Ash-flow tuffs of Neogene age exposed over 2,500 km2 in the Macusani region of southeastern Peru are the volcanic equivalent of S-type granites. The strongly peraluminous tuffs contain phenocrysts of andalusite, sillimanite, and muscovite and have high 87Sr/86Sri (0.7258 and 0.7226) and δ18O (+11‰). Elevated concentrations of Li, Cs, Be, Sn, B, and other minor elements compare with those in “tin granites.” Mineral phase relations and composition are indicative of low magmatic temperatures and oxygen fugacities and high aHF/aH2O. The chemical, isotopic, and mineralogical features and regional geologic relations are consistent with models of magma generation involving the incorporation of large amounts of pelitic rock.

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