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

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

San Miguel de Tucuman

Simposio; 1er Simposio sobre Petrología Ígnea y Metalogénesis Asociada, Instituto Superior de Correlación Geológica; 2011

Instituto Superior de Correlacion Geologica

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 North Patagonian Massif (NPM), Argentina. LEPC comprises: a) foliated Giménez Granite b)Prieto Granodiorite U-Pb SHRIMP dated in 273±2 Ma (Pankhurst et al., 2006); and c) Donosa Granite, dated using the Rb-Sr method in ~259±16 Ma by Pankhurst et al. (1992). The younger plutono-volcanic unit, the 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±2Ma, Pankhurst et al., 2006). LEPC intrudes the very low grade metasedimentary rocks of the Early Cambrian (?) Colo Niyeu Formation and is unconformably overlain by 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 Valcheta area. Previous geological and geochemical studies (Rapela and Llambías, 1985; Rapela and Caminos, 1987) indicate that LEPC rocks are calk-alkaline to alkaline and late to post-tectonic. Other authors (Ramos 1984) interpreted LEPC as the magmatic arc related to a Carboniferous to Permian collision of the Patagonia terrane with the Gondwana margin. Previous works have considered LEPC as undeformed since the majority of the 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 fabric is mostly absent in the Prieto Granodiorite and Donosa Granite. This study reports results on the petrography, magnetic susceptibility, mesostructural features, geochemistry and cooling ages of the LEPC. The Giménez Granite is characterized by its equigranular medium to fine light grey to pink. It is mostly foliated with low-dipping foliation planes. It has a paramagnetic to slightly ferromagnetic character depending on the grain size (K 10-700E-5 SI). Prieto Granodiorite (ferromagnetic, K 1000-4000E-5 SI) is the most extended facies; and varies from equigranular to porphyric by increasing the amount and size of the larger crystals of plagioclase and subordinate K-feldspar towards the border of the pluton. Donosa Granite (slightly ferromagnetic, K 500-1500E-5 SI) is a pink megacrystic granite. It exhibits oversized K-feldspar grains (12 x 3cm). W-E to NNW-SSE decimeter- length internal shear zones have been detected in the Donosa Granite nearby the contacts to the Prieto Granodiorite.