MICRO TO NANO FEATURES OF (NB,TA, SN)-OXIDE MINERALS REVISITED: EVIDENCE FOR DISEQUILIBRIUM CRYSTALLIZATION IN GRANITIC PEGMATITES

Disequilibrium-induced crystallization exists as an integral process in the development of primary textures (e.g., skeletal and comb-structured crystal morphologies) and late-stage mineral assemblages in complex granitic pegmatites. Rare-element granitic pegmatites are often found to contain a number of rock-forming (e.g., feldspar and mica) and accessory (e.g., garnet and tourmaline) mineral phases that exhibit varying degrees of disequilibrium features. Complex rare-element pegmatites enriched in the high-field strength elements Nb, Ta, and Sn, frequently carry accessory minerals of the columbite, tapiolite, wodginite and pyrochlore groups, and cassiterite. These minerals may display micro- to nano-scaled textures that include mineral zoning (progressive, oscillatory or patchy), symplectite, exsolution, overgrowths, corroded relict phases, resorption, and replacement by later generations of Nb, Ta, Sn minerals, all of which provide evidence of disequilibrium crystallization. These textures develop largely in response to mineral-melt and mineral-fluid interaction at magmatic, subsolidus and hydrothermal conditions and can occur during any stage of pegmatite consolidation.

Disequilibrium crystallization conditions in granitic pegmatites may result in extremely variable chemical compositions of the Nb, Ta, and Sn oxide mineral assemblage and therefore affect the pattern of enrichment of Nb, Ta, Fe, Mn and Sn. Traditional evolutionary paths of columbite group minerals, for example, that are generally characterized by systematic increases in Ta and Mn at the expense of Nb and Fe, respectively, may be dramatically disturbed as a result of mineral-melt or mineral-fluid interaction. Discontinuous or apparent “anomalous” fractionation trends (e.g., late-stage Nb enrichment at the expense of Ta), and broad chemical overlaps across (and along) pegmatite zones may be indicative of chemical disequilibrium. Although Nb, Ta and Sn oxide minerals can provide insight into the geochemical signature of pegmatite melts, caution is advised when using fractionation trends to assess the evolution of Nb, Ta and Sn during the crystallization history of complex pegmatites.