Carbonatite evolution at St Honoré (Canada), the apatite record

Abstract

The purpose of our study was to test the hypothesis that apatite provides a detailed record of the evolution of carbonatitic magmas. To this end, we investigated the chemistry and textures of apatite in the various rock units (banded carbonatite, biotitised syenite, biotitite, magnetite-biotite rock and massive carbonatite) of the St Honoré carbonatite using a combination of micro-analytical and imaging techniques. Subtle changes in the uptake of a variety of trace elements during growth led to corresponding changes in the cathodoluminescence response that are recorded as distinct zones in the apatite. In the banded carbonatite, the first apatite to crystallise was fluid/mineral inclusion-bearing and was followed by apatite displaying oscillatory zoning and, in turn, by apatite that replaced the earlier apatite through dissolution-reprecipitation. In contrast, the earliest apatite in the biotitised syenite displays oscillatory zoning and was variably replaced by later apatite. The subsequent crystallisation stages duplicate those of apatite in the banded carbonatite. Apatite crystallisation in the magnetite-biotite rock duplicated the stages recorded by apatite in the banded carbonatite. Finally, the apatite of the massive carbonatite contains representatives of all the apatite types mentioned above.

A model is presented in which inclusion-rich apatite records aqueous‑carbonic fluid exsolution from the magma, oscillatory zoned apatite records periods of quiescent growth and replacement apatite records dissolution-reprecipitation induced by the differential stresses that accompanied fluid overpressures. Apatite in the massive carbonatite was incorporated from the other units. Based on the above model, we propose that banded carbonatites at St Honoré and other similar complexes formed during an early stage of carbonatitic magma emplacement, when thermal gradients between the magma (hot) and the host rocks (cooler) were steep and the calcite liquidus was reached before significant biotitisation. With the emplacement of additional batches of magma, the thermal gradient was gradually flattened and biotitisation was extensive, producing massive biotitite. This hypothesis explains the occurrence, spatial distribution and genesis of the biotitite (glimmerite) and banded carbonatite observed in many carbonatite complexes.