Petrogenesis of Archean lamprophyres in the southern Vermilion Granitic Complex, northeastern Minnesota, with implications for the nature of their mantle source

Abstract

Petrogenetic modeling of major and trace element and isotopic data is used: 1. to define probable modes of petrogenesis of Archean spessartitic lamprophyric rocks in the southern portion of the Vermilion Granitic Complex (VGC) of northeastern Minnesota, and 2. to place constraints on the nature of the mantle source of these rocks. The lamprophyres range from olto qtz-normative and are associated with cumulate hornblendites and pyroxenites. The silica-rich lamprophyres are shown to be the result of low-pressure fractionation upon emplacement. On the other hand, the composition range of the ol-normative lamprophyres is explained by approximately 40% polybaric fractionation of elinopyroxene + olivine yielding ne-normative liquids. The fractionation explains low Cr, Ni and Sc concentrations compared to primary mantle-derived melts. Modeling of the lamprophyre MgO−FeO compositions using the olivine saturation surface (Hanson and Langmuir 1978) suggests that the 0.42 to 0.55 Mg/(Mg+Fe) ratios of most of the lamprophyres can be explained by the high-pressure fractionation. The model parent melt composition is similar to sanukitoid-type rocks found in Japan and elsewhere in the Superior Province. The lamprophyres have e Nd 2700 values of +1.4 to +2.0, indicating derivation from a depleted mantle source. Growth curves on an e Nd vs. age diagram are consistent with the extraction of the lamprophyres from a depleted source (Sm/Nd>chondrite) just prior to 2700 Ma, the accepted age of the VGC. The lamprophyres have fractionated REE patterns (Ce/Ybn=10–15) that indicate genesis by a) 1% to 3% fusion of a pristine garnet lherzolite or b) ∼10% fusion of an enriched mantle source. However, consideration of the pressure of melting and elemental plots of Al and Ti indicate that garnet was not a residual phase during lamprophyre genesis. Thus, the enrichment of the LREE (80–100 x chondrite), Sr (580–1400 ppm), and Ba (590–1600 ppm) indicate derivation from an enriched mantle. These apparently contradictory chemical characteristics can be reconciled if the source region of the lamprophyres was depleted over a period of time but subsequently enriched just prior to genesis of the lamprophyre magmas. It is suggested that the source of the enriched component may have been fluids derived from dehydration of a subducting ocean crust.