A petrologic and structural analysis of metavolcanic rocks in the western Piedmont and eastern Blue Ridge provinces, central Virginia

Author

Ciara M. Mills, College of William and MaryFollow

Date Thesis Awarded

4-2015

Access Type

Honors Thesis -- Access Restricted On-Campus Only

Degree Name

Bachelors of Science (BS)

Department

Geology

Advisor

Christopher Bailey

Committee Members

Brent E. Owens

Marylou Zapf

Abstract

The Ediacaran Catoctin Formation forms a distinctive and areally extensive unit in the central Appalachians that erupted as continental flood basalts during rifting of Rodinia. These metabasalts flank both sides of the Blue Ridge anticlinorium in northern and central Virginia. In the eastern Blue Ridge to the southwest of Charlottesville, the Catoctin Formation is exposed in a 1-3 km wide NE-SW trending belt. A number of small (km²) mafic metavolcanic outliers occur to the southeast of the main Catoctin belt within Evington Group phyllites.

Metabasalts in the eastern Blue Ridge (EBR) and Evington Group are mineralogically similar, and are typically composed of chlorite, epidote, magnetite, plagioclase, and quartz ± actinolite ± biotite. Whole-rock compositions for eight samples show a range in SiO₂ from 43.8 to 50.2 wt.% (with one exception at 55.9 wt.%) and Mg#s from 34 to 49. In general, whole-rock compositions for the metabasalt outliers are indistinguishable from those of the main Catoctin belt, suggesting that they are equivalent.

Well-foliated greenstone breccias are common in the outliers, and at one location a distinctive hyaloclastite occurs. We interpret these rocks to have been extruded subaqueously, perhaps reflecting the incipient formation of the Iapetus Ocean in the eastern extent of the Catoctin volcanic province during volcanism (570 to 550 Ma).

Cover units in the eastern Blue Ridge are characterized by a NE-SW striking foliation with down-dip mineral lineations, and NW-directed contractional kinematic indicators. Foliation in the outliers also strike NE-SW, but lineations plunge obliquely to gently, foliations dip steeply, and kinematic indicators record general dextral shear. Therefore, in central Virginia, the boundary between the eastern Blue Ridge and western Piedmont provinces is a 3-5 km wide high-strain zone of dextral transpression.

Recommended Citation

Mills, Ciara M., "A petrologic and structural analysis of metavolcanic rocks in the western Piedmont and eastern Blue Ridge provinces, central Virginia" (2015). Undergraduate Honors Theses. William & Mary. Paper 214.
https://scholarworks.wm.edu/honorstheses/214

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