Nb-Ta-Ti-W-Sn-oxide minerals as indicators of a peraluminous P- and F-rich granitic system evolution: Podlesí, Czech Republic |
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Authors: | K Breiter R ?koda P Uher |
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Institution: | (1) Czech Geological Survey, Praha, Czech Republic;(2) Academy of Sciences of the Czech Republic, Institute of Geology, v.v.i., Praha, Czech Republic;(3) Institute of Geological Sciences, Masaryk University, Brno, Czech Republic;(4) Department of Mineral Deposits, Comenius University, Bratislava, Slovakia |
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Abstract: | Summary The strongly peraluminous, P- and F-rich granitic system at Podlesí in the Krušné Hory Mountains, Czech Republic, resembles
the zonation of rare element pegmatites in its magmatic evolution (biotite → protolithionite → zinnwaldite granites). All
granite types contain disseminated Nb-Ta-Ti-W-Sn minerals that crystallized in the following succession: rutile + cassiterite
(in biotite granite), rutile + cassiterite → ferrocolumbite (in protolithionite granite) and ferrocolumbite → ixiolite →
ferberite (in zinnwaldite granite). Textural features of Nb-Ta-Ti-W minerals indicate a pre-dominantly magmatic origin with
only minor post-magmatic replacement phenomena. HFSE remained in the residual melt during the fractionation of the biotite
granite. An effective separation of Nb + Ta into the melt and Sn into fluid took place during subsequent fractionation of
the protolithionite granite, and the tin-bearing fluid escaped into the exocontact. To the contrast, W contents are similar
in both protolithionite and zinnwaldite granites.
Although the system was F-rich, only limited Mn-Fe and Ta-Nb fractionation appeared. Enrichment of Mn and Ta was suppressed
due to foregoing crystallization of Mn-rich apatite and relatively low Li content, respectively. The content of W in columbite
increases during fractionation and enrichment in P and F in the melt. Ixiolite (up to 1 apfu W) instead of columbite crystallized
from the most fluxes-enriched portions of the melt (unidirectional solidification textures, late breccia). |
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