Igneous Petrogenesis of Tequila Volcano, Western Mexico
Abstract
Tequila volcano belongs to a Quaternary volcanic chain that runs in parallel to the Middle American Trench, but that have been constructed within the so-called Tepic-Zacoalco rift: an extensional tectonic structure that has been active for the past 3.5 Ma. 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 at least four different magmatic series around Tequila: 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, probably indicating melt-crust interactions. 2) A group of vitreous domes and flows of dacitic to rhyolitic compositions, mostly contemporaneous to the Santa Rosa basalts, that were emplaced on the periphery of Tequila volcano. These rocks can have very low Sr and Eu contents but their isotopic compositions are remarkably constant and similar to the Santa Rosa basalts, probably indicating a genetic link through low pressure fractionation in the stability field of plagioclase. 3) The main edifice of Tequila volcano (~0.2 Ma) is made of two pyroxene andesites and dacites with strong subduction signatures (Ba/Nb= 53-112), that inversely correlate with MgO contents, but that follow a diverging evolutionary trend as the rest of the sequences. The isotopic compositions of Tequila main edifice can extend to slightly more enriched values, but do not correlate with fractionation indexes, thus indicating provenance from a different source. 4) The youngest activity on Tequila volcano (~0.09 Ma) is represented by amphibole bearing andesites that erupted through the flanks of the main cone. These rocks have similar trace-element and isotopic compositions as the Tequila main edifice, but display an independent evolutionary trend in major element diagrams. Tequila is thus an unusual stratovolcano that appears to have evolved in pulses of petrologically unrelated magmas. Andesites may have interacted with the crust during ascent, but the geochemical data cannot be explained by mixtures between the Santa Rosa basalts and the local crust. Direct melting of an underplated mafic crust also seems implausible because the Mg# are higher in andesites (54-57) than in basalts (52-41). Since no parental calc-alkaline basalts have been found, the geochemical data thus indicate a probable mantle origin for the Tequila andesites. If true, this mantle source appears to form discrete and well localized hydrous domains that underwent higher extents of melting than the surrounding and mostly dry peridotite.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V43C2590V
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes;
- 1037 GEOCHEMISTRY / Magma genesis and partial melting