Formation mechanism of Archean oceanic and continental crusts

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Through the detailed field mapping in the Archean orogenic belts of the 3.8 Ga Isua supracrustal belt, West Greenland, 3.5Ga Marble Bar and North Pole regions in eastern Pilbara, 3.3Ga Cleaverville region in western Pilbara, and 2.5Ga Hamersley region in southem Pilbara in Western Australia, Archean MORB,OIB and TTG were identified and collected systematically. For the identification of tectonic setting for each igneous rock, the principle of accretionary geology was applied. Primary magma composition, its temperature, and source mantle composition for each tectonic setting in the Archean time, were estimated, and the following conclusions were led ;(1) Formation mechanism of Archean oceanic crust at spreading center is essentially same as that of modern Earth, however, the details are slightly different depending on hotter Archean mantle. For example, the thickness of oceanic crust was 2-3 times thicker (ca.20km) than today.(2) Archean hot spot magma was generated from ascending mantle plume about 400 C higher than that of modern Iceland, by which specific magma low-Al komatiite was formed only in Archean time.(3) Source mantle had already been differentiated even at 3.8Ga among mid-oceanic ridge, mantle wedge, and hot spot.(4) Archean mantle was richer in crustal component than today, e.g., Mg value of olivine was 85 in the Archean (cf.89 in the modern Earth).(5) Archean continental crust was formed through the slab-melting. REE abundance and concentration of Archean TTG zoned plutons suggest that the Phanerozoic TTG was also formed by slab-melting by subduction of young oceanic lithosphere.