Lab%205---Metamorphic%20Rocks%2c%20Processes%2c%20and%20Resources

Lab%205---Metamorphic%20Rocks%2c%20Processes%2c%20and%20Resources

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Unformatted text preview: LABORATORY SEVEN Metamorphic Rocks, Processes, and Resources oCONTRIBUTING AUTHORS- Harold E. Andrews - Wellesley College James R. Besancon - Wellesley College Margaret D. Thompson - Wellesley College OBJECTIVES A. Be able to describe and interpret textural and compositional features of metamorphic rocks. B. Be able to determine the names, parent rocks (protoliths), and uses of common metamorphic rocks, based on their textures and mineralogical compositions. C. lnfer the relative grades of metamorphism that common metamorphic rocks have undergone. MATERIALS Pencil, eraser, laboratory notebook, hand magnifying lens (optional), metric ruler, mineral identification materials of your choice, and samples of metamorphic rocks (obtain as directed by your instructor). INTRODUCTION The word metamorphic is derived from Greek and means "of changed form.” Metamorphic rocks are rocks changed from one form to another (metamor- phosed) by intense heat, intense pressure, or the ac- tion of watery hot fluids (Figures 7.1, 7.2, 7.3). Think of metamorphism as it occurs in your home. Heat can be used to metamorphose bread into' toast, pressure can be used to compact an aluminum can into a flatter and more compact form, and the chemical action of watery hot fluids (boiling water, steam) can be used to change raw vegetables into cooked forms. Inside Earth, all of these metamorphic processes are more in- tense and capable of changing a rock from one form (size, shape, texture, color, and / 0r mineralogy) to an- other. Therefore, every metamorphic rock has a parent rock (or protolith), the rock type that was meta— morphosed. Parent rocks can be any of the three main rock types: igneous rock, sedimentary rock, or even metamorphic rock (i.e., metamorphic rock can be metamorphosed again). Figure 7.1 illustrates how a regional intrusion of magma (that cooled to form granite) has metamora phosed parent rocks to new metamorphic forms of rock. Mafic and ultramafic igneous rocks were meta- morphosed to serpentinite. Sedimentary conglomer— ate, sandstone, and limestone parent rocks were metamorphosed to metaconglomerate, quartzite, and marble. Shale was metamorphosed to slate, phyllite, schist, and gneiss, depending on the grade (intensity) of metamorphism from low-grade (slate) to medium- grade (phyllite, schist), to high-grade (gneiss). Hornfels formed only in a narrow zone of contact metamorphism next to the intrusion of magma. Different grades of metamorphism produce characteristic changes in the texture and mineralogy of the rock, which you will study below. Some com- mon metamorphic rock-forming minerals include quartz, feldspars, muscovite, biotite, chlorite, gar— net, tourmaline, calcite, dolomite, serpentine, talc, kyanite, sillimanite, and amphibole (hornblende). 133 134 ' Laboratory Seven ._ 'L.»-' Il-__’ 1___ ___I__._‘:-I'.'\I _I___‘. nged rocks ———'L—— ZONE OF ——'-I ‘ REGIONAL CONTACT METAMORPHISM METAMORPHISM ' (Hamish) FIGURE 7.1 Metamorphism of a region by the heat, pressure, and chemical action of watery hot (hydrothermal) fluids associated with a. large magma intrusion that cooled to form granite (granite intrusion); Some-of the pmexisting parent rocks are far removed from the intrusion and remain unchanged. closer to the intrusion, the parent rocks were changed in form within a zone of regional metamerphlsm. Mafic-igne‘eus rocks were meta- morphosed-to se‘rpentlnit‘e. Sedimentary conglomerate. sandstone, and limestone parent rocks-Were metamorphosedto-metaconglemerate.squartzite. and marble. Shale was meta— morphosed to slate, phylllte. schist. and males depending an thegrade (intensity)- of meta- morphism from low-grade {slate} to medium-grads (phytll'te, seliisitl, to high-grade (gages). Notice two scales of metamorphism in the figure. Contact metamorphism occurred in narrow zones next to the contact between parent rock and intrusive magma and along fractures in the parent rock that were. intruded by hydrothermal fluids. The zone of con- tact metamorphism next to the intrusive magma-was changed to hornfels by intense heat and chemical reaction with the magma. Zones of contact metamorphism have widths on the order of millimeters to tens-of-meters. Regional metamorphism occurred over a larger region, throughout the mountain belt, and was accompanied by folding and shearing of rock layers. You should familiarize yourself with all of these minerals by reviewing their distinctive properties in the Mineral Database (see Figure 3.28). PART 7A: METAMORPHIC PROCESSES AND ROCKS There are two main scales at which metamorphic processes occur: contact and regional (see Figure 7.1). Contact metamorphism occurs locally, adjacent to igneous intrusions and along fractures that are in con— tact with watery hot (hydrothermal) fluids. The latter process is called hydrothermal metamorphism, and it in- volves condensation of gases to form liquids, which may precipitate mineral crystals along the fractures such as in Figure 7.2. Contact metamorphism is caused by conditions of low to moderate pressure, in- tense heating, and reaction With the metamorphosing magma or hydrothermal fluids over days to thou- sands of years. The intensity of contact metamor- phism is greatest at the contact between parent rock FIGURE 7.2 Hydrothermal mineral deposits. The dark part of this rock is chromite (chromium ore) that was pre- cipitated from hydrothermal fluids (watery hot fluids). The light-colored minerals form a vein of zeolites (a group of light—colored hydrous aluminum silicates formed by low— grade metamorphism). The vein formed when directed pressure fractured the chromite deposit, hydrothermal flu- ids intruded the fracture, and the zeolites precipitated from the hydrothermal fluids as they cooled (making a healed fracture and a vein of zeolites). and intrusive magma or hydrothermal fluids. The in- tensity then decreases rapidly over a short distance from the magma or hydrothermal fluids. Thus, zones of contact metamorphism are narrow, on the order of millimeters to tens-of-meters thick. Regional metamorphism occurs over very large areas (regions), such as deep within the cores of rising mountain ranges (see Figure 7.1), and generally is ac- companied by folding of rock layers (see Figure 7.3). Regional metamorphism is caused by large igneous intrusions that form and cool over long periods (thou- sands to tens-of-millions of years), the moderate to ex- treme pressure and heat associated with deep burial or tectonic movements of rock, and/ or the very wide- spread migration of hot fluids from one region to an- other along rock fractures and pore spaces. The distinction between contact and regional metamorphism often is blurred. Contact metamor- phism may be caused by small igneous intrusions, or by the local effects of hydrothermal fluids from some distance away that are traveling along fractures or 135 Metamorphic Rocks, Processes, and Environments ‘ i FIGURE 7.3 Folded and foliated (layered) gneiss. The dark minerals are muscovite, and the white minerals are quartz. Some of the quartz has been stained brown by iron. Regional metamorphism caused this normally rigid and brittle rock to be bent into folds without breaking. The flat mica mineral grains have been sheared (smeared) into layers called foliations. Metamorphic rocks with a layered appearance or texture are foliated metamorphic rocks. Figure 7.2 is a nonfoliated metamorphic rock because it lacks layering. other voids. Regional metamorphism may be caused by large intrusions, tectonism, and / or the action of abundant and widespread hydrothermal fluids asso- ciated with large intrusions. One kind of metamor- phism replaces another, so that rocks undergo both regional and contact metamorphism. Most major in- trusions are preceded by contact metamorphism and followed by regional metamorphism. The mineralogical composition of a metamorphic rock is a description of the kinds and relative abun- dances of mineral crystals that comprise the rock. In- formation about the relative abundances of the minerals is important for constructing a complete name for the rock and understanding metamorphic changes that formed the mineralogy of the rock. Miner- alogical composition of a parent rock may change dur— ing metamorphism as a result of changing pressure, changing temperature, and / or the chemical action of hydrothermal fluids. Mineralogical composition may also stay the same, whereas the texture of the rock changes. Recrystallization is a process whereby small FOLIATED METAMORPHIC ROCK: SLATE SEDIMENTARY ROCK: MUDSTONE Edges of well—developed slaty cleavage surfaces (shear planes) along which the rock prefers to break Clay Metamorphl'sm , 1><, See Figure 7.5 Clay minerals have been changed- to chlorite and muscovite mica. which are weakly foliated (aligned). Deformed bedding plane Slaty cleavage surfaces 5;? * gag-g J l i II @231! I:I . '| .. ' .'_||.l .l ' . i| ' I'I Iii . _, ':"i|i' ' Bedding -. ,«T J -. .1”. planes 1-. ,e: planes _ ‘6 r.- Outcrop shows sedimentary “'27 , rocks with horizontal bedding Outcrop shows bedding planes and strata. planes and strata have been Hand sample folded and sheared. Slaty cleavage has developed by shearing parallel to the axes of the folds like a plane of creases in a folded and NONFOL|ATED creased deck of cards. METAMORPHIC SEDlMENTARY ROCK: ROCK: QUARTZITE SANDSTONE -- .. Iwnh Sand grains . -- ' " and grains fused S no pore space Pore spaces - Metamorphism I FIGURE 7.4 Foliated and nonfoliated metamorphic rock formed by regional metamorphism. The mudstone and sandstone (sedimentary rocks) occur in layers separated by relatively flat, horizontal, bedding planes. Regional metamorphism com— presses the sedimentary rock layers and bedding planes until they are folded (bent) and sheared across the layering into flat, parallel sheets of slate that slide past one another. The flat parallel surfaces between the layers of slate are called slaty cleavage surfaces (because they resemble cleavage in minerals). Photomiorograph illustrations (in circles) show microscopic effects of the metamorphism. The layers of mudstone are meta- morphosed to slate (Figure 7.5), in which the chlorite and muscovite mineral crystals are also fo/iated (aligned and layered sub— parallel to the shear planes). The sandstone (comprised of quartz sand grains and pore spaces) is metamorphosed to the hard— er, more dense, nonfoliated metamorphic rock quartzite, which is comprised of fused quartz sand grains. Notice that the shear planes are not obvious in the quartzite, because the sand grains roll and move about easily as the rock deforms. crystals of one mineral will slowly convert to fewer, larger crystals of the same mineral, without melting of the rock. For example, microscopic calcite crystals in seashells that comprise a limestone can recrystal- lize to form a mass of visible calcite crystals in meta- morphic marble. Neomorphism is one way that mineralogical composition actually changes during metamorphism. In this process, minerals not only recrystallize but also form different minerals from the same chemical ele- ments. For example, shales composed mainly of clay minerals, quartz grains, and feldspar grains may change to a metamorphic rock composed mainly of muscovite and garnet. The most significant mineralogical changes occur during metasomatism. In this process, chemicals are added or lost. For example, anthracite coal is a rela- tively pure aggregate of carbon that forms when the volatile chemicals like nitrogen, oxygen, and methane are driven off from peat or bituminous coal by pres- sure and heating. Hornfels sometimes has a spotted appearance caused by the partial decomposition of just some of its minerals. In still other cases, one min- eral may decompose (leaving only cavities or molds where its crystals formerly existed) and be simul- taneously replaced by a new mineral of slightly or wholly different composition. A. Hand samples, 1X (shear planes) along Metamorphic Rocks, Processes, and Environments ° 1 37 Textures of Metamorphic Rocks Texture of a metamorphic rock is a description of its constituent parts and their sizes, shapes, and arrange- ments. Two main groups of metamorphic rocks are distinguished on the basis of their characteristic tex- tures,foliated and nonfoliated. Foliated metamorphic rocks (foliated textures) ex— hibit foliations—layering and parallel alignment of platy (flat) mineral crystals, such as micas. All meta- morphic rocks with a layered appearance are foliated. This usually forms as a result of pressure (shearing and smearing of crystals) and recrystallization. Crystals of minerals such as tourmaline, hornblende, and kyanite can also be foliated because their crystalline growth oc- curred during metamorphism and had a preferred ori- entation in relation to the directed pressure. Specific kinds of foliated textures are described below: - Slaty rock cleavage—a very flat foliatz'on (resem- bling mineral cleavage) developed along flat, par- allel, closely spaced shear planes (microscopic faults) in tightly folded clay- or mica-rich rocks (Figure 7.4). Rocks with excellent slaty cleavage are called slate (Figure 7.5), which is used to make roofing shingles and classroom blackboards. The flat surface of a blackboard or sheet of roofing slate is a slaty cleavage surface. Clay minerals have been changed to chlorite and muscovite mica, which are weakly foliated (aligned). well-developed slaty cleavage which the rock prefers to break B. Side view, 30X FIGURE 7.5 Slate—a foliated metamorphic rock with dull luster, excellent slaty cleavage, and no visible grains. Slate forms from low-grade metamorphism of mudstone (shale, claystone). Clay minerals of the mudstone parent rock change to foliated chlorite and muscovite mineral crystals. Slate splits into hard, flat sheets (usually less than 1 cm thick) along its well-developed slaty cleavage (Figure 7.4). It is used to make roofing shingles and classroom blackboards. 138 ° Laboratory Seven Weakly developed slat cleava e y g Well-formed muscovite crystals cause development of a strong foliation along which the rock Foliation surfaces ' prefers to break. Weakly developed slaty cleavage (poorly developed compared to slate) V B. Side view, 30X FIGURE 7.6 Phyllite—a foliated, fine-grained metamorphic rock, with a satiny, green, silver, or brassy metallic luster and a wavy foliation with a wrinkled appearance (phyllite texture). Phyllite forms from low- grade metamorphism of mudstone (shale, claystone), slate, or other rocks rich in clay, chlorite, or mica. When the very fine—grained mineral crystals of clay, chlorite, or muscovite in dull mudstone or slate are metamorphosed to form the phyllite, they become recrystallized to larger sizes and are aligned into a wavy and/or wrinkled foliation (phyllite texture) that is satiny or metallic. This is the wavy foliation along which phyllite breaks. Slaty cleavage may be poorly developed. It is not as obvious as the wavy and/or wrinkled foliation surfaces. The phyllite grade of metamorphism is between the low grade that produces slate (Figure 7.5) and the intermediate grade that produces schist (Figure 7.7). O Phyllite texture—a wavy and/0r wrinkled foliation of fine-grained platy minerals (mainly muscovite 0r chlorite crystals) that gives the rock a satiny or metallic luster. Rocks with phyllite texture are called phyllite (Figure 7.6). The phyllite texture is normally developed oblique or perpendicular to a weak Slaty cleavage, and it is a product of inter- mediate-grade metamorphism. 0 Schistosity—a scaly glittery layering of Visible (medium- to coarse-grained) platy minerals (main- ly micas and chlorite) and/0r linear alignment of long prismatic crystals (tourmaline, hornblende, kyanite). Rocks with schistosity break along scaly, glittery foliations and are called schist (Figure 7.7). Schists are a product of intermediate-to-high grades of metamorphism. U Gneissic banding—alternating layers or lenses of light and dark medium- to coarse—grained minerals. Rock with gneissic banding is called gneiss (Figures 7.3 and 7.8). Ferromagnesian minerals usually form the dark bands. Quartz or feldspars usually form the light bands. Most gneisses form by high-grade metamorphism (including recrys- tallization) of clay- or mica-rich rocks such as shale (see Figure 7.1), but they can also form by metamorphism of igneous rocks such as granite and diorite. Nonfoliated metamorphic rocks have no obvious layering (i.e., no foliations), although they may exhibit stretched fossils or long, prismatic crystals (tourma- line, amphibole) that have grown parallel to the pres- sure field. N onfoliated metamorphic rocks are mainly characterized by the following textures: 0 Crystalline texture (nonfoliated)—a medium— to coarse-grained aggregate of intergrown, usually equal-sized (equigranular), visible crystals. Marble is a nonfoliated metamorphic rock that typically exhibits an obvious crystalline texture (Figure 7.9). 'Visable wall-farmed muscovlts crystals Edges of weak slaty cleavage surfaces FIGURE 7.7 Schist—a medium- to coarse-grained, scaly (like fish scales), foliated metamorphic rock formed by intermediate-grade meta- morphism of mudstone, shale, slate, phyllite, or other rocks rich in clay, chlorite, or mica. Schist forms when clay, chlorite, and mica mineral crys- tals are foliated as they recrystallize to larger, more visible crystals of chlorite, muscovite, or biotite. This gives schist its scaly foliated appear- ance called schistosity. Slaty cleav— age or crenu/ations (sets of tiny folds) may be present, but schist breaks along its scaly, glittery schistosity. It often contains porphyroblasts of gar— net, kyanite, sillimanite, or tourmaline mineral crystals. The schist grade of metamorphism is intermediate between the lower grade that pro- duces phyllite (see Figure 7.6) and the higher grade that produces gneiss (see Figure 7.8). Also see chlorite schist in Figure 7.14. FIGURE 7.8 Gneiss—a medium- to coarse—grained meta- morphic rock with gneissic banding (alternating layers or lenses of light and dark minerals). Generally, light—colored layers are rich in quartz or feldspars and alternate with dark layers rich in biotite mica, hornblende, or tourmaline. Most gneisses form by high-grade metamorphism (includ- ing recrystallization) of clay or mica-rich rocks such as shale (see Figure 7.1), mudstone, slate, phyllite, or schist. However, they can also form by metamorphism of igneous rocks such as granite and diorite. The compositional name of the rock in this picture is biotite quartz gneiss. 139 140 ’ Laboratory Seven FIGURE 7.9 Marble—a fine- to coarse-grained, nonfoliat- ed metamorphic rock with a crystalline texture formed by tightly interlocking grains of calcite or dolomite. Marble forms by intermediate- to high-grade metamorphism of limestone or dolostone. Marble is a dense aggregate of nearly equal-sized crystals (see photograph), in contrast to the porous and random-sized crystal aggregate of its par- ent rock, limestone (see Figure 6.5). Calcite crystals Photomicrograph (X 26.6) Original sample width is 1.23 mm ' Microcrystalline texture—a fine-grained aggregate of intergrown microscopic crystals (as in a sugar cube). Hornfels (Figure 7.10) is a nonfoliated meta- morphic rock that has a microcrystalline texture. 0 Sandy texture—a medium- to coarse-grained ag- gregate of fused, sand-sized grains that resembles sandstone. Quartzite is a nonfoliated metamorphic rock with a sandy texture (Figure 7.11) remaining from its sandstone parent rock. 0 Glassy texture—a homogeneous texture with no Visible grains or other structures and breaks along glossy surfaces; said of materials that resemble glass, such as anthracite coal (Figure 7.12). Besides the main features that distinguish foliated and nonfoliated metamorphic rocks, there are some features that can occur in any metamorphic rock. They inclu...
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