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Anorthite megacrysts from island arc basalts

Published online by Cambridge University Press:  05 July 2018

Mitsuyoshi Kimata
Affiliation:
Institute of Geoscience, The University of Tsukuba, Ibaraki 305, Japan
Norimasa Nishida
Affiliation:
Chemical Analysis Centre, The University of Tsukuba, Ibaraki 305, Japan
Masahiro Shimizu
Affiliation:
Institute of Geoscience, The University of Tsukuba, Ibaraki 305, Japan
Shizuo Saito
Affiliation:
Institute of Materials Sciences, The University of Tsukuba, Ibaraki 305, Japan
Tomoaki Matsui
Affiliation:
Institute of Geoscience, The University of Tsukuba, Ibaraki 305, Japan
Yoji Arakawa
Affiliation:
Institute of Geoscience, The University of Tsukuba, Ibaraki 305, Japan

Abstract

Anorthite megacrysts are common in basalts from the Japanese Island Arc, and signally rare in other global fields. These anorthites are 1 to 3 cm in size and often contain several corroded Mg-olivine inclusions. The megacrysts generally range from An94Ab4Ot2 to An89Ab6Ot5 (Ot: other minor end-members, including CaFeSi3O8, CaMgSi3O8, AlAl3SiO8, □Si4O8) and show no chemical zoning. They often show parting. Redclouded megacrysts contain microcrystals of native copper with a distribution reminiscent of the shape of a planetary nebula. Hydrocarbons are also present, both in the anorthite megacrysts and in the olivines included within them. Implications of lateral variations in the Fe/Mg ratio of the included olivines, in Sr-content and in Sr-isotope ratio of the anorthite megacrysts with respect to the Japanese island arc, relate to mixing of crustal components and subducted slab-sediments into the basaltic magmas.

Information

Type
Mineralogy
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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