Paper No. 3
Presentation Time: 9:30 AM


BIRSIC, Erin M., SINGER, Brad S. and JICHA, Brian R., Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St, Madison, WI 53706,

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.