FELSIC MAGMATISM IN FLOOD BASALT PROVINCES: A STUDY OF THE SOUTH MOUNTAIN (CATOCTIN) HIGH SILICA RHYOLITES

The early Cambrian Catoctin Volcanic Province of the central Appalachians represents a sequence of extension related magmas associated with the opening of the Iapetus Ocean. In the South Mountain region of Pennsylvania and Maryland, the Catoctin Formation is composed of intercalated, sub-aerial rhyolites and basalts. This bimodal association of the volcanic rocks is mostly absent in the rest of the Catoctin Formation rocks further south, which are dominated by basalts. The rhyolites are predominantly glassy lava flows or welded tuffs with feldspar (dominant), quartz, biotite and ilmenite phenocrysts. Preliminary geochemical data identify these rocks as high silica (72-79 wt. %) and low Ti (0.1-0.4 wt %) rhyolites. They are dominantly peralkaline (Na2O + K2O ~ 8-10%), with A-type affinities. Trace element signatures suggest K-feldspar fractionation as the dominant mechanism of the observed REE patterns. Ba contents can be used to sub-divide the rhyolites into two groups – high Ba (660-840 ppm) and low Ba (<310 ppm). The high-Ba group exhibits elevated Nb, Ta, Ga, Eu, TiO2, Al2O3, and MgO and lower SiO2 values. The Mount Rogers Formation (750 Ma) rhyolites (types A & B) are similar to the low-Ba Catoctin rhyolites. Petrogenetic models of rhyolites in other continental flood basalt provinces like the Paranás (Brazil), the Karoo Province (Africa) and the Deccan Traps (India) suggest partial melting of underplated mafic rocks near the crust-mantle boundary. The mafic source regions for these rhyolites is supported by distinct trace elemental properties (e.g., ratios for Tb/Yb ~ 0.38, Nb/Ta > 15 and Y/Nb < 1.20). In contrast, rhyolites from South Mountain exhibit an average Nb/Ta ratio (11.1) similar to average crustal values. This crustal signature is also suggested by analysis of Yb/Ta vs. Y/Nb correlations, where the Catoctin rhyolites exhibit measurable differences from OIB-like source field. The understanding of the chemical and temporal evolution of rhyolites in basaltic provinces may provide clues to the development of rifted continental margins.