文摘
New U–Pb perovskite ages reveal that diamondiferous ultramafic lamprophyre magmas erupted through the Archean crust of northern Labrador and Quebec (eastern Canada) between ca. 610 and 565 Ma, a period of strong rifting activity throughout contiguous Laurentia and Baltica. The observed Torngat carbonate-rich aillikite/carbonatite and carbonate-poor mela-aillikite dyke varieties show a large spread in Sr–Nd–Hf–Pb isotope ratios with pronounced correlations between isotope systems. An isotopically depleted component is identified solely within aillikites (87Sr/86Sri = 0.70323–0.70377; εNdi = +1.2–+1.8; εHfi = +1.4–+3.5; 206Pb/204Pbi = 18.2–18.5), whereas some aillikites and all mela-aillikites range to more enriched isotope signatures (87Sr/86Sri = 0.70388–0.70523; εNdi = −0.5 to −3.9; εHfi = −0.6 to −6.0; 206Pb/204Pbi = 17.8–18.2). These contrasting isotopic characteristics of aillikites/carbonatites and mela-aillikites, along with subtle differences in their modal carbonate, SiO2, Al2O3, Na2O, Cs–Rb, and Zr–Hf contents, are consistent with two distinctive metasomatic assemblages of different age in the mantle magma source region.