MAGMATIC DIFFERENTIATION IN THE DIKE SECTION OF THE MORGANTOWN SHEET, SOUTHEASTERN PENNSYLVANIA

The Morgantown Sheet is located in the central part of the Newark-Gettysburg Basin in southeastern Pennsylvania. The Jurassic-age intrusion is associated with the failed rift of the Central Atlantic Magmatic Province (CAMP). To the eastern side of the saucer-shaped diabase sill is a nearly vertical dike section. In the dike section, cm-scale rhythmic modal layering is oriented vertically. Preliminary field studies and sampling in the dike section reveal an approximately 10-m-wide zone of visually distinctive rocks in vertical contact with the normal gray diabase near the center of the dike. Unlike the normal gray diabase of the Morgantown Sheet, the rocks are dense, coarse-grained, slightly reddish-brown and spheroidally weathered. Initial investigations of these rocks indicate that they are coarse-grained segregation sheets of late-crystallizing minerals that formed during the last stages of crystallization. The coarse-grained rocks are three times more abundant in magnetite and six times more abundant in granophyre and apatite than the normal diabase rocks present in the dike. Whole-rock chemical analysis of the coarse-grained rocks show that they contain ~2.55 wt. % TiO­2, compared to the normal diabase that contains ~0.88 wt. % TiO2. The high concentration of TiO­2 in the coarse-grained rocks indicates that these rocks formed from highly-fractionated liquid. Late-crystallizing liquids in lateral sheets of magma are thought to mobilize by either buoyancy or a mechanism such as compaction of crystals that expels residual liquid upward within the intrusion. Because rhythmic modal layers and the contact relationship of the coarse-grained rocks observed in the dike section of the Morgantown Sheet are vertical, they present an excellent opportunity to study magmatic differentiation in an igneous setting contrary to laterally extensive intrusions. Chemical and textural analyses of these rocks provide clues to their origin and the igneous processes that may have played a role in their formation.