Mantle-derived Archaean monozodiorites and trachyandesites

The 2,680-2,750-Myr old^1-5 intrusive granite-greenstone terranes^6-8 in the Rainy Lake region, Ontario, and in, and adjacent to, the Vermilion District, Minnesota, of the Superior Province in North America, include a wide variety of volcanic and intrusive rocks with initial Pb, Sr and Nd isotope ratios close to those for the mantle^4,5,9-12. The isotopic constraints require the production of large volumes of silica-oversaturated magmas from silica-undersaturated mantle within 100-200 Myr. We present here geochemical data on monzodiorites and trachyandesites from the Rainy Lake area which are strongly enriched in large-ion-lithophile elements (LILE). We conclude that they are derived by direct melting of the mantle at depths of <50km in either anhydrous or hydrous conditions. Their rare-earth element (REE) abundances and initial ¹⁴³Nd/¹⁴⁴Nd ratios suggest that their mantle sources were enriched in LILE shortly before melting. The monzodiorites-trachyandesites and their granodioritic derivatives may comprise up to 20% of the exposed igneous rocks in the Rainy Lake region and Vermilion District. If these rocks are as abundant in other Archaean terranes, a significant part of the early continental crust could have formed by direct melting of LILE-enriched mantle.