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Journal of Petrology Volume 42 Number 3 Pages 459-486 2001
© Oxford University Press 2001

Magmatic Evolution of the La Pacana Caldera System, Central Andes, Chile: Compositional Variation of Two Cogenetic, Large-Volume Felsic Ignimbrites

J. M. LINDSAY1,*, A. K. SCHMITT1, R. B. TRUMBULL1, S. L. DE SILVA2, W. SIEBEL1,{ddagger} and R. EMMERMANN1

1GEOFORSCHUNGSZENTRUM POTSDAM, TELEGRAFENBERG, 14473 POTSDAM, GERMANY
2INDIANA STATE UNIVERSITY, TERRE HAUTE, IN 47089, USA

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 km3. 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°C with melt water contents of 3·1–4·4 wt %. The pre-eruptive Toconao magma was cooler (730–750°C) and its melt more water 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 (87Sr/86Sri 0·7094–0·7131; 143Nd/144Nd 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.

KEY WORDS: Central Andes; crystal-rich dacite; eruption trigger; high-silica rhyolite; zoned magma chamber


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