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Upper Pleistocene rhyolitic ash-flow and air-fall tuffs, erupted from several centers, were sampled in 23 pumice-filled basins over an area of 16,000 km2. Fifty pumice-matrix samples were analyzed for as many as 20 trace elements. Ba, Fe, Hf, Rb, Sm, Sr, Th, Ti, and Zr were particularly useful in “fingerprinting” correlations between basins and in corroborating the stratigraphy previously established within individual basins. On the basis of similar trace elements, a tephra and an overlying ash-flow sheet (together, a unit here named the Los Chocoyos Ash) appear to have formed from a multiphase eruption. The tephra, whose volume exceeded 100 km3, blanketed an area greater than 1 × 106 km2. The second phase of the eruption produced an ash flow of greater than 200 km3. Areal geochemical patterns within the ash-flow sheet are probably related to sequentially less explosive eruptions of progressively more mafic ash flows. Changes in chemical composition, size of pumice and lithic fragments, thickness, and elevation all suggest a source for the Los Chocoyos Ash in the Lake Atitlán cauldron. Chemical data suggest correlation of the H-tephra member of the Los Chocoyos Ash with the most prominent D layer of the Worzel ash of the equatorial Pacific.

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