A RE-APPRAISAL OF THE STRATIGRAPHY AND VOLCANOLOGY OF THE CERRO GALÀN VOLCANIC SYSTEM, NW ARGENTINA

From detailed fieldwork and high precision ⁴⁰Ar/³⁹Ar biotite dating correlated with paleo-magnetic analyses of lithic clasts, we present a revision of the stratigraphy, areal extent and volume estimates of ignimbrites in the Cerro Galán volcanic complex. We find evidence for nine distinct outflow ignimbrites, including two newly identified ignimbrite units in the Toconquis Group (the Pitas and Vega ignimbrites). We separate the Real Grande (4.68 Ma) and Cueva Negra (3.77 Ma) ignimbrite units. Toconquis Group ignimbrites (5.60 – 4.51 Ma) have been discovered to the southwest and north of the caldera, increasing their spatial extents from previous estimates. The form and collapse of the Cerro Galán caldera is also revised. Based on re-interpretation of the margins of the caldera we find evidence for a strongly fault-bounded piecemeal trapdoor collapse that was hinged along a regional N-S fault on the eastern side of the caldera and accommodated on a N-S fault on the western margin. The collapsed area defines a roughly isosceles trapezoid shape with maximum dimensions 26 x 18 km, elongated E-W. The Cerro Galán Ignimbrite (CGI; 2.56 Ma) outflow sheet extends to 40 km in all directions from the inferred structural margins, with a maximum runout distance of 79 km to the north of the caldera. We show the Galán system erupted increasing deposit volumes over time, which we attribute to the thermal and mechanical maturation of the crust. The Toconquis Group ignimbrites increase in volume from ~7 km³ (Blanco Ignimbrite; ~6 Ma) to a maximum of 384 km³ (DRE) with the Real Grande Ignimbrite. The climactic CGI has a revised volume of 547 km³ (DRE), just over half of the commonly quoted value.