Discussed are two probable models for lateral growth of continental crust in the Early Precambrian: accretion of island arcs and accretion of oceanic plateaus. Important constraints on geodynamic settings in which Archean crust was generated can be obtained from the study of magmatic products found in granite-greenstone provinces-tholeiitic basalts and rocks of the tonalite-trondhjemite-granodiorite series TTG. The komatiite-tholeiite and tholeiitic-basalt series are differentiated into island-arc and plume oceanic plateau assemblages based on their geochemical signatures and the character of associated lithologies. Because of the geochemical similarity between Archean TTG suites and adakites which are generated through oceanic slab melting in subduction zone, the tonalite-trondhjemite-granodiorite series is widely considered to be subduction related. Our case study in the Early Archean TTG suite of the Onot block SW margin of the Siberian craton shows that tonalite-trondhjemite suites, unlike adakites, bear no major element evidence that their initial melts interacted with mantle wedge peridotites. The TTG-associated amphibolites, just like model metabasite sources of tonalite melts, have trace element signatures identical to modern and Archean basalts of plume related oceanic plateaus, and not to MORB-like abyssal tholeiites. Archean TTG suites were not restricted solely to subduction related settings, but could also have formed through melting at the bottom a of thick crust generated by accretion of oceanic plateaus. The most promising criteria to reconstruct geodynamic settings can be provided by i compositional characteristics of metabasite enclaves in TTG suites and the model source for tonalite-trondhjemite melts, ii Mg#'s of TTG rocks and their similarity to experimental melts, and iii isotope parameters of TTG rocks and their associated amphibolites.
tonalite-trondhjemite suites, plume, subduction, continental crust, accretion, island arcs, oceanic plateaus.
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