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Introduction to Metamorphism Chemical Systems Definition Of Chemical Systems § An assemblage of coexisting phases Introduction to Metamorphism (thermodynamic equilibrium and the phase rule) l A basaltic composition can be either: Reading: F Melt F Cpx + plag (± olivine, ilmenite…) Winter Chapter 21 F Or any combination of melt + minerals along the liquid line of descent F If uplifted and eroded ® surface, will weather ® a combinations of clays, oxides… Definition of Metamorphism Lower Limit of Metamorphism “Metamorphism is a subsolidus process leading to l Low-temperature limit changes in mineralogy and/or texture (for example grain F Grades into diagenesis size) and often in chemical composition in a rock. These F The boundary is somewhat arbitrary changes are due to physical and/or chemical conditions s Diagenetic/weathering processes are that differ from those normally occurring at the surface indistinguishable from metamorphic s Metamorphism begins in the range of 100- of planets and in zones of cementation and diagenesis 150 oC for the more unstable types of protolith below this surface. They may coexist with partial s Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary melting.” Upper Limit of Metamorphism Metamorphic Agents & Changes l High-temperature limit grades into melting Temperature: typically the l Over the melting range solids and liquids most important factor in coexist metamorphism l If we heat a metamorphic rock until it melts, at what point in the melting process does it Estimated ranges of oceanic and continental steady-state become “igneous”? geotherms to a depth of 100 km using upper and lower l Xenoliths, restites, and other enclaves are limits based on heat flows considered part of the igneous realm because measured near the surface. After Sclateret al. (1980), melt is dominant Earth. Rev. Geophys. Space Sci., 18, 269-311. l Migmatites (“mixed rocks”) are gradational 1 Increased Temperature High Temperature Effects l Promotes recrystallization which increases grain size l Reactions occur that consume unstable mineral(s) and produces new minerals l Larger surface/volume ratio of a that are stable under the new mineral has lower stability conditions l Increasing temperature eventually l Overcomes kinetic barriers that might overcomes kinetic barriers to otherwise preclude the attainment of recrystallization, and fine aggregates equilibrium coalesce to larger grains Effect of Pressure “Normal” gradients may be perturbed in several ways, typically: s High T/P geotherms in areas of plutonic activity or rifting s Low T/P geotherms in subduction zones Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill. Metamorphic Grade A general increase in degree of metamorphism without Classification of specifying the exact relationship Metamorphic Rocks between temperature and pressure Reading: Winter Chapter 22: 2 Classification of Metamorphic Rocks Foliated Metamorphic Rocks l Metamorphic rocks are classified on the basis of texture and composition (either mineralogical or chemical) l Foliation: and planar fabric element l Unlike igneous rocks, which have been plagued by l Lineation: any linear fabric elements a proliferation of local and specific names, F They have no genetic connotations metamorphic rock names are surprisingly simple F Some high-strain rocks may be foliated, but and flexible they are treated separately l May choose some prefix-type modifiers to attach to names if care to stress some important or unusual textural or mineralogical aspects Foliated Metamorphic Rocks Foliated Metamorphic Rocks Cleavage Schistosity F Traditionally: the property of a rock to split along a regular set of sub-parallel, closely-spaced F A preferred orientation of inequaint mineral planes grains or grain aggregates produced by metamorphic processes F A more general concept adopted by some geologists is to consider cleavage to be any type F Aligned minerals are coarse grained enough to of foliation in which the aligned platy see with the unaided eye phyllosilicates are too fine grained to see F The orientation is generally planar, but linear individually with the unaided eye orientations are not excluded Foliated Metamorphic Rocks Foliated Metamorphic Rocks Slate: compact, very fine- grained, metamorphic rock with a well- developed cleavage. Freshly cleaved surfaces Gneissosestructure are dull Phyllite: a rock with a a F schistosity in which very Either a poorly-developed schistosity or fine phyllosilicates segregated into layers by metamorphic (sericite/phengite and/or chlorite), although rarely b processes coarse enough to see unaided, impart a silky F Gneissose rocks are generally coarse grained sheen to the foliation surface. Phyllites with both a foliation and lineation are very common. Figure 22-1. Examples of foliated metamorphic rocks. a. Slate. b. Phyllite. Note the difference in reflectance on the foliation surfaces between a and b: phyllite is characterized by a satiny sheen. Winter (2001) An Introduction to Igneous and Metamorphic Petrolo gy. Prentice Hall. 3 Foliated Metamorphic Rocks Foliated Metamorphic Rocks Gneiss: a metamorphic rock displaying gneissose structure. Schist: a metamorphic Gneisses are typically rock exhibiting a schistosity. By this layered (also called definition schist is a banded), generally with broad term, and slates alternating felsic and and phyllites are also darker mineral layers. types of schists. In Gneisses may also be common usage, schists lineated, but must also are restricted to those show segregations of metamorphic rocks in felsic-mineral-rich and which the foliated minerals are coarse dark-mineral-rich enough to see easily in concentrations. hand specimen. Figure 22-1c. Garnet muscovite schist. Muscovite crystals are visible and s ilvery, garnets occur as large Figure 22-1d. Quartzo-feldspathic gneiss with obvious layering. Winter (2001) An dark porphyroblasts. Winter (2001) An Introduction to Igneous a nd Metamorphic Petrology. Prentice Hall. Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Non-Foliated Metamorphic Rocks Specific Metamorphic Rock Types Marble: a metamorphic rock composed Simpler than for foliated rocks predominantly of calcite or dolomite. The Again, this discussion and classification applies only to protolith is typically limestone or dolostone. rocks that are not produced by high-strain metamorphism Granofels: a comprehensive term for any isotropic Quartzite: a metamorphic rock composed rock (a rock with no preferred orientation) predominantly of quartz. The protolith is Hornfels is a type of granofels that is typically very typically sandstone. Some confusion may fine-grained and compact, and occurs in contact result from the use of this term in sedimentary aureoles. Hornfelses are tough, and tend to splinter petrology for a pure quartz sandstone. when broken. Specific Metamorphic Rock Types Specific Metamorphic Rock Types Greenschist/Greenstone: a low-grade metamorphic Serpentinite: an ultramafic rock metamorphosed at rock that typically contains chlorite, actinolite, low grade, so that it contains mostly serpentine. epidote, and albite. Note that the first three minerals are Blueschist: a blue amphibole-bearing green, which imparts the color to the rock. Such a rock metamorphosed mafic igneous rock or mafic is called greenschist if foliated, and greenstone if graywacke. This term is so commonly applied to not. The protolith is either a mafic igneous rock or such rocks that it is even applied to non-schistose graywacke. rocks. Amphibolite: a metamorphic rock dominated by Eclogite: a green and red metamorphic rock that hornblende + plagioclase. Amphibolites may be contains clinopyroxene and garnet (omphacite + foliated or non-foliated. The protolith is either a pyrope). The protolith is typically basaltic. mafic igneous rock or graywacke. 4 Specific Metamorphic Rock Types Specific Metamorphic Rock Types Skarn: a contact metamorphosed and silica Migmatite: a composite silicate rock that is metasomatized carbonate rock containing heterogeneous on the 1-10 cm scale, calc-silicate minerals, such as grossular, commonly having a dark gneissic matrix epidote, tremolite, vesuvianite, etc. Tactite is a (melanosome) and lighter felsic portions synonym. (leucosome). Migmatites may appear layered, Granulite: a high grade rock of pelitic, mafic, or or the leucosomes may occur as pods or form quartzo-feldspathic parentage that is a network of cross-cutting veins. predominantly composed of OH-free minerals. Muscovite is absent and plagioclase and orthopyroxene are common. Metamorphic Rock Classification Metamorphic Rock Classification Additional Modifying Terms: Porphyroblastic means that a metamorphic rock has one or more metamorphic minerals that grew much larger than the others. Each individual crystal is a porphyroblast Some porphyroblasts, particularly in low-grade contact metamorphism, occur as ovoid “spots ” If such spots occur in a hornfels or a phyllite (typically as a contact metamorphic overprint over a regionally developed phyllite), the terms spotted hornfels, or spotted phyllite would be appropriate. Figure 23-14b. Spotted Phyllite. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Metamorphic Rock Classification Metamorphic Rock Classification Additional Modifying Terms: Some gneisses have large eye-shaped grains (commonly feldspar) that are derived
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