The process and mechanisms of metamorphism and deformation of hornblende—An example from the Hengshan Mountains, Shanxi Province

Abstract

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°C 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.