1988 Fiscal Year Final Research Report Summary

Genetic Environments of High-magnesian andesite and Related Rocks.

Research Project

Project/Area Number	61540593
Research Category	Grant-in-Aid for General Scientific Research (C)
Allocation Type	Single-year Grants
Research Field	鉱物学(含岩石・鉱床学)
Research Institution	HIROSHIMA UNIVERSITY
Principal Investigator	SATO Hiroaki Fac. Integ. Arts & Sci., Hiroshima Univ. Assoc. Prof., 総合科学部, 助教授 (60019495)
Project Period (FY)	1986 – 1988
Keywords	High-magnesian andesite / Boninite / Sanukitoid / Sr-Nd isotopic ratios / Trace element composition / Element partitioning / Oxygen fugacity meter / 火山岩石基組織
Research Abstract	Gelogical, petrographical and chemical studies were carried out on high-magnesian andesites of northeast Shikoku, Noto Peninsula, and northwest Kyushu, and boninite of Ogasawara Islands. High-magnesian andesite (sanukitoid) of northeast Shikoku may repesent primary andesite derived from hydrated mantle peridotite, though the source area may be aeverely affected by subducted sediments. Bronzite andesites of Noto Peninsula and NW Kyushu were probably formed by mixing of primitive basalt and dacite or rhyolite magmas. Trace element composition of boninite of Ogasawara Islands is characterized by low HFSE and rather high alkali contents with intermediate alkali earths. Such enrichement (e.g. high Rb/Ba and Ba/La ratios) is common among the high-magnesian andesites presently investigated, and is diagnostic from island arc tholeiites. 1-atmosphere melting studies of some volcanic rocks afforded a new oxygen fugacity meter. It uses Mg-Fe partition relationship between plagioclase and magma. The oxygen fugacity meter is sensitive at ca. log fo_2=-6 - -11, and may have wide applicability to plagioclase-phyric volcanics.