TWO CATEGORIES OF A-TYPE RHYOLITE: EXAMPLES FROM THE SNAKE RIVER PLAIN AND TRANS-PECOS TEXAS

The CSRP and TPVP volcanic fields are known for high-grade ignimbrites as well as extensive silicic lavas. Both areas are intraplate provinces. The CSRP, the manifestation of a continental hotpsot, is strongly bimodal with metaluminous to slightly peraluminous rhyolites. In contrast, TPVP rocks have alkaline affinities, include both trachytes and peralkaline rhyolites, and were erupted during a transition from subduction-related compression to Basin-and-Range extension. The TPVP rocks exhibit lower Mg, Ca, Sr, Ba and Eu/Eu*, and typically higher Fe and HFSE compared to CSRP units at the same SiO2 content. Rhyolites from both regions plot as A-type and intraplate on tectonic discrimination diagrams, but are sufficiently distinct geochemically that they warrant recognition as distinct subtypes. We propose the designation of extreme A-type (XA-type) for the TPVP units, which have the more strongly developed A-type characteristics.