The Mesozoic La Mula Island, Coahuila, Mexico
Granitic rocks at Potrero de La Mula and Sierra del Fuste, Coahuila, Mexico, are the only known exposures of a Late Triassic pluton that, by the Late Jurassic, stood as a topographically high area until it was inundated by the Cretaceous sea. The principal igneous rock at Potrero de La Mula is a two-feldspar, biotite-hornblende, I-type granite that is cut by comagmatic dikes of six ages and by several shear zones. A Rb-Sr age of 213 ± 14 m.y. and an initial ratio of 0.705 ± .0004 were obtained from nine whole-rock samples. A shallow depth of emplacement is inferred from textural and mineralogical evidence. Mineralogy, chemistry, and isotopes suggest emplacement above a subduction zone, which is consistent with the hypothesis that a late Paleozoic-earliest Mesozoic convergent plate boundary existed in northern Mexico. After being unroofed, the pluton contributed sediment to the adjoining Sabinas Gulf until the mid-Neocomian. By the Hauterivian, the pluton supported, a deeply weathered terrain of moderate relief flanked on its northeastern side by lagoonal environments (Oballos Member, Padilla Formation). Farther south, uplift by faulting along the northern flank of the Coahuila Island produced a clastic wedge, the finer-grained paralic components of which (La Mula Formation) prograded northeastward and eventually covered La Mula Island. Subsequent deposits at Potrero de La Mula, beginning with gypsum, reflect gradually deepening water and include the La Virgen, Cupido, La Pena, and Tamaulipas Formations.