Granulite
Photographed by Michael P. Klimetz
[Feldspar-Pyroxene-Sillimanite Granulite with Pyrope Porphyroblasts]
Winding Stair Gap Roadcut
Franklin
Macon County
NORTH CAROLINA
Photographed by Michael P. Klimetz
[Feldspar-Pyroxene-Sillimanite Granulite with Pyrope Porphyroblasts]
Winding Stair Gap Roadcut
Franklin
Macon County
NORTH CAROLINA
Photographed by Michael P. Klimetz
[Feldspar-Pyroxene-Sillimanite Granulite with Pyrope Porphyroblasts]
Winding Stair Gap Roadcut
Franklin
Macon County
NORTH CAROLINA
Photographed by Michael P. Klimetz
[Feldspar-Pyroxene-Sillimanite Granulite with Pyrope Porphyroblasts]
Winding Stair Gap Roadcut
Franklin
Macon County
NORTH CAROLINA
Granulites are a class of high-grade metamorphic rocks 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. 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 crust to shallower crustal levels at high temperature, while others cooled remaining at depth. 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 (metamorphosed basalt) defines the granulite facies. Granulites form at crustal depths, typically during regional metamorphism at high thermal gradients of greater than 30°C/km. 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 dehydration melting conditions include sapphirine, spinel, sillimanite, and osumilite. Some assemblages such as sapphirine + quartz indicate very high temperatures of greater than 900°C. Some granulites may represent the residues of partial melting at extraction of felsic melts in variable amounts, and in extreme cases represent rocks that all constituent minerals are anhydrous and thus look as if they did not melt at ultrahigh temperature conditions. Therefore, very high temperatures of 900 to 1150°C are even necessary to produce the granulite-facies mineral assemblages. Such high temperatures at crustal depths only can be delivered by upwelling of the asthenospheric mantle in continental rifting settings, which can cause the regional metamorphism at the high thermal gradients of greater than 30°C/km.
Photographed By Michael P. Klimetz
Mylonitic Granulite Gneiss
[Lowermost Continental Crust Structurally Above the Finero Ultramafic Complex]
Insubric Line
Finero
ITALY
Photographed By Michael P. Klimetz
Mylonitic Granulite Gneiss
[Lowermost Continental Crust Structurally Above the Finero Ultramafic Complex]
Insubric Line
Finero
ITALY
