Geochronology and geochemistry of Grenvillian igneous suites in the northern Oaxacan Complex, southern Mexico: tectonic implications

Abstract

Chemical and U � /Pb isotopic analyses of metaigneous rocks in the northern Oaxacan Complex in southern Mexico indicate that they form part of two granitic � /gabbroic suites intruded at � /1157 � /1130 and � /1012 Ma, which were metamorphosed under granulite facies conditions between � /1004 and 980 Ma. Although the older suite has both within-plate and arc geochemical signatures, the arc characteristics (enrichment of La and Ce relative to Nb, Ta, and Th) are inferred to result from crustal contamination, a conclusion consistent with their negative oNd signatures. The younger suite is spatially associated with anorthosites (from which we were unable to acquire a protolith age), suggesting that collectively it forms part of anorthosite � /mangerite � /charnockite � /granite (AMCG) suites. The tholeiitic nature of the mafic rocks along with the within-plate character of the felsic rocks suggests that they were intruded during extension related to either farfield backarc rifting, rifting above a slab window, or anorogenic intercontinental rifting. Potentially correlative AMCG suites are widespread in Mexico, the Grenville Province of eastern Canada and northeastern USA, and the Andean massifs of Colombia, however, Pb isotopic data most closely resemble those in South America. These data are consistent with published hypotheses that suggest Oaxaquia represents an exotic terrane derived from Amazonia. # 2002 Elsevier Science B.V. All rights reserved.