SUBDIVISIONS AND FIELD RELATIONS IN THE COMPOSITE PETERSBURG BATHOLITH NEAR RICHMOND, VA

The Petersburg batholith (PB) underlies ~1400 km² in the central Virginia Piedmont. It is bounded to the NW by the Mesozoic Richmond basin, and overlain by Coastal Plain strata to the SE. Although typically portrayed on regional geologic maps as a single, homogenous pluton, five years of detailed field mapping on six 7.5-minute quadrangles near Richmond, VA (through USGS STATEMAP funding to the Virginia Division of Geology and Mineral Resources) subdivides the composite PB into five units – granite gneiss, foliated granite, megacrystic granite, porphyritic granite, and subidiomorphic granite.

Collectively, granite gneiss and foliated granite constitute the largest exposed surface area of the PB. Field relationships between these two units are ambiguous: unit contacts are gradational; locally, foliated granite occurs as enclaves within granite gneiss; granite gneiss is locally overprinted by a younger foliation that similar in mineralogy and regional orientation to foliation in foliated granite. Megacrystic granite (K-spar phenocrysts as much as 30 cm-long) is interlayered with granite gneiss along the eastern margin of the PB. Porphyritic granite (2.5 cm-long K-spar phenocrysts) occurs along the northwestern flank of the batholith, and cross-cuts all other units as thin dikes. Subidiomorphic granite occupies the central-northern portion of the PB outcrop belt north and west of Richmond. This unit is peraluminous (biotite + muscovite + garnet) and chemically appears to be the most evolved.

Biotite-muscovite gneiss, talc-amphibole-biotite schist, and amphibolite occur locally as xenoliths, screens, or possibly roof pendants within the PB. These epidote-amphibolite- and amphibolite-facies lithologies are not common to the metavolcanic and metasedimentary rocks assigned to the Roanoke Rapids terrane (RRT), which the PB supposedly intruded.

A 330±8 Ma discordant U-Pb zircon age from 2 samples of subidiomorphic granite and granite gneiss approximately 50km apart published in 1975 historically has been applied for the Petersburg Granite and the entire PB. Subidiomorphic granite in the core of the PB, flanked by foliated granite and granite gneiss suggests a complex intrusive and deformational history during the Carboniferous Alleghanian orogeny.