PROCEEDINGS OF THE TWENTY-FOURTH ANNUAL KECK RESEARCH SYMPOSIUM IN GEOLOGY
April 2011 Union College, Schenectady, NY
Dr. Robert J. Varga, Editor Director, Keck Geology Consortium Pomona College
Dr. Holli Frey Symposium Convenor Union College
Carol Morgan Keck Geology Consortium Administrative Assistant
Diane Kadyk Symposium Proceedings Layout & Design Department of Earth & Environment Franklin & Marshall College
Keck Geology Consortium Geology Department, Pomona College 185 E. 6th St., Claremont, CA 91711 (909) 607-0651, [email protected], keckgeology.org
ISSN# 1528-7491
The Consortium Colleges The National Science Foundation ExxonMobil Corporation
KECK GEOLOGY CONSORTIUM PROCEEDINGS OF THE TWENTY-FOURTH ANNUAL KECK RESEARCH SYMPOSIUM IN GEOLOGY ISSN# 1528-7491
April 2011
Robert J. Varga Keck Geology Consortium Diane Kadyk Editor and Keck Director Pomona College Proceedings Layout & Design Pomona College 185 E 6th St., Claremont, CA Franklin & Marshall College 91711
Keck Geology Consortium Member Institutions: Amherst College, Beloit College, Carleton College, Colgate University, The College of Wooster, The Colorado College, Franklin & Marshall College, Macalester College, Mt Holyoke College, Oberlin College, Pomona College, Smith College, Trinity University, Union College, Washington & Lee University, Wesleyan University, Whitman College, Williams College
2010-2011 PROJECTS
FORMATION OF BASEMENT-INVOLVED FORELAND ARCHES: INTEGRATED STRUCTURAL AND SEISMOLOGICAL RESEARCH IN THE BIGHORN MOUNTAINS, WYOMING Faculty: CHRISTINE SIDDOWAY, MEGAN ANDERSON, Colorado College, ERIC ERSLEV, University of Wyoming Students: MOLLY CHAMBERLIN, Texas A&M University, ELIZABETH DALLEY, Oberlin College, JOHN SPENCE HORNBUCKLE III, Washington and Lee University, BRYAN MCATEE, Lafayette College, DAVID OAKLEY, Williams College, DREW C. THAYER, Colorado College, CHAD TREXLER, Whitman College, TRIANA N. UFRET, University of Puerto Rico, BRENNAN YOUNG, Utah State University.
EXPLORING THE PROTEROZOIC BIG SKY OROGENY IN SOUTHWEST MONTANA Faculty: TEKLA A. HARMS, JOHN T. CHENEY, Amherst College, JOHN BRADY, Smith College Students: JESSE DAVENPORT, College of Wooster, KRISTINA DOYLE, Amherst College, B. PARKER HAYNES, University of North Carolina - Chapel Hill, DANIELLE LERNER, Mount Holyoke College, CALEB O. LUCY, Williams College, ALIANORA WALKER, Smith College.
INTERDISCIPLINARY STUDIES IN THE CRITICAL ZONE, BOULDER CREEK CATCHMENT, FRONT RANGE, COLORADO Faculty: DAVID P. DETHIER, Williams College, WILL OUIMET. University of Connecticut Students: ERIN CAMP, Amherst College, EVAN N. DETHIER, Williams College, HAYLEY CORSON-RIKERT, Wesleyan University, KEITH M. KANTACK, Williams College, ELLEN M. MALEY, Smith College, JAMES A. MCCARTHY, Williams College, COREY SHIRCLIFF, Beloit College, KATHLEEN WARRELL, Georgia Tech University, CIANNA E. WYSHNYSZKY, Amherst College.
SEDIMENT DYNAMICS & ENVIRONMENTS IN THE LOWER CONNECTICUT RIVER Faculty: SUZANNE O’CONNELL, Wesleyan University Students: LYNN M. GEIGER, Wellesley College, KARA JACOBACCI, University of Massachusetts (Amherst), GABRIEL ROMERO, Pomona College.
GEOMORPHIC AND PALEOENVIRONMENTAL CHANGE IN GLACIER NATIONAL PARK, MONTANA, U.S.A. Faculty: KELLY MACGREGOR, Macalester College, CATHERINE RIIHIMAKI, Drew University, AMY MYRBO, LacCore Lab, University of Minnesota, KRISTINA BRADY, LacCore Lab, University of Minnesota Students: HANNAH BOURNE, Wesleyan University, JONATHAN GRIFFITH, Union College, JACQUELINE KUTVIRT, Macalester College, EMMA LOCATELLI, Macalester College, SARAH MATTESON, Bryn Mawr College, PERRY ODDO, Franklin and Marshall College, CLARK BRUNSON SIMCOE, Washington and Lee University.
GEOLOGIC, GEOMORPHIC, AND ENVIRONMENTAL CHANGE AT THE NORTHERN TERMINATION OF THE LAKE HÖVSGÖL RIFT, MONGOLIA Faculty: KARL W. WEGMANN, North Carolina State University, TSALMAN AMGAA, Mongolian University of Science and Technology, KURT L. FRANKEL, Georgia Institute of Technology, ANDREW P. deWET, Franklin & Marshall College, AMGALAN BAYASAGALN, Mongolian University of Science and Technology. Students: BRIANA BERKOWITZ, Beloit College, DAENA CHARLES, Union College, MELLISSA CROSS, Colgate University, JOHN MICHAELS, North Carolina State University, ERDENEBAYAR TSAGAANNARAN, Mongolian University of Science and Technology, BATTOGTOH DAMDINSUREN, Mongolian University of Science and Technology, DANIEL ROTHBERG, Colorado College, ESUGEI GANBOLD, ARANZAL ERDENE, Mongolian University of Science and Technology, AFSHAN SHAIKH, Georgia Institute of Technology, KRISTIN TADDEI, Franklin and Marshall College, GABRIELLE VANCE, Whitman College, ANDREW ZUZA, Cornell University.
LATE PLEISTOCENE EDIFICE FAILURE AND SECTOR COLLAPSE OF VOLCÁN BARÚ, PANAMA Faculty: THOMAS GARDNER, Trinity University, KRISTIN MORELL, Penn State University Students: SHANNON BRADY, Union College. LOGAN SCHUMACHER, Pomona College, HANNAH ZELLNER, Trinity University.
KECK SIERRA: MAGMA-WALLROCK INTERACTIONS IN THE SEQUOIA REGION Faculty: JADE STAR LACKEY, Pomona College, STACI L. LOEWY, California State University-Bakersfield Students: MARY BADAME, Oberlin College, MEGAN D’ERRICO, Trinity University, STANLEY HENSLEY, California State University, Bakersfield, JULIA HOLLAND, Trinity University, JESSLYN STARNES, Denison University, JULIANNE M. WALLAN, Colgate University.
EOCENE TECTONIC EVOLUTION OF THE TETONS-ABSAROKA RANGES, WYOMING Faculty: JOHN CRADDOCK, Macalester College, DAVE MALONE, Illinois State University Students: JESSE GEARY, Macalester College, KATHERINE KRAVITZ, Smith College, RAY MCGAUGHEY, Carleton College.
Funding Provided by: Keck Geology Consortium Member Institutions The National Science Foundation Grant NSF-REU 1005122 ExxonMobil Corporation
Keck Geology Consortium: Projects 2010-2011 Short Contributions— Volcán Barú, Panama
LATE PLEISTOCENE EDIFICE FAILURE AND SECTOR COLLAPSE OF VOLCÁN BARÚ, PANAMA Project Faculty: THOMAS GARDNER, Trinity University, KRISTIN MORELL, Penn State University
PETROLOGIC EVIDENCE FOR MAFIC RECHARGE AT VOLCÁN BARÚ, PANAMA SHANNON BRADY, Union College Research Advisor: Holli Frey
VOLUME CONSTRAINT AND POTENTIAL SECONDARY VOLUME INPUTS OF LATE PLEISTOCENE AGE SECTOR COLLAPSE, VOLCÁN BARÚ, PANAMA LOGAN SCHUMACHER, Pomona College Research Advisor: Eric Grosfils
VOLCÁN BARÚ DEBRIS AVALANCHE FACIES AND AGES HANNAH ZELLNER, Trinity University Research Advisor: Thomas Gardner
Keck Geology Consortium Pomona College 185 E. 6th St., Claremont, CA 91711 Keckgeology.org
24th Annual Keck Symposium: 2011 Union College, Schenectady, NY PETROLOGIC EVIDENCE FOR MAFIC RECHARGE AT VOLCÁN BARÚ, PANAMA
SHANNON BRADY, Union College Research Advisor: Holli Frey
INTRODUCTION et al. 2007). Volcán Barú lies inboard of the Panama The purpose of this study is to understand a large Triple Junction (PTJ), where there is convergence of sector collapse at Volcán Barú that occurred dur- the Cocos, Nazca, and Caribbean plates (Gardner, Fig. ing the Late Pleistocene and destabilized the south- 1, this volume). The tectonic setting of Volcán Barú is western flank of the edifice. The most profound complex due to oblique, aseismic, shallow slab subduc- physical evidence of this sector collapse is the vast tion of the Nazca plate beneath Panama, where a clear deposit of hummocks and lahars. The initial purpose Benioff zone does not exist below the volcanic arc of this project was to determine the distribution of in western Panama (Sherrod et al. 2007, Defant et al. different lithologies at the cores of hummocks and 1992). The edifice of Volcán Barú, which was built up correlate them to a pre-collapse edifice. However, by numerous lava flows, pyroclastic flows, and lahar the geographic distribution of lithologies, defined deposits, may have reached up to 4,000 m in eleva- by geochemistry and mineralogy, does not provide tion and 250-350 km3 in volume prior to the collapse significant insight to understand the mechanism of of its western flank (Sherrod et al. 2007). Volcanism is emplacement by debris avalanches. Yet, an interest- thought to have commenced ca. 0.5 Ma and Quaternary ing petrologic story has emerged from disequilibrium products at Volcán Barú are andesite-dacite in composi- mineral textures observed in samples taken from these tion (Sherrod et al. 2007, Hidalgo and Rooney 2010). hummocks. Reaction rims on hornblende phenocrysts There have been four major episodes of dome growth, in conjunction with crystal aggregates and dusty explosive eruptions, and block-and-ash flows over the sieve-textured plagioclase suggest disequilibrium, past 1,600 years, and recent seismicity since 2006 sug- possibly due to the injection of a hotter mafic batch of gests that it is still active (Sherrod et al., 2007). magma into the andesitic storage chamber of Volcán Barú (Rutherford and Devine, 2003, Plechov et al., FIELD METHODS 2008, Seaman, 2000). Opacitization of hornblende Rock samples were collected from the cores of hum- phenocrysts further suggests that hornblende was mocks and from the modern volcanic edifice of Vol- raised out of its stability field by decompression dur- cán Barú. Large hummocks proximal to the modern ing ascent. The combination of an increase in tem- volcanic edifice consist of closely spaced brecciated perature and a decrease in pressure of the pre-existing megaclasts of andesite composition that are shattered andesite magma may have possibly triggered eruptive throughout (Fig. 1C). Hummocks distal to the volcano activity at Barú (Plechov et al. 2008, Eichelberger and consist of loosely spaced brecciated megaclasts with Izbekov 2000). an inter-clast matrix of silt and mud. Megaclasts are inferred to have been previously intact wall rock from GEOLOGIC SETTING the pre-collapse volcanic edifice, transported during Volcán Barú is an active, calc-alkaline, stratovolcano the sector collapse (Glicken, 1991). Fresh exposures located in the Chiriquí Province of western Panama of hummocks were located in quarries, hydroelectric (Gardner, Fig 2, this volume), 35 km from the Costa dam construction sites along rivers, and road cuts, yet Rican border at the southern end of the Central they are rare because the longevity of these exposures American Volcanic Arc (CAVA) (Fig. 1). The sum- is limited due to rapid vegetation growth and chemical mit of Volcano Barú reaches 3,374 m in elevation weathering in the tropical climate of western Panama. and the edifice covers an area of 280 km2 (Sherrod Twenty-six samples were collected in the field and se
286 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY point-counter. Samples were also crushed using RockLabs® Laboratory Hydraulic Crusher/Breaker and powdered using RockLabs® Laboratory ring and puck mill. Major element geochemistry was performed at ACME Analytical Laboratory LTD in Vancouver, British Columbia, Canada. Trace element geochemistry was performed at Union College us- ing Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) following high-pressure HF digestion. Photomicrographs were taken using PAXcam digital microscope camera. Backscattered Electron images were acquired using the Zeiss EVO50XVP Scanning Electron Microscope.
RESULTS & OBSERVATIONS Samples from the hummocks and edifice range in composition from 51.8 to 61.7 wt. % SiO2. Samples from a newly discovered basaltic cinder cone and lava Figure 1. (A) Location of Volcán Barú in western Pana- flow have 43.7 and 45.9 wt. % SiO2 respectively (Fig. ma. (B) Location map of samples from hummocks within the debris avalanche zone (blue shaded region) and 2). Based on field relations, the cinder cones appear from the edifice of Volcán Barú. Purple squares indicate to post-date the sector collapse, but their relationship hornblende-bearing samples with fine-grained cores, to Volcán Barú and the sector collapse remains un- yellow triangles indicate hornblende-bearing samples der investigation. Mineral phases present within the with fine-grained rims, cyan circles indicate samples andesite debris avalanche blocks include plagioclase, without hornblende. Mafic samples are green diamonds clinopyroxene, orthopyroxene, ± hornblende, Fe-Ti and the orange hexagon outside the debris avalanche is a oxides, and trace amounts of olivine, apatite, quartz, hornblende-bearing sample without any reaction rims or and biotite in decreasing abundance. Based on point- evidence of disequilibria. (C) Hummock exposure proxi- counts, the average phenocryst total is 56.8%. Horn- mal to the edifice of Volcán Barú displaying an intact, yet blende bearing samples have concentrations that vary well-shattered andesite block. from 0.4-18.7 %. Three lithofacies are defined in this study based on the mineralogy and textures observed lected for analysis based on their location with respect in the Barú samples: 1) samples without hornblende, to the edifice and the type of facies comprising the 2) samples with fine-grained pyroxene dominant rims hummock. The UTM coordinates, size of the hum- around hornblende grains, and 3) samples with fine- mock or edifice exposure, the size of debris-avalanche grained crystal aggregates at the interior of horn- blocks, degree of shattering, largest clast, smallest blende grains (Fig. 1, 2). clast, and mineral phases present in hand sample were recorded at each outcrop. Sample sites were plotted Plagioclase on ESRI ArcMap 9.3 to record their geographic loca- Plagioclase phenocrysts are calcic in composition tion within the debris avalanche zone and with respect based on SEM EDS (energy dispersive spectrometry) to the edifice (Fig 1B). analyses. Plagioclase crystals display a dusty sieve- texture, common throughout most samples from Barú. LAB METHODS This sieve texture appears as a corroded interior with Samples were prepared for geochemical and petro- a clean outer rim. The core may consist of tiny glass graphic analyses at Union College in Schenectady, particulates (Fig 3A). New York. Rocks were cut into thin sections for observation under a petrographic microscope and Mafic Glomerocrysts to determine mineral phase percentages using a Mafic glomerocrysts occur as fine-grained cumulates 287
24th Annual Keck Symposium: 2011 Union College, Schenectady, NY of microcrystalline clinopyroxene, orthopyroxene, plagioclase, and Fe-Ti oxides in the groundmass. Clinopyroxene appears to be the dominant mineral phase. Crystals within the clots do not appear to form along euhedreal faces, rather they are subhedral to anhedral.
Figure 2. Total alkalies v. silica discriminant diagram for Volcán Barú samples. Classification grid from Lebas et al. (1986).
Hornblende The first type of reaction rim on hornblende in the an- Figure 3. (A-D) Photomicrographs of disequilibrium desite samples from Volcán Barú is defined by crystal mineral textures from Volcán Barú hornblende-bearing aggregates: it is a fine-grained rim of clinopyroxene, samples in transmitted light. (A) Hornblende phenocrysts orthopyroxene, plagioclase, and Fe-Ti oxides, with displaying opacitization and fine-grained core, dusty sieve- clinopyroxene as the dominant mineral phase (Fig. textured plagioclase. (B) Opactized hornblende pheno- 3E-F). These rims appear to shield hornblende phe- cryst with fine-grained core. (C) Opacitized hornblende phenocrysts with high-Ca fine-grained rims. (D) Clino- nocrysts where the crystal is in contact with the melt pyroxene crystal aggregate. (E-H) Backscattered electron (Fig. 3C). Where fine-grained rims are absent, horn- images from the SEM. (E) Hornblende phenocryst with blende-bearing samples display fine-grained microlite opacitization along crystal cleavage planes (white). (F) cores also consisting of pyroxene, plagioclase and ox- Magnified fine-grained rim of cpx, plag, and Fe-Ti oxides ide crystals (Fig. 3B, 3H). Overall, crystal aggregates from (E). (G) Fine-grained rim around hornblende. (H) occur as fine-grained reaction rims around hornblende Hornblende displaying good cleavage, thin reaction rim, grains, within the interior of hornblende grains, and and fine-grained core. also as individual glomerocrysts (Fig. 3D). The second type of reaction rim around hornblende grains is characterized by varying degrees of oxida- All hornblende-bearing samples from Barú demon- tion (Fig. 3A-C). Rutherford and Devine (2003) refer strate opacitization to some degree and the opacitized to this reaction as opacitization. Opacitization is not hornblende grains can be organized into three stages. limited to crystal rims where the hornblende is in con- [1] The first degree of opacitization is a distinct and tact with the melt because this reaction occurs along easily identifiable oxide rim around a hornblende crystal margins, fractures, and cleavage planes of grain, which is also clearly recognizable with its 56º individual hornblende phenocrysts. Some hornblende and 124º cleavage planes. This oxide rim can be eas- grains also display an oxide core (Fig. 3C). ily measured. [2] The second degree is characterized 288 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY by hornblende phenocrysts that have been almost en- an increase in temperature due to a mafic injection tirely consumed by oxides and there is no distinct rim and/or a decrease in pressure that causes dehydration around the crystal. Hornblende appears “moth-eaten” of the melt during ascent (Rutherford and Hill, 1993) due to oxidation along fractures and cleavage planes; (Fig. 5). however, the brown-greenish hue of the amphibole grain is still visible in the core of the crystal. [3] The third and final stage of opacitization is characterized by psuedomorphism. Hornblende phenocrysts have become entirely oxidized to the core, displaying an opaque appearance. Psuedomorphs are distinguish- able from Fe-Ti oxides because they have a fuzzy appearance and maintain the diamond or lath shape of the original hornblende crystal. All three stages of opacitization have been observed to coexist in a single sample. Smaller hornblende crystals become completely opacitized first, as op- posed to coexisting larger crystals that demonstrate Figure 4. Chondrite-normalized versus rare earth element thin oxide rims. The distribution of varying degrees diagram for each lithofacies defined in this study, includ- of opacitization appears to occur randomly within ing the mafic samples, plotted in comparison to the summit each thin section. domes unit (fine-grained cumulates + debris avalanche block) addressed by Hidalgo and Rooney (2010). INTERPRETATIONS Hidalgo and Rooney (2010) have previously ad- dressed the presence of amphibole-cumulates as evidence for deep crystal fractionation processes beneath Volcán Barú. They have proposed a crystal mush model for an amphibole sponge deep within the Panamanian arc. The geochemical values and mineral contents of samples in this study compare well with the findings of Hidalgo and Rooney (2010). How- ever, Hidalgo and Rooney’s sample/chondrite versus REE values for the summit domes unit are slightly more enriched in HREE than Barú samples (Fig. 4). Fine-grained and coarse-grained amphibole cumulates are present in Barú samples from this study; how- ever, they are in trace amounts. Hidalgo and Rooney (2010) do not have any record of reaction rims on amphiboles from Quaternary volcanism at Barú. Therefore, new evidence for hornblende disequilibria in this study can provide more information about the petrologic conditions operating beneath Volcán Barú. Figure 5. Pressure-temperature phase diagram for Volcán Barú andesite illustrating the stability field of hornblende Empirical studies reveal that the crystallization in red at pressures S>1000 bars based on the partial conditions of hornblende require pressures greater pressure of water and temperatures <950oC. The arrow than 1 kbar and an upper temperature limit of 950oC represents the adiabatic ascent path of magma leaving the (Plechov et al. 2008). Therefore, the disequilibrium stability field of hornblende. Modified from Moore and textures observed in Barú samples may imply that Carmichael (1998). hornblende was raised outside of its stability field by 289 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY Crystal aggregates, especially in the form of fine- al. 2000). Mafic recharge into a silicic chamber can grained rims on hornblende phenocrysts, and the occur via dykes and is likely to result in an effusive dusty sieve-texture of plagioclase suggest disequi- style eruption. The two end member magmas can libria by reheating (Rutherford and Devine, 2003, either become well-blended within the storage cham- Plechov et al., 2008). The presence of crystal aggre- ber or the denser mafic batch of magma can pond at gates and fine-grained rims are products of amphibole the bottom and simply heat the more silicic magma breakdown and replacement (Garcia and Jacobson, (Eichelberger and Izbekov, 2000). 1979). As the host magma is heated to temperatures above the stability limit of hornblende, there is a CONCLUSION decrease in water in the melt and that promotes the Mafic recharge is a commonly accepted mechanism growth of anhydrous minerals by the exchange of for eruption trigger in arc volcanoes. Replenishment components from hornblende phenocrysts (Murphy et of a silicic magma chamber with more mafic magma al., 2000). has been used to explain the eruptive episodes at According to Seaman (2000), dusty plagioclase Bezymyannyi, Karymsky, and Soufriere Hills Volca- phenocrysts form by resorption of more albitic plagio- noes to name a few (Plechov et al. 2008, Eichelberger clase due to an influx of calcic magma into the storage and Izbekov 2000, Murphy et al. 2000, Rutherford chamber. The “dusty” interior of the plagioclase may and Devine 1993). Based on the disequilibrium have formed by an initial injection of hotter magma mineral textures observed in andesite samples from that disrupted the equilibrium of the crystal. After Volcán Barú, it is possible that Barú has also expe- the injection of a hotter, more calcic, mafic magma rienced episodes of mafic recharge that remobilized into the cooler andesite magma, an overgrowth rim host magmas and possibly triggered an eruption. may have formed due to the crystallization of An-rich plagioclase during cooling (Murphy, 2000) Opacite rims are attributed to the partial breakdown REFERENCES of hornblende as the magma was raised outside the hornblende stability field by means of a decrease in Defant, M.J., Jackson, T.E., Drummond, M.S., DeBoer, pressure during ascent. 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