Magmatic evidence for craton reactivation: an Sr-Nd-Hf-Pb isotope study of lamproites, aillikites/carbonatites, and nephelinites from the Labrador Sea margin

The once-continuous North Atlantic craton (NAC; easternmost Canadian-Greenland Shield) is crosscut by the Labrador Sea, which opened at 60 Ma as a consequence of continental lithosphere delamination. The isotopic compositions of Mesoproterozoic lamproites (1400-1200 Ma), Neoproterozoic aillikites/carbonatites (610-550 Ma) and Cretaceous-Palaeogene alkaline basalts such as nephelinites (150-55 Ma), which have erupted through the same NAC crust along the present-day Labrador Sea margins, offer a relatively continuous record of the fate of the NAC lithosphere. At Aillik Bay on the southern NAC edge, Mesoproterozoic olivine lamproites are characterized by unradiogenic Nd (epsilon Nd -5.3 to -8.4), Hf (epsilon Hf -7.8 to -10) and Pb (206Pb/204Pb 14.2-14.8), but moderately radiogenic Sr isotope compositions (87Sr/86Sr > 0.7047). These isotopic compositions fingerprint ancient long-term enriched SCLM, which must have reached to depths of more than 150 km and remained isolated from mantle convection at this time. In contrast, Late Neoproterozoic carbonate-rich aillikites and carbonatites from Aillik Bay have fairly radiogenic Nd (0.1-1.8), Hf (-0.9 to +2.6), and Pb (206Pb/204Pb 17.4-18.8) but unradiogenic Sr isotope compositions (87Sr/86Sr < 0.7040) that point to the involvement of convective upper mantle material during melting. Simple binary mixing calculations and the fact that this carbonate-rich magmatism prevailed for over 30 Myr in the area imply a complex pattern of lithosphere - asthenosphere interaction at depths between 180 and 140 km. The Cretaceous suite of nephelinitic dyke rocks has slightly unradiogenic Nd (-3.9 to -1.3), moderately radiogenic Sr (0.7043-0.7062), but Hf (-3.3 to +1.4) similar to the aillikites and highly radiogenic Pb (206Pb/204Pb 19.0-20.7) isotope compositions. This reflects partial melting of a different (shallower) part of the SCLM that appears to have been thoroughly carbonate metasomatized during the prolonged Neoproterozoic carbonatite magmatism. The new age and isotopic data demonstrate that reactivation of the mantle lithosphere beneath the NAC commenced at the end of the Precambrian (Rodinia break-up) and continued intermittently until the opening of the Labrador Sea at 60 Ma. It is estimated that between 550 Ma, when an up to 180 km thick diamond-bearing lithospheric root was present, and 150 Ma, at least 30 km of the cratonic lithosphere had been replaced by the hotter underlying asthenosphere. This lithosphere delamination occurred prior to a more typical continental rift development during the Cretaceous as a response to incipient passive upwelling of the convective mantle beneath a stagnant supercontinent. Nevertheless, the accompanying volatile-rich alkaline magmatism played a key role in destroying the structural integrity of cratonic lithospheric mantle thereby facilitating the later delamination process.