Evolución magmática y geotectónica de la "Serie Andesítica" andina en la Cordillera Norpatagónica

RAPELA, C.W.; SPALLETTI, L.A.; MERODIO J.C.

Revista Asociacion Geologica Argentina

Asociación Geológica Argentina

Lugar: Buenos Aires; Año: 1983 vol. 38 p. 469 - 484

0004-4822

The Patagonian “Andesitic Series” is a polyciclic volcanic volcanoclastic complex (Rapela et al., 1982) evolved along subparallel belts in the western continental margin of the South American plate. Geological, geochemical and tectonic characteristics of the westernmost belt of the “Andesitic Series”, known as the “Andean Andesitie Series” (AAS) and better recognized between 40º 00’ and 43º 30’ SL, are described. The volcanic rocks of the AAS are calc-alkaline and high K calc-alkaline series ranging in composition from basalt to rhyolite; but andesites are dominant. Lava flows of basalts and basic andesites besides pulsatory pyroclastic-lava flow facies are widely distributed, while high silica ignimbritic and explosive plinian facies are restricted to Bariloche, Pilcaniyeu and Sierra de Cuyín Manzano areas. The volume ratio of basic to intermediate and acidic members of the AAS increases with time while the sequences show progressive iron enrichment. In some sequences there are also local variations from acid-to-basic composition. New K-Ar ages corroborate that the volcanic activity of the AAS started (at least) in the Middle Paleocene and lasted a protracted period of 15 m.y. Paleosubduction geometries inferred from K-h relationships suggest that the probable depths of magma origin during the Paleocene-Eocene could be located around 140-200 km deep. Compositional variations of Oligocene and Pliocene-Recent volcanics of the same latitude also suggest a variable dip-variable-depth subduction regime with flattening of the subducting slab during the Oligocene. The magmatic evolution of the AAS seems to have ocurred during a progressively extensional environment that ended with block faulting of the arc and development of the basins filled with thick volcanoclastic deposits in the Oligocene (Ñirihuau Formation). As an indirect consequence of the subduction process, the regime developed alkaline lava series in the back-arc area (Middle Eocene to Recent).