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Granulites are a class of high grade metamorphic rocks of the granulite-facies that have experienced high-temperature and moderate pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure.〔D.R. Bowes (1989), ''The Encyclopedia of Igneous and Metamorphic Petrology''; Van Nostrand Reinhold ISBN 0-442-20623-2〕 They are of particular interest to geologists because many granulites represent samples of the deep continental crust. Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature; others cooled while remaining at depth in the Earth. 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. Both clinopyroxene and orthopyroxene may be present, and in fact, the coexistence of clino- and orthopyroxene in a metabasite (metamorphed basalt) defines the granulite facies. A granulite may be visually quite distinct with abundant small pink or red pyralspite garnets in a 'granular' holocrystalline matrix. Concentrations of garnets, micas, or amphiboles may form along a linear pattern resembling gneiss or migmatite banding. ==Formation== Granulites form at high-temperature conditions at a range of pressure conditions, typically during regional metamorphism. In some cases, the high temperatures are difficult to account for at the inferred depths at typical geothermal gradients. In extreme cases, granulites may form at temperatures in excess of 1000 °C. Some granulites may represent the residues of partial melting, and in other cases represent rocks that never melted, despite extremely high temperatures, because the minerals are anhydrous and therefore do not melt even at high temperature conditions. In continental crustal rocks, biotite may break down at high temperatures to form orthopyroxene + potassium feldspar + water, producing a granulite. Other possible minerals formed at dry conditions include sapphirine, spinel, sillimanite, and osumilite. Some assemblages such as sapphirine + quartz indicate very high temperatures. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「granulite」の詳細全文を読む スポンサード リンク
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