Deformational History of the Granjeno Schist, Ciudad Victoria, Mexico: Constraints on the Closure of the Rheic Ocean?

Exposed in the core of a NNW-trending frontal anticline of the Laramide fold-thrust belt of northeastern Mexico, the Granjeno Schist comprises a polydeformed assemblage of Paleozoic metasedimentary and metavolcaniclastic rocks and serpentinized mafic-ultramafic units. The earliest deformation (D₁) predates emplacement of a leucogranite at 351 ± 54 Ma and may record obduction of this oceanic unit. Subsequent deformations (D_2a–D_2c) record tectonic juxtapositioning of the Granjeno Schist against the ~1 Ga Novillo Gneiss by NNW-directed dextral shear under conditions of decreasing temperature. Cooling ages of 313 ± 13 Ma and 300 ± 4 Ma are considered to date the onset of dextral motion, which continued into the Permian. These events are linked to the Late Paleozoic closing of the Rheic Ocean.