CHEMICAL AND TEXTURAL ANALYSIS OF MODAL LAYERING IN THE MORGANTOWN SHEET, PENNSYLVANIA

The Jurassic Morgantown Sheet is an igneous intrusion associated with the failed rift of the Central Atlantic Magmatic Province (CAMP). The intrusion is located in southeastern Pennsylvania in the central part of the Newark-Gettysburg Basin and consists of a ~400-m-thick saucer-shaped sill with a nearly vertical dike section on the eastern side. The intrusion is composed of diabase that exhibits characteristics of magmatic differentiation including cm-scale modal layering and sporadically distributed “dark channels” that cut across the modal layers. Each modal layer consists of a plagioclase-rich portion that overlies a pyroxene-rich portion. A visually sharp boundary divides the two portions, and each modal layer grades into normal diabase above and below the layer. The “dark channels” appear to be a similar in composition to that of a pyroxene-rich portion of a modal layer and may represent liquids that mobilized within the crystal mush during crystallization. Chemical and textural analysis of pyroxene and plagioclase in modal layers and “dark channels” provides clues to the nature of igneous processes that operate in thick mafic intrusions.