A petrologic and geochemical study of some amphibolite bodies in the Smith River Allochthon, Virginia

The Smith River allochthon is located in the Inner Piedmont of Virginia. Distinct amphibolite units are present in the lowermost Bassett Formation of the allochthon. The amphibolite units are sandwiched between metasedimentary rocks: an underlying gneiss and an overlying mica schist. One hundred amphibolite samples were collected from different bodies exposed in three adjacent quadrangles: Sanville, Philpott, and Bassett. On the basis of petrographic studies of these amphibolite samples, 20 samples were selected for chemical analysis. The data are presented in this study.

Microprobe analyses have shown the amphiboles in the amphibolites to be calcic and chemically similar. Plagioclase composition is An₅₅-An₇₀ with smaller amounts of An₉₂-An_95.. The major oxide phase is ferrian-ilmenite. T and P conditions of metamorphism for the amphibolites, based on phase assemblages and mineral compositions, are 535°- 650°C and 3.5-4.5 Kb, respectively.

The amphibolites are interpreted to be metaigneous in origin on the basis of Niggle trend and bulk chemistry. The protolith of the amphibolites was probably a quartz tholeiite, as judged from the AFM diagram and normative mineralogy. Discriminant diagrams based on major and trace elements suggest that the parental basalt was formed within a continental rift environment. This is consistent with the Late Precambrian tectonic history inferred for this part of the Appalachian orogeny.