The Cerro Guacha Caldera complex: An Upper Miocene-Pliocene polycyclic volcano-tectonic structure in the Altiplano Puna Volcanic Complex of the Central Andes of Bolivia

Four multicyclic complex calderas and smaller ignimbrite shields located within the Altiplano Puna Volcanic Complex of the Central Andes erupted 13000 km3 of magma within the last 11 Ma. One of the largest and most complex of these is the 5.6 to 1 Ma Cerro Guacha Caldera complex in SW Lipez, Bolivia. Ar-Ar age determinations and paleomagnetic signatures suggest that the Cerro Guacha Caldera was formed by two major eruptions, caldera collapse, resurgence cycles and several smaller eruptions. Two major ignimbrites of super-eruption magnitude are found with 40Ar-39Ar from biotites and sanidines of 5.65±0.01Ma for the 1300 km3 (magma volume) Guacha ignimbrite and 3.49±0.01Ma for the 800 km3 Tara Ignimbrite. The last major eruption occurred on the western flank producing the 1.72±0.02 Ma Puripica Chico Ignimbrite with a volume of approximately 10 km3. Characteristic remanent magnetization data (ChRM) for these ignimbrites show that the Guacha has reverse polarity, while the Tara is normally polarized and the magnetic fingerprints have allowed their current full extents to be identified. The Guacha ignimbrite extends over 70 km north and south of the caldera and is distributed over an area of almost 5000 km2. The Tara ignimbrite covers an area of almost 4000 km2 and extends southwards almost 70 km, where it ponded in La Pacana caldera. A conspicuous lineament of volcanic structures towards the caldera's easternmost edge, along with a flat surface (moat), welded ignimbrites, and sedimentary lacustrine sequences suggest an earlier 60x40 km outer collapse associated with the Guacha explosive episode. A central graben consisting of Guacha welded ignimbrite is related to the first episode of resurgence. Evidence of a second 30x15 km inner collapse scarp includes offset of welded Guacha ignimbrites and alignment of lava domes equivalent in age to the Tara ignimbrite. A second resurgence episode is suggested by the presence of a central block consisting primarily of welded Tara ignimbrite. Located along the W-NW edge of the inner collapse, the third explosive episode erupted a 20 m thick sequence of pumice-rich ignimbrites overlain by 10m thick lava flows. Preliminary field work, dating and geochemistry suggest a polycyclic magmatic system that decreases in size with time. A strong link with the La Pacana system just to the south is suggested by a congruent chronology, shared bounding structures and volcanic features. However, the magmatic systems of the Tara and Atana ignimbrites were clearly quite distinct from each other.