流体作用下石榴石溶解-沉淀蠕变过程——以红河-哀牢山剪切带内石榴夕线片麻岩研究为例

石榴夕线片麻岩是中、下地壳主要组成岩石之一，岩石内石榴石和夕线石的结晶学优选方位会显著影响地壳深部流变性质，因此探讨特征变质矿物的变形机制和主要受控因素对构造带深部演化过程有深远意义。本文选取红河-哀牢山韧性剪切带内石榴夕线片麻岩为研究对象，通过定向切片内显微构造、电子探针、X-ray成分扫描、电子背散射衍射（EBSD）和相平衡模拟综合研究，揭示出石榴石在溶解沉淀反应过程中具有明显的粒度敏感性，不同粒径石榴石表现出截然不同的长宽比、成分环带、包裹体排列方式和压力影发育情况。石榴石表面流体活动明显截切早期生长环带。EBSD分析揭示石榴石破碎颗粒以绕〈112〉轴机械旋转为主，溶解过程主要集中于颗粒表面和裂隙内高曲率位置。夕线石的EBSD结果表明基质内夕线石以绕〈010〉轴旋转为主，而流体作用明显区域夕线石则以（100）[001]滑移为主。岩石相平衡模拟限定岩石变质峰期P-T条件达高压麻粒岩相，退变过程中同剪切变形导致大量流体渗入而形成降温降压退变轨迹，由~9.5kbar、760℃演化至~6.0kbar、500~600℃，并在粗粒石榴石内保存早期进变质环带，剪切抬升过程中石榴石内普遍发育垂直剪切方向的裂隙，并在流体作用下进一步改造其形态。此研究揭示红河-哀牢山剪切带内除前人报道的石榴石高温韧性变形外，还存在大量中-上地壳层次同变质反应下的溶解-沉淀蠕变作用。因此，石榴石变质-变形的综合研究有助于揭示变质杂岩带挤压-剪切-伸展多阶段构造演化过程。

Fluid-assisted dissolution-precipitation creep of garnet: An example from garnet-sillimanite gneiss in the Red River-Ailao Shan shear zone

Garnet-sillimanite gneiss is one of dominate rock types in middle-lower crust and the crystallographic preferred orientation(CPO)of garnet and sillimanite will significantly affect the rheology features in deep site of tectonic zone.Focus on garnet and sillimanite in paragneiss from the Red River-Ailao Shan shear zone,we exhibit detailed petrographic observation,microstructures,Xray mapping,electron microprobe analysis and electron back scattered diffraction techniques(EBSD)investigation in this study.Our data demonstrated that the deformation of garnet show evident grain-size sensitive and was controlled by local fluid flux.Different grain size porphyroblast exhibit distinct aspect ratios,compositional zoning pattern,inclusions arrangement and pressure shadow styles.Moreover,along the long borders of garnet,part of the growth zoning is truncated by synkinematic fluid activity on the grains surface.EBSD result reveals that the deformation of garnet mainly by subgrains rotation around<112>axis.The prismatic sillimanite in matrix show grain rotation around<010>axis,while those sillimanite in strong fluid flux area display the(001)[100]slip systems.Phase equlibria simulation demonstrate the garnet-sillimanite gneiss experienced high-pressure granulite facies metamorphism and followed by a decompression-cooling trajectory due to huge amount fluid infiltrate during synkinematic exhumation and the P-T condition from~9.5 kbar,760℃turn to~6.0 kbar,500~600℃.The fast cooling/exhumation rate also let the coarse garnets preserved prograde growth zoning pattern.In the latest extension stage fluid along permeable cleavage reformed the shape of garnet grains.Our study also demonstrated that,in the Red River-Ailao Shan shear zone,apart from high temperature dislocation deformation in early pure-shear stage,abundant garnets show simultaneous reaction dissolution-precipitation creep at middle-upper crust site.Consequently,combine with deformation and metamorphic study of garnet is crucial to reveal the compressive-strike slip-stretching evolution history of metamorphic complex.