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Petrographic descriptions and electron microprobe analyses of minerals are presented for 35 specimens from seven suites chosen to examine the transition from magnetite series to ilmenite series granitoids along two transects across the Cretaceous‐Paleocene Inner Zone batholith of southwestern Japan. Regularities in chemical compositions of amphiboles, biotites, and feldspars suggest that fundamentally similar processes produced the magmas that formed the two series. Constant or decreasing Fe/(Fe+Mg) for biotites and amphiboles with increasing host rock silica content, coupled with the absence of early formed magnetite and sphene, suggest that magnetite series rocks may have become oxidized during crystallization near the level of intrusion, through the processes of second boiling and differential loss of hydrogen. For the Daito‐Yokota, magnetite series suite, Fe/(Fe+Mg) for biotites decreased from 0.48 to 0.37 as SiO2 content of the host rock increased from 55.3 to 75.5 wt %; for an ilmenite series suite from the Takanawa Peninsula, Fe/(Fe+Mg) for biotites increased from 0.51 to 0.77 with an increase in host rock SiO2 from 53.4 to 75.5. Detailed consideration of amphibole chemistry shows predominance of edenitic and tschermakitic substitution schemes as well as coupling between substitutions of Ti in octahedral sites and AlIV. Interrelations between amphibole and biotite chemistry show that Fe/(Fe+Mg) and Mn contents can be interpreted in terms of equilibration, whereas Ti content cannot. The chemistry of chlorites correlates well with that of biotites; primary and secondary muscovites are distinct in composition. Plagioclase in all studied suites shows igneous zoning appropriate to host rock composition; perthitic alkali feldspars in all samples have lost albite component, and temperatures based on the two‐feldspar geothermometer are low. The biotite‐apatite geothermometer is also inoperative for this group of samples because fully fluorinated apatites typically occur in biotites of modest F content. Whereas magnetites have reequilibrated, analyses of ilmenites for the representative Daito‐Yokota and Takanawa suites corroborate biotite compositional data and suggest that fO2 probably differed by 2 to 3 orders of magnitude during crystallization of silica‐rich magnetite and ilmenite series granites. Whole‐rock chemistry supports mineral chemistry in suggesting that the studied granitoids have crystallized from magmas generated in a lower crustal environment in which mantle‐derived magmas partially melted source materials with igneous characteristics.

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Late Cretaceous plutonic rocks of 6 gabbroids, and 75 granitoids, all belonging to ilmenite series, were re-examined by polarized XRF method. The gabbroids are relatively abundant in the south of the studied region, implying more in-put of heat and mafic magmas from the upper mantle to the lower crust. The MgO/Fe2O3 ratio of the gabbroids reveal regional variation being MgO-rich to the south, which would reflect generation depth of the mafic magmas. The granitoids are classified into I type and S type. The I-type granitoids occur most widely and divided into five zones. Their average silica contents vary from the Median Tectonic Line to the north as follows: Shinshiro-Shitara zone (60.1 % SiO2), Asuke zone (64.2 %), Toyota-Akechi zone (70.0 %), Sanagesan-Obara zone (73.9 %) and Seto zone (75.2 %). This regional variation was considered to reflect chemical heterogeneity of the infracrustal source rocks, rather than the magmatic differentiation.Granitoids of the Okazaki-Busetsu zone, represented by garnet-bearing muscovite-biotite granites, occur in the highest metamorphic grade zone, and is felsic (average SiO2 70.2 %), peraluminous (A/CNK over 1.1), and thus be called S type. Because of the highest heat flow rate in the OkazakiBusetsu zone, even supracrustal rocks were converted to magmas and formed the S-type granites. Both I and S type granitoids are often heterogeneous in the foliated parts, as compared with massive granitoids of non-metamorphic terrains. The heterogeneity can be considered due to different original compositions of the source materials and/or formed by differential regional stress during the flow movement of the solidifying magmas. Lack of hydrothermal mineralization in the south where metamorphic roof-pendent still remain, can be explained by a deep emplacement level ( 15 km) and small degree of fractionation of these granitic magmas.