MUSCOVITE-RICH PSEUDOMORPHS AFTER STAUROLITE AS A RECORD OF FLUID INFILTRATION DURING PROGRADE METAMORPHISM

During metamorphism, evidence of the prograde path is often obliterated by peak metamorphic conditions. However, pseudomorphs that form during prograde metamorphism may provide insights into heat and mass (fluid) transport along this portion of the P-T-X-t path. Fifteen sillimanite zone samples each containing 2 - 5 muscovite-rich pseudomorphs after staurolite were investigated from pelitic schists of the Farmington area, west-central Maine, USA. Pseudomorphs are easily recognized by an increase in average grain size (~cm) and a change to randomly-oriented muscovite-rich (~30-75%) modal mineralogy. SEM cathodoluminescence images, X-ray maps, and electron microprobe analyses characterize the mineral distribution, modes, and compositions within the pseudomorphs and the surrounding matrix. These data provide the basis for textural modeling studies and for testing open versus closed system behavior of pseudomorph formation.

Three compositional types of muscovite-rich pseudomorphs are observed: quartz-rich (15-30% qz with <1% pl, sil); plagioclase-rich (20-30% pl with < 1% qz, sil); and plagioclase- sillimanite-rich (~30% pl, 15-20% sil with <1% qz); in addition to those that are ms-rich (~75% ms). A biotite-rich ms-poor mantle surrounds most pseudomorphs. Irreversible thermodynamic modeling of pseudomorph development using analyzed mineral compositions and previously determined diffusion coefficients for metapelitic rocks suggests modes of ~75% ms, ~10% bio, ~10% pl, and ~5% qtz replace the staurolite in a system closed to fluids. These modes are substantially different than those observed. If the system is allowed to be open to H₂O, K, Na, and Al, modal variations in biotite, sillimanite, plagioclase and quartz comprising the pseudomorphs can be reproduced. Additionally, changes in cathodoluminescence from core to rim in pseudomorph quartz grains suggest two growth stages characterized by differing trace element distribution. These data suggest that metapelitic pseudomorphs retain evidence for an infiltrating fluid phase and its composition during prograde metamorphism.