EVIDENCE FOR KILOMETER-SCALE, FLUID-CONTROLLED REDISTRIBUTION OF GRAPHITE IN PELITIC SCHIST IN THE TACONIC THRUST BELT ON MOUNT GREYLOCK, MASSACHUSETTS
The distinction between graphite-rich and graphite-poor rocks is not a reliable stratigraphic tool. Typically graphite-rich (1-2 wt %) and graphite-poor (0.1-0.2 wt%) rocks are interlayered on scales ranging from 10- to 100-m. Examination of 200 thin sections reveals that 55% of WF samples are graphite-rich, but 45% are graphite-poor. Although only 10% of the GS samples are graphite-rich, 55% contain plagioclase grains with abundant graphite inclusions surrounded by a graphite-poor matrix, indicating that graphite was removed from the matrix by aqueous fluids.
We suggest that C was widely and more evenly distributed throughout the pelitic schist, and that large-scale redistribution of graphite occurred during metamorphism and thrusting. Aqueous fluids dissolved C in large volumes of rock, which are now graphite-poor, but commonly contain plagioclase with abundant graphite inclusions. Faults focused fluid flow and retrograde reactions, commonly observed in fault zone rocks, consumed water and decreased C solubility. C precipitated from these fluids to produce graphite-rich schist. Graphite weakened fault zone rocks and created a positive feedback between faulting, fluid flow, and graphite precipitation. We map the main thrust between graphite-rich schist and marble and include all of the pelites in the hanging-wall.