Anatexis in geology

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...

, refers to the differential, or partial, melting of rocks

Rock (geology)

In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

, especially in the forming of metamorphic rocks such as migmatite

Migmatite

Migmatite is a rock at the frontier between igneous and metamorphic rocks. They can also be known as diatexite.Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally...

s.

Optimum Temperature Conditions for Crustal Melting

In order to have partial melting in the middle to lower continental crust

Continental crust

The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...

, the continental geotherm must be steepened towards much higher temperatures. The minimum temperature needed to produce partial melting in metasedimentary rocks is about 650°C. Under these conditions, water saturated metapelites

Pelite

Pelite is old and currently not widely used field terminology for a clayey fine-grained clastic sediment or sedimentary rock, i.e. mud or mudstone. It is equivalent to the Latin-derived term lutite. More commonly, metamorphic geologists currently use pelite for a metamorphosed fine-grained...

 reach their solidus

Solidus (chemistry)

In chemistry, materials science, and physics, the solidus is the locus of temperatures below which a given substance is completely solid...

 and produce a melt of granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

 composition. The "standard" geotherm at the Moho

Mohorovičić discontinuity

The Mohorovičić discontinuity , usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. Named after the pioneering Croatian seismologist Andrija Mohorovičić, the Moho separates both the oceanic crust and continental crust from underlying mantle...

 is in the 500-600°C range which would not be optimally hot enough for anatectic melting.

Extending the Continental Geotherm

Situations when the continental geotherm is hotter and can induce partial melting of the crust is where "orogenic processes such as crustal thickening, lithospheric mantle thinning, and underplating of mafic magma" occur (the transfer of heat from magma into the base of the continental crust).

The Role of Water (H₂O) in the Production of Anatectic Melts

The amount of water in granitic systems controls the degree of melting at a given temperature. Very high temperatures are required to generate substantial melt if the water content of the system is low.

Examples of Anatexis

Typical examples of anatexis would be the generation of granitic

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

 melts (partially melted aluminous crustal rocks), basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

s (partially melted mantle peridotite

Peridotite

A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...

), and migmatitic

Migmatite

Migmatite is a rock at the frontier between igneous and metamorphic rocks. They can also be known as diatexite.Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally...

 rock.

Granitic rocks that come from the crust commonly contain xenolith

Xenolith

A xenolith is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption...

s of metamorphic or sedimentary rocks when pieces of the wall rock are included into the magma during ascent or site of placement.

Although a source of petrologic controversy, migmatites are thought to represent partial melting where melt and residual unmelted material are intertwined in layers (melt segregation). Migmatitic rocks "provide an example of the close relation between metamorphism, deformation, and melt generation and emplacement."

"The Granite Problem" - Where Igneous and Metamorphic Petrology Overlap

Granites "are undoubtedly the result of extreme fractionation of mantle-derived parental basaltic magma, but it is also possible to derive granite magmas from the melting of metasedimentary and metavolcanic rocks of various compositions in orogenic belts (also called S-type granites)." Two questions that implicate "the granite problem" are:

1. "How does one distinguish a granite (or granodiorite or tonalite) derived from fractionation of more mafic melt from one that originated by secondary melting within the continental crust?"





2. "Which of the two processes is more likely to have generated sizable volumes of truly granitic magma?"

Some in the field of petrology have considered migmatitic terrane

Terrane

A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...

s as "source areas for magmas, even referring to them as 'baby batholith

Batholith

A batholith is a large emplacement of igneous intrusive rock that forms from cooled magma deep in the Earth's crust...

s.'" Some regard "granitic liquids [that] are produced directly from partial melting of mantle peridotite or subducted oceanic crust [chemically unlikely]; otherwise we should find granitoids in the oceanic lithosphere."

Others have disputed it:

"No direct genetic connection between migmatite formation and larger plutonic masses has ever been established. An increasing number of petrologists who study migmatites now believe that no connection exists and that migmatite formation is an end point of the metamorphic process rather than the beginning of a larger-scale magmatic process. Magmas undoubtedly originate in the deep continental crust, but the nature of their source areas has not been established."