Assimilation–fractional crystallization origin of Archean Sanukitoid Suites: Western Superior Province, Canada

Abstract

Geochemical and Sm–Nd and Pb–Pb isotopic data are presented for post-tectonic diorites, monzodiorites, granodiorites and granites of late Archean age from the western portion of the Superior Province. These suites possess relatively high Mg numbers (0.43–0.62), high abundances of Cr, Sr, Ba and P 2O 5, low Rb/Sr ratios and steeply fractionated, light rare earth element (REE)-enriched patterns relative to pre-tectonic plutonic or volcanic suites. The geochemical signatures of these suites are characteristic of Archean sanukitoid suites which contain diorites–monzodiorites–granodiorites and granites. The dioritic end-members are interpreted to be mantle-derived and felsic portions are produced by subsequent crystal fractionation. Indeed, Nd isotopic data indicate an evolution from depleted, mantle-like values in dioritic samples to crustal values among granitic samples, implying that crustal assimilation played a major role in the magmatic evolution of the felsic members. In addition, Pb–Pb isotope data from K-feldspar separates in the felsic rocks are consistent with crustal contamination. The combination of enrichment in light rare earth and large ion lithophile elements, large negative Nb anomalies and time-integrated depleted Nd isotopic values for the dioritic members suggest derivation from a sub-arc, depleted mantle which was enriched (metasomatized) shortly prior to melting. However, because the suites were intruded up to 50 Ma following the end of subduction/collision in the Western Superior Province, a more complex petrogenetic model is required to explain the source of heat needed to generate the magmas. Alternatively, heat for the sanukitoid melting event could be derived from upwelling mantle as a result of either delamination of the subducting lithosphere following accretion of the Superior Province, or post-accretion strike-slip faults which penetrated to mantle depth.