Igneous Modal Layering in a Jurassic Basaltic Dike-Sill Complex, Southeastern Pennsylvania, USA

Igneous layering is a fundamental part of the crystallization repertoire for basaltic magmas. Here, we explore modal layering in dike and sill-like portions of the same hypabyssal basaltic sheet. The Jurassic Morgantown Sheet, associated with the failed rift of the Central Atlantic Magmatic Province, is located in SE PA between the Newark and Gettysburg Basins. The east side of the sheet is a dike striking NW-SE, dipping 80º SW. The south and north sides of the sheet are more sill-like and dip gently north. Layering occurs within 50 meters of the contact and in the present interior of the dike, and about a third of the distance up from the bottom of the “sill.”

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.