GEOCHEMICAL COMPARISON OF TWO RHYOLITE PROVINCES IN THE CENTRAL APPALACHIANS ASSOCIATED WITH RIFTING

The Catoctin (CF) and the Mount Rogers (MRF) Formations of the central Appalachians represent two volcanic events associated with rifting and the eventual breakup of Laurentia to form the Iapetus Ocean. The two volcanic provinces are separated temporally by ~ 200 m.y. and spatially by ~ 300 km. The CF (564 ± 9 Ma) represents a rifting event that lead to the breakup of Laurentia while the MRF (758 ± 12 Ma) represents an earlier, failed rifting event. These two volcanic provinces are both bimodal in composition comprising of basalts and stratigraphically younger rhyolites. The CF consists of 800 m thick rhyolite sequences exposed over an area of 350 km² and is divided into 6 lithologic units – red porphyritic, blue, orange porphyritic, vitreous, mottled and hematitic rhyolite units (Fauth, 1978). The MRF consists of 500-1000 km³ of rhyolite aggregating to almost 1800 m in thickness and has 3 stratigraphic members – the Buzzard Rock Member (BRM), the Wilburn (WRM) and Whitetop (WTRM) Rhyolite Members (Rankin, 1993). The CF rhyolites are predominantly glassy lava flows or welded tuffs with feldspar (dominant) and quartz phenocrysts. The MRF rhyolites are also lava flows to welded ash flow tuffs containing quartz (dominant) and perthite phenocrysts. Geochemical data identify the felsic rocks from both provinces as high silica (68-79 wt. %) and low Ti (0.1-0.5 wt %) rhyolites. They are dominantly peralkaline (Na₂O + K₂O ~ 7-10%), within-plate, with A-type affinities. Ba contents have been used to sub-divide the CF rhyolites into two groups – high Ba (660-840 ppm) and low Ba (<310 ppm). The WRM and WTRM of the MRF are similar to the low-Ba CF rhyolites while the BRM rhyolites with higher Ba contents (1700-2200 ppm) are comparable to the high-Ba group. Trace element signatures suggest K-feldspar fractionation as a dominant mechanism. The rhyolites of the CF high-Ba group and BRM exhibit elevated Sr, Eu, TiO₂, Al₂O₃, and MgO and lower Nb, Ta, Ga and SiO₂ values. The CF and MRF rhyolites exhibit an average Nb/Ta ratio (11.1) similar to average crustal values. This crustal signature is also suggested by Yb/Ta vs. Y/Nb and Y-Nb-Ce correlations. The geochemical similarity exhibited by the two rhyolite provinces points to the fact that the geochemical signature is not affected by the nature of rifting, i.e., unsuccessful or successful in forming an ocean.