CORONITIC TROCTOLITE OF PROBABLE JURASSIC AGE, NORTH CAROLINA PIEDMONT

An unusual spinel-bearing troctolite occurs in a single outcrop located along Smith Creek in southern Granville County, North Carolina. It lies on the trace of the Jonesboro fault, a major west-dipping normal fault that constitutes the eastern border of the Durham sub-basin of the Deep River Triassic basin in this area.

Petrographic examination shows that phenocrysts of olivine to 0.5 cm in diameter are surrounded by calcic plagioclase in a likely cumulate texture. Mineral analyses and imaging (done at the SENC-MIC, located at Fayetteville State University) reveal discontinuous corona reaction zones between the olivine and plagioclase. These zones consist of varying proportions of orthopyroxene, pargasitic amphibole, and possible sparse clinopyroxene, as well as green Al-spinel, which occurs in symplectic intergrowth.

Although the rock has not been dated, it may represent a cumulate fraction related to the Jurassic-age olivine diabase dikes common in the area. The mineral content and chemistry, the implied bulk composition, and the lack of greenschist to amphibolite facies metamorphic overprint all provide evidence for this conclusion. The reactions likely occurred at near-solidus conditions.