Late Palaeozoic-Early Triassic magmatic activity of Argentina and the Significance of new Rb-Sr Ages from northern Patagonia

CAMINOS, R.; LLAMBÍAS, E.; RAPELA, C.W.; PARICA, C.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 1988 vol. 1 p. 137 - 145

0895-9811

The main outcrops of late Paleozoic-Early Triassic magmatic rocks of Argentina are found on the eastern side of the Andes between 28°S and 36°S (Frontal Cordillera) and in the northern part of the Patagonian Platform (North Patagonian Massif) between 39°S and 44°S. These rocks belong to three magmatic episodes ? dated, respectively, at about 350-330, 280-210, and 240-210 Ma ? and consist of granitic bodies of batholitic size associated with coeval silicic to intermediate volcanics. In both regions, late underformed granites and granodiorites predominate over tonalites and diorites; gabbros are not common, but microdioritic dikes are abundant. All sequences show typical calc-alkaline trends, but although the older units are metaluminous, the younger events become peraluminous with slight transitions to peralkaline composition in the more differentiated types. Despite the magmatic similarities between the Frontal Cordillera and the North Patagonian Massif, these areas differ in other aspects of geologic history. A comparative scheme of the igneous and tectonic evolution of both regions during late Paleozoic-Early Triassic times is given, and the principal geologic features of two eruptive complexes studied in the North Pantagonian Massif are summarized. New Rb-Sr whole rock isochron ages obtained from the North Patagonian Massif eruptive complexes confirm the existence of Early to Middle Carboniferous (332-317 Ma) plutonic and plutonic-volcanic activity in northern Patagonia. These ages are older than those previously reported for the same sequences, which were considered Permian or Triassic according to K-Ar data and Rb-Sr errorchrons. Hence, a re-evaluation of some Permian or Triassic ages previously obtained in other sectors of the North Patagonian Massif is suggested. Moreover, the new ages, together with field observations, indicate that the plutonic episode had a volcanic counterpart not reported in former studies. These data support the hypothesis that a major magmatic cycle developed in the North Patagonian Massif during the Early-Middle Carboniferous, and that since that time a process of progressive uplift and cratonization has taken place in the region.