Lab 8: Metamorphic Rock Identification FOLIATION: Number these different foliation types in order of metamorphic grade from lowest(1) to highest (5)
Gneissic, Migmatitic, Phyllitic, Schistose, Slatey
INDEX MINERALS IN FOLIATED METAMORPHIC ROCKS: most foliated rocks are rich in silicate minerals. Some minerals form only in limited pressure and temperature conditions, and so are indicative of a particular metamorphic grade. Use the available Index Mineral chart in your lab reading and list which of these silicate minerals are most likely to be found in these foliated metamorphic types. Some minerals may occur in multiple grades of metamorphic rock:
Amphibole, Biotite, Chlorite, Garnet, Kyanite, Muscovite, Pyroxene, Sillimanite, Staurolite
SLATE
PHYLLITE
SCHIST
GNEISS
NON-FOLIATED METAMORPHIC COMPOSITIONS: match which minerals are found in which non-foliated metamorphic rocks. Consult the non-foliated metamorphic rock identification chart in your lab reading for assistance.
a. Quartz b. Garnet c. Dolomite d. Calcite e. Amphibole
Amphibolite
Quartzite
Marble
Metaconglomerate
PARENT ROCK COMPOSITION: fill in likely parent rock(s) for each of the following metamorphic rocks: Use your text chapter or the Metamorphic Rock Identification Charts for assistance.
Slate
Phyllite
Schist
Gneiss
Migmatite
Marble
Quartzite
Metaconglomerate
Hornfels
Amphibolite
Greenstone
Anthracite
Reading: Metamorphic Grades / Index Minerals Tectonics is the driving force behind all rock and mineral changes. Remember: minerals form at specific temperatures and pressures given a limited mixture of chemical elements. The minerals are only stable at the temperature and pressure at which they form. Tectonic processes move the rocks and minerals to a different location and the minerals become unstable, breaking down to form new minerals. When tectonics deforms the rock and forces it into the interior of massive mountain chains, it greatly alters the environment. Under the increased temperatures and pressure the rock deforms and recrystallizes, in the solid state, to form new metamorphic rocks. The temperatures and pressures that produce metamorphic rock are highly variable. There are many different processes that can create metamorphic rock. These processes range from meteorite impacts, fault movement, deep burial, and mountain building tectonics to igneous activity and more. The goal of a Metamorphic Petrologist is to determine what kind of geologic event created the new metamorphic rock. Sometimes the only evidence the geologist has is the texture and mineral composition of the metamorphic rock. From this evidence, the geologist must determine what the original rock was and what tectonic forces were present to produce the new rock.
Metamorphic Grades/Index MineralsThere are generally two major changes produced by metamorphism: altered textures and changes in mineral content. As temperatures and/or pressures are applied to the original ‘parent’ rock the rock starts to recrystallize. If the chemical composition is simple (all one mineral, Fig. 6.1) the new minerals produced will be the same composition as the original one. The new crystals may be larger in size and/or deformed into a preferred orientation. This is common in Quartz Sandstones or Limestones, where the mineral composition Fig. 6.1 A fine crystalline quartzite. The rock consists of one mineral, quartz, with a nonfoliated texture. The parent rock is a sandstone. 72 Fig. 6.2 Slate showing the original bedding of the parent shale (horizontal stripes). Notice that the ‘layering’ in the slate is not the same as the original bedding. is almost pure. Rocks with many different types of minerals produce more complex metamorphic alterations. The elements are rearranged and bonded to produce new mineral types and distinctive textures. Under different metamorphic conditions, for example, shales and basalts will form different metamorphic rock types. How do you track a parent rock and its many styles of alteration? Two simple ways are metamorphic grade and index minerals. (Metamorphic facies, a more complex system, will not be discussed in this course.) Metamorphic grades represent a quick way to determine how much the metamorphic rock has change from its original parent. Metamorphic grades use crystal sizes, mineral changes and texture alterations to compare the rock to its original state. Lowgrade metamorphic rocks are only slightly altered in character (Fig. 6.2). They generally have smaller crystals and the minerals present grew at lower temperatures or pressures. High-grade metamorphic Fig. 6.3 A folded gneiss. The minerals in this rock are greatly altered from their original parent. The high-grade nature of this rock shows bands of felsic and mafic minerals that have also been folded. conditions occur when rocks have been intensely altered (Fig. 6.3). The crystals are larger in size and may be altered to a preferred orientation. Minerals present within high-grade rocks are stable at high temperatures and pressures and are very different from the original minerals found in the parent rock. As a ‘parent’ rock deforms from low- to high- grades, a distinctive pattern of new minerals develops. These minerals are referred to as Index Minerals. The index minerals are formed from elements found only in the parent rock. The elements are released and ‘rearrange’, in the solid state, to form new minerals. Index minerals give a detailed record of the geologic changes that occur. Each represents a ‘code’, either a ‘thermostat’ or ‘barometer’, recording the tectonic history of the region. A metamorphic petrologist can determine tectonic environments by identifying the new index minerals (Fig. 6.4).
73 Fig. 6-4: Generalized Index Minerals for Metamorphic Grades Grades of Metamorphism (Clay) Low Medium High (Melt) <<<<<<<<<<<<<<<<<<<<<<<<< Quartz >>>>>>>>>>>>>>>>>>>>>>>>>>>>>> <<<<<<<<<<<<<<<<<<<<<<<<< Feldspars >>>>>>>>>>>>>>>>>>>>>>>>>>>> Chlorite >>>>>>>>>>>> <<<<< Muscovite >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> <<<<<<<< Biotite >>>>>>>>>>>>>>>>>>>>>>>>>>>>> <<<<< Hornblende >>>>>>>>>>>>>>>>>>>>>> <<<<< Garnet >>>>>>>>>>>>>>> <<<< Pyroxene >> The common parent rock for the above index minerals is a general type of Shale. Other common index minerals include: << Staurolite >>>> <<<< Kyanite >>> << Sillimanite >> Shale Progressive Metamorphic Rocks: >>Slate>>> Phyllite >>>>>>>>>> Schist >>>>>>>>>>>>>>>>>>>> Gneiss >>>>> Migmatite 74
Common Minerals In Metamorphic Rocks
Rock Fragments with Quartz Matrix; well fused together.
Quartz of various colors: milky quartz (left) and pink tinted quartz (right); fine and coarse textures possible; either foliated or nonfoliated textures.
Feldspars; sodium plagioclase and potassium feldspar; common in foliated textures
Muscovite; silver flakes; common in foliated textures, especially low to medium grade rocks.
Biotite; black flakes; common in foliated textures, especially medium and high grade rocks.
Talc; white to light green minute flakes; soft, soapy feel; possible in both foliated and nonfoliated textures.
Chlorite; small green flakes; common in foliated textures, especially in low grade rocks; possible in nonfoliated textures. Amphibole; black, rectangular crystals with shiny luster; possible in both foliated and nonfoliated textures of medium to high grades.
Garnet; wine red colored, round crystals; found mostly in foliated medium to high grade rocks.
Calcite and dolomite; white, pink, greens and grays; common in nonfoliated rocks.
Organic components; black, light weight, conchoidal fracture; nonfoliated textures only. Fig. 6.5
Each rock type results in a unique series of index minerals. To keep this course simple, we will use only one series of index minerals based on a clay-rich parent (Fig. 6.4). Notice that they include the common mafic minerals we have already studied. Keep in mind that not all minerals in a metamorphic rock are index minerals. Quartz and feldspars, for example, can occur at any grade and will not help in determining the grade of a metamorphic rock. (Fig. 6.5) Remember that different parent rocks result in different index mineral assemblages. Index minerals that form when rhyolite is metamorphosed will not be the same index minerals as when basalt is altered. Changes in rock textures are also possible during tectonic deformation. The development of textures in metamorphic rock occurs at different temperature and pressure conditions. Remember that the rock never melts and that the crystal growth occurs in the solid state. Therefore, higher grades of metamorphism (more intense deformation) result in greater alteration of crystal shapes. This may be evident by the development of crystals in ‘preferred orientations’ when pressure is applied. Types of Metamorphism The textures developed in the rocks are often based on the type of metamorphism that occurs. As new crystals develop, their growth is influenced by changes in pressure and the direction they come from. There are many ways to produce metamorphic alteration, in this lab we will only look at two types: contact and regional metamorphism. Contact Metamorphism The internal forces churning within the Earth’s interior are what drive tectonic processes. These forces are driven by the convection of heat radiating upward through Fig. 6.6 Amphibolite with crystals that are relatively large in size. The amphibole implies a mafic rich parent rock (basalt?) that has been altered to a medium or high grade. No foliation is visible. the mantle. Changes in heat and pressure result in bodies of molten magma forming in the Earth’s lithosphere. Volcanic activity, and the igneous rocks produced, are evidence of these processes. The molten material passes through miles of rock as it travels upward toward the Earth’s surface. The magma is significantly warmer than the surrounding rock (country rock/parent rock). The heat radiates into the country rock and alters the rock by a process known as contact metamorphism. Contact metamorphism occurs as the minerals recrystallize in the increasing temperatures (trying to stabilize to the new environment). New metamorphic minerals will develop through crystal growth, in the solid state. The grade to which the rock alters will be dependent on the temperature of the molten magma, the size of the magma body and the mineral composition of the parent rock. The higher the temperature of the adjacent magma (heat source) the more intense the contact metamorphism will be. The rock closest to the igneous intrusion will have the highest metamorphic grade, larger crystal sizes and higher temperature index minerals. Pressure is not an important factor for identifying this type of metamorphism. The crystals grow in a random pattern and show no alignment or preferred orientation. This rock texture is referred to as a nonfoliated texture (Fig. 6.7 & 6.8). Crystal sizes will depend on the grade of metamorphism and the amount of time in the metamorphic environment (Fig. 6.6). Regional Metamorphism The same internal forces that drive igneous activity also produce tectonic motion. Large masses of rock are forced into massive mountain chains where tectonic Metamorphic Rock Textures A B C D E F G Fig. 6.7 Metamorphic Rock Textures Foliated Textures: crystal alignment A) Slaty Cleavage: No visible minerals; dull luster; ‘layers’ B) Phyllitic Texture: No visible minerals; shiny luster; ‘layers’ some may be crenulated (folded). C) Schistosity: Visible minerals; visible layers and possible folding. D) Gneissic Texture: Visible minerals; banding with separation into felsic and mafic bands; folding possible. Nonfoliated Texture: random crystal orientation E) Large rock fragments and fused crystalline groundmass. F) Crystalline mafic minerals (amphibole). G) Crystalline, normal crystal sizes (ex. Calcite) 77 Fig. 6.8 Metaconglomerate, a nonfoliated rock made of rock fragments in a fused crystalline quartz matrix. The parent is a sedimentary conglomerate. plates collide. These tectonic forces deform rock through increased pressure and temperature. Minerals crystallizing under pressure produce a distinctive crystal orientation. Under a ‘squeezing’ pressure, all the crystals grow in the same direction: away from the greatest pressure. This rock texture is known as foliation (Fig. 6.7). Under low-pressure conditions (low grades) the crystals are microscopic and foliation is not well developed. The rock breaks along flat planes and has a texture known as slaty cleavage (Fig. 6.2 and 6.9). As the grade increases, the crystals grow larger and the texture more developed. Minerals are still not visible but large enough that the cleavage planes reflect light. Crenulations (small folds, Fig. 6.7B) are possible in the rock. This stage of foliation is called phyllitic texture (Fig. 6.10). Rocks formed at medium grade pressures contain crystals that have grown large enough to see with the naked eye. The texture is known as schistosity or a schistose texture (Fig. 6.11a & b). Micaceous minerals are common and the rock easily breaks along the micaceous layers. Fig. 6.9 Slate has microscopic minerals present in thin ‘layers’ of foliation (usually at an angle to bedding). Varying compositions result in many colors: red, gray and green are common. The highest-grade of foliation is called gneissic. Here the minerals have separated into felsic-rich bands alternating with maficrich bands. The high-grade metamorphic rocks appear to be stripped with light/ Fig. 6.10 A Phyllite. The minerals are microscopic micas that are still too small to see but large enough to reflect light. 78 Fig. 6.11a A Schist with visible muscovite and biotite micas. Micaceous minerals allow the rock to break apart easily. Garnets may be present (close up). dark bands and are solid, well formed rocks (Fig. 6.12). Note that the series of foliated rocks, Slate to Phyllite to Schist to Gneiss, is referred to as progressive metamorphism. The textures present in metamorphic rocks are indicators of the type of metamorphism and the metamorphic grade. Fig. 6.12 A gneiss showing high-grade foliation. The felsic bands (quartz and potassium feldspar) alternate with mafic bands (biotite mica). Fig. 6.11b Close up of a mica schist showing small red, ’round’ garnets. Nonfoliated textures are associated with increases in temperatures of contact metamorphism. The lack of significant pressures allows crystals to grow in random orientations. Foliated textures are developed through increased directed (‘squeezing’) pressure during regional metamorphism and mountain building events. The greater the pressure, the higher the alteration and the more developed the foliated texture. Higher pressures and temperatures result in melting and the development of igneous material. Parent Rocks The goal of studying metamorphic rock is to determine the tectonic history of the region (by determining the temperature and pressure ranges). An important part of this is understanding what the original, parent rock was. Metamorphic rocks that consist of a single mineral usually have a unique parent rock (Fig. 6.13). These rocks have been altered through simple recrystallization with no mineral changes. Usually the crystal orientations are difficult to see without a microscope. For this reason we assume any metamorphic rock with only one mineral type contains crystals that are random in character, i.e., nonfoliated textures. 79 Fig. 6.13 A white marble. The parent was a pure limestone. Marbles are nonfoliated rocks rich in either calcite or dolomite. Parent rocks are limestones or dolostones, respectively. Rocks with many different mineral types, go through metamorphic change by altering textures, mineral compositions or both. Parent rocks that contain multiple mineral types include most igneous rocks, sedimentary shale, and even other metamorphic rocks. Note that this list Fig. 6.14 A pink marble. The pink coloration is due to impurities in the parent rock. Note the minor ‘foliation’ represented by the darker (biotite?) region. consists of rocks that are composed of silicate minerals. Because regional metamorphism (with increased pressure) is the most common type of metamorphism, let’s say that the majority of silicate-rich rocks will alter into rocks showing foliation. The minerals that develop will be different for different parent rocks and metamorphic grades. Rocks containing ferromagnesian rich minerals will alter to rocks with index minerals. Without performing chemical analyses, the original minerals/parent rock types will be hard to determine. In most introductory courses, it is easier to just say that the parent rock for foliated metamorphic rocks is a clay-rich shale (the most abundant rock on the Earth’s surface). Classification of Metamorphic Rocks The first step in classifying metamorphic rocks is to determine the texture. Foliated textures have preferred crystal orientations. This means that the rocks will have ‘layers’ or stripes throughout the rock. (The ‘layers’ are not bedding, Fig. 6-2.) The higher the grade, the more developed the foliation. Nonfoliated rocks will be more massive in character, having random crystal growth. The minerals show no preferred orientation in the crystal habit. The crystals vary in size from microscopic to highly visible, depending on the metamorphic grade. (Nonfoliated rocks may develop during tectonic mountain building events. Minor foliation may be observed in some samples.) Nonfoliated rocks include the following: A carbonate-rich metamorphic rock, marble (Fig. 6.13 & 6.14), may consist of either calcite or dolomite. Color is commonly pink, white or green. Quartzite consists of pure quartz, whose colors may be red, purple, green or milky white (Fig. 6.1 & 6.15). The coloration in both marble and quartzite is due to inclusions in the parent rock. 80 Fig. 6.15 A pink quartzite. The mineral composition is quartz; the pink coloration is from minor inclusions of hematite in the sandstone parent. Mafic-rich nonfoliated rocks vary in composition and are typically dark in coloration. They may contain micas, amphiboles and feldspars with minor quartz. When the minerals are microscopic, they are identified by color: green rocks are called greenstone; dull, black rocks are called hornfels. With visible crystals (higher grades), ‘mafic’ nonfoliated rocks contain black hornblende and/or garnet and are called amphibolites (Fig. 6.6). Other nonfoliated metamorphic rocks include anthracite coal, a high-grade variety of coal (Fig. 6.16). Anthracite coal is shinier than its bituminous parent and commonly breaks with a conchoidal fracture. Metaconglomerates are metamorphosed conglomerates or breccias. They consist of solid, quartz-rich rocks with visible ‘rock fragments’ (Fig. 6.8). The rock is well fused and will not break easily; unlike its grainy, crumbly parent. If the ‘rock fragments’ are flattened, the metaconglomerate is called a stretched pebble metaconglomerate. (There are other nonfoliated metamorphic rocks, which we will not be covering in this lab.) Fig. 6.16 Anthracite coal is a high-grade form of coal that is very brittle and shiny in luster. Some samples will have conchoidal fracture. Foliated metamorphic rocks display progressive metamorphic foliation: slaty cleavage – phyllitic texture – schistosity – gneissic textures. Low grade foliated rocks are named using their textures: a rock with slaty cleavage is referred to as a slate (Fig. 6.2 & 6.9). A rock with phyllitic texture is known as a phyllite (Fig. 6.10 & 6.17). Identification of minerals in low-grade foliated rocks is difficult because the minerals are microscopic. The major difference is the way the rock reflects light: slates are usually dull ‘luster’, while phyllites have a sheen (reflect light). Medium to high-grade rocks with a schistose texture are called schists (Fig. 6.11 & 6.18). The visible minerals in schistose textures are easier to identify. Schistose rocks rich in silicates often contain one or more types of micas. The micas form weak zones in the rock and the rock may break along micaceous zones. Other mafic minerals, such as amphiboles and garnets, can occur in higher-grade schists. Index minerals are included in the rock name. 81 Fig. 6.17 Phyllite with a bright sheen. Some phyllites are crenulated (tightly folded). Examples of common schistose rocks include: Muscovite Biotite Schist or Amphibole Garnet Schist. (Quartz and feldspars may occur but are not index minerals and, therefore, not used in identification.) The highest grade of foliated rock types is known as gneiss (Fig. 6.3, 6.12 & 6.19). The gneissic texture is distinguished by the banding that appears as thin streaks or prominent stripes in the rock. The bands consist of visible minerals that are separated into felsic and mafic regions. The felsic minerals are quartz, and sodium or potassium-rich feldspars. The mafic bands include micas, amphibole, garnet, pyroxene and other mafic minerals. Gneissic rocks are common as grave markers and ‘granite’ counter tops because they are so sturdy and solid in character. Fig. 6.18 A muscovite garnet schist with highly visible garnets. Fig. 6.19 A gneiss with biotite and muscovite bands alternating with quartz and sodium plagioclase. Fewer mafic minerals mean the parent rock contained fewer iron and magnesium rich minerals (a rhyolite or granite?). Prelab: Answer additional questions on prelab page. Lab Assignment: Metamorphic rocks can be identified using textures, grades and compositions, when necessary. Charts 6.1 & 6.2 give the Classification of Metamorphic Rocks. Using the following procedure, identify the samples provided by your instructor. Duplicates are possible. 82 CHART 6-1: Metamorphic Rock Classification; Foliated Textures Foliation Diagnostic Features Type of Parent Rock Rock Name Microscopic crystals; dense with rock cleavage that breaks I Slaty Cleavage in layers; variable colors: gray, black, green and red; dull luster. Shale Slate N Low grade. R C Very fine crystals; dense with rock cleavage that may be E Phyllitic crenulated; variable colors: gray, greens; sparkling luster. Slate Phyllite A Low grade. I S Visible crystals: micas common, garnets, amphiboles; N Schistose rocks commonly break along micaceous layers; shiny luster; Phyllite or *Schist G Porphyroblasts (garnets) possible. Medium grade. fine igneous rocks *Name modified with visible index minerals. R G Coarse crystals separated into felsic/mafic bands: felsic A minerals include quartz and feldspars, mafics include D Gneissic biotite, amphiboles, pyroxenes, garnets; solid rock with Schist or *Gneiss E no rock cleavage. High grade. any igneous rock *Name modified with visible index minerals. (Migmatites are rocks that are a mixture of igneous and metamorphic textures. folded, distorted mafic bands. Highest grade; partial melting occurs.) Features: areas of coarse crystalline quartz and feldspars between zones of commonly used in the metamorphic rock name. Index minerals are important rock name modifiers. *Note: Though quartz and feldspar may be present in medium and high grade rocks, they are not Ex: muscovite garnet schist CHART 6-2: Metamorphic Rock Classification; Nonfoliated Textures Composition Diagnostic Features Parent Rock Rock Name Variable colors; white, red, buff, browns; hard; fused Quartz quartz grains. Crystal size varies with metamorphic Sandstone Quartzite grade: fine = lower grades; coarse = higher grades. Calcite or Dolomite Variable colors; white, green, pinks; soft; effervesces Limestone or Dolomite (may need to be powdered); coarse crystalline. Dolostone Marble Rock fragments Metamorphic grade varies. fused in quartz background; Conglomerate or Rock Fragments pebbles may be stretched (foliation); breaks across Breccia Metaconglomerate pebbles; hard. Metamorphic grade varies. Organic matter Black; shiny luster; low density; conchoidal fracture. Bituminous Coal Anthracite Coal Low to medium metamorphic grade. Mafic minerals Green; dense, microscopic minerals (chlorite?, amphiboles?, Basalt (or Shale?) Greenstone feldspars?); Low to medium metamorphic grade. Mafic minerals Black to dark gray; dense, microscopic minerals Varies, usually Hornfels (amphibole, feldspars); Low to medium metamorphic grade. Basalt or Shale Black with white and/or red; glossy luster; coarse Mafic minerals crystalline: amphibole (hornblende), feldspars +/- garnets; Basalt (or Shale?) Amphibolite May be foliated. Medium to high metamorphic grade. 84
Classification of Metamorphic Rock
Determine the Metamorphic Texture: Nonfoliated – random crystals; no apparent ‘layers’; higher grades have coarser crystal sizes. Foliated – may appear ‘layered’; minerals aligned/parallel. Folds may be present Determine the Metamorphic Composition: Foliated Textures: Named by grade/type of texture Slaty Cleavage -low grade -thin rock layers -microscopic minerals Phyllitic -low to medium grade -thin rock layers -shiny microscopic minerals Schistose -medium grade to high grade -layered; may be folded -visible minerals; often micaceous Gneissic -highest grade -alternating felsic/mafic banding -visible minerals Determining minerals in foliated textures: Only if visible minerals are present. Identify only index minerals. Do not include quartz or feldspars. Modify the rock with the most abundant mineral, when necessary. Ex: garnet schist; muscovite schist Nonfoliated: -Use mineral properties to identify mineral types: -Usually one or two minerals. -Dark microscopic textures: -refer to as ‘mafic’ minerals. Suggest the likely Parent Rock for every rock sample
