Contrasting mechanisms of fluid flow through adjacent stratigraphic units during regional metamorphism, south-central Maine, USA

Abstract

The flow pattern of reactive metamorphic fluid through six outcrops of micaceous, carbonate-bearing sandstones from the Vassalboro Formation was determined by calculating and mapping fluid-rock ratios for numerous samples within each outcrop. The ratio of maximum to minimum measured fluid/rock varied by factors of only 1.3-22.9 in each outcrop. Fluid flow was pervasive at metamorphic grades ranging from the biotite through the sillimanite zones. Average fluid-rock ratio for the outcrops increases with increasing grade of metamorphism from ∼0.4 in the biotite zone to ∼ 1.4 in the sillimanite zone. The flow pattern of reactive fluid through impure sandstones of the Vassalboro Formation was different at low and medium grades from fluid flow through the limestone member of the adjacent Waterville Formation. In the biotite and garnet zones, fluid flow through the Waterville Formation was channelized with channelways corresponding to individual lithologic layers that acted as metamorphic aquifers. Fluid-rock ratios recorded by the aquifers are greater than those recorded by the intervening beds by factors of up to 50–60. At the highest grades of metamorphism (sillimanite zone), however, flow through the Waterville Formation was as pervasive as through the Vassalboro Formation. The Waterville and Vassalboro Formations experienced the same metamorphic event. The difference in pattern of fluid flow through the two formations therefore reflects the important control that lithology exerts on the permeability of rocks during metamorphism. Micaceous, carbonate-bearing sandstones evidently were more permeable than argillaceous carbonate rocks. The greater permeability of the sandstones may result from a greater concentration of grain boundaries between unlike minerals in the rocks.