Tonalite-Trondhjemite-Granodiorite

Tonalite-trondhjemite-granodiorite rocks or TTG rocks are intrusive rocks with typical granitic composition (quartz and feldspar) but containing only a small portion of potassium feldspar. Tonalite, trondhjemite, and granodiorite often occur together in geological records, indicating similar petrogenetic processes. Post Archean (after 2.5 Ga) TTG rocks are present in arc-related batholiths, as well as in ophiolites (although of a small proportion), while Archean TTG rocks are major components of Archean cratons. Tonalite-trondhjemite-granodiorite rocks or TTG rocks are intrusive rocks with typical granitic composition (quartz and feldspar) but containing only a small portion of potassium feldspar. Tonalite, trondhjemite, and granodiorite often occur together in geological records, indicating similar petrogenetic processes. Post Archean (after 2.5 Ga) TTG rocks are present in arc-related batholiths, as well as in ophiolites (although of a small proportion), while Archean TTG rocks are major components of Archean cratons. The quartz percentage among felsic minerals in TTG rocks is usually larger than 20% but less than 60%. In tonalite and trondhjemite, more than 90% of the feldspars are plagioclase, while in granodiorite, this number is between 65% and 90%. Trondhjemite is a special kind of tonalite, with most of the plagioclase in the rock being oligoclase. The major accessory minerals of TTG rocks include biotite, amphiboles (e.g. hornblende), epidote, and zircon. Geochemically, TTG rocks often have a high silica (SiO2) content (commonly over 70 percent SiO2), high Na2O content (with low K2O/Na2O ratio) compared to other plutonic rocks, and low ferromagnesian element content (the weight percentage of Fe2O3, MgO, MnO2, TiO2 added together commonly smaller than 5%). Post Archean TTG rocks are commonly found in arc settings, especially in continental arcs. Ophiolite also contains a small amount of TTG rocks. Continental arc TTG rocks are often associated with gabbro, diorite, and granite, which forms a plutonic sequence in batholiths. They are formed by hundred of plutons that directly related to subduction. 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 (i.e. fractional crystallisation) of the subduction induced mantle wedge melt at depth. 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. 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, but they are rarely exposed. Tonalites (including trondhjemites) can be found above the layered gabbro section in ophiolites, below or within sheeted dykes. They are often irregular in shape and produced by magma differentiation. Archean TTG rocks appear to be strongly deformed grey gneiss, showing banding, lineation, and other metamorphic structures, whose protoliths were intrusive rocks. TTG rock is one of the major rock types in Archean cratons. In terms of trace element characteristics, Archean TTGs exhibit high light rare earth element (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 and amphibole, but no plagioclase in the residual phase during partial melting or precipitation phase during fractional crystallization. Confirmed by geochemical modelling, TTG type magma can be generated through partial melting of hydrated meta-mafic rocks. 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, Na2O, 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 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 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.