THE GEOCHEMISTRY AND CRYSTAL-CHEMISTRY OF COLUMBITE-GROUP MINERALS FROM GRANITIC PEGMATITES, SOUTHWESTERN GRENVILLE PROVINCE, CANADIAN SHIELD

Granitic pegmatites from the Grenville Province predominantly carry a NYF-type geochemical signature. Their accessory minerals includes a va riety of (Nb,Ln,U)-oxide minerals, of which columbite-group minerals a re infrequent members (nine occurrences to date). As compared to bette r-studied granitic pegmatites of LCT-type suites, the chemistry of the se columbite-group minerals is anomalous: (1) there is a general shift toward (Fe,Nb)-rich compositions, (2) Ti is normally present in subor dinate amounts, (3) levels of Zr, Mg, Sc, and W are significant, (4) U contents are several orders of magnitude higher than reported elsewhe re, and (5) many lanthanons occur in concentrations detectable by elec tron microprobe. The structural state of the samples is variable, rang ing from dominantly cation-disordered to dominantly ordered. For the Q uadeville Beryl pegmatite, the structural state of columbite-group min erals shows a systematic variation with pegmatite zonation. Although t he individual bodies of pegmatite have achieved low to moderate degree s of fractionation, the geochemical expressions of fractionation are d etectable and can be used to evaluate the internal evolution of each b ody. Trace-element behavior is occasionally the reverse of expectation s, and is not consistent from pegmatite to pegmatite; as a suite, NYF pegmatites have to be much better characterized before such discrepanc ies can be properly evaluated.