The Vaasa migmatite area in western Finland is thought to have formed by in situ anatexis of the Evijärvi schist belt rocks. The hypothesis is supported by the appearance and geochemistry of the rocks, as well as by petrographic features of the gradual contact between the two areas. Diatexite migmatites dominate the central parts of the area, whereas metatexites and mica-gneisses are more common on the margin. To complement previous research, the origin of the Vaasa migmatite area was studied by using whole-rock Sm-Nd and zircon Lu-Hf and U-Pb data in conjunction with elemental geochemistry. The concordia ages of the analyzed Vaasa area samples are 1.85-1.87 Ga. These ages are in line with previously reported U-Pb monazite ages (~1.86 Ga). The ages of inherited zircon cores in the samples show two clear zircon populations at 1.92-2.02 and 2.7 Ga, which corresponds with ages yielded by a sample from the Evijärvi belt, as well as with literature values presented for the lower sedimentary group of central Svecofennia. The initial εNd values of the Vaasa area samples are relatively unradiogenic (from -3.0 to -2.0). These values are comparable to a value (-3.6) calculated for an Evijärvi mica schist, as well as to literature values (from -0.5 to -3.0) for the Evijärvi belt. The average initial εHf values calculated for magmatic zircon range from -6.5 to -9.0 in the south, whereas the northmost sample has a near-chondritic value (+0.6). The variation is suggested to be caused by the participation of varying amounts of detritus rich in radiogenic Hf in the leucosome melts. Sources of the juvenile material have been the volcanic rocks of the Ylivieska arc and Evijärvi area. The REE signatures of the samples are variable indicating that of some of the Vaasa area diatexites have not formed in situ. The terms in-source diatexite and anatectic granite are proposed for Lu-depleted diatexites, depending on the presence of paleosome inclusions. Based on the high abundance of garnet in the Vaasa area rocks and geochronological data, the Vaasa area migmatization was caused by fluid-absent melting of micas during lithospheric extension and increased heat flow from the mantle at 1.87-1.85 Ga.
