Sm-Nd isotope whole rock and garnet from the Southwestern Grenvillian Oaxacan Complex, Mexico: a review of garnet closure temperature and structural implications

Abstract

The Oaxacan Complex represents the largest outcrop of late Mesoproterozoic age, granulite-facies metamorphic rocks in Mexico. The southwestern Oaxacan Complex was studied using the Sm-Nd isotopic system in whole-rocks and garnets. The use of garnet for dating granulites is a good approach because garnet is usually a rock-forming mineral in this facies, it strongly prefers heavy over light rare earth elements, and its closure temperature is close to the range of the granulite facies. Since the studied garnets display resorbed borders, they are interpreted as pre- or syn- Grenvillian-age granulitic peak. Their closure temperatures, calculated using the Dodson equation and the Nd diffusion coefficients, are 720–770ºC (5– 30ºC/my). The whole-rock Sm-Nd evolution lines intersect the depleted mantle model at ca. 1.4–1.7 Ga, although two paraderivate samples intersect at 2.1–2.2 Ga, suggesting a protolith from an older continental crust. In nearby outcrops, Sm-Nd isochron garnet-whole rock ages follow a certain NW-SE trend, displaying two hypothetical age groups: old ages (954–976 Ma) and young ages (907–920 Ma). The younger age garnet's group display Ti and LREE-MREE rim enrichments with respect to their cores that imply diffusional resetting processes. Folding and faulting structures suggest that these two age groups correspond to different structural levels during the early cooling history of the Oaxacan Complex.