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

Tappe, Sebastian and Foley, Stephen F. and Kjarsgaard, Bruce A. and Romer, Rolf L. and Heaman, Larry M. and Strackle, Andreas and Jenner, George A. (2008) Between carbonatite and lamproite—Diamondiferous Torngat ultramafic lamprophyres formed by carbonate-fluxed melting of cratonic MARID-type metasomes. Geochimica et Cosmochimica Acta, 72 (13). pp. 3258-3286. ISSN 1872-9533

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Abstract

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 peridotite substrate. Isotopic modelling suggests that only 5–12% trace element contribution from such geochemically extreme MARID-type material is required to produce the observed compositional shift from the isotopically most depleted aillikites/carbonatites towards enriched mela-aillikites. We conclude that cold cratonic mantle lithosphere can host several generations of contrasting vein assemblages, and that each may have formed during past tectonic and magmatic events under distinctively different physicochemical conditions. Although cratonic MARID-type and carbonate-bearing veins in peridotite can be the respective sources for lamproite and carbonatite magmas when present as the sole metasome, their concomitant fusion in a complex source region may give rise to a whole new variety of deep volatile-rich magmas and we suggest that orangeites (formerly Group 2 kimberlites), kamafugites, and certain types of ultramafic lamprophyre are formed in this manner.

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