峨眉地幔柱-岩石圈的相互作用:来自低钛和高钛玄武岩的Sr-Nd和O同位素证据

Emei Mantle Plume-Subcontinental Lithosphere Interaction: Sr-Nd and O Isotopic Evidences from Low-Ti and High-Ti Basalts

峨眉山玄武岩总体具有较高的87Sr/ 86Sr比值和较低的εNd(t)值 ,并具有富集地幔源区的特点。而低钛玄武岩 (LT)与高钛玄武岩 (HT)间又表现出一定的差异性 ,即早期低钛玄武岩(LT1)的87Sr/ 86Sr比值最高 (0 .70 6 3～ 0 .70 78) ,而其εNd(t)最低 (- 6 .74～ - 0 .34) ;晚期高钛玄武岩 (HT)具有最低的87Sr/ 86Sr比值 (0 .70 4 9～ 0 .70 6 4 )和最高的εNd(t)值 (- 0 .71～ 1.5 )。峨眉山低钛玄武岩中单斜辉石的氧同位素变化范围为 6 .2‰～ 7.86‰ ,高于洋岛拉斑玄武岩的平均值5 .4‰。研究样品较地幔岩石偏高的δ18O值说明在其形成和演化过程中有壳源物质的参与。结合前人的研究成果和对元素地球化学的研究认为 ,壳源物质可能主要来自于新元古代富集的扬子西缘次大陆岩石圈地幔。地幔柱 -岩石圈的相互作用过程中表现在时间和空间的系统变化 ,即早期西岩区形成含大量壳源组分的低钛玄武岩 ,晚期为壳源组分相对较少的高钛玄武岩。空间上低钛玄武岩仅分布在西岩区 ,而中、东岩区皆为高钛玄武岩。壳源组分随着时间演化逐渐减少 ,在空间上由西而东也逐渐减少。表明峨眉山火成岩省形成早 (主 )期地幔柱头卷入并熔融了较多交代富集的次大陆岩石圈物质 ,晚期则有较少的壳源物质参与。

The Emeishan flood basalts have high 87 Sr/ 86 Sr ratios and low ε Nd ( t ) values, suggesting that they were generated from a enriched mantle source. The diversity of Sr and Nd isotopes between early stage low Ti (LT) and late stage high Ti (HT) basalts implies that they were generated from distinct mantle source and experienced different crustal contamination or assimilation. The earliest formed LT1 basalts have highest 87 Sr/ 86 Sr ratios (0.7063～07.78) and lowest ε Nd ( t ) values(-6.74～-0.34), while late stage HT basalts have lowest 87 Sr/ 86 Sr ratios (0.7049～0.7064) and highest ε Nd ( t ) values(-0.71～1.5). Along with oxygen isotopes of clinopyrxene from the LT basalts ranging from 6.2 ‰ to 7.86 ‰, higher than oceanic island basalts (5.4 ‰), this study suggests that The Emeishan flood basalts well recorded the interaction processes in both temporal and spatial aspects. The early stage formed low Ti basalts mainly distributed in the western division of Emeishan large igneous province contain many crustal material signatures. The metasomatized subcontinental lithospheric mantle might be the major source for its generation. Late stage formed high Ti basalts occurring mainly in the middle and eastern divisions show some plume signatures and little crustal contamination or assimilation. These factors suggest that the interaction between plume head and the enriched subcontinental lithospheric mantle played an important role in the formation of the low Ti basalts. However, it was less important for the generation of high Ti basalts at later stage.