The temperature/pressure conditions of grenville-age granulite-facies metamorphism of the Oaxacan Complex, Southern Mexico.
Mora, Claudia Ines
Valley, John W.
Master of Arts
Coexisting feldspar thermometry for charnockites and quartzofeldspathic gneisses in the Grenville-age Oaxacan Complex, southern Mexico, yields an average temperature of T=762+5°C at 7kb. Reintegration of coarsely-exsolved perthites yields internally consistent peak temperature estimates. Texturally-homogeneous alkali-feldspar has reequilibrated at low temperature. Unusually extensive solid-solution of albite (Abs to Ab5§) and anorthite (Ang.3 to An^ç) necessitated extrapolation of the Stormer (1975) determinative curves beyond their explicit limits. This results in unreasonably high temperature estimates of up to 975°C for feldspar pairs in which X/yj /^f>.4 and suggests that extrapolation is unwarranted. The distribution of albite between coexisting feldspars is consistent with the shape of the Brown and Parsons (1981) general form for the graphical two-feldspar thermometer. Peak metamorphic pressure was estimated using the Newton and Perkins (1982) anorthite+enstatite=pyrope 2/3 grossular 1/3 + quartz geobarometer which yields P=7.5+lkb. The experimentally determined geobarometer anorthite+ferrosilite=almandine 2/3 grossular 1/3 + quartz (Bohlen et al., 1983) yields P=7.25+lkb. The assemblage dolomite+diopside+forsterlte+calcite restricts the metamorphic temperature to 764°C at PQ) =7kb, calculating from the 5kb experimental reversal of the buffering reaction 3dolomite+ldiopside= 2forsterite+4calcite+2C2 by Kase and Metz (1981). Using the temperature estimated by two-feldspar thermometry, at P=7kb the assemblage buffers the fluid composition to Xco =.8. The estimated pressures and temperatures are consistent with metamorphic conditions indicated by the occurrence of quartz+K-feldspar+garnet +sillimanite+biotite in the pelitic gneisses and orthopyroxene+plagioclase+ garnetjMclinopyroxene in the mafic gneisses of the terrane.