Tonalite-Trondhjemite-Granodiorite
Encyclopedia
Tonalite
–trondhjemite
–granodiorite
(TTG) series are an aggregation of rocks that are formed by melting of hydrous mafic
crust
at high pressure. It is widely accepted that most Archaean granite
–greenstones
are dominated by TTG, although Late Archaean terrane
s, such as in the Yilgarn Craton
, are dominated by potassium
-rich granitoid
rocks that are derived through remelting of older felsic
TTG-dominated crust. According to this model, a much greater degree of crustal reworking has occurred in the Pilbara craton
than is required by TTG-dominated crust.
The origin of TTG suites is debated; their chemistry is similar to modern-day subduction zone magmas, but there is disagreement as to whether modern plate tectonic processes operated during the Archaean. Some authors have suggested alternate styles of subduction, while others attribute the development of TTG to direct melting of the lithosphere
by mantle plumes.
Tonalite
Tonalite is an igneous, plutonic rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.In older references tonalite is...
–trondhjemite
Trondhjemite
Trondhjemite is a leucocratic intrusive igneous rock. It is a variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites are sometimes known as plagiogranites....
–granodiorite
Granodiorite
Granodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...
(TTG) series are an aggregation of rocks that are formed by melting of hydrous mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
at high pressure. It is widely accepted that most Archaean 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...
–greenstones
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....
are dominated by TTG, although Late Archaean 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, such as in the Yilgarn Craton
Yilgarn craton
The Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...
, are dominated by potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
-rich granitoid
Granitoid
A granitoid or granitic rock is a variety of coarse grained plutonic rock similar to granite which mineralogically are composed predominately of feldspar and quartz. Examples of granitoid rocks include granite, quartz monzonite, quartz diorite, syenite, granodiorite and trondhjemite. Many are...
rocks that are derived through remelting of older felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
TTG-dominated crust. According to this model, a much greater degree of crustal reworking has occurred in the Pilbara craton
Pilbara craton
The Pilbara craton , along with the Kaapvaal craton are the only remaining areas of pristine Archaean 3.6-2.7 Ga crust on Earth...
than is required by TTG-dominated crust.
The origin of TTG suites is debated; their chemistry is similar to modern-day subduction zone magmas, but there is disagreement as to whether modern plate tectonic processes operated during the Archaean. Some authors have suggested alternate styles of subduction, while others attribute the development of TTG to direct melting of the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
by mantle plumes.