An enigmatic report of unmixing of calcic plagioclase to oligoclase and anorthite finally resolved
O’Brien, Patrick J.; Mcconnell, J. Desmond C.
European Journal of Mineralogy Volume 11 Number 6 (1999), p. 915 - 918
published: Nov 29, 1999
manuscript accepted: Jun 17, 1999
manuscript received: Jan 4, 1999
ArtNo. ESP147051106001, Price: 29.00 €
Abstract A new explanation is presented for a reported replacement, in a metagabbro, of calcic plagioclase (An50-55) by discrete more calcic (An65-93) and more sodic (An18-33) 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.