Verification of superplastic flow in the crust and mantle

本文利用电子背散射衍射（EBSD）技术，对武库马山脉中沿Hatagawa剪切带的花岗岩糜棱岩，以及来自Hidaka变质带的辉长岩糜棱岩和橄榄岩糜棱岩进行了详细的显微结构观察和组成矿物的晶体取向分析。糜棱岩分别形成于中地壳、下地壳和上地幔。虽然应变在糜棱岩中局部形成以反应产物为主要成分的细粒（小于等于50 μm）多矿物聚集体，但其主要组成矿物颗粒既无形态，也无结晶取向。这表明糜棱岩中的细粒多矿物聚集体受到晶界滑动的变形，验证了壳幔的超塑性流动。细粒多矿物聚集体在花岗岩超长玄武岩和橄榄岩糜棱岩中形成应力支撑网络，表明糜棱岩具有超塑性流动。辉长岩超长棱岩中形成应力支撑网络的是斜长石集合体。由于斜长石晶粒在形状和晶体学上都具有择优取向，因此斜长石集合体在位错蠕变作用下发生变形。因此辉长岩超长岩的流变学可以用斜长石的晶体塑性流动来近似，而不能用超塑性流动来近似。

We have made detail microstructural observations and crystallographic orientation analyses of constituent minerals using an electron backscatter diffraction (EBSD) technique on granite mylonites along the Hatagawa Shear Zone in the Abukuma Mountains, and gabbro mylonites and a peridotite mylonite both from the Hidaka metamorphic belt. These mylonites were formed in the middle crust, the lower crust and the uppermost mantle, respectively.Although strain is localized in these mylonites into fine-grained (【less than or equal】50 μm) polymineralic aggregate mainly composed of reaction products, its main constituent mineral grains show neither shape nor crystallographic preferred orientations. This indicates that the fine-grained polymineralic aggregate in these mylonites is deformed by grain boundary sliding, verifying superplastic flow in the crust and mantle.The fine-grained polymineralic aggregate is forming a stress-supporting network in the granite ultramylonite and in the peridotite mylonite, implying superplastic flow of these mylonites. In contrast, it is plagioclase aggregate which is forming a stress-supporting network in the gabbro ultrarmylonite. Because plagioclase grains develop both shape and crystallographic preferred orientations, the plagioclase aggregate is deformed by dislocation creep. Hence the rheology of the gabbro ultramylonite is approximated by crystal plastic flow of plagioclase, but not by superplastic flow.