西藏冈底斯带叶巴组火山岩地球化学及成因  

Geochemistry and Genesis of the Yeba Volcanic Rocks in the Gangdise Magmatic Arc, Tibet

作　　者：耿全如[1] 潘桂棠[2] 金振民[1] 王立全[2] 朱弟成[2] 廖忠礼[2] 

机构地区：[1]中国地质大学地球科学学院,湖北武汉430074 [2]成都地质矿产研究所,四川成都610082

出　　处：《地球科学（中国地质大学学报）》2005年第6期747-760,共14页Earth Science-Journal of China University of Geosciences

基　　金：国土资源大调查项目(No.1212010310102).

摘　　要：叶巴组早侏罗世双峰式火山岩分布在拉萨、达孜至墨竹工卡之间,岩性为浅变质玄武岩、玄武质熔结凝灰岩、英安岩、酸性凝灰岩及火山角砾岩等.火山岩SiO2含量集中在41%～50.4%和64%～69%两个区间,为钙碱性系列的玄武岩和英安岩2类.玄武岩的显著特征是TiO2含量极低,仅为0.66%～1.01%,远低于大陆拉斑玄武岩.玄武岩的稀土总量REE=60.3～135μg/g,英安岩的稀土总量REE=126.4～167.9μg/g.玄武岩和英安岩具有相似的稀土和微量元素特征,两者均为轻稀土富集型,分布特征相似,轻、重稀土的分馏较明显,Eu异常均不显著;均表现为LILE、LREE富集,HFS、HREE亏损的特点.玄武岩亏损Ti、Ta、Nb、Zr,Nb和Ta仅略负亏损,Nb=0.54～1.17,平均为0.84;英安岩亏损HFS中P、Ti,Ta、Nb略负异常,Nb=0.74～1.06,平均为0.86.玄武岩类的εNd(t)=0.96～10.03、(87Sr/86Sr)i=0.7043～0.7064,英安岩的εNd(t)=-1.42～1.08、(87Sr/86Sr)i=0.7038～0.7049.从微量元素和同位素成分看,玄武岩和英安岩浆起源于俯冲带之上的地幔楔不同程度的部分熔融,源岩可能是亏损的尖晶石二辉橄榄岩.源区曾受到具地壳成分特征的流体不均匀交代.后期变质作用对岩石大离子亲石元素含量有影响.叶巴组双峰式火山岩形成于成熟岛弧后期的短暂拉张环境,是印支期冈底斯岩浆弧演化的结果.The Early Jurassic Yeba bimodal volcanic rocks occur among Lhasa, Dazi and Mozhugongka, with major rocks of metabasalt, basaltic ignimbrite, dacite, silicic tuff and volcanic breccia. SiO2 contents in lava rocks are 41%-50. 4% and 64%--69%, belonging to calc-alkaline basalt and dacite. One notable feature of basalt is its low TiO2 contents, 0. 66%-1.01%, much lower than those of continental tholeiite. The ZREE contents of basalt and dacite are 60. 3-135 μg/g and 126. 4--167. 9μg/g respectively. Both rocks have similar REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE plots and without Eu anomaly. The basalts have depleted Ti, Ta, Zr and Nb and slightly negative Nb and Ta anomalies, with Nb^＊ =0. 54--1.17 and an average of 0.84. The dacites have depleted P and Ti and also slightly negative Nb and Ta anomalies, with Nb^＊ =0. 74--1.06 and an average of 0. 86. The end （t）, （87Sr/ 86Sr）i values for basalts are 0. 96--10. 03 and 0. 704 3--0. 706 4. These values for dacites are （-1.42）-1.08 and 0. 703 8- 0. 704 9 respectively. Trace elemental and isotopic studies suggest that both basalt and dacite originated from partial melting of the mantle wedge at different degrees above subducted belt. The spinel lherzolite in upper mantle is likely the source rocks for the bimodal volcanic rocks, which might have undergone selective metasomasis of crustal fluids. Metamorphism at late stage made influence on the LILE contents. The Yeba bimodal volcanic rocks formed in temporal extensional environment in mature island arc settings, resulting from the Indosinian Gangdise magmatic arc.

关 键 词：双峰式火山岩 地球化学 成因 冈底斯岩浆弧 印支晚期-燕山早期 

分 类 号：P588.14[天文地球—岩石学]