Ring-fracture rhyolites of the Valles Caldera (VC) were examined to determine the evolution of the magma system following eruption of the upper Bandelier Tuff (UBT) and subsequent caldera collapse. Volcanism began with eruption of Deer Canyon (DC), Redondo Creek, and Del Medio (DM) rhyolites during the interval 1140--1133 ka. Quartz Δ18O for the UBT, (average +8.3%0, DC (+7.9%0), and DM (+8.7%0 rhyolites indicate no significant lowering of Δ18O following caldera collapse. In contrast, DM rhyolites record low ϵNd of -3.6 to -3.8 relative to the UBT (-2.7) and variable initial 87Sr/86Sr (0.70923--0.71307). Del Abrigo (DA) and Santa Rosa I rhyolites (973--915 ka) exhibit lower ϵNd (-4.4 to -4.6) and initial 87Sr/86Sr (0.70707--0.71009), as well as shifts in compatible and incompatible trace element ratios. Seco, San Luis, and Santa Rosa II rhyolites (800--787 ka) have ϵNd (-4.0 to -4.3) similar to the 973 to 915 ka rhyolites but lower initial 87Sr/86Sr (0.70616--0.70747). After a hiatus of 230 ka, San Antonio (SA), South Mountain (SM), and La Jara (LJ) rhyolites (557--521 ka) were erupted with ϵNd of -3.7 to -4.3, distinctly lower initial 87Sr/86Sr of 0.70513--0.70553 and less evolved trace element compositions.

The youngest rhyolites, the El Cajete-Banco Bonito group (EC-BB) (300--170 ka) are petrographically and geochemically distinct with the highest &egr;Nd (-2.7 to -3.0) and the lowest initial 87Sr/86Sr (0.70464--0.70478) of postcollapse rhyolites. The isotopic data indicate that Valles rhyolites are not direct melts of Proterozoic basement (ϵNd of -10 to -15) and indicate a significant mantle-derived (basaltic) component. Elevated initial 87Sr/86Sr is restricted to rhyolites depleted in Sr (<10 ppm) and probably reflects minor upper crustal assimilation. Calculated magma Δ18O values (+6.6 to +7.0%0) indicate no substantial supracrustal sediment in the source region, nor interaction with hydrothermal fluids (or altered wall rock) in the upper crust.

The intermittent volcanic activity and the heterogeneity of petrographic, geochemical, and isotopic compositions from one eruptive interval to the next suggests that these rhyolites were vented from a series of temporally and spatially separated magma chambers. Parental melts supplied to these magma chambers were most probably produced by basalt-induced fusion events in a hybridized deep crustal environment. ¿ American Geophysical Union 1993