HT-deformation of garnet: an EBSD study on granulites from Sri Lanka, India and the Ivrea Zone

Abstract

Garnets from three regional granulite terrains (Sri Lanka, Eastern Ghats, Ivrea Zone) have been analyzed and the results indicate, that garnet can be plastically deformed by dominant dislocation glide/creep with variable dominant slip systems.Garnets from a granulite facies fold hinge in a quartzite of the Highland Complex of Sri Lanka, unequivocally attest to plastic deformation of garnet. Outside the fold hinge the garnets form flat discs elongated parallel to a pronounced stretching lineation and show deformation microstructures like boudinage and pinch and swell structures. In the fold hinge these garnet discs are folded within the quartzitic matrix at temperatures of about 800°C. In spite of the preservation of deformed shapes, intracrystalline deformation microstructures are scarce. The garnets have distinct crystallographic preferred orientations, which can be interpreted by a dominant 1/2〈111〉{110} slip system approaching ‘easy slip’ positions. These are reached rapidly under moderate strains as necessary lattice rotations are small.Similar slip systems were derived for a garnet-bearing quartzite from the Eastern Ghats deformed at temperatures of 750–800°C, but with more influence of initial orientation of garnets. Completely different textures indicative of dominant 〈100〉 [010] slip were found in a restitic garnet–sillimanite granulite from the Ivrea Zone. The variable deformation features and variable slip systems with 1/2〈111〉 and 〈100〉 Burgers vectors prove that garnets bear significant potential for contributing additional information on the HT-deformation of rocks.