Dawsonite, NaAlCO 3 (OH) 2 , occurs as a replacement, cement, and fracture filling in continental, zeolitized and silicified vitric tuffs and litharenites of the Cenomanian Cerro Castaño Member, Cerro Barcino Formation, Chubut Group, Patagonia, Argentina. Analcime is the only associated zeolite; it replaces the vitric masses and also fills fractures and cavities. Dawsonite and analcime display an inverse ratio. Textural relationships indicate that dawsonite is a pseudomorph after oligoclase, quartz, vitric shards, and vitric masses; it coexists with diagenetic quartz cement and postdates analcime, calcite, and hematite cements. Unaltered crystals of high sanidine postdate all other cements generated. Silicification of the tuffs is likely to have occurred in various stages during the diagenetic history of the sequence. Hypabyssal bodies of alkaline basic rocks of the El Buitre -El Canquel Formation, which intruded regionally the Cerro Castaño Member during Eocene time, are interpreted to be responsible for the introduction of CO 2 gas at high partial pressures, together with sodium, which led to dawsonite formation. Diagenetic quartz, at a late stage of silicification, is related to the transformation of oligoclase and analcime to dawsonite, also releasing Na and Ca into the system. The d 13 C (PDB) values of dawsonite, in the range -1.2 to -2.4‰, attest to alkaline igneous activity in the Cerro Castaño Member during the Eocene and, in addition, set a limit on the age of the dawsonite.
