古代中央海嶺火成活動 : 太古代マントルの組成と温度の束縛条件

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1:5000 scale detailed mapping was performed in the Isua, southern West Greenland. The result shows that the Isua NE part is divided into three units by two low-angle thrusts, northern, middle, southern unit, respectively, The southern unit with the best exposures is underlain by twelve subunits (horses) with similar lithostratigraphy to one another and all of them are bounded by layer-parallel thrusts. The reconstructed lithostratigraphy of each horse reveals a simple pattern, in ascending order, greenstone with chemical affinities of low-K tholeiite, chert/BIF and mafic-felsic turbidite. The cherts and the underlying low-K tholeiites never include continent-or arc-derived materials. The lithostratigraphy is quite similar to the Phanerozoic oceanic plate stratigraphy forming accretionary complex. In addition, the faults formed by layer-parallel shortening are recognizable bounding each accretionary prism, and hence the so-called duplex structure is widespread all over the Isua belt, varying in size from a few meters to kilometers in scale. As recognized well on the map, basaltic flow layers tend to converge to the south. The similarities of field evidences in the mapped area to those in Phanerozoic accretionary complexes, for example, in terms of both duplex structure and oceanic plate stratigraphy. indicate that oceanic crust subducted beneath oceanic island arc to form an accretionary complex in the Early Archean. The greenstones could be classified into two types based on geological criteria on the analogy of the Phanerozoic accretionary complexes. One is MORB-type rocks (IMORBs) that are overlain by thick, up to 50 m, bedded chert and often characterized by well-preserved pillow structure, suggesting that they were formed at mid-oceanic ridge. The other is OIB-type rocks (IOIBs), which are interlayered with thin chert or appears as sills or dikes into chert but not into the overlying terrigenous sediment, suggesting that they were derived from oceanic island volcanism. Major element geochemistry shows that IMORBs and IOIBs are high-Fe and low-K tholeiites. IMORBs are enriched in FeO and Al_2O_3 compared with modern MORB, suggesting that source mantle of IMORBs was enriched in basaltic components. That is consistent with incompatible trace element chemistry. Siderophile elements (Ni and P) are not enriched compared with modern MORBs, suggesting that the formation of metallic core of the Earth had been completed by 3.8Ga Rare earth elements (REE) abundance of IMORBs and IOIBs is ten times more than that of chondorite respectively. The former is depleted in light to middle REE, and by contrast, the latter displays flat REE pattern, indicating that mantle had been already differentiated between MORB (shallow) and OIB (deep) source mantle through the formation of oceanic crust.
特定非営利活動法人日本火山学会の論文
1997-04-25

古代中央海嶺火成活動 : 太古代マントルの組成と温度の束縛条件

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