PROCESSES OF FLUID-ROCK INTERACTION REVEALED BY K LOSS FROM METAMORPHOSED CARBONATE ROCKS, WEPAWAUG SCHIST, CT USA

The geochemical behavior of K during regional metamorphism of metacarbonate layers within the Wepawaug Schist was investigated using mass balance analysis. Progressive metamorphism formed regional oligoclase, biotite, amphibole, and diopside index mineral zones during the Acadian Orogeny. In some amphibole zone and all diopside zone rocks, near total depletion of K, Rb, and Ba is observed in metacarbonate layers. In the amphibole zone, depletions are most marked in alteration selvages around quartz veins and at lithologic contacts with surrounding metapelitic and metapsammitic rocks, whereas in the diopside zone, depletions may occur throughout the layers. Fluid flow in a direction of increasing temperature would be expected to cause gains in Na and possibly Ca coupled to loss of K, but such gains are not observed. Instead, it is concluded that the K loss was a consequence of reactions that destroyed biotite. Model reactions that are isochemical except for volatile loss destroy biotite to make K-feldspar (Kfs) along with amphibole or diopside. Thermodynamic analysis indicates that the chemical potentials of K species in fluids coexisting with Kfs-bearing metacarbonate rock are greater than those coexisting with metaclastic schists or the syn-metamorphic, tonalitic magmas that intruded the higher-grade parts of the Wepawaug. Consequently, infiltration of fluids (by diffusion-dispersion or advection) derived from the schists or the intrusions would tend to strip out K from the metacarbonate layers. The near total absence of Kfs in the altered metacarbonates indicates that the rate of K removal was greater than the rate of Kfs nucleation and growth. Balanced open system reactions show that most of the Al in the biotite was used to form clinozoisite/zoisite instead of Kfs. Local enrichments of K in rocks adjacent to the K-depleted metacarbonate layers are not observed, suggesting significant transport distances for K larger than the thickness of individual layers. The K systematics indicate that fluid-rock interactions between the metacarbonate layers and surrounding fluid sources occurred. Consequently, quantification of fluid fluxes, devolatilization budgets, and other geochemical behavior must model the metacarbonate reaction progress by considering mass transfer between different lithologies.