The 6163 m high volcano San Pedro in the Western Cordillera of the central Andes of North Chile, is a composite of two cones, each representing a different structural and chemical stage in the evolution of the volcano. Eruption of thin lavas and scoria, dominantly of basaltic andesite composition, built up an older cone, which was subsequently modified by both glacial erosion and a major collapse event, akin to that of Mt. St. Helens in May 1980. A younger cone has subsequently grown within the crater, obscuring much of the detailed structure of the older cone. The younger cone was built up in four distinct phases of eruption which produced andesite and dacite lavas and pyroclastic flow deposits. The latter include both pumice flows and hot avalanche deposits. All of these products from the younger cone are chemically distinct from those of the older cone.

Whilst much of the whole rock geochemistry can be explained by simple crystal-liquid fractionation involving the observed phenocryst phases, this process cannot adequately account for the petrographic textures and mineralogical variations, which show evidence for considerable thermal and chemical disequilibrium. Textural, mineralogical and chemical variations are consistent with models of late stage mixing between magmas of the same fractionation series, with chemical and thermal inputs from hot dense basalt or basaltic andesite injected into the base of the magma chamber.

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