Igneous Modal Layering in a Jurassic Basaltic Dike-Sill Complex, Southeastern Pennsylvania, USA
Modal layering is similar in texture, scale, and mineral compositions in both the dike and sill-like portions of the Morgantown Sheet. Layering is contact-parallel, dipping N in the sill and steeply SW in the dike. Pyroxene-rich and plagioclase-rich layers alternate on a scale of a few cm to decimeters, and are continuous for at least several meters. Similarities can be explained by processes that capture and sort entrained pyroxene phenocrysts in progressing solidification fronts. The most laterally extensive layering is located in a thickened part of the sill that may lie over a feeder into the system.
Layer widths and grain sizes are somewhat smaller in the dike than in the sill. Minerals are zoned to more extreme compositions in the dike than the sill, e.g., augite Mg# 70-20 (dike) vs. 76-60 (sill); plagioclase An 75-20 vs. 74-49. Differences suggest more rapid cooling, less exchange of trapped liquid with the magma interior, and less diffusional re-equilibration in the dike than the sill. We are mapping the 3-D distribution of the layering and comparing CSDs of the dike and sill to constrain models of layering development.