
Tonalite–trondhjemite–granodiorite (TTG) rocks are
intrusive rock
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: Marc ...
s with typical
granitic
A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quart ...
composition (
quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
and
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feld ...
) but containing only a small portion of
potassium feldspar Potassium feldspar refers to a number of minerals in the feldspar group, and containing potassium:
*Orthoclase
Orthoclase, or orthoclase feldspar ( endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock ...
.
Tonalite
Tonalite is an igneous, plutonic ( intrusive) rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the tota ...
,
trondhjemite
Trondhjemite is a leucocratic (light-colored) intrusive igneous rock. It is a variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites that occur in the oceanic crust or in ophiolites are usually called pla ...
, and
granodiorite
Granodiorite () is a coarse-grained ( phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar.
The term banatite is sometimes used informally for various rocks ranging from gr ...
often occur together in
geological records, indicating similar
petrogenetic processes.
Post
Archean
The Archean Eon ( , also spelled Archaean or Archæan) is the second of four eon (geology), geologic eons of History of Earth, Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozo ...
(after 2.5 Ga) TTG rocks are present in
arc-related
batholith
A batholith () is a large mass of intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, su ...
s, as well as in
ophiolite
An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks.
The Greek word ὄφις, ''ophis'' (''snake'') is found ...
s (although of a small proportion), while Archean TTG rocks are major components of Archean
craton
A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and ...
s.
Composition
The quartz percentage among
felsic
In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, wh ...
minerals in TTG rocks is usually larger than 20% but less than 60%.
In tonalite and trondhjemite, more than 90% of the
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feld ...
s are
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...
, while in
granodiorite
Granodiorite () is a coarse-grained ( phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar.
The term banatite is sometimes used informally for various rocks ranging from gr ...
, this number is between 65% and 90%.
Trondhjemite
Trondhjemite is a leucocratic (light-colored) intrusive igneous rock. It is a variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites that occur in the oceanic crust or in ophiolites are usually called pla ...
is a special kind of
tonalite
Tonalite is an igneous, plutonic ( intrusive) rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the tota ...
, with most of the
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...
in the rock being
oligoclase
Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite ( Na Al Si3 O8) and anorthite ( CaAl2Si2O8). The albite:an ...
. The major
accessory mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...
s of TTG rocks include
biotite
Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more alum ...
,
amphibole
Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is ...
s (e.g.
hornblende
Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic ro ...
),
epidote
Epidote is a calcium aluminium iron sorosilicate mineral.
Description
Well developed crystals of epidote, Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH), crystallizing in the monoclinic system, are of frequent occurrence: they are commonly prismatic in hab ...
, and
zircon
Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...
.
Geochemically
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing th ...
, TTG rocks often have a high
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is o ...
(SiO
2) content (commonly over 70 percent SiO
2), high
sodium oxide
Sodium oxide is a chemical compound with the formula Na2 O. It is used in ceramics and glasses. It is a white solid but the compound is rarely encountered. Instead "sodium oxide" is used to describe components of various materials such as glass ...
(Na
2O) content (with low
potassium oxide
Potassium oxide ( K O) is an ionic compound of potassium and oxygen. It is a base. This pale yellow solid is the simplest oxide of potassium. It is a highly reactive compound that is rarely encountered. Some industrial materials, such as fertili ...
/Na
2O ratio) compared to other
plutonic rock
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: March ...
s, and low
ferromagnesian
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
element content (the weight percentage of
iron(III) oxide
Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally ...
,
magnesium oxide
Magnesium oxide ( Mg O), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2− ...
,
manganese dioxide
Manganese dioxide is the inorganic compound with the formula . This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese and a component of manganese nodules. The principal use for is for dry-cel ...
, and
titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insoluble ...
added together commonly smaller than 5%).
Post Archean TTG Rocks
Post Archean TTG rocks are commonly found in
arc settings, especially in
continental arcs.
Ophiolite
An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks.
The Greek word ὄφις, ''ophis'' (''snake'') is found ...
also contains a small amount of TTG rocks.
Continental arc TTG rocks
Continental arc TTG rocks are often associated with
gabbro
Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ...
,
diorite
Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-sil ...
, and
granite
Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies und ...
, which forms a plutonic sequence in
batholith
A batholith () is a large mass of intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, su ...
s.
They are formed by hundred of
pluton
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 ...
s that directly related to
subduction
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
.
For example,
Coastal Batholith of Peru consists of 7 ~ 16% gabbro and diorite, 48 ~ 60% tonalite (including trondhjemite), and 20 ~ 30% granodiorite, with 1 ~ 4% granite. These TTG rocks in continental arc batholiths may partially originate from the
magma differentiation
In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of ...
(i.e.
fractional crystallisation) of the subduction induced
mantle wedge
A mantle wedge is a triangular shaped piece of mantle that lies above a subducting tectonic plate and below the overriding plate. This piece of mantle can be identified using seismic velocity imaging as well as earthquake maps. Subducting oceanic ...
melt at depth. However, the large volume of such TTG rocks infer their major generation mechanism is by the crustal thickening induced
partial melting
Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the soli ...
of the former gabbroic
underplate at the base of the continental crust.
Tonalitic composition rock crystallised first before the magma differentiated to granodioritic and later granitic composition at a shallow depth. Some island arc plutonic roots also have TTG rocks, e.g.
Tobago
Tobago () is an island and ward within the Republic of Trinidad and Tobago. It is located northeast of the larger island of Trinidad and about off the northeastern coast of Venezuela. It also lies to the southeast of Grenada. The offici ...
, but they are rarely exposed.
TTG rocks in ophiolite
Tonalites (including trondhjemites) can be found above the layered gabbro section in
ophiolite
An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks.
The Greek word ὄφις, ''ophis'' (''snake'') is found ...
s, below or within sheeted dykes.
They are often irregular in shape and produced by
magma differentiation
In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of ...
.
Archean TTG rocks

Archean TTG rocks appear to be strongly deformed grey
gneiss
Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...
, showing banding, lineation, and other
metamorphic structures, whose
protolith
A protolith () is the original, unmetamorphosed rock from which a given metamorphic rock is formed.
For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and t ...
s were
intrusive rock
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: Marc ...
s.
TTG rock is one of the major rock types in
Archean
The Archean Eon ( , also spelled Archaean or Archæan) is the second of four eon (geology), geologic eons of History of Earth, Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozo ...
craton
A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and ...
s.
Geochemical features
In terms of trace element characteristics, Archean TTGs exhibit high light
rare earth element
The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides ( yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous si ...
(LREE) content yet low heavy rare earth element (HREE) content. However, they do not show
Eu and
Sr anomalies. These features indicate the presence of
garnet
Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different ...
and
amphibole
Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is ...
, but no plagioclase in the residual phase during
partial melting
Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the soli ...
or precipitation phase during
fractional crystallization.
Petrogenesis and classification
Confirmed by geochemical modelling, TTG type magma can be generated through partial melting of hydrated meta-
mafic rocks
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
. To produce the very low HREE pattern, the melting should be conducted under a garnet-stable pressure-temperature field.
Given that garnet temperature stability rises dramatically with increasing pressure, strongly HREE-depleted TTG melts are expected to form under relatively high pressure.
Besides the source composition and the pressure, the degree of melting and temperature also influence the melt composition.
Detailed studies classified Archean TTGs into three groups based on geochemical features, that are low, medium, and high pressure TTGs, although the three groups form a continuous evolution.
The low pressure subseries shows relatively low
Al2O3, Na
2O,
Sr content and relatively high
Y,
Yb,
Ta, and
Nb content, corresponding to melting under 10-12
kbar with the source rock mineral assembly of plagioclase,
pyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
and possibly amphibole or garnet.
The high pressure group shows the opposite geochemical features, corresponding to melting at a pressure over 20 kbar, with the source rock containing garnet and
rutile
Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.
Rutile has one of the highest refractive indices at visible wa ...
but no amphibolite or plagioclase.
The medium pressure group has transitional features between the other two groups, corresponding to melting under a pressure around 15 kbar with the source rock containing amphibole, much garnet, but little rutile and no plagioclase.
Medium pressure TTGs are the most abundant among the three groups.
Geodynamic settings
The
geodynamic setting of the Archean TTG rock generation is currently not well understood. Competing hypotheses include subduction related generation involving plate tectonics and other non-plate tectonic models.
Plate tectonic setting

Geochemical similarity shared between TTGs and
adakites have long been noted by researchers.
Adakites are one type of modern arc lavas, which differ from common arc lavas (mostly granitoids) in their felsic and sodic nature with high LREE but low HREE content.
Their production is interpreted to be the partial melting of young and hot subducting oceanic slabs with minor interaction with surrounding mantle wedges, rather than mantle wedge melts like other arc-granitoids.
Based on geochemical features (e.g.
Mg,
Ni, and
Cr contents), adakites can be further divided into two groups, namely high SiO
2 adakites (HSA) and low SiO
2 adakites (LSA). It was then noted that the Archean TTGs were geochemically almost identical to high silica adakites (HSA), but slightly different from low silica adakites (LSA).
This geochemical similarity let some researchers infer that the geodynamic setting of Archean TTGs was analogous to that of modern adakites.
They think that Archean TTGs were generated by hot subduction as well. Although modern adakites are rare and only found in a few localities (e.g.
Adak Island
Adak Island ( ale, Adaax, russian: Адак) or Father Island is an island near the western extent of the Andreanof Islands group of the Aleutian Islands in Alaska. Alaska's southernmost town, Adak, Alaska, Adak, is located on the island. The isl ...
in Alaska and
Mindanao
Mindanao ( ) ( Jawi: مينداناو) is the second-largest island in the Philippines, after Luzon, and seventh-most populous island in the world. Located in the southern region of the archipelago, the island is part of an island group of t ...
in the Philippines), they argue that due to a higher mantle potential temperature of the Earth, a hotter and softer crust may have enabled intense adakite-type subduction during Archean time.
TTGs packages were then generated in such settings, with large scale proto-continents formed by
collisions
In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great fo ...
at a later stage.
However, other authors doubt the existence of
Archean subduction by pointing out the absence of major plate tectonic indicators during most of the Archean Eon. It is also noted that Archean TTGs were
intrusive rock
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: Marc ...
s while the modern adakite is
extrusive
Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. In contras ...
in nature, thus their magma should differ in composition, especially in water content.
Non-plate tectonic settings

Various evidence has shown that Archean TTG rocks were directly derived from preexisting mafic materials.
The melting temperature of meta-mafic rocks (generally between 700 °C and 1000 °C) depends primarily on their water content but only a little on pressure.
Different groups of TTG should therefore have experienced distinct
geothermal gradient
Geothermal gradient is the rate of temperature change with respect to increasing depth in Earth's interior. As a general rule, the crust temperature rises with depth due to the heat flow from the much hotter mantle; away from tectonic plate bo ...
s, which corresponding to different geodynamic settings.
The low pressure group has formed along
geotherms around 20-30 °C/km, which are comparable to those during the
underplating of plateau bases.
Mantle upwellings add mafic basement to the crust and the pressure due to the cumulation thickness may reach the requirement of low pressure TTG production.
The partial melting of the plateau base (which can be induced by further mantle upwelling) would then lead to low pressure TTG generation.
The high pressure TTGs have experienced geotherms lower than 10 °C/km, which are close to modern hot subduction geotherms experienced by young slabs (but around 3 °C/km hotter than other modern subduction zones), whilst the geotherms for the most abundant TTG subseries, medium pressure group, are between 12 and 20 °C/km.
Other than hot subduction, such geotherms may also be possible during the
delamination
Delamination is a mode of failure where a material fractures into layers. A variety of materials including laminate composites and concrete can fail by delamination. Processing can create layers in materials such as steel formed by rolling an ...
of mafic crustal base.
The delamination may be attributed to
mantle downwelling or an increase in density of the mafic crustal base due to metamorphism or
partial melt extraction.
Those delaminated meta-mafic bodies then sink down, melt, and interact with surrounding mantle to generate TTGs. Such delamination induced TTG generation process is petrogenetically similar to that of
subduction
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
, both of which involves deep burial of mafic rocks into the mantle.
See also
*
Eoarchean geology
Eoarchean geology is the study of the oldest preserved crustal fragments of Earth during the Eoarchean era from 4 to 3.6 billion years ago. Major well-preserved rock units dated Eoarchean are known from three localities, the Isua Greenstone Belt ...
*
Archean subduction
References
{{DEFAULTSORT:Tonalite-trondhjemite-granodiorite
Igneous rocks