The Petrology and Geochemistry of Calc-Alkaline Andesites on Shodo-Shima Island, SW Japan |
| |
Authors: | TATSUMI Y; NAKASHIMA T; TAMURA Y |
| |
Institution: | 1INSTITUTE FOR FRONTIER RESEARCH ON EARTH EVOLUTION, JAPAN MARINE SCIENCE AND TECHNOLOGY CENTER, YOKOSUKA 237-0061, JAPAN
2INSTITUTE FOR GEOTHERMAL SCIENCES, KYOTO UNIVERSITY, BEPPU 874-0903, JAPAN |
| |
Abstract: | Petrographical and geochemical characteristics of calc-alkalineandesites on Shodo-Shima Island, SW Japan, having bulk compositionslargely identical to the continental crust, are presented. Thefollowing petrographic observations suggest a role for magmamixing in producing such andesite magmas: (1) two types of olivinephenocrysts and spinel inclusions, one with compositions identicalto those in high-Mg andesites and the other identical to thosein basalts, are recognized in terms of NiMg and CrAlFe3+relations, respectively; (2) the presence of orthopyroxene phenocrystswith mg-number >90 suggests the contribution of an orthopyroxene-bearinghigh-Mg andesite magma to production of calc-alkaline andesites;(3) reversely zoned pyroxene phenocrysts may not be in equilibriumwith Mg-rich olivine, suggesting the involvement of a differentiatedandesite magma as an endmember component; (4) the presence ofvery Fe-rich orthopyroxene phenocrysts indicates the associationof an orthopyroxene-bearing rhyolitic magma. Contributions fromthe above at least five endmember magmas to the calc-alkalineandesite genesis can also provide a reasonable explanation ofthe PbSrNd isotope compositions of such andesites. KEY WORDS: calc-alkaline andesites; high-Mg andesites; magma mixing; continental crust; SW Japan |
| |
Keywords: | |
本文献已被 Oxford 等数据库收录! |
|