Mafic to intermediate c. 1.86 Ga
magmatism has been discovered within the boundary area between central and southern
Svecofennia in southern Finland (Kara et al. 2016). The magmatism occurs as NW-SE
trending dykes and small intrusions which crosscut the surrounding migmatitic country
rocks. The magmatism can be divided into three different varieties based on their
geochemical and isotopic characteristics: i) high niobium basaltic dykes (HNB),
ii) high magnesium basaltic dykes (HMB) and iii) high silica adakite-like rocks
(HSA).

            The
HNB are alkalic and enriched in HFSE, particularly Nb (>20 ppm) and show
high LREE, P₂O₅, TiO₂ and F contents. In-situ
zircon Hf-analyses show very high average ε_Hf value of c. +10. The
HMB exhibit high MgO (up to 12 wt%) and Cr concentrations and show high average
ε_Hf values of c. +3 and +5 (two dykes). The HSA group is enriched in
Sr and LREE and depleted in Y (<20 ppm) and Yb (mostly <1.9 ppm) among
the other high silica adakite characteristic signatures (Martin et al. 2005).
Two HSA-intrusions exhibit average ε_Hf value of zero and c. -3.

            The
ages as well as geochemical and isotope data are clearly different from the
adjacent, c. 1.90-1.88 Ga, arc-related igneous rocks suggesting a different
kind of petrogenesis for these two age groups. The similar ages and close
spatial occurrences support a genetic link between the 1.86 Ga groups. However,
separate sources are likely due to differences in geochemistry and isotope
signatures.