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Metamorphic Rocks
Metamorphic Rocks and Processes
| Topic | Material |
|---|---|
| Metamorphic Rocks | Rocks that have been changed from a pre-existing rock (protolith) by heat, pressure, and/or chemical action of fluids (agents of metamorphism) while in the solid state. |
| Agents of Metamorphism | HEAT: From deep burial or intrusion by magma. PRESSURE: also from deep burial or tectonic stresses. CHEMICALLY ACTIVE FLUIDS: Derived from the rock itself of from intruding magmas. |
| Types of Metamorphism 1 | CONTACT METAMORPHISM: occurs at the contact between intruding magma and the country rock, and is caused primarily by the heat and magmatic fluids. |
| Types of Metamorphism 2 | BURIAL METAMORPHISM: Caused by the high heat and pressure that deeply buried rocks are subjected to. |
| Types of Metamorphism 3 | DYNAMIC METAMORPHISM: Occurs when tectonic processes create strong directional stesses along faults. |
| Types of Metamorphism 4 | REGIONAL METAMORPHISM: Affects rocks over a large area that is subjected to large scale tectonic processes such as mountain building. It may include all of the above types of metamorphism. |
| Metamorphism 1 | Can cause changes in the texture of a rock, its composition, or both. Recrystallization of mineral grains tends to produce coarser textures and/or reorientation of the grains. |
| Metamorphism 2 | When mineral grains recrystallize, new minerals may form, either from the same chemical constituents rearranged in new crystal forms, or by the addition of new chemical constituents from chemically active fluids. |
| Foliation 1 | If a rock is composed of flaky or platy minerals, such as mica, directed stress can cause these mineral grains to be oriented perpendicular to the direction of stress |
| Foliation 2 | The preferred orientation of platy or flaky minerals in metamorphic rocks. (definition) |
| Foliation 3 | A simple way of classifying metamorphic rocks first separates them into foliated and non-foliated rocks. |
| Foliated Rocks 1 | Several of the foliated metamorphic rocks can be thoughtof as representing stages in the progressive metamorphism of a shale protolith. Shale is composed of flaky clay minerals that have preferred orientation evident in its fissility. |
| Foliated Rocks 2 | Directed stress can reorient the clay minerals normal to the stress axis. The resulting rock, SLATE, will tend to break parallel to this new direction. Known as SLATY CLEAVAGE cause the pressure eliminates pore spaces. |
| Foliated Rocks 3 | Under continues pressure and high heat, clay minerals will recrystallize to fine grained muscovite known as sericite. The shiny micas give the rock a silky luster. This met. rock, which also exhibits slaty cleavage, is PHYLLITE. |
| Foliated Rocks 4 | If the right chemical constituents are present, more intense met. con produce feldspar minerals and biotite, which tend to form in segregated bands normal to the stress direction. |
| Foliated Rocks 4 Cont'd. | This preferred orientation of the light and dark minerals is referred to as GNEISSIC BANDING, and is the characteristic of the met. rock GNEISS, which usually has a similar composition to granite. |
| Lineation | Elongate minerals also may have been oriented parallel to one another in a preferred orientation. EX: amphibolite, composed primarily of elongate amphibole minerals (common product of the met. of basalt.) |
| Non-Foliated Rocks | Met. rocks that are NOT composed of flaky minerals or have not been subjected to directed stress do not develop foliation. Marble is the result of met. of carbonate rocks. It is still composed of calc. carb., but is usually coarsely crystalline. |
| Non-Foliated Rocks Cont'd.1 | A sandstone that has been met. is a quartzite. All pore spaces are lost, although quartz grains of the sand are still present, they become welded together. A broken quartzite goes right across grains rather than around them as in SS. |
| Non-Foliated Rocks Cont'd. 2 | If instead of being squeezed, shales or mudstones are baked by hot magma along intrusive contacts, they form a dense, fire grained rock called HORNFELS. |
| Intensity of Metamorphism 1 | Different minerals form from the same original constituents as different temperatures and pressures. Specific index minerals can be used to recognize different zones w/i which similar temp. and pressure conditions prevailed during metamorphism. |
| Intensity of Metamorphism 2 | Lines that separate metamorphic zones are lines of constant metamorphic grade, ISOGRADS, along which temperature and pressure conditions were constant and across which they differed. |
| Intensity of Metamorphism 3 | Metamorphic zones are apparent in the CONTACT AUREOLE surrounding an igneous intrusive body, within which the metamorphic grade is higher (higher temp.) closer to contact and lower farther from it. |
| Metamorphic Grade 1 | MET. FACIES are associations of index minerals that each characterize rocks formed under similar temperature and pressure conditions. They are usually named for an included mineral or rock type. |
| Metamorphic Grade 2 | The GREENSCHIST FACIES indicated low grade metamorphism (low T and P) and may be recognized by clorite, a green mica, or biotite. Higher grade met. produces the EPIDOTEAMPHIBOLITE, and then the amphib. facies rocks, which contain amphiboles. |
| Metamorphic Grade 3 | Still higher grade metamorphism is indicated by the PYROXENE GRANULITE FACIES with pyroxene and olivine. |
| Prograde and Retrograde Met. 1 | Met. that transforms sedimentary rocks into feldspars and other minerals w/relatively high melting pts is PROGRADE. A rock can be met. to amphibolite=RETROGRADE. |
| Where do met. occur? | Where there is igneous activity or the deformation that builds mountain ranges. Associated with plate boundaries, esp. subduction zones. |
| Where do met. occur? | The high temp., moderate pressure facies forms under a volcanic arc when it is active. A parallel band of the unusual blueschist facies marks the place wher |
Created by:
ebrinklow
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