Analcime-bearing pyroclastic rocks, mostly tuffs, are described from the Chubutian group (Cretaceous) of the Senguerr river region, Argentina. The analcime occurs as a replacement of the tuffaceous matrix and as crystals, cavity fillings, veinlets, and cement. Origin is authigenic and closely related to a lacustrine or marshy environment with arid or semiarid climatic conditions; association with volcanic ash or its alteration products is apparently a prerequisite.

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