Abstract
The progressive metamorphism of a single stratigraphic unit of impure carbonate rock from conditions of the chlorite zone to conditions of the sillimanite zone involved the following sequence of prograde reactions: muscovite + ankerite + albite → biotite + calcite + quartz + anorthite component of plagioclase; ankerite + quartz + plagioclase → calcic amphibole + calcite; biotite + calcite + quartz ± amphibole → diopside + clino-zoisite + K-feldspar. Prograde mineral reactions liberated a CO2-rich mixture of volatiles. Pressure during metamorphism was near 3500 bars (∼13 km); and temperature varied between ∼390°C in the chlorite zone and ∼530°C in the sillimanite zone. Metacarbonate rocks were in equilibrium during metamorphism at all grades with an H2O-rich, CO2-H2O fluid with XCO2 = ∼0.1–0.2.
The flow pattern of reactive metamorphic fluids through six large outcrops of metacarbonate rock in the chlorite, biotite, garnet, staurolite-andalusite, and sillimanite zones was determined by calculating and mapping fluid-rock ratios for numerous samples within each outcrop. Two components of fluid flow were detected: a pervasive component and a channelized component. In the chlorite, biotite, and garnet zones, fluid flow was highly channelized along bedding with enhanced flow occurring within certain layers that acted as metamorphic aquifers. A smaller but detectable amount of pervasive fluid flow occurred within the intervening layers that acted as metamorphic aquitards. Fluid-rock ratios calculated for some aquifers and aquitards in the same outcrop differed by over a factor of 50 at grades lower than the staurolite-andalusite zone. Fluid-rock ratios were uniform within any given layer over distances of up to at least ∼ 15 m parallel to bedding. In the sillimanite zone, flow was more pervasive, and fluid-rock ratios differed by no more than a factor of 1.5 between any beds within the same outcrop. In the staurolite-andalusite zone, patterns of fluid flow were transitional between those in the sillimanite zone and those at lower grades: metamorphic aquifers and aquitards could still be identified, but the difference in their fluid-rock ratios was much less than at lower grades (less than a factor of ∼6). Two generalizations can be made about the control of metamorphic grade on patterns of metamorphic fluid flow: (1) with increasing metamorphic grade, fluid flow becomes less channelized and more pervasive and (2) with increasing grade, average fluidrock ratios increase.