Geochemistry and petrogenesis of the Early Jurassic diabase from the central Newark Basin of New Jersey and Pennsylvania

Petrographic, structural, and whole-rock geochemical data from eight diabase sheets and one associated dike in the central Newark basin indicate a common petrogenesis. The petrogenetic model also can be applied to the Palisades sill in the northern Newark basin, that together with some of the central Newark basin diabases make up a single "megasheet" extending for approximately 150 km. Regional chill-margin compositions are extremely constant and are high-titanium, quartz-normative tholeiites (HTQ) typical of the Mesozoic eastern North America (ENA) magmatic province. On the basis of their compositional uniformity and their close similarity to many other ENA-HTQ basalts, the HTQ chills are believed to best approximate the typical diabase parental magma composition. A few coarse-grained samples may have been derived from an ENA low-titanium, quartz-normative tholeiite (LTQ) basalt. Mass-balance models show that the early fractionation of the HTQ parent was dominated by clinopyroxene with lesser amounts of orthopyroxene and plagioclase; accumulations of these phases in parental magmas produced MgO-rich compositions. Plagioclase fractionation became dominant after approximately 20%-25% crystallization; Fe-Ti oxide and apatite fractionation increased dramatically at the latter stages of differentiation. No in situ olivine fractionation is required throughout the entire diabase differentiation sequence. Residual granophyric compositions were produced by approximately 70%-80% total crystallization and they exhibit no geochemical evidence for being the result of large-scale crustal anatexis. However, a local, selective contamination by Triassic sedimentary wall rocks or xenoliths resulted in alkali enrichment and calcium depletion in a few diabase samples and the Quarry dike. Typically, diabase sheets with MgO-rich rocks lack abundant granophyres; conversely, granophyre-rich sheets usually lack any significant amounts of MgO-rich rocks. Granophyre-rich zones are consistently found at higher structural levels than exposures with MgO-rich units. This distribution is consistent with a dynamic fractionation model where low-density residual liquids are displaced both laterally and up-dip along a sheet. The injection of multiple magma pulses is recognized in at least one sheet exposure, and this process, along with cumulate compaction, could have been the driving force for the migration of residual liquids.