DO VEINS REPRESENT LARGE-SCALE OR LOCAL MASS TRANSFER DURING REGIONAL METAMORPHISM? EVIDENCE FROM THE WAITS RIVER FORMATION, EAST-CENTRAL VERMONT INDICATES LARGE-SCALE MASS TRANSFER

Quartz veins in regional metamorphic terrains have been interpreted either as fossilized paths of fluid flow and large-scale advective mass transfer or as segregations produced by local diffusive mass transfer. Veins and associated alteration selvages in the Waits River Formation were investigated to determine which is the better explanation. Samples of the vein-selvage system and adjacent wallrock were collected in both pelite and carbonate hosts from outcrops in the chlorite and the kyanite zones. Vein abundances are 4.5 volume % in the chlorite zone and 12.2% in the kyanite zone. Selvages are thin (usually <5 mm up to ≈2.5 cm thick) and of very small volume, 0.4% of both outcrops. Modes show that the mineralogy of selvages and unaltered wallrock are similar, but that proportions of minerals change abruptly over ≈1mm at the boundary between selvage and unaltered wallrock. Selvages are depleted in quartz relative to unaltered wallrock. Veins are composed primarily of quartz with variable amounts of calcite and minor muscovite, biotite and pyrrhotite. Bulk compositions of selvages and unaltered wallrocks, measured by XRF and ICP-MS, were compared using a geochemical reference frame defined by combinations of Zr, Ti, REEs and U. All selvages from both outcrops exhibit losses of Si, K, Ba, Cs, and Rb relative to unaltered wallrock. Kyanite zone selvages also show a loss in Mg. Differences in K, Rb, Ba, Cs, and Mg between the vein-selvage system as a whole and adjacent unaltered wall rock are not significant, but differences in Si and Ca are. Vein-selvage systems, specifically, contain more Si than unaltered wall rock, with an addition of ≈47 mg Si/cm³ for both low and high-grade outcrops. Mass balance of SiO₂ requires 85-100% of vein SiO₂ to be derived externally. Quartz veins in the Waits River Formation formed primarily by fluid flow and large-scale advective mass transfer with a small component of local mass transport by diffusion. The minimum time-integrated fluid flux necessary to produce the observed amount of quartz in an entire outcrop is 5.8*10⁶ cm³/cm². The abundances and modes of mineral inclusions in garnet indicate that formation of selvages and veins initiated prior to formation of garnet and continued after the end of garnet growth.