EVOLUTION OF THOLEIITIC DIABASE SHEET SYSTEMS IN THE EASTERN UNITED-STATES - EXAMPLES FROM THE CULPEPER BASIN, VIRGINIA MARYLAND, AND THE GETTYSBURG BASIN, PENNSYLVANIA

High-TiO2, quartz-normative (HTQ) tholeiite sheets of Early Jurassic a ge have intruded mainly Late Triassic sedimentary rocks in several ear ly Mesozoic basins in the eastern United States. Field observations, p etrographic study, geochemical analyses and stable isotope data from t hree HTQ sheet systems in the Culpeper basin of Virginia and Maryland and the Gettysburg basin of Pennsylvania were used to develop a genera l model of magmatic differentiation and magmatic-hydrothermal interact ion for HTQ sheets. The three sheet systems have remarkably similar ma jor-oxide and trace-element compositions. Cumulus and evolved diabase in comagmatic sheets separated by tens of kilometers are related by ig neous differentiation. Differentiated diabase in all three sheets have petrographic and geochemical signatures and fluid inclusions indicati ng hydrothermal alteration beginning near magmatic temperatures and co ntinuing to relatively low temperatures. Sulfur and oxygen isotope dat a are consistent with a magmatic origin for the hydrothermal fluid. Th e three sheet systems examined apparently all had a similar style of c rystal-liquid fractionation that requires significant lateral migratio n of residual magmatic liquid. The proposed magmatic model for HTQ she ets suggests that bronzite-laden Magma was intruded in an upper crusta l magma chamber, with bronzite phenocrysts collecting in the lower par t of the magma chamber near the feeder dike. Early crystallization of augite and Ca-poor pyroxene before significant plagioclase crystalliza tion resulted in density-driven migration of lighter residual magmatic liquids along lateral and vertical pressure gradients towards the upp er part of the sheet. The influence of water on the physical propertie s of the residual liquid, including density, viscosity and liquidus te mperature, may have facilitated the lateral movement more than 15 km u p dip in the sheets. Exsolution of a Cl- and S-rich metal-bearing aque ous fluid from residual magma resulted in concentration and redistribu tion of incompatible and aqueous-soluble elements in late-stage differ entiated rocks. This proposed hydrothermal mechanism has important eco nomic implications as it exerts a strong control on the final distribu tion of noble metals in these types of diabase sheets.