Correlation of ignimbrites using characteristic remanent magnetization and anisotropy of magnetic susceptibility, Central Andes, Bolivia

Abstract

Large ignimbrite flare‐ups provide records of profound crustal modification during batholith formation at depth. The locations of source calderas and volumes and ages of the eruptions must be determined to develop models for the tectonomagmatic processes that occur during these events. Although high‐precision isotopic ages of the ignimbrites are critical, less expensive and more rapid techniques, such as paleomagnetism, can extend the temporal information from dated outcrops. Paleomagnetic and rock magnetic data, including characteristic remanent magnetization (ChRM) and anisotropy of magnetic susceptibility (AMS), from the Altiplano‐Puna Volcanic Complex of the Central Andes reliably identify calderas and eight associated Mio‐Pliocene ignimbrites. ChRM results indicate a larger between‐site error for most ignimbrites, in comparison to within‐site scatter. Part of this dispersion may be due to tumescence/detumescence associated with the caldera‐forming eruptions, but most of the effect is probably due to the recording of paleosecular variation during cooling and vapor‐phase crystallization of the thick ignimbrites. AMS data identify the source calderas for four ignimbrites and provide limits on possible post‐emplacement rotations of the deposits. AMS data indicate significant topographic control on inferred flow directions, implying that the flows were dense and/or of low mobility.