Microstructures, petrofabrics and seismic properties of ultra high-pressure eclogites from Sulu region, China: implications for rheology of subducted continental crust and origin of mantle reflections
Microstructures, petrofabrics and seismic properties of ultra high-pressure eclogites from Sulu region, China: implications for rheology of subducted continental crust and origin of mantle reflections
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DOI：
10.1016/s0040-1951(03)00177-x
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发表时间：
2003-07
期刊：
影响因子：
2.9
通讯作者：
S. Ji;K. Saruwatari;D. Mainprice;R. Wirth;Zhiqin Xu;B. Xia
中科院分区：
文献类型：
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作者：
S. Ji;K. Saruwatari;D. Mainprice;R. Wirth;Zhiqin Xu;B. Xia
来自苏鲁地区(中国)的超高压榴辉岩代表大陆地壳的镁铁质成分，它们首先被俯冲到大于10 0公里的地幔深度，然后被折返到地表。通过对超高压榴辉岩的显微构造、化学成分、岩石组构和地震性质的详细研究，可以为研究俯冲大陆地壳的运作变形机制和流变性以及上地幔内地震反射的成因提供重要信息。本文介绍了中国大陆科学钻探计划(CCSD)钻探现场(江苏省东海县茅北)新露头榴辉岩的野外观测、光学和透射电子显微镜观测、电子背散射(EBSD)测量和地震性质的数值计算结果。榴辉岩分为两类：第一类榴辉岩是在变质高峰期由恢复调节位错蠕变变形而成的粗粒榴辉岩；第二类榴辉岩是在超高压变质岩折返过程中活跃的剪切带中重结晶调节位错蠕变的第二类榴辉岩(细粒榴辉岩)。榴辉岩中的石榴石和绿辉石都发生了塑性变形，在超高压变质条件下，两种矿物之间的流动强度差明显小于一个数量级。榴辉岩在超高压条件下的塑性可以有效地促进板间剪切带的沟道流动。大陆岩石圈中残留的地壳物质的保存可能会在上地幔产生区域性广泛的、强烈的地震反射。这可能解释了在世界上许多地区观察到的地幔反射的起源。
Ultra high-pressure (UHP) eclogites from Sulu region (China) represent mafic components of the continental crust, which were first subducted to mantle depths greater than 100 km and then exhumed to the earth's surface. Detailed investigation of microstructures, chemical compositions, petrofabrics and seismic properties of the UHP eclogites can provide important information on the operating deformation mechanisms and rheology of subducted continental crust and on the origin of seismic reflections within the upper mantle. We present here results from field, optical and TEM observations, electron back-scattered diffraction (EBSD) measurements and numerical computations of the seismic properties of UHP eclogites collected from fresh surface outcrops at the drill site (Maobei, Donghai County, Jiangsu Province) of the Chinese Continental Scientific Drilling Program (CCSD). Two types of eclogites have been distinguished: Type-1 (coarse-grained) eclogites deformed by recovery-accommodated dislocation creep at the peak metamorphic conditions, and Type-2 (fine-grained) eclogites which are composed of reworked Type-1 materials during recrystallization-accommodated dislocation creep in shear zones which were active during the exhumation of the UHP metamorphic rocks. Both garnet and omphacite in these eclogites deformed plastically and the flow strength contrast between these two constituent minerals is apparently much less than an order of magnitude under the UHP metamorphic conditions. Plasticity of eclogites under UHP conditions can effectively facilitate channeled flow along the interplate shear zone. The preservation of the relict crustal materials within the continental lithosphere may produce regionally extensive, strong, seismic reflections in the upper mantle. This may explain the origin of mantle reflections observed in many areas of the world.