MAGMA CRYSTALLIZATION WITHIN A CAMP UPPER-CRUSTAL PLUMBING SYSTEM: INSIGHTS FROM PHASE COMPOSITION MAPS AND GEOCHEMICAL MODELING

The Central Atlantic Magmatic Province (CAMP) includes voluminous basalt lava flows and mafic intrusions about 201 Ma in age. We present new data from the Morgantown-Jacksonwald System, at the western end of the Newark Basin, Pennsylvania. The Jacksonwald basalt and chilled margins from underlying sills and dikes have virtually identical REE and incompatible trace element compositions that correlate with high-Ti, quartz-normative tholeiite (HTQ, York Haven or Orange Mtn. type). Both lavas and chills contain phenocrysts (plagioclase, augite, olivine) with the same high-temperature compositions consistent with crystallization from a parental HTQ liquid. By contrast, sill and dike interiors have lower-temperature, more evolved mineral compositions even in samples with abundant orthopyroxene phenocrysts and cumulate-like bulk compositions. The data suggest the entire plumbing system was built by an initial influx of more primitive magma, then expanded by the intrusion of multiple batches of more evolved magmas. To better constrain the composition and crystallization history of the magma batches we have developed a new technique, the Phase Composition Map, which shows the spatial distribution of minerals or specific compositional ranges of minerals with solid solution in rock thin sections. We use energy-dispersive mineral analyses to relate grayscale values in backscattered electron images directly to mineral identification and composition through regression analysis. Phase Composition Maps can reveal spatial-compositional patterns that are too subtle to be recognized in standard light or electron images. Results from dike samples with vertical modal layering reveal the presence of crystals with more primitive compositions that were carried in by more-evolved host magma. By combining the regression model with MELTS thermodynamic models we can produce Phase Composition Maps that show the spatial arrangements of mineral compositions that crystallized together at specific temperature steps. We use these Phase Composition Maps to quantitatively test how well MELTS models using a range of starting compositions fit observed mineral modes and compositions at the thin section scale.