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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

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dc.contributor.advisor de Silva, Shanaka L
dc.creator Iriarte, Rodrigo
dc.date.accessioned 2012-07-03T15:55:43Z
dc.date.available 2012-07-03T15:55:43Z
dc.date.copyright 2012-05-22
dc.date.issued 2012-05-22
dc.identifier.uri http://hdl.handle.net/1957/30475
dc.description Graduation date: 2012 en_US
dc.description.abstract Four multicyclic complex calderas and smaller ignimbrite shields located within the Altiplano Puna Volcanic Complex of the Central Andes (APVC) erupted 13000 km³ of magma within the last 11 Ma. One of the largest and most complex of these is the Cerro Guacha Caldera. Ar-Ar age determinations and paleomagnetic directions suggest that the Cerro Guacha Caldera was formed by two major eruptions, caldera collapse, resurgence cycles and several smaller eruptions. Two major ignimbrites (> 600 km³) 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. A conspicuous lineament of volcanic structures in the eastern part of the caldera, bordering a caldera moat, filled out welded ignimbrites and sedimentary lacustrine sequences suggest an earlier 60x40 km outer collapse associated with the Guacha explosive episode. A central graben formed on the Guacha welded ignimbrite is related to a first episode of resurgence. Evidence of a second 30 x15 km inner collapse includes offset of welded Guacha ignimbrites and alignment of lava domes associated with the Tara ignimbrite. A second resurgence episode is suggested by the presence of an uplifted central block consisting primarily of welded Tara ignimbrite. As a whole the three ignimbrites (Guacha, Tara and Puripica Chico) share the same petrological and geochemical characteristics: high-K series, compositional ranges from dacite to rhyolite, with andesitic members present as lavas (for the Guacha and Puripica Chico Ignimbrites) and as pumices (for the Tara Ignimbrite). Highest silica content is found in the Chajnantor dome. Rayleigh modeling for Ba, Rb and Sr suggests at least 60% of crystal fractionation to account for the compositional variation between the Guacha andesite and the Chajnantor dome. Dy/Hb ratio increases with time from the Guacha andesite to the Negreal andesite suggesting stabilization of garnet owing to crustal thickening. Fe-Ti exchange geothermometry for the Tara Ignimbrite yielded log fO₂ values ranging from -13.06 to -13.38 and temperatures of 714° to 801°C. Amphibole geobarometry yielded pressures ranging from 150 to 180 MPa equivalent to 5.3 and 6.4 km depth respectively for the Tara Ignimbrite; the pressures range between 133 to 242 MPa, equivalent to 5.0 to 9.2 km depth for the Guacha Ignimbrite. The zircon saturation method yielded saturation temperatures of 716° and 705°C for the Guacha and Chajnantor dome respectively and 784°C for the Tara Ignimbrite. The zircon crystallization range for the magmas of the Cerro Guacha Caldera is 1.25 Ma for the Guacha Ignimbrite; 1.09 Ma for the Puripica Chico Ignimbrite and 0.95 Ma for the Tara Ignimbrite. Recycling of antecrystic zircons within the caldera magmas is continuos through time. en_US
dc.language.iso en_US en_US
dc.subject Andes en_US
dc.subject caldera en_US
dc.subject Guacha en_US
dc.subject ignimbrite en_US
dc.subject.lcsh Ignimbrite -- Altiplano en_US
dc.subject.lcsh Calderas -- Altiplano en_US
dc.subject.lcsh Volcanism -- Altiplano en_US
dc.subject.lcsh Ignimbrite -- Bolivia
dc.subject.lcsh Calderas -- Bolivia
dc.subject.lcsh Volcanism -- Bolivia
dc.subject.lcsh Geology, Stratigraphic -- Miocene
dc.subject.lcsh Geology, Stratigraphic -- Pliocene
dc.title 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 en_US
dc.type Thesis/Dissertation en_US
dc.degree.name Master of Science (M.S.) in Geology en_US
dc.degree.level Master's en_US
dc.degree.discipline Oceanic and Atmospheric Sciences en_US
dc.degree.grantor Oregon State University en_US
dc.contributor.committeemember Dilles, John
dc.contributor.committeemember Kent, Adam
dc.contributor.committeemember Haley, Brian
dc.description.peerreview no en_us

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