A minimum age for high-grade metamorphism and granite intrusion in the Piedmont of the Potomac River gorge near Washington, D.C

Near Great Falls on the Potomac River, metamorphosed Piedmont rocks of the Glenarm Group were intruded by small bodies of granite and pegmatite after the peak of high-grade metamorphism in the area. Muscovites from two of the feldspar-quartz-muscovite pegmatites were dated by the Rb-Sr method. They...

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Bibliographic Details
Published in:Geology (Boulder) 1979-07, Vol.7 (7), p.349-350
Main Authors: Muth, K. G, Arth, J. G, Reed, Jr
Format: Article
Language:English
Subjects:
absolute age
age
Appalachians
dates
Fairfax County Virginia
Falls Church Quadrangle
geochronology
Glenarm Series
granites
Great Falls
high-grade metamorphism
igneous rocks
intrusions
Lower Ordovician
lower Paleozoic
metamorphism
mica group
muscovite
North America
Ordovician
orogeny
Paleozoic
pegmatite
Piedmont
plutonic rocks
Potomac River
Rb/Sr
sheet silicates
silicates
Taconic Orogeny
United States
Virginia
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Summary:Near Great Falls on the Potomac River, metamorphosed Piedmont rocks of the Glenarm Group were intruded by small bodies of granite and pegmatite after the peak of high-grade metamorphism in the area. Muscovites from two of the feldspar-quartz-muscovite pegmatites were dated by the Rb-Sr method. They show ages of 469 plus or minus 20 m.y. and 469 plus or minus 12 m.y. The ages may represent the time of intrusion or cooling of the rocks below about 500 degree C. The pegmatites crosscut the foliation of the Glenarm metasediments, and thus their age places a younger limit on the time of deposition of the series and of high-grade metamorphism in the area.
ISSN:0091-7613
1943-2682
DOI:10.1130/0091-7613(1979)7<349:AMAFHM>2.0.CO;2