Abstract

The alkalic rocks of the Morotu district occur as sheets, laccoliths, and dikes in the Tertiary formations and are probably Pliocene. The rocks vary from dolerite, the most abundant, through monzonite to syenite. Most of the intrusive bodies, except small sheets and dikes, are composed of dolerite in the marginal parts and monzonite or syenite or both in the central parts, with gradational boundaries. Monzonite and syenite also occur as irregular veinlets or schlieren in the doleritic parts, with sharp boundaries. Thus differentiation in situ is strikingly displayed. These intrusive bodies are, therefore, formed by composite intrusion. Contact effects of the intrusions on the Tertiary formations are weak and are confined to the immediate contact zone. Phenomena of assimilation have not been observed.

Principal minerals of these rocks are plagioclase, anorthoclase, microperthite, olivine, titan- and alkali-pyroxenes, alkali-amphiboles, titanbiotite, analcite, and iron ores. Feldspar evolves from labradorite to oligoclase and is joined by anorthoclase and microperthite in the later stage. The trend is always toward enrichment in the Or molecule. Pyroxene, the most important mafic mineral, forms a continuous reaction series from diopsidic augite through titanaugite, soda-augite, and aegirinaugite to aegirine. Amphibole also forms a reaction series from barkevikite through kaersutite and hastingsite to arfvedsonite, but it is not clear whether their relation is continuous or discontinuous. Biotite is abundant in the dolerites, but very low in the syenites. Analcite is abundant in all rocks, apparently formed hydrothermally in the later stages.

Chemically these rocks range from 46 to 61 per cent SiO2. Their alkali-lime index is 50.0, placing the suite in the “alkalic series.” The parental magma is inferred to be olivine basaltic magma with moderately alkalic affinity. The various rock types are believed to have been formed chiefly by crystallization differentiation from the parental magma after it intruded into the present position. Volatile components have also played an important role during crystallization, especially in the later stage.

There is no absolute enrichment in iron throughout the crystallization course of the Morotu rocks. It is inferred that not only the shape of the intrusive bodies but also the composition of the parental magma determine whether iron enrichment or alkali and silica enrichment will prevail in the intrusive bodies.

Its geographical position, its period of activity, and its petrological character justify the conclusion that the Morotu district constitutes the northeastern end of the Circum-Japan Sea province of Cenozoic alkalic rocks.

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