Rock-Forming feldspars of the Khibiny alkaline pluton,Kola Peninsula,Russia |
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Authors: | G Yu Ivanyuk Ya A Pakhomovsky N G Konopleva A O Kalashnikov Yu A Korchak E A Selivanova V N Yakovenchuk |
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Institution: | 1.Geological Institute, Kola Scientific Center,Russian Academy of Sciences,Apatity,Russia |
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Abstract: | This paper describes the structural-compositional zoning of the well-known Khibiny pluton in regard to rock-forming feldspars.
The content of K-Na-feldspars increases inward and outward from the Main foidolite ring. The degree of coorientation of tabular
K-Na-feldspar crystals sharply increases in the Main ring zone, and microcline-dominant foyaite turns into orthoclase-dominant
foyaite. The composition of K-Na-feldspars in the center of the pluton and the Main ring zone is characterized by an enrichment
in Al. This shift is compensated by a substitution of some K and Na with Ba (the Main ring zone) or by an addition of K and
Na cations to the initially cation-deficient microcline (the central part of the pluton). Feldspars of volcanosedimentary
rocks occurring as xenoliths in foyaite primarily corresponded to plagioclase An15–40, but high-temperature fenitization and formation of hornfels in the Main ring zone gave rise to the crystallization of anorthoclase
subsequently transformed into orthoclase and albite due to cooling and further fenitization. Such a zoning is the result of
filling the Main ring fault zone within the homogeneous foyaite pluton with a foidolite melt, which provided the heating and
potassium metasomatism of foyaite and xenoliths of volcanosedimentary rocks therein. The process eventually led to the transformation
of foyaite into rischorrite-lyavochorrite, while xenoliths were transformed into aluminum hornfels with anorthoclase, annite,
andalusite, topaz, and sekaninaite. |
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