Leucogranite is a light-colored,

granitic A granitoid is a broad term referring to a diverse group of coarse-grained igneous rocks that are widely distributed across the globe, covering a significant portion of the Earth's exposed surface and constituting a large part of the continental ...

igneous rock Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

containing almost no dark

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

s. Leucogranites have been reported from a variety of orogenies involving continental collisions. Examples include the Black Hills (

Trans-Hudson orogeny The Trans-Hudson orogeny or Trans-Hudsonian orogeny was the major mountain building event (orogeny) that formed the Precambrian Canadian Shield and the North American Craton (also called Laurentia), forging the initial North American conti ...

Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

age),Peter I. Nabelek and Mian Liu, ''Petrologic and thermal constraints on the origin of leucogranites in collisional orogens,'' Transactions of the Royal Society of Edinburgh: Earth Sciences, 95, 73–85, 2004
the Blue Ridge basement complex (

Grenville orogeny The Grenville orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, ...

of Proterozoic age),''Alkali feldspar leucogranite,'' Mineral Resources, Online Spatial Data, Virginia, USGS
/ref> the

Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...

Appalachian orogeny in

Maine Maine ( ) is a U.S. state, state in the New England region of the United States, and the northeasternmost state in the Contiguous United States. It borders New Hampshire to the west, the Gulf of Maine to the southeast, and the Provinces and ...

, and the currently active Himalayan orogeny. The leucogranite

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

s are interpreted to have been derived by partial melting of pelitic rocks in the upper portions of thickened crust. These melts result following deformation and

metamorphism Metamorphism is the transformation of existing Rock (geology), rock (the protolith) to rock with a different mineral composition or Texture (geology), texture. Metamorphism takes place at temperatures in excess of , and often also at elevated ...

, but the heat source is uncertain. Shear-heating associated with large shear zones in the crust has been proposed as the mechanism. Mont Saint-Michel in

Normandy Normandy (; or ) is a geographical and cultural region in northwestern Europe, roughly coextensive with the historical Duchy of Normandy. Normandy comprises Normandy (administrative region), mainland Normandy (a part of France) and insular N ...

, France is made of leucogranite, which solidified from an underground

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

of molten magma about 525 million years ago during the

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

period. A study of sodium-rich

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

- alkali feldspar— biotite gneiss granulite facies terrane in the

Kerala Kerala ( , ) is a States and union territories of India, state on the Malabar Coast of India. It was formed on 1 November 1956, following the passage of the States Reorganisation Act, by combining Malayalam-speaking regions of the erstwhile ...

Khondalite Belt near Manali in south India found that ''in situ'' leucosomes (light colored segregations) within the gneiss showed the development of

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

replacing the dark biotite. The study indicated localized melting or migmatization within the gneiss. This was followed by intrusion of the gneiss by garnet-bearing leucogranitic melts.

Strontium Strontium is a chemical element; it has symbol Sr and atomic number 38. An alkaline earth metal, it is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is exposed to ...

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

ratios of the leucogranite intrusives are distinct from that of the gneiss and associated leucosomes. This indicates the leucogranite melts were not derived from the local gneiss, however the gneiss is isotopically variable and the leucogranite could have derived from subjacent gneiss.Braun I., G.R. Ravindra Kumar, and M. Raithi, (1996) ''Dehydration—Melting Phenomena in Leptynitic Gneisses and the Generation of Leucogranites: a Case Study from the Kerala Khondalite Belt, Southern India,'' Journal of Petrology, Vol. 37, No. 6, pp. 1285-1305
/ref> Alaskite is leucocratic alkali feldspar granite.

References

Ultrapotassic rocks Granitic rocks {{petrology-stub