COMPOSITIONAL AND STRUCTURAL VARIATIONS IN COLUMBITE-GROUP MINERALS FROM GRANITIC PEGMATITES OF THE BRUNSWICK AND OXFORD FIELDS, MAINE: DIFFERENTIAL TRENDS IN F-POOR AND F-RICH ENVIRONMENTS

The Coastal Lithotectonic Block of the Brunswick pegmatite field and the Central Maine Synclinorium of the Oxford pegmatite field, in Maine, are intruded by numerous rare-element granitic pegmatites that carry columbite-group minerals (CGM) as the principal (Nb,Ta)-bearing phases. Generally, the CGM from Maine are hosted by pegmatites characterized by a LCT (Li–Cs–Ta) geochemical signature and are infrequent in pegmatites with NYF (Nb–Y–F) characteristics ( e.g ., Topsham area of the Brunswick pegmatite field). Significant chemical-compositional and structural differences are observed between the CGM from the Brunswick and Oxford pegmatite fields. Columbite-group minerals from the Brunswick pegmatite field mainly consist of columbite-(Fe), commonly with elevated Ti and Mg contents. By comparison, pegmatites from the Oxford field carry columbite-(Fe), columbite-(Mn) and tantalite-(Mn) with low Ti and Mg contents. The structural state of CGM from the Brunswick pegmatite field ranges from highly cation-disordered to cation-ordered, in contrast to predominantly highly ordered CGM in the Oxford pegmatite field. Limited concurrent fractionation of Mn from Fe and Ta from Nb observed in the Brunswick pegmatite field is characteristic of F-poor environments in general, and of F-enriched pegmatites with high μHF/μalkF (stability field of topaz). However, the extensive Fe–Mn fractionation preceding moderate to strong enrichment in Ta is typical of the Oxford pegmatite field, and apparently promoted by lepidolite-generating environments with prominent chemical potential of alkali fluorides and low μHF/μalkF. This observation finds support in data on CGM from numerous other pegmatite populations worldwide. In contrast, the structural state of CGM does not seem to be linked to the activity of different forms of fluorine. The overall abundance, diversity and temporal extent of the volatile components were undoubtedly different in the two pegmatite fields under study, and were apparently enhanced in the more fractionated pegmatites of the Oxford pegmatite field.