CONTINENTAL BACKARC VOLCANISM OF NORTHERN PATAGONIA: A COMPARATIVE ANALYSIS

The Cenozoic of northern extra-Andean Patagonia is characterized by numerous episodes of volcanism that began in the Paleocene and extended to the Quaternary. These volcanics are in a back arc position to the Andes. The duration of volcanism, superb exposures in this area and variety of structural and crustal features make it an important area to study to understand back-arc volcanism. This paper reviews the Quaternary volcanism of northern Patagonia based on previously published and new geochemical data. Comparisons are made with volcanism at other similar tectonic settings ranging in age from Silurian to Recent, with interpretations for the similarity and differences.

Quaternary volcanic rocks of northern Patagonia include the Crater Basalt, the Moreniyeu, and the Mojón Formations. The distribution of volcanic centres is controlled by the Gastre mega-fracture system. This structure separates a southern transpressional segment of the Andes from a northern transitional segment where the Liquine-Ofqui fault zone controls Quaternary volcanism. The Villarica-Quetrupillan-Lanin volcanic lineament forms the extension of the Gastre fault zone from the backarc into the Andean arc.

The basalts form cinder cones, flood-type basalt flows and tube-fed pahoehoe flows, covering lateral distances up to 40 km from a vent area. Additional geochemical analyses are in progress, but current data show that the rocks are relatively homogeneous trachybasalts to basanites that are LREE enriched (La/Sm = 4-6), with enrichment in the LILE relative to NMORB, but with low Ba/La values of 13-17. Chemical variations are attributed to fractionation at a volcanic centre. There is no evidence for crustal contamination or prior metasomatism of the mantle related to subduction. These rocks are interpreted to form by 3-7% partial melting of an enriched mantle source.

The northern Patagonia Quaternary basalts are very similar to those erupted in a backarc setting in southern Patagonia, but lack the variability of basalts in other backarc positions. These differences are interpreted within a tectonic context.