PETROLOGICAL AND STRUCTURAL ANALYSIS OF METAVOLCANIC ROCKS IN THE EASTERN BLUE RIDGE AND WESTERN PIEDMONT PROVINCES, CENTRAL VIRGINIA

The late Neoproterozoic 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 SW of Charlottesville, the Catoctin Formation is exposed in a 1 to 3 km wide NE-SW trending belt. A number of small (<1 to 3 km²) mafic metavolcanic outliers occur to the southeast of the main Catoctin belt. The purpose of this research is to determine if these outliers are Catoctin equivalents, and evaluate the structural geometry and kinematics across the boundary between the eastern Blue Ridge and western Piedmont in central Virginia.

Metabasalts in both the eastern Blue Ridge (EBR) and western Piedmont 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. Most samples have moderately fractionated REE patterns (La_N/Yb_N = 3.8-6.4), although one sample has a much flatter pattern (La_N/Yb_N = 1.6). The samples display only slight negative or positive Eu-anomalies. In general, whole-rock compositions for the western Piedmont 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 EBR during Catoctin volcanism (550 to 570 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 strikes NE-SW, but lineations plunge obliquely to gently and kinematic indicators record dextral general shear. In central Virginia, the boundary between the eastern Blue Ridge and western Piedmont is a 3 to 5 km wide zone of dextral transpression.