Magmatic evolution of the La Pacana caldera system, Central Andes, Chile: Compositional variation of two cogenetic, large-volume felsic ignimbrites

La Pacana is one of the largest known calderas on Earth, and is the source of at least two major ignimbrite eruptions with a combined volume of some 2700 km(3). These ignimbrites have strongly contrasting compositions, raising the question of whether they are genetically related. The Toconao ignimbrite is crystal poor; and contains rhyolitic (76-77 wt % SiO2) tube pumices. The overlying Atana ignimbrite is a homogeneous tuff whose pumice is dacitic (66-70 wt % SiO2), dense (40-60% vesicularity) and crystal rich (30-40% crystals). Phase equilibria indicate that the Atana magma equilibrated at temperatures of 770-790 degreesC with melt mater contents of 3.1-4.4 wt %. The pre-eruptive Toconao magma wets cooler (730-750 degreesC) and its melt more mater rich (6.3-6.8 wt % H2O). A pressure of 200 MPa is inferred from mineral barometry for the Atana magma chamber Isotope compositions are variable but overlapping for both units (Sr-87/Sr-86(i) 0.7094-0.7131; Nd-143/Nd-144 0.51222-0.51230) and are consistent with a dominantly crustal origin. Glass analyses from Atana pumices are similar in composition to those in Toconao tube pumices, demonstrating that the Toconao magma could represent a differentiated melt of the Atana magma. Fractional crystallization modelling suggests that the Toconao magma can be Produced by 30% crystallization of the observed Atana mineral phases. Toconao melt characteristics and intensive parameters are consistent with a volatile oversaturation-driven eruption. However the low H2O content, high viscosity and high crystal content of the Atana magma imply an external eruption trigger.