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REBECCA A. LANGE, IAN S. E. CARMICHAEL, Hydrous Basaltic Andesites Associated with Minette and Related Lavas in Western Mexico, Journal of Petrology, Volume 31, Issue 6, December 1990, Pages 1225–1259, https://doi.org/10.1093/petrology/31.6.1225
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Abstract
The convergent margin of western Mexico is uniquely characterized by a volcanic front of lamprophyric and related lavas located ˜70 km closer to the Middle America trench than the main axis of andesitic volcanism. This front, defined by small volcanic centers ranging in age from ˜1 ka to 3 Ma, contains several lava types: minette, absarokite, leucitite, spessartite, and kersantite, all without feldspar phenocrysts. Many of the lavas contain hydrous phenocrysts; they are enriched in potassium and other incompatible elements, and they are more oxidized relative to the andesitic suite of the main axis. Intimately associated are flows of basaltic andesite of comparable volume. They range in composition from 53 to 58 wt.%SiO2, have 5–9 wt.%MgO and contain phenocrysts of olivine, sparse augite, and varying amounts of plagioclase. Their alkali contents are typical of calc-alkaline varieties, with average Na2O and K2O concentrations of 4·2 and 1·1 wt.% respectively. The basaltic andesites often contain olivine of unusually high forsterite content, reflecting crystallization under oxidizing conditions, and they have oxygen fugacities up to 3·3 log units above the Ni-NiO buffer. Manifestations of high water contents are (1) the ubiquitous occurrence of groundmass olivine rather than orthopyroxene, and (2) the suppression of plagioclase as an early crystallizing phase. Both features reflect the role of water in reducing the activity of silica in the melt. The progressive influence of water during crystallization is also seen in the continuum between the two intermediate lava types, basaltic andesite and kersantite, as plagioclase is suppressed and hornblende is stabilized in the phenocryst assemblage. Thus, despite the absence of hydrous minerals in the basaltic andesites, their phenocryst assemblages reveal the influence of substantial amounts of water, and thereby show a genetic link to the various lamprophyric lavas.
