METADOLERITE DIKE SWARM IN BAKERSVILLE-ROAN MOUNTAIN AREA, NORTH CAROLINA
A Precambrian metadolerite dike swarm, emplaced in Cranberry gneiss, extends from Roan Mountain toward Bakersville, North Carolina, but loses its identity in the hornblendic Roan gneiss and schist. Ortho-amphibolites exposed near Toecane in the boundary zone between the Cranberry and Roan formations are equivalents of the metadolerites exposed in Cranberry gneiss. Age relationships of the region indicate that the country rocks have been affected by at least two, and probably three, plutonic cycles. Magnetic, petrographic, mineralogical, chemical, and spectrographic data of the dike rocks are given. The metamorphic origin of the rocks is established by textural and mineralogical criteria. Chemically the dike rocks are typical olivine dolerites belonging to the middle- and late-stage basalts of Wager. The ortho- and para-amphibolites cannot be distinguished by their bulk chemical compositions. Comparisons of contents of copper, gallium, barium, nickel, cobalt, and chromium lead to the same conclusion, but it is believed possible that ortho and para-amphibolites can be differentiated by their strontium contents. Metamorphism was of amphibolite facies; temperatures were probably about 450°–500° C. Differences between subophitic plagioclase-pyroxene and granoblastic plagioclase-hornblende dike rocks are attributed to differences in water content during recrystallization at essentially the same temperatures and total pressures. The abnormal granulitic trend of metamorphic recrystallization of basaltic rocks is due to water-deficient conditions. This type of recrystallization can occur both in the granulite and the amphibolite facies of metamorphism. Four stages of metamorphism, each of different water content, are detailed. Amphibolite problems are reviewed briefly.