REVISED PRESSURE –TEMPERATURE CONDITIONS FOR THE GRENVILLE-AGED ZAPOTECAN OROGENY, OAXACAN COMPLEX, SOUTH-CENTRAL MEXICO

The Oaxacan Complex is a Grenville-aged terrane in Southern Mexico, ~600 km South of Mexico City. It is a poly-metamorphic terrane, which experienced two tectono-thermal events; the Olmecan event, dated at 1106 +/- 6Ma and the younger Zapotecan orogeny, dated between 1004-978 Ma (Keppie et. al., 2004). Mora et al. (1986) determined pressure temperature conditions of metamorphism using the orthopyroxene – garnet – plagioclase geobarometer combined with two feldspar geothermometry. These workers calculated pressure and temperature conditions of metamorphism of 710° +/- 50°C and 730 +/- 100 MPa (Mora et al. 1986). Using newly collected samples of biotite+garnet+kyanite (sillimanite)+ k-spar (perthite) +quartz gneiss, pressure-temperature conditions of the Oaxacan Complex have been calculated using garnet biotite geothermometry combined with the GASP geobarometer. Our results yield recrystallization conditions of 609 +/-50°C and 576 +/ 82 MPa. Our calculated pressure-temperature conditions for Zapotecan recrystallization are significantly lower (100+°C; 154Mpa) than those reported by Mora (1986). Substituting our (biotite-garnet) temperature into the OPX-GAR-PLAG geobarometric results reported by Mora et al. (1986) reduces the calculated pressure from 730 MPa to 580 MPa which is consistent with our results. Our calculated P-T conditions (within error) straddle the sillimanite-kyanite boundary in agreement with the presence of kyanite and sillimanite in our suite of samples. Both kyanite and sillimanite-bearing gneisses contain k-feldspar in equilibrium with an aluminosilicate which requires reduced aH2Ofluid during metamorphism to drive the KSPAR +SILL/KY + H2O = MUSC + QTZ reaction to lower temperatures. Several of our samples collected from the El Catrín Migmatite Unit, show evidence for biotite melting (biotite+aluminosilicate +plagioclase+quartz = k-feldspar + melt) which should not occur at temperatures below 750-780°C. Based on our data, Zapotecan–age temperatures are not sufficiently high to drive biotite melting and, therefore, we assign migmatite formation to the earlier Olmecan Event which is in agreement with age dates (1106+/-6Ma) and field relationships reported by Solari (2002).