東北日本弧,第四紀玄武岩組成の水平変化
1988 年 83 巻 1 号 p. 9-25
詳細
1988 年 83 巻 1 号 p. 9-25
The lateral variation of the basaltic rocks from four volcanic zones, the Aoso-Osore, the Sekiryo, the Moriyoshi and the Chokai zone, arranged from the volcanic front to the back arc region of the Northeast Japan arc is investigated. These basaltic rocks are enriched in Sr, K and Rb and depleted in HFS elements as compared with N-type MORB. On the basis of the abundance of HFS elements, chemical trend on the MgO-FeO/MgO diagram and Na2O contents of each basalt, the degree of partial melting in the source mantle has been estimated. Using the degree of partial melting, the abundances of incompatible elements of the source mantle are calculated.
The source mantle of Aoso-Osore zone is enriched in Sr. On the contrary, the source mantle of the back arc region is enriched in K and Rb. The change of Sr, K and Rb abundance in the source mantle is continuous. If the enrichment of the LIL elements is caused by mantle metasomatism owing to dehydration of hydrous minerals, the decomposed hydrous mineral is pargasite at the volcanic front side, and phlogopite at the back-arc side. Lateral variation of the volcanic rocks depends not only on the degree of partial melting but also on the lateral heterogeneity of the wedge mantle in the Northeast Japan arc.
