変成帯の超塩基性岩類 Origin of Ultrabasic Rocks in the Metamorphic Zones in Japan
The associations of ultrabasic rocks in metamorphic zones of Japan are as follows: dunite-wehrlite generally with a great deal of gabbro (W-series, Sambagawa zone), dunite-lherzolite-plagioclase-lherzolite with a fairly amount of gabbro (L-series, Hidaka zone) and dunite-harzburgite with a little amount of leucocrate (H-series, Sangun and Kamuikotan zones). It is the most noteworthy that in one metamorphic zone only one ultrabasic rock series appears. Other examples of the Alpine type peridotite intrusions in some matamorphic zones in the world are informed. Especially the examples of Sikhote-Alin, Ural, New Zealand, Cornwall and southeast Alaska support our conclusion in Japan. In Fig. 3, the chemical analyses of the ultra-basic rocks of each zone in Japan are plotted on the norm Cpx-Ol-Q diagram. Three series are well divided by this diagram. Chemical analyses of peridotite xenoliths in kimberlites are also plotted in this diagram (Fig. 4). If we consider that chemical composition of the upper mantle closely resembles to that of the peridotite xenoliths in kimberlites, it may be said there are three types in the composition of the upper mantle, corresponding to W-, L- and H- series of ultrabasic rock series. Peridotite intrusion is never found in the Ryoke metamorphic zone, where basic volcanic activity is scarcely recognized in the geosynclinal stage. In other metamorphic zones mentioned above, of course, a large amount of basaltic and basic pyroclastic rocks, which have more or less metamorphosed, is extruded. Although the characters of orogenesis and metamorphism of the Kamuikotan and Sambagawa zones are the same, the ultrabasic rocks series of these zones are different to each other; that is, the former is H-series and the latter is W-series. The chemical compositions of basic or pyroclastic rocks of geosynclinal stage of both zones are compared (Fig. 5, 6 and 7), but no critical difference is found. According to the experimental data of KUSHIRO and DAVIS & SCHAIRER (Fig. 8 and 9), at first melt of basaltic or picritic compositionre presented by P-point begins to be produced from any ultrabasic composition such as A or B in Figs. 8 or 9. If the melt of the composition of P is taken place from the ultrabasic mantle of composition A or B, the composition of residual solid fraction shall approach to the composition of A' or B' (Fig 8. or 9). This may indicate the similarity of the compositions of basic volcanic and pyroclastic rocks in the Kamuikotan are Sambagawa zones, even though the ultrabasic rock series in both zones are H- series and W-series respectively. In the Kitakami and Abukuma mountaineous districts various types of ultrabasic rocks, dunite, wehrlite, harzburgite, lehrzolite occur intermingly. This may show the complicated history of these districts. The ultrabasic rocks in the sedistricts have probably been modified from their original characters by the polygeological events.
地質学論集 (4), 99-108, 1969