Estudio geoquímico y petrológico del vulcanismo Paleógeno en la zona de San Carlos de Bariloche y la Sierra de Cuyín Manzano Provincias de Rio Negro y Neuquen, Argentina

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

Buenos Aires

Congreso; V Congreso Latinoamericano de Geología; 1982

Subsecretaría de Minería

Geochemical and petrological investigations of stratigraphic cross sections in the lower-Tertiary volcanic of the Northpatagonian Cordillera (San Carlos de Bariloche and sierra de Cuyin Manzano areas) were carried out. Major and trace (Rb, Sr, Ba, Zr, Nb, Zn, Ni) element analyses were performed in 53 samples. The calc-alkaline andesitic complex is composed of a variety of rocks ranging from basalts to rhyolites and contains 1.5% K2O at 57% SiO2. Immobile element plots suggest that the calc-alkalic complex is composed (at least) by two magmatic cycles erupted at different times (Magmatic Cycles I and II). Each cycle is composed by a group of trends deriving from an initial composition different for each cycle. These trends are better defined in the Magmatic Cycle II where three main trends were recognized (IIa, IIb and IIc). The fractionating minerals in the initial, stages of crystallization included plagioclase, pyroxenes, olivines and magnetite. Hornblende + biotite play an important role as fractionating minerals when the liquid composition rises above 55% SiO2. The extent of fractionation decreases with time in both cycles and the less evolved rocks appear at the top of each sequence.   Provisional minimum ages Paleocene-lower Eocene and mid-Eocene- lower Oligocene have been assigned respectively to the magmatic Cycles I and II on the basis of new published K-Ar age determinations.