rock index: metamorphic
Metamorphic rocks are classified according to the parent rock and the degree of deformation to which the rocks have been subjected. Metamorphism occurs locally (contact, dynamic, hydrothermal) or on a large scale (regional). Altered rocks undergo textural/mineralogical changes in regions of contact metamorphism ۷, dynamic metamorphism ۷, hydrothermal metamorphism ۷, and regional metamorphism ۷. Metamorphic rocks can occur in zones where degree of alteration to index minerals passes through chlorites → biotite → garnets → staurolite → kyanite → sillimanite. Metamorphic rocks are also classified by metamorphic grade or metamorphic facies (image above left - click to view larger image in metamorphic facies item). Z = Zeolite : PP = Prehnite-Pumpellyite : H = Hornfels : GS = Greenschist : A = Amphibolite G = Granulite B = Blueschist E = Eclogite | ||
Contact metamorphism typically manifests as heat-induced metamorphic aureoles in the country rock surrounding intrusive igneous rocks (plutons). The metamorphic grade of hornfels rocks produced in the aureole is that of the peak metamorphic mineral which has formed within the aureole. | ||
| muds & clays | Pelites are are derived from muds or clays, and form andalusite hornfels, sillimanite hornfels, or pyroxene hornfels. | |
| sandstones | Psammites are derivatives of micaceous sandstones. | |
| Dynamic metamorphism results from frictional heat and confining pressures associated with major fault zones. Textures generated in dynamic metamorphic zones depend on the depth of formation where confining pressures determine the predominant deformation mechanisms. | ||
| Tectonic or monomictic breccia | At depths of less than 5km, confining pressures are to low to produce dynamic metamorphism, and a zone of breccia or cataclasite is formed instead, with the rock milled and broken into a mélange of random fragments. | |
| Pseudotachylites | Pseudotachylites form at depths from 5-10 km, where confining pressures are focused into discrete fault planes and are sufficient to prevent brecciation and milling. The frictional heating at these depths can melt the rock to form pseudotachylite glass or mylonite, and adjacent to these zones, can result in growth of new mineral assemblages. | |
| Mylonites | Mylonites are fine-grained, compact, laminated, metamorphic rocks with textures greatly modified by predominantly ductile, plastic flow due to dynamic recrystallization in fault zones deeper than 10-20km. | |
| Hydrothermal metamorphism results from the interaction of a rock with warm fluids, producting metamorphic and metasomatic reactions that depend upon temperature and compositional differences between the country rock and the invading fluid. | ||
| Serpentinites | Serpentinites contain serpentine minerals generated by low-grade hydrothermal serpentinization of mantle-derived ultramafic rocks, such as peridotites and dunites. | |
| Pegmatites | Pegmatites are very coarse-grained igneous rocks in which crystal grains are larger than 20 mm, usually larger than 50 mm. They are produced when mineral laden water leaks from crystallizing magma into surrounding country rock. | |
| Mineral ore deposits | Rich ore deposits often form through hydrothermal metamorphism. Hydrothermal metamorphism can also produce Mg-Fe rich hydrous minerals, such as talc, chlorites, serpentines, actinolite, tremolite, zeolites, and clay minerals from basaltic rocks that normally lack hydrous minerals. | |
| Regional metamorphism involves large volumes of rock subjected to heat and intense compression during tectonic events such as subduction and orogeny. Depending upon the location, regional metamorphism follows Buchan (Abukama), Barrovian, Sanbagawa, or Franciscan trajectories, producing metamorphic rocks of characteristic facies. | ||
| Amphibolite facies | Amphibolites arise through Buchan, Barrovian, and or advanced Abukuma regional metamorphic temperature and pressure conditions | |
| Blueschist facies | Blueschists form at moderate temperatures and low pressures diagnostic of former subduction zones, corresponding to burial at a depth of 15 to 30 kilometers (>0.6 GPa) at 200~500 ºC. | |
| Eclogite facies | Eclogites are coarse-grained mafic metamorphic rocks of basaltic composition that form under conditions of Franciscan metamorphism (high temperature (>400–1000 °C) and very high pressure (1.2 GPa) at depths below that of the crust). | |
| Granulite facies | Granulites are regional metamorphic rocks with a granuloblastic texture that form at high temperatures and moderate through high pressures. | |
| Greenschist facies | Greenschists or greenstones are schistose or foliated basalt-derived rocks tinted by the greenish chlorite phyllosilicates, actinolite, and epidote. Greenschists define low temperature (400-500 °C), moderate pressure metamorphism (depths > 8km) of Barrovian and the lower-pressure, high temperature Abukuma regional metamorphism (mid-oceanic ridges). | |
| Prehnite-Pumpellyite facies | The prehnite-pumpellyite facies is a metamorphic facies diagnostic of temperatures of 250-350 °C and pressures of 2-kb associated with low-grade alteration of basaltic oceanic crust at mid-ocean ridge spreading centres. | |
| Sanidinite facies | Rocks of the sanidinite facies, which results from temperatures higher than the pyroxene hornfels facies, are relatively rare in contact metamorphic aureoles, although they are found as xenoliths in igneous rocks. | |
| Zeolite facies | The zeolite facies results from low grade metamorphism due to low temperature/low pressure (T ~50 - 150 °C, burial at 1 - 5 km depth). | |
| At depths and temperatures greater than Barrovian and temperatures greater than Franciscan regional metamorphism, metamorphism merges with magma formation, producing partial melt migmatites. | ||
