花岗质超糜棱岩的微观构造与变形机制

Microstructural characteristics and deformation mechanismsof granitic ultramylonite during ductile rheology

花岗质岩石的变形方式和过程决定大陆地壳的流变学特性。本文聚焦藏南拆离系超糜棱岩化的花岗质岩石,借助传统显微构造分析方法和扫描电镜、阴极发光、矿相自动分析系统和电子背散射衍射等新技术手段,开展微观组分、结构、组构定量化观测和分析。超糜棱岩主要造岩矿物为钾长石、斜长石、石英、黑云母等,显微构造呈现为单矿物相域与多相矿物混合域交织结构。相平衡模拟与斜长石钙含量等值线变形温度估算结果为390~410℃。单相域的矿物集合体条带主要分为钾长石条带与石英条带,其中钾长石条带内变形颗粒呈现典型的核-幔构造。组构分析表明钾长石颗粒具有强烈的晶格优选定向,残斑与动态重结晶的钾长石颗粒具有相似的晶格优选方位(CPOs)特征。施密特因子法分析揭示钾长石残斑变形过程中主要活动的滑移系为(100)[010]、(010)[001]和(001)[100],基质钾长石颗粒形成机制主要为位错蠕变驱动的亚颗粒旋转重结晶。在混合相域,矿物颗粒发生强烈细粒化而只含有少量残斑,基质颗粒主要为斜长石,斜长石颗粒间广泛分布微米级黑云母颗粒。斜长石无组构或弱组构,主导变形机制为颗粒边界滑动。在单相域条带与混合相域基质内,石英颗粒均发生强烈细粒化,颗粒表面发育溶蚀结构以及细小的新晶晶核,石英轴晶格优选定向及形态学长轴优选定向皆平行于线理X方向,变形机制为溶解-沉淀蠕变。这显示在由单相域向混合相域的演化过程中,流体作用至关重要,流体与单相域钾长石进行交代使其分解为细粒的斜长石与石英,并导致花岗质岩石变形机制由位错蠕变向非位错蠕变转换,并诱发岩石的流变弱化。

The deformation patterns and processes of granitic rocks determine the rheological behavior in the continental crust.In this contribution,we focused on the granitic ultramylonite in the South Tibetan Detachment System to investigate the microstructures,fabrics and compositions by various methods,including scanning electron microscopy,cathode luminescence,automatic mineral analyzer and electron backscattered diffraction.The mainly mineral compositions of the ultramylonite are K-feldspar,plagioclase,quartz and biotite,etc.The microstructure is characterized by significant mylonitic foliation defined by compositional banding,namely,a combination of monophase domains and polyphase domains.The deformation temperatures estimated by phase equilibrium simulation and compositional isopleths of Ca component in plagioclase range from 390℃to 410℃.The monophase domains are mainly divided into K-feldspar domains and quartz domains.The microstructures of K-feldspar domains show typical core-and-mantle structures.K-feldspars show intense fabrics and the similar CPO patterns for both porphyroclasts and matrix grains.The Schmid factor analysis for the porphyroclasts of K-feldspar indicates that active slip systems are(100)[010],(010)[001]and(001)[100]during ductile deformation.The formation mechanism of K-feldspar matrix grains is the dynamic recrystallization by subgrain rotation.Dislocation creep is the dominant deformation mechanism for K-feldspar in the monophase domains.In the polyphase domains,the minerals are featured by fine-grained matrix and only a few porphyroclasts exist.The matrix grains are mainly plagioclases and the micron-sized biotites are widely distributed at the grain boundaries of the plagioclases.Plagioclases show random fabrics or weakly fabrics in the polyphase domains.The deformation mechanism is dominated by grain boundary sliding in plagioclase.In the monophase and polyphase domains,the fine-grained quartz crystal show dissolution structures on their surfaces.The crystallized preferred orientation of the quartz c-axes and the preferred orientation of the quartz morphology long axis are both parallel to the lineation direction.Dissolution-precipitation creep is the major deformation mechanism in quartz.Fluid can make an important impact on the evolution from monophase domains to polyphase domains during ultramylonization.The fluid promotes the monophase K-feldspar to break into fine-grained plagioclase and quartz,which can further lead to the deformation mechanism transition from dislocation creep to non-dislocation creep and the rheological weakening of the rock.