CONTRASTING STRUCTURES RESULTING FROM MAGMA SUPPLY AT DIFFERENT LEVELS WITHIN A DIABASE SHEET

Two localities in the Morgantown Sheet, western end of the Newark Basin, SE PA, contain structures indicating multiple magma inputs. Post-magmatic tilting by ~20°N exposes a deeper level in the PA Granite Quarry (PAGQ) and a shallower level in the Rt 625 outcrop, separated vertically by ~1.5-3.5 km. Although both localities have similar mineral core compositions and layers of similar thickness (~0.3-0.6 m) the igneous structures are otherwise different. The evidence is consistent with supply of more primitive magma and settling of opx in the PAGQ, and flow of magma to a higher sill now exposed in the 625 outcrop.

The PAGQ has ~50-m vertical exposure of layered diabase with thicker mafic layers rich in orthopyroxene phenocrysts and thinner plagioclase-rich layers. Basal load-cast structures in mafic layers and opx-rich mafic channels that transect the layering and have upward flow indicators are evidence of magma replenishments entering the chamber within or at the top of crystal mush. Plag-rich layers are finer-grained and have compositions different from plag inclusions in opx, suggesting they crystallized elsewhere, possibly detached from the roof, and accumulated on the growing mush. Limited intracrystalline zoning is consistent with migration of liquid through crystal mush; mineral alignments suggest some compaction or flow; rock chemistry reflects crystal accumulations and loss of liquid with dissolved fluids.

The 625 outcrop exposes at least a dozen layers separated by internal chilled margins a few mm thick. In the 4 layers studied, chilled margins are at the top with some crystals growing downwards. Opx phenocrysts are absent; augite and pigeonite have patchy domains suggesting amalgamation of many nuclei. Mineral core compositions do not vary much across a layer, but individual crystals are zoned to more evolved compositions than in the PAGQ (An 72-50 and Mg 77-62 for aug). Pegmatitic diabase lenses are located 0-2 cm beneath the chilled margin, and contain more Na-rich plag (An 67-37) and Fe-rich aug (Mg 65-51), and more hbl, bio, interstitial qtz and K-spar, and hydrothermal alteration. These features suggest magmatic inputs that intruded solid rock, lost heat from the roof, and crystallized relatively rapidly as a closed system with limited re-distribution of slightly evolved liquid.