Blueschist Rocks Geology Worksheet-George Mason University Museum.

METAMORPHIC ROCKS OBJECTIVE To recognize the major metamorphic rock types and to understand the significance of their texture and composition. MAIN CONCEPT Metamorphic rocks are classified on the basis of texture and composition. Two main groups are recognized: (1) foliated and (2) nonfoliated. Regional metamorphism occurs in the roots of mountain belts formed at convergent plate margins and along the rift zone at divergent plate margins. Contact metamorphism develops at the margins of igneous intrusions. SUPPORTING IDEAS 1. Foliation results from recrystallization and the growth of new minerals. 2. The three main types of foliation are (a) slaty cleavage, (b) schistosity, and (c) gneissic layering. 3. Nonfoliated texture develops by the recrystallization of rocks composed predominantly of one mineral, such as sandstone (quartz) or limestone (calcite). THE NATURE OF METAMORPHIC ROCKS Many igneous and sedimentary rocks have recrystallized in the solid state— without melting—to such an extent that the diagnostic features of the original rock have been greatly modified or obliterated. Recrystallization occurs because of changes in temperature, pressure, and the chemical composition of the fluids that flow through them. We call these solid-state processes metamorphism (Greek, “changed form”). These solid state reactions are akin to those that a potter uses to convert soft clay into hard ceramic. When a soft clay pot is placed in a kiln at a temperature near 1200 °C, the clay minerals change into other minerals that are stable under those conditions. In other words, the clay is metamorphosed. The recrystallization occurs without melting but is sufficient to create a new material radically different than its precursor. During metamorphism of rocks, most structural and textural features in the original rock—such as stratification, graded bedding, vesicles, and porphyritic textures—are destroyed. New minerals replaced those originally in the rock to create a new rock texture. These are metamorphic rocks, a major group of rocks that results largely from the constant motion of tectonic plates (Figure 5.1, next page). Metamorphic rocks can be formed from igneous, sedimentary, or even previously metamorphosed rocks. Many people know something about various igneous and sedimentary rocks but only vaguely understand the nature of metamorphic rocks. All of us have seen many environments where new sedimentary rocks are forming. Most have also seen a few igneous rocks form—when volcanoes erupt, for example. But the formation of metamorphic rocks takes place so deep within the crust that we are not familiar with these processes. Perhaps the best way to become acquainted with this group of rocks, and to appreciate their significance, is to study carefully Figure 5.1. Originally these were sedimentary and volcanic layers deposited horizontally. They have been deformed so intensely, however, that it is difficult to determine the original bottom or top of the rock sequence. The effects of metamorphism include (1) chemical recombination and the growth of new minerals, with or without the addition of new elements from circulating liquids and gases; (2) deformation and rotation of the constituent mineral grains; and (3) the recrystallization of minerals to form larger grains. The net result is rock of greater crystallinity, increased hardness, and new structural features that commonly exhibit the effects of flow or other expressions of deformation. All three major rock types can be metamorphosed, but intrusive igneous rocks and previously metamorphosed rocks are affected less by metamorphic processes than are the sedimentary rocks that developed at Earth’s surface. METAMORPHIC TEXTURE Foliated Textures Foliation is a planar element in metamorphic rocks. It may be expressed (1) by closely spaced fractures (slaty cleavage), (2) by the parallel arrangement of platy minerals (schistosity), or (3) by alternating layers of different mineral composition (gneissic layering). Foliation is usually developed during metamorphism by directed stresses that cause differential movement or recrystallization. It is a fundamental characteristic of metamorphic rocks and is a basic criterion of the classification system. The various types of foliation are shown in Figure 5.2A-D. Slaty Cleavage. Slaty cleavage is a type of foliation expressed by the tendency of a rock to split along parallel planes. Do not confuse this planar feature with bedding planes, which are a sedimentary structure. Slaty cleavage results from the parallel orientation of microscopic platy minerals such as mica, talc, or chlorite. In a metamorphic environment, these minerals grow with flat surfaces perpendicular to the applied forces. The perfect cleavage within each tiny mineral grain is thus oriented in the same direction. This creates definite planes of weakness throughout the entire rock and causes it to break along nearly parallel planes. Such a surface is shown in Figure 5.2A. Note that slaty cleavage commonly cuts across bedding planes, the thin light and dark lines in the specimen in the figure. It is the product of a relatively low intensity of metamorphism. Schistosity. Schistosity develops from more intense metamorphism. Mica, chlorite, and talc form larger, visible crystals and, as a result, the rock develops a distinctly planar element (Figure 5.2B). Schistosity is thus similar to slaty cleavage, but the platy mineral crystals are much larger, and the entire rock appears coarse grained. The increase in crystal size represents a higher grade of metamorphism in which garnet, amphibole, and other nonplaty minerals also develop. Schistosity typically occurs in mica-rich rocks. Gneissic Layering. Gneissic layering is a type of foliation in which the planar element is produced by alternating layers of different mineral composition. Rocks with gneissic layering are characteristically coarse grained and represent a higher grade of metamorphism in which the minerals are recrystallized, stretched, crushed, and rearranged completely. Feldspar and quartz commonly form light-colored layers that alternate with dark layers of ferromagnesian minerals (Figure 5.2C). Nonfoliated Textures Some metamorphic rocks do not possess foliation but appear massive and structureless, except for elongated grains or other linear features resulting from directional stress. Examples of nonfoliated texture are shown in Figure 5.3A-C. Nonfoliated rocks are commonly formed from a parent rock composed largely of a single mineral, such as sandstone or limestone. Lineation, such as the elongated pebbles in Figure 5.3A, may develop in nonfoliated rocks. The pebbles were originally round but have been stretched by the directed stress within the rock. Sand grains in a metamorphosed sandstone often show similar expressions of deformation, or the grains may be fused into a dense, compact mass of interlocking particles. Deformation of limestone will produce streaks of material that contains large amounts of organic matter. CLASSIFICATION OF METAMORPHIC ROCKS The formation of metamorphic rocks is so complex that developing a satisfactory classification system is difficult. The most convenient scheme is to group metamorphic rocks by structural feature, with further subdivision based on composition. Using this classification, two major groups of metamorphic rocks are recognized: (1) those that are foliated (possess a definite planar structure), and (2) those that are non-foliated, that is, massive and structureless. The foliated rocks can then be subdivided further, according to the type of foliation. Finally, a large variety of rock types can be recognized in each group, according to the dominant minerals. The basic framework for this classification system is shown in Figure 5.4. Examples of the major types of metamorphic rocks are shown in Figures 5.5 and 5.6. They are shown in the same order in which they are listed in Figure 5.4. Serpentinite is a fine-grained metamorphic rock that differs from the more conventional metamorphic rocks shown in the classification chart above. It may be massive or it may be
weakly foliated with weak schistosity. Serpentinite is derived from sea-water circulation through oceanic crust near the oceanic ridge. Large volumes of cold seawater seep downward through fractures and fissures in the pillow basalts and sheeted dikes and are heated and then rise, altering the surrounding basalt. Serpentinite is thus one of the most abundant metamorphic rocks on Earth, although most is hidden beneath the ocean. PHOTOGRAPHS OF ROCK SPECIMENS The photographs in Figures 5.5 and 5.6 illustrate the characteristics of the major types of metamorphic rocks, as seen in hand specimens and under a microscope. The hand specimens are actual size and the photomicrographs show a magnification of approximately 20 times. Some specimens have a polished surface, which better shows their textual features. Others have a natural fractured surface. Remember, the photographs are not a key to rock identification! Use them as a visual reference only. listed, identify the name of the rock Composition: K-Feldspar 30% Plagioclase 20% Mica 20% Amphibole 30% O Slate Marble Schist O Phyllite Garnet Mica Schist O Quartzite Anthracite Gneiss O Metaconglomerate Using the photograph/micrograph along with the mineral composition listed, identify the name of the rock Composition: CaCO3 90% Chlorite 5% Slate Marble Schist Phyllite Garnet Mica Schist O Quartzite O Anthracite Gneiss Metaconglomerate Using the photograph/micrograph along with the mineral composition listed, identify the name of the rock Composition: Chlorite 80% Other minerals 20% Slate O Marble Schist Phyllite Garnet Mica Schist O Quartzite O Anthracite O Gneiss Metaconglomerate Using the photograph/micrograph along with the mineral composition listed, identify the name of the rock Composition: Quartz 95% Rock Fragments 5% Slate O Marble Schist O Phyllite Garnet Mica Schist O Quartzite O Anthracite Gneiss Metaconglomerate