西藏东巧方辉橄榄岩的显微构造特征及其流变学意义

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 

Microstructures and Rheology of Harzburgites from Dongqiao, Northern Tibet

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.