Microprobe analyses of plagioclases from metamorphic carbonate rocks of the Central Alps
Wenk, Eduard; Schwander, Hans; Wenk, Hans-Rudolf
European Journal of Mineralogy Volume 3 Number 1 (1991), p. 181 - 192
published: Feb 21, 1991
manuscript accepted: Jun 14, 1990
manuscript received: Jan 8, 1990
ArtNo. ESP147050301007, Price: 29.00 €
Abstract Microprobe analyses of plagioclase in carbonate rocks from the Tertiary metamorphic belt of the Central Alps demonstrate a selective frequency distribution of An-content, with maxima 0-3, 29-39, 65-71, 89-96 and minima 4-22 (peristerite), 47-62 (Boggild), 74-84 (Huttenlocher). Rational microscopic intergrowths of sodic andesine, calcic labradorite and anorthite are observed in the same rocks. These data are significant for the equilibrium phase diagram of the plagioclase series at temperatures of metamorphism which cannot be determined experimentally. The regional distribution of An-groups shows a wide high-grade zone in the centre, with intermediate to very calcic plagioclase, surrounded by a marginal oligoclase-andesine zone without plagioclase An > 50, bordering the pure albite zone of the greenschist facies. Peristerite feldspars are confined to the border greenschist/amphibolite facies. The results of this microprobe study are compared with those obtained by U-stage method. Accuracy and precision of the two methods are discussed.