Rb-Sr Geochronology of La Esperanza Eruptive complexes, Northpatagonian Massif 68° 28'W, 40° 28'S

LLAMBÍAS, E.J.; RAPELA, C.W.; PARICA, C.

COMUNICACIONES - DEPARTAMENTO DE GEOLOGIA. FACULTAD DE CIENCIAS FISICAS Y MATEMATICAS. UNIVERSIDAD DE CHILE

Universidad de Chile

Lugar: Santiago; Año: 1985 vol. 35 p. 149 - 153

0069-357X

The north Patagonian Massif (NM) Is essentially composed by 1) a basement, formed by low to medium grade metamorphic schists and migmatites and some magmatic intrusions of Late Precambrian to Early Paleozoic age; and 2) plutonic and plutonic-volcanic sequences of Late Paleozoic age. Regional descriptions of these units have been recently reviewed by Caminos and Llambías (1984) and Llambías et al. (1984) respectively. The Upper Paleozoic eruptive sequences are widespread along the NM. They are composed by postorogenic granitic intrusion emplaced near the surface, and the later events are often related to ignimbritic shields. During the Late Paleozoic the (NM). became rigid and its petrologic and tectonic features may be important in order to understand the evolution of the pre-andean terranes in the eastern side of the Cordillera. The eruptive complexes of La Esperanza are representative of most of the units described for the Upper Palaeozoic eruptive sequences of the (NM). The stratigraphic relations and petrographic characteristics of these eruptive complexes have been recently described by Llambías and Rapela (1984) and their chemical evolution by Rapela and Llambías (1985). A synthesis of their most important features is given below. Two igneous complexes have been recognized 1) the oldest one, La Esperanza plutonic complex, is characterized by distinct petrographic units that in order of decreasing age consist of hornblende-biotite granodiorites and hornblende-biotite granites with K-feldspar megacrysts (Prieto Granodiorite); biotite granites, and biotite granites with K-feldspar megacrysts. The contacts between units are sharp, while within individual units there are often transitional facies. Several microdioritic to spessartitic dikes intruded these units. 2) Dos Lomas plutonic-volcanic complex, which 15 composed by dacitic ignimbrite sheets; a rhyolitic dome; rhyolitic aplitic dykes; rhyolitic ignimbrite sheets, and leucogranites (CalvoGranites.the leucogranites intruded the lower units of the volcanic sequence. Field relationships and chemical similarities between the volcanic rocks and the leucogranitic plutons suggest that they are evolving from the same magmatic source. Both complexes are separated by an erosion surface elaborated on the granitic rocks of La Esperanza complex. The dacitic ignimbrites of the Dos Lomas complex flowed over this almost horizontal surface. However, the similarities in chemical and evolutive trends between the two complexes, probably indicate that they belong to a single eruptive cycle. Both complexes are mostly composed by high-K calc-alkalic associations showing metaluminous to peraluminous transition Nevertheless, mildly peralkaline compositions, acmite normative, were found iii the final magmatic episode of the Dos Lomas complex, represented by the leucogranites and associated dykes. Four main magmatic episodes with distinctive chemical trends that show an overall increasing acidity and alkalinity were recognized within each complex. Clear consanguinity relationships relate rocks of a given petrographic unit, but the trends of the different units of a complex do not follow a co-linear partern. Each complex appear then to be composed by several ‘progressively more acidic magmatic pulses emplazed at epizonal levels (La Esperanza complex) and subvolcanic to surface levels (Dos Lomas complex). Samples for Rb-Sr analyses from Prieto Granodiorite and Calvo Granite were obtained from single intrusions iii each case, taking into consideration their internal! variations in order to obtain appropiate Rb/ Sr ratios. Al! the samples were processed in the INGEIS laboratories, and Rb-Sr whole rock isochrons were obtained iii both units. The method used is the same as that reported by Parica (1983).   The isotopic determinations were performed in a Micromass 30 mass spectrometer commanded by a HP-9845A, and the measured 8lSr/86Sr ratios were normalized to a 86Srf88Sr ratio of 0.1 194 (Faure and Powell, 1974) included in the I-IP’s program (V.G. Micromass, 1979). The age, regression, MSWD, initial ratio and slope were determined according to Mclntyre et al. (1966). A good alignment of points and low value of MSWD were obtained in both isochron diagrams, allowing to consider them as real isochrons. The Prieto Granodiorite, which is the oldest unit of La Esperanza complex, yielded an age of 332 ± 6 Ma, with an initial 87Sr/86Sr ratio of 0.705 5. The latest unit of the Dos Lomas complex, the Calvo Granite, gave 317 ± 4 Ma, with an initial 87Sr/86Sr ratio of 0.7078. The separation of both units by a short period of time is consistent with previous results based on fielid and geochemical grounds suggesting that both complexes represent a single major magmatic event. The difference in the initial 87Sr/86Sr ratio require a more detailed explanation, however, it is probable that the ultimate source for the Calvo Granite would have been influenced by rocks with high Rb/Sr ratios. Similar Rb-Sr isochron ages and initial 87Sr/86Sr ratios are reported by Caminos and Parica (this volume) for plutonic units located 140 km east of La Esperanza.   Upper Palaeozoic magmatic activity in the (NM) had been recognized previously by Groeber (1929) and Stipanicic (1967). In La Esperanza area reported radiometric ages range from Permian to Triassic (Halpern, 1968; Stipanicic and Linares, 1969; Rosenman, 1979; Devicenzi, personal communication). Whole rock Rb-Sr analyses of widely spaced granitic plutons yielded Late Permian, while K-Ar gave Early Permian to Early Triassic ages.   The new radiometric data support the hypothesis that a major magmatic cycle developed during the Early Carboniferous and that it will be necessary to carry out more detailed research to determine the extension of the probable Permian cycles reported earlier. Taking into account that the Upper Paleozoic complexes show postorogenic features and that each magmatic sequence is emplaced at higher levels of the crust with decreasing age, it may be concluded that since Early Carboniferous a process of progressive uplifting and cratonization of the (NM) has taken place.