DETAILED STRUCTURAL AND PETROLOGIC ANALYSIS OF THE DEEP PART OF THE MORGANTOWN INTRUSION PROVIDES INSIGHTS INTO MAGMA PROPAGATION AND CRYSTALLIZATION

The Morgantown intrusion (~201.5 Ma), part of the Central Atlantic Magmatic Province (CAMP), occurs in a narrow, early Mesozoic rift basin in SE PA west of the Newark basin. Both petrologic and structural data at km to µm scales are needed to fully understand its development. Here, we focus on the structurally deepest exposed segment of the Morgantown intrusion (the St. Peters sheet, SPS) that forms its southern side today.

Four petrologic units are defined in drill cores sampling ~90% of the SPS. Units 1 & 4 are porphyritic margins with inward-increasing grain size; the abundance of pyroxene (PX) phenocrysts increases then decreases inwards. Unit 2 in the lower to middle interior has upward-increasing frequency of mms-thick plagioclase (PLAG) +/- PX layers. Unit 3, middle to upper interior, has intervals of cms-thick PLAG +/- PX layers and very coarse-grained and granophyric diabase. These same petrologic units occur in other segments of the Morgantown intrusion and in a connected sheet to the west, consistent with magma emanating from inferred feeder dikes and propagating kilometers vertically and laterally. Similar units and modal layering occur at proportionally similar positions in the Palisades sill, and are also in correlative CAMP intrusions in PA, MD, and VA. The SPS is notable for its more complete record of modally layered interior units up to >300 m thick with evidence for lateral shear flow and melt migration, important aspects of its crystallization history.

We have reconstructed the 3D geometry of the SPS at the time of emplacement by correcting for syn- and post-CAMP deformation and erosion, using the above drill-core data plus data from quarries, field exposures, historic iron-ore mines, and lidar imagery. The reconstructions indicate that the SPS intruded pre-rift Precambrian and Paleozoic rocks at >4 km depth, forming a continuous sheet with an average dip of ~20°NNW that both followed and cut the Precambrian foliation. As the sheet entered overlying syn-rift Triassic sedimentary rocks, it branched into three lobes dipping ~20°NNW in the west, ~10°NNW in the center, and ~35°NNW in the east. The central lobe was ~500 m thick, ~100 m thicker than the E and W lobes. The reconstructed shape resembles intrusion shapes from 3D seismic data and provides evidence for lateral, up-dip magma propagation from NNW to SSE.