New Mexico Geological Society Annual Spring Meeting — Abstracts

Petrographic and geochemical characteristics of rhyolites erupted from the Jemez Mountains during the past 3 Ma do not support the notion of a single long-lived silicic magma chamber beneath the Valles Caldera. Pre-caldera andesites and dacites have trace-element abundances indicating substantial involvement of upper crust in petrogenesis. In contrast, the Bandelier group of rhyolites (pre-Bandelier ignimbrites, Bandelier Tuffs, and Cerro Toledo rhyolites) may either be lower crustal melts or extreme fractionates of mantle-derived basalts. While the Lower Bandelier Tuff appears to represent a single variably-fractionated batch of high-silica rhyolite magma similar in many respects to the Bishop Tuff of California, the upper Bandelier displays chaotic internal chemical variations indicating a complex origin. It may in part be derived from remelted Lower Bandelier cumulates. However, the post-upper Bandelier eruptives (valles Rhyolite) mark a return towards the earlier upper-crustal dominated compositons. The msot recent unit, the El Cajete Series rhyolite (El Cajete Pumice, Battleship Rock ignimbrite,
the Banco Bonito lava) is in internal petrographic disequilibrium; most of the "phenocrysts" are interpreted as derived from a pre-existing igneous body. The El Cajete Series magma was erupted in the act of being generated by crustal melting. The Quaternary history of Jemes Mountains magmatism is thus a succession of independently generated silicic magma bodies, some of which were erupted during or very soon (10 -100 ka) after generation.