New U-Pb zircon ages of volcanic deposit from the Angastaco Formation (middle to upper Miocene) in the Tonco and Amblayo valleys, Salta province, NW Argentina

GALLI, C.; COIRA, B.; MATTEINI, M.

Medellín, Colombia

Simposio; VIII SSAGI; 2012

Servicio Geológico Colombiano

In the last decades Miocene continental sediments from intermountain basins in the NW Argentina have been extensively studied by several authors to understand the tectonic evolution of the Eastern Cordillera in the Central Andes. Cenozoic deposits of Payogastilla Group (Middle Eocene ? Late Miocene) in the localities of el Tonco and Amblayo valleys, Salta province, were studied The strata comprise of clastic deposits, defined, from base to top, by Los Colorados and Angastaco formations. Mineralogic and petrographic analyses of the constituent facies of Los Colorados and Angastaco formations show that the main sandstone facies component is characterized by quartz grains; potassium feldspar, altered plagioclase, perthite and abundant mica. The compacted and matrix supported conglomerate facies exhibit clasts mostly of pegmatite, granite, quartz, slate-clay, gneiss and migmatites. These data together with the results of major and trace elements suggested a similar affiliation to the basement rocks of the eastern edge of the Puna (Eruptivo Oire Complex) and Sierras Pampeanas. These interpretations are in agreement with paleocurrent data suggesting primary sediment contribution from the west of the basin throughout the middle Eocene to late Miocene.   Two distal fall deposits from two different localities (El Tonco and Amblayo valleys) were collected for geochronological studies. The samples represent volcanic ash beds at stratigraphic levels located respectively 700m and 550m above the bottom of the Angastaco Fm. In situ U-Pb analyses on zircons from the two samples, using a LA-MC-ICP-MS (Neptune) at the Geochronology Laboratory of Brasilia University, provided two ages of 13.2 ±.0.1 (El Tonco valley sample) and 14.0 ± 0.3 Ma (Amblayo valley sample). These two U-Pb ages for the Angastaco Fm. are consistent with other available data from literature and provided new temporal constraints for the sedimentation in intermountain Miocene basins in the Central Andes of NW Argentina.