KECK GEOLOGY CONSORTIUM

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 & 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, & 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, 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— Mountains

KECK SIERRA: MAGMA-WALLROCK INTERACTIONS IN THE SEQUOIA REGION Project Faculty: JADE STAR LACKEY, Pomona College, STACI L. LOEWY, California State University—Bakersfield

ORIGIN OF MIGMATITIC ROCKS IN THE SEQUOIA PENDANT, SIERRA NEVADA, CALIFORNIA MARY BADAME, Oberlin College Research Advisor: Steve Wojtal

PLUTON-WALLROCK INTERACTION OF THE EMPIRE QUARTZ DIORITE, SOUTHERN SIERRA NEVADA: IMPLICATIONS FOR SKARN FORMATION IN THE PENDANT MEGAN D’ERRICO, Trinity University Research Advisor: Dr. Benjamin Surpless

TEMPORAL VARIATION IN PLUTON-WALLROCK INTERACTION IN THE SIERRAN ARC STANLEY HENSLEY, California State University, Bakersfield Research Advisor: Dr. Staci Loewy

THE PETROGENESIS OF THE ASH MOUNTAIN INTRUSIVE COMPLEX: IMPLICATIONS FOR SIERRAN MAGMATISM JULIA HOLLAND, Trinity University Research Advisor: Ben Surpless

EARLY SIERRA NEVADA MAGMATISM EXAMINED USING SHRIMP-RG U-PB AGES AND TRACE ELEMENT COMPOSITIONS OF ZIRCONS FROM THE MINERAL KING ROOF PENDANT RHYOLITE UNITS JESSLYN STARNES, Denison University Research Advisor: Dr. Erik Klemetti

STABLE ISOTOPE GEOCHEMISTRY OF MARBLES IN THE KINGS SEQUENCE, SIERRA NEVADA, CA JULIANNE M. WALLAN, Colgate University Research Advisor: William H. Peck

Keck Geology Consortium Pomona College 185 E. 6th St., Claremont, CA 91711 Keckgeology.org

24th Annual Keck Symposium: 2011 Union College, Schenectady, NY PLUTON-WALLROCK INTERACTION OF THE EMPIRE QUARTZ DIORITE, SOUTHERN SIERRA NEVADA: IMPLICATIONS FOR SKARN FORMATION IN THE EMPIRE MOUNTAIN PENDANT

MEGAN D’ERRICO, Trinity University Research Advisor: Dr. Benjamin Surpless

INTRODUCTION GEOLOGIC SETTING

Skarns are recognized as important products and The Lower Mesozoic metavolcanic and metasedimen- indicators of metamorphic conditions, with the most tary rocks of the Mineral King roof pendant are located common skarns resulting from the intrusion of felsic among the calc-alkaline granitoid belt of the former magmas into calcareous wallrocks. Skarn deposits southern Sierra Nevada arc (Lackey and Loewy, this contain critical information regarding the sources volume, Fig. 1). Busby and Saleeby (1987) summa- of fluids and dissolved components, as well as the rized the stratigraphic and magmatic evolution of the physio-chemical processes by which pre-existing Mineral King area, documenting the Triassic-Jurassic crust and intruding magma interact. The nature of metamorphic units which are highly deformed, steeply this interaction is fundamentally controlled by the dipping pendants between Cretaceous plutons. The protolith composition, temperature, spatial distance hypabyssal intrusion of the Early Cretaceous Empire from the contact and temporal evolution of fluids (e.g. quartz diorite into calcareous wallrocks resulted in

Clechenko and Valley, 2003). In the Mineral King the pervasive calcic skarn mineralization (unit CS5) of pendant, we characterize the metasomatic fluid flow the Empire Mountain pendant. The protoliths of calc- processes controlling growth, zoning and nucleation silicate units CS1 to CS5 are thought to have recorded of skarn minerals as well as the igneous history of the period of volcanic and tectonic quiescence (Busby and exposed Empire quartz diorite pluton and associated Saleeby, 1987). Busby and Saleeby (1987) reported an skarn to determine the nature of intrusion and evolu- U-Pb zircon age of 107 Ma for the Empire pluton, and tion of the hydrothermal system (e.g., Einaudi and overlapping ages of 99 Ma and 98 Ma for the east and Burt, 1982; Bowman, J. R., 1998; and Meinert et al., west binding plutons, the Eagle Lake quartz monzodio- 2005). rite and the granite, respectively (Fig.1). Mackenzie (1983) conducted an extensive study of Excellent outcrop exposure at Empire Mountain (Fig. the Mineral King mining district, and reported mineral 1), provides a rare opportunity to examine the shallow phases and field relationships consistent with those (1-2 kbar; Nadin and Saleeby, 2008) intrusion of the documented in this study. Pb-Zn-Ag sulfide depos- Early Cretaceous Empire quartz diorite into calcare- its and associated skarns of the Empire Mine (Fig. 1) ous wallrocks. The three dimensional exposure at classified by Mackenzie (1983) were thought to have Empire Mountain displays the relationships between formed through a two-stage process of prograde skarn the intruding Empire pluton, skarns and calc-silicate development followed by fracturing of roof pendant wall rocks. We present new U-Pb geochronology data rocks, where declining temperature and pH fluctua- and Ti-in-zircon temperature estimates for the Empire tions in meteoric water controlled the development and quartz diorite pluton, and we evaluate trace element replacement of skarn mineral assemblages with later and oxygen isotope analyses of skarn-forming miner- stage open-space precipitation. We intend to build upon als and the pluton that record rock-fluid interactions at his interpretation and develop a model of skarn forma- Empire Mountain. tion and retrograde mineralization.

313 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY FIELD RELATIONSHIPS color change is observed within the calc-silicate rock. Composed of small (~1 mm), euhedral garnet, the up- At Empire Mountain, there is a well-exposed, three- per unit of the pendant is a red-brown garnetite (Site dimensional view of the relationships and contacts C; Fig. 2), while the lower unit is tan-brown garnetite between the three major rock types: the Empire quartz (Site B; Fig. 2). The tan-brown garnetite is fine- diorite, skarns dominated by red-brown or tan-brown grained and mostly granoblastic grossular garnet; in garnetite, and calc-silicate rock (Fig. 2). Representa- contrast, the red-brown unit displays early granoblas- tive light to medium grey quartz diorite samples from tic andradite and late grossular vein fillings, where the unaltered cupola zone of the Empire pluton show the observed growth direction displays end member little modal variation, with ~40% plagioclase, ~20% oscillations and fills in late vein areas (Fig. 3D). A alkali feldspar, ~10 % quartz, and ~30 % ferromagne- sub-horizontal, 1 – 3 m thick zone of vug-filling, sian minerals (biotite, hornblende and orthopyroxene, large, euhedral garnets (approximately 10 - 15 cm in decreasing order). Map unit CS5 is a massive, iron- in diameter) and late stage quartz (Fig. 3C) marked rich calc-silicate rock with a very fine average grain the approximate boundary between the two garnetite size (<0.2 mm), so mineral identification is difficult. units (Fig. 1). The large garnet crystals within the However, garnet is ubiquitous and abundant proximal quartz matrix display macroscopic 2 – 5 cm zoning to the Empire Mine (Fig. 1), consistent with the find- ings of Mackenzie (1983).

In contrast, former originally calcareous wallrock from the ridgetop pendants has been completely replaced by calc-silicate skarn minerals, with gar- net significantly more abundant than clinopyroxene (Site B, C; Fig. 2). In the field, a vertically stratified

Figure 2. Geologic map of Empire Mountain pendants draped over topography in Google Earth and oblique Figure 1. Geologic map of Mineral King (modified from photo displaying field relationships. Site A) West contact Saleeby et. al, 1993) draped over topography in Google between calc-silicate (CS5) rock and Empire quartz diorite Earth. Map shows the dominantly Triassic-Jurassic (QD); Site B) Tan-brown garnetite pendants; Site C) Em- metavolcanic and metasedimentary units intruded by three pire Mountain consisting mostly of red-brown garnetite; Cretaceous plutons that make up the metamorphic wall- Site D) Later stage concentrated subhedral to euhedral rocks of the King Sequence. Empire Mountain pendant epidote and calcite, observed at high angles to garnetite study area is boxed in white. contact. 314 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY in color from bright red cores to tan-brown rims. The diorite pervasively intruded and appears to have initi- position of the sub-horizontal boundary zone between ated the ductile deformation of the garnetite. How- garnetite units of different compositions suggests that ever, west of this transition zone (<50 m from Fig fluid pressures might have opened space between 3A) in the cupola zone of the quartz diorite, apparent protolith bedding planes and precipitated garnet and products of spalling from the overlying calc-silicate quartz in that space. The macroscopic zoning ob- pendant, occur as xenoliths of the garnetite within the served in garnet, observed both on the pendant scale Empire quartz diorite. In these exposures, the calc- and in large garnet crystals from the boundary zone, silicate rock becomes progressively dark red-brown indicates that variations in fluid composition played garnet rich toward the quartz diorite contact over 5 an important role in the hydrothermal system. The cm – 0.5m (Fig. 3B), suggesting that color zonation well-exposed sub-horiztonal contact of the Empire is not due to major element compositional variations quartz diorite with supradjacent CS5 garnetite in the original calc-silicate, but more likely formed by is highly variable. In the sub-horizontal transition progressive fluid infiltration and intense metasomatic zone between these two units, the calcareous wallrock reactions (Atkinson and Einaudi, 1978). Endoskarn exhibits sharp contacts with several lens-like blobs of mineralization (mineralization within the pluton) is quartz diorite (Fig. 3A) and is coarse-grained, garnet- not widespread and is only observed at a few locali- rich, with large calcite filled vugs (1 – 3 cm diameter). ties adjacent to the contact, where it is likely that fluid Where this interaction is best exposed, the quartz flow from the wallrock affected the intrusion.

Several features near the garnetite–Empire quartz diorite contact appear to post-date original skarn for- mation. For example within 3 or 4 m of the contact, concentrated subhedral to euhedral epidote and calcite occur at high angles to the contact within the garnetite (Site D; Fig. 2). Within the pendant garnetite, garnet is a solid mineral, with angular fractures that suggest brittle deformation post-crystal growth; while the

presence of actinolite [(Ca2)(Mg,Fe)5(Si8O22)(OH)2],

calcite (CaCO3), quartz (SiO2), and epidote [(Ca2)

(Al2Fe)(Si2O7)(SiO4)O(OH)] within the garnetite ap- pear to have mineralized utilizing a post-skarn forma- tion fracture system (Fig. 4).

The tabular shape of these 2 – 4 m wide zones, the brecciated garnetite, the frequent euhedral nature of epidote and calcite mineralization, and the outcrop Figure 3. distribution relative to the quartz diorite contact sug- A. Field relationships between Empire quartz diorite gest that this mineralization occurred due to chan- (outlined in yellow) and the surrounding Kings Sequence nelized flow of magmatic, meteoric, and/or connate wall rocks. fluids within the brittlely-deforming calc-silicate rock. B. Field relationships of Kings Sequence xenoliths spall- These zones probably were cored by faults or frac- ing from the overlying pendant within the Empire quartz tures within the metamorphic rock, which provided diorite. The calc-silicate becomes progressively garnet channel pathways for fluids and may have been cre- rich towards the contact. ated by hydraulic fracturing of the rocks. C. Euhedral garnet of up to 15 cm in diameter exhibit zon- ing and changes in color from core to rim. ANALYTICAL METHODS D. Polished slab photo of red-brown garnetite displaying early granoblastic andradite cores and later grossular vein fillings. We collected samples of the Empire quartz diorite 315 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY

Figure 5. A. BSE image of garnet crystal 26A with associ- ated EMPA analytical transects (red and yellow dots). B. Crystal nucleation sites display grossular rich cores and concentric zoning, based on elemental geochemistry. Each of the transects were correlated relative to the prominent white growth rim (red star). C. From geochemical data, BSE image brightness was linearly correlated with grossular content. Generally, the darker zones are grossular rich, while light gray or white zones are more andradite rich. Figure 4. Tabular zones of epidote, quartz and calcite min- eralization (1 – 2 m wide). Brecciated garnetite (circled in SUMAC facility at Stanford University. Ion beam red) occurs within calcite vein fillings, suggesting that late working conditions and quantitative analysis of iso- stage mineralization occurred due to channelized flow of topic ratios and of trace elements was performed as fluids within the brittley-deforming calc-silicate rock. described in Barth and Wooden (2010). pluton, associated skarns and calc-silicate units near Qualitative and quantitative composition data were the Empire Mountain roof pendant at various distanc- gathered using a JOEL JXA-8200 electron micro- es (<1 m to >1000m) from the exposed pluton-wall- probe (EMPA) at the University of Texas, in Austin. rock contact (Fig. 2). Fifteen of these samples were Numerous highly magnified (40x - 130x) backscat- cut into billets for thin sections and sent to Spectrum tered-electron images (BSE) were obtained from Petrographics Inc. in Vancouver, Washington. Major polished thin sections from two samples of skarn and minor, and 17 trace elements were determined mineralization (Fig. 5). The UT electron microprobe for eleven whole rock powdered samples by X-ray analysis and image collection was conducted in spot fluorescence (XRF) at Pomona College following the mode, running at 15 kV and 1.5 e-7 A, following method of Johnson et al. (1999). similar method as Page et al. (2010). For geochronology and elemental zircon analyses with the Sensitive High Resolution Ion Micro Probe Oxygen isotope analyses of the zoned garnet, associ- Reverse Geometry (SHRIMP-RG), a representative ated skarn minerals and calc-silicates were performed sample of the Empire quartz diorite (10MD-17) was using a 30W CO2 laser at University of Texas, at crushing and zircon was separation using standard Austin, following techniques described by Sharp methods of gold table, heavy liquid and Frantz mag- (1990). Oxygen isotope ratios were measured with a netic separation. Zircon grains were handpicked, ThermoElectron MAT 253 mass spectrometer. Sample mounted, and loaded into the SHRIMP-RG at the δ18O values are reported in standard per mil (‰) nota- 316 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY tion relative to Vienna Standard Mean Ocean Water of Empire garnets implies high mineral growth rates (VSMOW). The analyses were standardized against and low temperatures (Jamtveit and Anderson, 1992) the University of Wisconsin Gore Mountain garnet followed by slower epitaxial growth parallel to earlier standard (UWG-2, Valley et al., 1995). crystal surfaces. The sharp compositional disconti- nuities recorded during garnet growth are thought to SHRIMP-RG U-PB AND TI-IN-ZIRCON occur due to the narrow range of aqueous solution compositions in equilibrium with intermediate solid- Sensitive High Resolution Ion Micro Probe Reverse solution members (Prieto et al. 1997; 2009). Geometry (SHRIMP-RG), analysis of the represen- tative sample of Empire quartz diorite (10MD17) δ18O ISOTOPIC SIGNATURES OF THE SKARN yielded a single age population within error and com- SYSTEM bine to give a weighted mean 206Pb/238U age of 108.5 ±1.0 Ma (with MSWD = 1.8 and probability = 0.033). Skarn minerals from different parts of the system This study also analyzed Ti-in-Zircon for the Empire were sampled for δ18O analysis to determine the rela- pluton sample and an estimate of emplacement tem- tive magmatic and meteoric components in hydrother- perature was proposed based on the Ferry and Watson mal fluids during the evolution of the skarn system. (2007) calibration. The high Ti concentration (28 ± 6 Samples of andraditic garnet, grossular garnet, epi- ppm) of zircons from the magnetite - bearing pluton dote, calcite, and quartz were sampled, with the hope indicate likely magmatic temperatures of 874±23°C. that the relative timing of mineralization determined This temperature is the hottest Ti-in-zircon tempera- by field relationships could provide context for the ture yet reported from a Sierran granitoid (Fu et al., changes in fluid composition. For reference, zircon 2008). from the Empire quartz diorite display values of above 6.0‰ δ18O, which is a good proxy value for GARNET MORPHOLOGY AND CHEMISTRY magmatic fluid.

The massive Empire Mountain garnetite found in δ18O data from zoned skarn garnet reveal early cores the Mineral King roof pendant contains spectacular of andraditic garnet have a meteoric signature (near zoned garnets. Back-scatted electron (BSE) images 0‰ δ18O), while rims from the same crystals are and ion microprobe spot analyses reveal oscilla- grossular-rich and indicate an increasing magmatic tory compositional zonation of variable thickness component (2 - 3‰ δ18O). These values are signifi- (20 mm to 500 mm) (Fig. 5). These analyses reveal cantly below the expected δ18O values for garnet that garnet composition is a binary mixture between precipitation in equilibrium magmatic fluids (above 18 isotropic grossular-rich (Ca3Al2Si3O12) and anisotro- 4.0‰ δ O). Epidote and calcite values, which pre- pic andradite-rich (Ca3Fe2Si3O12) garnet phases, with cipitate late in the system’s evolution, also display only minor almandine (Fe3Al2Si3O12) and spessartine strong meteoric fluid signatures for each mineral (3‰ 18 (Mn3Al2Si3O12) components (<0.06 and <0.08, re- and 5 - 7‰ δ O, respectively). spectively) (Fig. 5B). Greyscale intensity is linearly correlated to composition, with darker areas reflect- SKARN EVOLUTION ing grossular phase (Fig. 5C). The oscillatory zoning and composition of these euhedral garnet crystals are Mineralization in the Empire Mountain skarn system strikingly similar to those studied in other shallow records two stages of metamorphism that were con- (2-3 km) contact aureole intrusion settings (Jamtveit trolled by the shallow (1-2 kbar) and hot (>800oC) and Anderson, 1992). intrusion of the Empire quartz diorite. The proximal garnet>clinopyroxene exoskarn is the most proximal The garnets morphological characteristics of Em- zone of classic skarn zoning, relative to the intrud- pire Mountain suggest variable crystal growth rates ing pluton (Meinert et al., 2005). Since other more and system fluctuations (or departure) from equilib- distal zones (with increasing vertical distance from rium. Initial cellular growth of grossular-rich cores the pluton, clinopyroxene>garnet, wollastonite, then 317 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY unaltered marble), it is likely that most of the skarn dominant. The zoned Empire Mountain garnets are deposit has been eroded since formation. a new low-δ18O locality and the first recognized in the western U.S. These data support our hypothesis The granoblastic andraditic cores observed in the gar- that very hot crystallizing pluton was able to ascend netite pendant are evidence of the early crystallization to shallow and therefore relatively cool crustal levels, of this anhydrous calc-silicate mineral during pro- permitting the unusually early initiation of brittle de- grade metamorphism, while the more grossular-rich formation, which allowed the involvement of meteor- rims and veins most likely filled in during a second ic fluids throughout the evolution of the hydrothermal period of garnet growth during retrograde mineraliza- system, yielding the large garnetite skarn pendant at tion. The initial formation of garnet and lesser diop- Empire Mountain. side during prograde metamorphism likely promoted permeability due to the strength of garnet (Meinert REFERENCES et al., 2005), permitting the complete replacement observed in the Empire Mountain pendant. Field Atkinson, W., Einaudi, M, 1978, Skarn formation relationships suggest that this period of mineralization and mineralization in the contact aureole at Carr was synchronous with a change from ductile to brittle Fork, Bingham, Utah: Economic Geology and deformation, suggesting that even during prograde the Bulletin of the Society of Economic Geolo- metamorphism and intrusion, cooling of the system gists, v. 73, issue 7, p. 1326-1365. was already taking place. Barth, A. P., and Wooden, J. L., 2010, Coupled el- During retrograde mineralization in skarn systems emental and isotopic analyses of polygenetic zir- that are dominated by brittle deformation expected cons from granitic rocks by ion microprobe, with in a cooling system, andradite is often overgrown by implications for melt evolution and the source of quartz, calcite, magnetite and pyrite, whereas diopside granitic magmas, Chemical Geology, v. 277, p. is replaced by actinolite, quartz and calcite (Meinert 149-159. et al., 2005). The observed tabular zones of abun- dant hydrous skarn minerals (epidiote and actinolite), Bowman, J. R., 1998, Basic Aspects and Applications quartz and calcite in the Empire pendant therefore are of Phase Equilibria in the Analysis of Metaso- hypothesized to represent retrograde mineralization matic Ca-Mg-Al-Fe-Si Skarns: In Mineralized structurally controlled by faults, joints or intrusive Intrusion-Related Skarn Systems (D.R. Lentz, contacts. ed.): Min. Assoc. Can. Short Course, 26, p. 1-49.

Oxygen isotope data strongly indicate that both pro- Busby, S. C., and Saleeby J. B., 1987, Geologic guide grade skarn mineralization and later retrograde min- to the Mineral King area, , eralization were never in equilibrium with magmatic California: Field Trip Guidebook - Pacific Sec- fluids, but were instead in equilibrium with meteoric tion, Society of Economic Paleontologists and fluids, completely unlike any major skarn system Mineralogists, Los Angeles, CA, United States: worldwide (e.g., Einaudi and Burt, 1982; Meinert Society of Economic Paleontologists and Miner- et al., 2005). The δ18O values of Empire Mountain alogists, Pacific Section, 44 p. skarns are highly unusual and only four other loca- tions in the world report low δ18O values of primary Busby-Spera, C., and Saleeby, J. B., 1990, Intra-arc garnet minerals (Jamtveit and Hervig, 1994; Crowe strike-slip fault exposed at batholithic levels in et al., 2001; Clechenko and Valley, 2003). In the southern Sierra Nevada, California: Geology, v. shallow (<10 km) intrusion of a massif anorthosite, 18, issue 3, p. 255–259. Clencheko and Valley (2003) document the distinct morphology and (0.8 – 6.3‰) δ18O values of Wills- Clechenko, C. C., and Valley, J. W., 2003, Oscillatory boro skarn garnets, and propose an open system in zoning in garnet from the Willsboro Wollastonite which shallow circulation of meteoric water was Skarn, Adirondack Mts, New York: a record of 318 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY shallow hydrothermal processes preserved in a granulite facies terrane: Journal of Metamorphic Mackenzie, D., 1983, Sulfide mineral deposits of the Geology, v. 21, issue 8, p. 771-784. Mineral King mining district, Tulare County, California: M.S. Thesis, California State Univer- Crowe, D., Riciputi, L., Bezenek, S., Ignatiev, A., sity, Los Angeles, 100 p. 2001, Oxygen isotope and trace element zoning in hydrothermal garnets: Windows into large- Meinert, L., Dipple, G., Nicolescu, S., 2005, World scale fluid-flow behavior: Geology, v. 29, p. skarn deposits: United States: Economic Ge- 479-482. ology 100th Anniversary Volume, Society of Economic Geologists, Littleton, Colorado, USA, Einaudi, M., Burt, D., 1982, Special Issue Devoted to p. 299-336. Skarn Deposits: Introduction-Terminology, Clas- sification, and Composition of Skarn Deposits: Nadin, E. and Saleeby, J., 2008, Disruption of region- Economic Geology, v. 77, p. 745-754. al primary structure of the Sierra Nevada Batho- lith by the Kern Canyon fault system, California: Ferry, J. M., Watson, E. B., 2007, New thermodynam- Special Paper - Geological Society of America, ic models and revised calibrations for the Ti-in- v. 438, p. 429-454. Zircon and Zr-in-Nitrile termometers: Contribu- tions to Minerology and Petrology, v. 154, no. 4, Page, F. Z., Kita, N. T., Valley, J. W., 2010, Ion micro- p. 429-437. probe analysis of oxygen isotopes in garnets of complex chemistry; Chemical Geology, v. 270, Fu, B., Page, F., Cavosie, A., Fournelle, J., Kita, N., p. 9-19. Lackey, J.S., Wilde, S., Valley, J.W., 2008, Ti- in-zircon thermometry: applications and limited, Prieto, M., 2009, Thermodynamics of Solid Solution- Contributions to Mineralogy and Petrology, v. Aqueous Solution systems: Reviews in Mineral- 156, no. 2, p. 197-215. ogy & Geochemistry, v. 70, p. 47-85.

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