Tonalite–trondhjemite–granodiorite

[3] The major accessory minerals of TTG rocks include biotite, amphiboles (e.g. hornblende), epidote, and zircon.

Confirmed by geochemical modelling, TTG type magma can be generated through partial melting of hydrated meta-mafic rocks.

Although modern adakites are rare and only found in a few localities (e.g. Adak Island in Alaska and Mindanao 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.

[13] It is also noted that Archean TTGs were intrusive rocks while the modern adakite is extrusive in nature, thus their magma should differ in composition, especially in water content.

[4][8] The partial melting of the plateau base (which can be induced by further mantle upwelling) would then lead to low pressure TTG generation.

[8] The delamination may be attributed to mantle downwelling[19] or an increase in density of the mafic crustal base due to metamorphism or partial melt extraction.

Such delamination induced TTG generation process is petrogenetically similar to that of subduction, both of which involves deep burial of mafic rocks into the mantle.

[1] Continental arc TTG rocks are often associated with gabbro, diorite, and granite, which forms a plutonic sequence in batholiths.

[21] 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.

[22] These TTG rocks in continental arc batholiths may partially originate from the magma differentiation (i.e. fractional crystallisation) of the subduction induced mantle wedge melt at depth.

[23] However, the large volume of such TTG rocks infer their major generation mechanism is by the crustal thickening induced partial melting of the former gabbroic underplate at the base of the continental crust.

Archean TTG rock outcrop in Kongling Complex, South China Craton. The white TTG rock body is intruded by dark mafic dikes, as well as light color felsic dikes. The mafic minerals in the TTG rock body, possibly biotite , were weathered, which introduced a brownish coating on the TTG rock surface.
TTG rock sample (Tsawela gneiss) with foliation from the Kaapvaal Craton, South Africa. The white minerals are plagioclase; the light grey ones are quartz; the dark, greenish ones are biotite and hornblende, which developed foliation.
Hypothesized Archean hot subduction induced Archean TTG generation model. The heavier oceanic crust sinks into the lighter mantle. The subducting slab is young and hot, thus when it is heated, it partially melts to generate TTG magmas, which rise and intrude into the continental crust. Light green: continental crust; dark green: oceanic crust; red: TTG melts; orange: mantle. Modified from Moyen & Martin, 2012. [ 4 ]
The delamination and underplating induced Archean TTG generation models. In the upper figure, heavier mafic crust delaminates into the lighter mantle. The pressure and temperature increases induce the partial melting of the delaminated mafic block to generate TTG magma, which rises and intrudes to the crust. In the lower figure, mantle plume rises to the base of the mafic crust and thicken the crust. The partial melting of the mafic crust due to the plume heating generates TTG magma intrusions. Modified from Moyen & Martin, 2012. [ 4 ]