- 著者
-
藤縄 明彦
- 出版者
- Japan Association of Mineralogical Sciences
- 雑誌
- 岩石鉱物鉱床学会誌 (ISSN:00214825)
- 巻号頁・発行日
- vol.77, no.12, pp.419-437, 1982
- 被引用文献数
-
1
2
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.<br> 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.<br> The crystallization sequence obtained through the study is as follows.<br> Stage A: Plagioclase (An<sub>90</sub>+), olivine (Fo<sub>78</sub>), bronzite (Wo<sub>4.5</sub> En<sub>76</sub>Fs<sub>19.5</sub>) and pigeonite (Wo<sub>9.5</sub>En<sub>71</sub>Fs<sub>19.5</sub>) 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.<br> Stage B: Mafic minerals gradually enrich in Fe<sup>2+</sup>. 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 (Fo<sub>75</sub>) is fractionated from the liquid. An mole % of the plagioclase is nearly constant.<br> Stage C: The three pyroxenes further enrich in Fe<sup>2+</sup>. Three pyroxene coexistence continues until the Mg/(Mg+Fe) of the pigeonite reaches 0.26. Chemical composition of the plagioclase is nearly constant (An<sub>90</sub>).<br> 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.