MASS-TRANSFER DURING BARROVIAN METAMORPHISM OF PELITES, SOUTH-CENTRALCONNECTICUT .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 chemi cal 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. Measur ed vein densities increase from about 2 to 4 percent in the lowest gra de rocks to about 20 to 30 percent in the kyanite zone. In the amphibo lite facies, veins are commonly surrounded by a highly aluminous selva ge rich in staurolite +/- kyanite and micas and poor in quartz and pla gioclase. Staurolite and kyanite are typically absent from the less al uminous wallrocks located beyond the selvage margins. The selvage widt h 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 reg ional scale movement of metamorphic fluids. (1) Mass balance analysis of two selvages and their protoliths indicates that major and trace el ements were mobilized during selvage development by reactions that des troyed quartz, plagioclase, and micas and produced staurolite and kyan ite. The reactions suggest significant hydrogen metasomatism. (2) Esti mated f(HCl)/f(H2O) is highest in the selvages directly adjacent to ve ins, which suggests that the most altered rocks were infiltrated by fl uids with elevated f(HCl)/f(H2O) and that the avenues for infiltration were quartz veins. (3) The amount of silica loss from local pelitic w allrocks 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 inf iltration down regional T and P gradients. (4) Because previously publ ished stable isotopic studies have shown that quartz in amphibolite fa cies veins may be significantly out of oxygen isotopic equilibrium wit h walIrock quartz, the fluid that precipitated quartz was probably in part externally-derived. Selvage formation resulted in significant dec reases in Na/Al and, in some cases, K/Al which stabilized aluminous in dex minerals. The staurolite and kyanite isograds in Barrovian terrane s 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 associa ted staurolite and kyanite growth may mark regions of major fluid outf low and advective heat transport in orogenic belts.