Mantle-derived Archaean monozodiorites and trachyandesites

Abstract

The 2,680–2,750-Myr old1–5 intrusive granite–greenstone terranes6–8 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 mantle4,5,9–12. 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.