| 俯冲带流体作用的地球化学示踪 |
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| 引用本文： | 黄德志,高俊,戴塔根.俯冲带流体作用的地球化学示踪[J].地学前缘,2001,8(3):131-140. |
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| 作者姓名： | 黄德志 高俊 戴塔根 |
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| 作者单位： | 1. 中南大学资源环境与建筑工程学院
2. 中国科学院地质与地球物理研究所 |
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| 基金项目： | 国家自然科学基金资助项目 ( 49972 0 79)，博士后基金资助项目 |
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| 摘 要： | 俯冲带流体包括俯冲带岩石脱水形成的水流体和熔融作用产生的熔体。俯冲带流体循环及伴随的化学作用是壳幔演化和物质交换的重要机制。岛弧火山岩地球化学研究可以示踪流体的性质、成分。造山带变质岩内高压脉是俯冲带流体活动的直接记录。通过高压脉及其主岩地球化学研究 ,可以示踪流体成分和来源。俯冲带水流体成分与流体释放深度有关 ,俯冲带深处 ( >5 0km)形成的水流体内溶质相当可观。水流体流动方式有大规模沟道式和小规模弥散式 ,与其相应的流体来源分别为外来的和内部的。俯冲带流体组分、流动方式、物质交换机制与俯冲带深度、热状态、物质结构的定量关系 ,以及矿物 /水流体间元素分配系数的高压实验可能将是俯冲带流体今后研究的重要课题。中国西天山高压变质带榴辉岩及蓝片岩中广泛发育高压脉 ,根据脉的矿物组合 ,高压脉可分成 4种类型。高压脉氧同位素研究表明 ,脉体与流体间氧同位素交换宏观与局部平衡 ,流体活动为大规模的 ;流体活动方式既有沟道式 ,又有渗透式 ;流体的δ18O值为 + 10 .8‰左右 ,流体来源于主岩经过海水低温蚀变的大洋玄武岩。西天山广泛发育的高压脉 (尤其是蓝片岩中的高压脉的发现目前在世界范围尚属首例 ) ,是研究古俯冲带流体作用良好的野外天然实验室。
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| 关 键 词： | 俯冲带流体 地球化学示踪 高压脉 岛弧火山岩 |
| 文章编号： | 1005-2321(2001)03-0131-10 |
| 修稿时间： | 2001年4月1日 |
GEOCHEMICAL TRACING OF THE FLUIDS IN SUBDUCTION ZONES |
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| HUANG De-zhi ,GAO Jun ,DAI Ta-geng.GEOCHEMICAL TRACING OF THE FLUIDS IN SUBDUCTION ZONES[J].Earth Science Frontiers,2001,8(3):131-140. |
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| Authors: | HUANG De-zhi GAO Jun DAI Ta-geng |
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| Institution: | HUANG De-zhi 1,GAO Jun 2,DAI Ta-geng 1 |
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| Abstract: | There are two kinds of fluids in subduction zones, the aqueous fluid released by dehydration and the melt created by melting. Cycling processes and simultaneous chemical reactions of fluids in subduction zones play an important role in the evolutions of Earth crust and mantle and in the substance exchange. The geochemistry of island-arc volcanics can trace the feature and composition of fluids. High-pressure veins cutting metamorphic rocks in the orogenic belt have recorded the fluid activity directly. The composition and the source of fluids could be traced by the geochemical study on the high-pressure veins and related host rocks. The composition of fluids is related to the depth at which the fluids were released. The aqueous fluids have a considerably high solubility within the depth to a subduction zone (>50km). Fluids can migrate in a channel-controlled way on a large scale or in a pervasive penetration way on a limited scale. The source of fluids is external or internal respectively. The quantitative relationships between chemical composition, migration way and substance exchange mechanism of fluids in subduction zones on the one hand and the depth, thermal state and construction of subduction zones on the other will be the important subjects for future studies on the fluids in subduction zone. Southwestern Tianshan Mountains, China, show a very good outcrop of high-pressure veins. Four different types of high-pressure veins can be identified according to the mineral assemblages of veins. Variations in δ 18 O values of omphacites between the different veins is less than 0.9‰. The omphacites at different locations of a vein almost have an equal δ 18 O values(variations in δ 18 O omp ≤0.31‰).Except for one sample,the δ 18 O values of quartz separated from different veins vary between 12‰ and 13.9‰.For a certain host-vein pair, the calculated δ 18 O values of whole-rock of the host rock under the metamorphic temperature of the eclogites in the area using the δ 18 O values of quartz or omphacites of the veins, are almost the same as the analytical values. The authors suggested that, on either a large or microscopic scale, the oxygen isotope exchange between host rocks and the fluids in the ancient subduction zones nearly equilibrated during the formation of the high-pressure veins.The δ 18 O values of the fluids in the subduction zones is about +10.8 ‰.The fluids were released by host rocks-the oceanic basalts which had undergone low temperature alteration by sea water.The results of oxygen isotope of high-pressure veins and host rocks in Tianshan indicate that there were certain amounts of fluids ever released by host rocks at blueschist to eclogite transition, and the movement of the fluids was on a large scale. |
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| Keywords: | fluids in subduction zones geochemistry trace high-pressure veins island-arc volcanics |
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