Contact zone interaction of metabasites with metapelites: Amphibolite facies mineral assemblages, chemical profiles and their origin, the Puolankajärvi Formation, Finland

The amphibolite facies, iron-rich metapelites (garnet-staurolite mica schists) of the early Proterozoic Puolankajärvi Formation are intercalated with metamorphosed basic rocks which vary from iron-rich amphibolites to magnesium-rich layered sills and serpentinites. Garnet-cummingtonite-hornblende assemblages are abundant at the contacts of the metapelites with amphibolites, while garnet-chlorite, garnet-gedrite±chlorite, garnet-gedrite-cummingtonite±chlorite and chlorite-cummingtonite-hornblende assemblages occur at their contacts with the layered magnesium-rich sill.

The bulk composition profiles of these contacts, normalised with respect to chromium and/or aluminium, show conspicuous enrichment in Fe, S, Ga and Co and depletion in Si, K, Ba and Rb. Some elements (e.g. Mg, Mn, Ni, P, Zr and Hf) are depleted or enriched in only small amounts, while others (e.g. Ca, Na, Sr and the rare earth elements) may show both enrichment and depletion in the course of a profile.

Although early infiltration during the hydration of metabasites and tectonic reworking may have produced some interaction of the basites with the pelites, the major metasomatic event is considered to have bile elements. The composition of the metapelites, for example, approached that of magnesium-rich ultramafites through potassium depletion, while the ultramafites became markedly enriched in iron. To conserve the volume, silica was depleted from the contact which underwent iron enrichment.