The Pastos Grandes Caldera Complex of SW Bolivia: The building of a composite upper crustal batholith

The Pastos Grandes Caldera Complex in SW Bolivia is one of the longest-lived in the Altiplano Puna Volcanic Complex (APVC) of SW Bolivia. This nested caldera complex has erupted over 2500 km3 of ignimbrite from at least two caldera-forming supereruptions and an ignimbrite shield depositing the 5.45±0.02 Ma Chuhuilla, the 5.22±0.02 Ma Alota, and the 2.89 ± 0.01 Ma Pastos Grandes Ignimbrites. Late stage lava domes of the Chascon Runtu-Jarita complex in the southwest of the caldera represent the most recent volcanic activity (~85 ka). Each eruption represents a discrete pulse of magma injected into the shallow crust beneath the complex mapping out the incremental construction of a large upper crustal batholith. Field observations and paleomagnetic data complement Ar-Ar dating to reveal the details of ignimbrite stratigraphy, distribution, and the relationship to structure of the caldera. Characteristic remnant magnetism in the Chuhuilla ignimbrite records a reversed polarity, while Alota and Pastos Grandes ignimbrites record normal polarity, helping identify the Chuhuilla and Pastos Grandes outflow extents of over 3000 km2. The former is exposed only to the north and northwest of the caldera to a distance of 40 km and has a total volume (DRE) of ~1200 km3, while the latter buries the caldera and its flanks as far as 50 km east with a total volume of ~1500 km3. The structure of the Pastos Grandes caldera is of comparable size and geometry to other calderas of the Altiplano-Puna Volcanic Complex, measuring 35 km by 60 km. Evidence for collapse exists only on the eastern margin of the complex with an arcuate scarp overlain by lava flows and domes cut by concentric normal faults. Based on stratigraphic relations between the lavas and the on-lapping Pastos Grandes ignimbrite (PGI), this collapse feature appears to be an older scarp of Chuhuilla age that was reactivated by the Pastos Grandes eruption. Much of the current topography and morphology in the central region of the caldera complex is the result of the 2.89 Ma Pastos Grandes eruption and subsequent events. The resurgent dome, a ~ 1 km high structural dome with a central graben, is composed primarily of the PGI and is flanked by post-resurgent lava domes of PGI age. The high K ignimbrite pumice (65 wt% - 70 wt% SiO2) contains variable amounts of quartz, plagioclase, and biotite and sanidine. The presence of zircon is enabling investigations of the development of the magmatic system. Of particular interest are granitic blocks recovered from the Chascon-Runtu Jarita complex. These are similar in composition to the surrounding volcanic units and preliminary U/Pb ages from zircon separates in the granitic clasts have yielded ages of 2.56 - 3.34 Ma that bracket the Ar/Ar ages of the Pastos Grandes ignimbrite (2.89 Ma). This reveals, for the first time in the APVC, direct evidence for the development and longevity of a pluton related to a supereruption. Our preliminary interpretation that individual plutons grow to supervolcanic proportions, erupt, and solidify in less than 1 Ma remains to be tested further.