云南会泽峨眉山玄武岩特征及成因

发布时间：2018-05-23 08:21

  本文选题：峨眉山玄武岩 + 岩石地球化学　； 参考：《东华理工大学》2017年硕士论文

【摘要】：峨眉山玄武岩主要分布于中国四川、云南、贵州等地,关于峨眉山玄武岩的研究一直是国内外地质学家热点之一。云南会泽县位于扬子板块西南缘,区内峨眉山玄武岩广泛发布,出露厚度1314m左右,本文结合野外实测剖面数据,对该地区峨眉山玄武岩的岩相学及岩石地球化学等方面进行了的研究,进而分析了其岩浆源区、演化及其岩石成因。会泽峨眉山玄武岩系岩石主要以基性熔岩(溢流相)为主,含少量火山角砾岩(爆发相)、凝灰岩(空落相)。韵律旋回性特征明显,其中熔岩、火山碎屑类的喷发总体表现为:致密状玄武岩→斑状玄武岩→杏仁状玄武岩组合→凝灰岩,熔岩类(溢流相)岩石厚度比较巨大,熔岩类多具有少斑状-斑状结构和微晶质结构,部分具有隐晶结构,斑晶矿物以单斜辉石为主,局部为斜长石,主要发生低温蚀变。火山角砾岩局部出露,凝灰岩分布较为广泛,但厚度较薄。会泽峨眉山玄武岩SiO_2含量在46.02%~50.91%之间,含量变化较小,平均值48.5%;TiO2含量变化较大,变化于3.32%~4.84%之间;Na2O/K2O变化范围为0.53-5.70;Mg#变化于37.01%~67.86%,均值为50.47%,其Mg#值均相对较低。稀土元素特征展现出轻稀土元素富集,重稀土相对亏损,轻重稀土分馏特征明显(∑REE=170.16×10-6~286.02×10-6,LREE/HREE=7.23~9.56),稀土配分模式表现出一致的陡右倾型,Eu、Ce具弱的正异常或弱的负异常。相容及不相容元素特征显示:高场强元素富集(Rb、Th、U、Nb、La、Ce、Ti、Sr、Zr、Sm等),而K、Ti、P明显亏损,Mg#与Co、Cr、Ni、Nb呈明显的负相关关系。不相容元素比值与EM-1板内洋岛玄武岩或EM-2板内洋岛玄武岩相当。根据岩相学及主量、轻重稀土元素及分布特征分析,会泽峨眉山玄武岩形成的构造环境为板内拉张环境,研究区峨眉山玄武岩TiO2介于2.15%~4.84%之间,Ti/Y介于440~620,说明研究区的玄武岩中既有高钛玄武岩,也有低钛玄武岩,为地幔柱中带的产物。研究区峨眉山玄武岩Nb/U平均值为27.71,处于地壳与大洋中脊玄武岩(MORB)之间,同时玄武岩Th、U、K、Ti、P明显亏损,部分样品还有明显的Ta负异常,说明岩浆上升过程中受到了地壳混染作用。研究区的峨眉山玄武岩基本不显示Eu异常,δEu=0.95~1.08,表明斜长石结晶程度低,岩浆结晶分离作用很弱,末期岩浆上升过程中受到了地壳混染作用。
[Abstract]:Emeishan basalt is mainly distributed in Sichuan, Yunnan, Guizhou and other places in China. The study of Emei mountain basalt has always been one of the hot spots of geologists at home and abroad. Huize County in Yunnan Province is located in the southwest margin of Yangtze plate. The Emeishan basalt is widely released in this area, and the exposed thickness is about 1314m. The petrographic and petrogeochemical aspects of the Emeishan basalt in this area are studied, and the magmatic source, evolution and petrogenesis of the basalt are analyzed. The basalt rocks in Huize Emeishan are mainly composed of basic lava (overflow facies) and a small amount of volcanic breccia (explosive facies and tuff). The characteristics of rhythmic cyclicity are obvious, among which the eruptions of lava and pyroclastic are as follows: dense basalt, porphyry basalt and almond-like basalt assemblage tuff, and the thickness of lava (overflow phase) rock is very large. Most of the lava have less porphyry-porphyry structure and microcrystalline structure and some have cryptic structure. The porphyry minerals are mainly clinopyroxene and some are plagioclase. Volcanic breccia is localized, tuff is widely distributed, but its thickness is thin. The content of SiO_2 in the basalt of Huize Emeishan is between 46.02% and 50.91%, the average value of which is 48.5%, and the average value is 48.5%. The variation range of Na _ 2O / K _ 2O is between 3.32% and 4.84%, ranging from 0.53-5.70% to 37.01% and the mean value is 50.47.The Mg# value is relatively low. The REE features show light REE enrichment, relative depletion of heavy REE, and obvious fractionation characteristics of LREE / R (鈭，

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