Effects of sillimanite-grade metamorphism and shearing on Nb-Ta oxide minerals in granitic pegmatites, Marsikov, northern Moravia, Czechoslovakia

Abstract

The (Be,Nb,Ta)-bearing albite-muscovite pegmarites at Marsftov, in the Hrubf Jesenik Mountains pegmatite field of northern Moravia, Czechoslovakia, *ete iubjected to a two-stage, sillimanite-grade, regional metamorphism tlat reached about 600oC and 4-6 kbar, P(rotal) > > P(fluid). The first stage generated chrysoberyl at the expense of beryl, under relatively staiic conditions (pegmatite I), whereas extensive shearing during a second stage produced a second generation of chrysoberyl, abundant sillimanite and a foliation (pegrnatite III). The primary.assemblage of the M-Ta oxide minerals consisted of manganocolumbite-manganotantalite displaying oscillatory zoning in terms of M and Ta, replaced by fersmite and microlite (rarely pyrochlore). The columbite-tantalite was frst recrystallized into ,,bird'j-eye oscillitory-zoned textures or heterogeneous granular aggxegates. The original range of,Tal(Ta + Nb) values was commonly reduced, although locally expanded by contribution of additional Nb or Ta from incipient breakdown of fersmite or microlite, respectively. Mn/(Mn + Fe) largely decreased, by as much as 0.40, by introduction of Fe from the host hornblende gneisi; Mg and Ti also were enriched. During the second metamorphic stage' the Tal(Ta + |r{!) value of columbite-tintalite wis locally further homogenized, but Mn/(Mn + Fe) did not change. Fersmite and microlite were almost completely obliterated. Significant ordering of the originally intermediate structural slate of columbite-tantalite was piomoted during metamorphic recrystallization. This study documents extensive compositional and structural re-equilibration of M-Ta oxide minerals at medium-gxade regional metamorphic conditions. This pattern of behavior contrasts with their virtual inertness in an environment of mylonitization.