EVIDENCE FOR KILOMETER-SCALE, FLUID-CONTROLLED REDISTRIBUTION OF GRAPHITE IN PELITIC SCHIST IN THE TACONIC THRUST BELT ON MOUNT GREYLOCK, MASSACHUSETTS

During the Ordovician Taconic orogeny, Neoproterozoic to Ordovician shale and siltstone from the Laurentian slope and rise were thrust over coeval shelf rocks. Flysch, eroded from the advancing thrust sheets, blanketed the shelf rocks, and was locally overridden by the thrust sheets. On Mount Greylock in MA graphitic and non-graphitic schist are indistinguishable except for their C content. The graphite-rich schist was mapped as flysch of the Walloomsac Fm (WF), and the contact that separates WF, from graphite-poor rocks of the Greylock Sch (GS) was interpreted as a thrust. This assumed that the present distribution of graphite reflects the primary preservation of organic material in sediments. Based on our mapping, the gradational contact between WF and GS is difficult to locate and we found no strain gradients near it. In contrast, a wide zone within the WF especially near the contact with the structurally lower marble is a zone of intense deformation and shares many characteristics of mélange.

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.