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Anorthite megacrysts from island arc basalts

1995; Cambridge University Press; Volume: 59; Issue: 394 Linguagem: Inglês

10.1180/minmag.1995.59.394.01

1471-8022

Mitsuyοshi Kimata, Norimasa Nishida, Masahiro Shimizu, Shizuo Saito, Tomoaki Matsui, Yoji Arakawa,

Mineralogy and Gemology Studies

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 An 94 Ab 4 Ot 2 to An 89 Ab 6 Ot 5 (Ot: other minor end-members, including CaFeSi 3 O 8 , CaMgSi 3 O 8 , AlAl 3 SiO 8 , □Si 4 O 8 ) 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.