EVALUATING LATERAL AND VERTICAL FLOWS AND CRYSTALLIZATION IN A COMPLEXLY SHAPED UPPER-CRUSTAL DIABASE INTRUSION

Studies of ancient igneous complexes provide time-integrated summaries of magmatic systems that complement studies of active volcanoes. We focus on basaltic upper-crustal intrusions in the Mesozoic basins of the Central Atlantic Magmatic Province (CAMP) associated with rifting of Pangaea at ~201.5 Ma. The tilted, eroded Morgantown intrusion (Narrow Neck west of the Newark basin, SE PA, USA) is exposed in a cross-sectional view from below the rift basin to the paleo-surface. We use 3D reconstructions (companion poster by Patel et al.) to identify possible feeder conduits and magma flow paths, estimate depths of emplacement, and provide context for petrologic data.

The complex shape of the Morgantown intrusion (interconnected gently dipping and inclined sheet segments and steeply dipping dikes) requires three parameters to express sample position: 1) original emplacement level (0.5–7 km depth); 2) segment orientation; and 3) position relative to country rock contacts (marginal or interior). Drill cores (Smith, 1973; Polycor, Inc.) provide an ~80% complete section of the most deeply emplaced sill segment. We define rock units that are laterally continuous for a few kilometers across the cores: marginal units with grain size increasing inwards; units in the bottom half enriched in pyroxene phenocrysts; thick middle units with modal layering and evidence for lateral shear flow; and units in the upper third that include minor pegmatitic diabase (granophyre).

The bulk chemical composition of our more complete core dataset, corrected for intrusion geometry, is consistent with pyroxene accumulation in bottom and middle units reflecting transport, sorting and repacking of crystal cargo during emplacement and inflation. The presence of similar bottom/middle units in all segments at emplacement levels from ~7-3 km constrains distances of lateral and vertical flows within the rift basin to ~10 km and ~5 km, respectively. No similar rocks occur in inclined sheets and dikes at the highest emplacement levels. In more steeply dipping segments, convection after emplacement may have enhanced separation of crystals and evolved liquids. We present petrographic and geochemical data to assess connectivity, lateral and vertical flow models, and crystallization/reactive flow processes within the connected segments.