Document Type
Thesis
Abstract
The Blue Ridge province of central Virginia hosts a suite of biotite-rich mafic dikes. These dikes intrude both Mesoproterozoic basement compiex as well as Neoproterozoic granitoid plutons. Various workers have speculated that these dikes may be lamprophyres, however, no study has confirmed this idea. We studied the mineralogy, geochemistry, and structure of these dikes to determine their classification, origin, and likely tectonic association. Thirteen dikes from ten localities were analyzed in Nelson and Albemarle counties. All are steeply dipping and vary in size from < 10 cm to ~ 2.5 m in width. Three petrographic types were distinguished in the study: a) biotite + amphibole-bearing, b) muscovite + carbonate-bearing, and c) garnet-bearing. Additionally, most contain abundant epidote + titanite. These petrographic types show no correlation spatially. SiO2-content varies from 43.6 to 61.2 wt. % with 8 falling below 48%. Na.O + KO = 3.2 to 8.8 wt. %, so all dikes are alkaline. Additionally, most are potassic (K2O > Na2O). Mg-numbers are low and a few are very low (40.1-7.5). Because of their potassic character, the dikes contain high Ba and Rb (up to 1940 ppm and 427 ppm, respectively). However, Ni and Cr concentrations are very low, and half of the dikes analyzed are below detection. REE patterns are moderately fractionated (Lay/Y by = 3-11; Lay = 53-296), and most show negligible Eu-anomalies. Despite their alkalic nature, these rocks are dissimilar from typical lamprophyres, which are often enriched in Ni and Cr, have more highly fractionated REE patterns, and higher Lay/Luy ratios. Instead, these rocks are a collection of basanites, basaltic trachy-andesites, alkalic basalts, and a trachyte. The dikes of this study are likely unrelated to the Catoctin Formation which is tholetitic and has lower K2O concentrations. We propose that these dikes are instead related to the same Neoproterozoic rifting event that emplaced alkalic plutons of the Robertson River Igneous Suite (~720 Ma). One of these plutons is intruded by a dike of this study, and may reflect the latest stage of this rifting event.
Date Awarded
2014
Department
Geology
Advisor 1
Brent E. Owens
Advisor 2
Christopher M. Bailey
Recommended Citation
Marshall, Aaron A., "Petrological and Structural Analysis of Mafic Dikes in the Central Virginia Blue RIdge" (2014). Geology Senior Theses. William & Mary. Paper 320.
https://scholarworks.wm.edu/geologyseniors/320
