Between carbonatite and lamproite—Diamondiferous Torngat ultramafic lamprophyres formed by carbonate-fluxed melting of cratonic MARID-type metasomes

• Main Authors: Tappe, Sebastian, Foley, Stephen F., Kjarsgaard, Bruce A., Romer, Rolf L., Heaman, Larry M., Strackle, Andreas, Jenner, George A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Pergamon 2008
• Subjects: QE Geology; North Atlantic
• Online Access: https://research.library.mun.ca/410/; https://research.library.mun.ca/410/1/between_carbonatite.pdf; https://research.library.mun.ca/410/3/between_carbonatite.pdf; http://www.sciencedirect.com/science/article/pii/S0016703708001361

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.Integration of petrologic, geochemical, and isotopic information leads us to propose that the isotopically enriched component originated from a reduced phlogopite-richterite-Ti-oxide dominated source assemblage that is reminiscent of MARID suite xenoliths. In contrast, the isotopically depleted component was derived from a more oxidized phlogopite-carbonate dominated source assemblage. We argue that low-degree CO2-rich potassic silicate melts from the convective upper mantle were preferentially channelled into an older, pre-existing MARID-type vein network at the base of the North Atlantic craton lithosphere, where they froze to form new phlogopite-carbonate dominated veins. Continued stretching and thinning of the cratonic lithosphere during the Late Neoproterozoic remobilized the carbonate-rich vein material and induced volatile-fluxed fusion of the MARID-type veins and the cold ...