Hornblende's plastic and superplastic deformation mechanisms were studied on the basis of some new information obtained from amphibolite samples in the Hengshan Mountains (Hengshan, Shanxi, China).For this purpose, the samples were measured and analyzed by optical microscope (OM), electron probe microanalysis (EPMA), transmission electron microscope (TEM) and electron backscattered diffraction (EBSD) respectively.Because localized strong strain is more than 1000%, it is presumed that structural superplastic deformation was developed during the ductile shearing process.It is calculated that deformation occurred at 650-679℃ and 0.770-0.914 GPa, based on the study of plagioclase-hornblende geothermometry and geobarometry.TEM images show that new hornblende grains developed few dislocations and hornblende crystals exhibit straight grain boundaries.EBSD of strongly deformed hornblendes reveals a fabric dominated by a {100} <001> lattice preferred orientation (LPO), indicative of new grain recrystallization along the lineation direction (X axis).Thus superplastic deformation of hornblendes from the Hengshan Mountains was developed by mainly solid-state diffusive mass transfer and grain boundary sliding.
JI MoHU LingLIU JunLaiSHEN LiangZOU YunXinSONG HongLinMIAO PeiSen
Multiple methods were applied to study the deformation characteristics of hornblende in Archean plagioamphibolite mylonite from the Western Hills(Beijing),including optical microscopy(OM),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),and electron probe microanalysis(EPMA).The hornblendes are σ and δ type porphyroclasts with the new-born needle shaped grains as their tails.The analysis of lattice preferred orientation(LPO) of both the porphyroclasts and the new-born grains shows that the main slip system of the deformed hornblende is(100)<001>,suggesting that the fabric characteristics of new-born grains inherit that of porphyroclasts.Sub-microstructures show the porphyroclast core is dominated by dislocation tangle,little or no dislocations in the new-born grains,and the subgrains confined by dislocations in the transition zone between porphyroclasts and new-born grains.By using plagioclase-hornblende geothermometry and hornblende geobarometry,the estimated temperature and pressure of porphyroclasts are 675.3-702.9℃ and 0.29-0.41 GPa and those of new-born grains are 614.1-679.0℃ and 0.11-0.31 GPa.The bulging recrystallization is summarized as deformation mechanisms of hornblende by the discussions of the microstructures,EBSD fabric,sub-microstructures,and the deformed temperature and pressure.
