Grenville orogenic affinities in the Red Mountain area, Llano Uplift, Texas

Abstract

The Red Mountain area in the southeast corner of the Llano Uplift contains evidence of a Grenville-age, polyphase, non-coaxial deformational history. Four synmetamorphic deformational phases affected the Precambrian Click Formation of the Packsaddle Group, and a single shear-related deformation is observed in the Big Branch and Red Mountain gneisses. The first phase of deformation produced tight folds with an axial–planar metamorphic foliation. The second, and most intense, pervasive deformational phase produced the regional metamorphic layering, isoclinal folds, transposition of bedding, and mylonitic textures in the gneisses. The third deformation is characterized by tight to isoclinal, north-verging folds with an axial–planar differentiation crenulation cleavage. The fourth phase of deformation resulted in a north-northwest-trending differentiation crenulation cleavage that is axial–planar to small east-northeast-verging folds. The growth of cordierite over all foliations indicates that low-pressure metamorphism postdated all four deformational phases.The second deformation had begun at the latest by 1167 ± 2 Ma, the age of metamorphism of the gneisses, and before the intrusion of melarhyolite dikes dated at 1080 ± 30 Ma. Thus, the orogenic event(s) responsible for the complex deformational and metamorphic history is Grenvillian in age.