Petrology of the Sumiyoshizaki alkali dolerite sheet in the Atsumi district, Yamagata Prefecture, Japan.
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概要
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The petrological study on the Sumiyoshizaki alkali dolerite sheet in Atsumi area, Northeast Japan, has been carried out to clarify the in situ crystallization process of alkali basalt magma. The sheet is divided into an upper intrusive and a lower intrusive. In this paper, the author mainly investigated the lower Sumiyoshizaki sheet. The lower Sumiyoshizaki sheet is composed of alkali basalt of the chilled margin, alkali dolerite up to the middle of the body and analcime dolerite to the top. Many differentiated veins and patches are found in the body. They comprise pegmatite, monzonite and syenite. The main constituent minerals of the dolerite are olivine, clinopyroxene, plagioclase, magnetite and ilmenite, and the differentiated products lack olivine. Major elements abundances of the dolerite, especially Mg, change from the base to the top, whereas all the chemical compositions of the minerals do not change significantly through the body, nor do the incompatible element concentrations. The chemical compositions of minerals in the dolerite pegmatite are nearly the same as those in the dolerite main body, whereas the major and trace element abundances recorded the fractionated nature. The chemical compositions of clinopyroxene and plagioclase in the monzonite and syenite are FeO-rich and Na2O-rich, respectively, and these differentiated rocks are highly enriched in incompatible elements. Two different chemical trends were observed in the differentiates: one shows K2O-enrichment, and the other shows Na2O-enrichment as well as K2O-depletion. With respect to latter, K2O might have become lack by alteration. The alkali dolerite, analcime dolerite and dolerite pegmatite were formed by cumulated olivine, clinopyroxene and plagioclase mixed with residual liquid. Their chemical compositions represent neither the variation trend of the cumulated rocks nor the liquid line of descent of alkali basalt magma. Instead, the monzonite and syenite which belong to the trend of K2O-enrichment are probably late differentiation products showing residual liquid.
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