Xenoliths from central Mexico: Experimental, mineralogical and petrological evidence for high temperature metamorphism of the lower crust.

Abstract

The assemblage orthopyroxene + rutile $\pm$ quartz $\pm$ ilmenite, common in suites of lower crustal xenoliths from central Mexico, provides a potential but untested thermobarometer for garnet-free granulites. Equilibria were calibrated with a piston-cylinder apparatus at pressures of 12-26 kbar and temperatures of 800-1100$\sp\circ$C for the compositional range Fs$\sb{40}$En$\sb{60}$ to Fs$\sb{70}$En$\sb{30}.$ The tightly reversed equilibria yield lnK$\sb{\rm D}$ = 2457.7/T + 0.270 $\{$T in Kelvins; K$\sb{\rm D}$ = (XMg/XFe)$\sp{\rm opx}$(XMg/XFe)$\sp{\rm ilm}\}$ at 800-1100$\sp\circ$C. The equilibria are sensitive to small changes in composition and more applications are needed to evaluate the utility of the equilibria for barometry. Activity/composition relations of orthopyroxene may be estimated from the data if an ilmenite mixing model is specified. Results from $\sim$20 experiments were fit to a symmetric regular solution model to constrain mixing relations in orthopyroxene. X-ray diffraction analysis of synthetic orthopyroxene powder indicates that the volume of mixing is linear. Slight to moderate positive (activity coefficients of 1.03-1.30) deviations from ideality are calculated for FeSiO$\sb3$-MgSiO$\sb3$ orthopyroxene at 800-1100$\sp\circ$C with large errors (2$\sigma$ of $\pm$5 kJ/mol in W$\sb{\rm G}$) in mixing parameters and activity coefficients when realistic errors in the variables P-T-X and regression statistics are considered. Pressure-temperature conditions of equilibration were estimated for garnet granulite xenoliths from central Mexico. A new barometer based on the reaction almandine + grossular + rutile = anorthite + ilmenite + ferrosilite is calculated for mafic, garnet-bearing granulites that lack quartz and applied to the lower crustal xenoliths from central Mexico. Calculated pressures of 10-13 kbar are consistent with pressures calculated from other barometers for samples from the same localities. Feldspar thermometry indicates that peak metamorphic temperatures of $\ge$1000-1100$\sp\circ$C are preserved in the xenoliths with near isobaric cooling to ${\sim}900\sp\circ$C. Garnet-pyroxene and two-pyroxene thermometry generally yield temperatures of 950-1050$\sp\circ$C. The petrological data are consistent with available geological and geophysical data and indicate that basaltic underplating was the heat source for the last metamorphism of deep crustal xenoliths in central Mexico during the mid-Tertiary.