Mineral infillings in alkali basalts as petrogenetic indicators. Sierra de Valle Fértil, San Juan

Abstract

Zeolites and smectites fill cavities in alkaline basalts of Triassic age in the Sierra de Valle Fértil, San Juan. We have carried out an integrated analysis applying optical microscopy, scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction techniques to determine the nature of cavity-filling minerals. The basaltic layers in which these minerals have been identified were differentiated according to their mineralogy and textural relationships in: lower aphyric, upper porphyritic or aphyric. The lower aphyric and upper porphyritic layers have amygdales filled with an intergrowth of smectites, zeolites, plagioclase and iron oxides. Replacement of olivine by iddingsite in the lower aphyric and upper porphyritic layers suggests deuteric alteration under oxidizing conditions. Alteration to iddingsite releases some chemical elements (Si, Mg) to form stable phases. The upper aphyric layer has both ocellar and amygdaloid texture. The ocelli cores are filled with calcite, feldspar or zeolites, green clinopyroxene and amphibole; the ocelli rims have clinopyroxene and apatite. The origin of ocelli is related to early-stage (liquid immiscibility) and late-stage (liquid segregation) magmatic crystallization processes. The amygdales mineralogy (zeolites and/or calcite) would result from crystallization of residual fluids during cooling of the lava. Low-temperature metamorphic events or hydrothermal alteration are discarded as causes for the infilling mineralogy.