The morphology of zircon crystals from some Cretaceous granitoids in northeastern Kyushu is described. They are Hirao granodiorite, Kawara granodiorite, Masaki granite, Katsuyama granite, and Kaho granite in geologic ascending order. The Kawara granodiorite and the Kaho granite, distributed on the western side of the Kokura-Tagawa Tectonic Line, contain the 100 dominant prismatic zircon. On the eastern side, the Hirao granodiorite and the Katsuyama granite are of the intermediate type (prismatic to long-prismatic), and the Masaki granite (long-prismatic) is of the 110 dominant type. The difference in the prism index can be explained that the western granitoids were formed at a little higher temperature than the eastern ones. Similarly, judging from the difference in the elongation index, the western Kawara granodiorite is considered to have intruded first, followed by the Hirao granodiorite and the Masaki granite on the eastern side. The Katsuyama granite of S type (Chapell and White, 1974) contains many acidental zircon crystals of the 110 type which exhibit developed cracks.

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In the Sanchu belt, detrital chromian spinels are found in the Cretaceous sandstone and conglomerate of the Ishido Formation near the southern marginal faults, along which small serpentinite bodies are emplaced. The detrital chromian spinels can be classified into two, low-Ti and high-Ti, groups in terms of chemistry. The low-Ti spinels (TiO₂ < 0.4 wt%; Cr # (=Cr/(Cr+Al) atomic ratio), 0.5-0.9) occur as coarse discrete grains. They are relatively low in Mg # (=Mg/(Mg+Fe²⁺) atomic ratio) and were probably derived from low-temperature peridotites. The high-Ti spinels (TiO₂, ca.1 wt%; Cr #, 0.5 to 0.6) sometimes occur as small euhedra in serpentine or chlorite aggregates, which are alteration products either of olivine phenocryst or of volcanic glass. They are relatively high in Mg #. The low-Ti spinels in the Cretaceous sediments are slightly different from those in the currently exposed serpentinite: the former spinels are similar in TiO₂ level and Cr # range to, but are slightly higher both in average Cr # and in Fe³⁺ contents than, the latter ones. Spinels in metavolcanics associated with the serpentinite bodies are definitely higher both in TiO₂ and in Fe³⁺ contents and are lower in Mg # than the detrital high-Ti spinels.
Based on the spinel chemistry, the serpentinite-volcanics complex which had supplied detritus to the Sanchu Cretaceous sediments are proved to be slightly but clearly different in lithology from that currently exposed along the southern margin of the Sanchu belt. The Sanchu serpentinites were most probably derived from arc to fore-arc upper mantle. The spinel chemistry and preliminary bulk rock chemistry indicate that the volcanics associated with the serpentinite, on the other hand, have characteristics of within-plate basalt or E-type MORB. A transcurrent fault (plate boundary) formed in a fore-arc region was possibly responsible for the protrusion of the serpentinite-volcanics complex, which had dammed up the terrigenous sediments as the ocean-ward border of the Sanchu sedimentary basin.

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The pseudobinary system diopside (CaMgSi₂O₆)-gehlenite (Ca₂A1₂SiO₇) has a pseudoeutectic point, where diopside solid solution and melilite solid solution coexist with liquid, at 1295±3°C and Di₇₅Geh₂₅. Below solidus temperatures (1225±10°C) anorthite appears and coexists with melilite_ss and/or diopside_ss.
The system diopside-akermanite (Ca₂MgSi₂O₇)-gehlenite dose not have the point at which three crystalline phases and liquid coexist, but a phase boundary along which diopside_ss and melilite_ss coexist with liquid. Temperature minimum seems to exist at the central part of the primary field of melilite_ss. The phase assemblages below solidus temperatures are diopside_ss+ melilite_ss in the Ca₂MgSi₂O₇-rich region and diopside_ss+ melilite_ss+anorthite in the Ca₂Al₂SiO₇-rich region.

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