~990 Ma peak granulitic metamorphism and amalgamation of Oaxaquia, Mexico: U–Pb zircon geochronological and common Pb isotopic data

Abstract

The ~1 Ga Oaxacan Complex is the largest exposure of the Oaxaquia terrane of Mexico. Microprobe analysis of co-existing ortho- and clino-pyroxene, garnet, ilmenite and plagioclase indicate metamorphic conditions of 735 ± 5° C and 7.7 ± 0.1 kbar in the granulite facies, which was dated between ~998 and ~979 Ma using U–Pb isotopic analyses of zircon populations. Peak temperatures >800° C and isobaric cooling are indicated by the sporadic presence of sapphirine in metasediment and garnet–cordierite– sillimanite–K-feldspar–rutile assemblages, and garnet coronas around pyroxene and titanomagnetite. Common Pb isotopes from acid-leached whole-rock samples define an errorchron that intersects the Stacey/Kramers crustal growth curve at 1,187 ± 63 Ma indicating a major crust-forming event at this time, and suggesting that the granulite facies metamorphism did not significantly reset the common Pb isotopic composition. These data are comparable with those from other ~1 Ga inliers in Mexico and is consistent with a single Oaxaquia terrane by ~1 Ga.