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Title: Petrological and geochronological constraints on magma storage conditions beneath Uturuncu volcano, SW Bolivia
Author: Muir , D. D.
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2013
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Uturuncu is an effusive, dacitic volcano in the Bolivian altiplano of the Central Andes. A 70 km diameter InSAR anomaly associated with the volcano is thought to be caused by ongoing magma intrusion in the mid-crust. Detailed petrological study of Uturuncu lavas and domes has been conducted to better understand the sub-volcanic magma system. Phenocryst proportions, phase assemblage and bulk chemistry are relatively constant but wide ranging mineral compositions and the presence of mafic andesite enclaves suggest magma mixing plays a major role in the genesis of Uturuncu magmas. Norite cumulates testify to the role of fractional crystallisation. Dacite magmatic temperatures vary from 760 to 900 degree C with hottest temperatures most likely occurring due to recharge of andesitic magmas. Volatile compositions of plagioclase- hosted melt inclusions provide minimum trapping pressures of 50 to 120 MPa. Phase equilibria experiments using dacite bulk-rock and rhyolitic groundmass glass compositions constrain dacite pre-emptive storage conditions to <100±50 MPa «5.7 km deep). Existing geodetic inversions of deformation data require a deformation source> 17 km deep, therefore such shallow storage of dacites precludes intrusion of magmas in pre-eruptive storage regions as the cause of ongoing deformation at Uturuncu. Uturuncu dacites ultimately arc sourced from the Altiplano-Puna Magma Body (APMB), the largest known active magma body in the Earth's continental crust. Phase equilibria experiments using a mafic andesitic enclave composition at mid-crustal conditions have been performed. These experiments test whether dacites erupted at Uturuncu call be sourced from residual melts of mafic andesite’s at APMB levels. Results indicate that dacite melts form at mid-crustal pressures (0.5-1.1 GPa) from equilibrium crystallisation of mafic andesite. Melts in volatile-undersaturated experiments are most similar to erupted dacite compositions, mainly as a result of plagioclase crystallisation. In detail however, major and trace element concentrations do not match those of natural dacites. Melt fractions are too high suggesting drier or cooler conditions are required. Biotite and groundmass separates from 27 Uturuncu lavas and domes were dated using 4oAr/39Ar. Results show that eruptions between 1.05 and 0.25 Ma formed the 72 to 80 km3 edifice. Four eruptive hiatuses >50 kyrs one possibly as long as 240 kyrs, occurred in the past. Extrusive centres have migrated several kilometres throughout Uturuncu's history and are commonly marked by large domes. There are no systematic variations in textural and chemical data of erupted products through time and unpredictable temporal changes in trace element compositions preclude magma evolution from a single parental source. Instead multiple ephemeral , shallow crustal reservoirs with distinct trace element compositions are envisaged.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available