Nd isotope evidence for crust-mantle interaction in the generation of A-type granitoid suites in Labrador, Canada

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Early Proterozoic “A-type” granite suites in Labrador, northeastern Canada, have initial Nd isotopic compositions that lie between those of a depleted mantle reservoir and the local basement rocks. One of these A-type granite suites consists of a linear array of discrete plutons that crosses the boundary between an Archean craton and an adjoining Early Proterozoic crustal block. The ϵ Nd of these granites (at formation, t = 1720 Ma) changes from −8 to +4 across this boundary, reflecting changes in the nature and antiquity of the continental crust. However, in both domains, the granites cannot be derived entirely by anatexis of the local basement rocks, and apparently require significant amounts of “juvenile” (mantle-derived) material. A-type granites have been widely interpreted as anatectic derivatives of lower-crustal rocks that were depleted by previous melt extraction. The Nd isotopic data suggest that these Labrador examples were instead mixtures of mantle-derived magmas and overlying lower-crustal rocks, and that new material may have been the volumetrically dominant component (50–90%) in most cases. As an alternative to the prevalent “residual-source” model, it is proposed that the distinctive geochemical characteristics of these A-type granites are a function of the high liquidus temperatures and primitive, Mg-rich character of subcrustal magma fluxes in post-orogenic and anorogenic environments, where such magmas are emplaced into stabilized continental crust. Further Nd isotope studies of other A-type granite suites are required to ascertain if models invoking crust-mantle interaction are more widely applicable.