The Archaean tonalite-trondhjemite-granodiorite (TTG) series is not an analogue of Cenozoic adakite

The tonalite-trondhjemite-granodiorite (TTG) series comprises silicic and s odic rocks that form a major component of preserved Archaean crust. TTG are widely considered to have formed during subduction by partial melting of h ydrated oceanic crust. This hypothesis relies primarily on compositional si milarities with Cenozoic subduction-related felsic rocks called adakites. H owever, simple geochemical parameters, such as silica content and Mg-# [= M g2+/(Mg2++Fe-Total) x 100, with Fe-Total as Fe2+], show that TTG are distin ct from adakite. Most adakite suites comprise, or include, high Mg-# and lo w SiO2 samples which suggest that these slab melts interacted with the mant le, whereas virtually all samples of pre-3.0 Ga TTG, and more than half of the samples of post-3.0 Ga TTG, show no evidence for such interaction. Conv incing evidence for a direct mantle component in TTG is primarily restricte d to samples from the late Archaean Superior Province of Canada, where ther e is also independent evidence for subduction and arc-accretion. Consequent ly, most early Archaean, and many late Archaean, TTG suites are not analogu es of Cenozoic adakite. The composition of these TTG suites does not provid e evidence for modern-style subduction processes. Tectonic models that acco mmodate TTG production through melting of hydrous basaltic material at the base of thickened crust may be more appropriate to the Archaean. Crown copy right (C) 2000 Published by Elsevier Science B.V. All rights reserved.