(Fe,Mn)-(Ti,Sn)-(Nb,Ta) oxide assemblage in a little fractionated portion of a mixed (NYF + LCT) pegmatite from Piława Górna, the Sowie Mts. block, SW Poland

Abstract

* �Correspondingauthor T extural relationships and compositional variations of (Fe,Mn)-(Ti,Sn)-(Nb,Ta) oxides and associated minerals from low- to moderately fractionated, mixed (NYF + LCT) anatectic pegmatites, which belong to the Julianna pegmatitic system, exposed at Pilawa Gorna, Lower Silesia, Poland, were studied to elucidate the geochemical evolution of the pegmatite-forming melt. The primary Nb-Ta oxides crystallized in the following sequence: (1) columbite-(Fe) evolving to columbite-(Mn), associated with Nb- and Ta-bearing ilmenite-pyrophanite + almandine-spessartine garnet + Ti-poor schorl, (2) columbite-(Fe), (3) tantalite-(Fe), (4) Ti- and Sn-bearing ixiolite ± ferrowodginite ± Nb- and Ta-bearing cassiterite ± columbite-(Fe), all crystallizing along with Ti-enriched schorl evolving to dravite. The most characteristic features of the mineral assemblages are the reversed Mn-Fe fractionation trends and increasing Ca concentrations at the final stage of crystallization. These compositional variations are explained by a possible pre-emplacement contamination of the pegmatite-forming melt and re-equilibration of biotite and plagioclase from the early-crystallized outer zones of the pegmatites with rapidly cooling pegmatitic melt. Local changes in melt composition were of minor importance. Late alterations, such as chloritization and muscovitization of biotite or albitization of Ca-rich plagioclase were significant only at the hydrothermal stage.