DOCSLIB.ORG

Silicic Magma Chambers and Mafic Dikes a Dissertation Submitted to the Department Of

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Silicic Magma Chambers and Mafic Dikes a Dissertation Submitted to the Department Of INVESTIGATIONS OF MAGMATIC END-MEMBERS: SILICIC MAGMA CHAMBERS AND MAFIC DIKES A DISSERTATION SUBMITTED TO THE DEPARTMENT OF GEOLOGICAL AND ENVIRONMENTAL SCIENCES AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Gwyneth Retta Hughes May 2010 © 2010 by Gwyneth Retta Hughes. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/cf090yt6229 Includes supplemental files: 1. Caldera references for Chapters 2 and 3 (Caldera_index_ref.pdf) 2. Bayes Classifier Code for Chapter 3 (bayes_classifier.zip) 3. Caldera data for Chapter 2 (Arc_caldera_data.csv) 4. Caldera data for Chapter 3 (All_caldera_data.csv) ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Gail Mahood, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. David Pollard I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Paul Segall Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii Abstract Approximately 10% of the global population, some 550 million people, live within 100 km of an active volcano, making it imperative that the causes of magma accumulation in the crust and the factors affecting the scale of subsequent volcanic eruptions are investigated and modeled. This PhD dissertation examines magmatic processes at two scales: silicic calderas represent the large but infrequent end of the continuum, whereas mafic dikes are small in scale but common. Many previous authors have presented hypotheses for how large silicic systems develop. In order to test and refine these hypotheses, I undertook a global compilation that empirically examines how the characteristics of 140 young silicic calderas reflect their crustal-tectonic setting. Results indicate that the size and geochemistry of silicic calderas are affected by the nature of the underlying crust, the tectonic setting, and the local stress regime. For example, large, rhyolitic calderas tend to occur in continental settings under extension. There are, however, few true “rules,” and exceptions may prove useful in analyzing how silicic magma chambers form. Based on this compilation, I present a probabilistic method for determining the tectonic-crustal setting of a given caldera from its diameter, and eruption geochemistry. Focusing specifically on arc settings, this study demonstrates that (1) the abundance of silicic calderas in a given arc is proportional to the trench-normal convergence rate, except in arcs with back-arc spreading; and (2) silicic calderas in continental arcs tend to occur farther behind the volcanic front than do more typical iv arc volcanoes, possibly because of the abundance of pre-existing structures in continental margins. At the opposite end of the volcanic spectrum, this dissertation examines the intrusion of two mafic dikes. The first lies beneath Mammoth Mountain, California, and was associated with a 1989 seismic swarm. Based on an inversion of leveling data constrained by relocated earthquakes, I propose that a dike 2 km long, 8 km high, with 1 m of opening was intruded at 9 km depth beneath the south side of Mammoth Mountain. The second dike investigation focused on the eruption of Miyakejima, Japan, in 2000, associated with more than 10,000 earthquakes, several small eruptions and progressive caldera collapse. Displacements recorded by GPS stations and pre- and syn- event seismicity were used to determine a geological explanation of the event. In the proposed model, a shallow dike propagated ~30 km away from Miyakejima for one week, stopped propagating laterally after intersecting a pre- existing fault zone, and then continued to open and grow vertically for nearly two months. v Acknowledgements I would like to thank my advisors Gail Mahood and Paul Segall for leading me into interesting branches of research and providing extensive academic support over the past five years. In addition, as committee members, David Hill and David Pollard have been invaluable as sources of feedback and encouragement. A great deal of this thesis involved reading papers in other languages. While I was able to cope with those written in romance languages, Uwe Martens, Yo Fukushima, and Curran Hughes helped translate papers in German, Japanese, and Russian. Members of the Segall research group were a valuable resource of advice on programming and data analysis. The Stanford Earth Sciences library staff also helped me in obtaining a variety of data and resources. My officemates Karen Knee and Matt Coble provided much helpful feedback and camaraderie. A special thanks is extended to my parents Sue Cornish and Steve Hughes and all family members who have unendingly encouraged (and financially supported) my academic endeavors. Finally, I could not have come this far without the support, help, advice and encouragement of my wonderful husband, Michael Cardiff. vi Table of Contents Abstract ................................................................................................................... iv Acknowledgements ............................................................................................. vi Table of Contents.................................................................................................vii List of Tables.......................................................................................................... xi List of Figures .......................................................................................................xii 1 Introduction..................................................................................................... 1 2 Silicic Calderas in Arc Settings: Characteristics, Distribution and Tectonic Controls.............................................................................................. 7 2.1 Abstract ................................................................................................................. 7 2.2 Introduction......................................................................................................... 8 2.3 Background: Choosing the Examined Parameters ...............................10 2.4 Data and Methods ............................................................................................13 2.4.1 Data Specific to Each Caldera............................................................................ 13 2.4.2 Data Specific to Arcs and Arc Sections .......................................................... 16 2.4.3 The Control Group: “Normal” Arc Volcanoes ............................................. 17 2.4.4 Across‐arc Distribution of Silicic Calderas .................................................. 18 2.5 Results and Discussion...................................................................................18 2.5.1 Caldera Attributes.................................................................................................. 19 2.5.2 Crustal Attributes................................................................................................... 19 vii 2.5.3 Tectonic Controls on Silicic Caldera Formation........................................ 22 2.5.4 Distance from Volcanic Front............................................................................ 29 2.5.5 Association With Structural Features............................................................ 31 2.6 Conclusions and ImPlications......................................................................32 2.6.1 Conclusions............................................................................................................... 32 2.6.2 Further Implications............................................................................................. 33 2.7 Tables...................................................................................................................37 2.8 Figures .................................................................................................................40 3 Tectonic settings of silicic calderas: analysis of a global compilation.......................................................................................................57 3.1 Abstract ...............................................................................................................57 3.2 Introduction.......................................................................................................59 3.3 Previous Work ..................................................................................................61 3.4 Data.......................................................................................................................63 3.4.1 Caldera‐specific Parameters.............................................................................
Load more

Recommended publications
  • A Chronology of Middle Missouri Plains Village Sites
    Smithsonian Institution Scholarly Press smithsonian contributions to botany • n u m b e r 9 2 Smithsonian Institution Scholarly Press TaxonomicA Chronology Revision of of the MiddleChiliotrichum Missouri Group Plains Villagesensu stricto Sites (Compositae: Astereae) By Craig M. Johnson Joséwith Mauricio contributions Bonifacino by Stanley A. Ahler, Herbert Haas, and Georges Bonani SERIES PUBLICATIONS OF THE SMITHSONIAN INSTITUTION Emphasis upon publication as a means of “diffusing knowledge” was expressed by the first Secretary of the Smithsonian. In his formal plan for the Institution, Joseph Henry outlined a program that included the following statement: “It is proposed to publish a series of reports, giving an account of the new discoveries in science, and of the changes made from year to year in all branches of knowledge.” This theme of basic research has been adhered to through the years by thousands of titles issued in series publications under the Smithsonian imprint, com- mencing with Smithsonian Contributions to Knowledge in 1848 and continuing with the following active series: Smithsonian Contributions to Anthropology Smithsonian Contributions to Botany Smithsonian Contributions in History and Technology Smithsonian Contributions to the Marine Sciences Smithsonian Contributions to Museum Conservation Smithsonian Contributions to Paleobiology Smithsonian Contributions to Zoology In these series, the Institution publishes small papers and full-scale monographs that report on the research and collections of its various museums and bureaus. The Smithsonian Contributions Series are distributed via mailing lists to libraries, universities, and similar institu- tions throughout the world. Manuscripts submitted for series publication are received by the Smithsonian Institution Scholarly Press from authors with direct affilia- tion with the various Smithsonian museums or bureaus and are subject to peer review and review for compliance with manuscript preparation guidelines.
    [Show full text]
  • Dynamics of Large Pyroclastic Currents Inferred by the Internal Architecture of the Campanian Ignimbrite Claudio Scarpati*, Domenico Sparice & Annamaria Perrotta
    www.nature.com/scientificreports OPEN Dynamics of large pyroclastic currents inferred by the internal architecture of the Campanian Ignimbrite Claudio Scarpati*, Domenico Sparice & Annamaria Perrotta Large ignimbrites are the product of devastating explosive eruptions that have repeatedly impacted climate and life on global scale. The assemblage of vertical and lateral lithofacies variations within an ignimbrite sheet, its internal architecture, may help to determine how the parental pyroclastic current evolves in time and space. The 39 ka Campanian Ignimbrite eruption, vented from Campi Flegrei caldera, laid down a thick ignimbrite over an area of thousands of km2. A detailed reconstruction of the vertical and lateral variation of the seven lithofacies recognised in the ignimbrite medial sequence constrains the behaviour of this event. The pyroclastic current fowed over a wide area around Campi Flegrei without depositing (bypass zone), and inundated a huge area during most of the paroxysmal, waxing phase, emplacing a mainly incipiently- to strongly- welded ignimbrite. Following this waxing phase, the leading edge of the current retreated back towards the source as the current waned, impacting a progressively smaller area and leaving an unconsolidated ash and lapilli deposit, later lithifed. Our study illustrates how large pyroclastic currents can evolve in time and space and the importance of both internal (eruptive and transport mechanisms) and external (topography, surfcial water and rain) factors in governing their behaviour. Catastrophic pyroclastic currents impact huge regions and represent one of the most devastating natural phenomena1. Large pyroclastic currents emplace thick sequences of ash- and vesiculated juvenile-rich deposits (ignimbrite2,3). Large ignimbrites show changes in facies on a regional scale 4.
    [Show full text]
  • Full-Text PDF (Final Published Version)
    Pritchard, M. E., de Silva, S. L., Michelfelder, G., Zandt, G., McNutt, S. R., Gottsmann, J., West, M. E., Blundy, J., Christensen, D. H., Finnegan, N. J., Minaya, E., Sparks, R. S. J., Sunagua, M., Unsworth, M. J., Alvizuri, C., Comeau, M. J., del Potro, R., Díaz, D., Diez, M., ... Ward, K. M. (2018). Synthesis: PLUTONS: Investigating the relationship between pluton growth and volcanism in the Central Andes. Geosphere, 14(3), 954-982. https://doi.org/10.1130/GES01578.1 Publisher's PDF, also known as Version of record License (if available): CC BY-NC Link to published version (if available): 10.1130/GES01578.1 Link to publication record in Explore Bristol Research PDF-document This is the final published version of the article (version of record). It first appeared online via Geo Science World at https://doi.org/10.1130/GES01578.1 . Please refer to any applicable terms of use of the publisher. University of Bristol - Explore Bristol Research General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/red/research-policy/pure/user-guides/ebr-terms/ Research Paper THEMED ISSUE: PLUTONS: Investigating the Relationship between Pluton Growth and Volcanism in the Central Andes GEOSPHERE Synthesis: PLUTONS: Investigating the relationship between pluton growth and volcanism in the Central Andes GEOSPHERE; v. 14, no. 3 M.E. Pritchard1,2, S.L. de Silva3, G. Michelfelder4, G. Zandt5, S.R. McNutt6, J. Gottsmann2, M.E. West7, J. Blundy2, D.H.
    [Show full text]
  • Bluestone Resources Inc. Initiating Coverage 2 May 2019
    Bluestone Resources Inc. Initiating Coverage 2 May 2019 Cerro Blanco - a golden opportunity Bluestone Resources Inc. (‘Bluestone Resources’) is a TSX-V listed, mine development company, developing the Cerro Blanco underground gold deposit in Guatemala. Cerro Blanco will be a high- Target price (CAD) 2.4 grade, low cost, gold mine that could produce 113koz/yr for an Share price (CAD) 1.1 initial eight years, at the low AISC of USD 579/oz, commencing in Q4 2021. Previous owners, Goldcorp, spent over USD 170m on the project, reducing the initial capital requirement while largely de- Ticker BSR.V, BSR.CN Sector Metals & Mining risking underground mine development and de-watering. We Shares fully diluted (m) 103.7 initiate coverage of Bluestone Resources with a BUY Market cap (USDm) 63.5 recommendation and a target price of CAD 2.40/sh. Net debt Q1'19 (USDm) (20) Minority interests (USDm) - Enterprise value (USDm) 44 Free float (%) 33 High grade – high margin – near term production The Cerro Blanco gold deposit is one of the highest-grade gold Performance deposits in the world, containing reserves of 0.94Moz at 8.5g/t gold CAD and 32.2g/t silver. The fully permitted mine will produce 113koz/yr at 1.50 the low AISC of USD 579/oz for its 8-year mine life, commencing in Q4 2021. 1.40 1.30 Resource upgrade to increase valuation by 10-12% 1.20 2019 drilling is targeting conversion of 0.20-0.25Moz of inferred resources, 60% of which could be converted to reserves for inclusion 1.10 within the mine plan.
    [Show full text]
  • Lombok Island, Sumbawa Island, and Samalas Volcano
    ECOLE DOCTORALE DE GEOGRAPHIE DE PARIS (ED 4434) Laboratoire de Géographie Physique - UMR 8591 Doctoral Thesis in Geography Bachtiar Wahyu MUTAQIN IMPACTS GÉOMORPHIQUES DE L'ÉRUPTION DU SAMALAS EN 1257 LE LONG DU DÉTROIT D'ALAS, NUSA TENGGARA OUEST, INDONÉSIE Defense on: 11 December 2018 Supervised by : Prof. Franck LAVIGNE (Université Paris 1 – Panthhéon Sorbonne) Prof. HARTONO (Universitas Gadjah Mada) Rapporteurs : Prof. Hervé REGNAULD (Université de Rennes 2) Prof. SUWARDJI (Universitas Mataram) Examiners : Prof. Nathalie CARCAUD (AgroCampus Ouest) Dr. Danang Sri HADMOKO (Universitas Gadjah Mada) 1 Abstract As the most powerful event in Lombok’s recent eruptive history, volcanic materials that were expelled by the Samalas volcano in 1257 CE covered the entire of Lombok Island and are widespread in its eastern part. Almost 800 years after the eruption, the geomorphological impact of this eruption on the island of Lombok remains unknown, whereas its overall climatic and societal consequences are now better understood. A combination of stratigraphic information, present-day topography, geophysical measurement with two-dimensional resistivity profiling technique, local written sources, as well as laboratory and computational analysis, were used to obtain detailed information concerning geomorphic impacts of the 1257 CE eruption of Samalas volcano on the coastal area along the Alas Strait in West Nusa Tenggara Province, Indonesia. This study provides new information related to the geomorphic impact of a major eruption volcanic in coastal areas, in this case, on the eastern part of Lombok and the western coast of Sumbawa. In the first place, the study result shows that since the 1257 CE eruption, the landscape on the eastern part of Lombok is still evolved until the present time.
    [Show full text]
  • Undiscovered Papua National Mask Festival
    UNDISCOVERED PAPUA NATIONAL MASK FESTIVAL One of the world’s most astonishing man-made spectacles, the Papua New Guinea national mask festival celebrates the unique mask cultures of the tribes of East New Britain: the Tolai, Baining, Pomio and Sulka. The festival itself is a four day extravaganza of storytelling, music, cultural dancing, and ritual performance. On the first day at dawn, the Kinavai is held, signalling the arrival of the Tolai Tubuans who arrive on canoes amidst the chanting and beating of kundu drums. Traditional masks will be on display during the festival – some many decades old, brought over from New Ireland Province and other parts of Papua New Guinea. Many of these masks are sacred and are not meant to be viewed, and the opening ceremony acts as a cleansing or appeasement ritual for the broken taboos. The highlight of the evening performances are the spectacular fire dances in which young initiated Baining men perform exhilarating rituals through blazing fires as their elders chant in haunting tones. During our stay we will also have time to visit wartime sites and WWII relics, travel through colourful welcoming villages and tour the rim of an active volcano. Attend the astonishing Kinavai ceremony on Kokopo beach at dawn. Witness the arrival of the Tolai people on canoes with wild drum beats. See traditional masks from all over Papua New Guinea. Be immersed by the performance of the Asaro mud-men warriors from the central highlands. Climb an active volcano and visit a vulcanology research station. Travel to the landing spot of Australian forces in World War One.
    [Show full text]
  • Geophysical and Geochemical Signals at Yellowstone and Other Large Caldera Systems
    Phil. Trans. R. Soc. A (2006) 364, 2055–2072 doi:10.1098/rsta.2006.1813 Published online 27 June 2006 Monitoring super-volcanoes: geophysical and geochemical signals at Yellowstone and other large caldera systems 1, 2 1 BY JACOB B. LOWENSTERN *,ROBERT B. SMITH AND DAVID P. HILL 1US Geological Survey, Volcano Hazards Team, MS 910, 345 Middlefield Road, Menlo Park, CA 94025, USA 2Department of Geology and Geophysics, University of Utah, 135 South, 1460 East, Room 702, Salt Lake City, UT 84112, USA Earth’s largest calderas form as the ground collapses during immense volcanic eruptions, when hundreds to thousands of cubic kilometres of magma are explosively withdrawn from the Earth’s crust over a period of days to weeks. Continuing long after such great eruptions, the resulting calderas often exhibit pronounced unrest, with frequent earthquakes, alternating uplift and subsidence of the ground, and considerable heat and mass flux. Because many active and extinct calderas show evidence for repetition of large eruptions, such systems demand detailed scientific study and monitoring. Two calderas in North America, Yellowstone (Wyoming) and Long Valley (California), are in areas of youthful tectonic complexity. Scientists strive to understand the signals generated when tectonic, volcanic and hydrothermal (hot ground water) processes intersect. One obstacle to accurate forecasting of large volcanic events is humanity’s lack of familiarity with the signals leading up to the largest class of volcanic eruptions. Accordingly, it may be difficult to recognize the difference between smaller and larger eruptions. To prepare ourselves and society, scientists must scrutinize a spectrum of volcanic signals and assess the many factors contributing to unrest and toward diverse modes of eruption.
    [Show full text]
  • Chiodi Et Al 2019.Pdf
    Journal of South American Earth Sciences 94 (2019) 102213 Contents lists available at ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames Preliminary conceptual model of the Cerro Blanco caldera-hosted geothermal system (Southern Puna, Argentina): Inferences from T geochemical investigations ∗ A. Chiodia, , F. Tassib,c, W. Báeza, R. Filipovicha, E. Bustosa, M. Glok Gallid, N. Suzañoe, Ma. F. Ahumadaa, J.G. Viramontea, G. Giordanof,g, G. Pecorainoh, O. Vasellib,c a Instituto de Bio y Geociencias del NOA (IBIGEO, UNSa-CONICET), Av. 9 de Julio14, A4405BBA Salta, Argentina b Department of Earth Sciences, University of Florence, Via La Pira 4, 50121 Florence, Italy c CNR-IGG Institute of Geosciences and Earth Resources, Via La Pira 4, 50121 Florence, Italy d Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires (CIFICEN), Pinto 399, 7000, Buenos Aires, Argentina e Universidad Nacional de Jujuy, Argentina f Department of Sciences, University Roma Tre, 00146 Rome, Italy g CNR-IDPA Institute for Dynamics of Environmental Processes, Via M. Bianco, 20131 Milan, Italy h Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Palermo, Via Ugo La Malfa 153, 90146, Palermo, Italy ARTICLE INFO ABSTRACT Keywords: The Cerro Blanco Caldera (CBC) is the youngest collapse caldera system in the Southern Central Andes (Southern Hydrothermal system Puna, Argentina). The CBC is subsiding with at an average velocity of 0.87 cm/year and hosts an active geo- Fluid geochemistry thermal system. A geochemical characterization of emitted fluids was carried out based on the chemical and Geothermal prospection isotopic compositions of fumaroles, and thermal and cold springs discharged in this volcanic area with the aim of Quaternary caldera constructing the first hydrogeochemical conceptual model and preliminary estimate the geothermal potential.
    [Show full text]
  • Assessing Pyroclastic Fall Hazard Through Field Data And
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Earth-prints Repository JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B2, 2063, doi:10.1029/2001JB000642, 2003 Assessing pyroclastic fall hazard through field data and numerical simulations: Example from Vesuvius Raffaello Cioni,1 Antonella Longo,2 Giovanni Macedonio,3 Roberto Santacroce,2 Alessandro Sbrana,2 Roberto Sulpizio,2 and Daniele Andronico4 Received 12 April 2001; revised 2 April 2002; accepted 7 May 2002; published 1 Feburary 2003. [1] A general methodology of pyroclastic fall hazard assessment is proposed on the basis of integrated results of field studies and numerical simulations. These approaches result in two different methods of assessing hazard: (1) the ‘‘field frequency,’’ based on the thickness and distribution of past deposits and (2) the ‘‘simulated probability,’’ based on the numerical modeling of tephra transport and fallout. The proposed methodology mostly applies to volcanoes that, by showing a clear correlation between the repose time and the magnitude of the following eruptions, allows the definition of a reference ‘‘maximum expected event’’ (MEE). The application to Vesuvius is shown in detail. Using the field frequency method, stratigraphic data of 24 explosive events in the 3–6 volcanic explosivity index range in the last 18,000 years of activity are extrapolated to a regular grid in order to obtain the frequency of exceedance in the past of a certain threshold value of mass loading (100, 200, 300, and 400 kg/m2). Using the simulated probability method, the mass loading related to the MEE is calculated based on the expected erupted mass (5 Â 1011 kg), the wind velocity profiles recorded during 14 years, and various column heights and grain-size populations.
    [Show full text]
  • 12. Eruption Alert at Rabaul Caldera: 1971–1994
    12. Eruption Alert at Rabaul Caldera: 1971–1994 In a statement issued in Papua New Guinea on Monday [23 January 1984], the principal volcanologist, Dr P. Lowenstein, said that ‘evidence is accumulating to suggest that the volcano has embarked on an irreversible course towards the next eruption and that it is only a matter of time before this occurs … the eruption that was previously only a possibility is now much more likely to occur within the next few months’. Peter Hastings (1984) Crisis Build-up and Stage-2 Alert Villagers living near the south-eastern end of Matupit Island, Rabaul, were by 1970–1971 aware that nearby coastal clis had encroached perilously close towards their homes as a result of sea-wave erosion. Their concerns were alleviated after 1971, however, when a new beach began to form at the foot of the pumice clis which gradually became stranded inland. The south-eastern end of the island was rising episodically, following the two, major, Solomon Sea earthquakes that had shaken Rabaul in July 1971. Rabaul Volcanological Observatory (RVO) sta led by Rob Cooke began measuring the amount of ground uplift in 1973 using a survey line that ran southwards from Rabaul town to the end of the island. About 60 centimetres of uplift had been detected by 1979, the year of Cooke’s death, and to more than a metre by 1983.1 Matupit is a low-lying island that in 1971 was already well known for its vertical oscillations, most noticeably at times of major earthquakes or tsunamis when a causeway linking the island with the shore might disappear and then reform.2 The island is made up of at-lying pumice beds, but there is no persuasive geological evidence that it is, or ever was, an eruptive centre.
    [Show full text]
  • Geology and Petrology of Rabaul Caldera, Papua New Guinea
    Geology and Petrology of Rabaul Caldera, Papua New Guinea R. F. HEMING* Department of Geology and Geophysics, University of California, Berkeley, California 94720 ABSTRACT parison with experimental data for high- off the north coast of New Guinea and ter- alumina basalt also suggests a shallower minates at Rabaul. Only the New Britain Rabaul caldera is unusual in that it was origin. A progressive increase in Ti02, section has the classical features of an is- formed by two episodes of construction and A1203, and alkalis along the New land arc: a deep submarine trench to the collapse on an older basalt volcano. One Britain—New Guinea arc toward Rabaul at south, an inclined seismic zone dipping collapse occurred around 3,500 yr B.P. and its eastern end cannot be explained. Key northward, and a string of volcanoes on the the latest around 1,400 yr B.P. Both were words: igneous petrology, volcanology. side of the island opposite the trench. West accompanied by the eruption of daciiic of New Britain, the trench disappears, and pumiceous ash flows. Following initial col- INTRODUCTION in the vicinity of Long Island (Fig. 1), the lapse, volcanism was confined to a large Rabaul caldera, situated at the northeast- simple Benioff zone is replaced by a north- andesite volcano in the southern part of the ern tip of the island of New Britain, is the and a south-dipping zone (Denham, 1969). caldera, but a renewal of basaltic volcanism easternmost volcano of the New West of here, the volcanic arc lies well occurred at a point on the eastern ring frac- Guinea—New Britain arc (Fig.
    [Show full text]
  • Fisher Caldera, Unimak Island, Aleutians
    Journal of Volcanology and Geothermal Research 111 (2001) 35±53 www.elsevier.com/locate/jvolgeores Low-d 18O tephra from a compositionally zoned magma body: Fisher Caldera, Unimak Island, Aleutians Ilya N. Bindeman*, John H. Fournelle, John W. Valley Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, WI 53706, USA Received 13 August 2000; revised 21 January 2001; accepted 20 February 2001 Abstract We present the results of an oxygen isotope study of phenocrysts in pumice clasts and ash layers produced by the 9100 yr BP composite dacite-basaltic andesite climactic eruption that formed Fisher Caldera in the eastern Aleutians. Products of the eruption represent a low-d 18O magma with d 18O plagioclase (14.79 ^ 0.24½) and clinopyroxene (3.81 ^ 0.23½) correspond- ing to equilibrium at magmatic temperatures. Dacitic and overlying basaltic±andesitic tephra of the climactic eruption, subsequent intracaldera basaltic to andesitic lavas, and a cumulate inclusion, are similarly low in d 18O. Other analyzed lavas and pyroclastics of Unimak island and the lower Alaska peninsula, as well as precaldera Fisher basalt, have normal d 18O magmatic values (.15.5½). We propose a model in which prior to 9100 yr BP, normal mantle-derived basaltic magma coalesced in a large shallow precaldera magma chamber during Late Wisconsin glaciation. Lowering of magmatic d 18O resulted then from long-term assimilation of ,5±10% of syn-glacial hydrothermally-altered country rocks. Differentiation of basaltic magma was concurrent with this assimilation and produced low-d 18O Fisher dacites, cumulates, and post-caldera crystal-richer lavas.
    [Show full text]