩石圈地幔的混染。因此，总体上，苦橄岩的地球化学特征的研究结果支持了峨眉山大火成岩省是地幔柱头部熔融的成因模型。

Picritic lava flows near Lijiang in the late Permian Emeishan flood basalt province are associated with augite-phyric basalt, aphyrie basalt, and pyroclastie units. The dominant phenocryst in the pieritie flows is Mg-rieh olivine （up to 91.6% forsterite component） with high CaO （to 0.42wt% ） and glass inclusions, indicating the olivine crystallized from a melt. Associated chromite has high Cr-number （73-75 ）. The estimated MgO content of the primitive pieritie liquids is about 22wt%, and initial melt temperature may have been as high as 1630-1680℃. The basaltic lavas appear to be related to the pieritie ones principally by olivine and elinopyroxene fraetionation. Age-eorreeted Nd-Sr-Pb isotope ratios of the pieritie and basaltie lavas are indistinguishable and eover a rather small range （ e. g. , 6Na （t） = -1.3 to ＋ 4.0）. The higher-εNd（t） lavas are isotopieally similar to those of several modern oceanie hotspots, and have ocean-island-like patterns of alteration-resistant ineompatible elements. Heavy rare-earth element eharaeteristies indicate an important role for garnet during melting and that the lavas were formed by rather small amounts of partial melting. Rough eorrelations of isotope ratios with ratios of alteration-resistant highly ineompatible elements （e. g. , Nb/La） suggest minor amounts of eontamination involving eontinental material or eontinental lithospherie mantle with a relatively low-εNd eomponent in the source. Overall. our results are consistent with other evidenee suggesting a plume-head origin for the Emeishan province.