剪切带中变斑晶的生长及包裹体痕迹的演化

韧性剪切带中,由于变形分解作用的存在,岩石发生递进变形过程中,产于共轴或非共轴递进缩短带内的变斑晶不发生旋转,而变斑晶内的包裹体痕迹是递进变形过程中遗留在变斑晶内的变形变质痕迹。利用未旋转斑晶中的包裹体痕迹可以确定早期面理的取向,寻找构造演化的时间标志,确定变形变质的关系及其演化史。对北祁连托勒牧场大型走滑韧性剪切带中石榴石、黑云母等变斑晶及包裹体痕迹的研究,揭示了变斑晶的生长和包、裹体痕迹与褶劈理的演化有着重要联系以及剪切变形过程中变形变质演化史、应变速率的变化。递进变形相应地发生递增变质,但两者存在着一定的差异性。

PORPHYROBLAST GROWTH AND EVOLUTION OF THE INCLUSION TRAIL IN DUCTILE SHEAR ZONES

Deformation partitioning occurring in ductile sheac zones gives rise to the growth of noorotated porphyroblasts within the zoae dominated by coaxial or non-coaxial progressivc shortening during progressive deformation.Accordiagly,the iaclusion trails withia the porphyroblasts represent tbe remnant structures and textures of previous deformation and metamorphjsm preserved ia the porphyroblasts.Thus,detailed observation and studies of tbe inclusion trails in the noa-rotated porphyroblasts help to define tbe oricntation of early foliation,uncover the temporal indicator of.tectonic evolution and determine the correlationship between deformation and metamorphism and tbeir evolution history to a more realistic situation.Inclusion trails,within the porphyroblasts such as garaet and biotite ia the large scale Tuole Pasture strike-slip ductile shear zone,North Qilian orogenic belt,suggest that the porphyroblasts did not rotate during ductile deformation,therefore,silimoid inclusion trails and multi-stages of foliation formcd as a result of deformation partitioning and replacement of fbliation in response to progressive deformation.Though with certain extent of delay,progressive deformatioo is concurreat with pro-grade metamorphism in large.The zone undergone most cxtcnsive deformation and metarnorphism is usually referred to be located in the centre of ductile shear zone,but in this ductile shear zone,the most intensive metamorphism and deformation occurred just next to the central part,which is demonstrated by the varia- tion of structure pattern of folding from type B,via type AB to type A,the transitioa of characterized mineral assemMages from Gt +Bi→ Gt+Bi→ Sta→ Gt+Bi+Ky+Chd→Gt+Bi,and thc variation Of grain size Of prophyroblast from small→ larger→largest→fine (but with high conteat) from rim to centre of the ductile shear zone.Fluid process involving in the shearing may have certain effect on strain rate,for example the intecgranular water will facilitate the shcaring and increase tbe strain rate.Under the pro-grade metamorphism,dehydration reactioos would release a nurnber of watcr giving rise to the increasiag of intergranular water,which would enhance the ductile shearing.while metamorphism had rcachcd to the.peak metatrnorphism,hydration reactions would lead to the reduction of iotergraaular water,thererore,the straln rate would greatly decrease.It is showm that there is important relationship between folded cleavage and porphyrobast growth.The geometry and structure patterns of the inclusion trails developed within the porphyroblast do help to reveal the variations of strain rate,unt ravel tectonic evolution history and determine the relation between deformation and metamorphism during ductile shearing.