The mechanisms of petrogenesis of the Archaean continental crust—Comparison with modern processes

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The petrographic and chemical composition of magmatic rocks generated during the Archaean appears to be different from that of post-Archaean rocks. Komatiites are widespread before 2.5 Ga and rarely occur afterwards. In addition the Archaean continental crust is primarily TTG (Tonalitic, Trondhjemitic and Granodioritic) in composition, exhibiting typical trondhjemitic differentiation trends; whereas modern equivalents are granodioritic to granitic following classical calc-alkaline differentiation trends. This distinction becomes more prominent when rare-earth elements (REE) are taken into account: Archaean TTG are Yb-poor (Yb N < 8.5) and have high ( La Yb ) ratios ( 5 < ( La Yb ) N < 150 ), in comparison, the post-2.5 Ga granitoids, emplaced in subduction-zone geodynamic environments have high Yb content (4.5<Yb N <20) with very low ( La Yb) N ratios (⩽ 20). Theoretical calculations and experimental petrology have shown that the TTG can be produced by partial melting of an Archaean tholeiite transformed into garnet-bearing amphibolite. Consequently, the low heavy REE content of the TTG is explained by the influence of both residual garnet and hornblende in their source. After 2.5 Ga the role of these minerals in calc-alkaline magma genesis becomes progressively less important, which is interpreted in terms of a cooling Earth model. In modern subduction zone environments the subducted oceanic slab is relatively “old and cold” and the geothermal gradient along the Benioff plane in low (ca. 10°C/km). Consequently, the down-going lithosphere undergoes dehydration before partial melting is able to occur. The liberated fluids are light REE and LILE-enriched and ascend into the overlying mantle wedge where they induced partial fusion. The produced magmas separate from their mantle source region leaving a residue mainly composed of olivine and pyroxenes. Mantle derived magmas typically exhibit high Yb contents due to low KD Yb values for olivine and pyroxenes. During the Archaean, the subducted lithosphere was relatively “young and hot” providing high geothermal gradients along the Benioff zone. Thus, partial melting of the subducted slab was possible at lower temperatures before dehydration would take place. Garnet and hornblende are the main residual phases accounting for the low Yb contents of the Archaean TTG. This model can be tested using a modern analogue of Archaean-like subduction processes. In south Chile an oceanic ridge has subducted and all thermodynamic calculations indicate that this creates locally high geothermal gradients along the Benioff zone. Thus in very small areas, Archaean-like environments may be simulated in modern subduction zones. The modern andesites produced in this environment show Archaean geochemical characteristics with low Yb N (<5), whereas the majority of andesites along the Andean arc have modern patterns with Yb N ranging from 8 to more than 17. This conclusion was generalised to all young subducted lithospheres all over the world. In conclusion, it appears that since the Archaean there has been a change in the site of continental crust genesis. The location of calc-alkaline magma source in subduction-zone environments has migrated through time from the subducted slab to the mantle wedge. This is a direct consequence of the progressive cooling of the Earth.