Hornblende-rich gabbro-diorite sequence in the Oura Igneous Complex, Maizuru City, Japan, part II: Hydrothermal events and genesis of igneous albite.

AGATA TAKASHI

doi:10.2465/ganko1941.76.35

The Main Appinitic Sequence is a suite of hornblende-rich gabbroic and dioritic rocks, late differentiates of the õura Igneous Complex. Hydrothermal minerals including hornblende, epidote, biotite, plagioclase, and quartz are spread out in the Main Appinitic Sequence, and they form such mineral assemblages as hornblende-rich, epidote-rich, and biotite-rich. An appreciable amount of copper sulfide concentration occurs in a biotite-rich altered area. Fluid inclusions in the quartz crystals from the mineralized area show fairly high homogenization temperatures, suggesting that the mineralization and alteration were caused essentially by fluid which escaped from the magma forming the Main Appinitic Sequence. The variation of hydrothermal assemblages may be the result of fractional separation of fluid: the earlier separated fluid brings about the hornblende-rich and epidoterich assemblages in the earlier formed appinitic rocks, and the biotite-rich assemblage and copper sulfide concentration in leuco albite tonalite are formed by the fluid escaping from the later differentiated magma.<br> The albite with the igneous textures from the Main Appinitic Sequence is sometimes replaced by the hydrothermal calcic plagioclase and presumably is an original product crystallized from the magma. The generation of albite may be accounted for by the large amount of hornblende concurrently crystallizing from the magma: in view of incongruent melting of hornblende, hornblende in liquid comes out at the expense of anorthite, clinopyroxene, and orthopyroxene components in the final stage of crystallization of clinopyroxene and brings down the activity of anorthite in liquid.

Hornblende-rich gabbro-diorite sequence in the Oura Igneous Complex, Maizuru City, Japan, part II: Hydrothermal events and genesis of igneous albite.

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