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

By
LeeAnn Srogi
LeeAnn Srogi
Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19383, USA
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Tim Lutz
Tim Lutz
Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19383, USA
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Loretta D. Dickson
Loretta D. Dickson
Department of Geology and Physics, Lock Haven University, Lock Haven, Pennsylvania 17745, USA
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Meagen Pollock
Meagen Pollock
Department of Geology, College of Wooster, Wooster, Ohio 44691, USA
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Kirby Gimson
Kirby Gimson
Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19383, USA
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Nicole Lynde
Nicole Lynde
Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19383, USA
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Published:
January 01, 2010

Abstract

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.

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Contents

GSA Field Guide

The Mid-Atlantic Shore to the Appalachian Highlands

Gary M. Fleeger
Gary M. Fleeger
Pennsylvania Geological Survey 3240 Schoolhouse Road Middletown, Pennsylvania 17057-3534 USA
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Steven J. Whitmeyer
Steven J. Whitmeyer
Department of Geology and Environmental Science James Madison University 800 S. Main Street, MSC 6903 Harrisonburg, Virginia 22807 USA
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Geological Society of America
Volume
16
ISBN electronic:
9780813756165
Publication date:
January 01, 2010

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