LAYERING IN DIABASE OF THE JURASSIC MORGANTOWN SHEET: PRELIMINARY RESULTS AND A WORKING HYPOTHESIS

The Morgantown sheet in southeast Pennsylvania is a Jurassic diabase intrusion in the failed rift related to opening of the Atlantic Ocean. A continuous section approximately 600 meters long occurs in a NNW-trending roadcut along the original I-176 about a kilometer north of Morgantown. This section is near the southwestern corner of the sheet as presently exposed, and probably closer to the floor than the roof. Subtle centimeter to decimeter-scale mafic-felsic layering is visible in most of the outcrop and has not been previously reported. Any single layer is not laterally continuous for more than a few meters. Layering dips NNE typically at a shallow angle, but is locally moderately steep.

Mafic layers are consistently more coarse-grained, and contain augite, inverted pigeonite, and orthopyroxene as subhedral crystals and crystal clusters, or as more anhedral grains in subophitic intergrowth with plagioclase. Felsic layers are predominantly small plagioclase grains with much less pyroxene, interstitial micrographic K-feldspar-quartz intergrowths (lacking in mafic layers), and more hydrothermal alteration. Pyroxene in felsic layers is anhedral and interstitial, or subophitic. Compositional and grain size variations are not consistent with an origin for the mafic layers as influxes of new magma or the felsic layers as accumulations of residual liquid.

Our working model suggests that differences in nucleation and growth rates of plagioclase and pyroxenes during crystallization at a solidification front led to oscillating deviations from a cotectic surface. Pyroxene initially nucleated more readily and grew more quickly than plagioclase, which drove the liquid into the plagioclase field and triggered plagioclase nucleation. Existing plagioclase in the mafic layer grew fairly rapidly and residual liquid was expelled upwards where it was trapped in a rapidly-growing mesh of numerous plagioclase nuclei. We are testing this hypothesis with detailed analysis of contiguous thin sections through five mafic and four felsic layers. We will report on results of grain size and modal analysis, as well as mineral compositions obtained with a Quanta SEM with Oxford EDX system.