Three magmatic events in the Los Menucos area of the Central- North Patagonian Massif, Argentina?

REIMOLD UWE; HAUSER NATALIA; FALCO JUAN IGNACIO

Cochabamba

Simposio; 11th South American Symposium on Isotope Geology; 2018

The Los Menucos volcanic complex (LMVC) was developed in the centralNorth Patagonian Massif of Argentina. It was assigned to the Middle to UpperTriassic on the basis of K/Ar dating, or alternatively to the Upper Permian toLower Triassic based on recent U-Pb on zircon dating. Outcrops in the basin displayignimbrites of dacitic to rhyolitic composition, with minor interbedded sedimentarydeposits occasionally showing fossil imprints assigned to the Dicroidium flora. The best outcrops arelocated close to Los Menucos village in Rio Negro province. Five detailedstratigraphic profiles allowed us to better define the main units of thecomplex and to select potential samples for U-Pb on zircon dating by LA-ICPMSat the Geochronology Laboratory, University of Brasília. Zircon crystals are mainlyprismatic; between 100-350 µm, Th/U values are lower than 0.8, typical from amagmatic origin. The results from the ignimbrites from the LMVC indicate thatthe age of this complex is between 252±2 Ma, a 206Pb/238Uweighted average age for ten zircon crystals, and 248±1 Ma, a Concordia age forsix zircon crystals. Two additional samples, one an ignimbrite apparently lyingunconformably below the LMVC, and a second sample from the caldera structureoccupying the central-south part of the basin, yielded Concordia ages of 256±1 and211±2 Ma, respectively. These results indicate that apparently three magmaticevents can be identified during Permian-Triassic times in the central NorthPatagonian Massif.