The Archaean high-Mg diorite suite: Links to tonalite-trondhjemite-granodiorite magmatism and implications for early Archaean crustal growth

The 2.95 Ga Pilbara hip high-Mg diorite suite intrudes the central part of the Archaean granite-greenstone ten ain of the Pilbara Craton, Western Aust ralia, and shows many features typical of high-Mg diorite (sanukitoid) suit es from other late Archaean terrains. Such suites form a minor component of Archaean felsic crust, They are typically emplaced in late- to post-kinema tic settings sometimes in association with felsic alkaline magmatism, and a re either unaccompanied by, or post-date, tonalite-trondhjemite-granodiorit e (TTG) magmatism, which comprises a much greater proportion of Archaean fe lsic crust. The TTG series comprises sodic, Sr-rich rocirs with high La/Yb and Sr/Y ratios, thought to result from partial melting of eclogite facies basaltic crust. High-Mg diorite shares these characteristics but has signif icantly higher mg-number (similar to 60), and Cr and Ni concentration, sugg esting a mantle source. Many compositional features of TTGs are also shared by Cenozoic felsic magmas called adakites. Adakites from by melting of a y oung, hot, subducting slab and provide an a priori reason to invoke a subdu ction origin for TTG. During ascent through the mantle wedge. adakite commo nly assimilates, or is contaminated by peridotite, and the resulting 'wedge -modified adakite' bears strong compositional similarity! to Archaean high- Mg diorite. Nevertheless, the latter are not simply an Archaean analogue of 'wedge-modified adakite' (i.e. 'wedge-modified TTG') because their intrusi on is post-tectonic and unaccompanied by TTG magmatism. The petrogenesis of the Pilbara high-Mg diorite suite requires remelting of a mantle source. e xtensively metasomatized Dy addition of about 40% TTG-like melt. However al though the generation of this metasomatized source appears to require a sub duction environment, many Archaean TTG suites show no clear chemical eviden ce of having interacted with a mantle wedge, and on that basis are more lik ely to represent pastiad melts of basaltic lower crust rather than of subdu cted slab. High-Mg diorite suites appear to concentrate in the Late Archaea n suggesting that subduction may have become an important process only afte r similar to3.0 Ga.