COMPARATIVE PETROLOGY AND TEXTURAL ANALYSIS OF LATE JURASSIC, PHLOGOPITE-BEARING ULTRAMAFIC DIKES IN PENNSYLVANIA AND WEST VIRGINIA: INSIGHTS INTO XENOLITH AND XENOCRYST SAMPLING AND TRANSPORT

Late Jurassic ultramafic dikes occur in a discontinuous belt from Upstate New York through Pennsylvania and West Virginia to the Carolinas approximately parallel to the structural grain of the Grenville and Appalachian orogens. These dikes have received limited study compared to Eocene mafic intrusions and diatremes that they are co-located with in SE West Virginia and west-central Virginia. The Late Jurassic intrusions are compositionally and mineralogically diverse including type-I (?) and -II kimberlites, pyroxenities, nephelinites, and basanites; the majority are xenolith-rich and contain abundant phlogopite xenocrysts. We present textural and petrographic analyses of two ~146 Ma dikes: the Masontown (type-II) kimberlite, Fayette County, PA, and a 'mica-pyroxenite' dike from Sugar Grove, Pendleton County, WV. Both contain a diverse assortment of mantle and crustal xenoliths, including shallow sedimentary lithologies, and contain abundant, 0.5 - 1 cm-diameter, rounded phlogopite xenocrysts. Using emplacement models derived from type-I kimberlites, we describe the textures within the two dikes and interpret the relative timing of different xenocryst / xenolith sampling, rounding, and lagging events during emplacement of the dikes.