Back-reaction of K-feldspar + sillimanite assemblages: Insights from SIMS and EMP analysis of muscovite + quartz symplectites

In magmatic arc settings, the regional background geotherm is episodically perturbed by pluton emplacement to produce the anomalously high P/T assemblages that typify Buchan-style metamorphism. Reaction of muscovite + quartz to form K-feldspar + sillimanite assemblages signifies equilibration at upper amphibolite facies metamorphic conditions. The kinetics of the retrograde version of this reaction that produce symplectic intergrowths of muscovite + quartz are governed by diffusion and can be used to constrain both pressure-temperature trajectory during cooling and the ambient background geotherm. In this study, we have examined anatectic wallrocks of the eastern Peninsular Ranges batholith. Pluton emplacement occurred at 95 ± 2 Ma and caused upper amphibolite facies metamorphism (675 ± 25°C; 5 ± 1 kbar; water activity ~ 0.7) defined by pervasive growth of sillimanite + K-feldspar assemblages in graphitic pelitic to psammitic bulk compositions. Thermochronologic data indicate that ca. 450°C temperatures persisted from 94-86 Ma and that conditions remained above 400°C until 78 Ma. This near-isobaric episode of slow-cooling created an ideal environment for retrograde muscovite growth. Pervasive, coarse, unoriented muscovite commonly overgrew fibrolitic sillimanite and finer grained muscovite + quartz symplectites grew with myrmekite at the expense of K-feldspar. We have undertaken ion microprobe Ti-in-quartz measurements and electron microprobe analysis of Ti in cogenetic muscovite. These results reveal that the symplectic intergrowths grew at 60-80°C lower than peak grade conditions and constrain the background geotherm to conditions near the sillimanite-kyanite equilibrium boundary. We find that grain size variation among symplectic intergrowths reflects reaction progress as temperatures relaxed toward the ambient geotherm such that the finest symplectites most closely approximate the ambient geotherm.