PETROLOGY OF VOLCAN-TEQUILA, JALISCO, MEXICO - DISEQUILIBRIUM PHENOCRYST ASSEMBLAGES AND EVOLUTION OF THE SUBVOLCANIC MAGMA SYSTEM

Volcan Tequila is an extinct stratovolcano in the western Mexican Volc anic Belt that has erupted lavas ranging from andesite to rhyolite dur ing the last 0.9 Ma. Following an early period of rhyolitic volcanism, the main edifice of the volcano was constructed by central vent erupt ions that produced similar to 25 km(3) of pyroxene-andesite. At about 0.2 Ma central activity ceased and numerous flows of hornblende-bearin g andesite, dacite, and rhyodacite erupted from vents located around t he flanks of the volcano. Bimodal plagioclase phenocryst rim compositi ons in lavas from both the main edifice and the flanks indicate that m agma mixing commonly occurred shortly prior to or during eruption. Com positions of endmember magmas involved in mixing, as constrained by wh ole-rock major and trace element abundances, phenocryst compositions, and mineral-melt exchange equilibria, are similar to those of some lav as erupted from the central vent and on the flanks of the volcano. Est imated pre-eruptive temperatures for hornblende-bearing lavas (970 deg rees-830 degrees C) are systematically lower than for lavas that lack hornblende (1045 degrees-970 degrees C), whereas magmatic H2O contents are systematically higher for hornblende-bearing lavas. In addition t o stabilizing hornblende, high magmatic water contents promoted crysta llization of calcic plagioclase (An(70-82)). Frequent injections of ma gma into the base of the subvolcanic plumbing system followed by erupt ion of mixed magma probably prevented formation of large volumes of si licic magma, which have caused paroxysmal, caldera-forming eruptions a t other stratovolcanoes in western Mexico. The later stages of volcani c activity, represented by the flank lavas, indicate a change from a l arge magma storage reservoir to numerous small ones that developed alo ng a NW-trending zone parallel to regional fault trends. Sr and Nd iso topic data for lavas from the Tequila region and other volcanoes in we stern Mexico demonstrate that differentiated calc-alkaline magmas are formed primarily through crystal fractionation of mantle-derived calc- alkaline basalt coupled with assimilation of crustal material.