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Late Paleozoic to Triassic magmatism is an important event along the western margin of South America. Active margin and subsequent collisional events were envisaged for the Late Paleozoic tectonomagmatic evolution of northern Patagonia. Particularly, the Mamil Choique Granitoids (MCG) and related rocks have been interpreted as related to a postcollisional stage associated to the ca 330 Ma collision of the Deseado Massif (Pankhurst et al., 2006).The mainly peraluminous to weakly peraluminous and calc-alkaline MCG (López de Luchi and Cerredo,2008) crop out in an area of more than 350 km2 in the Sierra de Mamil Choique (41º 50.6?S 70º 09.0?W), to the north in the nearby Viuda de Gallo stall and Loma Guacha hill, and further south in the Sierra del Medio(40º 07.0?S 69º 47.0?W) as indicated in Fig. 1.In the western and northwestern of the Sierra de Mamil Choique (Fig. 1) these granitoids comprise two banded facies (1) the Huenchuquil Tonalite?Granodiorite (HTG; biotite ± hornblende ± muscovite ± epidote tonalite?granodiorite) and the Viuda de Gallo Granite (biotite ± muscovite granodiorite- granites), and (2)Cerro Mojón Granite (CMG; biotite?muscovite partially foliated monzogranites). The banded CMG exhibits not only magmatic features such as synmagmatic folding (folding axis N60ºE), but also an overprinted foliation. In the easternmost area of the Sierra crops out the Nahuelfil Granite (NG) a K-feldspar porphyritic biotite?muscovite monzogranite lacking any penetrative planar fabric but locally showing megacryst isorientation. Further descriptions can be found in Cerredo and López de Luchi (1998). A fourth minor unit, La Pintada Granite (LPG, sensu López de Luchi et al., 2013 this volume) crops out as small stocks as well as pegmatite stocks and dykes in the northernmost part of the Sierra, that make up the core of the Sierra south of the town of Mamil Choique (Fig. 1). LPG is composed of garnet lepidolite bearing leucomonzogranites and clearly intrudes the HTG. However, the pegmatite core does not intrude the CMG. The NO-SE trending Sierra del Medio is built up of banded biotite bearing- granodiorite-granite orthogneisesas well as leucocratic massive or locally foliated granites and muscovite bearing pegmatite dykes (Llambías et al., 1984). On the other hand, the rocks of the Laguna del Toro range in composition from tonalite to two-mica granite orthogneisses (Rapela et al., 1992). In this contribution eight new K-Ar muscovite and biotite cooling ages are provided for six critical localities whose ages were not previously constrained such as the Loma Guacha hill to northeast of the Quintuleu stall, and the southeasternmost tip of the Sierra del Medio. Methodology K-Ar mica dating were performed in the Geowissenschaftliches Zentrum of the University in Göttingen (Table 2). Selected samples from muscovite pegmatoids associated to leucogranites were crushed in a steel jaw crusher and sieved to isolate the 300?400 μm size fraction. After the fractions were carefully processed by magnetic separation, mica grains from each sample were handpicked under a binocular microscope to obtain homogeneous microcrystalline groundmass separates. Details of argon and potassium analyses for the laboratory in Göttingen are given in Wemmer (1991). Mica flakes were obtained from samples LG8 and 640 from muscovite pegmatoids associated to muscovite bearing leucogranite. Samples C7, MC2 and MC88 belong to muscovite- biotite foliated and banded granites from the CMG in the Sierra de Mamil Choique whereas the muscovite grains of sample MCH52 were isolated from a foliated ms- bearing leucogranite that intrudes the CF in the easternmost entrance of the Cañadón La Angostura (41º 44.4?S 70º 28.0?W). Table 1. Set of new cooling ages for the Sierra de Mamil Choique and Sierra del Medio areas. Rb-Sr isochrons for La Pintada Granite, several biotite bearing monzogranite dykes from the easternmost side (Figure 2) and Sierra del Medio Granites (not shown in Figure 2) were rebuilt using ISOPLOT 3.0 (Ludwig 2003) with data previously published in Dalla Salda et al. (1994) and Pankhurst et al. (2006) as well as unpublished data from the INGEIS repository. Figure 2. New Rb-Sr Isochrons for high Rb/ very low Sr La Pintada Granite (left) and several biotite monzogranite dykes that intrude Nahuelfil Granite (right) using data from Dalla Salda et al. 1994(samples RC), INGEIS repository and Pankhurst et al. 2006 (MAC-126). Discussion Available and new geochronological data for the main plutonic units of the Sierra de Mamil Choique, Sierra del Medio are surroundings is summarized in Figure 1. Interpreted from a conservative point of view, this set of ages allow recognizing at least two main episodes of cooling in the contruction of the Somuncura Batholith. The first episode could be stablished in the northern part of the Sierra where the Lower Permian crystallization (≈ 290-280 Ma at >750ºC, U-Pb SHRIMP for a banded granodiorite, Pankhurst et a., 2006) for the Huenchuquil amphibole- biotite tonalites/granodiorites are associated with Upper Permian cooling ages (≈ 255-250 Ma; at 250ºC) whereas the La Pintada garnet and lepidolite bearing granites and other types of acidic pegmatites (such as Sierra del Medio sample) exhibit Upper Permian cooling ages (≈ 260-250 Ma; at 350ºC). Even though HTG and La Pintada and associates are contemporary, FC geochemical or isotopical linkage between them is highly unlikely. The second episode is still restricted to the southeastern part of the Sierra de Mamil Choique and seem to be constrained only to the banded (and foliated) Cerro Mojón granitoids. Muscovite and biotite cooling ages (~350ºC; 250ºC respectively) for two different localities of this facies located nearby the Río Chico Lineament point towards Middle to Upper Triassic cooling. There is no constrain available to tight up the crystallization age of the Cerro Mojón Granitoids. However, a Middle to Upper Triassic cooling age WR Rb-Sr isochron (≈400ºC) was calculated for a bt- monzogranite that intrude Nahuelfil porphyritic granitoids, which may be indicating not only a maximum age for the host but also could be representative of the magmatic pulse that also had lead to the intrusion of Cerro Mojon Granitoids. These Triassic cooling ages were not only detected for the south external part of the Sierra Mamil Choique but also in a foliated Ms bearing leucogranite in Río Chico and a banded biotite tonalite (tentatively addresed to HTG facies). However it is not clear if this 230±5 Ma (bio)-tonalite is not an yet intrusive in HTG host. Hence, more data is necessary in order to understand if this set ?delayed? Triassic ages could be interpreted as a result of the beggining of the exhumation process of the NW-SE Sierra de Mamil Choique- Sierra del Medio system through the well known Río Chico ? Gastre Fault and associated lineaments through shear zones and other magmatic artifacts, or they truly represent an steady- cooling an Upper Permian unit. References Cerredo, M.E., López de Luchi, M.G., 1998. Mamil Choique Granitoids, southwestern North Patagonian Massif, Argentina: magmatism andmetamorphism associated with a polyphasic evolution. Journal of South American Earth Sciences 11 (5): 499?515. Dalla Salda, L., Varela, R., Cingolani, C. y Aragón, E., 1994. The Río Chico Paleozoic Cristaline Complex and the evolution of Northen Patagonia. Journal of South American Earth Sciences, 7 (3): 1-10. Llambías, E., Llano, J., Rossa, N., Castro, C. y Puigdomenech, H., 1984. Petrografía de la Formación Mamil Choique en la sierra del Medio -departamento Cushamen-provincia de Chubut. 9 Congreso Geológico Argentino, Actas 2: 554-567, Argentina. López de Luchi, M.G., Cerredo, M.E., Wemmer, K., Pawling, S., 2005. Cooling and Nd model ages of the Devonian to Late Paleozoic units of the SW corner of the North Patagonian Massif. 16th Congreso Geológico Argentino, La Plata, Actas I, pp. 273?278. Ludwig, K.R., 2003. Isoplot 3.0 A Geochronological Toolkit for Microsoft Excel. Berkeley Geochronology center Special Publication No. 4., 74 p. Pankhurst, R.J., Rapela, C.W. Fanning, C.M., Márquez, M., 2006.Gondwanide continental collision and the origin of Patagonia. Earth-Science Reviews 76: 235-257. Rapela, C.W., Pankhurst, R.J., Harrison, S.M., 1992. Triassic ??Gondwana??granites of the Gastre district, North Patagonian Massif.Transactions of the Royal Society of Edinburgh: Earth Sciences 83: 291?304. Varela, R., Basei, M.A.S., Cingolani, C.A., Siga, O., Passarelli, C., 2005. El basamento cristalino de los Andes norptago´nicos en Argentina:Geocronología e interpretación tectónica. Revista Geológica de Chile 32:167?187. Wemmer, K., 1991. K/Ar-Altersdatierungsmöglichkeiten für retrograde Deformationsprozesse im spröden und duktilen Bereich-Beispiele aus der KTB-Vorbohrung (Oberpfalz) und dem Bereich der Insubrischen Linie (N-Italien). Göttinger Arbeiten zur Geologie und Paläontologie 51, 1-61.