20 Mesozoic Magmatism, Postrift Tectonics, and Passive Margin Development of Northeastern North America

The northeastern region experienced protracted Mesozoic (Triassic-Cretaceous) magmatism associated with continental rifting and subsequent passive margin development. We welcome contributions related to the occurrence, tectonics, or driving mechanisms of these syn- and postrift events.

Monday, 23 March 2026
8:00 AM - 12:00 PM
CCC, Room 27

CAMP Intrusions in Southeastern Pennsylvania: 2) Magma Emplacement, Flows and Differentiation in a Structural Context

Paper number: 20-4
Program: Technical Sessions
Time: 9:05 AM – 9:25 AM

Authors

Abstract

This presentation follows Withjack et al. on the Eastern North American Central Atlantic Magmatic Province (ENA-CAMP), southeastern Pennsylvania. We focus on the petrology of the upper-crustal Adamstown, Morgantown (MS), and Jacksonwald diabase sheets, Narrow Neck and Newark basins. Our structural analysis provides a semi-quantitative spatial framework for interpreting our petrologic dataset of ~700 samples.

We reconstruct original intrusion geometry at emplacement and identify dikes in basement as potential feeders at ~6-7 km depth. We define an igneous stratigraphy based on a nearly complete top-to-bottom transect from drill cores in the MS. Characteristic features are traceable at similar positions (% distance from an intrusion contact) across MS segments with different emplacement depth (~5.5 to ~1 km), emplacement dip (<10° to ~55°), thickness (~250 to ~450 m), and distance from the feeder (~3 to ~20 km laterally, up to 7 km vertically). Accumulations of PX phenocrysts from <5 to >30 modal% occur in the lower third and upper 10%. Modal layering is found throughout the upper ~90%. Irregular, mms-thick, plagioclase-rich layers (PRLs) occur near the base and increase in frequency upwards. A dimension stone quarry reveals PRLs in gently-dipping, sub-planar to lobate patterns over 10’s of meters, crosscut by quasi-cylindrical mafic channels. Near the intrusion midpoint layering transitions to m-scale intervals of cms-thick, parallel, felsic-mafic layers. Crosscutting veins/lenses of coarse-grained diabase with evolved (Fe-, Si-rich) compositions occur in the upper ~25-30% of the entire MS; however, the largest masses and most evolved rocks are localized in the NW corner (shallowest depth, farthest from the feeder) and the NNE side (steeply-dipping Birdsboro dike). Nearly identical features occur in the Adamstown, Jacksonwald, and correlative ENA-CAMP intrusions.

First-phase CAMP intrusions (~201.5 Ma) followed weak zones (faults, bedding) indicating overpressure which helped bring PX crystals/clusters up from deeper in the plumbing system. Preservation of internal chill boundaries and features related to magmatic flows and crystal-liquid separation suggest multiple magma pulses and rapid cooling. Igneous structures suggest both gravity and lateral up-dip shear flow distributed phenocrysts and formed PRLs during emplacement. Mafic channels that crosscut PRLs exemplify post-emplacement, sub-vertical liquid migration in crystal mush with ~30-45% liquid (from mineral compositions and rhyolite-MELTs modeling). Migration and coalescence of liquid ± hydrothermal fluids after ~50-80% crystallization (MELTs models) formed the evolved diabase.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411211

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