GEOCHEMISTRY AND GEOCHRONOLOGY OF A MONZODIORITIC PHASE OF THE PETERSBURG BATHOLITH, EASTERN PIEDMONT PROVINCE, VIRGINIA

The Petersburg batholith in east-central Virginia forms one of the largest Alleghanian plutonic complexes in the southern Appalachians, underlying an elongate area of more than 1800 km² that extends from near Ashland, VA to south of Dinwiddie, VA. Like other Alleghanian plutons, generation and emplacement of Petersburg magmas was ultimately related to convergence of Laurentia and Gondwana, although details regarding both the timing and nature of emplacement as syn- or post-tectonic have thus far been poorly constrained by the currently available geochronology. Most previous studies have shown that the batholith is comprised primarily of granite sensu stricto, although the full range of compositions has never been systematically investigated. We report here results for a newly recognized monzogabbroic to monzodioritic phase of the batholith. It is the dominant rock type exposed in an active quarry operated by Luck Stone in Richmond, VA. The rock is medium- to coarse-grained, dark gray and massive, and is locally intruded at a variety of scales by pink, medium- to coarse-grained granite or pegmatite dikes. Dominant minerals include plagioclase, hornblende, and biotite, along with accessory epidote (locally cored by prominent allanite) and titanite. Notably, the rocks are poor in K-feldspar and quartz. The rocks (seven samples) are mafic (48-54 wt% SiO₂) and mostly shoshonitic (2.2-3.6 wt% K₂O). Our initial hypothesis was that this rock represents an early, more-mafic phase of the batholith or perhaps an older, unrelated unit. To test this, we obtained U-Pb ages on both zircon and titanite from one sample by LASS. Although the analyzed zircon population appears slightly older (309 ± 6 Ma; n=36) than the titanites (298 ± 10 Ma; n=23), the two dates are not statistically different, indicating co-crystallization following emplacement. Somewhat surprisingly, this ~300 Ma age is essentially the same as other, more recently published Petersburg ages, i.e., this phase is approximately coeval with the rest of the batholith (thus ruling out both of our initial hypotheses). The shoshonitic character of this unit, along with other high-K to shoshonitic compositions reported by others elsewhere in the batholith (but at much higher SiO₂) may be a reflection of dominantly post-collisional magmatism.