Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province

Abstract

In southwestern Superior Province, diorite, monzodiorite, and trachyandesite ("sanukitoids") occurring within syn- to post-tectonic intrusive complexes and within greenstone belts have the following chemical characteristics: 55–60 wt.% SiO2, MgO > 6 wt.%, Mg# > 0.60, Ni and Cr both > 100 ppm, Na2O + K2O = 6 wt.%, Sr and Ba both 600–1800 ppm, and rare-earth-element (REE) patterns that are strongly light rare-earth-element (LREE) enriched (Cen = 80–250, Ybn = 4 – 10) and show no Eu anomalies. Sanukitoids and their granodioritic derivatives constitute at least 5% of the exposed crust in the study area. The sanukitoids cannot be derived by melting, fractionation, or crustal contamination of basalts or lamprophyres that are coeval with the sanukitoids. Crustal contamination of komatiites fails to explain the high large-ion-lithophile-element (LILE) contents of the sanukitoids. Rather, we suggest that the sanukitoids were derived by hydrous melting of LILE-enriched mantle peridotite at pressures between 10 and 15 kbar. The sanukitoids with steepest REE patterns have the lowest FeO contents, indicating that the part of the mantle source with the highest Mg# had the most fractionated REE pattern prior to melting. Mantle source regions to the sanukitoids had different Mg#'s and were enriched in LILE's (metasomatized) to varying extents by fluids of crustal or mantle origin prior to melting.