佐渡のビクライト

概要

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Picrite sills occur at the southwestern end of the Sado Island. Picrites are composed of phenocrysts of olivine and spinel with matrix of ophitic texture common to dolerite. Modal proportion of olivine is constantly high, 40 to 60%, in the lower part of the sills and it rapidly decreases to less than 20% in the upper parts. Such variation is similar to the picrites reported from the Skye and shows that the picrites were formed by crystal settling of olivine from olivine basalt magma. There is no evidence to show that they were derived from picrite or picritic parent magma. Most of olivine grains have chemically homogeneous cores. They have always very narrow Fe-rich rims as a product of rapid cooling after emplacement. Compositions of olivine cores are diverse, ranging from Fo 89 to Fo 79 even in a single thin section. Some olivines have reverse zoning; Mg-rich mantle on Fe-rich core or monotonous increase of Mg towards the rim. The variation of core compositions and reverse zonings may be attributed to heterogeneity of magma composition due to crystallization in a magma chamber with temperature gradient, rather than contamination of peridotite and wall rocks or multiple injection of various-type olivine basalts.
1992-03-22