Granulite 1226995 221323087 2008-06-24T00:48:58Z Fangjian 4807131 [[Image:Mineraly.sk - granulit.jpg|right|thumb|250px|A sample of granulite-facies metamorphic rock of [[felsic]] composition]] '''Granulites''' are fine to medium–grained [[metamorphic rock]]s that have experienced high [[temperature]]s of metamorphism, composed mainly of [[feldspar]]s sometimes associated with [[quartz]] and anhydrous [[ferromagnesian minerals]], with granoblastic texture and gneissose to massive structure. <ref> D.R. Bowes (1989), ''The Encyclopedia of Igneous and Metamorphic Petrology''; Van Nostrand Reinhold ISBN: 0-442-20623-2 </ref> They are of particular interest to [[geologist]]s 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 [[continent]]s contains [[pyroxene]], [[plagioclase]] feldspar and accessory [[garnet]], [[oxides]] and possibly [[amphibole]]. Both clinopyroxene and orthopyroxene may be present, and in fact, the coexistence of clino- and orthopyroxene in a metamorphosed [[basalt]] defines the Granulite facies. ==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 degrees [[Celsius]]. Some granulites may represent the residues of partial melting, and in other cases represent [[Rock (geology)|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. ==1911 Definition== '''Granulite''' ([[Latin]] ''granulum'', "a little grain") is a name used by petrographers to designate two distinct classes of [[Rock (geology)|rock]]s. According to the terminology of the [[France|French]] school it signifies a [[granite]] in which both kinds of [[mica]] (muscovite and biotite) occur, and corresponds to the [[German language|German]] ''Granit'', or to the [[English language|English]] muscovite biotite granite. This application has not been accepted generally. To the German petrologists granulite means a more or less banded fine-grained [[metamorphic rock]], consisting mainly of [[quartz]] and [[feldspar]] in very small irregular [[crystal]]s and usually also containing a fair number of minute, rounded, pale-red [[garnet]]s. Among English and American [[geologist]]s the term is generally employed in this sense. The granulites are very closely allied to the [[gneiss]]es, as they consist of nearly the same [[mineral]]s, but they are finer-grained, have usually less perfect foliation, are more frequently garnetiferous, and have some special features of microscopic structure. In the rocks of this group the minerals, as seen in a microscopic slide, occur as small rounded grains forming a closely-fitted mosaic. The individual crystals never have perfect form, and indeed traces of it area rare. In some granulites they interlock, with irregular borders; in others they have been drawn out and flattened into tapering lenticles by crushing. In most cases they are somewhat rounded with smaller grains between the larger. This is especially true of the quartz and feldspar which are the predominant minerals; mica always appears as flat scales (irregular or rounded but not hexagonal). Both muscovite and biotite may be present and vary considerably in abundance; very commonly they have their flat sides parallel and give the rock a rudimentary [[schist]]osity, and they may be aggregated into bands in which case the granulites are indistinguishable from certain varieties of gneiss. The garnets are very generally larger than the above-mentioned ingredients, and easily visible with the eye as pink spots on the broken surfaces of the rock. They usually are filled with enclosed grains of the other minerals. The feldspar of the granulites is mostly [[orthoclase]] or cryptoperthite; microcline, oligoclase and [[albite]] are also common. Basic feldspars occur only rarely. Among accessory minerals, in addition to [[apatite]], [[zircon]], and [[iron oxide]]s, the following may be mentioned: [[hornblende]] (not common), [[riebeckite]] (rare), [[epidote]] and [[zoisite]], [[calcite]], [[sphene]], [[andalusite]], [[sillimanite]], [[kyanite]], [[hercynite]] (a green spinel), [[rutile]], [[orthite]] and [[tourmaline]]. Though occasionally we may find larger grains of feldspar, quartz or epidote, it is more characteristic of these rocks that all the minerals are in small, nearly uniform, imperfectly shaped individuals. On account of the minuteness with which it has been described and the important controversies on points of theoretical geology which have arisen regarding it, the granulite district of [[Saxony]] (in the area of Rosswein and Penig) in [[Germany]] may be considered the typical region for rocks of this group. It should be remembered that though granulites are probably the commonest rocks of this country, they are mingled with granites, gneisses, [[gabbro]]s, [[amphibolite]]s, mica schists and many other petrographical types. All of these rocks show more-or-less [[metamorphism]] either of a thermal character or due to pressure and crushing. The granites pass into gneiss and granulite; the gabbros into flaser gabbro and amphibolite; the [[slate]]s often contain andalusite or [[chiastolite]], and show transitions to mica schists. At one time these rocks were regarded as Archean gneisses of a special type. [[Johannes Georg Lehmann]] propounded the hypothesis that their present state was due principally to crushing acting on them in a solid condition, grinding them down and breaking up their minerals, while the pressure to which they were subjected welded them together into coherent rock. It is now believed, however, that they are comparatively recent and include [[sedimentary rock]]s, partly of [[Palaeozoic]] age, and [[Intrusion (geology)|intrusive]] masses which may be nearly massive or may have gneissose, flaser or granulitic structures. These have been developed largely by the injection of semi-consolidated highly [[Viscosity|viscous]] intrusions, and the varieties of texture are original or were produced very shortly after the crystallization of the rocks. Meanwhile, however, Lehmanns advocacy of post-consolidation crushing as a factor in the development of granulites has been so successful that the terms granulitization and granulitic structures are widely employed to indicate the results of dynamometamorphism acting on rocks at a period long after their solidification. The Saxon granulites are apparently for the most part igneous and correspond in composition to granites and [[Porphyry (geology)|porphyries]]. There are, however, many granulites which undoubtedly were originally [[sediment]]s ([[arkose]]s, grits and [[sandstone]]s). A large part of the highlands of [[Scotland]] consists of paragranulites of this kind, which have received the group name of Moine gneisses. Along with the typical [[acid]] granulites above described, in Saxony, [[India]], Scotland and other countries there occur dark-colored basic granulites (trap granulites). These are fine-grained rocks, not usually banded, nearly black in color with small red spots of garnet. Their essential minerals are pyroxene, plagioclase and garnet: chemically they resemble the gabbros. Green augite and [[hypersthene]] form a considerable part of these rocks, they may contain also biotite, hornblende and quartz. Around the garnets there is often a radial grouping of small grains of pyroxene and hornblende in a clear matrix of feldspar: these centric structures are frequent in granulites. The rocks of this group accompany gabbro and [[serpentine]], but the exact conditions under which they are formed and the significance of their structures is not very clearly understood. ===Granulite facies=== '''Granulite facies''' is determined by lower temperature boundary of 700 +/- 50 °C and pressure range 5-15 kb. The most common mineral assemblage of granulite facies consists of antiperthitic [[plagioclase]], alkali [[feldspar]] containing up to 50% [[albite]] and <tt>Al<sub>2</sub>O<sub>3</sub></tt>-rich [[pyroxene]]s. Transition between [[amphibolite]] and granulite facies is defined by these reaction [[isograd]]s: <tt>amphibole -> pyroxene + H<sub>2</sub>O</tt> <tt>biotite -> K-feldspar + garnet + orthopyroxene + H<sub>2</sub>O</tt>. '''Hornblende granulite subfacies''' is transitional coexistence region of anhydrous and hydrated ferromagnesian minerals, so above mentioned isograds mark the boundary with '''pyroxene granulite subfacies''' – facies with completely anhydrous mineral assemblages. <ref> D.R. Bowes (1989), ''The Encyclopedia of Igneous and Metamorphic Petrology''; Van Nostrand Reinhold ISBN: 0-442-20623-2 </ref> ==See also== * [[Migmatite]] and [[granite]] origin * [[Metamorphic]] rocks * [[Metamorphism|Metamorphic facies]] * [[Ultrahigh-temperature metamorphism]] ==References== <references/> *{{1911}} {{Metamorphic Facies}} [[Category:metamorphic rocks]] [[Category:structural geology]] [[bg:Гранулит]] [[cs:Granulit]] [[de:Granulit]] [[es:Granulita]] [[eu:Granulita]] [[fr:Granulite]] [[ko:백립암]] [[hr:Granulit]] [[lt:Granulitas]] [[hu:Granulit]] [[nl:Granuliet]] [[pl:Granulit]] [[ru:Гранулит]] [[fi:Granuliitti]] [[uk:Грануліт]] [[zh:麻粒岩]]