1.90-1.88 Ga arcmagmatism of central Fennoscandia: geochemistry, U-Pb geochronology, Sm-Nd andLu-Hf isotope systematics of plutonic-volcanic rocks from southern Finland

The earliest Svecofennian magmatism in southern Finland has been dated to 1.90-1.88Ga. As an example of this, the Orijärvi ( ca. 1.89Ga) and Enklinge ( ca. 1.88Ga) volcanic centres comprise bimodal plutonic batholiths surrounded by volcanic rocks of comparable ages and chemical compositions. The rock types range from gabbros to granites and indicate a subduction-related continental margin setting. The zircons from the Orijärvi granodiorite define an age of 1892±4Ma whereas the Enklinge granodiorite yields an age of 1882±6Ma. Several inherited ages of 2.25-1.95Ga as well as younger metamorphic ages of 1.86-1.80Ga were found in the Enklinge granodiorite. The initial εNd values of the mafic rocks from both locations fall in the range +1.1 to +2.9, whereas the felsic rocks exhibit initial εNd values of -0.4 to +1.2. The magmatic zircons from the Orijärvi and Enklinge granodiorites show average initial εHf values of -1.1 (at 1892Ma) and zero (at 1882Ma), respectively, both with a spread of about 7 ε-units. The initial εHf values for the inherited zircons from Enklinge range from +3.5 to +7.6 with increasing age. The Sm-Nd data indicate that the mafic rocks were derived from a “mildly depleted” mantle source while the felsi rocks show some crustal contribution. Also, the variation in ε Hf values indicates minor mixing between mildly depleted mantle-derived magmas and crustal sources. U-Pb ages and Hf isotopes for inherited zircons from the Enklinge granodiorite suggest the presence of juvenile Svecofennian “proto-crust” at depth