Origin of Archean lamprophyre dykes, Superior Province, Canada: rare earth element and Nd?Sr isotopic evidence

Abstract

Hornblende- and clinopyroxene-phyric lamprophyre dykes exposed in the Roaring River Complex, Superior Province are alkaline, nepheline-normative, basaltic compositions (>50 wt% SiO2), that range from primitive to fractionated [Mg/(Mg + total Fe)=0.66–0.40; Ni=200–35 ppm], and which have high abundances of light rare earth elements (REE) [(Ce/Yb)n=16–26, Cen=60–300; n = chondrite normalized], Sr (870–1,800 ppm), P2O5 (0.4–1.3 wt%), and Ba (150–900 ppm). Crystal fractionation of the lamprophyres produced coeval gabbro and clinopyroxenite cumulate bodies. A whole-rock Sm−Nd isochron for lamprophyres and gabbro-pyroxenite yields a crystallization age of 2,667±51 Ma Ma (I=0.50929±0.0004; ɛNd = + 2.3 α 0.7). Whole-rock Sr isotope data are scattered, but suggest an initial 87Sr/86Sr ratio of ≈ 0.7012, similar to bulk Earth. The elevated levels of light REEs and Sr in the lamprophyres were not due to crustal contamination or mixing with contemporaneous monzodioritic magmas, but a result of partial melting of a mantle source which was enriched in these and other large-ion-lithophile elements (LILEs) shortly before melting. The lamprophyres were contemporaneous with mantle-derived, high-Mg, LILE-enriched monzodiorite to granodiorite of the Archean “sanukitoid” suite. Both suites have concave-downward light REE profiles, suggesting that depleted mantle was common to their source regions, but the higher light REE abundances, higher Ba/La ratios, and lower ɛNd values (+1.3±0.3) of the parental monzodiorites suggest a more enriched source. The lamprophyres and high-Mg monzodiorites were derived from a mineralogically and compositionally heterogeneous, LILE-enriched mantle lithosphere that may have been part of a mantle wedge above a subducting plate in an arc environment.