Basement of the Sierra Madre Oriental in NE Mexico: New field data of the Paleozoic Granjeno Schist in the Cañón de Caballeros area

The basement of the Sierra Madre Oriental (SMO) is well exposed in the core of the laramidic Huizachal-Peregrina Anticlinorium near Ciudad Victoria. It comprises 4 contrasting units: Grenville-age Novillo Gneis (~1 Ga), Paleozoic Granjeno Schist (~ 300 Ma), leucocratic Tonalite (~350 Ma) and a Silurian to Permian deformed sedimentary sequence. They are NW-SE oriented and in tectonic contact through vertical faults. The basement is covered by a very thick Mesozoic sedimentary column mainly constituted by red beds, evaporites, limestones and shales.

Granjeno schist (GS) outcrops particularly in the Cañones Novillo, Peregrina and Caballeros. A GS narrow (~1.6 km) NW-SE trending band is exposed in the Cañón de Caballeros. Here the schist is massive, black to gray, with interstratifications of pelitic to psammitic lithologies. The general mineralogy is: quartz, mica, albite, graphite, chlorite and garnet. The pelitic levels are strongly deformed. The psammitic beds are more competent and display lighter colors. They could also represent metapyroclastic beds.

Quartz segregations are more abundant in the pelitic levels. They are parallel to the main schistosity and commonly folded, recording the deformation phases. At least three schistosities are preserved: 150º/55º, 320º/60º and 99º/58º. The first fabric is bedding parallel, as described by other authors (e.g. Dowe et al., 2005) for the other GS localities.

The GS represents the late stages of the formation of Pangea.