| 西藏雄梅地区晚白垩世江巴组火山岩岩石成因及对加厚地壳减薄的指示 |
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| 引用本文： | 孙渺,陈伟,曲晓明,马旭东,丁吉顺.西藏雄梅地区晚白垩世江巴组火山岩岩石成因及对加厚地壳减薄的指示[J].地球科学,2018,43(9):3234-3251. |
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| 作者姓名： | 孙渺 陈伟 曲晓明 马旭东 丁吉顺 |
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| 作者单位： | 1.中国地质科学院矿产资源研究所, 北京 100037 |
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| 基金项目： | 中国地质调查局项目DD20160026 |
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| 摘 要： | 江巴组火山岩是西藏雄梅地区近年来发现的火山岩，但其岩石成因尚不明确.通过开展系统的岩相学、地球化学、同位素定年示踪研究，结果表明英安质火山岩锆石U-Pb年龄为85.1±1.0 Ma，为晚白垩世.安山质和英安质火山岩显示出明显的埃达克质岩特征，主量元素显示出较高的SiO2和Al2O3含量及Mg#值；稀土元素整体呈轻稀土元素富集，重稀土元素强烈亏损，轻重稀土元素分异明显，（La/Yb）N值较高，无明显Eu异常；微量元素具有明显的高Sr，低Y、Yb和高Sr/Y值，相对富集Th、Zr和Hf，亏损Nb、Ta和Ti的特点；英安质火山岩锆石εHf（t）值均为正值，在+2.7~+7.1之间，指示有亏损地幔物质参与成岩作用；以上表明安山质和英安质火山岩可能形成于加厚新生下地壳拆沉的部分熔融并有亏损地幔物质的加入.流纹质火山岩具相对低的MgO、TiO2含量；LREEs富集、HREEs亏损，但轻重稀土元素分馏明显较安山质和英安质火山岩弱，微量元素富集Rb、Th和K，强烈亏损Eu，Sr，Ba，P和Ti，明显的负Eu异常，表明流纹质火山岩为地壳发生深熔而形成.综合对比江巴组火山岩的地球化学性质，表明班公湖-怒江缝合带中段昂龙岗日-班戈弧地区在晚白垩世期间存在一期板内加厚下地壳拆沉减薄事件.
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| 关 键 词： | 班公湖-怒江缝合带 江巴组火山岩 埃达克质岩 加厚新生下地壳 地球化学 |
| 收稿时间： | 2017-12-15 |
Petrogenesis of the Late Cretaceous Jiangba Volcanic Rocks and Its Indications for the Thinning of the Thickened Crust in Xiongmei Area,Tibet |
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| Abstract: | The Jiangba Formation volcanic rocks are found in the Xiongmei area in recent years, however its petrogenesis is still unclear. This paper has carried out a systematic petrography, geochemistry and isotopy study on the intermediate-acid Jiangba Formation volcanic rocks in the Xiongmei area, Tibet. The zircon U-Pb age of 85.1±1.0 Ma has been obtained from the dacitic volcanic rocks. The dacitic and andesitic volcanic rocks show the typical geochemical characteristics of adakite, such as high contents of SiO2, Al2O3, Sr concentrations, Mg# values, and Sr/Y ratios, and low contents of Y and Yb. The dacitic and andesitic volcanic rocks are relatively enriched in LREEs (light rare earth elements), depleted in HREEs (heavy rare earth elements), and high (La/Yb)N values without obvious Eu anomalies. The spider diagram of the trace elements of the dacitic and andesitic volcanic rocks display enrichment in Th, Zr and Hf, and depletion in Nb, Ta and Ti. In addition, these dacitic volcanic rocks have positive zircon εHf(t) values (+2.7~+7.1), indicative of the participation of the depleted mantle in the magmatism. These signatures indicate the dacitic volcanic rocks and andesitic volcanic rocks were most likely derived from partial melting of a delaminated young lower crust. The rhyolitic volcanic rocks have relatively lower contents of MgO and TiO2 relative to the dacitic and andesitic volcanic rocks. These rocks are enriched in LREEs, depleted in HREEs, but show less fractionation between LREEs and HREEs than the dacitic and andesitic volcanic rocks, with strongly negative Eu anomalies. On spider diagram of the trace elements, the rhyolitic volcanic rocks show enrichment in Th, Rb and K, and strongly negative Eu, Sr, Ba, P and Ti. These indicate the rhyolitic volcanic rocks were likely derived from remelting of pure crust. These facts indicate a magmatic event caused by the delamination of the thickened lower crust in the Anglonggangri-Bangor magmatic arc of the middle Bangonghu-Nujiang suture zone during the Late Cretaceous. |
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