THE ORIGIN AND SIGNIFICANCE OF A LARGE MAFIC GNEISS XENOLITH IN THE PETERSBURG GRANITE, PIEDMONT PROVINCE, VIRGINIA

A large (approximately 350 by 250 m), distinct mass of metamorphosed mafic rock occurs within the Petersburg granite near its western margin (south of Midlothian, VA). On the geologic map of the Chesterfield 7.5’ quadrangle (Carter et al., 2010, VDGMR open-file report), the mass (labeled mafic gneiss) appears to represent a sizable xenolith, or possibly a screen or roof pendant. Given the lack of intrusive contacts of Petersburg granite with adjacent rocks (most contacts are faulted or covered by Coastal Plain sediments), this xenolith provides a rare opportunity to investigate the nature of rocks intruded by the batholith. The mass is actively being quarried, and thus fresh samples are readily available. Observations at the quarry show that the rocks, although typically “mafic,” are quite heterogeneous, and intruded at a variety of scales by felsic veins and dikes. We focused our sampling on what appears to be the most mafic, and homogeneous variety of the rock. Eight samples collected from a small area of the quarry are all foliated, fine-grained amphibolites, dominated by hornblende and plagioclase, with minor amounts of biotite, chlorite, titanite, Fe-Ti oxides, and apatite. Whole-rock compositions of all samples are similar, with SiO₂ ranging from 46 to 50 wt%, but with a considerable range of Mg# (64-38). All represent sub-alkaline, tholeiitic basalts, and strong correlations between immobile high-field strength elements (e.g., Zr, Y), suggest that these samples are co-genetic. Overall concentrations of REE are low (La_N = 8.1-18.7 x chondrites) and chondrite-normalized patterns are similar and typically flat (La_N/Lu_N = 0.9-1.5), with negligible Eu-anomalies. On numerous tectonic discriminant diagrams based on immobile trace elements, all samples consistently plot in the field for volcanic arc basalts, and their chondrite-normalized patterns are consistent with such an origin. Thus, these samples probably represent arc-related volcanism, followed by amphibolite facies metamorphism, that pre-dated intrusion by the Petersburg granite at ~300 Ma. A likely possibility is that these rocks formed in an arc setting as part of the “Iapetan Realm,” an interpretation that could be evaluated by future geochronological investigations.