La secuencia andesítica terciaria de Andacollo, Neuquen, Argentina

RAPELA, C.W.; LLAMBÍAS, E.J.

Antofagasta

Conferencia; IX Congreso Geológico Chileno; 1985

Sociedad Geológica de Chile

Field relations and the petrographic and chemical composition (major and trace elements) of the Lower Tertiary volcanic sequence of Andacollo (Cayanta Formation -nov.-; 37º09’S; 70°43’W) have been described. This homocline sequence (2.300 m. thick) with a 20-22º westward plunge is composed of flows, dikes and volcanic agglomerates. The lithological composition, arranged in decreasing abundance, is: andesite, basaltic andesite and basalt. Silica variation in the whole sequence is between 51-64%. The dominant paragenesis is: a) pl + ho + cpx + opx + opaque minerals and in a less proportion: b) pl + cpx + opx + opaque minerals. Rhyolitic dikes (73-74% Si02) are intruded in the upper part of the andesitic sequence. The Cayanta Formation is the product of a single volcanic cycle with a typical calc-alkaline evolution. This sequence presents clear, mineralogical and geochemical differences compared with the Paleocene—Eocene volcanism (40° 42°30’S) of the Andean Patagonian Volcanic Province (Rapela et al., 1984). The Tertiary volcanism at 37°S might form part of a different volcanic province together with the mineralogically similar Lower Tertiary volcanics from the southern part of Mendoza and northern of Neuquén (35°-38°s).