In-situ determination of major and trace elements in calcite and apatite, and U–Pb ages of apatite from the Oka carbonatite complex: Insights into a complex crystallization history

This study reports in-situ determination of major and trace element compositions of calcite and apatite, and U–Pb ages of apatite for carbonatites and associated okaites and ijolites from the Oka carbonatite complex (Canada). Both apatite and calcite are rare earth element (REE)-enriched (total REEs > 4 wt.%), with REE and SiO2 contents that correlate positively in apatite. This result supports the previously proposed substitution scheme of REE3+ + Si4+ = Ca2+ + P5+. With the exception of Ba, Sr, and Mn, the other trace elements investigated preferentially partition into apatite relative to calcite. Trace element abundances for apatite and calcite both within and between different samples exhibit large variations, which cannot be reconciled with equilibrium or fractional crystallization from a single parental melt using available partition coefficient data. In-situ U–Pb ages determined by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) on individual apatite grains (total of 215 analyses) from carbonatites (n = 7), okaites (n = 4), and ijolites (n = 2) define a relatively large range, from ~ 111 and ~ 129 Ma. Evidence for protracted magmatic history is also present within individual samples. Apatite from one ijolite defines two distinct ages; an older age of 127 ± 3.6 Ma is recorded in apatite inclusions within early-formed melanite, whereas apatite within matrix calcite is characterized by a younger age of 115 ± 5.1 Ma. U–Pb ages obtained for all apatite from carbonatites, okaites and ijolites yield bimodal distributions with peaks at ~ 113 to ~ 117 and ~ 125 to ~ 127 Ma. Our multi-faceted investigation of individual minerals has documented, for the first time, a bimodal age distribution and simultaneous emplacement of the carbonatites and associated okaites and ijolites at the Oka carbonatite complex. The combined large variations in major and trace element compositions recorded by both apatite and calcite, and petrographic textural evidences are consistent with a complex crystallization history involving mixing of discrete partial melts.

► Pb/U ages for apatite indicate a protracted magmatic history for the Oka complex.
► The geochronological results show a bimodal distribution.
► Trace element data cannot be reconciled with closed system behavior.
► Emplacement of carbonatites and associated silicate rocks was coeval.