In the Sanbagawa metamorphic belt in the northwestern part of the Kii Peninsula, occurrences of biotite and its altered products are newly recognized. Pelitic schists in the area within the previously accepted garnet zone occasionally contain biotite and/or a mineral, which is optically almost identical to biotite. The latter mineral has variable K₂O contents less than 6 wt%, and has similar stoichiometry to biofite except low-K content. EPMA and XRD analyses suggest that the mineral is weathered biotite, which contains interlayered vermiculite. Taking the localities of such altered biotite-bearing pelitic schists into consideration, the biotite zone is newly established as a narrow region along the east-west trending axis of the Dainichi synform. The Fe-Mg partition coefficients for garnet-chlorite pairs in the pelitic schists of biotite zone indicate that the biotite zone in the Kii Peninsula corresponds to the lower-grade part of the albite-biotite zone in central Shikoku.

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Detailed stratigraphical investigation revealed that pigeonite-phyric basalts from Kurohana-Yama, Sendai, Japan erupted at late Miocene or early Pliocene, with the age of the eruption is younger than 8 Ma. It is because K-Ar ages of dikes which are stratigraphicaly older than Kurohana-Yama lavas are about 8 Ma (Tsunakawa et al., 1983).
     The pigeonite basalts from Kurohana-Yama are chemically devided into two types; type A basalts contain higher MgO (7 wt%) and lower Al₂O₃ (17-18 wt%) than type B basalts (MgO 5.2-5.5 wt% and Al₂O₃ 19-20 wt%). All these basalts contain plagioclase and olivine phenocrysts .Assemblages of phenocryst pyroxenes are various and most of the basalts contain large pigeonite phenocrysts.

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Miocene volcanic rocks were discovered in the Kurahashi Jima and Hashira Jima districts. The volcanic rocks compose of MgO-rich olivine-clinopyroxene andesite, biotite-hornblende dacite, and high-alkali tholeiitic olivine basalt, occurring as dikes or necks with minor pyroclastic rocks.
     Whole rock K-Ar age determinations were carried out on two magnesian andesites, a daciteand a basalt. The magnesian andesites and the dacite yielded Middle Miocene ages of 14.9±0.7, 13.3±0.7, and 12.2±0.6 Ma respectively. These results indicate that the andesites and the dacite belong to the Setouchi volcanic rocks, whereas the basalt gives a Late Miocene age of 8.4±0.4 Ma suggesting another younger volcanic activity.

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Partition coefficients for Hf and Zr (D_Hf, D_Zr) have been determined experimentally between HFSE-bearing mafic minerals such as amphibole, clinopyroxene and garnet and basanitic andandesitic melts doped with Hf and, in some cases Zr as well, in a range of pressure from 0.5 to 3.0 GPa, and 1000-1050°C (basanite) or 900-950°C (andesite). D_Hf and D_Zr for amphibole/melt decrease with increasing P, and this pressure effect on D's is most noteworthy between 0.5 and 1.5 GPa. The D_Hf and D_Zr correlate positively with the relevant D_Ti. Also amphibole/melt pairs show D_Hf>D_Zr consistently through the pressure range 0.5-2.5 GPa. Thus Zr and Hf are decoupled to some extent by amphibole crystallization. A similar partitioning behaviour of Hf and also possible Zr/Hf fractionation occurs for clinopyroxene/melt pairs. In contrast, garnet/melt pairs indicate D_Zr>D_Hf. This implies that a distinctive Hf/Zr fractionation would occur with garnet involvement, compared with clinopyroxene (or amphibole). Also a modest change in Hf/Zr ratios is supposable in magmas, especially during initial stages of the partial melting process in which clinopyroxene is postulated as a residual phase and/or if amphibole fractionation occurs during shallower-level differentiation. However, results of our partial melting and fractionation modeling indicate that the change is only minor to negligible through such mineral(s)/melt separation processes involving clinopyroxene and amphibole. Co-existence of garnet with clinopyroxene in the residue further restrains the change of Hf/Zr ratios in partial melts, because the two minerals tend to fractionate Hf and Zr in opposite directions.

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