GEOCHEMICAL DISTINCTION AND TECTONIC SIGNIFICANCE OF MESOZOIC AND LATE NEOPROTEROZOIC DIKES, BLUE RIDGE PROVINCE, VIRGINIA

Extension-related mafic dike swarms invaded basement rocks of the Blue Ridge province in Virginia during Mesozoic and Late Neoproterozoic encratonic rifting. Dikes composed of fine- to medium-grained diabase (Mesozoic) and metabasalt (Neoproterozoic) produced during each event are locally indistinguishable in the field, posing problems for field geologists involved in mapping studies. Primary mineral assemblages composed of plagioclase + one or two pyroxenes + Fe-Ti oxides are common to both populations; however, secondary mineral assemblages are useful in distinguishing dikes of the two swarms. Dikes of Late Neoproterozoic age contain actinolite + biotite + serpentine as alteration products of primary pyroxene; Mesozoic dikes contain assemblages dominated by chlorite. Dikes constituting both swarms are dominantly quartz-normative tholeiites; however, Late Neoproterozoic rocks can be distinguished compositionally by generally lower SiO₂ and higher TiO₂ contents and by relative enrichment in generally incompatible high-field-strength elements, including Zr, Nb, and Y, and by depletion in typically compatible elements, including Ni and Cr. Both dike swarms were emplaced into extensional tectonic environments associated with encratonic rifting that ultimately resulted in formation of ocean basins. Dikes produced during each event can be correlated using minor- and trace-element characteristics to probable cogenetic, stratigraphically constrained lava flows of the Late Neoproterozoic cover sequence of the Virginia Blue Ridge and the Early Jurassic volcanic section of the nearby Culpeper basin, thus providing a first-order age determination for dikes that are petrologically similar in the field. However, the Late Neoproterozoic swarm includes both metabasalt and greenstone, both of which correlate compositionally to lava flows of the Catoctin Formation. Greenstone dikes occur most commonly in spatial association with faults and other structures that acted as pathways for fluids that enhanced retrograde reactions involving primary minerals of the original mafic protoliths. Geochemical characteristics of the dike swarms preserve a record of source compositions involved in generation of magmas at similar stages in consecutive plate tectonic cycles associated with Late Neoproterozoic rifting of Rodinia and, about 370 m.y. later, Mesozoic fragmentation of Pangea.