Leucogranite
Encyclopedia
In geology
, leucogranites are amongst the youngest intrusions related to anatexis
of continental crust anywhere in the world. Leucogranites are commonly found in deformed metapelitic/metagraywacke sequences
that have been thrusted over basement
s during crustal thickening associated with continental collisions. Several examples include the leucogranites of the High Himalayas, the Avalon
terrane of Maine, and the Black Hills, South Dakota. Leucogranites are typical examples of S-type melts, their composition are strongly influenced by the protolith
composition and melt volume depends on the amount of volatiles
available during melting and protolith
composition
Leucogranites are common features of convergent orogens
and consist of rich-potassium (K2O5.5 wt%), (Ba400 p.p.m.) and Sr (300 p.p.m.), and exhibit low concentrations of Zr (~ 40 ppm), Th (< 1 ppm) and Y (< 10 ppm). The chondrite-normalized spectra
show that they have a low abundance of (LaN20, LuN3) and are moderately fractionated (LaN/LuN7). An Eu anomaly is absent or weakly negative. The higher 87Sr/86Sr ratio at 550 Ma (0.7345) compared with the migmatite (0.7164) precludes a direct genetic relationship between leptynitic gneisses and leucogranites at Manali. The chemical and mineralogical compositions of the leuocogranites strongly favour a derivation through fluid-absent biotite
melting of isotopically distinct but chemically comparable Manali-type gneisses. The undersaturation of Zr, Th and REE, a typical feature of leucogranitic melts generated during granulite facies anatexis of psammo-pelitic lithologies. (Braun et al., 1996).
The source of Leucogranites in the Himalayas are generally ascribed to the aluminous schists and gneisses, partly because of structural relation and partly because of isotopic systematics for Rb, Sr, Nd, O. The monazite and zircon upper intercept ages around 500 Ma of the Zanskar leucogranite clearly indicate that they must be partly derived from the melting of the Cambro-Ordovician Granites. As these old granites are relatively poor in muscovite, the leucogranite must also have had a more pelitic source.
The origin of Himalayan leucogranite was believed to be related to water-saturated melting. Fluid advection from the footwall of the Main Central Thrust (Lesser Himalaya) into the relatively hot hanging wall (HHCS) was invoked as the driving mechanism for the production of water-saturated melt in the upper structural levels of the HHCS (Le Fort et al. 1987). There is however little evidence to support the notion of pervasive aqueous fluids during metamorphism of the mid-lower crust and recent trace element studies argue against fluid present melting in the formation of crustal melts. Also in Zanskar, metamorphic conditions and textural evidences observed in the migmatitic zone rather indicate that melts were derived from the incongruent melting of muscovite.
Lately, Himalayan leucogranitic magmas are thus increasingly thought to be initially water-undersaturated, indicating either that a fluid phase with aH2O<< 1 was present during melting or that the melting reactions were fluid-absent. In the absence of free water, melting depends on the availability in the source region of hydrous minerals like muscovite or biotite which may release their water during anatexis. The amounts of water released by the breakdown of these minerals is usually not sufficient to saturate the magma and is dissolved in the melt without formation of a vapour phase: dehydration melting or vapor-absent melting. Aluminous schists and gneisses are generally considered to be the likely source for peraluminous granites.
The breakdown of muscovite occurs through the reaction
: 22 Muscovite + 7 plagioclase + 8 quartz = 5 K-feldspar + 5 Al2SiO5 + 2 biotite + 25 melt, which is also known as the second sillimanite isograd. This reaction can however only produce 10-15 vol. % melt between 750° and 850°C and at 10 kbar. Up to 50 vol. % melt can however be produced at higher temperatures (850°-900°C) with the breakdown of biotite through the reaction: biotite + Al2SiO5 + plagioclase + quartz = Garnet + K-feldspar + melt. The amount of partial melting required before melts begin to segregate and forms plutons is called critical melt percentage. The value of this critical melt percentage is generally believed to be of the order of 25%." (Braun et al. 1996).
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, leucogranites are amongst the youngest intrusions related to anatexis
Anatexis
Anatexis in geology, refers to the differential, or partial, melting of rocks, especially in the forming of metamorphic rocks such as migmatites.-Optimum Temperature Conditions for Crustal Melting:...
of continental crust anywhere in the world. Leucogranites are commonly found in deformed metapelitic/metagraywacke sequences
Séquences
Séquences is a French-language film magazine originally published in Montreal, Quebec by the Commission des ciné-clubs du Centre catholique du cinéma de Montréal, a Roman Catholic film society. Founded in 1955, the publication was edited for forty years by Léo Bonneville, a member of the Clerics...
that have been thrusted over basement
Basement Rock
Basement or Basement Rock music was a sub-genre coined in 2006 in an article by music magazine TGR. This was first in relation to the existence of underground record label Criminal Records but more for the independent bands they represent. The roots of the sub-genre are noted to be as far back as...
s during crustal thickening associated with continental collisions. Several examples include the leucogranites of the High Himalayas, the Avalon
Avalonia
Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, and the eastern coast of North America. It is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States...
terrane of Maine, and the Black Hills, South Dakota. Leucogranites are typical examples of S-type melts, their composition are strongly influenced by the protolith
Protolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
composition and melt volume depends on the amount of volatiles
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
available during melting and protolith
Protolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
composition
Leucogranites are common features of convergent orogens
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
and consist of rich-potassium (K2O5.5 wt%), (Ba400 p.p.m.) and Sr (300 p.p.m.), and exhibit low concentrations of Zr (~ 40 ppm), Th (< 1 ppm) and Y (< 10 ppm). The chondrite-normalized spectra
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
show that they have a low abundance of (LaN20, LuN3) and are moderately fractionated (LaN/LuN7). An Eu anomaly is absent or weakly negative. The higher 87Sr/86Sr ratio at 550 Ma (0.7345) compared with the migmatite (0.7164) precludes a direct genetic relationship between leptynitic gneisses and leucogranites at Manali. The chemical and mineralogical compositions of the leuocogranites strongly favour a derivation through fluid-absent biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
melting of isotopically distinct but chemically comparable Manali-type gneisses. The undersaturation of Zr, Th and REE, a typical feature of leucogranitic melts generated during granulite facies anatexis of psammo-pelitic lithologies. (Braun et al., 1996).
The source of Leucogranites in the Himalayas are generally ascribed to the aluminous schists and gneisses, partly because of structural relation and partly because of isotopic systematics for Rb, Sr, Nd, O. The monazite and zircon upper intercept ages around 500 Ma of the Zanskar leucogranite clearly indicate that they must be partly derived from the melting of the Cambro-Ordovician Granites. As these old granites are relatively poor in muscovite, the leucogranite must also have had a more pelitic source.
The origin of Himalayan leucogranite was believed to be related to water-saturated melting. Fluid advection from the footwall of the Main Central Thrust (Lesser Himalaya) into the relatively hot hanging wall (HHCS) was invoked as the driving mechanism for the production of water-saturated melt in the upper structural levels of the HHCS (Le Fort et al. 1987). There is however little evidence to support the notion of pervasive aqueous fluids during metamorphism of the mid-lower crust and recent trace element studies argue against fluid present melting in the formation of crustal melts. Also in Zanskar, metamorphic conditions and textural evidences observed in the migmatitic zone rather indicate that melts were derived from the incongruent melting of muscovite.
Lately, Himalayan leucogranitic magmas are thus increasingly thought to be initially water-undersaturated, indicating either that a fluid phase with aH2O<< 1 was present during melting or that the melting reactions were fluid-absent. In the absence of free water, melting depends on the availability in the source region of hydrous minerals like muscovite or biotite which may release their water during anatexis. The amounts of water released by the breakdown of these minerals is usually not sufficient to saturate the magma and is dissolved in the melt without formation of a vapour phase: dehydration melting or vapor-absent melting. Aluminous schists and gneisses are generally considered to be the likely source for peraluminous granites.
The breakdown of muscovite occurs through the reaction
Metamorphic reaction
A metamorphic reaction is a chemical reaction that takes place during the geological process of metamorphism wherein one assemblage of minerals is transformed into a second assemblage which is stable under the new temperature/pressure conditions resulting in the final stable state of the observed...
: 22 Muscovite + 7 plagioclase + 8 quartz = 5 K-feldspar + 5 Al2SiO5 + 2 biotite + 25 melt, which is also known as the second sillimanite isograd. This reaction can however only produce 10-15 vol. % melt between 750° and 850°C and at 10 kbar. Up to 50 vol. % melt can however be produced at higher temperatures (850°-900°C) with the breakdown of biotite through the reaction: biotite + Al2SiO5 + plagioclase + quartz = Garnet + K-feldspar + melt. The amount of partial melting required before melts begin to segregate and forms plutons is called critical melt percentage. The value of this critical melt percentage is generally believed to be of the order of 25%." (Braun et al. 1996).