COMPARISONS IN A SUITE OF CRYSTAL-RICH AND CRYSTAL-POOR LAVAS IN THE SANGANGUEY VOLCANIC FIELD, NAYARIT, MEXICO: IMPLICATIONS FOR THE ORIGINS OF ANDESITE AND DACITE LIQUIDS

To evaluate if intermediate magmas erupting from Volcán Sanganguey (Mexico) and the surrounding volcanic field are formed by mixing of basalts and rhyolites or if they initially exist as intermediate liquids, a detailed petrological study is presented for eight andesite and dacite magmas. Six are crystal-rich, erupted from the central edifice, and two are crystal poor, erupted from monogenetic vents in the peripheral volcanic field. All samples are multiply saturated in five to seven mineral phases, with plagioclase spanning a 30-40 mol% An range and orthopyroxene spanning a Mg# range of 5-10. Pre-eruptive temperatures and oxygen fugacites range from 853-1085 °C and -0.1 to 0.9 ∆NNO. Application of the plagioclase-liquid hygrometer reveals maximum H₂O contents from 1.7-6.2 wt%. Comparison with phase equilibrium experiments shows that all plagioclase and orthopyroxene compositions in the crystal-poor samples could have grown from their respective whole rock compositions, but comparison of crystal rich samples reveals the presence of potential sodic xenocrysts and an estimated excess plagioclase crystal cargo. No calcic plagioclase xenocrysts are observed, so the excess plagioclase in the crystal-rich samples were likely originally formed in intermediate magmas.

For the crystal-poor samples, we propose that the complex phenocryst assemblages are produced by degassing (±cooling), as it may shift equilibrium plagioclase compositions, kinetically inhibit crystal-growth, and increase melt viscosity, leading to complex textures. This requires that the intermediate melts initially exist as liquids prior to crystallization, supporting the hypothesis that these melts are generated in the deep crust. For the crystal-rich samples, degassing (±cooling) may also generate a portion of the variation in the assemblages, and some incorporation of antecrysts or xenocrysts must occur; however, we find no definitive evidence supporting the incorporation of crystals initially grown in basalts or rhyolites. Given the similarities in phase assemblage, mineral compositions, mineral textures, and intensive variables between the crystal-poor and –rich samples, we conclude that the melts arriving into the upper crust in this region are initially formed as intermediate-composition liquids in the deep crust.