DEFORMATION MECHANISMS AND MIXED CRYSTALLOGRAPHIC PREFERRED ORIENTATIONS IN HORNBLENDE-RICH AMPHIBOLITE FROM THE CENTRAL ASSAM-MEGHALAYA GNEISSIC COMPLEX: NORTHEAST INDIA

Amphibolite, a diagnostic rock commonly found in regional metamorphic belts, constitutes minerals from the amphibole group, plagioclase (Plg), biotite (Bt), quartz (Qtz) ± garnet (Grt). Despite its prevalence, the mechanism of deformation of hornblende (Hbl) in amphibolite remains insufficiently understood due to the scarcity of supporting data for Crystallographic Preferred Orientation (CPO) patterns in Hbl. This study addresses this knowledge gap by investigating the highly strained amphibolite within the central Assam-Meghalaya Gneissic Complex (AMGC). Microstructurally, the Hbl aggregates are aligned along the foliation in a matrix of strongly aligned Bt fabric, while no preferred orientation is observed for Qtz and Plg. The CPO of Hbl obtained from EBSD analysis, displayed a pronounced pattern in the lineation parallel section, indicative of deformation. The study reveals that Hbl, as one of the major anisotropic phases alongside the rheologically weak Bt, accommodates strain rather Qtz and Plg. Notably, (100) [001] is the primary slip system of Hbl in naturally deformed amphibolite, exhibits at high temp > 800°c. However, in this study, the (100) pole is subnormal to the slip plane, while the [010] axis aligns along the slip plane, resulting in a weak girdle formation for the [001] axis. Remarkably, a mixed CPO pattern (type-II + IV) is identified, indicating rigid body rotation in a cataclastically flowing matrix to be the major mechanism of deformation. The CPO pattern observed by the (100) [010] slip system suggests deformation in a southeast directed extensional regime. The experimental studies suggest that both the CPO pattern formed in a temperature range of 500-600⁰c.

The presence of this new type of mixed CPO pattern in hornblende suggests formation of Hbl fabric in an extensional tectonic environment under amphibolite facies conditions. Additionally, the possibility of two distinct deformation events is proposed, with an earlier higher-temperature (>600°c) regime being overprinted by a subsequent low-temperature (500°c) deformation event. In conclusion, this study sheds light on the deformation mechanism of hornblende in naturally deformed amphibolite and provides valuable insights into the tectonic environment and deformation history of the middle-lower crust of the AMGC.