PRECAMBRIAN AND PALEOZOIC METAMORPHIC BASEMENT IN NE MEXICO. NORTHERN AND SOUTHERN CONNECTION

The framework of Mexico´s geological basement has been a controversial issue. Former authors have proposed hypotheses in which the Proterozoic basement in the NE region represents an extension of the Ouachita Orogenic Belt, displaced through major transform movements during Mesozoic times (Mojave-Sonora Megashear) to its present position. In this case, it could correspond to the possible Southern Laurentia margin.

Field relationships, isotopic, seismic and gravimetric data, suggest that the Proterozoic rocks of Northeastern and Southern Mexico represent fragments of the North American Grenville Province. There are some theories about the origin of the Mexican grenvillean rocks. Some of them indicate a relationship with the Grenville Orogen from Southeastern Laurentia. In the other hand, other ideas point out to an origin linked to exotic terranes derived from Amazonia. The more recent theory, based upon on paleontological, geochronological and paleomagnetic data, postulates that the ~ 1 Ga granulitic rocks in Northeastern, Center and Southern Mexico belong to a common microcontinent: Oaxaquia. It was accreted to Rhodinia during the Grenville Orogeny.

Metasedimentary rocks are juxtaposed against Proterozoic rocks belonging together to the basement. These rocks comprise a Paleozoic unit related to the evolution of the Iapetus Ocean during the Appalachian Orogenic Belt (Ouachita Marathon). In contrast to this theory, the latest research on Paleozoic outcrops from Northeastern (Granjeno Schist) and South Central (Acatlán Complex) Mexico, proposed an origin associated to the opening of the Rheic Ocean, from the Northern margin of Gondwana during the early Ordovician, and its subsequent closing by the amalgamation of these margin against Laurussia during the Permo-Carboniferous Pangea evolution.

More work has to be done to understand the petrological evolution of the single metamorphic units as well as the relationship between the geographical divided Mexican metamorphic terranes.