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.

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The 12 ka eruption of Nantai Volcano, central Honshu, Japan, produced a succession of pyroclastic deposits followed by the emplacement of a lava flow. The 12 ka eruption began with phreatic eruptions, forming a fine ash deposit (unit A). Explosive magmatic eruption followed forming a scoria-fall (unit B) and partially welded scoria-flow deposits (unit C). The scoria eruption was followed by a pumice eruption producing a pumice-fall deposit (unit D) and a consecutive voluminous pumice-flow deposit (unit E). The 12 ka eruption finished with the extrusion of a heterogeneous dacite-andesite mingled lava flow (unit F).
The early scoriae are phenocryst-poor (∼3-15 vol.%) tholeiitic dacite and andesite with an anhydrous phenocryst assemblage (plagioclase+orthopyroxene+clinopyroxene+Fe-Ti oxides±olivine), whereas unit D-F eruptives are porphyritic calc-alkaline dacite and andesite with a hydrous phenocryst assemblage (plagioclase+hornblende+quartz+orthopyroxene+clinopyroxene+Fe-Ti oxides±olivine). This suggests that two magma types, firstly tholeiitic and secondly calc-alkaline, fed the 12 ka eruption. Although major and trace elements are broadly similar in all dacites erupted between unit D and F, there are considerably differences in phenocryst contents: unit-D and unit-E dacites have lower crystal contents (∼17 vol.%) whereas unit-F lava has higher crystal contents (∼50 vol.%). The unit D-E dacite (pumice) and the unit-F dacite (lava) may have been derived from different parts of a magma chamber, perhaps from inner melt-rich and marginal mushy parts, respectively.

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This paper concerns the occurrence and chemical compositions of “in-situ” greenstones in the Nago and Kerama Formations in the Okinawa and Kerama Islands, which belong to the Shimanto Belt of Cretaceous-Paleogene age. The in-situ greenstones are composed of metabasaltic pillow lava, pillow breccia, hyaloclastite and sills. These are closely interbedded with, or grade upward into, pelitic rock (in Nago Fm.) and sandstone (in Kerama Fm.), indicative of a volcanism near trench. The greenstones have geochemical features of N-MORB in terms of high-field strength element (HFSE) and rare-earth element (REE) chemistry. The relict Cr-spinel composition also support this bulk chemical characteristics. They may have been produced at the spreading ridge between the Kula and Pacific plates, which is thought to have passed northward along the SW Japan arc through Cretaceous and Paleogene. Assuming a ridge migration rate of 4 cm/year, we infer that ages of the Nago and Kerama Formations are about 95-100 Ma.

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Epidote group is important rock-forming minerals which form in a variety of geological conditions. Crystal structure of epidote contains 9-coordinated A1 and 10-coordinated A2 sites, and three kinds of octahedral sites, M1, M2, and M3. Various cations can occupy the A and M sites, and consequently epidote group minerals have a wide range chemical compositions. Natural Ca-epidote group minerals contain several transition elements, including Fe, Mn, Cr and V, and their occurrence, chemical composition and crystal chemistry have thus been studied in some detail. Crystallographic studies of synthetic epidote group minerals on the Ca₂Al₃Si₃O₁₂(OH)(Cz)-Ca₂Fe₃³⁺Si₃O₁₂(OH) and Cz-Ca₂Mn₃³⁺Si₃O₁₂(OH) joins have shed much light on the contribution of each ionic substitution to their crystal structures. In this paper, we review studies of the ionic substitutions in the octahedral M1, M2, and M3 sites in Ca-epidote group minerals, and the influence of these substitutions on structural changes.

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