西藏东巧方辉橄榄岩的显微构造特征及其流变学意义

王永锋; 郑有业; 金振民

西藏东巧方辉橄榄岩的显微构造特征及其流变学意义

1.
中国地质大学地球科学学院, 湖北武汉 430074
2.
中国地质大学资源学院, 湖北武汉 430074

基金项目:

国家自然科学基金项目 40372093

国土资源大调查项目 0100154095

详细信息

作者简介:
王永锋(1978-), 男, 博士生, 从事岩石流变学和显微构造研究.E-mail: luckyyfwang1@sina.com

中图分类号: P585.1
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- 被引次数: 0
出版历程
- 收稿日期: 2004-11-23
- 网络出版日期: 2022-06-22
- 刊出日期: 2005-01-25

Microstructures and Rheology of Harzburgites from Dongqiao, Northern Tibet

1.
Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China
2.
Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China

摘要

摘要: 西藏东巧地区蛇绿岩套中橄榄岩是青藏高原出露的为数不多的地幔岩体之一, 对于揭示该地区的上地幔流变学特征具有重要意义.报道了该地区方辉橄榄岩中橄榄石的位错显微构造特征, 估算了上地幔流变学参数.显微构造研究表明, 东巧方辉橄榄岩发育残斑结构, 橄榄石中位错组态类型比较丰富, 包括自由位错、位错壁(包括宽阔型和紧密型)、位错弓弯和位错网, 表明橄榄石的主导变形机制可能为位错蠕变.该区地幔岩变形大致分为2个阶段: (1) 地幔缓慢塑性流动变形, 形成宽阔型位错壁; (2) 蛇绿岩侵位过程中的变形, 产生紧密型位错壁.根据2种位错壁估算的流动应力平均值分别为3 9.3MPa (宽阔型)和113.9MPa (紧密型), 计算结果获得东巧地区上地幔流动速率为1.13× 10^-12 ~ 2.95× 10^-11s^-1, 有效粘度为4.44×10¹⁷~ 1.16×10¹⁹Pa°s, 这些参数为东巧地区上地幔物理特征提供了流变学的约束条件
- 方辉橄榄岩 /
- 橄榄石 /
- 组构 /
- 位错 /
- 东巧
Abstract: The peridotite section of the Dongqiao ophiolite is one of the few peridotite bodies exposed in Tibet, which may play an important role in revealing the rheological properties of the upper mantle in this area. In this paper, the microstructural and rheological characteristics of harzburgites from Dongqiao are investigated. Microstructural analysis indicates that harzburgites exhibit porphyroclastic texture and undulose extinction and kink bands are common in olivine and orthopyroxene. The oxidation decoration technique is used to examine the dislocation microstructure in olivine. The results show olivines are abundant in dislocation substructures, including free dislocation; dislocation wall, which can be subdivided further into widely and closely spaced types; and dislocation bow and net, indicating that the deformation of olivine may be dominated by dislocation creep. Following the microstructural analysis, two deformation stages are recognized. The first is deformation due to slow upper mantle flow, which results in a wide dislocation wall. The second is deformation related to the emplacement of ophiolite, which generates a closely spaced dislocation wall. The average flow stress calculated is 39.3 MPa (for the wide dislocation wall) and 113.9 MPa (for the closely spaced dislocation wall) respectively. Based on Chopra and Paterson's olivine flow law, the creep rate estimates of the upper mantle in this area are 1.13×10^-12-2.95×10^-11 s^-1 and effective viscosity estimates are 4.44×10¹⁷-1.16×10¹⁹ Pa°s. All these provide fundamental constraints on the rheological properties of the upper mantle in the Dongqiao area.
- harzburgite /
- olivine /
- petrofabric /
- dislocation /
- Dongqiao

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图 1 研究区(东巧)和西藏北部湖区蛇绿岩分布示意图(据王希斌等, 1987略作修改)

Fig. 1. Schematic map showing the study area (Dongqiao) and distribution of ophiolite in northern Tibet

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图 2 东巧地区方辉橄榄岩中橄榄石组构

a.样品TD0021共79颗粒; b.样品TD0024-86颗粒.下半球等面积投影, 等密线: 1%~2%~3%~4%~5%~6%;S.面理; L.线理

Fig. 2. Petrofabric diagram of olivines

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图 3 代表性橄榄石位错壁间距对数频数分布

Fig. 3. Representative histograms displaying the number of dislocation walls versus natural logarithm of the wall spacing

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图版Ⅰ 图版Ⅰ

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表 1 东巧方辉橄榄岩岩石主量元素化学成分和部分主要矿物电子探针分析结果

Table 1. Bulk-rock compositions and representative mineral analyses of harzburgite from Dongqiao

表 2 东巧地区橄榄石的位错壁间距统计和差异应力计算结果

Table 2. Dislocation wall spacing and differential stress of olivine in Dongqiao

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