Geochronology and stratigraphy of the ignimbrites from the 21°30'S to 23°30'S portion of the Central Andes of northern Chile

The Central Volcanic Zone (CVZ) of the Andes, which extends from 14° to 28° S is one of the largest provinces of Late Tertiary to Recent ignimbrite volcanism in the world. In the 21°30'S to 23°30'S portion of the CVZ in northern Chile, ignimbrite volcanism was initiated at the beginning of the Late Miocene, ∼10.4 Ma ago, and continued until the Recent. The stratigraphy is dominated by the products of five major dacitic ignimbrite eruptions (> 100 km ³); the Artola ignimbrite member (9.4 Ma), the Sifon ignimbrite member (8.3 Ma), the Pelon ignimbrite member (5.5 Ma), the Puripicar ignimbrite member (4.2 Ma) and the Atana ignimbrite member (4.1 Ma). At least three of these represent only portions of larger ignimbrite sheets erupted from centres of SW Bolivia. Two of the ignimbrites, the Sifon and Atana ignimbrites, must represent erupted volumes in excess of 1000 km ³ each. The major units, which represent ∼90% of the total volume of ignimbrite in this area, are intercalated with smaller (< 50 km ³), less extensive volcanic units, some of which were erupted from source structures within the study area. The Upper Tertiary to Recent volcanic stratigraphy has been revised using the large ignimbrites as the major stratigraphic horizons. The ignimbrite formations can be divided into two groups on a geographic basis; those in the northern and central portions of the study area are known as the San Bartolo Group, while those in the southern portion belong to the Silapeti Group. These two groups appear to be contemporaneous, although the Silapeti Group may represent a period of ignimbrite volcanism which was initiated later, about 6 Ma ago. The Ignimbrite Formation of SW Bolivia and the Punense Arancaniano Formation of NW Argentina are equivalent to the San Bartolo and Silapeti groups based on correlation of the stratigraphy into adjacent areas in SW Bolivia and NW Argentina. The beginning of the Late Miocene (10.4 Ma) marks the onset of the most intense phase ignimbrite volcanism in the CVZ. This phase of volcanism is suggested to be the result of crustal melting in response to crustal thickening during the Quechua phase of deformation and subduction. Approximately 10,000 km ³ of ignimbrite erupted over a period of 9 Ma in the 21°30'-23°30' S portion and led to the development of a major ignimbrite province.