New U-Pb data for Sierra de Umango, Andean Foreland at 29º S, and geodynamic implications.

VARELA, RICARDO; BASEI, MIGUEL; SATO, ANA MARÍA; SIGA, JR., OSWALDO; GONZÁLEZ, PABLO D.; CAMPOS NETO, MARIO; CINGOLANI, CARLOS

San Carlos de Bariloche

Simposio; 6th South American Symposium on Isotope Geology.; 2008

INGEIS-CIG

Recent field work and six new U-Pb zircon ID-TIMS data from several geological units of the Umango Range, allow us to confirm previous tectonic interpretations and present new ones. Juchi Orthogneiss is a well established unit cropping out in the south of the Umango Range, and previous and three new U-Pb zircon data for tonalitic and granitic gneiss and granitic mylonite constrain the magmatic crystallization age of the igneous protolith in the 1.100-1.200 Ma interval. Rb-Sr whole rock age of ca. 1.000 Ma is interpreted as a result of a metamorphic imprint. In addition, Early Mesoproterozoic Sm-Nd TDM model ages and positive εNd(t) values, together with low 87Sr/86Sr inicial ratios point out the character of juvenile Mesoproterozoic crust. Tambillito Metamorphite is a newly proposed unit, cropping out in the Tambillito range, east sector of the Umango Range. It is characterized by phyllites, schists and amphibolites, with absence of marbles. The U-Pb zircon age of ca. 1.100 Ma from an amphibolite gives it the character of Mesoproterozoic metasedimentary sequence. The original stratigraphic relation with Juchi Orthogneiss is unknown because it constitutes a tectonic slice of a complex imbricate system in the east sector of the Umango range. Tambillo Metamorphite is a widespread sequence of schists, marbles and metabasites cropping out in the central and west sectors of Umango range. The new U-Pb zircon age of ca. 480 Ma interpreted as magmatic, is from an amphibolite lens related with eclogite and high-pressure granulite facies assemblages exposed in the Cacho range. Together with previous ca. 470 Ma Rb-Sr and U-Pb ages for synorogenic El Peñón Granite and 450 Ma U-Pb age in monazite of metasedimentites, they are related to Famatinian Orogeny. El Cordobés Metabasite is another proposed unit cropping out in a belt in the central zone of the Umango range. Gabbros and amphibolites are emplaced in marbles and schists of the Tambillo Metamorphite. The new U-Pb Zr age of ca. 450 Ma for the metagabbro is a little older than the previous Sm-Nd isochron of 400 Ma age (whole rock, feldspar, amphibole). Together with the swarm pegmatites well exposed in the cross section of Umango river, we consider them as related to Chanic tectonics. Finally the Los Guandacolinos and Cerro Veladero granites are discordant stocks of Carboniferous age related with the Gondwanian Orogeny, which is conspicuous in Frontal Cordillera to the west. Juchi Orthogneiss and Tambillito Metamorphite are Grenvillian-age crustal fragments, allochtonous or parauthoctonous. The other units, except Los Guandacolinos and Cerro Veladero Granites, are related to the collisional history of Famatinian Orogeny.