MINERALOGY AND GEOCHEMISTRY OF ALKALINE MAFIC DIKES IN THE BLUE RIDGE PROVINCE OF CENTRAL VIRGINIA: THE PRODUCT OF EARLY NEOPROTEROZOIC, PRE-IAPETAN RIFTING?

Biotite-rich mafic dikes intrude both Mesoproterozoic basement rocks and Neoproterozoic (~680-715 Ma) granitic plutons in the Blue Ridge Province of central Virginia. Such dikes have previously been speculated to be lamprophyres by various workers, but have never been investigated in detail. In this study, we report on the mineralogy and geochemistry of 13 dikes from ten separate localities throughout Nelson and Albemarle Counties. Dikes vary from ~10 cm up to 2.5 m in width; strikes are variable, but all are steeply dipping. In addition to biotite, most contain abundant epidote ± titanite. One variety contains additional hornblende; a second contains muscovite + carbonate, and another contains garnet. Thus, these rocks have all been metamorphosed. The mineralogical groups show no correlation with locality. The total range in SiO₂-content is from 43.6 to 61.2 wt%, but most (8 of 13) are below 48%. All are alkaline (Na₂O + K₂O = 3.2 to 8.8 wt%), and most are potassic (K₂O > Na₂O). Mg-numbers are low to very low (40.1-7.5). In keeping with their potassic character, the dikes have high Ba (up to 1940 ppm) and Rb (up to 427 ppm). In contrast, levels of Cr and Ni are below detection in most cases. REE patterns for all samples are moderately fractionated (La_N/Yb_N = 3-11; La_N = 53-296), and most show negligible Eu-anomalies; those with garnet have the highest REE concentrations, and two of this type have pronounced negative Eu-anomalies. Although alkalic, these dikes are unlike typical lamprophyres, which have more primitive compositions, much higher Cr and Ni, and more highly fractionated REE patterns. These dikes are probably not related to Catoctin metabasalts, which are typically tholeiitic and have much lower K₂O. Furthermore, unlike the Catoctin, such dikes do not intrude the Lynchburg Group. We consider it more likely that these dikes are related to an earlier Neoproterozoic (failed) rifting event, which also produced alkalic plutons (e.g., the Robertson River Igneous Suite). Some dikes intrude these plutons, and thus may reflect the latest stage of this rifting event.