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Migmatitic rocks are widely distributed in the northwest and centre of Argentina and most of them belong to the Sierras Pampeanas metamorphic basement. This basement was generated during the evolution of two superposed orogenic cycles (Aceñolaza and Toselli, 1976; Aceñolaza et al., 2000): the Pampean (Late Precambrian Lower Cambrian) and the Famatinian (Upper Cambrian Devonian). These oregenies have been interpreted as the product of subduction and continental collision of several terranes along the margin of Gondwana (e.g. Ramos et al, 1986; Ramos, 1988; Kraemer et al., 1995; Rapela et al, 1998, 2001) or as contiguous stages in the evolution of an intra-cratonic mobile belt along the margin of Gondwana (Lucassen et al., 2000), this last interpretation mainly in the northwest of Argentina. In the study area, localized between the ranges of Ancasti, Ambato y Aconquija in the Northwestern Sierras Pampeanas, three new main lithostratigraphic units were defined: El Portezuelo Metamorphic-igneous Complex (EPMIC), La Chilca Shear Zone (LChSZ) and Quebrada del Molle Metamorphic Complex (QMMC). The EPMIC includes the greatest part of the study area and it is composed mainly of migmatites. The migmatites are mostly metatexites of stromatitic type but also diatexites has been recognized, defining a regional migmatitic belt in N-S direction. The migmatites have lithologic continuity with gneisses, schists, calcsilicates and marbles, this assembly define the EPMIC. Concordant syntectonic tonalites complete this unit. Both rocks, of the lowest metamorphic grade (schists) and the highest metamorphic grade (migmatites) are considered from the same continental crust block but developed in different structural levels. The migmatites contain the mineral assemblage Qtz-Pl-Bt-(±Kfs)-(±Sil)-(±Grt)-(±Crd). This mineralogy is consistent with amphibolite to granulite metamorphic facies conditions. A regional S2 foliation is present in the migmatites with a NNW-SSE /N-S strike and ENE/E dip. To determine P-T conditions of the metamorphism (M2) that produced the widespread anatexis of metasedimentary rocks of the EPMIC four samples of migmatite leucosomas were selected. Temperatures and pressures were calculated using analysis from minerals (garnet, biotite, plagioclase and cordierite) in contact with each other. Multiphase equilibria method was applied using the software TWQ 2.32. For the thermal peak, we estimated P-T conditions of 760 ºC and 5-6 kbar on core analysis for the sample 7649 (diameter of garnet > 1 mm). Lower P-T conditions of 630-670 ºC and 4-5 kbar were estimated in samples 7295 and 7621 with garnets of diameters < 1 mm affected probably by diffusion process during the cooling. A highest pressure close to 9 kbar (T of 670 ºC) was estimated for the sample 7444. P-T conditions of retrograde metamorphism were calculated using mineral rim compositions. The results obtained were 650 ºC and 4 kbar for the sample 7649, 630 ºC and 5 kbar for the sample 7295, 615 ºC and 4 kbar for the sample 7621, and 640 ºC and 5 kbar for the sample 7444. Geothermobarometric results of core and rim analysis allowed deducing two types of post peak P-T paths: isobaric cooling paths with P/T gradients of 0.51 kbar/100 ºC and 0.91 kbar/100 ºC, and a strong isotermic decompression path with a P/T gradient of 14.88 kbar/100 ºC. The differents P-T paths observed could be interpreted as a consequence of differential movements of the crust, where a strong isotermic decompression path occur possibly related to the orogenic front. In addition to this, the isobaric cooling paths were related to internal areas of the orogen or the retro-arc associated to thermal sources. To constrain the timing of metamorphism, monazites from trhee different migmatite leucosomes from the El Portezuelo Metamorphic-igneous Complex (EPMIC) were dated by using single mineral U-Pb geochronology. Monazite analysis yield a mean 206Pb/238U age of 471.3 ± 1.3 Ma using three monazite fractions from the sample 7444-leu, a mean 206Pb/238U age of 476.7 ± 5.0 Ma from the sample 7550-leu (two monazite fractions), and a mean 206Pb/238U age of 470 ± 12 Ma from the sample 7649-leu (trhee monazite fractions). These results are interpreted as the timing of peak metamorphism. In conclusion, the migmatites from the basement of Northwestern Sierras Pampeanas evolved in amphibolite to granulite facies conditions with medium to high pressures (4 to 9 kbar) and high temperatures (630 to 760 ºC) as a consequence of a progresive regional metamorphism (M2) that was product of an convergent orogenic event during the Early Ordovician. Even though the origin of migmatites of the EPMIC respond to a convergent orogenic model and they could be relationed to additional thermal sources that allowed to reach the peak metamorphic temperatures. Differential movements of the crust determined high exhumation rates relationed to the orogenic front. However, there is not any clear evidence of this decompression in the migmatites of internal areas of the orogen. The metamorphism of the EPMIC was part of the Famatinian Orogeny developed on the Gondwana proto-pacific margin during the Early Ordovician. Because of this, it could be relationed towards the north with rocks of high metamorphic degree of the Sierra de Quilmes (≈ 470 Ma; Büttner et al., 2005), and towards the east with the metamorphic basement of Sierra de Ancasti (El Portezuelo Formation, 472 ± 26 Ma; Knüver, 1983). Furthermore it has continuity to the south with the rocks of high metamorphic degree of the Sierra de San Luis (Nogolí Metamorphic Complex, ≈ 478 Ma; Steenken et al., 2006) and towards the south-west with the migmatites of the Sierra de Valle Fértil (466 Ma; Rapela et al., 2001). This previous data and the new geochronological results presented here put in evidence the importance and magnitude of the Famatinian metamorphism in the evolution of metamorphic basement of Sierras Pampeanas during the Early Palaeozoic times.

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