The enigmatic Mount Grace extrusive carbonatites, southeastern Canadian Cordillera: volcanic architecture, mantle source, and tectonomagmatic framework

The Late Devonian Mount Grace carbonatites outcrop as thin, laterally discontinuous strata-bound lenses within the deformed and metamorphosed Monashee cover sequence of the Canadian Cordillera. The identified lithofacies range from carbonatite tuffs to tuff breccias, in line with a pyroclastic origin resulting from phreatomagmatic eruptions forming in a field of maar volcanoes. Carbonatitic eruptions occurred in a transgressive shallow marine platform setting along the western margin of paleo-North America. The timing and distribution of the Mount Grace carbonatites correlate with regional Late-Devonian back-arc extension and other rift-related sedimentary exhalative deposits, alkalic volcanism, and carbonatite and alkalic intrusions. Whole-rock geochemistry of the Mount Grace carbonatites shows a strong positive Nb anomaly, moderate REE contents with high LREE/HREE, and strongly negative Zr-Hf anomalies that overlap with global rift-related carbonatite compositions. A positive correlation exists between REE content and LREE/HREE, and Nb/Ta and Zr/Hf in Mount Grace samples that indicate primary carbonate and pyrochlore crystallization, increasing in the more evolved compositions. The evolved compositions correspond with higher proportions of phoscorite (apatite, biotite, ferrorichterite, and pyrochlore). We invoke an ascent model for the Mount Grace carbonatites, similar to other carbonatite/phoscorite complexes where an ascending parental carbonate-phosphate/Fe oxide-rich melt undergoes liquid immiscibility of phoscorite-enriched carbonatitic magma, allowing the lower density residual phoscorite-poor carbonatite magma to ascend rapidly through the crust. Zircon Hf and apatite and pyrochlore Nd and Sr isotopes from the Mount Grace carbonatites have relatively depleted mantle signatures at 360 Ma, very close to Focal Zone (FOZO) and high mu (238U/204Pb; HIMU)-like mantle reservoir endmembers similar to global ocean island basalts and indicates a mantle source for these melts, potentially of great depths. Apatite and pyrochlore Nd and Sr isotopes from the Mount Grace carbonatites fall within the range of values for a depleted mantle reservoir presumably developed at ~3 Ga that sits beneath the Canadian Cordillera based on data from carbonatites emplaced from 2700 to 110 Ma in the Cordillera and Canadian Shield. Zircon Hf from Mount Grace carbonatites overlap with this depleted FOZO-HIMU mantle endmember and extend towards more enriched Hf values, reflecting their metasomatic origins. We infer that reactivated crustal-scale rifts allowed the emplacement of small-degree partial melts of an underlying, moderately depleted mantle source.