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The highest indicated isograd is the solidus of hydrated granite, at higher metamorphic grade partial melting occurred in the quartzite. Metamorphic zones in Scottish Highlands , the region where they were first recognized by geologist George Barrow in 1912.
The Moldanubicum or Moldanubian sensu stricto consists of high grade metamorphic rocks. The protoliths may have a Proterozoic age, but their origins or ages are mostly unknown. [2] The tectonostratigraphy of the Moldanubian Zone sensu stricto is divided into three main units: At the top the Gföhl unit, a stack of south-vergent crystalline nappes.
Metamorphic grade is an informal indication of the amount or degree of metamorphism. [ 78 ] In the Barrovian sequence (described by George Barrow in zones of progressive metamorphism in Scotland), metamorphic grades are also classified by mineral assemblage based on the appearance of key minerals in rocks of pelitic (shaly, aluminous) origin:
Of the three nappe stacks the Penninic nappes have the highest metamorphic grade. They contain high grade metamorphic rocks of different paleogeographic origins. They were deposited as sediments on the crust that existed between the European and Apulian plates before the Alps were formed.
In other words, it is a metamorphic rock composed of mineral grains easily seen with the unaided eye, which form obvious compositional layers, but which has only a weak tendency to fracture along these layers. In Europe, the term has been more widely applied to any coarse, mica-poor, high-grade metamorphic rock. [1]
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 metamorphic grade of such a rock is a rough measure of the degree of metamorphism it has undergone, as characterised by the presence of certain index minerals. An isograd is a theoretical surface comprising points all at the same metamorphic grade, and thus separates metamorphic zones whose rocks contain different index minerals. [1] [2] [3]
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 ...