TYPES OF METAMORPHISM AND METAMORPHIC ROCKSRecall that three conditions cause metamorphism: rising temperature, rising pressure, and changing chemical environment. In addition, tectonic deformation develops foliation and thus strongly affects the texture of a metamorphic rock. These conditions occur in four geologic environments.
CONTACT METAMORPHISM
Contact metamorphism occurs where hot magma intrudes cooler country rock. The country rock may be of any type – sedimentary, metamorphic, or igneous. The highest-grade metamorphic rocks form at the contact, closest to the magma. Lower-grade rocks develop farther out. A metamorphic halo around a pluton can range in width from less than a meter to hundreds of meters, depending on the size and temperature of the intrusion and the effects of water or other fluids. Contact metamorphism commonly occurs without deformation. As a result, the metamorphic minerals grow with random orientations, and the rocks develop no metamorphic layering.
Common Contact Metamorphic Rocks The hornfels is a hard, dark, fine-grained rock usually formed by contact metamorphism of shale. Mica and chlorite are common in the cooler, outer parts of a hornfels halo. Hornblende and other amphiboles occur in the middle of the halo, and pyroxenes can form next to the pluton, in the highest-temperature zone. Quartz and feldspar are common throughout the halo, because they are stable over a wide temperature range.
BURIAL METAMORPHISM
Burial metamorphism results from deep burial of rocks in a sedimentary basin. A large river carries massive amounts of sediment to the ocean every year, where it accumulates on a delta. Over tens or even hundreds of millions of years, the weight of the sediment becomes so great that the entire region sinks isostatically. Younger sediment may bury the oldest layers to a depth of more that 10 kilometers in a late basin. Because rocks are heavy, pressure within the Earth increases rapidly with depth. Over time, temperature and pressure increase within the deeper layers until burial metamorphism begins. Burial metamorphism occurs without tectonic deformation. Consequently, metamorphic minerals grow with random orientations, and, like contact metamorphic rocks, burial metamorphic rocks are unfoliated.
Common Burial Metamorphic Rocks Because of the lack of deformation, rocks formed by burial metamorphism often retain sedimentary structures. Shales and siltstones become harder and better lithified to form argillite which looks like the parent rock although new minerals have replaced the original ones. Quartz sandstone becomes quartzite. When sandstone is broken, the fractures occur in the cement between the sand grains. In contrast, quartzite becomes so firmly cemented during metamorphism that the rock fractures through the grains. Burial metamorphism converts limestone and dolomite to marble.
REGIONAL METAMORPHISM
Regional metamorphism occurs in and near a subduction zone, where tectonic forces build mountains and deform rocks. It is the most common and wide-spread type of metamorphism and affects broad regions of the Earth’s crust. Magma forms in a subduction zone, where oceanic lithosphere is sinking beneath a continent. As the magma rises, it heats large regions of the crust. The high temperatures cause new metamorphic minerals to form throughout the region. At the same time, the tectonic forces squeeze and deform rock. The rising magma further deforms the hot, plastic country rock as it forces its way upward. As a result of all of these processes acting together, regionally metamorphosed rocks are strongly foliated and are typically associated with mountains and igneous rocks. Regional metamorphism produces zones of foliated metamorphic rock tens to hundreds of kilometers across.
Common Rocks Formed by Regional Metamorphism Shale consists of clay minerals, quartz, and feldspar and is the most abundant sedimentary rock. Shale undergoes a sequence of changes as metamorphic grade increases. At definite temperatures certain metamorphic minerals are stable. Thus, it shows the sequence in which minerals appear, and then decompose, as metamorphic grade increases. As regional metamorphism begins, the clay minerals break down and are replaced by mice and chlorite. These new, platy minerals grow perpendicular to the direction of tectonic squeezing. As a result, the rock develops slaty cleavage and is called slate (аспидный сланец). With rising temperature and continued deformation, the micas and chlorite grow larger, and wavy or wrinkled surfaces replace the flat, slaty cleavage, giving phyllite a silky appearance. As temperature continues to rise, the mica and chlorite grow large enough to be seen by the naked eye, and foliation becomes very well developed. Rock of this type is called schist (кристаллический сланец). Schist forms approximately at the transition from low to intermediate metamorphic grades. At high metamorphic grades, light- and dark-colored minerals often separate into bands that are thicker than the layers of schist to form a rock called gneiss (pronounced “nice”). At the highest metamorphic grade, the rock begins to melts, forming small veins of granitic magma. When metamorphism wanes and the rock cools, the magma veins solidify to form migmatite, a mixture of igneous and metamorphic rock. Under conditions of regional metamorphism, quartz sandstone and limestone transform to foliated quartzite and foliated marble, respectively.
HYDROTHERMAL METAMORPHISM
Water is a chemically active fluid; it attacks and dissolves many minerals. If the water is hot, it attacks minerals even more rapidly. Hydrothermal metamorphism (also called hydrothermal alteration and metasomatism) occurs when hot water and ions dissolved in the hot water react with a cork to change its chemical composition and minerals. In some hydrothermal environments, water reacts with sulfur minerals to form sulfuric acid, making the solution even more corrosive. The water responsible for hydrothermal metamorphism can originate from three sources. Magmatic water is given off by a cooling magma. Metamorphic water is released from rocks during metamorphism. Most hydrothermal alteration, however, is caused by circulating ground water – the water that saturates soil and bedrock. Cold ground water sinks through bedrock fractures to depths of a few kilometers, where it is heated by the hotter rocks at depth or, in some cases, by a hot, shallow pluton. Upon heating, the water expands and rises back toward the surface through other fractures. As it rises, it alters the country rock through which it flows.
Rocks Formed by Hydrothermal Metamorphism Hydrothermal metamorphism is like an accelerated from of weathering. As in weathering, feldspars and many other minerals of the parent rock dissolve. The hot water carries away soluble components, such as potassium, sodium, calcium, and magnesium. Aluminum and silicon remain because they have low solubilities. They combine with oxygen and water to from clay minerals. Hydrothermally metamorphosed rocks often have a white, bleached appearance and a soft consistence because the clays are white and soft. Most rocks and magma contain low concentrations of metal such as copper, gold, lead, zinc, and silver. Although metals are present in very low concentrations, hydrothermal solutions sweep slowly through vast volumes of country rock, dissolving and accumulating the metals as they go. The solutions then deposit the dissolved metals when they encounter changes in temperature, pressure, or chemical environment. In this way, hydrothermal solutions scavenge and concentrate metals from average crustal rocks and then deposit them locally to form ore.
|
