TY - JOUR A1 - Novak,M. A1 - Chladek,S. A1 - Uher,P. A1 - Gadas,P. T1 - Complex magmatic and subsolidus compositional trends of columbite-tantalite in the beryl-columbite Sejby granitic pegmatite, Czech Republic: role of crystal-structural constraints and associated minerals JF - Journal of Geosciences JA - J. Geosci. Y1 - 2018 VL - 63 IS - 3 SP - 253 EP - 263 CY - Prague PB - Czech Geological Society SN - 1803-1943 (online), 1802-6222 (print) AV - Free UR - http://dx.doi.org/10.3190/jgeosci.269 L1 - http://www.jgeosci.org/content/jgeosci.269_novak.pdf KW - columbite-group minerals KW - tapiolite KW - compositional trends KW - Nb-Ta and Fe-Mn fractionation KW - granitic pegmatite AB - The simply zoned Sejby pegmatite of beryl-columbite subtype is enclosed in migmatitized gneisses - country rocks of the southern part of the Central Moldanubian Plutonic Complex, Bohemian Massif, Czech Republic. The columbite-group minerals (CGM) occur mostly in blocky K-feldspar unit. Primary magmatic CGM [columbite-(Fe)—tantalite-(Fe)], show an interesting compositional and textural evolution. Subhomogeneous cores of columbite-(Fe)—tantalite-(Fe) (A) are surrounded by heterogeneous intermediate zones (B) and (C) with irregular to oscillatory zoning and mutually comparable compositions. Zone (B) contains grains of tapiolite-(Fe) and inclusions of pyrite decomposed into a mixture of secondary jarosite-group minerals. In the next zone (C), irregular Mn-enriched patches occur and in the outermost thin zone (D) CGM exhibit fine oscillatory zoning typically developed at crystal terminations. The late, subsolidus CGM include patchy-zoned Mn-enriched columbite-(Fe) and late columbite-(Fe) veinlets both formed at the expense of their magmatic precursors in the zone C; the veinlets are also present in the zone D. Similar but fine patches and veinlets are developed around jarosite pseudomorphs after pyrite. The compositional evolution of CGM shows a slight increase in Ta/Nb and Mn/Fe in the zones A and B followed by a reverse trend to elevated Nb and Fe in the zone C and, in particular, in the zone D. Complex subsolidus fluid-melt interactions that generated patches and veinlets caused only minor changes in Ta/Nb and slight Mn-enrichment. Tantalum depletion in veins close to jarosite pseudomorphs after pyrite indicates higher mobility of Ta in acidic low-temperature fluids. ER -