Chemical Alteration and Spilitization of the Catoctin Greenstones, Shenandoah National Park, Virginia

The Catoctin Formation of Precambrian age is composed predominantly of sodium-rich greenstone derived from metamorphism of basalt. Intimately associated with the spilitic greenstone are irregular masses of quartz-epidote rock (epidosite), estimated to make up about one-third of the total volume of the meta-volcanic rocks. The calculated composition of the greenstone-epidosite complex is almost identical with that of dikes in the underlying basement rocks that are believed to be feeders from which the Catoctin lavas were erupted. Evidently the spilitic greenstone and epidosite are the products of chemical alteration of basalt flows with compositions like those of the feeder dikes. The principal changes involved are enrichment of the greenstone (and depletion of epidosite) in sodium, magnesium, and hydrogen, and enrichment of epidosite (and depletion of greenstone) in oxygen and calcium. Chemical alteration apparently resulted from circulation of fluids with high oxidation potential through a fissure system within the flows either shortly after eruption or during Paleozoic regional metamorphism.