Retrograde cataclastic gneiss north of Motagua fault zone, East Central Guatemala

Abstract

An areally extensive quartz-monzonitic gneiss, characterized by a variety of cataclastic textures, has been mapped as the structurally lowest member of the Chuacus Group in east-central Guatemala. This apparent orthogneiss is separated from overlying Chuacus Group metasediments (phyllite and marble) by a south-dipping, low-angle thrust. The appearances of the gneiss is variable, ranging in texture from ultramylonite to blastomylonite gneiss. Retrograde mineral assemblages characteristic of the upper-greenschist facies (quartz-microclineoligoclase-biotite-chlorite) developed in response to cataclastic deformation adjacent to the Motagua fault zone, and do not represent regional metamorphism in the classic sense. The areal extent of the gneiss suggests that it may be a fundamental basement unit north of the Motagua fault zone in Guatemala's Cordillera Central. It has not been recognized south of the fault zone.