Emplacement temperature and flow direction analysis of large dimension calderas ignimbrites: the Cerro Galan and Toconquis group ignimbrites (Puna plateau, NW Argentina)

Abstract

Estimates of pyroclastic flow emplacement temperatures in the Cerro Galàn ignimbrite and Toconquis Group ignimbrites and flow directions were determined using thermal remanent magnetization (TRM) of lithic clasts embedded within the deposits and the anisotropy of magnetic susceptibility, respectively. These ignimbrites belong to the Cerro Galàn volcanic system, one of the largest calderas in the world, in the Puna plateau, NW Argentina. Temperature estimates for the 2.16 ± 0.16 Ma Cerro Galán ignimbrite are retrieved from 40 sites in 14 localities (176 measured clasts), distributed at different distances from the caldera and different stratigraphic heights. Additionally, temperature estimates were obtained from 27 sample sites (125 measured clasts) in seven ignimbrite units forming the older Toconquis Group (5.60±0.20 - 4.51±0.11 Ma), mainly outcropping along a type-section at Rio Las Pitas, Vega Real Grande. Flow directions are defined in 35 sites (403 measures specimens), distributed again at different distances and azimuth from the caldera and at different stratigraphic heights along the same section. The flow direction assessment was carried out on the Cerro Galàn ignimbrite and not on the older units, as the outcrops of the Toconquis Group are limited in extension mainly to the west of the caldera. The paleomagnetic data obtained by progressive thermal demagnetization (PTD), show that most of the clasts of the Cerro Galán ignimbrite have one single magnetic component, oriented close to the expected geomagnetic field at the time of emplacement. Results show therefore that most of the clasts acquired a new magnetization oriented parallel to the magnetic field at the moment of the ignimbrite deposition, suggesting that the clasts were heated up to or above the highest blocking temperature (Tb) of the magnetic minerals (Tb=580°C for magnetite; Tb=600-630°C for titanohematite). We obtained similar emplacement temperature estimations for five out of six volcanic units belonging to the Toconquis Group, with the exception of one unit (Lower Merihuaca), where we found two distinct magnetic components. The estimation of emplacement temperatures in this latter case is constrained between 580°-610°C. The study of the AMS was performed in order to analyse the ignimbrite fabric and to evaluate the flow structure and emplacement mechanism, relating the magnetic fabric to the paleotopography. Flow directions are defined in 35 sites (403 measures specimens), distributed at different distances and azimuth from the caldera and at different stratigraphic heights along the same section. The comparison of magnetic fabric with mineralogical fabric is also examined to assess the reliability of the AMS as flow direction indicator.

AMS results show a strong uniformity throughout the ignimbrite, with the exception of sites where the topographic control on the emplacement mechanism is higher. Flow directions results show a radial pattern around the caldera, in proximal sites, while in distal sites the directions are deflected by the paleotopography. The strong control of the paleotopography revealed in this study, together with field evidences of low level of turbulence and high emplacement temperatures estimation found, indicate that the flow was highly concentrated throughout the flow path. We conclude that the Cerro Galán ignimbrite and Toconquis Group ignimbrites were emplaced at temperatures equal to or higher than 620°C and that in distal sites the flow, besides having the capacity of travel up to 80 Km, follows passively the paleotopography. The homogeneity of high temperatures from proximal to distal facies, and the behaviour in presence of topographic obstacles in distal sites, provide constraints for an emplacement model for the Cerro Galán ignimbrite, marked by a relatively low eruption column, low levels of turbulence during deposition, air entrainment, surface-water interaction, and a high level of topographic confinement, all ensuring minimal heat loss and high concentration flow.