SUCCESSIVE PLEISTOCENE IGNIMBRITES: FORERUNNERS TO THE ACTIVE RHYOLITIC MAGMA SYSTEM BENEATH LAGUNA DEL MAULE VOLCANIC FIELD, SOUTHERN ANDES, CHILE?

The Andean Southern Volcanic Zone (SVZ), Chile hosts large silicic calderas in its rear-arc region including the Diamante and Calabozos calderas. East of the Tatara-San Pedro frontal arc volcano, the Laguna del Maule (LdM) volcanic field includes the welded Laguna Sin Puerto pyroxene-dacite ignimbrite and non-welded biotite-rhyodacite ignimbrite of Cajon de Bobadilla. Both units reflect the LdM volcanic field’s largest eruptions prior to post-glacial 19 to <2 ka rhyolite eruptions. An 80 km² caldera produced the >500 m thick, 45 km² Bobadilla ignimbrite that likely obliterated the Laguna Sin Puerto vent that produced 300 m thick ignimbrite exposures over 15 km (Hildreth et al. 2010). Understanding the Pleistocene ignimbrites’ development is key to determining the crustal state at the onset of the current, shallow, silicic magma system presumably producing post-glacial rhyolite flows (Singer et al., this meeting). From single pumice blocks or fiamme, biotite yields an ⁴⁰Ar/³⁹Ar age of 876 ± 70 ka for the Bobadilla ignimbrite, whereas plagioclase yields an age of 1830 ± 140 ka for the Laguna Sin Puerto ignimbrite. Previous ⁴⁰Ar/³⁹Ar dating of plagioclase yielded ages of 950 ± 7 ka and 1484 ± 150 ka respectively. Bobadilla ignimbrite pumice blocks and Laguna Sin Puerto ignimbrite fiamme range from 66 to 72% and 64 to 70% SiO₂ respectively. It is unclear whether variability in either ignimbrite reflects a chemically-zoned magma. ⁸⁷Sr/⁸⁶Sr ratios of 0.7043 for both ignimbrites are higher than any post-glacial rhyolite (Hildreth et al., 2010), indicating a larger continental crust contribution than the current magmatic system. Several approaches will be used to constrain the magmatic development of both ignimbrites including: core to rim An and trace element content analyses of zoned plagioclase to determine differentiation rates, magma mixing processes, and phase equilibria, two-oxide and pyroxene thermometry to determine pre-eruptive T and fO₂ using magnetite-ilmenite pair and ortho-clinopyroxene compositions to derive FeO/Fe₂O₃ and Ca-Mg-Fe contents respectively, and Al-in-hornblende barometry to constrain pre-eruptive pressures. EC-AFC modeling of both ignimbrites will assess their magmatic evolution using the aforementioned data, comparing them with current magmatic processes in the LdM volcanic field.