STRUCTURE, PETROLOGY, AND GEOCHRONOLOGY OF MESOZOIC DIABASE DIKES IN CENTRAL VIRGINIA

In central Virginia, a regionally extensive suite of diabase dikes intrude Paleozoic to Proterozoic rocks. These dikes are traditionally associated with the Central Atlantic Magmatic Province (CAMP), a brief but widespread episode of tholeiitic magmatism that occurred at ~200 Ma prior to the opening of the Atlantic Ocean. We focused on diabase dikes in a 250 km² area at the boundary between the Piedmont and Blue Ridge. Our goals were to: 1) map the orientation, width, and length of the diabase dikes, 2) analyze the petrology and geochemistry of these dikes, and 3) determine the crystallization age and thermal history of diabase using U-Pb zircon and apatite geochronology.

Field mapping and magnetic surveys indicate seven map-scale (1:24k) in the area. Dikes vary in length from 3 to ~13 km with average widths of ~30 m (max. 200 m). Dikes form distinctive NNW trending lineaments, overall dikes strike 320 to 350 ^˚ and dip steeply. A few dikes form an en-echelon map patterns and two dikes are linked by a bridge of diabase striking 310^˚. Diabase typically crops out as massive large spheroidal boulders, however at one location is densely fractured. Fine-grained granophyric dikelets (up to 2 cm wide) cut diabase in the Norwood dike.

Most samples are fine-grained to microcrystalline while some appear nearly gabbroic with massive fabric. Samples also exhibit porphyritic texture with subhedral phenocrysts of olivine and/or pyroxene. Chemistry results for the diabase show a range of SiO₂ from 47 to 52% and Fe₂O₃ values from 11 to 13%. The granophyre contains 64% SiO₂, 7% Fe₂O₃, and ~6% K₂O + Na₂O, with an anomalous drop in Eu values. Granophyre exhibits a myrmekitic texture with abundant saussurite suggesting plagioclase alteration through fractionation. Normative mineralogy reveals two geochemical types: olivine-normative and quartz-normative with high TiO­₂. The geochemical types suggest different processes of crystallization and cooling, quartz-normative as a result of continuous fractionation versus a slower process of olivine-normative crystallization and differentiation. Preliminary results of U-Pb zircons and apatite dating are consistent with CAMP age magmatism and cooling of these dikes.