秦岭商丹带沙沟糜棱岩带的显微构造及其(p)-T-t演化路径的再认识

对沙沟糜棱岩带的78个样品进行了显微构造与组构分析。石英以动态重结晶Ⅱ型条带为主,其C-轴组构型式为极密Ⅰ型,同时可见Ⅲ型石英条带残存。长石均显脆性碎裂变形,仅钾长石略具韧性变形。糜棱岩面理普遍绕过石榴石斑晶分布。存在多次后期脆性变形构造。这些显微构造与组构特征表明,该带糜棱岩化阶段处于中─高绿片岩相条件、并大致发生在晚白垩世以后。糜棱岩化阶段之前该带可能存在一个角闪岩相左行韧性剪切变形阶段。糜棱岩化阶段之后,该带直接进入脆性变形阶段。据此,笔者对前人有关沙沟糜棱岩带(p)-T-t演化路径提出修正意见。

MICROSTRUCTURES AND RE-RECOGNIZATION OF (p)-T-t PATH IN THE SHAGOU MYLONITE ZONE OF THE SHANGDAN ZONE,QINLING BELT,CHINA

nalyses of microstructures and fabrics in 78 samples from the ShagouMylonite Zone have been carried out.In this zone, quartz dominantly corresponds to Type Ⅱ ribbons of dynamically recrystallized grains, which are producedby grain boundary migration and progressive subgrain rotation.The c-fabrics ofthese recrystallized quartz grains are of the pattern of Maximum Ⅰ.In some seriously deformed parts, quartz ribbons show the transition from Type Ⅱ to Type Ⅰ.Simultaneously, some Type Ⅲ quartz ribbons remain in some weakly mylonizedlocations, where some traces of sinistral shearing are kept.All of feldspars showtypical features of brittle deformation of cataclasis, but K-feldspar shows a littleof ductile deformation, which is represented by recrystallized grains and myrmekiteexisting on the boundaries and cracks of K-feldspar porphyroclasts.Themylonitic foliation generally spreads around garnet porphyroblasts.Otherwise,there exist some pseudotachylite caused by part-melting and tectonic breccia produced by multiple deformation of extension.Microstructures and fabrics of theShagou Mylonite Zone obviously indicated that the mylonization phase happenedunder conditions of mid-upper greenschist grade and ought to be after LateCretaceous.Before the mylonization phase, there possibly existed a ductiledeformation of sinistral shearing under amphibolite grade.After the mylonizationphase, this zone went immediately into a brittle deformation phase ofmultiactivation.Depending on above facts, it is considered that the possible(p)-T-t path of the Shagou Mylonite Zone should have four major phasesas following: (1) Granite intrusion phase happened approximately duringIndo-China epoch (211± 8 Ma), (2)Ductile deformation phase of sinistralshearing under amphibolite grade during Late Triassic to Early Cretaceous,(3) Mylonization phase under mid -upper greenschist grade after Early Cretaceous;(4) Brittle deformation phase relative to general up -lift of this zone approximatelyduring Late Cretaceous.