TWO PETROLOGIC TYPES OF HIGH-GRADE IGNIMBRITE? A COMPARISON BETWEEN RHYOLITES OF THE TRANS-PECOS TEXAS AND CENTRAL SNAKE RIVER PLAIN VOLCANIC FIELDS

The central Snake River Plain (CSRP) and the Trans-Pecos Texas (TPTX) volcanic fields are known for high-grade rheomorphic ignimbrites that are often indistinguishable from lavas in outcrop, as well as extensive true silicic lavas. Both modes of emplacement are favored by high magmatic temperatures and low water contents compared to rhyolites that form 'normal' non- to densely welded ignimbrites and lava domes. Nonetheless, high-grade rocks worldwide display considerable compositional diversity, exemplified by these two North American volcanic fields. CSRP rocks are exclusively rhyolites erupted in association with a continental hotspot, and are dominantly metaluminous to slightly peraluminous. TPTX rocks have alkaline affinities and include both trachytes and peralkaline rhyolites, erupted in a continental back-arc setting during a time of transition from lithospheric compression to extension. On average, TPTX units have lower Mg, Ca, Sr Ba and Eu and higher HFSE and trivalent REE at similar SiO2 compared to CSRP units. Low La/Nb and Th/Nb in TPTX lavas are inconsistent with typical arc signatures. Depletion in Ca, Sr and Eu suggests that separation from a large quantity of residual plagioclase is involved in the petrogenesis of TPTX liquids, while Ba depletion additionally requires fractionation from alkali feldspar. The most likely origin of these units is from partial melting of mafic crustal rocks of intraplate geochemical affinity to form a trachytic magma. Subsequent fractionation of feldspar then drives the liquid toward peralkaline rhyolitic compositions. In contrast, the CSRP units show a geochemistry more consistent with melting of granitoid crust. In both cases, the source rocks were probably water-poor.