LAMPROPHYRES IN VIRGINIA? USING THE VIRGINIA ENERGY ROCK REPOSITORY ARCHIVES TO INVESTIGATE THE PETROLOGICAL AND GEOCHEMICAL CHARACTERISTICS OF MAFIC DIKES IN THE BLUE RIDGE PROVINCE OF VIRGINIA

Mafic dikes found in the Blue Ridge geologic province of Virginia were identified as possible lamprophyres in historic Virginia Department of Energy mapping publications. Fourteen samples of these dikes from five counties archived in the Virginia Energy Rock Repository were petrographically and geochemically examined. Geochemical analyses were funded by a National Geological and Geophysical Data Preservation grant for critical mineral investigation of legacy samples and provided the impetus for this study.

These mafic rocks are reported in the field as cross-cutting dikes ranging in size from several inches to several feet in width intruding within Mesoproterozoic to Neoproterozoic rocks. They are dark colored, fine to medium crystalline, very micaceous (biotite), and have a weak to strong foliation. In thin section, the mineral assemblage is biotite and plagioclase feldspar with minor quartz and opaque minerals. Several samples are amphibole-bearing while the others are muscovite+carbonate-bearing and one sample includes garnet. They have variable foliation and metamorphism is at least chlorite-grade.

SiO₂ content ranges from 37 to 54 wt.% and a LeMaitre (2002) TAS plot shows a wide range of rock types: Foidite, Basanite, Phonotephrite, Basalt, Trachybasalt, and Basaltic Trachyandesite. Many samples have elevated Ba (388-1079 ppm) and Mn (1058-1993 ppm). Sample R-08111, with 37% SiO₂, has enriched Y (111 ppm), Th (25.5 ppm), Rb (1395 ppm) while R-08005, with 48% SiO₂, has enriched Cr (964 ppm) and Ni (600 ppm). Most samples have LREE enrichment (80-1000X chondrites), slight negative to positive Eu anomalies, and smoothly declining HREE depletion. A Hf+Th+Nb/2 discrimination diagram (Krmíček et al., 2011), used to evaluate orogenic and anorogenic petrogenesis, reveals that many samples fall into the anorogenic field while four are near the boundary for the orogenic field.

Although our samples are alkalic none of these samples can be truly identified as lamprophyres which have much higher Cr and Ni and more highly fractionated REE patterns. A previous study by Marshall et al. (2014) for samples in Nelson and Albemarle County describes similar rocks and reaches similar conclusions. These dikes may represent intrusive igneous activity during the Neoproterozoic Robertson River Igneous Suite emplacement.