The 70 samples were collected from Neogene pelitic rocks in the region of the River No, Niigata Prefecture, Japan. The < 2 μm fraction of these samples were studied by X-ray diffractometry and scanning electron microscopy (SEM). X-ray diffraction studies reveal that the constituent minerals were montmorillonite, illite/smectite (I/S), chlorite, clay-size mica and clinoptilolite. As authigenicc minerals, montmorillonite, I/S and clinoptilolite were observed by SEM. The assemblage of authigenic minerals gradually changes with increase of depth of the sediments. A remarkable feature is continuous decrease of the smectite proportion in I/S and its structure changes from g=0 to g=3 continuously. This continuous conversion was confirmed in eastward of this area, too.
These results suggest that the authigenic minerals have been formed in diagenetic alteration process.

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Highly potassic mafic rocks are rarely found in Southwest Japan Arc. Recently, we found highly potassic lamprophyre (minette) from the northeastern part of the Sera Plateau in the Chugoku Mountains. It consists of phenocrysts of phlogopite (mg 85.6, TiO₂ 4.08-4.18), clinopyroxene (diopside-salite), olivine (mg 84.5) and apatite set in a groundmass of poikilitic sanidine ?? anorthoclase, phlogopite, clinopyroxene, apatite and titanomagnetite. The minette is high in K₂O content (5.88 wt%) and K₂O/Na₂O ratio (3.51). The rock shows relatively high concentration of MgO, Ni and Cr, and is strikingly enriched in incompatible elements, particularly Rb, Sr, Ba, Zr and P. The MORB-normalized pattern of the minette is most closely matched by certain alkali basalts of island arcs and continental margins, having negative Nb-anomalies. However, it is highly enriched in incompatible elements compared with island arc basalts. This suggests that the source for the minette is similar to that for the island arc alkali basalt but spiked with K, Rb, Sr, Ba, Zr and P prior to melting.

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Greenstones form an important part of both the Cretaceous and Paleogene mélange terrains in the Shimanto Belt of eastern Kyushu, Japan.
The Cretaceous greenstones are characterized by thrust sheets or slivers of pillowed and massive basalts, metamorphosed to actinolite-greenschist grades, associated with bedded cherts and umber-like metalliferous sediments. In contrast, the Paleogene greenstones examined in this study typically occur as isolated ‘float’ blocks of pillow basalt and pillow breccia, chaotically disposed together with blocks of massive sandstone and alternating sandstone-shale, in a weakly metamorphosed (upper zeolite facies) pebbly shale matrix.
In this paper, field descriptions, petrographic observations and geochemical data for both major and minor elements are presented. Minor elements include those considered to be essentially immobile during secondary alteration and metamorphism (Ti, Zr, Y and Nb) and thus useful in classifying the original magma type and tectonic setting.
Geochemical and field relationships suggest that the Cretaceous greenstones represent accreted slivers of uppermost oceanic crust (MORB) underplated to the base of the accretionary complex together with highly deformed pelagic/hemi-pelagic sedimentary sequences.
The Paleogene greenstones exhibit similar MORB-type geochemical characteristics, but their field relationships require a different model to explain their emplacement. The chaotic juxtaposition of various lithologies within a muddy/pebbly matrix may be generated by a variety of sedimentary processes such as debris flows, slides or rock falls. However, in this study field evidence suggests that the primary mechanism responsible for disruption was mud diapirism. The greenstone blocks are thought to represent fragments of previously accreted oceanic crust which became dismembered and chaotically mixed, together with trench or near-trench lithologies during mud diapirism at the toe of the accretionary prism.

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Spessartine garnet + Na-pyroxene-bearing schists occur as boudins or lenses in the highpressure schists of the Kamuikotan zone in the Horokanai-Kamietanbetsu area. Their mineral assemblages are spessartine garnet + Na-pyroxene + Na-amphibole + quartz ± chlorite. The Fe-Mg partition coefficient for garnet-pyroxene equilibrium is 91, and the Fe-Mg, Mn-Fe and Mn-Mg partition coefficient for garnet-chlorite equilibrium are 35, 55 and 1913, respectively. Formation temperature obtained is about 300°C, assuming 9.5 kbar pressure. The mineral assemblage of spessartine garnet + Na-pyroxene in lower temperature zone is favored by high-pressure metamorphism.

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