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 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 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— Big Sky Orogen

EXPLORING THE PROTEROZOIC BIG SKY OROGENY IN SOUTHWEST MONTANA Project Faculty: TEKLA A. HARMS, JOHN T. CHENEY, Amherst College, JOHN BRADY, Smith College

PROTOLITH DETERMINATION OF PRECAMBRIAN MYLONITIC ROCKS ADJACENT TO THE MADISON MYLONITE ZONE, MOUNTAINS, SOUTHWEST MONTANA AND JESSE DAVENPORT, College of Wooster Research Advisor: Shelley Judge

PETROGRAPHIC AND GEOTHERMOBAROMETRIC ANALYSES OF METASEDIMENTARY ROCKS IN THE , IDAHO AND MONTANA KRISTINA DOYLE, Amherst College Research Advisor: Tekla Harms

GEOCHRONOLOGY OF PRECAMBRIAN META-GABBRO IN THE HENRYS LAKE MOUNTAINS, SOUTHWEST MONTANA AND IDAHO B. PARKER HAYNES, University of North Carolina - Chapel Hill Research Advisor: Drew S. Coleman

PETROGENESIS AND DEFORMATION OF PRECAMBRIAN QTZ-DOL MARBLE UNITS IN THE GRAVELLY RANGE AND REYNOLDS PASS, HENRYS LAKE MTNS, SW MT AND ID DANIELLE LERNER, Mount Holyoke College Research Advisor: Steve Dunn and Michelle Markley

PETROGENESIS OF PRECAMBRIAN IGNEOUS AND META-IGNEOUS ROCKS SOUTH OF THE MADISON MYLONITE ZONE, HENRYS LAKE MOUNTAINS, SW MONTANA AND IDAHO CALEB O. LUCY, Williams College Research Advisor: Reinhard A. Wobus

STRUCTURAL ANALYSIS OF PRECAMBRIAN MYLONITE ZONES, HENRYS LAKE MOUNTAIN, SOUTHWEST MONTANA AND IDAHO ALIANORA WALKER, Smith College Research Advisor: H. Robert Burger

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

24th Annual Keck Symposium: 2011 Union College, Schenectady, NY STRUCTURAL ANALYSIS OF PRECAMBRIAN MYLONITE ZONES, HENRYS LAKE MOUNTAINS, SOUTHWEST MONTANA AND IDAHO

ALIANORA WALKER, Smith College Research Advisor: H. Robert Burger

INTRODUCTION METHODS

Through collaborative field and lab work, the 2010- During a four week field season, oriented mylonitic 2011 Montana Big Sky Keck Project hopes to discov- rock samples were collected from a wide range of er a better understanding of the metamorphic, struc- locations within the study area (Fig. 1). In addition to tural and geochronologic histories of the Precambrian samples, foliation and lineation measurements from metamorphic rocks that make up the field area in the the mylonites, were collected in the field. Data from Henrys Lake Mountains as they evolved through the kinematic indicators were collected wherever pos- late history of the Wyoming province. In doing so, the sible. project seeks to understand if, and how, these histo- ries vary across the field area and how they fit into a Upon returning to Smith College, lineation-parallel, regional context. Erslev and Sutter (1990) studied the foliation-perpendicular thin sections were cut from all metamorphic and structural history of the Madison 24 samples. Using a polarizing light microscope, op- mylonite zone outside of the Keck field area to the tical techniques were used to the identify mineralogy northeast. Similar mylonites along strike in the Keck and any kinematic indicators present in the sample field area, both within and beyond the areas mapped suite. In addition, some work was done on the Scan- as Madison mylonite zone, have not been studied in ning Electron Microscope (SEM) to determine more detail. Through kinematic analysis, this project hopes precise mineral compositions. Once kinematics were to determine whether these mylonites have or have determined in thin sections, the samples were reori- not experienced the same history as the mylonites of ented and shear sense was determined in real space. the Madison mylonite zone. PETROGRAPHIC AND PETROLOGIC RE- Many questions can be asked about the specific his- SULTS tories of the mylonites within the field area. These include: Do the mylonites vary across the field area A range of textures occurs in the study samples, there- and if so, how? Are all the mylonites of the study area fore textures have been ranked (Fig. 1) from 1, highly related to the Madison mylonite zone, or are some brittle cataclasites, to 2, protomylonites to 3, ductile related to a different phase of deformation? What do mylonites. Rocks with cataclasitic textures, ranked mylonite mineral assemblages tell us about the pres- with a degree of deformation of 1 (Fig. 1, Sample sures and temperatures the mylonites experienced at 10-AW-23B), are composed of a complex of altered, the time of formation? What large and small scale broken grains of mafic minerals, feldspar, and quartz, structures exist in the field area? How do these same in a matrix of chlorite, biotite and other alteration structures fit within the regional geologic history of minerals. These rocks contain few to no kinematic the study area? What is the sense of shear in the my- indicators. More of these rocks were found in the lonite zones and what is the regional transport direc- southwestern field area (Figure 1: stations 13 and 23). tion? From what protolith did these mylonites form? In other cases, such as, but not unique to, station 3, samples with brittle textures (such as 10-AW-3C) ap- pear to represent the edge of a shear zone. More 87 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY

111°35'0"W 111°32'30"W 111°30'0"W 111°27'30"W 111°25'0"W

44°47'30"N 44°47'30"N 25 10-­AW-­7 3 10 10-­AW-­8U 3 24 10-­AW-­3AD 3 4910-­AW-­3B D2 10-­AW-­11U2 10-­AW-­3C D2 46 10-­AW-­20 U2

54 10-­AW-­19 2

44°45'0"N 44°45'0"N

10-­JD-­17 2 10-­AW-­5 D2 37 66 10-­AW-­141 52 410-­AW-­15A1 44°42'30"N 49 44°42'30"N 2 10-AW-1A 10-­AW-­17U3

U 3 10-­AW-­421 50 10-­AW-­1B U3 10-­AW-­1A U2 45 10-­AW-­4 U 2

3 10-AW-21 10-­AW-­21 U 3 44 44°40'0"N 44°40'0"N

10-­AW-­22 U2

26

53 10-­AW-­23A1 10-­AW-­13B1 14 10-­AW-­23B1 10-­AW-­13A1 010-­AW-­18 2 1 10-AW-23B

44°37'30"N 44°37'30"N

111°35'0"W 111°32'30"W 111°30'0"W 111°27'30"W 111°25'0"W 3 Mylonite 2 Lineations Protomylonite 1 Cataclasite Miles Sample L ocations U Shear Sense 0 0.5 1 2 3 4 Figure 1. Study area sample location map, showing the degree of deformation denoted on a scale of 1 to 3, and linea- tion orientation of each sample. Each diagnostic sample is accompanied with a D for top down or U for top up sense of shear determined from each sample. Photomicrographs show examples of each grade of deformation. Based on USGS 3D scaled topographic quadrangle maps: Earthquake Lake, Cliff Lake, Hidden Lake Bench, Targhee Peak, Targhee Pass, Mount Jefferson, Sawtelle from http://www.arcgis.com/home/ with a scale of 1:86,439. 88 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY ductilely deformed samples (such as 10-AW-3A and It is proposed that symplectite grew as a result of 10-AW-3B) also occur. shearing because the symplectite fabric is systemati- cally aligned with relation to orientation with respect Rocks that represent the mylonitic end member, Tex- to the shear fabric. Although variation in texture ture 3, are found in the centers of shear zones. These exists, wavy lamellae within the symplectite are per- rocks range in composition, but are predominantly pendicular or at high angle to the principal mylonitic quartzofeldspathic rocks with muscovite, biotite, foliation. In many cases, entire lithoclasts are formed some chlorite and some small percentage of mafic of individual grains, each made up of symplectite, minerals. They have well developed foliations and in which case the symplectite intergrowth is coarser shear bands and can contain diagnostic kinematic grained at lithoclast boundaries than at the interior indicators. Pressures and temperatures in these rocks of the grain. In cases where only part of a lithoclast were such that quartz was able to flow and dynamical- has symplectic texture, symplectite is present in all ly recrystallize, but feldspar was left undeformed or cases at the edge of the lithoclast and not in the center broken in response to strain. In many of these rocks (Fig. 2D, 2E). In many cases, symplectite textures there are particles with multiple mineral grains, pos- can be seen to curve from high angle to the C-fabric sibly once representing lithic fragments, here referred to parallel to the C-fabric the closer to the margin to as lithoclasts, commonly with well formed tails and of the lithoclast. As temperatures were not suffi- surrounded by comparatively fine grained micaceous cient to allow feldspar to bend or ductilely deform, matrix. These lithoclasts commonly contain large symplectite must have grown while mylonites were amounts of symplectite. In contrast to the quartz- being sheared. In lithoclasts only partially composed ofeldspathic rocks, some of the mylonites in the field of symplectite, symplectite growth typically exists area have low silica content and are predominantly on grain edges that are parallel to the S-fabric, and composed of mafic minerals (Samples 10-AW-17 and is not present in the tails of the lithoclast (Fig. 2D, 10-AW-21). 2E). This pattern of growth (Fig. 2D) with a principal alignment of symplectite wavy lamellae along folia- Yet a third group of protomylonitic like rocks occur tion parallel edges of lithoclasts is believed to result within the field area, and are ranked with a degree of from shearing because the replacement of K-feldspar deformation of 2 (Fig. 1, Sample 10-AW-1A). These by symplectite leads to volume reduction in the rock intermediate rocks show both brittle and ductile (Simpson and Wintsch, 1987). characteristics. In these rocks, S-fabrics are generally better developed that C-fabrics. Grains are gener- In all locations where intermediate intrusives are pres- ally angular and rotated into alignment with the shear ent, mylonites are also found in localized shear zones. fabric. Lithoclasts and strain shadow tails are poorly As at Station 3 (fig. 1), a gradual transition within formed. These rocks can be highly altered and con- intrusive bodies from mylonite (10-AW-3A) in the tain a higher percentage of mafic minerals forming center of the zone, to remnant primary igneous texture lithoclasts and matrix than in the more ductile rocks. (10-AW-3C), to apparently intact igneous rock was Distribution of the cataclasites, mylonites, and proto- typical. Where shear zones are present in metaigne- mylonites can be seen in Figure 1. ous bodies, the grain size of the mylonites is always larger than the surrounding metaigneous rock. Symplectic texture of quartz intergrown with perthite, composed of orthoclase and pure albite (Fig. 2B) is It is not known whether the same brittle to ductile very prevalent in quartzofeldspathic mylonites stud- transition exists for other potential protoliths. Other ied. In almost all cases where symplectite is absent, mylonites are so intensely strained or were initially so quartz and plagioclase are not present or are in very fine grained that no obvious protolith could be de- low abundance within the rock. Although this inter- termined in the field. Despite mylonites’ sometimes growth can only be seen at thin section scale, it repre- quite close proximity to metasedimentary rocks, a sents up to 80 percent of lithoclast composition in the gradational transition from metasedimentary rock to mylonites, but it is not present in the matrix. mylonites was never observed. 89 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY formation. The ductile behavior of quartz accompa- Ph A nied with the brittle behavior of feldspar in quartz-

Bi ofeldspathic mylonites also supports this conclusion.

Ph A method of phengite geobarometry, described in Massonne and Schreyer (1987), was applied to Sample 10-AW-1B. As a chemical equilibrium with phengite and the correct limiting mineral assemblage Qz of quartz, K-feldspar and phlogopite is present in the rock (Fig. 2A, 2B), a ratio of 3.2 to 3.3 silicon per

B C formula unit in phengite is interpreted based on the model (Fig. 2C) to give pressure for these rocks of 5 kb (0.5 GPa). This is considered a minimum pressure Ab Or Ab Ab Mz because of uncertainty about the temperature at the time of shearing. As phengite was identified in sym- plectite (Fig. 2A) that likely grew during shear, this pressure applies to the time when rocks in Antelope Basin (Fig. 1) were actively accommodating large amounts of strain. A D E Sym Ms STRUCTURAL RESULTS

Fsp Bt Fsp Sym Qz Several structural trends are clear from analysis of Sym Qz mylonite field data. Mylonitic foliation is closely Sym Sym clustered around a mean of 237/38 (Fig. 3A). Only Ms three measurements do not follow this trend, and those were all taken on the far eastern side of the field Figure 2. Symplectic texture and phengite geobarometry. area (Fig. 3). If this indicates some large scale over- A. Back scattered SEM image showing locations of phen- turning or folding within the field area, insufficient gite used for geobarometry. B. Back scattered SEM image data exist to support that conclusion. The NE trend- showing symplectite intergrowth of quartz and albite/ orthoclase perthite. C. Diagram modified from Massonne ing, NW dipping mylonitic foliation in the Madison and Schreyer (1987) showing temperature pressure plot mylonite zone of the southern Madison Range (Fig with Si isopleths for phengite Si contents per formula unit 3C) (Erslev and Sutter, 1990) is parallel to mylonitic in the limiting assemblage with K-feldspar, quartz and foliation in the Henrys Lake Mountains (Fig 3A). phlogopite. Pressure solution for a temperature of 500 Mylonitic Lineations are not as closely clustered as degrees C and 3.2 to 3.2 Si per formula unit is shown by a foliations, but show a mean vector of 47/304, very red ellipse. D. Idealized pattern of symplectite growth on similar to that in the Madison mylonite zone (Fig. porphyroclasts as a result of shear. Modified form Simpson 4A). In general, lineations are parallel, or close to and Wintsch (1989) E. Cross-polarized light photomicro- parallel, to the dip of foliation. Lineations measured graph of a lithoclast in 10-AW-8 showing similar pattern in the field were typically white minerals, possibly of symplectite growth, as in D where only part of the litho- quartz or feldspar, and elongate black mafic min- clast is composed of symplectite. erals. Plotted separately, (Fig. 4D) both lineation Mylonite samples appear to be greenschist facies types show similar means. There is little meaningful based on the mineral assemblage chlorite, biotite, variation across the field area. For this reason it is muscovite, epidote, albite, orthoclase, quartz and believed that all mylonites in the field area are related unidentified mafic minerals. No index minerals have to the Madison mylonite zone. been found within the mylonites to give a more spe- cific possible temperature range for during mylonite 90 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY Figure 3: Montana, Big Sky Keck Project Study Area Foliation Map Figure 4: Montana, Big Sky Keck Project Study Area Lineation Map

111°35'0"W 111°32'30"W 111°30'0"W 111°27'30"W 111°25'0"W 111°35'0"W 111°32'30"W 111°30'0"W 111°27'30"W 111°25'0"W C A 44°47'30"N 44°47'30"N 44°47'30"N 44°47'30"N A 25 60 10 24 36 14 25 24 40 75 40 49 Lineation = Poles = N = 53 N = 69 51 46 34 54

44°45'0"N 44°45'0"N 44°45'0"N 44°45'0"N Stations:3, 7-11, 19, 20 N = 24; Poles = Stations:1, 2, 4, 5, 14-17, B N = 24; Poles = B Stations:6,12, 13, 18, 21-23, JD19 N=21; Poles =

62 Stations: 3, 7-11, 19, 20, 37 74 50 N = 20; Lineation =

61 Stations: 1, 2, 4, 5, 14-17, 52 50

59 N = 22; Lineation = 52 44°42'30"N 44°42'30"N 44°42'30"N Stations: 6, 12, 13, 18, 21-23, JD19 4 44°42'30"N 48 N=10; Lineation = 32 34 63 54 46 38 41 50 51 51 45

47 44°40'0"N 44°40'0"N 44 44°40'0"N 44°40'0"N C D

30 29 23 26 53 15 52 14 62 0 72

Aligned Mafic Minerals: N=20; Lineation= 44°37'30"N 44°37'30"N 44°37'30"N Aligned Qz Rods: 44°37'30"N N=37; Lineation=

111°35'0"W 111°32'30"W 111°30'0"W 111°27'30"W 111°25'0"W 111°35'0"W 111°32'30"W 111°30'0"W 111°27'30"W 111°25'0"W

Miles Lineation Miles Foliation Strike and Dip 0 0.5 1 2 3 4 0 0.5 1 2 3 4 Figure 3. Study area foliation map. A. Equal area lower Figure 4. Study area lineation map A. Equal area lower hemisphere stereoplot of Kamb contoured poles to folia- hemisphere stereoplot of Kamb contoured lineation data tion with a contour interval of 2 sigma. B. Equal area with a contour interval of 2 sigma. B. Equal area lower lower hemisphere stereoplot of poles to foliation sepa- hemisphere stereoplot of lineations separated into along rated into along strike domains. Mean vectors with 95% strike domains. C. Mylonite lineations of the Madison confidence cone small circles shown in blue. C. Poles mylonite zone from Erslev and Sutter, (1990). D. Equal to mylonite foliations of the Madison mylonite zone from area lower hemisphere stereoplot of lineations separated (Erslev and Sutter, 1990) Based on USGS 3D scaled topo- by composition. Mean vectors with 95% confidence cone graphic quadrangle maps: Earthquake Lake, Cliff Lake, small circles shown in blue. Based on USGS 3D scaled Hidden Lake Bench, Targhee Peak, Targhee Pass, Mount topographic quadrangle maps: Earthquake Lake, Cliff Jefferson, Sawtelle from http://www.arcgis.com/home/ Lake, Hidden Lake Bench, Targhee Peak, Targhee Pass, with a scale of 1:86,439 Mount Jefferson, Sawtelle from http://www.arcgis.com/ home/with a scale of 1:86,439

KINEMATIC RESULTS For the range of brittle to ductile textures observed, kinematics were determined in different ways. In Although variation exists in the data set, kinematic rocks with cataclastic texture (1, Fig. 1), no diagnostic indicators around the field area give a principal kinematic indicators were found. These rocks show transport direction verging to the SE (Fig. 1) or up brittle offsets in grains, but a consistent pattern of dip (Fig. 3A). Exceptions to this general trend are offsets was not found to give a sense of shear for the three samples at station 3 (10-AW-A, 10-AW-B, and rocks. Protomylonites (2, Fig. 1) typically yielded 10-AW-C) in the north part of the field area, as well as indicators. In these rocks, S-C fabrics are commonly sample 10-AW-5 to the south (Fig. 1). Further work good to poor. Although the rocks do commonly is being done to understand the reason for exceptions contain lithoclasts with sigmoidal tails, most litho- in the results. clasts have tails with no clear asymmetry due to the 91 24th Annual Keck Symposium: 2011 Union College, Schenectady, NY angular form of the lithoclasts. Mylonites (3, Fig. 1) CONCLUSION yield the most diagnostic indicators. S-C fabrics are well formed, lithoclasts have well formed sigmoidal Mylonites in the Henrys Lake Mountains are present tails and rotated grains are not uncommon (Fig. 5). in a range of textures and compositions. Textural dif- In many cases, tail composition is different from the ferences in these mylonites are interpreted as a result composition of the center of the lithoclast, and so was of difference in protolith as well as difference in the classified as strain shadows. As the orientation of degree of deformation as a result of distance from shear zone boundaries are not known for any of the the shear zone centers. Although symplectite may samples, all potential S-C’ fabrics are here described have been present in the protolith, symplectite has as S-C fabrics. grown during shear, potentially as a means for volume reduction (Simpson and Wintsch, 1989). Mineral as- semblages typical of greenschist facies and phengite equilibrium with K-feldspar, quartz and phlogopite suggests a minimum pressure during mylonite forma- tion of 5kb (0.5 GPa) (Fig. 2). Mylonites are present in NE trending, NW dipping, SE verging shear zones parallel to those in the Southern Madison Range, dat- S ed by Erslev and Sutter (1990) at 1.8 Ga, and believed C here to be the result of the Big Sky orogeny.

REFERENCES 4mm A 10-AW-1B Erslev, E.A., and Sutter, J.F., 1990, Evidence for Pro- terozoic mylonitization in the northwestern Wyo- ming province: Geological Society of America Bulletin, v. 102, p. 1681–1694. C S Massonne, H.J. and Schreyer, W., 1987, Phengite geo- barometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz: Contributions to Mineralogy and Petrology, v. 96, p. 212-224.

2mm B Simpson, C. and Wintsch, R.P., 1989, Evidence for 10-AW-3A deformation-induced K-feldspar replacement by myrmekite: Journal of Metamorphic Geology, v. 7, p. 261-275.

C

S

2mm C 10-AW-17 Figure 5. Photomicrographs of three mylonite samples from different locations in the field area in plain polarized light showing shear sense determinations and orientations of well formed S-C fabrics. 92