The Effects of Contact Metamorphism by Diabase Intrusion on the Carbon and Sulfur Bearing Phases in the Siltstones of the Culpeper Basin

Many of the large igneous provinces during the Phanerozoic have been temporally linked to mass extinction events. The intrusion of magma into country rock has been hypothesized to facilitate the release of carbon and sulfur bearing volatiles and has been proposed as one of mechanisms that drove these mass extinctions. In this study I examine a dike of the Central Atlantic Magmatic Province and its interaction with adjacent sedimentary rocks in the Culpeper Basin of Virginia. Sampling was done at the 0.5 m scale along transects of sedimentary lithologies perpendicular to the ~170 m wide diabase intrusion. The observed mineralogical and geochemical changes in sedimentary rocks occur in a much narrower zone from the intrusion than predicted by the applied thermal model. Carbon isotopes of organic matter within the sedimentary rocks are enriched in 13C toward the intrusion indicating the generation of thermogenic methane within the first meter from the intrusion. Additionally, geochemical and petrologic textures suggest both the addition of magmatic sulfur into the country rock, shown through the isotopic signatures of sulfide minerals with mantle compositions. The possible thermal break down of sedimentary pyrite is evidenced by highly negative isotopic composition of sulfide minerals and general lack of pyrite. I suggest that sedimentary pyrite initially reacted to pyrrhotite, which was then converted to chalcopyrite through reactions with copper in fluids derived from the magma. These reactions also allowed for the formation of magnetite, which is elevated near the dike-sedimentary contact and at the end of the transect. A simple illustrative model of a hypothesis of fluid flow along the bed is used to explain the observed isotopic signatures and mineralogical changes along the transect. This study highlights how models for volatile generation through magma-country rock interaction may have overestimated the volatile fluxes from these environments and the roles that heterogeneity of sedimentary rocks and kinetic factors may have in the variance in these fluxes.