[B-CG05] 地球史解読：冥王代から現代まで

[BCG05-P10] Petrogenesis of the mid-Archaean tonalite-trondhjemite-granodiorite rocks in the Barberton granitoid-greenstone terrain: constraints from geochemical and isotopic signatures of zircon

*青木 一勝¹、三宅 志歩¹、福山 繭子²、小笠原 正継³、小宮 剛⁴ (1.岡山理科大学　、2.秋田大学、3.産総研、4.東京大学)

The mid-Archean Barberton Granitoid-Greenstone terrain (BGGT) contains not only a series of mafic metamorphic rocks from low to high grade but also the Tonalite-Trondjhemite-Granodiorite (TTG) suite, indicating that the tectonics of BGGT is essential for understanding the crustal evolution throughout Earth history. The tectonic evolution of the BGGThas been interpreted by three contrasting processes: plate tectonic subduction-accretion, crustal partial convective overturn, and deformable lid. It is still uncertain which process was contributed to the formation of the BGGT. Igneous zircon reflects the geochemical characteristics of parental magma as trace elements composition. Therefore, geochemical analysis based on trace elements of zircon can be used to estimate the crustal evolution at the time of TTG formation in the BGGT. In this study, trace elements composition, and U-Pb and Lu-Hf isotope analyses were conducted for zircons separated from three TTG gneisses around the Inyoni Shear Zone in the BGGT, using LA-ICP-MS. The studied samples are tonalite as protolith definition. The cathode luminescence images of zircons showed a clear oscillatory zoned structure without an inherited core. The weighted mean U-Pb ages of each sample were ca. 3.2 Ga. The chondrite-normalized REE patterns obtained from all samples showed positive Ce anomalies, slightly negative Eu anomalies, and enrichments in HREE relative to LREE and MREE. The εHf(t) values were within 2.0 to -2.1. The Ti-in-zircon thermometer indicated that the zircon crystallization occurred at about 850-700 ℃. In addition, the analyzed zircons were plotted above the mantle array in the figure of Nb/Yb vs. U/Yb. These data indicate the contribution of the enriched juvenile source to the TTG magma in the BGGB. Probably, the TTG rocks were formed by partial melting of the tholeiitic basaltic crust at the depth of 10-30 km, and fluid was involved in its generation. This suggests that the vertical tectonics alone cannot explain the formation of TTG in the BGGT.