Anorthite megacrysts from island arc basalts

Authors:
  • MKMitsuyoshi Kimata
    Mitsuyoshi Kimata
  • NNNorimasa Nishida
    Norimasa Nishida
  • MSMasahiro Shimizu
    Masahiro Shimizu
  • SSShizuo Saito
    Shizuo Saito
  • TMTomoaki Matsui
    Tomoaki Matsui
  • YAYoji Arakawa
    Yoji Arakawa
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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.

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