Clement, Emily

Virtual Commons - Bridgewater State University

Undergraduate ReviewVolume 2 Article 282006Origin of Gabbroic Xenoliths within the LoneMountain Dacite Intrusion, Big Sky, Montana: AField and Petrographic AnalysisEmily ClementFollow this and additional works at: http://vc.bridgew.edu/undergrad_revPart of the Geochemistry Commons, and the Geology CommonsThis item is available as part of Virtual Commons, the open-access institutional repository of Bridgewater State University, Bridgewater, Massachusetts.Copyright © 2006 Emily ClementRecommended CitationClement, Emily (2006). Origin of Gabbroic Xenoliths within the Lone Mountain Dacite Intrusion, Big Sky, Montana: A Field andPetrographic Analysis. Undergraduate Review, 2, 214-222.Available at: http://vc.bridgew.edu/undergrad_rev/vol2/iss1/28,>4Emily is an Earth Science major graduatingin 2006. Her project, funded with an AlPgran, allowed her to conduct research inthe lab at Big Sky, Montana. She and hermentor, Dr. Michael Krol, are continuingto research both geochemical and micro-structural analysis.TIlE UNDERGRADUATE REVIEWOrigin of Gabbroic Xenolithswithin the Lone Mountain DaciteIntrusion, Big Sky, Montana:A Field and Petrographic AnalysisBY EMILY CLEMENTAbstractLone Mountain represents a dacite laccolith that intruded in LateCretaceous time -68 Ma. This intrusion resulted in contact meta·morphism of the sedimentary country rock resulting in formationof a thin zone of black hornfels. Field work reveals the presence ofabundant, 1~9 em sized gabbro xenoliths and lesser amounts of siltstone inclu-sions within the dacite intrusion. Compositionally, the Lone Mountain daciteconsists of hornblende + plagioclase + biotite + quartz + opaques. Whereasthe gabbroic xenoliths consist of pyroxene + hornblende + plagioclase + minorchlorite.This study is concerned with the origin of the gabbroic xenoliths and theirrelation to the dacite intrusion. A major question we answer is, does the gabbroand dacite represent magmatic differentiation of an initial mafic magma or arethe xenoliths an older crystallized mafic pluton intruded by a younger interme-diate composition Lone Mountain dacite? Petrographic evidence suggest thetwo may be related based on their similar mineral assemblages.INTRODUCTIONThe geology ofsouthwest Montana has involved a long and complex historyof magmatic activity spanning tens of millions of years. Igneous rocks foundwithin the Gallatin Range record just a small piece ofa much larger geologic sto-ry. This portion of Montana was intruded by a series of medium-sized plutonsrelated to the subduction of the Farallon tectonic plate beneath western NorthAmerica some 70-50 million years ago. This was a time of intense and compo-sitionally diverse magmatism that affected a large area of the North AmericanCordilleras (Feeley, 2003). Current models for the origin and evolution of thesemagmatic bodies involve either; 1) the generation of mantle-derived mafic mag-mas that differentiate and change composition over time,or 2) magma mixing of a mafic magma with a felsic, crustalcomponent resulting in a hybrid magma (Feeley, 2003; Fee-ley & Cosca, 2003).Lone Mountain represents a large laccolith or mushroomshaped pluton that intruded and crystallized - 70 Ma (Kel-logg & Harlan, unpublished data, 1994; Tysdal et aI., 1986).It intruded a sequence of Cambrian to Cretaceous-agedsedimentary rocks that consist of sandstone, siltstone, andshale. The goal of this project is to determine the mineralogyand compositional variation in these rocks. In addition, weattempt to explain the presence of gabbroic xenoliths foundwithin the Lone Mountain dacite.MESOSCOPIC ANALYSISThe core of the mountain is composed of predominantlyigneous dacite with the flanks of the mountain composed ofmoderately dipping sedimentary rocks. Samples were col-lected mainly along the southern, eastern, and northeasternsides of the mountain (Figure 1). Due to the presence of >3feet of snow, much of the mountain was closed off due topossible avalanche threats.Most samples of the Lone Mountain dacite containedhornblende and plagioclase crystals set in a fine-grain ma-trix or groundmass. The dacite intrusion is completely un-deformed and possesses an original, igneous porphyritic tex-ture. On the east side of the mountain at lower elevations,sedimentary strata is tilted with an average strike and dipof bedding being N33E/53NW. This unit consists of alter-nating layers of fine-grained, tan colored siltstone and whitevolcanic ash (Figure 2). A minor fault cutting this sedimen-tary sequence was observed and had an orientation ofN33EI59NW. Along the most southeastern part of the mountain,a conglomerate unit was collected (Figure 3). This conglom-erate contained pebbles of well-rounded black chert andquartzite ranging in size from -1 cm to 5 cm. Strike and dipmeasurement of bedding indicate an orientation of N25EI21532SE. Beyond the intrusion zone, there was an outcrop withblack siltstone, sandstone, and some more volcanic ash. Thiswas at a higher elevation with a strike and dip ofN20E/19SE.The alternating bedding measurements with units dippingtowards the NW and SE suggest the sedimentary rocks weredeformed into a broad anticline syncline structure, prior tothe intrusion of the Lone Mountain dacite. Emplacementof the dacite may have additionally uplifted the surroundingsedimentary strata.PETROGRAPHIC ANALYSISDacite IntrusionSamples were collected from the dacite intrusion alongthe on the east and southeast side of Lone Mountain (Figure4). The matrix consists of an aphanitic to porphyritic tex-ture. Samples analyzed contain minerals such as plagioclase+ hornblende + biotite + quartz + opaques. Plagioclase oc-curs as subhedral phenocrysts within a finer grained ground-mass. It exhibits albite twinning with grains approximately 1-6 mm in size. Hornblende is found as subhedral crystals ap-proximately 1-3 mm in size (Figures 5 and 6). Biotite crystalswere found to be anhedral and only about 1-2 mm in size.Quartz occurs as anhedral grains with a much smaller grainsize than that observed in plagioclase. Grain size typicallyvaries between 1-2 mm. Figure 5 shows a representationof the distinct contact between the fine-grained dacite anda larger coarse-grained gabbroic xenolith. Figure 6 displaysa fine-grained dacite with plagioclase, quartz, biotite, andhornblende minerals.Gabbroic XenolithsSamples of dacite containing abundant mafic xenolithswere collected along the east and northeast side of LoneMountain (Figure 1). The gabbroic xenoliths are found to bepredominantly phaneritic in texture (Figure 7). Pyroxene isthe most abundant mineral present and occurs as subhedralto anhedral grains 0.5-1 em in size (Figure 8). Hornblende8RIDGEW",TER ST",TE COllEGE"6occurs as 0.5 to 0.7 em in size. Minor amounts of chloriteare also present as subhed.ra1 grains. SubhedraJ plagioclasecrystals with albite twinning are present as 1-4 mm grains(Figure 9).Two samples collected from the southeast side of LoneMountain also contain coarse-grained xenoliths with similarmineralogy. Figure 10 shows exsolution textures in plagio-clase grains. Minerals present include subhedral plagioclase,occurring as 4-6 mm in size, euhedral pyroxene, approxi-mately 2·4 mm in size, and finally minimal quartz and biotitegrains that were anhedral and only 1·2 mm in size.Sedimentary Country RocksSeveral samples were collected from the sedimentarycountry rock in order to evaluate the effects of thermalmetamorphism related to the intrusion of the Lone Moun-tain dacite (Figures 11-13). One sample was collected (roman interlayered sequence o( cross·bedded sandstone andshale. Petrographic analysis reveals a very fine-grained maotrix with quartz. plagioclase. minor orthoclase. and opaqueminerals. Grains were well rounded and moderate to mod-erately-sorted (Figures 12). Little petrographic evidence wasobserved indicating these rocks experienced any significantthermal effects.A sample collected on the eastern side of Lone Mountainwas composed almost entirely of fine-grained quartz withminor plagioclase (eldspar. This quartz sandstone consistsof angular grains approximately 1·3 mm in size (Figure 13).The furthest sample collected (rom the intrusion was a fine-grained sandstone composed almost entirely o( rounded,anhedral quartz. Grains range in size from 1-2 mm and arecross-cut by quartz veins.Ultramafic RockA small body of ultramafic rock was collected east ofLone Mountain at the lowest elevations. It contains plagiO-clase + hornblende + clinopyroxene + minor chlorite. Pia-TilE UNoERGItAOUATE REVIEWgioclase has albite twinning and occurs as subhedral grains24 mm in size. Hornblende is subhedral with clasts from3-6 mm in size. Oinopyroxene grains are subhedral withclasts 2-7 mm in size. The minor chlorite is found aroundthe edges of the plagioclase grains suggesting secondary al-teration (Figure 14). The significance of this unit is beyondthe scope of this project. but it could represent an even olderphase of more mafic magmatism.DICSUSSIONBased on petrographic analysis of 20 thin sections. it canbe determined that the mineralogy of the Lone Mountaindacite contains plagioclase. hornblende. quartz, and biotite.The gabbroic xenoliths found within the dacite contain pla-gioclase. hornblende. and pyroxene. The igneous texturespreserved in the dacite and xenoliths are distinguishable fromone another. The dacite is mainly aphanitic to porphyriticwhereas the xenolith is commonly phaneritic. On the basisof mineralogy. the Lone Mountain laccolith is classified com-positionally as a dacite whereas the xenoliths are gabbros.The question remains whether these gabbroic xenolithsrepresent an early magmatic differentiation from an initialmagma. or if these xenoliths are an older plutonic rock laterintruded by dacitic magma. MagmatiC differentiation is theprocess in which magma can change composition by crys-tallizing and segregating different minerals over time. thusaffecting the compositions of the residual magma. Somepossible magmatic differentiation events that could havehappened on Lone Mountain are assimilation; the mixingof two or more magmas; various degrees of partial meltingfrom the same source; or two distinct melting events fromtwo distinct sources.On lhe basis of mineral assemblages. both the dacite andgabbro xenoliths are similar. with only minor differencessuggesting they may in fact be genetically related. The nextstep in our project is to pursue a geochemical approach thisupcoming year and investigate the chemistry of these rocks.in an effort to determine if a genetic relation exists.FUTUREWQRKDuring the fall 2005 and spring 2006 semesters I will bepreparing samples for geochemical analysis. Samples of LoneMountain dacite and gabbroic xenoliths will be analyzed formajor and trace elements using x~ray fluorescence at Hart-wick College in New York. The goal will be to determinebulk rock chemical signatures of the xenoliths and dacite.and see if they are similar. Similar chemical signatures. es~pedally trace elements. would tend to support our hypoth-esis that the Lone Mountain dacite represents a magmaticdifferentiate of an initially mafic magma with the gabbroicxenoliths being the early crystallized phase.217ACKNOWLEDGEMENTSI would like to thank the Adrian TInsley Program forfunding this project. It has provided me with both a won-derful and educational experience. Also, I would like tothank Dr. Michael Krol for being my mentor and assistingme in lhe field. Special thanks to Katie and Jake Grimm fortheir generosity in allowing us to stay in the wonderful homeof Mom Haley during our field work in Big Sky. Montana.'.IDGEWATl.liTATE COLLEGl21.RGURE 1: Portion of the a 7\7 minute topographic map showing LoneMoun/oin ond sample locations used in this study.RGURE 3: Photograph ofa roa~-groiMdOOfIglomfflJre that isport of the C~r~s-ogtd country fed intn.J<kd by the Lonl"Mountain doore.THE UNDEIlGItAI>UATE REVIEWFIGURE 2: Photograph ofdeformed ond filled loyers ofalternatingsiltstone ond volcanic ash layers in country rock adjacent to theLOM Mountain doci~ intfusion.fIGURE 4: Photograph of typirolo~h~ pit Otposurr ofdocite pluton.FIGURE 5: PtIotomK:r09lOph of fi~.gram«ldoot~ and Iorg~r roo~·grain«! gabbro«: xmoJlth within doci~ inrrl/SlOftFIGURE 6: PhotomicroglOph of fint-groin«! dacit~ that consists ofplogioclo~.quartz. biotit~. and hombl~nde.219RGURE 7: PtIotogroph of mafic xmmith within a docit~ nxt.RGURE 8: Photomicrograph of coo~-grain«lgabbroic :«nolith (/~ft)and fin~-graintd docit~ (right). X~nolith consists of moinly pyrox~n~and plagioclo~. whtrras daciric rods consist ofplagioclo~andhornbl~nd~ with min()l PYfOJ(~ne..IIDGEWATEI STATE COllEGE110FIGURE 9: Photomicrograph of a large plagioclase phenocryst withina sample ofgabbro;c xenolith (LM-4-04).FIGURE 10: Photomicrograph of fine-grained plagioclase and largerhornblende phenocrysts-TIlE UNOERGRADUATE REVIEWFIGURE 11: Photomicrograph of the sedimentary country rockadjacent to the dacite intrusion. Angulor shaped quartz groins orecur by briWe fractures from top left to lower right..RGURE 12: Photomicrograph of ~frt:~/yfine-grai~day-richfJJ(Jtrix. with rounded quartz, plogiodo~. and opaque grains. Sampleis from the sedimentary sondstone country rock.FIGURE 13: Photomicrogroph ofquartz vein rutting a quartz-richsiltstone of the country rod221RGURE 14: PfIotomierogroph ofan ultramafic body thor contamscrystals ofplogiodose {gray} and c:/inopyroxene (bright blueinterfert:nr:e colors).lalOG£ ...."T£1 ST"T£ COU£G£222TIlE UNDERGRADU"TE REVIEWReferences CitedFeeley. T.e., 2003, Origin and tectonic implications of across-strike geochemi-cal variations in the Eocene Absaroka volcanic province, United States, Journalof Geology, v. Ill, p. 329-346.Feeley, T.e. & Cosca, M.A., 2003. Time vs. composition trends of magmatismat Sunlight volcano. Absaroka volcanic province, Wyoming, Geological Soci-ety of America Bulletin, v. 115, p. 714-728.Tysdal, R. G., Marvin, R. E, DeWitt, ED, 1986, Late Cretaceous stratigraphy,deformation, and ~ntrusion in the Madison Range of southwestern Montana.Geological Society of America Bulletin, v. 97, p. 859·868.

Origin of Gabbroic Xenoliths within the Lone Mountain Dacite Intrusion, Big Sky, Montana: A Field and Petrographic Analysis

http://vc.bridgew.edu/cgi/viewcontent.cgi?article=1043&context=undergrad_rev

Origin of Gabbroic Xenoliths within the Lone Mountain Dacite Intrusion, Big Sky, Montana: A Field and Petrographic Analysis

Abstract

Similar works

Full text

Available Versions

Virtual Commons - Bridgewater State University