The 2.08-Ma Cerro Galán Ignimbrite (CGI) represents a >630-km 3 dense rock equivalent (VEI 8) eruption from the long-lived Cerro Galán magma system (∼6 Ma). It is a crystal-rich (35-60%), pumice (<10% generally) and lithic-poor (<5% generally) rhyodacitic ignimbrite, lacking a preceding plinian fallout deposit.

The Pliocene‐Holocene Newer Volcanic Province in Victoria, southeast Australia, is a wide volcanic field site of the most recent volcanic activity of Australia. The intraplate signature and the localization on the northern side of the Southern Ocean have been used in the past to ascribe the volcanic activity either to the presence of a mantle hot spot or to thermal anomalies inherited from the seafloor spreading that separated Australia from Antarctica during the Gondwana breakup. The spacing and distribution of groups of eruption points suggests the presence of a 32 km deep mantle thermal anomaly. Alignments of eruption points indicate a strong tectonic control on magma emplacement mainly along NW‐SE Mesozoic‐Cenozoic structures and along N‐S Palaeozoic and E‐W Late Cretaceous structures. This volcano tectonic setting relates to the interference of the left‐lateral kinematics of the major N‐S trending faults associated with the Tasman Fracture Zone and the extensional structures of the Otway basin. This interpretation is also coherent with stress in situ data and focal mechanisms of earthquakes (SHmax oriented N150°). This interpretation enhances the role of the Tasman Fracture Zone, a major sinistral transform, on the reactivation of transtensional structures, and on triggering the magmatism of the Newer Volcanic Province.

From detailed fieldwork and biotite 40

Estimates of pyroclastic flow emplacement temperatures in the Cerro Galán ignimbrite and Toconquis Group ignimbrites were determined using thermal remanent magnetization of lithic clasts embedded within the deposits. These ignimbrites belong to the Cerro Galán volcanic system, one of the largest calderas in the world, in the Puna plateau, NW Argentina. Temperature estimates for the 2.08-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) from seven ignimbrite units forming the older Toconquis Group (5.60-4.51 Ma), mainly outcropping along a type section at Rio Las Pitas, Vega Real Grande. The paleomagnetic data obtained by progressive thermal demagnetization show that 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 (T b ) of the magnetic minerals (T b =580°C for magnetite;

Columnar jointing is thought to occur primarily in lavas and welded pyroclastic flow deposits. However, the non-welded Cerro Galán Ignimbrite at Paycuqui, Argentina, contains well-developed columnar joints that are instead due to high-temperature vapor-phase alteration of the deposit, where devitrification and vapor-phase crystallization have increased the density and cohesion of the upper half of the section. Thermal remanent magnetization analyses of entrained lithic clasts indicate high emplacement temperatures, above 630°C, but the lack of welding textures indicates temperatures below the glass transition temperature. In order to remain below the glass transition at 630°C, the minimum cooling rate prior to deposition was 3.0×10 −3 -8.5×10 −2°C /min (depending on the experimental data used for comparison). Alternatively, if the deposit was emplaced above the glass transition temperature, conductive cooling alone was insufficient to prevent welding. Crack patterns (average, 4.5 sides to each polygon) and column diameters (average, 75 cm) are consistent with relatively rapid cooling, where advective heat loss due to vapor fluxing increases cooling over simple conductive heat transfer. The presence of regularly spaced, complex radiating joint patterns is consistent with fumarolic gas rise, where volatiles originated in the valley-confined drainage system below. Joint spacing is a proxy for cooling rates and is controlled by depositional thickness/valley width. We suggest that the formation of joints in high-temperature, non-welded deposits is aided by the presence of underlying external water, where vapor transfer causes crystallization in pore spaces, densifies the deposit, and helps prevent welding.