INVESTIGATING THE ORIGIN OF PEGMATITE DIKES IN UNIONVILLE, PENNSYLVANIA AND THE GEOCHEMICAL INTERACTIONS WITH SURROUNDING SERPENTINITES

The Unionville Serpentine Barrens in Chester County, Pennsylvania host a group of pegmatite dikes intruding into a serpentinite body, giving way to complex geochemical reactions. Limited data exists on these pegmatites pertaining to their origin and composition except for a few publications regarding adjacent corundum mining that occurred from 1839 to 1892. Field mapping and sample collection of pegmatites, serpentinites, and intermediary reaction zone rocks was conducted. Thin sections of samples were examined to investigate textural and mineralogical changes across the pegmatite-serpentinite reaction gradient. Geochemical analyses of major and trace elements were used to classify the pegmatites by composition, to identify the origin of the melt that produced the pegmatites, and to observe the extent of elemental component exchange between the pegmatites and serpentinites.

Due to historical mining in the area, outcrop is rare; therefore, general field relationships were determined by presence of localized pits and tailings piles. Existing maps largely disagree with location of pegmatite mine pits, suggesting existing maps may require revision. Pegmatite samples varied in texture by location but maintained a consistent mineralogy (Ab>Qtz>Ms>Tur). Most pegmatite thin sections exhibit some foliation, representing strain imposed on the rocks. Intermediary ‘reaction rocks’ are generally microcrystalline with extensive veining and brecciation, although they exhibit a gradient of textures depending on proximity to the pegmatite or serpentinite.

Mineralogical and geochemical data indicates that the Unionville Pegmatites are within the Muscovite-Rare Element pegmatite class. Concentrations of Si, Ce, Sr, Ni, and Cr reveal a consistent compositional gradient from pegmatite to serpentinite in strong agreement with textures observed in thin section. Examination of this geochemical gradient allows for an understanding of component exchange between pegmatites and serpentinites. Plotting trace element concentrations on petrographic discrimination diagrams suggests that the Unionville Pegmatites originated in a volcanic arc setting rather than an anatectic/metamorphic setting.