ISOTOPIC AND GEOCHEMICAL CONSTRAINTS ON THE ORIGIN AND EVOLUTION OF POSTCOLLAPSE RHYOLITES IN THE VALLES CALDERA, NEW-MEXICO

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 deltaO-18 for the UBT, (average +8.3 parts-per-thousand), DC (+7.9 parts-per-thousand), and DM (+8.7 parts-per-thousand) rhyolites indicate no significant lowering of deltaO-18 following caldera collapse. In contrast, DM rhyolites record low epsilon(Nd) Of -3.6 to -3.8 relative to the UBT (-2.7) and variable initial Sr-87/Sr-86 (0.70923-0.71307). Del Abrigo (DA) and Santa Rosa I rhyolites (973-915 ka) exhibit lower epsilon(Nd) (-4.4 to -4.6) and initial Sr-87/Sr-86 (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 epsilon(Nd) (-4.0 to 4.3) similar to the 973 to 915 ka rhyolites but lower initial Sr-87/Sr-86 (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 epsilon(Nd) of -3.7 to -4.3, distinctly lower initial Sr-87/Sr-86 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 epsilon(Nd) (-2.7 to -3.0) and the lowest initial Sr-87/Sr-86 (0.70464-0.70478) of postcollapse rhyolites. The isotopic data indicate that Valles rhyolites are not direct melts of Proterozoic basement (epsilon(Nd) of -10 to -15) and indicate a significant mantle-derived (basaltic) component. Elevated initial Sr-87/Sr-86 is restricted to rhyolites depleted in Sr (<10 ppm) and probably reflects minor upper crustal assimilation. Calculated magma deltaO-18 values (+6.6 to +7.0 parts-per-thousand) 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.