PETROLOGIC AND GEOCHEMICAL INVESTIGATION OF CAMP DIKES FROM THE BLUE RIDGE AND VALLEY AND RIDGE IN CENTRAL VIRGINIA

The Central Atlantic Magmatic Province (CAMP) is a large igneous province (LIP) composed of basalts associated with the breakup of the supercontinent Pangea and the end-Triassic mass extinction (Davies et al. 2017). Though magmatism occurred over a short time period, there are regional differences in the geochemistry and mineralogy composition of the magmas. Three of six defined compositional groups are found in Virginia and overlap geographically. In this study, we compare composition and mineralogy of dikes from central Virginia to characterize the spatial heterogeneity of magma compositions. Samples were collected from the Blue Ridge province south of Charlottesville and from the Valley and Ridge province in Harrisonburg. Thin sections were imaged on a Phenom XL desktop SEM and semi-quantitative EDS data was collected using a 15kV beam. Whole rock chemical analyses were performed using a Malvern PANalytical Zetium 1kW WD-XRF. The mineralogy and composition of the Blue Ridge dike contrasts with that of the Valley and Ridge dike. The Blue Ridge gabbro contains zoned pigeonite, plagioclase (An₂₃-An₆₈), and micrographic quartz and K-feldspar. Accessory minerals include apatite and intergrown magnetite, ilmenite, and titanite. The Valley and Ridge olivine gabbro contains olivine (Fo₆₀-Fo₈₄), augite, and plagioclase (An₅₈-An₇₆) with trace chromite, pyrite, and magnetite. The Blue Ridge dike contains moderate MgO (6.15-6.16 wt%) and high TiO₂ (0.98-0.99 wt%) and fits into the Prevalent-CAMP group of lava flows and dikes that span across the entire LIP. The Valley and Ridge sample has high MgO (11.63-14.20 wt%) and low TiO₂ (0.47-0.55 wt%) and is part of the Carolina group spanning Georgia to Virginia. The dikes are located within a 60 km region implying localized heterogeneity of mantle source or variation in fractional crystallization history. Quantitative electron microprobe analyses of minerals and whole rock trace element compositions could help constrain the depth, temperature, and source composition for these dikes.