Evidence for Paleoproterozoic, Grenvillian, and Pan-African age Gondwanan crust beneath northeastern Mexico

Abstract

A number of previous studies including two recent reconstructions of Rodinia have considered the Grenvillian basement of Mexico to be continuous with the Laurentian basement of Texas. Isotopic studies and U–Pb dating of zircons from Precambrian granitoids that occur as cobbles and boulders in a Paleozoic conglomerate exposed in the state of Coahuila, northeastern Mexico, indicate the region is underlain by Gondwanan rather than Laurentian basement. Essentially nothing was known of the basement before this study because the region is blanketed with Phanerozoic sedimentary and volcanic deposits. Nine out of ten analyzed samples have Grenvillian crystallization ages (1232±7 to 1214±2 Ma). This phase of magmatic activity in northeastern Mexico was coeval with the construction of volcanic arcs in east-central and southern Mexico (∼1220–1150 Ma), but was later than the arc magmatism of Laurentia (e.g., Elzevirian arcs, >1230 Ma, in Texas and the Grenville Province). A subset of the Grenvillian samples contains xenocrystic ∼1.85 Ga zircons, which are the first evidence of Paleoproterozoic crust south of the Paleozoic Ouachita suture in Mexico. The remaining sample has a Pan-African crystallization age (580±4 Ma), and this is the first sample of Pan-African age basement found in Mexico. The Pan-African orogeny marked the assembly of Gondwana, and Laurentia was unaffected by that event. Two lines of evidence argue that some of the Grenvillian rocks of northeastern Mexico were affected by a Pan-African age event. First, two of the Grenvillian clasts have U–Pb lower intercept ages within error of 580 Ma. Second common Pb and Nd isotopic compositions of the Pan-African granite strongly suggest that it was derived either by melting of Grenvillian basement or by assimilating massive amounts of that basement. Thus far, no other Grenvillian terrane is known to have the exact combination of Nd–Pb isotopic compositions and absolute ages as the Coahuila samples although there are some similarities with the Santa Marta massif of Colombia and the Cordillera de Merida of Venezuela.