A granulite is a metamorphic rock formed under average conditions pressure and high temperature. Granulites are also used to define a facies high degree metamorphic. the metamorphism of the granulite facies is in the range of temperatures from 700 to more than 1,000°C and from 2 to 15 kbars of pressure.
Granulites are mainly composed of quartz and feldspars (orthoclases and plagioclases). Garnets and pyroxenes (hypersthene) can be found there. This mineralogical composition makes them rather light colored rocks, quite close to gneiss. The grains are however finer, the rock is less foliated and is more often composed of garnets. The mineralogical composition of a granulite will however vary depending on the composition of the original rock (the protolith), but also depending on the pressure and temperature conditions experienced during metamorphism. At the microscopic level, crystals are never perfectly crystallized.
Evidence of conditions at the base of the continental crust
Granulites are of particular interest to geologists because they are typical rocks of base of continental crust. Their observation in the field makes it possible to indirectly study a very deep region of the crust that cannot be reached by drilling methods. They are therefore witnesses to the pressure and temperature conditions that can prevail at depth. They are found at the outcrop thanks to tectonic processes which have the capacity, during collision events, to pile up deep geological units. Erosion then takes care of exhuming them.
The very high temperatures necessary for the formation of granulites are moreover not common for the base of the continental crust. These temperature conditions can only be explained by a anomaly temperature and a gradient temperature of more than 30°C/km, caused by the rise of the coat asthenospheric. This imbalanced thermal architecture, associated with this rise in the mantle, is observed in particular at continental rift zones.
Rift zones are regions of the globe where the continental crust begins to open up, leading to the splitting of a continent into two. The analysis of granulites thus makes it possible to reconstruct certain parts of the tectonic history of the Earth.
Previously, the term granulite was also used in France to designate certain mica-rich granites. However, this usage is no longer in effect.
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