Magmatic layering and intrusive plumbing in the Jurassic Morgantown Sheet, Central Atlantic Magmatic Province

This field trip explores igneous layering in the Morgantown Sheet, southeastern Pennsylvania, a Jurassic diabase intrusion that is part of the Central Atlantic Magmatic Province, formed during rifting of Pangea. The Pennsylvania Granite Quarry (Stop 1) is a dimension stone quarry in the southern side of the sheet, in which the cut walls display intermittent modal layering crosscut by channels of mafic diabase. Plagioclase-rich layers overlie pyroxene-rich layers in packages with slightly concave-up "wok" shapes ∼0.3-0.4 m in dimension and ∼0.35-0.5 m thick. Mafic diabase-both layers and crosscutting channels-contain 15-25 modal percent orthopyroxene phenocrysts and are interpreted as basaltic magma replenishments. Orientations of layering and channels suggest this part of the sheet was originally a horizontal sill ∼400 m thick, at about six kilometers depth, and that the sheet was tilted 20°-25° to the north after crystallization. The Dyer aggregate quarry (Stop 2) is in the northeast side of the sheet that dips ∼80° southeast (Birdsboro dike). Here, rhythmic plagioclase-pyroxene layering also dipping ∼80° is found in the interior and near the margin of the ∼255-m-wide dike. Augite and plagioclase compositions are very similar in samples from different vertical heights in the sheet, suggesting localized rather than sheet-wide fractionation. We compare the Morgantown Sheet layering to similar features in the Palisades sill, New Jersey, and Basement sill, Antarctica, and discuss models for their formation. © 2010 The Geological Society of America. All rights reserved.