雪球状石榴子石变斑晶的形成机制:以南迦巴瓦地区雅鲁藏布江缝合带西侧石英片岩为例

雅鲁藏布江缝合带米林地区的石英片岩糜棱岩化强烈,线理及面理构造发育。S-C组构、"σ"残斑以及不对称褶皱等指示了上盘相对下盘向NW下滑的剪切运动趋势。电子背散射衍射(EBSD)测试结果表明:雪球状石榴子石变斑晶边部面理(S2)中石英包裹体晶格优选方位模式图指示的运动指向与石英岩基质面理(或外部面理;S3)中石英包裹体晶格优选方位模式图指示的运动指向一致,都是上盘向NW正滑。然而,雪球状石榴子石的核部(S1)石英包裹体优选方位(LPO)模式图指示相反运动指向。能量色散显微分析(EDS)测试结果表明石榴子石的成分环带显示连续生长环带特征。连接石榴子石核部面理(S1)可以恢复得到石英岩早期不对称褶皱形状的面理轨迹。这些说明文章样品中雪球状石榴子石变斑晶是生长在不对称褶皱之上的。此过程主要是剪切方向发生了旋转,而不是石榴子石自身旋转。这种雪球状石榴子石变斑晶的存在说明南迦巴瓦地区雅鲁藏布江缝合带西侧岩石最初经历向SE的逆冲作用,后期经历由SE向NW的拆离滑脱事件。

The formation mechanism of garnet porphyroblast with snowball structure:A case study from the quartz schist in west Indus-Yarlung Tsangpo suture zone,Namche Barwa,Tibet

The quartz schist from Milin in the Indus-Yarlung Tsangpo suture zone has undergone extensive myIonization with well developed lineation,foliation;its S-C fabric,"σ"type porphyroblasts and asymmetrical folds indicate the north-westward norlnal slip ductile shear deformation.According to the electron backscattered diffraction(EBSD)test results,lattice preferred orientation(LPO)patterns for quartzes in the matrix foliation(or external foliation,S_3)and inclusion-trails at rims of the garnet porphyroblasts(S_2)both shoW atop-down-to-the-NW(normal)shear sense.While LPO patterns for quartzes in the inclusion-trails at the garnet porphyroblast cores(S_1)show an opposite shear sense.The garnet compositional zonation indicates a growth zoning characteristic from energy dispersive spectroscopy(EDS)analysis.By connecting the inclusiontrails in garnet cores,an early stage foliation trace with asymmetrical fold shape can be obtained.It suggests that these garnet porphyroblasts with snowball structure formed on the asymmetrical folds by the rotation of external shear sense rather than the rotation of garnet porphyroblasts.Based on the study of garnet porphyroblast with snowball structure,the author8 proposed that the rocks from the west of Indus-Yarlung Tsangpo suture zone have undergone south-eastward thrusting and then the late from south-eastward to north-westward detachment event.