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Proquest Dissertations VOLCANOLOGY, PETROLOGY AND GEOCHEMISTRY OF LASCAR VOLCANO, NORTHERN CHILE by Stephen John Matthews at University of London January 1994 ProQuest Number: 10017777 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10017777 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 ABSTRACT. This is the first comprehensive pétrographie and geochemical study of Lascar Volcano, an active calc-alkaline stratovolcano located in the Atacama Desert, Northern Chile. The volcano probably initially became active during the last glacial maximum, about 20,000 to 27,000 years ago. It has successively built up three eruptive centres which form a slightly elongated structure trending ENE-WSW. The lavas and pumices produced by this volcano are predominantly 2-pyroxene andésites and dacites. The chemical and pétrographie features of the eruption products are explained here in terms of a fractionating magma chamber at shallow depth which receives periodic influxes of basaltic andésite magma. This magma, which has already crystallised in middle to lower crustal magma chambers, mixes with the resident more evolved magmas to create a variety of disequilibrium textures including mafic inclusions and reaction coronae on olivine phenocrysts. These influxes of hotter magma lead to convective overturn of the magma chamber. The “primitive” magma which rises from deep levels is relatively rich in dissolved sulphur and chlorine as well a large amounts of water. A model is proposed in which these volatile phases are degassed from the magma on quenching, depressurisation and oxidation of the mafic endmember to form a separate mixed fluid phase in the magma chamber. This release of volatiles is an important contributor to the violence of the eruptions which frequently occur in such volcanoes. The discovery of anhydrite in a prehistoric dacitic pyroclastic flow, the Soncor Flow, and also in the April 1993 eruption products, indicates that sulphur is an important volatile phase and that the magma chamber is relatively oxidised, stabilising sulphate rather than sulphide in the magmas. This is confirmed by calculations of oxygen fugacity using the compositions of coexisting magnetite-ilmenite pairs. A trend of increasing f02 with decreasing temperature has been attributed to buffering by the SO 2 -H 2 S equilibrium in a coexisting H20-rich fluid phase. This model explains the common association of oxidised, anhydrite-bearing magmas with excessively high SO 2 emissions in some volcanoes, notably the 1991 eruption of Mount Pinatubo, Phillipines. The source of this sulphur is believed to be either the subducted oceanic crust below the volcanic front or the overlying mantle wedge. Important changes in the plumbing system are believed to have occurred in the past, producing magmas which appear to have bypassed the magma chamber on their way to the surface. This is based on whole rock geochemical trends and pétrographie analysis, which shows a lack of evidence for magma mixing in such eruption products. One of these, the andesitic Chaile Flow, lies stratigraphically between dacitic flows with very similar geochemical and pétrographie features indicative of magma mixing and typical of magma chamber derived products. A similar change is believed to have occurred in the present eruption cycle, as the 1986 and 1990 lavas are interpreted as having bypassed the volatile-rich magma chamber which gave rise to the 1993 eruption. This change is thought to be responsible for the switch in eruption style from shallow short-lived vulcanian explosions to a sustained sub-plinian eruptive style producing anhydrite bearing pumices. This petrological monitoring of an active volcano provides a new way of studying the magmatic system and predicting the future eruption style. ACKNOWLEDGEMENTS. I am indebted to many people for their assistance during this project. In particular, I am grateful to Adrian Jones for his careful supervision and to Claudio Vita-Finzi for advice and support. Moyra Gardeweg, Steve Sparks, Ana Espinoza, Sergio Manquez, Clive Oppenheimer and Mark Stasiuk are thanked for assistance in the field. Thanks also to Nena for looking after us in Toconao. I am grateful to Andy Beard for training and assisting me in the use of the electron microprobe at Birkbeck College and to Steve Mirons for carrying out XRD analysis and identification of some difficult minerals. Tony Osborn is thanked for assistance with whole rock analyses and for conducting ICP analyses on a number of samples. Peter Francis provided some vital samples of the 1993 eruption products and Marcos Zentilli arranged for sulphur isotope analyses of native sulphur samples. Mathew Thirlwall and Jerry Ingram assisted in the preparation of samples for Nd and Sr isotope analyses and conducted the analyses. Giselle Mariner helped in the preparation of samples for XRF analysis. Judith Milledge trained and assisted me in the use of the infrared microscope. Finally, I am grateful to my parents for their support, encouragement and advice over the period of study. This project was funded by Kingston Polytechnic and by the Natural Environment Research Council Grant no. GT4/90/GS/84. CONTENTS. PAGE ABSTRACT. 1 ACKNOWLEDGEMENTS. 3 CONTENTS. 4 TABLE OF FIGURES. 11 TABLES 18 CHAPTER I: INTRODUCTION. 19 1.1 GENERAL. 19 1.2 TECTONIC SETTING. 21 1.2.1 Tectonic Evolution of the Central Andes. 21 1.2.2 Upper Cenozoic and Neogene Tectonic and Voicanic Structure. 23 1.2.2.1 Tectonic Structure. 25 1.2.2.2 Volcanic History of the Central Andean Volcanic Zone. 27 1.3 LOCATION AND ACCESS. 30 1.4 GEOGRAPHICAL FEATURES. 31 1.5 PREVIOUS WORK. 34 1.6 CLIMATE. 35 1.6.1 Modern Climatic Conditions. 35 1.6.2 Pieistocene Ciimatic History. 36 1.6.3 Holocene Climatic History. 38 1.7 BASEMENT GEOLOGY. 38 CHAPTER II: GEOLOGY OF LASCAR VOLCANO. 40 11.1 INTRODUCTION. 40 11.2 CENTRE I. 45 11.2.1 The Early Andésite Lavas. 45 PAGE 11.2.2 The Saltar Flow. 46 11.3 CENTRE II 48 11.3.1 The Piedras Grandes Flow. 48 11.3.2 The Soncor Flow. 48 11.3.3 The Chaile Flow. 51 11.3.4 The Capricorn Lava. 54 11.3.5 The Tumbres Fiow. 54 11.4 CENTRE III. 56 11.4.1 The Talabre Lava. 56 11.4.2 The Collapse Craters. 56 11.5 HISTORIC ACTIVITY. 58 TER III: PETROGRAPHY OF LAVAS AND PUMICES. 66 III.1 GENERAL. 66 III.2 PETROGRAPHY OF ROCKS FROM LASCAR VOLCANO. 66 III.2.1 Plagioclase. 67 III.2.2 Orthopyroxene. 69 III.2.3 Clinopyroxene. 70 III.2.4 Olivine. 70 III.2.5 Amphibole. 73 III.2.6 Biotite. 75 III.2.7 Fe-Ti Oxides. 75 III.2.8 Apatite. 76 III.3 PETROGRAPHY OF INDIVIDUAL FLOWS. 76 III.3.1 Centre 1 Andésite Magmas. 76 III.3.1.1 Orthopyroxene. 77 III.3.1.2 Clinopyroxene. 77 III.3.1.3 Plagioclase. 79 III.3.1.4 Conclusions. 79 PAGE III.3.2 Chaile Flow. 81 III.3.2.1 Orthopyroxene. 83 III.3.2.2 Clinopyroxene. 83 III.3.2.3 Plagioclase. 83 III.3.2.4 Conclusions. 83 III.3.3 The Soncor Pyroclastic Fiow and Piedras Grandes Flow. 85 III.3.3.1 Orthopyroxenes. 88 III.3.3.2 Clinopyroxenes. 92 III.3.3.3 Amphiboles. 95 III.3.3.4 Plagioclase. 101 III.3.3.5 Spinels. 102 III.3.3.6 Anhydrite. 102 III.3.3.7 Glass compositions. 104 III.3.3.8 Conclusions. 109 III.3.4 The Capricorn Lava. 110 III.3.4.1 Orthopyroxene. 112 III.3.4.2 Clinopyroxenes. 114 III.3.4.3 Amphibole. 117 III.3.4.4 Plagioclase. 119 III.3.4.5 Spinels. 121 III.3.4.6 Glass compositions. 124 III.3.4.7 Conclusions. 124 III.3.5 The 1986-1993 Eruptive Sequence. 125 III.3.5.1 Orthopyroxene. 127 III.3.5.2 Clinopyroxene. 127 III.3.5.3 Plagioclase. 129 III.3.5.4 Glass compositions. 129 Ml.3.3.5 Conclusions. 134 III.4 DISCUSSION. 136 PAGE CHAPTER IV: GEOTHERMOMETRY, GEOBAROMETRY AND OXYGEN BAROMETRY. 145 IV.1 GENERAL. 145 IV.2 TEMPERATURE AND OXYGEN FUGACITY. 147 IV.2.1 Magnetite-ilmenite Geothermometry. 147 IV.2.2 2-Pyroxene Geothermometry and Oiivine-Spinei Oxygen Barometry. 153 IV.2.3 Melt Inclusion Homogenization Experiments. 155 IV.2.4 Pressure Calculations. 161 IV.4 CONCLUSIONS. 167 CHAPTER V: BULK CHEMICAL AND ISOTOPIC COMPOSITION. 169 V.1 CHEMICAL COMPOSITION. 169 V.2 RADIOGENIC ISOTOPE RATIOS. 185 CHAPTER VI: XENOLITHS AND THEIR RELATION TO KNOWN BASEMENT GEOLOGY. 191 V I.1 GENERAL. 191 VI.2 CENOZOIC LAVAS. 192 VI.3 PARTIALLY MELTED METASEDIMENTARY ROCKS. 193 VI.4 SKARN XENOLITHS. 198 VI.4.1 Petrography of Caic-Silicate Xenoliths. 198 VI.4.1.1 Pyroxenes. 200 VI.4.1.2 Garnets. 202 VI.4.1.3 Wollastonite. 202 VI.4.1.5 Plagioclase. 205 PAGE VL4.2 Geothermobarometry of Skarn and Buchite Xenoliths. 205 V I.4.3 Glass and Fluid Inclusions. 213 V I.4.4 Stable Isotopes. 220 VI.4.5 Source of Calc-Silicate Xenoliths. 224 CHAPTER VII: VOLATILE COMPONENTS AND THEIR CONTROLS ON MAGMA CHEMISTRY AND ERUPTION STYLE. 226 VII.1 MAIN VOLATILE COMPONENTS. 226 VII.2 WATER.
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