COMPOSITIONAL AND TEXTURAL VARIATIONS OF COLUMBITE-GROUP MINERALS FROM BERYL-COLUMBITE PEGMATITES IN THE MARSIKOV DISTRICT, BOHEMIAN MASSIF, CZECH REPUBLIC: MAGMATIC VERSUS HYDROTHERMAL EVOLUTION

We studied compositional variations in columbite group minerals (CGM) from several granitic pegmatites of the berylcolumbite subtype in the Marsikov district, Silesian Domain of the Bohemian Massif, Czech Republic. The CGM are characterized by distinct zoned patterns in BSE images. Primary magmatic homogeneous to oscillatory zoning is preserved in corroded crystal cores, whereas the majority of the crystal volume is replaced by secondary complexly zoned domains formed via post-magmatic processes. The primary domains show relatively uniform evolutionary trends from core to rim, generally with steeply increasing Ta/(Ta + Nb) and negligible to slightly increasing Mn/(Mn + Fe). In contrast, the compositions of secondary CGM domains indicate a reversed evolution, with: (1) steeply decreasing Ta/(Ta+Nb) and relatively constant Mn/ (Mn+Fe) characteristics for CGM in the Bienergraben and Scheibengraben pegmatites, and (2) insignificantly decreasing Ta/ (Ta + Nb) and strongly decreasing Mn/(Mn + Fe) characteristics for CGM in the Schinderhubel I and Lysa Hora pegmatites. Patchy zoning and secondary evolution in CGM result from metasomatic replacement processes related to fluids. These fluids are probably late-magmatic and exsolved from the residual melt and in later stages locally mixed with external Mg-enriched fluids derived from the host rocks. The presence of volatiles (mainly H2O, F) facilitated high mobility of the elements and replacement of the early CGM. Textural characteristics and compositional variations in CGM show the complex evolution of the pegmatite system from the magmatic stage to subsolidus-hydrothermal conditions.