MASS-TRANSFER DURING BARROVIAN METAMORPHISM OF PELITES, SOUTH-CENTRAL CONNECTICUT .2. CHANNELIZED FLUID-FLOW AND THE GROWTH OF STAUROLITE AND KYANITE

Petrologic study of quartz veins and their wallrocks from the Wepawaug Schist, Connecticut, constrains the role of fracture how in the chemical and mineralogical evolution of amphibolite facies (staurolite and kyanite zone) pelites during Barrovian metamorphism. Quartz veins may contain accessory calcite and pyrite in the chlorite and biotite zones and plagioclase, micas, kyanite, staurolite, apatite, and sulfides at higher metamorphic grades. Crack-seal textures are widespread. Measured vein densities increase from about 2 to 4 percent in the lowest grade rocks to about 20 to 30 percent in the kyanite zone. In the amphibolite facies, veins are commonly surrounded by a highly aluminous selvage rich in staurolite +/- kyanite and micas and poor in quartz and plagioclase. Staurolite and kyanite are typically absent from the less aluminous wallrocks located beyond the selvage margins. The selvage width increases as a function of vein width at a ratio of similar to 1.3. Most vein development occurred after garnet crystallization had begun but pre- to syn-staurolite +/- kyanite growth. Four critical lines of evidence suggest that the quartz veins were major conduits for the regional scale movement of metamorphic fluids. (1) Mass balance analysis of two selvages and their protoliths indicates that major and trace elements were mobilized during selvage development by reactions that destroyed quartz, plagioclase, and micas and produced staurolite and kyanite. The reactions suggest significant hydrogen metasomatism. (2) Estimated f(HCl)/f(H2O) is highest in the selvages directly adjacent to veins, which suggests that the most altered rocks were infiltrated by fluids with elevated f(HCl)/f(H2O) and that the avenues for infiltration were quartz veins. (3) The amount of silica loss from local pelitic wallrocks can only account for about 70 percent of the total volume of quartz in the average amphibolite facies vein. The other 30 percent is inferred to have been externally-derived through widespread fluid infiltration down regional T and P gradients. (4) Because previously published stable isotopic studies have shown that quartz in amphibolite facies veins may be significantly out of oxygen isotopic equilibrium with walIrock quartz, the fluid that precipitated quartz was probably in part externally-derived. Selvage formation resulted in significant decreases in Na/Al and, in some cases, K/Al which stabilized aluminous index minerals. The staurolite and kyanite isograds in Barrovian terranes therefore may reflect P, T, fluid composition, protolith lithology, and metasomatic shifts in bulk chemistry caused by fluid infiltration. Widespread syn-amphibolite facies quartz vein development and associated staurolite and kyanite growth may mark regions of major fluid outflow and advective heat transport in orogenic belts.