Petrology of Volc�n Tequila, Jalisco, Mexico: disequilibrium phenocryst assemblages and evolution of the subvolcanic magma system

Abstract

Volcan Tequila is an extinct stratovolcano in the western Mexican Volcanic Belt that has erupted lavas ranging from andesite to rhyolite during the last 0.9 Ma. Following an early period of rhyolitic volcanism, the main edifice of the volcano was constructed by central vent eruptions that produced ∼ 25 km3 of pyroxene-andesite. At about 0.2 Ma central activity ceased and numerous flows of hornblende-bearing andesite, dacite, and rhyodacite erupted from vents located around the flanks of the volcano. Bimodal plagioclase phenocryst rim compositions in lavas from both the main edifice and the flanks indicate that magma mixing commonly occurred shortly prior to or during eruption. Compositions of endmember magmas involved in mixing, as constrained by whole-rock major and trace element abundances, phenocryst compositions, and mineral-melt exchange equilibria, are similar to those of some lavas erupted from the central vent and on the flanks of the volcano. Estimated pre-eruptive temperatures for hornblende-bearing lavas (970°–830°C) are systematically lower than for lavas that lack hornblende (1045°–970°C), whereas magmatic H2O contents are systematically higher for hornblende-bearing lavas. In addition to stabilizing hornblende, high magmatic water contents promoted crystallization of calcic plagioclase (An70–82). Frequent injections of magma into the base of the subvolcanic plumbing system followed by eruption of mixed magma probably prevented formation of large volumes of silicic magma, which have caused paroxysmal, caldera-forming eruptions at other stratovolcanoes in western Mexico. The later stages of volcanic activity, represented by the flank lavas, indicate a change from a large magma storage reservoir to numerous small ones that developed along a NW-trending zone parallel to regional fault trends. Sr and Nd isotopic data for lavas from the Tequila region and other volcanoes in western 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.