A Grenvillian arc on the margin of Amazonia: evidence from the southern Oaxacan Complex, southern Mexico

The rocks in the southern Oaxacan Complex of Mexico form part of the baseme nt of Oaxaquia, a terrane that underlies most of eastern Mexico. They have undergone granulite facies metamorphism at similar to 1 Ga, that was initia lly overprinted by lower amphibolite facies metamorphism accompanied by ext ensive hydration, and then by greenschist facies metamorphism. Geochemical data indicate that protoliths include volcanic arc lavas and sediments intr uded by a rift-related granite. The metavolcanic rocks have SiO2 ranging be tween 55 and 72%, are typically enriched in highly incompatible elements su ch as Ba, Rb and light rare earth elements, and are accompanied by relative depletion of Nb and Ti characteristic of subduction-related magmas. The gr anite has SiO2 ranging between 67 and 70%, is enriched in strongly incompat ible elements, and depleted in Nb and Ti indicative of either a crustal sou rce or a subduction-related tectonic setting. Concordant, U-Pb analyses of single zircons from this granite yield an age of 1117 +/- 4 and 988 +/- 5 M a interpreted as the age of intrusion and the time of peak granulite facies metamorphism, respectively. Combined with published information from other parts of Mexico, these data indicate that Oaxaquia was a juvenile arc meta morphosed during a relatively young Grenvillian metamorphism at 990 Ma. Sim ilar characteristics may also be found in the Andean massifs of Colombia an d in the Appalachian terranes (Carolina-Piedmont and Avalon) of Amazonian p rovenance. This suggests that Oaxaquia may also have lain adjacent to north western Amazonia at similar to 1 Ga, a location that may allow Neoproterozo ic subduction as the origin of the hydrous, lower amphibolite facies metamo rphism. It would also provide a proximal source for low grade to unmetamorp hosed similar to 1 Ga and Neoproterozoic igneous boulders in northern Oaxaq uia and 1000-900 Ma detrital zircons in Neoproterozoic and Paleozoic, sedim entary rocks in Avalonia and Iberia. (C) 2001 Elsevier Science B.V. All rig hts reserved.