榴辉岩组构运动学与大陆深俯冲——中国大陆科学钻探主孔榴辉岩的EBSD研究

中国大陆科学钻探主孔位于苏鲁超高压带南部的东海县毛北榴辉岩体之上。主孔0-600米深度的榴辉岩的塑性变形以具中等倾角的东倾面理,近南北向的水平拉伸线理、“A”型剪切褶皱和一系列平行面理的微型韧性剪切带为特征。使用电子背散射（EBSD）技术测量的主孔7个榴辉岩样品的石榴石和绿辉石的晶格优选定向（LPO）表明：石榴石基本上无序排列,而绿辉石表现出强烈的LPO。绿辉石的[001]轴近平行于拉伸线理方向,（010）面的法线和[100]轴垂直面理分布,{110}的法线形成垂直面理的环带,反映绿辉石的位错蠕变由[001]（100）和1／2〈^-110〉滑移系控制,其不对称的LPO指示了由北向南的剪切指向。根据单斜辉石的高温实验结果,毛北榴辉岩经历了800-900℃的超高压变质作用。通过构造重塑,揭示毛北榴辉岩体为剪切流变褶皱,形成于扬子板块深俯冲时的超高压变质过程。因此榴辉岩中保留的早期岩石组构特征可以为板块的深俯冲运动学和俯冲极性提供重要信息。

Fabric kinematics of eclogite and deep continental subduction: EBSD study of eclogite from the main hole of the Chinese Continental Scientific Drilling Project.

The main hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) is located in the Maobei eclogite complex, south Sulu UHP terrane. Eclogites from 0 - 600m in depth of the CCSD-MH are characterized by east-dipping foliation with moderate dip angle, S-N stretching lineation, "A" type shear fold and a series of smallVductile shear zones. Lattice-preferred orientations ( LPO) of omphacite and garnet in 7 eclogite samples were measured using electron backscatter diffraction ( EBSD) technique. Garnet appears to be randomly oriented whereas omphacite displays very strong LPO. The [001 ]-axes of omphacite are concentrated parallel or subparallel to the lineation, the (OlO)-poles and [ 100]-axes normal to the foliation, and { 110 } -poles form the girdle perpendicular to the foliation, implying that the dominant slip systems in omphacite are [001 ] ( 100) and 1/2 < 110 >. Based on high-temperature deformation of clinopyroxene in laboratory, the Maobei eclogites experienced the dislocation creep at 800 -900 C and UHP conditions. The asymmetry of omphacite LPO with respect to foliation and lineation reveals top-to-south shear sense in eclogites, which was formed during the northward deep subduction of the Yangtz plate beneath the North China plate. The reconstructed architecture of the Maobei eclogite complex is a sheared flow fold developed under UHP conditions. Therefore early petrofabrics preserved in eclogites can furnish important kinematic information on the relative plate motions in deep subduction zones.