Abstract

This study is the first to report a detailed geochemical investigation and in situ U-Pb ages for niocalite, which occurs within carbonatite from the Bond Zone area of the Oka Carbonatite Complex (Canada). Niocalite is a Nb-disilicate member of the låvenite–cuspidine group. The major element composition of the niocalite studied here is relatively homogeneous with the average formula of: (Na0.34Fe0.06Mn0.19Mg0.09Ca13.40REE0.15Ti0.02)∑14.25Nb2.12Ta0.06(Si2O7)4O7.65F2.44. Niocalite is enriched in minor and trace elements [i.e., Ta, Ti, and rare earth elements (REEs) up to 4.35 wt.%], with double- and triple-valenced elements (i.e., Sr, Y, REEs) substituting at different Ca-occupied lattice sites. The chondrite-normalized REE patterns for niocalite are LREE-enriched (~104 times chondrite) and negatively sloped. In addition, niocalite has higher HREE contents compared to those for co-existing apatite from the identical carbonatite sample. This result is expected based on bond valence data because of differing cation sizes. In situ U-Pb ages for niocalite from three carbonatite samples obtained by LA-ICP-MS define a wide range of ages, between ~111 and ~133 Ma. Niocalite from one carbonatite sample yields a bimodal distribution with weighted mean 206Pb/238U ages of 110.1 ± 5.0 Ma and 133.2 ± 6.1 Ma, and overlap those of co-existing apatite for the same sample. Two other samples of carbonatite yield weighted mean 206Pb/238U ages of 110.6 ± 1.2 Ma and 115.0 ± 1.9 Ma. The geochronological data for niocalite reported here suggests a protracted petrogenetic history for Oka, which corroborates the relatively large range of ages (~20 Myr) previously determined for apatite from the Oka complex. In situ87Sr/86Sr isotope ratios for niocalite obtained by LA-MC-ICP-MS are variable and range between 0.70314 (±7) and 0.70343 (±8). Based on the combined U-Pb ages, major and trace element abundances, and Sr isotope ratios, the history of niocalite formation cannot be attributed to either simple closed-system fractional crystallization or carbonate-silicate liquid immiscibility. Rather, these are consistent with periodic carbonatite melt derivation from a heterogeneous mantle source, and suggest that some of niocalite investigated here represent cognate crystals.

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