Mineralogical Investigation of a 42 Ma Anorthoclase-Rich Dike, Pendleton County, West Virginia

Tertiary magmatism in the Appalachians and Eastern Midcontinent is comparatively rare and many questions regarding these rocks remain unresolved. Detailed investigations of mineralogy are lacking. One well recognized example is the Brushy Fork “albitite” dike in Pendleton County, West Virginia. This dike is exposed in the Brushy Fork stream and is approximately 9 m wide with a N. 83° E. trend. The country rock is the Devonian Brallier Formation consisting of siliciclastics, dipping steeply toward the southeast at approximately 50°. The dike intruded a fracture cross-cutting strike at an angle of 52°. Previous workers report an age of 42.8±0.5 Ma and describe the texture of the rock as aphanitic, resembling a very fine grained quartz sandstone. The mineralogy has been described as consisting of >95 percent of equigranular, euhedral crystals of albite with trace amounts of small, euhedral biotite crystals, and more rarely ilmenite grains.

Powder X-ray diffraction (XRD) and forty-four electron microprobe analyses (EMPA) indicate that the feldspar present in the rock is anorthoclase, not albite. Hypersolvus crystallization to produce anorthoclase indicates either low pressure or very dry conditions. Composition of the feldspar varied from Ab 0.39 to Ab 0.88 (Avg. Ab0.71) and Or 0.04 to Or 0.54 (Avg. Or 0.24). The average calcium and barium content in the anorthoclase, expressed as CaO and BaO, is 1.09 wt % and 0.28 wt%, respectively. Average iron content expressed as FeO is 0.40 wt%, which is quite high compared to most feldspars.

Fe-Ti oxides occur as do trace amounts of chalcopyrite and interstitial quartz. Anhedral to subhedral hedenbergite grains also occur and contain significant amounts of MnO (Avg. = 2.35 wt %) suggesting minor solid solution with johannsenite. This is the first detailed study of mineral chemistry of the rock and suggests a more complicated history than previously believed.