LOCAL AND REGIONAL GRADIENTS IN FLUID COMPOSITION DURING BARROVIAN-STYLE METAMORPHISM OF THE WEPAWAUG SCHIST, CT, USA

Metamorphic index mineral zones and fluid compositions were determined for metacarbonate layers within the Wepawaug Schist. The index minerals oligoclase, biotite, calcic amphibole, and diopside formed with increasing metamorphic grade during the Acadian orogeny. In the upper greenschist facies and in the amphibolite facies, prograde reaction progress is greatest along the margins of metacarbonate layers in contact with surrounding schists, or in reaction selvages bordering quartz veins. New index minerals typically appear first in these more highly reacted contact and selvage zones. Hewitt (1973) postulated that this spatial zonation of mineral assemblages resulted from infiltration, largely by diffusion, of water-rich fluids across lithologic contacts or away from fluid conduits like fractures. Significant local gradients in X_CO2 are predicted by the model, but new fluid composition estimates indicate that differences in X_CO2 preserved across layers or vein selvages were very small, ~0.02 or less. However, analytical solutions to the two-dimensional advection- dispersion- reaction equation show that only small fluid composition gradients across layers or selvages are needed to drive prograde CO₂ loss by diffusion and mechanical dispersion. These gradients, although small, are considered to be the best explanation for the observed patterns of reaction progress and would still be large enough to dominate the effects of fluid flow and reaction along regional T and P gradients. Larger local gradients may have existed transiently, but they would have been rapidly eliminated and thus rarely preserved in the rock record (Ague and Rye, 1999). Most of the fluid needed to drive prograde CO₂ loss probably came from regional dehydration of surrounding metapelitic schists, although very water-rich Diopside zone conditions may have also required an external fluid component derived from syn-metamorphic intrusions or the metavolcanic rocks that underlie the Wepawaug Schist.