QUATERNARY MINETTES AND ASSOCIATED VOLCANIC-ROCKS OF MASCOTA, WESTERNMEXICO - A CONSEQUENCE OF PLATE EXTENSION ABOVE A SUBDUCTION MODIFIEDMANTLE WEDGE

Pleistocene-Holocene volcanism in the Jalisco block of western Mexico is confined to two conspicuous grabens, where potassic eruptives range from absarokites (48-52% SiO2) and minettes (49-54% SiO2) through bas altic andesites (53-57% SiO2), the most voluminous type, to andesites and their lamprophyric equivalent spessartite (58-62% SiO2); there are no contemporary rhyolitic rocks. This suite has high concentrations o f Mg, Cr (<550 ppm) and Ni (<450 ppm) accompanied by large concentrati ons of K, P, Ba (<3000 ppm) and Sr (<5000 ppm) and elements such as LR EE and Zr (<600 ppm). No combination of crystal fractionation and/or c rustal contamination can reproduce the compositional range of these ma gmas, which nevertheless are believed to be genetically related becaus e of their proximity in time and space. Hydrous minerals in the lampro phyres and the typical absence of plagioclase phenocrysts in both basa ltic andesites and andesites reflect the relatively high concentration s of water in the magmas, which suppressed the crystallisation of feld spar. Experimental verification of the minimal amounts of water requir ed to reproduce the phenocryst assemblages in selected rocks range fro m 3.5 to 6%. During ascent in a volcanic conduit, andesitic magma may lose water and consequently precipitate plagioclase, or it may ascend more rapidly, retaining more of its initial water, which stabilises ph enocrysts of hornblende at the expense of plagioclase. Our estimates o f water concentrations, which are consistent with the various low pres sure phenocryst assemblages, will be minimal for the magmas in their s ource regions, and the process of magmatic dewatering on ascent may be typical in well established volcanic conduits. In accord with the com positions of phenocrystic olivine in the basaltic andesites and the mi nettes, the values of FeO and Fe2O3 of the bulk lavas and scoriae are demonstrably pristine. As a consequence, are two characteristic featur es of the Mascota the high range of relative oxygen fugacities (Delta NNO=1-5) and the high Mg # (MgO/MgO+FeO) that ranges from 0.70 to 0.91 (with only one andesite as low as 0.66). From the evidence of phlogop ite phenocrysts, a partial melt involving phlogopite would have a high er Mg # than one from olivine (Fo(90)) and pyroxene alone. As the Masc ota series shows a correlation between K2O and Mg #, we conclude that it was generated by partial fusion of the mantle wedge, with a variabl e contribution of phlogopite and apatite from veins throughout the lhe rzolitic assemblage. In conformity with an origin by varying increment s of partial fusion of a phlogopite-bearing mantle, all incompatible e lements vary linearly with Ti (or K) as if phlogopite (+apatite) in th e source dominated their contribution to the partial melts. Fluids fro m dehydration of the subducting slab presumably deposit hydrous and ot her minerals in veins in the mantle wedge and also increase its redox state. As the Mascota volcanism occurs in grabens closer to the trench than the main andesite are, it is concluded that the eruption of thes e small volumes of hydrous magmas require the tectonically favored asc ent paths offered by the extensional grabens to reach the surface from their mantle sources.