Geochemical characteristics of the Upper Paleozoic magmatism in the Eastern Sector of the North Patagonian Massif (40° 45'S, 67° OO'W)

RAPELA, C.W.; CAMINOS, R.

REVISTA BRASILEIRA DE GEOCIENCIAS

Revista Brasileira de Geociencias

Año: 1987 vol. 17 p. 535 - 543

0375-7536

Recent Rb-Sr geochronological studies in wide apart plutonic-volcanic complexes indicate that an extended carboniferous episode (332-317 Ma) is registered in the Northpatagonian Massif. Previous detailed studies in La Esperanza area (68°28’W. 40°28’S) and 35 new m4jor and trace element analyses of the Nahuel Niycu-Comicd area (66°45’W, 4O’38’S) are the base of a comparative study designed to point out the major geochemical characteristics of the Carboniferous super-units. La Esperanza super-unit is characterized by hornblende-biotite granitoids (tonalite, granodiorite, and granite facies with rnicrodioritic enclaves showing silica variation between 61-71%. Biotite granites and biotite granites with K-feldspar megacrysts intrude the former unit. A microdioritic-spessartitic (52-6O% SiO2) dyke swarn intrudes all the granitoid bodies, culminating the evolution of the super-unit. Andesitic lava flows and dacitic and rhyolitic calc-alkaline (high-K) ignimbrites overlie the granitoid rocks. This volcanic event is recognized as a second large super-unit (Dos Lomas) that culminates with high-silica mildly peralkalinc leucogranites and biotite granites. Clear consanguineity relationships relate rocks of a given unit but the trends of the different units that in4egrate the super-units do not follow a co-linear pattern. Each super-unit is composed by several progressively more acidic magmatic pulses emplaced at epizonal level (La Esperanza super-unit) and subvolcanic to surface levels (Dos Lomas super-unit). Regarding the tectonic environment, it is considered that the overall geological characteristics, calc-alkaline affinities, and trace element contents are consistent with a volcanic arc signature for the Carboniferous super-units.