Nb-Ta-Ti-W-Sn-oxide minerals as indicators of a peraluminous P- and F-rich granitic system evolution: Podlesi, Czech Republic

The strongly peraluminous, P- and F-rich granitic system at Podlesi in the Krusne 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 predominantly 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).