Petrology of Quaternary shoshonitic lavas of northwestern Argentina

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Abstract
Cerro Chorrillos and San Jerónimo Quaternary volcanoes of northwestern Argentina erupted on a metamorphic Precambrian basement mostly covered by Paleozoic sandstones. Thickness of the continental crust is about 50 km and depth to the central part of the subducted slab is about 220 km. The lava series consists of absarokite, shoshonite, and banakite. These lavas do not contain plagioclase phenocrysts. The absarokites and shoshonites contain phenocrysts of olivine (Fo 88 to 82 percent core) and clinopyroxene (with high AlVI and low Ti+AlIV). Orthopyroxene and pargasitic hornblende phenocrysts appear in the banakites. Ti-rich phlogopite occurs in some shoshonites and banakites. Xenocrysts of quartz (as much as 9 volume percent) and sieve-textured xenocrysts of feldspar (with Ab- or Or-rich core) occur in all the lavas. Lack of iron enrichment with differentiation is noteworthy. Mg numbers (≃70), Sc, Co, Cr, and Ni contents (22, 30, 300, and 200 ppm, respectively) in basic lavas indicate origin of the magma by partial melting of peridotite mantle. Compared to calc-alkaline lava series from the south-central Andes, the shoshonitic lavas have high Ba, Rb, Th, K, Ta, Zr, and Hf contents. They also exhibit enriched light rare earth element patterns. Major and trace element distribution in the series is consistent with fractionation of olivine + clinopyroxene at deep crustal levels followed by orthopyroxene and amphibole fractionation. Sr-isotope ratios are almost constant at 0.7068 to 0.7079. Mineralogy and geochemistry of the shoshonite lavas series from northwestern Argentina indicate a three-stage origin: (1) partial melting of a mica-bearing peridotite; (2) storage at deep-crustal levels, and possible assimilation of lower-crustal materials; and (3) contamination at upper-crustal levels by incorporation of solid felsic materials.

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