An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution

• DOI: 10.1016/j.lithos.2004.04.048
• Publication Date: 2004-12-11T19:47:11Z
• Authors: H. Martin; R.H. Smithies; R. Rapp; J.-F. Moyen; D. Champion
• Keywords: crustal evolution; Keywords: adakite; island arc; Archaean; granodiorite; trondhjemite Adakite; 01 natural sciences; tonalite; orthoandesite; 13. Climate action; Sanukitoid; TTG; 0105 earth and related environmental sciences

Examination of an extensive adakite geochemical database identifies two distinct compositional groups. One consists of high-SiO2 adakites (HSA) which is considered to represent subducted basaltic slab-melts that have reacted with peridotite during ascent through mantle wedge. The second group consists of low-SiO2 adakites (LSA) which we interpret to have formed by melting of a peridotitic mantle wedge whose composition has been modified by reaction with felsic slab-melts. The chemical composition of less differentiated (primitive) Archaean tonalite–trondhjemite–granodiorite (TTG) magmas evolvedfrom4.0to2.5Ga.Mg#(molecularMg/(Mg+Fe 2+ ),Ni,andCrcontentsincreasedoverthisperiodoftimeandweinterpret thesechangesintermsofchangesinthedegreetowhichtheTTGmagmasinteractedwithmantleperidotite.Overthesame period, concentrations of (CaO+Na2O) and Sr also increased, as the amount of plagioclase, residual from basalt melting, decreased in responsetoincreasedpressuresatthesiteofslab-melting. IntheEarlyArchaean,itappearsthattheseinteractionswereveryrareor absentthus leading tothe conclusion that subduction was typically flat andlacked the developmentofa mantle wedge.In contrast, therelativelylowerheatproduction by~2.5Gameant thatslab-melting occurredatgreaterdepth,whereplagioclasewasnolonger stable, and where the development of a thick mantle wedge ensured interaction between the slab-melts and mantle peridotite. Close compositional similarities between HSA and Late Archaean TTG (Tb~3.0 Ga) strongly suggest a petrogenetic analogy. However, an analogy between the older Archaean TTG and HSA is not complete because evidence for mantle wedge interaction is missing in most Early Archaean TTGs. Late Archaean sanukitoids and the compositionally similar Closepet-type granites have compositions significantly different from TTG of all ages. However, they show some affinity with LSAwhich could be considered as their possible analogue. These magmas are all thought to result from melting of a mantle peridotite whose composition has been modified by reaction with slab-melts.