Phosphorus and rubidium in alkali feldspars: case studies and possible genetic interpretation


Authors: Breiter K, Frýda J, Leichmann J

Published in: Bulletin of Geosciences, volume 77, issue 2; pages: 93 - 104; Received 6 March 2002; Accepted in revised form 8 April 2002;

Keywords: alkali feldspar granite, pegmatite, chemical composition, alkali feldspar group, magmatism, fractional crystallization, phosphorus, rubidium,

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Potassium feldspar (Kfs) and albite (Ab) are major hosts of phosphorus in peraluminous highly fractionated granitic systems. Kfs is also an important carrier of Rb. The absolute contents of P and Rb, their homogeneous, zoned, or disturbed distribution within individual Kfs and Ab crystals, the ratio of P contents in the whole rock (WR) vs. Kfs and the ratio of P contents in Kfs vs. albite can serve as useful indicators of the grade of parental magma fractionation and late- and/or post-magmatic crystal-fluid reaction. The P and Rb distributions in feldspars were studied in detail in four peraluminous granitic systems widely differing in intensity of influence of late- to post-magmatic fluids: dominantly magmatic Podlesí granite system (Bohemia), Beauvoir granite (France) influenced by late magmatic fluids, hydrothermaly altered Křížový Kámen granite (Bohemia - Bavaria) and a quartz-Kfs-amblygonite pegmatite-like dyke at Vernéřov (Bohemia). Rubidium is firmly bound in the Kfs lattice and preserves well its magmatic signature during post-magmatic processes. In contrast, phosphorus can be easily released from the feldspar lattice and P content serves as a sensitive indicator of the late- and/or post-magmatic fluid-related reactions.