High temperature emplacement of the caldera forming Campo de la Piedra Pómez ignimbrite (Puna plateau, NW Argentina) determined by paleomagnetic analyses.

WALTER BAEZ; MARCELO ARNOSIO; GUIDO GIORDANO; JOSÉ VIRAMONTE; MASSIMILIANO PORRECA; AGOSTINA CHIODI

Colima

Congreso; 7th Cities on Volcanoes.; 2012

IAVCEI

The Campo de la Piedra Pómez ignimbrite (CPPI) is an intermediate volume (~20km3; VEI6) collapse caldera ignimbrite unit, erupted ~72ka from the high-silica nested caldera Cerro Blanco Volcanic Complex (CBVC), located in the Southern Central Andes. In order to evaluate the emplacement temperature of the CPPI, we performed progressive thermal demagnetization (PDT) analysis on the lithic clasts incorporated into the deposits at different distances from the inferred caldera rim and different stratigraphic heights. Thermal Remanent Magnetization (TRM) data, show that most of the lithics have stable paleomagnetic behavior and are completely demagnetized within the temperature range between 580ºC and 630ºC. The TRM data demonstrate the clasts to have generally one single magnetic component, oriented close to the expected geomagnetic field in this locality at ~72Ka. Most of the clasts acquired a new magnetization oriented in the same direction at the moment of deposition of the ignimbrite. This suggests that they were heated up close to, or above, the Curie temperature of the magnetic minerals (T>=580ºC for magnetite, T>=630ºC for hematite). We conclude that the CPPI was emplaced at temperatures equal to, or higher than, 630ºC. High emplacement temperatures have been interpreted elsewhere that indicate minimal heat loss in highly concentrated flows originating from boil-over eruption columns, with little, or no interaction with the environment (e.g. Galan ignimbrite). The CPPI shares a similar Galan Ignimbrite thermal structure, but a much more variable facies, architecture and sedimentological features. This suggests that the deposit originated from the progressive aggradation of pyroclastic material from a density stratified flow, with a basal high concentration zone and an upper more diluted and turbulent zone. In order to better understand the origin of the thermal structure of the CPPI and of ignimbrites in general, we shall explore the many factors influencing the thermal dissipation within flows in relation to the fluid-dynamics of the pyroclastic flows.