Pyrochlore-group minerals are relatively common accessory constituents of calcite carbonatite at the Oka Carbonatite Complex, Quebec. This complex is a member of the Cretaceous Monteregian petrological province, and consists predominantly of calcite carbonatite and feldspathoidal silicate rocks. Mineralogical studies of the complex have identified ceroan pyrochlore, ceriopyrochlore, uranoan pyrochlore, uranpyrochlore, thorian pyrochlore and "thoriopyrochlore" as occurring within the NIOCAN and Bond Zone deposits. At Oka, pyrochlore [general structural formula: A 16-x B 16 O 48 (O,OH,F) 8-y ] typically occurs as euhedral to subhedral crystals, rarely as aggregates and clusters. Back-scattered electron images, coupled with results of energy-dispersion analyses, reveal complex compositional zoning in pyrochlore, which reflects changes in magma composition during growth. Uranium-rich pyrochlore from Oka exhibits either strong zonation or is devoid of zoning. Large compositional variations were observed for the major oxides of the pyrochlore: CaO (ranging from 4.1 to 34.8 wt.% oxide), TiO 2 (2.3-40.4%), Nb 2 O 5 (20.1-58.1%), ThO 2 (0.3-18.2%), and UO 2 (0.1-28.0%). Ceriopyrochlore, ceroan pyrochlore and uranpyrochlore exhibit the greatest A-site vacancies, ranging from 8.1 to 62.5%. Of the rare-earth elements, only Ce is present at high levels of concentration (ranging from 2.1 to 15.8% Ce 2 O 3 ). Of note is the significant content of ZrO 2 , which ranges from 0 to 16.3 wt.%. An A-site substitution in the pyrochlore-group minerals has been identified between (REE + U + Th) and (Na + Ca), as well as a B-site substitution between (Nb + Ti) and Zr. The wide variety of pyrochlore-group minerals at Oka, coupled with their textural features, indicate that the Oka calcite carbonatite has an extremely complex evolutionary history. The pyrochlore-group minerals crystallized from two or more carbonatitic magmas, enriched in Nb, Ti, LREE, U, and Th. The calcite carbonatites of the NIOCAN and Bond Zone areas are hybrid rocks. No simple hypothesis can be devised to explain the significant concentrations of pyrochlore in particular samples of host rock. Enrichment in specific zones is dependent upon rheological factors rather than compositional controls.Keywords: calcite carbonatite, pyrochlore-group minerals, magma mixing, Oka carbonatite complex, Quebec. SOMMAIRELes minéraux du groupe du pyrochlore sont des phases accessoires assez répandues dans la carbonatite calcitique du complexe d'Oka, Québec. Ce complexe fait partie de la province pétrographique Montérégienne, d'âge crétacé, et contient surtout des venues de carbonatite à calcite et des roches silicatées à feldspathoïdes. Nous avons identifié le pyrochlore enrichi en cérium, le cériopyrochlore, le pyrochlore uranifère, l'uranpyrochlore, le pyrochlore thorifère et le "thoriopyrochlore" dans les gisement de NIOCAN et de la zone Bond. A Oka, le pyrochlore [formule structurale générale: A 16-x B 16 O 48 (O,OH,F) 8-y ] se présente généralement en cristaux idoimorphes ...

[1] The crystallization ages and the Sr and Nd isotopic compositions of groundmass perovskite from a well-established, SE trending, Triassic-Jurassic corridor of kimberlite magmatism in central and eastern North America were determined to investigate the origin of this magmatism. The results obtained from kimberlite fields located along this corridor are interpreted to indicate that at least two distinct mantle sources contributed to this magmatism. The most primitive Rankin Inlet and Timiskaming kimberlites have a relatively unradiogenic strontium isotopic signature (0.7032-0.7036), interpreted to be derived from recycled and metasomatized oceanic lithosphere in the deep mantle. In contrast, the Attawapiskat and Kirkland Lake kimberlites have CHUR-like (Chondritic Uniform Reservoir) signatures (0.7040-0.7042) interpreted to have an origin in the asthenosphere. The progressive decrease in the age of magmatism from the Triassic Rankin Inlet kimberlites to the Miocene Great Meteor seamount, combined with the similarity in the isotopic composition of these diverse magmas along the proposed >3000 km long hot spot track, provides strong evidence in support of a common mantle plume origin for both the continental and oceanic components.

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