MASSIF ANORTHOSITE OF OAXAQUIA AND ITS ROLE IN THE TECTONIC EVOLUTION AND CORRELATION OF THE MEXICAN GRENVILLE

Massif anorthosite is an essential component of the continental crust that characterizes the Grenville orogens of Laurentia and Oaxaquia (Mexico); however, its origin remains problematic. In eastern Mexico, they occur within granulite facies inliers from Tamaulipas to Chiapas: Novillo (50% anorthositic rocks), Huiznopala (25%), northern Oaxaca (20 %), southern Oaxaca (60%), and Guichicovi complexes (not found or minor). U-Pb zircon dates on these rocks (interpreted as magmatic ages) range between 998 and1,010 Ma. Oaxaquia anorthosites are distinguished by the sodic composition of plagioclase (av. An35), and enrichment in P, Fe, Ti, Zr represented by nelsonitic bodies, and K and Si by modal quartz and antiperthite. Outcrops with plagioclase megacrysts and exsolved pyroxene, and inverted pigeonite (?) in thin section, demonstrate high temperatures and probably shallow levels of emplacement. These anorthosites are synchronous with intrusion of within-plate mafic and felsic rocks. The collisional Oaxacan orogeny, which immediately succeeded anorthosite emplacement in all cases, has been dated at 975-990 Ma, indicating 10-20 Ma between intrusion and burial to lower crustal depths. Accompanying granulite facies metamorphism produced granoblastic recrystallization of all minerals at intermediate to high pressures. These features of the Mexican anorthosites support correlation of all inliers of Oaxaquia and define a key element for the paleogeographic reconstruction of the Grenville belt. Considering their very young ages and pretectonic emplacement with respect to the Oaxacan orogeny, this block cannot represent a fragment of the Grenville belt detached from eastern Laurentia. Nor can it be correlated with the Australian and Texan belts, which lack anorthosite and have a different ages and protoliths. The anorthositic massifs in the Andean Grenville of Colombia and Venezuela, although not dated yet, may represent close correlatives of those in Oaxaquia, as supported by other data. The Oaxaquia anorthosites appear to have been emplaced during a rifting event of undetermined tectonic setting.