TEXTURAL STUDY REVEALS DYNAMIC DEVELOPMENT OF MINERAL LAYERING AND CRYSTAL ALIGNMENTS BY MAGMATIC FLOW IN A DIABASE SHEET, WESTERN NEWARK BASIN, PENNSYLVANIA, USA

Both magmatic and subsolidus processes are proposed in the literature for the formation of igneous structures such as mineral layering. We investigate these igneous structures in a diabase sill exposed in the cut walls of a dimension stone quarry. The sill is part of the Morgantown Intrusive Complex in the western Newark Basin, equivalent to the Palisades sill, which formed during rifting of Pangea at approximately 201.5 Ma.

Quarry walls show cm-thick light gray layers rich in plagioclase (up to 75% modal plag) underlain by darker pyroxene-rich layers. The most prominent plag-rich layers are spaced about 0.33 m apart. Multiple magmatic pulses are preserved. Later pulses (0.33-1m thick sheets) create drag folds and basal load-cast-like structures in the underlying crystal mush. These indicate that the mush was not fully solidified at the time of the later magma emplacements. These emplacements have prominent plag-pyx layers at their top, so we interpret all similar layers as having formed from magmatic flow and shear forces in a dense suspension of pyx and plag antecrysts. The plag-pyx layers also are cut by mostly vertical, pipe-like, mafic channels (mm to < 1m wide) formed by upward and lateral melt migration within the crystal mush.

This study focuses on the micro-scale textures of the plag-rich layers. Evidence for magmatic flow rather than compaction or deformation includes alignment and tiling of euhedral plag grains, chemical zoning patterns, and limited ductile and brittle deformation. Backscattered electron images, x-ray maps, and mineral chemistry are used to help document plag growth within the layering. Preliminary results show evidence of dissolution and reprecipitation of plag following impingement and tiling. We are measuring orientation, length, and width of tablet shaped plag in thin sections cut perpendicular and parallel to the layering. Preliminary results confirm visual inspection and show that plag grains have preferred orientations with long dimensions generally parallel to layer margins. However, orientations and flow direction as shown by plag tiling can be locally variable. Local domains showing different flow patterns suggest that micro-scale magmatic flow was complex and changed as the crystal mush solidified.