Anorthite Megacrysts from Island Arc Basalts

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

Keywords: anorthite • megacryst • island arc • Japan • basalt

Mineralogical Magazine; March 1995 v. 59; no. 394; p. 1-14; DOI: 10.1180/minmag.1995.59.394.01
© 1995, The Mineralogical Society
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