PRELIMINARY GEOCHEMISTRY AND MINERALOGY OF DIABASE FROM IRONSTONE RIDGE, CENTRAL PENNSYLVANIA: IMPLICATIONS FOR MAGMA STORAGE DEPTHS

Ironstone Ridge is part of the suite of Mesozoic mafic magmatism characterized in southern Pennsylvania by Smith et al. (1975) as having three different geochemical types: Quarryville-type olivine tholeiite with a TiO2 concentration between 0.35 and 0.45 weight percent; Rossville-type quartz tholeiite with a TiO2 concentration between 0.6 and 0.85 weight percent; and York Haven-type quartz tholeiite with a TiO2 concentration between 0.95 and 1.25 weight percent. This study focuses on Ironstone Ridge, classified by Smith et al (1975) as a Rossville-type diabase dike. The Ironstone Ridge dike trends north-south for approximately 20 km through the central part of the Cumberland Valley in south central Pennsylvania, intruding Ordovician-age limestones, dolostones, and shales; within the carbonate sequence the dike branches in two places. We have sampled the dike at six locations along the length of the dike. Most of the samples come from unconsolidated outcrops overlying the dike, but a full transect of the dike was sampled from an intact road cut along the Pennsylvania Turnpike. At that location the width of the dike is 23.72 m. A preliminary measurement of bulk rock density indicates that the bulk rock density is 3029.40 kg m-3. Preliminary thin section mineralogy shows an intergranular to subophitic groundmass textures with numerous subhedral plagioclase laths between 0.11 mm and 1 mm long; plagioclase phenocrysts are rare and are generally 1.25 mm -2.5 mm long and subhedral to euhedral. All plagioclase exhibits primary twinning. Pyroxene is of similar size and varies from euhedral to subhedral. Samples appear to be about 12% – 20% anhedral to subhedral opaque minerals from 0.4-0.6 mm in length. Collection of major and trace element geochemical analyses for all samples is in progress. The goals of this study is to better characterize source and storage depths for three Mesozoic diabase types in south-central Pennsylvania. References Smith II, R.C., Rose, A.W., and Lanning, R.M., 1975, Geology and Geochemistry of Triassic Diabase in Pennsylvania: Geological Society of America Bulletin, v. 86, p. 943-955.