Geochemistry of Picritic and Associated Basalt Flows of the Western Emeishan Flood Basalt Province, China
Geochemistry of Picritic and Associated Basalt Flows of the Western Emeishan Flood Basalt Province, China
复制标题
DOI：
10.1093/petrology/egl034
复制
发表时间：
2006-10
影响因子：
3.9
通讯作者：
Zhaochong Zhang;J. Mahoney;J. Mao;Fusheng Wang
中科院分区：
文献类型：
--
作者：
Zhaochong Zhang;J. Mahoney;J. Mao;Fusheng Wang
晚二叠世峨眉山洪泛玄武岩省丽江附近的泥质熔岩流与辉石-榴辉玄武岩、玄武岩和玄武岩火山碎屑单元伴生。苦橄榄岩流中的主要斑晶为高CaO含量(0.42%)的富镁橄榄石(镁橄榄石成分高达91.6%)和玻璃包裹体，表明橄榄石是从熔体中结晶出来的。伴生铬铁矿具有较高的铬离子数(73-75)。原始苦橄岩流体的氧化镁含量约为22wt%，熔体初始温度可能高达1630-1690�C。玄武岩熔岩与苦橄岩熔岩主要通过橄榄石和单斜辉石的分馏作用联系在一起。苦橄岩和玄武岩熔岩的年龄校正的Ndsr-Pb值很难区分，覆盖的范围也相对较小[例如End(T)≈�1.3To？4.0]。较高端(T)熔岩的同位素与几个现代海洋热点的熔岩相似，并具有海洋岛状的抗蚀变不相容元素模式。重稀土元素特征表明，石榴石在熔融过程中起着重要作用，熔岩是由较小程度的部分熔融形成的。同位素比率与抗蚀变高度不相容元素(如Nb/La)比率的粗略相关性表明，有少量的污染涉及大陆物质或来源中相对较低的成分。总体而言，我们的结果与其他证据一致，表明峨眉山省存在某种类型的烟羽起源。
Picritic lava flows near Lijiang in the late Permian Emeishan flood basalt province are associated with augite-phyric basalt, aphyric basalt, and basaltic pyroclastic units. The dominant phenocryst in the picritic flows is Mg-rich olivine (up to 91.6% forsterite component) with high CaO contents (to 0.42 wt %) and glass inclusions, indicating that the olivine crystallized from a melt. Associated chromite has a high Cr-number (73–75). The estimated MgO content of the primitive picritic liquids is about 22 wt %, and initial melt temperature may have been as high as 1630– 1690 � C. The basaltic lavas appear to be related to the picritic ones principally by olivine and clinopyroxene fractionation. Agecorrected Nd–Sr–Pb isotope ratios of the picritic and basaltic lavas are indistinguishable and cover a relatively small range [e.g. eNd(t) ¼� 1. 3t oþ4.0]. The higher eNd(t) lavas are isotopically similar to those of several modern oceanic hotspots, and have oceanisland-like patterns of alteration-resistant incompatible elements. Heavy rare earth element characteristics indicate an important role for garnet during melting and that the lavas were formed by fairly small degrees of partial melting. Rough correlations of isotope ratios with ratios of alteration-resistant highly incompatible elements (e.g. Nb/La) suggest modest amounts of contamination involving continental material or a relatively low-eNd component in the source. Overall, our results are consistent with other evidence suggesting some type of plume-head origin for the Emeishan province.