Browsing by Autor "Shanaka L. de Silva"

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40Ar/39Ar chronostratigraphy of Altiplano-Puna volcanic complex ignimbrites reveals the development of a major magmatic province
(Geological Society of America, 2010) Morgan J. Salisbury; Brian R. Jicha; Shanaka L. de Silva; Brad S. Singer; Néstor Jiménez; Michael H. Ort
Research Article| May 01, 2011 40Ar/39Ar chronostratigraphy of Altiplano-Puna volcanic complex ignimbrites reveals the development of a major magmatic province Morgan J. Salisbury; Morgan J. Salisbury † 1Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA †E-mail: salisbum@geo.oregonstate.edu Search for other works by this author on: GSW Google Scholar Brian R. Jicha; Brian R. Jicha 2Department of Geoscience, University of Wisconsin–Madison, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Shanaka L. de Silva; Shanaka L. de Silva 1Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA Search for other works by this author on: GSW Google Scholar Brad S. Singer; Brad S. Singer 2Department of Geoscience, University of Wisconsin–Madison, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Néstor C. Jiménez; Néstor C. Jiménez 3Instituto de Investigaciones Geológicas y del Medio Ambiente, Universidad Mayor de San Andrés, La Paz, Bolivia Search for other works by this author on: GSW Google Scholar Michael H. Ort Michael H. Ort 4School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2011) 123 (5-6): 821–840. https://doi.org/10.1130/B30280.1 Article history received: 22 Feb 2010 rev-recd: 22 Jun 2010 accepted: 19 Jul 2010 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Morgan J. Salisbury, Brian R. Jicha, Shanaka L. de Silva, Brad S. Singer, Néstor C. Jiménez, Michael H. Ort; 40Ar/39Ar chronostratigraphy of Altiplano-Puna volcanic complex ignimbrites reveals the development of a major magmatic province. GSA Bulletin 2011;; 123 (5-6): 821–840. doi: https://doi.org/10.1130/B30280.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Lípez region of southwest Bolivia is the locus of a major Neogene ignimbrite flare-up, and yet it is the least studied portion of the Altiplano-Puna volcanic complex of the Central Andes. Recent mapping and laser-fusion 40Ar/39Ar dating of sanidine and biotite from 56 locations, coupled with paleomagnetic data, refine the timing and volumes of ignimbrite emplacement in Bolivia and northern Chile to reveal that monotonous intermediate volcanism was prodigious and episodic throughout the complex. The new results unravel the eruptive history of the Pastos Grandes and Guacha calderas, two large multicyclic caldera complexes located in Bolivia. These two calderas, together with the Vilama and La Pacana caldera complexes and smaller ignimbrite shields, were the dominant sources of the ignimbrite-producing eruptions during the ∼10 m.y. history of the Altiplano-Puna volcanic complex. The oldest ignimbrites erupted between 11 and 10 Ma represent relatively small volumes (approximately hundreds of km3) of magma from sources distributed throughout the volcanic complex. The first major pulse was manifest at 8.41 Ma and 8.33 Ma as the Vilama and Sifon ignimbrites, respectively. During pulse 1, at least 2400 km3 of dacitic magma was erupted over 0.08 m.y. Pulse 2 involved near-coincident eruptions from three of the major calderas resulting in the 5.60 Ma Pujsa, 5.65 Ma Guacha, and 5.45 Ma Chuhuilla ignimbrites, for a total minimum volume of 3000 km3 of magma. Pulse 3, the largest, produced at least 3100 km3 of magma during a 0.1 m.y. period centered at 4 Ma, with the eruption of the 4.09 Ma Puripicar, 4.00 Ma Chaxas, and 3.96 Ma Atana ignimbrites. This third pulse was followed by two more volcanic explosivity index (VEI) 8 eruptions, producing the 3.49 Ma Tara (800 km3 dense rock equivalent [DRE]) and 2.89 Ma Pastos Grandes (1500 km3 DRE) ignimbrites. In addition to these large caldera-related eruptions, new age determinations refine the timing of two little-known ignimbrite shields, the 5.23 Ma Alota and 1.98 Ma Laguna Colorada centers. Moreover, 40Ar/39Ar age determinations of 13 ignimbrites from northern Chile previously dated by the K-Ar method improve the overall temporal resolution of Altiplano-Puna volcanic complex development. Together with the updated volume estimates, the new age determinations demonstrate a distinct onset of Altiplano-Puna volcanic complex ignimbrite volcanism with modest output rates, an episodic middle phase with the highest eruption rates, followed by a decline in volcanic output. The cyclic nature of individual caldera complexes and the spatiotemporal pattern of the volcanic field as a whole are consistent with both incremental construction of plutons as well as a composite Cordilleran batholith. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Catastrophic Caldera-Forming (CCF) Monotonous Silicic Magma Reservoirs: Geochemical and Petrological Constraints on Heterogeneity, Magma Dynamics, and Eruption Dynamics of the 3·49 Ma Tara Supereruption, Guacha II Caldera, SW Bolivia
(Oxford University Press, 2017) Stephanie B. Grocke; Shanaka L. de Silva; R. Iriarte; Jan M. Lindsay; Elizabeth Cottrell
The 5·65 to 1·80 Ma Cerro Guacha Caldera Complex (CGCC) in the Altiplano–Puna Volcanic Complex of SW Bolivia, with >90% of its >2500 km3 erupted volume consisting of crystal-rich dacite, has all the characteristics of a 'monotonous' magma system. However, it also records minor lithological heterogeneity. Such hand-sample scale heterogeneity is ubiquitous in dominantly 'monotonous' magmas, yet remains poorly investigated. Here we explore the heterogeneity in the CGCC, and its implications for the construction and evolution of 'monotonous' magma systems. We focus on the Guacha II Caldera (G2C), the younger of two calderas in the complex, because its pre- to post-climactic eruptive history is fully represented and, although the eruptive products are dominantly dacitic (66–72 wt % SiO2), the juvenile pyroclastic deposits and lavas erupted throughout the history of the G2C define a high-K, calc-alkaline suite of diverse compositions that range from andesite to high-Si rhyolite. The G2C cycle initiated with the effusive eruption of crystal-rich andesite lava. The subsequent explosive phase began with a short-lived plinian eruption of crystal-poor rhyolite pumice. This was immediately followed by the Catastrophic Caldera Forming (CCF) eruption at 3·49 ± 0·01 Ma and the deposition of dacite–rhyolite and banded pumice within the >800 km3 dense rock equivalent (DRE) Tara ignimbrite. A significant volume of magma remained and caused ∼1·5 km of resurgent uplift. Three crystal-rich dacite–rhyolite post-climactic lava domes (Chajnantor Dome, Rio Guacha Dome, and Chajnantor Lavas) subsequently erupted from separate coexisting melt-rich 'pods' within the G2C's remnant mush. Whole-rock isotope ratios across all lithologies span a significant range in 87Sr/86Sr (0·709380–0·713159) and a relatively narrow range in 143Nd/144Nd (0·512179–0·512297) and δ18O(qtz) (+8·38 to + 8·68‰), best reconciled with a two-stage assimilation–fractional crystallization (AFC) model. Stage 1 initiated with parental melts from the Altiplano–Puna Magma Body (APMB) fractionating and assimilating crustal lithologies in the upper crust (10–25 km depth) to generate the magma compositions recorded in the andesite lava and the ignimbrite banded pumice. These magmas subsequently accumulated and underwent a second stage of AFC in the uppermost crust (∼800–850 °C and 5–9 km depth) to produce the most differentiated magmas recorded in the ignimbrite rhyolite pumices. Although the two-stage AFC model presented here is non-unique, it implies that the basement composition is temporally or spatially variable throughout the ∼30 km of upper crust beneath the G2C. The origin of some of the minor lithologies erupted from the G2C requires further explanation beyond AFC. Recharge, binary mixing, and additional crystal fractionation of the plagioclase, quartz, sanidine, biotite, and Fe–Ti oxides identified within the ignimbrite dacite–rhyolite pumice are required to explain the geochemical diversity of these minor lithologies. Integrating geochemical modeling with the volcanological framework we find that the andesite magma played a large role in the development and evolution of the G2C magma system. At least one episode of andesite magma recharge occurred in the upper crustal magma reservoir prior to the eruption of the G2C and fractionation of this andesite magma formed residual rhyolitic melt sampled by the plinian eruption. Andesitic recharge therefore not only sustained the pre-eruption magma reservoir's dominant dacitic composition throughout its history, but also may have acted as an eruption trigger. The minor lithologies erupted from a 'monotonous' magmatic system record processes that are otherwise lost in the inexorable march to homogeneity that characterizes the long-term evolution of many CCF magma systems. Our findings complement investigations that have identified heterogeneity at the crystal- and micro-scale, but emphasize that upper crustal processes are recorded as hand-sample scale heterogeneity that is serendipitously sampled by eruption before being integrated into a 'monotonous' whole.
CERRO VOLCÁN QUEMADO: AN ENIGMATIC HOLOCENE MONOGENETIC PHREATOMAGMATIC VOLCANO IN THE ALTIPLANO OF BOLIVIA
(Geological Society of America, 2023) Verenice Becerril-Gonzalez; Shanaka L. de Silva; Alejandro Cisneros de León; Néstor Jiménez; Frank J. III Tepley
CERRO VOLCÁN QUEMADO: AN ENIGMATIC HOLOCENE MONOGENETIC PHREATOMAGMATIC VOLCANO IN THE ALTIPLANO OF BOLIVIA
(Geological Society of America, 2024) Verenice Becerril-Gonzalez; Shanaka L. de Silva; Alejandro Cisneros de León; Néstor Jiménez; Frank J. III Tepley
Correlation of ignimbrites using characteristic remanent magnetization and anisotropy of magnetic susceptibility, Central Andes, Bolivia
(Wiley, 2012) Michael H. Ort; Shanaka L. de Silva; Néstor Jiménez C.; Brian R. Jicha; Brad S. Singer
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.
HOLOCENE MONOGENETIC SILICIC PHREATOMAGMATIC VOLCANISM ON THE ALTIPLANO OF BOLIVIA: CERRO VOLCÁN QUEMADO
(Geological Society of America, 2024) Verenice Becerril-Gonzalez; Shanaka L. de Silva; Alejandro Cisneros de León; Néstor Jiménez; Frank J. III Tepley
Large ignimbrite eruptions and volcano-tectonic depressions in the Central Andes: a thermomechanical perspective
(Geological Society of London, 2006) Shanaka L. de Silva; G. Zandt; Robert B. Trumbull; J. Viramonte; G. Salas; Néstor Jiménez
Abstract The Neogene ignimbrite flare-up of the Altiplano Puna Volcanic Complex (APVC) of the Central Andes produced one of the best-preserved large silicic volcanic fields on Earth. At least 15 000 km 3 of magma erupted as regional-scale ignimbrites between 10 and 1 Ma, from large complex calderas that are typical volcano-tectonic depressions (VTD). Simple Valles-type calderas are absent. Integration of field, geochronological, petrological, geochemical and geophysical data from the APVC within the geodynamic context of the Central Andes suggests a scenario where elevated mantle power input, subsequent crustal melting and assimilation, and development of a crustal-scale intrusive complex lead to the development of APVC. These processes lead to thermal softening of the sub-APVC crust and eventual mechanical failure of the roofs above batholith-scale magma chambers to trigger the massive eruptions. The APVC ignimbrite flare-up and the resulting VTDs are thus the result of the time-integrated impact of intrusion on the mechanical strength of the crust, and should be considered tectonomagmatic phenomena, rather than purely volcanic features. This model requires a change in paradigm about how the largest explosive eruptions may operate.
Synthesis: PLUTONS: Investigating the relationship between pluton growth and volcanism in the Central Andes
(Geological Society of America, 2018) M. E. Pritchard; Shanaka L. de Silva; Gary Michelfelder; G. Zandt; Stephen R. McNutt; Joachim Gottsmann; M. E. West; Jon Blundy; D. H. Christensen; N. J. Finnegan
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The Cerro Guacha Caldera complex, SW Bolivia: A long-lived, multicyclic, resurgent caldera complex in the Altiplano-Puna Volcanic Complex of the Central Andes
(Elsevier BV, 2023) Alvaro Rodrigo Iriarte; Shanaka L. de Silva; Axel K. Schmitt; Néstor Jiménez