The southeastern boundary of the Middle Proterozoic microcontinent Oaxaquia [1], which underlies most of central and southern Mexico, is not well established because 1 Ga old granulites (Guichicovi complex) are exposed in the southwestern Maya terrane [2]. The Chiapas massif, which extents from the eastern side of the Tehuantepec isthmus as far as the border to Guatemala, is composed mostly of Permo-Triassic igneous rocks (Chiapas batholith). Recent studies [3, 4] have shown that medium to high-grade metasedimentary and metaigneous rocks are present in the Chiapas massif, and these may provide important information about the pre-batholithic history of the southern Maya terrane. We present the results of U-Pb zircon dating of 8 samples of metamorphic rocks from the Chiapas massif. All samples display ductile deformation, gneissic texture, and most of them can be described as augen gneisses. These orthogneisses are more deformed than the igneous-textured rocks of the Chiapas batholith. Zircons from all samples are strongly discordant but yield Late Permian lower intercept ages and >1 Ga upper intercept ages. Based on the distribution of data and zircon morphologies from orthogneisses, we interpret the lower intercepts as igneous crystallization ages and the upper intercepts as the average age of heterogeneous inherited components. An orthogneiss that crops out ~8 km east of Tonala yields intercept ages of 258 ± 2 Ma and 1046 ± 6 Ma, and an augen gneiss exposed ~12 km southeast of Villa Flores yields intercepts of 251 ± 3 Ma and 1015 ± 22 Ma. Results from all other samples (including paragneisses) are within error of these ages. These results demonstrate the existence of Grenvillian zircons in the Chiapas massif; and therefore, a southeastern prolongation of Oaxaquia including the southern Maya terrane should be considered. However, to date, no granulites similar to the Grenvillian rocks of the Oaxacan and Guichicovi complexes have been found exposed in the Chiapas massif. Our results indicate that the most important tectonothermal event there is Late Permian rather than Precambrian in age.

References: [1] Ortega, F. et al. 1995., Geology, 23, 1127-1130. [2] Weber, B., Köhler, H., 1999. Precamb. Res., 96, 245-262. [3] Schaaf, P. et al. 2002. N. Jb. Geol. Paläont. Abh. 225,1-23. [4] Weber, B. et al. 2002. Geos, 22, 1, 2-11.