Mafic and felsic magma interaction in the 1.64 Ga Ahvenisto complex : field relations, petrography, geochemistry, and melt viscosity modeling of monzodioritic and granitic rocks

Earlier field work in the 1.64 Ga Ahvenisto AMCG (anorthosite-mangerite-charnokite-granite) complex has revealed mafic and felsic magma interaction between hornblende granite and monzodioritic rocks, but a detailed description of this interaction has not previously been presented. In this study field observations, detailed mapping, petrography, geochemistry, and melt viscosity modeling are utilized to provide a comprehensive description of magma interaction processes and the related rock types. Two major mafic and felsic interaction processes, both also observed in the Ahvenisto complex, are mingling and mixing. Mingling is observed in the form of monzodioritic pillows with finger-like granitic veins protruding in them. Mixing is observed as a hybrid rock with monzodioritic groundmass and K-feldspar phenocrysts. The monzodioritic rocks are presumed to represent the last liquids of the mafic magmas of the complex, and the hornblende granite and the granitic veins are presumed to represent the first felsic melts of the granitic intrusion stage. Observed mixing and mingling indicates that these two magmas were, at least partly, liquids simultaneously at the time of intrusion. Petrography and geochemistry reveals at least three different monzodioritic rock types: olivine monzodiorite, ferrodiorite, and monzodiorite. These types represent partial melts of the monzodioritic magma, olivine monzodiorite being the least evolved and monzodiorite the most evolved. The vein granites originate from a more evolved partial melt of the hornblende granite superheated by the coeval mafic magma. Calculated mixing trends and melt viscosities with liquidus temperatures were used to evaluate the magma interaction styles between different rock types. Volatile contents of the two magmas played an important role: the felsic melts most likely contained higher amount of volatiles whereas the mafic melts were relatively dry. Mixing calculations reveal that the hybrid rock was produced by mixing of monzodioritic and hornblende granitic magmas. Mingling occur between ferrodiorite and the vein granites. The least evolved monzodioritic rock, olivine monzodiorite, shows minor interaction only and is probably a close representative of the 'primary' monzodiorite magma of the Ahvenisto complex.