A re-appraisal of the stratigraphy and volcanology of the Cerro Galan volcanic system, NW Argentina

From detailed fieldwork and biotite 40Ar/39Ar dating correlated with paleomagnetic analyses of lithic clasts, we present a revision of the stratigraphy, areal extent and volume estimates of ignimbrites in the Cerro Galan volcanic complex. We find evidence for nine distinct outflow ignimbrites, including two newly identified ignimbrites in the Toconquis Group (the Pitas and Vega Ignimbrites). Toconquis Group Ignimbrites (similar to 5.60-4.51 Ma biotite ages) have been discovered to the southwest and north of the caldera, increasing their spatial extents from previous estimates. Previously thought to be contemporaneous, we distinguish the Real Grande Ignimbrite (4.68 +/- 0.07 Ma biotite age) from the Cueva Negra Ignimbrite (3.77 +/- 0.08 Ma biotite age). The form and collapse processes of the Cerro Galan caldera are also reassessed. Based on re-interpretation of the margins of the caldera, we find evidence for a fault-bounded trapdoor collapse hinged along a regional N-S fault on the eastern side of the caldera and accommodated on a N-S fault on the western caldera margin. The collapsed area defines a roughly isosceles trapezoid shape elongated E-W and with maximum dimensions 27 Au 16 km. The Cerro Galan Ignimbrite (CGI; 2.08 +/- 0.02 Ma sanidine age) outflow sheet extends to 40 km in all directions from the inferred structural margins, with a maximum runout distance of similar to 80 km to the north of the caldera. New deposit volume estimates confirm an increase in eruptive volume through time, wherein the Toconquis Group Ignimbrites increase in volume from the similar to 10 km(3) Lower Merihuaca Ignimbrite to a maximum of similar to 390 km(3) (Dense Rock Equivalent; DRE) with the Real Grande Ignimbrite. The climactic CGI has a revised volume of similar to 630 km(3) (DRE), approximately two thirds of the commonly quoted value.