峨眉山大火成岩省:地幔柱活动的证据及其熔融条件

The Emeishan Large Igneous Province: Evidence for mantle plume activity and melting conditions

对苦橄岩中橄榄石斑晶及其中熔体包裹体的电子探针分析表明，峨眉山大火山岩省的原始岩浆具高镁（ MgO > 16％）特征。玄武岩的 REE反演计算揭示，参与峨眉山玄武岩岩浆作用的地幔具有异常高的潜能温度（ 1 550℃）。这些特征以及峨眉山玄武岩的大面积分布和一些熔岩所显示的类似于洋岛玄武岩 (OIB)的微量元素和 Sr- Nd同位素特征均为地幔热柱在能量和物质上参与峨眉山溢流玄武岩的形成提供了确凿证据。峨眉山两个主要岩类（高钛和低钛玄武岩）可能是不同地幔源区物质在不同条件下的熔融产物。低钛玄武岩形成于温度最高、岩石圈最薄的地幔柱轴部。地幔（ ISr≈ 0.705,ε Nd(t)≈＋ 2）熔融始于 140 km，并一直延续到较浅的深度（ 60 km,尖晶石稳定区 ),部分熔融程度为 16％，这类岩石可能代表了峨眉山玄武岩的主体。而高钛玄武岩的母岩浆的形成基本局限在石榴子石稳定区（ > 70 km），其源区特征为 : ISr≈ 0.704,ε Nd(t)≈＋ 5，可能代表了热柱边部或消亡期地幔小程度部分熔融（ 1.5％）的产物。

Electron microprobe analyses on olivine phenocrysts in picrites and their trapped melt inclusions confirmed the existence of high magnesian (MgO > 16％ ) primary magmas for the Emeishan basaltic province. The application of McKenzie and O Nions (1991) rare earth element inversion scheme to the geochemistry of the Emeishan basalts reveals an enhanced mantle potential temperature of 1 450～ 1 550° C. These, together with the vast volume of basalts and the OIB- like signatures preserved in some samples, strongly suggest the involvement of mantle plumes in the generation of the Emeishan basalts. It is further suggested that two distinct mantle components may have been involved in generation of the low- Ti and high- Ti lavas, respectively. Whereas the high- Ti basalts originated most likely from an OIB- like mantle source (ISr≈ 0.704,ε Nd(t)≈＋ 5), a slightly enriched mantle component (ISr≈ 0.705,ε Nd(t)≈＋ 2) is required for the low- Ti lavas. The low- Ti lavas were generated in the plume axis where the mantle temperature is high and the lithosphere is relatively thin. The melting was initiated at a relatively great depth (140 km, garnet stability) and continued to the shallow level (60 km, spinel stability). The total degree of partial melting is 16％ . The low- Ti lavas may thus represent the main phase of this large igneous province. In contrast, the high-Ti lavas resulted from melting of the mantle at plume periphery or during a waning stage of plume activity. The low mantle potential temperature and thick lithosphere led the depth to the top of melting column confined within the garnet stability field and a relatively low degree of melting (1.5％ ).