Color mantle-zoning of alkali feldspar from small pegmatites in the Tanakami Granite Pluton, Shiga Prefecture, southwest Japan

Abstract

Small alkali feldspars with mantle-zoning occur in miaroritic pegmatites in the fine-grained biotite granite as a roof facies of the Tanakami Granite pluton, Otsu, Shiga, southwest Japan. The mantle- zoning visible to the naked eye consists of white alkali feldspar in the core and non-colored, transparent alkali feldspar in the thin rim. The core alkali feldspar is lamellae-patch microperthite, but the rim alkali feldspar is microscopically featureless. The average composition of the core alkari feldspar is Or₇₆Ab₂₄. The compositions of the rim are comparable to the extreme compositions of the host Or-rich phase. The formation processes of the alkali feldspar are estimated as follows. The alkali feldspar in the core was crystallized at the temperature about 550∼600°C (P_H2O=2−3 kb), then was transformed to cryptoperthite at nearly 450°C. The cryptoperthite was coarsened to microperthite, and, eventually, the alkali feldspar in the rim was crystallized at nearly 200°C together with the microperthite in the core, which underwent coarsening and albitization. The last very-low temperatures reactions should provide a new insight to the understanding of the environments and cooling conditions of pegmatite.