NEW AGE CONTROLS ON THE EMPLACEMENT OF THE PETERSBURG GRANITE AND ON THE ROCKS INTO WHICH IT INTRUDES, CENTRAL-EASTERN VIRGINIA PIEDMONT, USA

Rocks traditionally assigned to the Petersburg Granite (PG) in the central-eastern VA Piedmont are bounded to the W-NW by Mesozoic rift basins, overlain by Coastal Plain strata to the E-SE, and are thought to be in fault contact with greenschist-facies rocks of the Roanoke Rapids terrane (RRt) to the SW. Detailed mapping has resulted in subdivision of the PG outcrop belt into five units – granitic gneiss, foliated granite, porphyritic granite, subidiomorphic (massive) granite, and pegmatite. The PG is described as a Pennsylvanian-Permian batholith, but due to its composite nature we conducted U-Pb zircon geochronology of different units of the PG to determine whether or not all of these rocks are part of the same plutonic complex.

Buchwaldt and Owens (2012) reported high precision U-Pb ID-TIMS zircon ages of ca. 300 Ma for porphyritic PG and ca. 296 Ma for massive PG. SHRIMP U-Pb zircon analysis of a massive, coarse-crystalline, smoky quartz-bearing phase in the southern portion of the PG outcrop belt yielded an age of 320.8 +/- 4.6 Ma, which is similar to that reported by Lee and Williams (1993). These rocks likely represent the earliest emplaced phase of the PG. Granitic gneiss and foliated granite constitute the largest exposed surface area of what has been correlated with the PG. These rocks are classified as granites both modally and geochemically. Owens et al. (2017) reported a ca. 400 Ma age for an enclave of foliated granite within pegmatite. New U-Pb zircon LA-ICPMS data from another foliated granite exposure yielded an age of 415.7 +/- 4.4 Ma. These data suggest that a significant portion of what was previously mapped as PG may be Devonian in age and represent intruded country rock rather than being a part of the PG batholith. Thus, PG plutonism spanned at least ~25 m.y. and intruded into Devonian granite bodies that were likely part of a regional infrastructure prior to the onset of and during Alleghanian-related partial melting.