An enigmatic report of unmixing of calcic plagioclase to oligoclase and anorthite finally resolved

Universitaet Bayreuth, Bayerisches Geoinstitute, Bayreuth, Federal Republic of Germany

A new explanation is presented for a reported replacement, in a metagabbro, of calcic plagioclase (An (sub 50-55) ) by discrete more calcic (An (sub 65-93) ) and more sodic (An (sub 18-33) ) plagioclase phases, by a process of exsolution. In contrast to a possible exsolution origin from unstable incommensurate plagioclase structures during long, slow cooling, it is proposed here that a two-stage metamorphic process is the most likely cause for the presence of the two different feldspars. An initial high-pressure breakdown of magmatic Ca-plagioclase produced branching bundles of lath-shaped zoisite crystals set within a residual more sodic plagioclase. Later breakdown of the zoisite, at lower pressure, gave rise to the present-day occurrence of discrete anorthite (or Ca-rich plagioclase) laths in a more sodic feldspar matrix.

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