白亜紀花崗岩類の風化にともなう化学組成およびSr・Nd同位体比組成の変化

抄録

We analyzed the changes of mineral, chemical and isotopic compositions of weathered Cretaceous granitic rocks from the Gozu Granite in the Gozu mountain, Niigata Prefecture, Kise Granodiorite in the Ina mountain, Nagano Prefecture, and Aji Granite in the Aji Peninsula, Kagawa Prefecture. The analyzed samples are fresh and weathered rocks and “masa” (sand originated from weathered granitic rocks) collected from the same outcrops for each granite. The change of mineral compositions of weathered granitic rocks is mainly characterized by the appearance of hematite and the increase of alteration degree of mafic minerals. Chemical composition of weathered granitic rocks is characterized by the depletion of major and trace elements. In particular, CaO and Na₂O, which are partitioned mostly into plagioclase, and S are reduced extremely. Fe₂O₃/FeO ratios increase extremely with increasing weathering degree. Variations of total Fe and Cr contents dyring weathering are little. Rb/Sr ratios increase and Sr isotopic ratios change in progress of weathering. Nd isotopic compositions are almost maintained during weathering. The variation of Sr isotopic composition is generally caused by dissolution of minerals and isotope exchenge.