那須火山帯，島弧型ソレアイト質マグマの早期結晶作用 船形火山ピジョン輝石玄武岩類の岩石学的研究

Abstract

In order to understand the generation and differentiation processes of the island arc tholeiite magma, it is essential to reveal the crystallization sequence of the minerals in the shallow magma chamber. For that purpose, a microprobe study has been undertaken on three Funagata pigeonite basalts which are the most primitive basalts among the island arc tholeiitic series in the Nasu zone, NE Japan. Firstly, the crystallization sequence recognized in each sample has been clarified based on chemical compositions and textural relations. Secondly, information on the near equilibrium crystallization in the shallow magma chamber has been extracted and connected. The crystallization sequence obtained through the study is as follows. Stage A: Plagioclase (An90+), olivine (Fo78), bronzite (Wo4.5 En76Fs19.5) and pigeonite (Wo9.5En71Fs19.5) coexist with liquid. At this stage, olivine has already been in reaction relation with liquid to form bronzite and/or pigeonite. Bronzite is also in reaction relation with liquid to form pigeonite. Stage B: Mafic minerals gradually enrich in Fe2+. When the Mg/(Mg+Fe) ratio of the pigeonite reaches 0.23, augite begins to crystallize. Penecontemporaneously with the start of the augite crystallization, olivine (Fo75) is fractionated from the liquid. An mole % of the plagioclase is nearly constant. Stage C: The three pyroxenes further enrich in Fe2+. Three pyroxene coexistence continues until the Mg/(Mg+Fe) of the pigeonite reaches 0.26. Chemical composition of the plagioclase is nearly constant (An90). Experimental data indicate that the crystallization sequence obtained here must reflect the notable change of the pyroxene phase relation in the early stage differentiation of the Funagata tholeiitic basalt magma. This type of change in pyroxene phase relation is interpreted as being due to the successive decrease of the liquidus and solidus temperatures of pyroxenes, accompanied with the proceeding of the differentiation.