柴达木北缘鱼卡-落凤坡榴辉岩-片麻岩单元的变质变形演化

    Metamorphic and deformational evolution of the Iqe-Luofengpo eclogite-gneiss unit in the North Qaidam Mountains

    • 摘要: 鱼卡-落凤坡榴辉岩-片麻岩单元位于柴北缘HP/UHP 变质带的西段。微构造分析和岩相学观察显示,榴辉岩及相关岩石经历了3期与俯冲和折返作用有关的变质变形阶段:①前榴辉岩相阶段,变质变形组构主要以包裹体的形式保存在具有生长环带的石榴子石核部,矿物组合为Ep+Pl+Amp,并局部显示出S形或反S形分布的特征,反映与俯冲作用有关的变形组构以不对称的旋转应变为特征。②榴辉岩相变质变形阶段,以绿辉石、多硅白云母等矿物围绕石榴子石定向分布为特征,构成榴辉岩相条件下的面理和拉伸线理。缺乏明显的不对称组构,显示榴辉岩相的变形作用以共轴变形为特征。③后榴辉岩相变质变形阶段,以角闪石、斜长石等矿物的定向分布为特征,其变形组构主要存在于围绕榴辉岩透镜体分布的退变榴辉岩(角闪石化榴辉岩)和围岩中,与区域上占主导地位的片麻岩中角闪岩相的变形构造一致,与榴辉岩的折返作用有关。榴辉岩及相关岩石的变质变形演化代表了鱼卡-落凤坡榴辉岩-片麻岩单元从俯冲到折返的构造热历史。

       

      Abstract: The Iqe-Luofengpo eclogite-gneiss unit is situated in the western segment of the northern Qaidam marginal HP/UHP metamorphic belt. Microstructures and petrographic data suggest that eclogite and associated rocks underwent three stages of metamorphism and deformation related to subduction and subsequent exhumation. (1) Pre-eclogite facies stage. This stage is inferred from garnet cores with growth zoning and locally with “S”- or anti-“S”-shaped inclusion trails, consisting of epidote, amphibole and plagioclase, suggesting an asymmetrical rotational deformational strain of deformation fabrics related to subduction. (2) Eclogite-facies metamorphism and deformation stage. This stage is defined by oriented distribution of omphacite, phengite and zoisite around garnet, forming stretching lineation and foliation under eclogite-facies conditions. Lack of distinct asymmetrical fabrics indicates that the deformation under eclogite-facies conditions was coaxial. (3) Post-eclogite facies metamorphism and deformation stage. This stage is characterized by oriented distribution of amphiboles and plagioclase, and the deformation fabric mainly occurs in retrogressive eclogite (hornblendized eclogite) distributed around eclogite lenses and country rocks, which is in agreement with widespread regional amphibole-facies structure and is related to exhumation of HP/UHP rocks. The deformation and metamorphic evolution of eclogite and related rocks reflects the tectono-thermal history from the subduction to exhumation of the Iqe-Luofengpo eclogite-gneiss unit.

       

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