The Archaean High-Mg Diorite Suite: Links to Tonalite-Trondhjemite-Granodiorite Magmatism and Implications for Early Archaean Crustal Growth

Abstract

The 2·95 Ga Pilbara high-Mg diorite suite intrudes the central part of the Archaean granite–greenstone terrain of the Pilbara Craton, Western Australia, and shows many features typical of high-Mg diorite (sanukitoid) suites from other late Archaean terrains. Such suites form a minor component of Archaean felsic crust. They are typically emplaced in late- to post-kinematic settings, sometimes in association with felsic alkaline magmatism, and are either unaccompanied by, or post-date, tonalite–trondhjemite–granodiorite (TTG) magmatism, which comprises a much greater proportion of Archaean felsic crust. The TTG series comprises sodic, Sr-rich rocks 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 significantly higher mg-number (∼60), and Cr and Ni concentrations, suggesting a mantle source. Many compositional features of TTGs are also shared by Cenozoic felsic magmas called adakites. Adakites form by melting of a young, hot, subducting slab and provide an a priori reason to invoke a subduction origin for TTG. During ascent through the mantle wedge, adakite commonly 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 intrusion is post-tectonic and unaccompanied by TTG magmatism. The petrogenesis of the Pilbara high-Mg diorite suite requires remelting of a mantle source, extensively metasomatized by addition of about 40% TTG-like melt. However, although the generation of this metasomatized source appears to require a subduction environment, many Archaean TTG suites show no clear chemical evidence of having interacted with a mantle wedge, and on that basis are more likely to represent partial melts of basaltic lower crust rather than of subducted slab. High-Mg diorite suites appear to concentrate in the Late Archaean, suggesting that subduction may have become an important process only after ∼3·0 Ga.