Petrography and cooling ages of the magmatic units of the Late Permian- Triassic La Esperanza Eruptive Complexes, North Patagonian Massif, Argentina.

MARTÍNEZ DOPICO, C.I.; RAPALINI, A.E.; LÓPEZ DE LUCHI, M.G.

San Miguel de Tucumán

Simposio; Primer Simposio de Petrología Ígnea y Metalogénesis asociada; 2011

Universidad Nacional de Tucumán

The La Esperanza Plutonic Complex (LEPC, Llambías and Rapela, 1984) is located 60 km N-NW of Los Menucos in the Río Negro Province, north of the North Patagonian Massif (NPM), Argentina. The LEPC comprises: a) the foliated Giménez Granite; b) the 273±2 Ma (U-Pb SHRIMP, Pankhurst et al., 2006) Prieto Granodiorite, and c) the 259±16 Ma (Rb-Sr, Pankhurst et al. 1992) Donosa Granite. The younger Dos Lomas Complex (DLC) is interpreted to be the eruptive counterpart of the LEPC and is mainly composed of dacitic and rhyolitic domes and ignimbrites and high-silica two mica leucogranites (250±2 Ma, Pankhurst et al., 2006). The LEPC intrudes the very low grade metasedimentary rocks of the Early Cambrian (?) Colo Niyeu Formation and is unconformably overlain by the DLC. Geochemical and limited geochronological evidence might indicate a possible correlation between LEPC-DLC and the Navarrete plutonic and Treneta volcanic complexes further east in the Valcheta area. Previous geological and geochemical studies (Rapela and Llambías, 1985) indicate that the LEPC rocks are calk-alkaline to alkaline and late to post-tectonic. Other authors (e.g., Ramos, 1984) interpreted the LEPC as a magmatic arc related to a Carboniferous to Permian collision of the Patagonia terrane with the Gondwana margin This study reports results on the petrography, magnetic susceptibility, mesostructural features, geochemistry and cooling ages of the LEPC. Previous works have considered the LEPC as undeformed since most rocks have a very weak internal fabric. However, our field mapping and mesostructural observations indicate that the Giménez Granite is variably foliated which lead us to propose a change in the plutonic stratigraphy because penetrative planar fabrics are mostly absent in the Prieto Granodiorite and in the Donosa Granite. The Giménez Granite is a light grey to pink equigranular, medium to fine grained mostly foliated granitoid. with low-dipping foliation planes. It has a paramagnetic to slightly ferromagnetic character depending on the grain size (K 10-700E-5 SI). The Prieto Granodiorite (ferromagnetic, K 1000-4000E-5 SI) is the most extended facies; it varies from equigranular to porphyritic by increasing the amount and size of the larger crystals of plagioclase and subordinate K-feldspar towards the border of the pluton. The Donosa Granite (slightly ferromagnetic, K 500-1500E-5 SI) is a pink megacrystic granite, with oversized K-feldspar grains (12 x 3 cm). W-E to NNW-SSE decimeter-length internal shear zones have been detected in the Donosa Granite close to the contacts with the Prieto Granodiorite. Llambías, E.J. and Rapela, C.W. 1984. Geología de los complejos eruptivos de la Esperanza, provincia de Río Negro. Revista de la Asociación Geológica Argentina 39: 220-243. Pankhurst, R.J., Rapela, C.W., Caminos R., Llambías E.J. and Parica, C. 1992. A revised age for the granites of the central Somuncura Batholith, North Patagonian Massif. Journal of South American Earth Sciences 5: 321-325. Pankhurst, R.J., Rapela, C.W., Fanning, C.M. and Márquez, M. 2006. Gondwanide continental collision and the origin of Patagonia. Earth-Science Reviews 76: 235-257. Ramos, V.A., 1984. Patagonia: ¿Un continente paleozoico a la deriva? 9° Congreso Geológico Argentino, San Carlos de Bariloche, Actas 2, 311-325. Rapela, C. and Llambías, E.J. Evolución magmática y relaciones regionales de los Complejos Eruptivos de La Esperanza. Provincia de Río Negro. Revista de la Asociación Geológica Argentina 40: 4-25.