| 青海南部三叠纪结扎群火山岩地球化学特征及其构造环境分析 |
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| 引用本文：马丽艳,白云山,牛志军,王建雄,段其发.2007.青海南部三叠纪结扎群火山岩地球化学特征及其构造环境分析[J].地球学报,28(5):428-437. |
| DOI：10.3975/cagsb.2007.05.03 |
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| 基金项目:中国地质调查局1:25万直根尕卡幅、曲麻莱县区调项目(编号:20001300009121、200313000006) |
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| 中文摘要:对青海南部治多-杂多一带晚三叠世结扎群火山岩进行了主量元素、稀土元素、微量元素及Sr、Nd同位素分析.结果表明:该火山岩是一套主要由玄武质粗面安山岩、粗面安山岩组成的偏碱性岩石,岩石化学上表现为低Ti,高Al;稀土元素特征表现为轻稀土元素富集,轻重稀土呈现较强的分馏并具有轻微的负铕异常;在微量元素地球化学方面,表现为富集大离子亲石元素,高场强元素Nb、Zr、P、Ti相对亏损,略具Ta、Nb槽;在Hf/3-Th-Nb/16及10TiO2-Al2O3-10K2O图解中,样品均落入岛弧火山岩区.火山岩的ISr、εNd(t)变化在0.70728～0.70818、-2.4～-4.2之间,表明形成晚三叠世火山岩的原始岩浆是壳幔混合型.综合上述火山岩的各种地球化学特征,表明该套火山岩形成于岛弧环境,由于晚三叠世班公湖-怒江洋发生海底扩张,甘孜-理塘洋向南西俯冲消减,产生强烈的碰撞造山,地幔物质在上涌过程中,受到陆壳的混染,结扎群火山岩为挤压造山机制下的陆缘火山弧岩石. |
| 中文关键词:三叠纪 结扎群 火山岩 地球化学特征 青海 |
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| Geochemistry and Tectonic Settings of Volcanic Rocks in Late Triassic Gyeza Group of Southern Qinghai |
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| Abstract:Whole rock analyses comprising major,REE and trace elements and Sr-Nd isotopic geochemistry were conducted for samples of volcanic rocks from Late Triassic Gyeza Group of southern Qinghai to reveal the magma source signature of these volcanic rocks in Qiangtang Block and their tectonic setting.The results show that the volcanic rocks are composed of basaltic trachyandesite,trachyandesite and trachybasalt,characterized by poor Al and high Ti.In REE elements,they form a LREE-enriched distribution pattern with a strong contrast between LREE and HREE and the existence of weak negative Eu anomaly.In trace elements,the rocks have rich LILE and depleted HFSE with slight Nb and Ta negative anomalies.All the samples are plotted in the region of island arc basalt on Hf/3-Th-Nb/16 and 10TiO2-Al2O3-10K2O diagrams.ISr,INd isotope ratios range from 0.70728 to 0.70818 and from 0.512132 to 0.512249,respectively,whereas εNd(t) values are negative,ranging from-2.4 to-4.2,which indicate that the primary magma forming late Triassic volcanic rocks might have been of the crust-mantle mixing type.These characteristics suggest that the rocks were formed in an island arc setting.Spreading of Bangong Co-Nujiang ocean-floor during Late Triassic induced southwestward subduction of Garzê-Litang ocean and strong collision between Qiangtang and Yangtze Blocks,and then the mantle substance erupted with significant upper crust contamination during its upward movement.All this implies that volcanic rocks from Gyeza Group were derived from the continental margin arc under compressional orogenic mechanism. |
| keywords:Triassic Gyeza Group volcanic rock geochemistry Qinghai |
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