USGS

Table 3. Geochronology and tectonic history of the Blue Ridge-South Mountain anticlinorium, northern Virginia and southern Maryland.
Epoch/Period/Era
Age (Ma)
Unit
Petrology
Tectonic event
Dating technique
Reference
Early Jurassic
200
Dikes
Diabase
Late-stage continental rifting
Ar/Ar hornblende
Kunk and others, 1992.
Mississippian
340


Greenschist-facies cleavage Alleghanian orogeny
Ar/Ar muscovite
Burton and others, 1992b. Kunk and others, 1993.
Early Cambrian

Tomstown Formation
Dolomite
Passive margin




Antietam Formation
Sandstone





Harpers Formation
Siltstone
Rift to passive margin transition




Weverton Formation
Quartzite





Loudoun Formation
Conglomerate,
phyllite



Late
Proterozoic
570-565
Catoctin Formation
Metabasalt,
metarhyolite
Continental rifting
Rb/Sr pyroxene
Badger and Sinha, 1988.


Dikes
Metadiabase,
metarhyolite
Continental rifting
U/Pb
Aleinikoff and others, 1995.


Swift Run Formation
Metasedimentary rocks
Alluviation during continental rifting



730-700
Robertson River
Igneous Suite
Granitoids
Anorogenic pluton of incipient rift
U/Pb
Tollo and Aleinikoff, 1992.
Middle
Proterozoic
1000-920
Diorite

Cooling age of granulite-grade Grenville metamorphism
Ar/Ar hornblende
Kunk and others, 1993.

1058
Leucocratic
metagranite


U/Pb
Aleinikoff and others, 1993.

1060
Charnockite
Diorite, monzonite

Pb/Pb
Herz and Force, 1984.

1070
Garnet monzogranite

Plutonism of Grenville orogeny
U/Pb
Aleinikoff and others, 1993.

1110
Hornblende gneiss
Quartz monzonite

U/Pb
Aleinikoff and others, 1993.

1127+-7
Biotite granite


U/Pb
Aleinikoff and others, 1993.

1092-1139
Granite gneiss


U/Pb
Aleinikoff and others, 1993.

1144
Porphyroblastic
granite gneiss


U/Pb
Aleinikoff and others, 1993.

1800
Paragneiss
Gneiss, schist, quartzite, and phyllonite

Pb/Pb
Herz and Force, 1984.

 

U.S. Geological Survey, U.S. Department of the Interior
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Contact: Scott Southworth
Last modified 08.29.00