Mixing between basaltic and granitic magmas in a rapakivi related quartz-feldspar porphyry, Åland, SW Finland

Southwest of the Middle Proterozoic Åland rapakivi batholith (1575 Ma) there occur dolerites closely associated with quartz-feldspar porphyries. The dolerites and the quartz-feldspar porphyries were formed during a bimodal igneous event characterizing the initial stages of the rapakivi intrusion. A coarse-porphyritic porphyry carries abundant mafic and hybrid enclaves interpreted as resulting from simultaneous intrusion and mixing of basaltic (dolerite) and granitic (porphyry) melts. The fine-grained mafic enclaves are a Fe-enriched differentiation product of the doleritic melts that intruded the country rock. Mixing tests indicate that the hybrid enclaves are a result of magma mixing between melts now represented by the mafic enclaves and the coarse-porphyritic porphyries. Discrepancies resulting from mixing tests at three localities within the porphyry are explained by variations in the degree of homogenization that are due to the incorporation into the hybrids of early phenocrysts formed from the two end member melts. The coarse-porphyritic porphyry also has petrographical features such as tiny mafic enclaves, labradorite xenocrysts and quartz ocelli, which indicate that the composition of the porphyry itself had been to some extent influenced by mixing. It is suggested that the incorporation of the tiny mafic enclaves and labradorite xenocrysts into the porphyry occurred during the very earliest stage of the bimodal intrusion event, when the mafic melt interacted with a porphyry melt newly formed by anatexis. New surges of mafic melt chilling against acid melt generated the fine-grained mafic enclaves. The hybrid magma, now represented by the hybrid enclaves, was formed by mixing of the contrasting magmas.