云南宾川地区峨眉山玄武岩地球化学特征:岩石类型及随时间演化规律  

GEOCHEMISTRY OF EMEISHAN FLOOD BASALTS AT BINCHUAN AREA, SW CHINA: ROCK TYPES AND TEMPORAL EVOLUTION

作　　者：肖龙[1] 徐义刚[1] 梅厚钧[1] 沙绍礼 

机构地区：[1]中国科学院广州地球化学研究所,广州510640 [2]云南省地质矿产局第三地质大队,大理671000

出　　处：《地质科学》2003年第4期478-494,共17页Chinese Journal of Geology(Scientia Geologica Sinica)

基　　金：中国科学院知识创新项目 (KZCX2 1 0 1 );国家 973项目 (G1 9990 43 2 0 5 )的成果

摘　　要：分布于云南宾川地区、厚逾 5 0 0 0m的晚二叠世峨眉山玄武岩以拉斑玄武岩为主 ,少量碱性玄武岩。根据它们的岩相学和主量元素、微量元素特征 ,将其划分为两个地球化学类型 :低钛玄武岩 (LT)和高钛玄武岩 (HT)。低钛玄武岩主要分布于岩石剖面的中 下部 ,其主要地球化学标志为低Ti/Y( <5 0 0 ) ,高Mg# ( 0 48～ 0 67) ,低的∑REE ( <1 2 0 μg/g)、(La/Yb) N( <6)和 (Sm/Yb) N( <2 5 )。高钛玄武岩分布于剖面的顶部 ,以高Ti/Y( >5 0 0 ) ,低Mg# ( 0 39～0 5 3)、高的∑REE ( >1 5 0 )、(La/Yb) N( >9)和 (Sm/Yb) N( >3)为特征。根据LT的分异指数和Th、U异常特征等 ,将其分为LT1和LT2两个亚类。其中LT1位于岩石剖面下部 ,表现为明显的Th和U正异常 ,高Mg# ( 0 61～ 0 67)和低Nb/U比值等。LT2位于岩石剖面中部 ,具Rb、Ba正异常 ,无Th和U异常和较低的分异指数 (Mg# =0 48～ 0 5 4 )。LT玄武岩可能是峨眉地幔热柱主活动期地幔柱头部熔融的产物 ,后经较弱的橄榄石 +单斜辉石±斜长石结晶分异形成。剖面最底部的LT1玄武岩显示较强的“壳源”印记 ,可能与岩石圈地幔中富集组份的活化有关。这些壳源物质的参与程度自底部向上有降低的趋势。晚期HT玄武岩为地幔热柱消亡期的产物 ,其在地壳浅部经历强烈的以?The Late Permian Emeishan flood basalts, with thickness over 5 000 meters at Binchuan area in western margin of the Yangtze craton, consisted mainly of tholeiitic basalts and minor alkaline basalts. Based on petrography, major and trace element data, the tholeiitic basalts could be classified into two major geochemical types: low Ti (LT) and high Ti (HT) basalts. The LT type lavas, occurred in the lower middle part of petrologic column, exhibited lower Ti/Y (<500), ∑REE (<120), (La/Yb) N (<6), (Sm/Yb) N (<2.0) and higher Mg # (0.48～0.67); whereas the HT type lavas on the top of the petrologic column had higher Ti/Y (>500), ∑REE (>150), (La/Yb) N (>9), (Sm/Yb) N (>3) and lower Mg # (0.39～0.53). The LT could be further divided into two subtypes: LT1 and LT2. They occurred in the lower and middle parts of the petrologic column respectively. LT1 lavas exhibited relatively higher Mg # (0.61～0.67) and lower Nb/U ratio than that of the LT2 type lavas, and had positive Th and U anomaly. The LT and HT lavas might be originated from different mantle source under various melting conditions and underwent distinct differentiation and contamination processes. The LT lavas, with significant 'crustal source' signatures, might associate with the mobilization of enrichment component in lithosphere mantle. 'Crustal source' signatures were getting less from the bottom (LT1) to the top (HT) of the petrologic column. LT lavas that might be formed at the main activity stage of the Emeishan plume, might be generated from the head of mantle plume with higher degree partial melting and then experienced weak garnet + clinopyroxene fractional crystallization. Late waning stage formed HT lavas experienced lower degree of partial melting, and then underwent strong plagioclase dominated fractional crystallization process in the upper crust and little crustal contamination.

关 键 词：云南 峨眉山玄武岩 地球化学 岩石类型 地幔柱 

分 类 号：P588.1[天文地球—岩石学]