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A shield is a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. [1] These rocks are older than 570 million years and sometimes date back to around 2 to 3.5 billion years.
This tends to produce low-grade metamorphic rock. [29] Much more common is metamorphic rock formed during the collision process itself. [30] The collision of plates causes high temperatures, pressures and deformation in the rocks along these belts. [31] Metamorphic rock formed in these settings tends to shown well-developed schistosity. [30]
The macroscopic colour is grey, being little darker than granite. There is high-grade metamorphic rock originated from nepheline syenite that is characterized by gneiss texture of very rare occurrence. It is called nepheline syenite gneiss or litchfieldite. An example is found at Canaã village, State of Rio de Janeiro, Brazil.
Semi pelite - A type of metamorphic rock that falls between pelitic (clay-rich) and psammitic (sand-rich) compositions; Serpentinite – Rock formed by transformation of olivine; Skarn – Hard, coarse-grained, hydrothermally altered metamorphic rocks; Slate – Metamorphic rock - A low grade metamorphic rock formed from shale or silt
Slate is an example of a foliated metamorphic rock, originating from shale, and it typically shows well-developed cleavage that allows slate to be split into thin plates. [36] The type of foliation that develops depends on the metamorphic grade.
Map of index minerals of the Barrovian metamorphic zones of Scotland. An index mineral is used in geology to determine the degree of metamorphism a rock has experienced. . Depending on the original composition of and the pressure and temperature experienced by the protolith (parent rock), chemical reactions between minerals in the solid state produce new mi
The minerals present in a granulite will vary depending on the parent rock of the granulite and the temperature and pressure conditions experienced during metamorphism. A common type of granulite found in high-grade metamorphic rocks of the continents contains pyroxene, plagioclase feldspar and accessory garnet, oxides and possibly amphiboles.
Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock that was reconstituted subsequently by partial melting ("paleosome"), while the alternate layer has a ...