SERPENTINIZATION OF THE UNIONVILLE SERPENTINE BARRENS: A GEOCHEMICAL AND PETROGRAPHIC STUDY

The Unionville Serpentine Barrens, located in Chester County, Pennsylvania hosts a serpentinite body that has not been fully characterized. Located in the Pennsylvania Piedmont, the Unionville serpentinite body is approximately 2 km by 1 km, surrounded by the Doe Run Schist, and is cut by several pegmatite dikes. The Unionville body is the southernmost ultramafic body in a series of early Paleozoic ultramafic bodies in the PA Piedmont which were deformed during the Taconic Orogeny. Several hypotheses are proposed to explain the occurrence of Piedmont ultramafics in this region, including: ophiolitic fragments, diapiric mantle, or arc-magmatic differentiation. Field relationships, thin sections, and geochemical data were analyzed to attempt to determine the nature of the protolith and its origin.

Over twenty-five samples were collected during field work and based on the rock textures (foliation, color, etc.) and field relationships. Five unique groups of serpentinite samples were identified. Eleven samples, representative of the groups, were chosen for thin section production. Petrographic analysis revealed the presence of various pseudomorphic textures, including mesh (lizardite after olivine), hourglass (mainly lizardite after olivine) and bastite (lizardite after pyroxene, amphibole, phyllosilicates). Based on thin section microstructures and textures, three distinct groups were identified: large mesh nets (1.0 – 0.5 mm), small mesh nets (<0.5 mm), and hourglass mixed with mesh. The field groups and thin section groups did not correlate and internally did not exhibit systematic trends. While bastite textures were observed in some thin sections, they were the minority component. Relict olivine can be seen in mesh centers in five of the samples, however, this texture is not restricted to an individual group.

Ten samples were selected for geochemical analyses of major and trace elements. Bulk rock loss on ignition data (used as a proxy for water content) indicates that the outer edge of the serpentinite body is more hydrous than the interior, however, this trend does not correlate with the presence of relict grains. Plotting on petrogenetic discrimination diagrams yields an island arc basalt signature suggesting that the Unionville serpentinite body was part of an arc system prior to serpentinization.