FINE- AND COARSE-GRAINED DIABASE LAYERS WITHIN THE ROOF SOLIDIFICATION ZONE OF THE JURASSIC MORGANTOWN SHEET, WESTERN NEWARK BASIN, PENNSYLVANIA

The Morgantown Sheet is a small (~400 km^2) intrusion in the western end of the Newark Basin within the 200-Ma Central Atlantic Magmatic Province. Post-magmatic tilting to the north exposes interconnected sills and dikes that fed lava flows at the surface. A roadcut through one sill on Rt 625 south of Knauers, PA, reveals at least 20 magmatic layers close to the roof contact. At the top of each layer is a thin (~1 mm) fine-grained selvage that overlies a thicker lenticular layer of pegmatitic diabase. There is no correlation between the thickness of the pegmatitic material (range 0.5 to 8.0 cm) and overall layer thickness (range 15 to 65 cm). With increasing distance from the roof, the layering disappears and pegmatitic segregations change from lenticular to blobs that become smaller and more dispersed.

We use textures and mineral compositions to investigate hypotheses for the layering: repeated magmatic injections at the roof of the sill (Srogi et al., 2012, GSA abstr. prog., 44, 68); detachment of crystal mush from the roof causing convective currents to draw more evolved liquid upwards (Philpotts & Dickson, 2002, J. Str. Geol., 24, 1171-7); or volatile enrichment and loss associated with magma eruption or migration (Puffer & Horter, 1993, GSA Bull., 105, 738-748). Thin sections reveal that plagioclase and augite crystals grew downward from the fine-grained selvage into the diabase pegmatite, while the contact between the fine-grained selvage and the overlying diabase is more abrupt. The fine-grained selvages are virtually devoid of glass or low-temperature phases; thus, they are not chilled parental or evolved liquids. The pegmatite layers contain late phases including hbl, bio, and granophyre. Preliminary SEM-EDX analyses indicate that plag and augite cores in pegmatitic diabase (An 60, Mg 55) are more evolved than either general diabase (An 70, Mg 70) or the slightly more mafic fine-grained selvages (An 75, Mg 75). Thus far, none of the three models explains all of the data. Additional textural and mineral composition data in sections across one entire layer are being analyzed for evidence of fractionation and juxtaposition of different magmas due to replenishment, migration of evolved liquid, or convective overturn.