THE MECHANISM OF HORNBLENDE SUPERPLASTIC DEFORMATION—A CASE STUDY FROM HENGSHAN AMPHIBOLITES IN SHANXI, CHINA

Study of samples from a Heng Mountain (Hengshan) amphibolite (Shanxi, China) provides new information on hornblende’s plastic and superplastic deformation mechanisms. The amphibolites, originally gabbroic dikes, were intruded into high-amphibolite-granulite grade metamorphic rocks of the Archean Wutai Group, prior to their deformation within localized ductile shear zones.

During the ductile shearing process, localized strong strain is more than 1000%. Following recrystallization, almost all new hornblende grains have no SPO (shape preferred orientation). Some (<10%) are weakly elongated (axial ratios 1:1~1:2) along the mineral aggregate-defined lineation direction. It suggests that the new grains of recrystallized hornblende developed during structural superplastic deformation. Deformation occurred at 650~679°C and 0.770~0.914 Gpa based on plagioclase-hornblende geothermometry and geobarometry.

TEM shows that new hornblende grains developed few dislocations; hornblende crystals exhibit straight grain boundaries. We, therefore, conclude that dislocation creep did not take the leading role during the deformation process. Dynamic recrystallization of hornblendes probably resulted from solid-state diffusive mass transfer.

EBSD (electron backscatter diffraction) of strongly deformed hornblendes reveal a fabric dominated by a {100} <001> LPO (lattice preferred orientation) that is indicative of new grain recrystallization along the lineation direction (X axis). The weak hornblende SPO was presumably affected by the recrystallization of secondary phases (primarily plagioclase with an equant mosaic of small new grains). Local domains of strong strain might be related to grain boundary sliding, which is supported by the existence of straight grain boundaries.

It is, thus, reasonable to conclude that superplastic deformation of the Hengshan hornblendes was mainly developed by solid-state diffusive mass transfer and grain boundary sliding, and it shows weak SPO but strong LPO.