EVIDENCE FOR KILOMETER-SCALE FLUID-CONTROLLED REDISTRIBUTION OF GRAPHITE IN THE TACONIC THRUST BELT ON MOUNT GREYLOCK, MASSACHUSETTS: IMPLICATIONS FOR STRAIN LOCALIZATION AND FAULT GROWTH

During the Ordovician Taconic orogeny, deep-water deposits from the Laurentian slope and rise were thrust westward over shelf rocks. On Mount Greylock graphite(C)-rich schist is interpreted as a flysch deposit, whereas C-poor schist is assigned to the Taconic thrust sheet by Ratcliffe et al. (1993). According to this interpretation, the major thrust is within the schistose rocks and separates C-rich Ordovician(?) Walloomsac Fm from structurally overlying, C-poor Late Proterozoic Greylock Schist. This interpretation assumes that the present distribution of C preserves primary variations in organic material.

The contact between C-rich and C-poor schist on Mount Greylock is diffuse and not marked by well-defined strain gradients. In contrast the contact between the C-rich schist and the structurally lower marble, is a mélange characterized by intense deformation. Our detailed field mapping indicates that the distinction between C-rich and C-poor rocks is not a reliable stratigraphic tool. Typically C-rich and C-poor rocks are interlayered on scales ranging from 10- to 100-m. Examination of 200 thin sections from 173 locations reveals that 55% of Waloomsac Formation samples are C-rich, but 45% are C-poor. Samples of Greylock Schist are more complex; although only 10% are C-rich; another 55% contain plagioclase porphyroblasts with abundant C inclusions surrounded by a C-poor matrix. This common texture suggests that C was once present in the matrix but was dissolved by aqueous fluids and transported out of the rock.

Thin section evidence suggests that C was formerly widely distributed in the schist on Mount Greylock, but that large-scale redistribution of C occurred during metamorphism and thrusting. We suggest that aqueous fluids dissolved C in large volumes of rock that are now C-poor, as shown by plagioclase porphyroblasts containing abundant C inclusions. Fluids later precipitated C in narrow zones to form C-rich schist. We suggest that a high density of fractures focused fluid flow parallel to thrust faults, and that C precipitated in these zones. C may have weakened the rocks and promoted faulting, thus creating a positive feedback between thrusting, fluid flow, and C precipitation.