Pocoshock Creek Gneiss and redefinition of the Petersburg Granite, central-eastern piedmont of Virginia

Abstract

The Petersburg Granite (sensu lato) was formerly interpreted to be a single late Paleozoic composite batholith in the central-eastern part of the Piedmont Physiographic Province of Virginia. Detailed mapping beginning in the mid 2000s resulted in the subdivision of these voluminous granitic rocks into five units—layered granitic gneiss, foliated granite, porphyritic granite, subidiomorphic (equigranular) granite, and pegmatitic granite. U-Pb zircon geochronology demonstrate that foliated granite and layered granitic gneiss currently assigned to the batholith are 100 million years (m.y.) older, and yield Silurian to Devonian ages (ca. 425 mega-annum (Ma) to ca. 403 Ma). We propose the name Pocoshock Creek Gneiss for these strongly foliated and layered gneisses. Geochronologic data also require redefinition of the Petersburg Granite (sensu stricto). Massive equigranular and porphyritic rock types of the Petersburg Granite (sensu stricto) are geochemically distinct from a subset of Pocoshock Creek Gneiss samples. Massive equigranular and porphyritic rock types are similar to another, more granitic subset of the Pocoshock Creek Gneiss, which indicates different magma sources for the Petersburg Granite (sensu stricto) and Pocoshock Creek Gneiss but also likely indicates assimilation of Pocoshock Creek Gneiss during late Paleozoic intrusion of these Petersburg Granite (sensu stricto) rock types. Equigranular granite southwest of Petersburg, Virginia is 15–20 m.y. older than porphyritic granite, massive granite, and monzodiorite (ca 300 Ma) near Richmond, Virginia. The Petersburg Granite (sensu stricto) consists of two regional plutons—the northern Richmond pluton and the southern DeWitt-Sutherland pluton. The DeWitt-Sutherland pluton of Early Pennsylvanian age is separated from Late Pennsylvanian to early Permian granite of the Richmond pluton by a map scale septum of Pocoshock Creek Gneiss. Geochemical analyses of 15 samples of Pocoshock Creek Gneiss are compared to other igneous suites throughout the Appalachian orogen. Whole rock geochemistry shows elevated average thorium and uranium values in porphyritic granite compared to massive equigranular granite and Pocoshock Creek Gneiss and suggest that homes and businesses built on that rock type of the Richmond pluton may be most at risk for uranium and (or) dissolved radon in groundwater or radon gas in confined spaces.