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The Musters stock which is located 20 km west of Valcheta town is a broadly elongated but curved 30 km2 unit with a major NNE trending branch (Figure 1a). This stock was considered as part of the Naverrete Plutonic Complex (Caminos, 2001; López de Luchi et al. 2008b) based on lithological affinities. A well developed contact aureole is recognized on the host Ms-Chl metarenites and meta- volcaniclastic facies of the Campo Martín Formation (Martínez Dopico, 2013). This strip of the Campo Martin Fm. is separated by two parallel reactivated shear zones associated with retrogression of the original mineral paragenesis from the higher grade orthogneises, gneisses and amphibolites that hosted the Ordovician Ms-leucogranites (López de Luchi et al., 2008a). The stock is made up by medium to fine grained dark grey Qz-monzodiorite/monzogabbro with 45-50% plagioclase, 10-15% K-feldspar, 15-20% quartz, 10-20 % biotite and up to 20% amphibole. Accessory minerals are magnetite and scarce acicular apatite. Biotite medium grained pink granodiorite dykes intrude the stock. Plagioclase forms subhedral to locally euhedral zoned crystal of variable size with rectangular cores partially affected by patchy zoning or replaced by white mica-epidote-clay aggregates. A well developed oscillatory zoning in a general normal trend is observed in some cases associated with patchy zoning in some bigger crystals. Plagioclase composition is highly variable from bytownite-labradorite but borders in all the measured cases are of sodic to normal andesine and contains acicular apatite. Isolated smaller crystals are also variable in composition for An39 to An57. Amphibole is dominantly magnesio-horblende. It contains magnetite and plagioclase (An45 to An69) inclusions (Figure 1b). Relict cores of clinopyroxene are mostly replaced by actinolitic horblende. Biotite composition is uniform with Mg# between 0.58-0.60 and AlIV between 2.38-2.47 a.p.f.u. Crystals are oversized, cribated and exhibit intense chlorite alteration or appear as skeletal crystals in amphibole. K-feldspar which is interstitial and mostly ameboidal with plagioclase and biotite inclusions appears incipiently twinned. Quartz is heterogeneously distributed in interstitial aggregates with locally chessboard pattern and grain boundary migration textures. Mafic microgranular enclaves of monzogabbric to monzodioritic composition are present. They are roughly spherical and normally do not exceed 30 cm, boundaries are sharp. Amphibole clots are characteristic together with amphibole or brown biotite megacrysts. Microstructural evidence of magma mixing is indicated by the zoning of plagioclase, amphibole clots in the enclaves, acicular apatite crystals and resorbed biotite inclusions in amphibole. Core composition in plagioclase varies from An86 to An56. Rims associated with the more basic cores are slightly more calcic, i.e up to An 92 whereas labradorite cores show calcic andesine (An 45) rims but slightly more calcic borders (An 51). Amphibole clots are made up essentially by actinolitic amphibole with relictic clinopyroxene and magnetite cores surrounded by green amphibole and brown biotite.Some clots are roughly rectangular and resemble a clinopyroxene crystal. Hybridization of a granodiorite magma and monzogabbro magmas and an exchange of partially crystallized phase (plagioclase and probably some biotite) can account for the mineralogical and textural features. Calcic cores in the plagioclase of the quartz monzogabbro/granodiorite may represent crystals entrapped from the monzogabbro magmas. The influx mafic magmas could also be responsible for the resorption of biotite which is present as inclusions in amphibole. Some of these amphibole plus skeletal biotite are also observed in the monzogabbro enclaves. Acicular apatite which appears mostly in the sodic andesine rims of plagioclase may result from the selective loss of the more mobile components to the granitic melt and the relative concentration of phosphorus in the mafic phase (Vernon 1990). No quartz ocelli are observed which probably could indicate that the interaction between the two magmas took place at the early stages of crystallization of the felsic end member. Dykes of biotite granodiorite could represent the felsic component of the hybridization since they exhibit a biotite similar that of the quartz monzogabbro together with some highly zoned plagioclase laths that coexist with andesine to oligoclase normally zoned crystals. Two chemical data of the quartz monzogabbro /monzodiorite are available. Based on the TAS classification of Middlemost (1985) they are quartz diorites (SiO2 59.29-59.60%) which belong to a high-K calc-alkaline series with ASI between 0.95-1.01. The samples display similar chondrite-normalized REE patterns that are characterized by LREE enrichment with (La/Sm)N=2.37- 3.40 and weakly fractionated HREE with (Tb/Yb)N=1.40-1.50, suggesting garnet-free sources. Moderate but very consistent negative Eu anomalies (Eu/Eu*=0.66-0.71) were calculated (Figure 1C). In primitive-mantle normalized trace element patterns light-REE enrichment, positive Pb anomaly and the Nb-Ti troughs are observed which are typical of calc-alkaline magmatism in active continental margins. References Caminos, R., 2001. Hoja Geológica N 4166-I Valcheta, provincia de Río Negro, Boletín 310, Servicio Geológico Minero Argentino, 78 pp. Buenos Aires. López de Luchi, M.G., Wemmer, K., Rapalini, A.E., 2008a. The cooling history of the North Patagonian Massif: first results for the granitoids of the Valcheta area, Río Negro, Argentina. In: Linares, E., Cabaleri, N.G., Do Campo, M.D., Ducós, E.I., Panarello, H.O. (eds.). Book of Abstracts, VI South American Symposium on Isotope Geology, San Carlos de Bariloche, Abstracts, 33. López de Luchi, M.G., Rapalini, A.E., Tomezzoli, R.N., 2008b. Magnetic Fabric and Microstructures of Late Paleozoic granitoids from the North Patagonian Massif: Evidence of a collision between Patagonia and Gondwana?. Tectonophysics 494:118-137. Martínez Dopico, C.I., 2013. Geología, petrogénesis y condiciones de emplazamiento de los granitoides permo-triásicos del área de La Esperanza, Macizo Norpatagónico: Inferencias sobre la construcción del borde SO del Gondwana. Tesis Doctoral (inédito), Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 520p. Middlemost, E. A. K., 1985. Magmas and Magmatic Rocks. London: Longman Sun, S.-S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A.D., Norry, M.J. Eds. , Magmatism in Ocean Basins. Geol. Soc. Spec. Publ., London, pp. 313?345. Vernon, R.H., 1990. Crystallization and hybridism in microgranitoid enclave magmas: Microstructural evidence. Journal of Geophysical Research, 95: 17849-17859.