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Título: The pre-andean roots of the north patagonian andes: early paleozoic low pressure - high temperature metamorphism of the colohuincul complex, Argentina.
Autor(es): Serra Varela, Samanta
González, Pablo Diego
Giacosa, Raúl Eduardo
Fecha de publicación: 2016
Es parte de: Whitehorse 2016
Resumen: New mapping and petrological studies conducted on metamorphic rocks of the Colohuincul Complex near San Martin de los Andes (40°S - 71°W) result in the characterization of low pressure–high temperature metamorphism. The metamorphic rocks are predominantly paragneisses and migmatites with minor schists and amphibolites. The migmatites are mainly diatexites with biotite rich schlieren and schollen of relictic migmatized paragneisses and schists. Leucosomes are equigranular, medium grained and composed of Qtz + Plg + Bt+ K feldspar +Ms. Stromatites are also recognized with leucosomes parallel to S2 main foliation of paragneisses and schists. Peak metamorphic assemblage of Qtz + Plg + Bt + Crd + Sill (fibrolite) + K-feldspar ± melt in paragneisess and schists is contained on the S2 axial-plane foliation. A preliminary P-T pseudosection constructed with Perple_X in the NCKFMASH system allowed us to constrain the peak metamorphic conditions between 560° and 640°C, and below 0.36 GPa. Field relations indicate that migmatization has occurred within the paragneisses and schists, and was followed by two generations of intrusive rocks: harmonic syn-kinematic leucogranites and discordant post-kinematic tonalitic plutons which cut the metamorphic fabric. The leucogranites are parallel to S2 foliation and are most likely the result of melt accumulation derived from the anatexis of the local paragneisses and schists. U-Pb SHRIMP zircon crystallization ages of the post kinematic plutons are ca. 390 Ma. The high rheological contrast between this post-kinematic intrusives and their country rocks indicate that the metamorphic rocks were already deformed and metamorphosed at the time of the intrusion. Therefore the metamorphism occurred before 390Ma. We could identify at least two processes of melt generation. The first and the most important one, is the generation of melt produced by the prograde breakdown reaction of Ms + Qtz. The second one is the leucosome formation due to decompression. After these processes a re-hydration stage is suggested by postkinematic muscovite porphyroblasts with reaction rims of Ms + Qtz symplectites which overprints the previous metamorphic fabric. Contact metamorphism produced by the intrusion of postkinematic plutons cannot be completely ruled out as responsible of this porphyroblast. On a regional context and considering the stratigraphic relations, the HT-LP metamorphism of the Colohuincul Complex on the Northpatagonian Andes seems to be related to an Early Paleozoic subduction zone. In this geotectonic setting, the heat source could be related either to magma intrusions or to underplating of mafic magmas beneath the magmatic arc.
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Este documento es resultado del financiamiento otorgado por el Estado Nacional, por lo tanto queda sujeto al cumplimiento de la Ley N° 26.899

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