Formation mechanism of Archean oceanic and continental crusts

通过对西澳大利亚东部皮尔巴拉3.8 Ga Isua上地壳带、西格陵兰、3.5Ga Marble Bar和北极地区、西部皮尔巴拉3.3Ga Cleaverville地区和南部皮尔巴拉2.5Ga Hamersley地区太古代造山带的详细野外填图，系统地识别和收集了太古代MORB、OIB和TTG。为确定各火成岩的构造背景，应用了增生地质学原理。对太古宙各构造背景下的原始岩浆组成、温度和源幔组成进行了估算，得到以下结论：(1)扩张中心太古宙洋壳的形成机制与现代地球基本相同，但由于太古宙地幔温度较高，在细节上略有不同。例如，海洋地壳的厚度是现在的2-3倍（约20公里）。(2)太古宙热点岩浆是由上升的地幔柱产生的，比现代冰岛高出约400℃，仅在太古宙时期形成了特定的岩浆低铝科马铁矿。(3)在3.8Ga时，源幔已在洋中脊、地幔楔和热点之间进行了分异。(4)太古宙地幔的地壳成分比今天丰富，如太古宙橄榄石的Mg值为85（现代地球为89）。(5)太古代大陆地壳是通过板块熔融作用形成的。太古宙TTG分带岩体的REE丰度和浓度表明显生宙TTG也是由年轻海洋岩石圈俯冲而形成的板块熔融形成的。

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.