IGNEOUS PETROGENESIS OF TEQUILA VOLCANO, WESTERN MEXICO

Tequila volcano belongs to a Quaternary volcanic chain that runs in parallel to the Middle American Trench, but that has been constructed within the realm of a continental rift. This unusual tectonic setting, and the existence of a high-resolution stratigraphy for the Tequila Volcanic Field (Lewis-Kenedi, 2005, Bull Volcanol), provide an excellent opportunity to study andesite petrogenesis. New comprehensive geochemical data allow the recognition of three different magmatic series in the Tequila Volcanic Field: 1) The Santa Rosa intraplate basalts (1.0 – 0.2 Ma), a volcanic plateau constructed along the Santiago River Fault north of Tequila volcano. These Na-alkaline basalts are olivine-phyric, have negligible subduction signatures (Ba/Nb= 11.75 – 49.36) and display Sr-Nd-Pb isotopic compositions that correlate with fractionation indexes, indicating melt-crust interactions. 2) A group of peripheral vitreous domes and flows of dacitic to rhyolitic compositions, mostly contemporaneous (0.9 – 0.23 Ma) to the Santa Rosa basalts. These rocks extent to very low Sr and Eu contents at remarkably constant isotopic compositions, indicating a genetic link through closed-system, low pressure fractionation in the stability field of plagioclase. 3) The main edifice of Tequila volcano (~0.2 – 0.09 Ma) is made of two pyroxene andesites and dacites with occasional later-stage hornblende. These rocks display strong subduction signatures (Ba/Nb= 50.3 – 112.1) and follow an independent isotopic evolution to the basalts, indicating provenance from a different source. Interestingly, Tequila andesites also extend to low Dy/Yb (2 – 1.4) and Nb/Ta (17 – 12) ratios with increasing silica at high and constant Mg# (~60), geochemical features that are consistent with melting of an amphibole-rich lithology located at shallow depths in the mantle wedge. Tequila is thus an unusual stratovolcano that appears to have evolved in pulses of petrologically unrelated magmas. Whereas the Santa Rosa basalts likely formed by low extents of melting of a relatively dry mantle, the Tequila andesites cannot be explained by simple crystal fractionation of basalts, direct crustal anatexis or by mixing between basalts and local rhyolites. We thus interpret their source to be a well-localized and discrete amphibolitic domain in the local upper mantle.