The 2.74 Ga Likamännikkö complex in Suomussalmi, East Finland: Lost between sanukitoids and truly alkaline rocks?

Abstract

The Likamännikkö quartz alkali feldspar syenite complex is located in eastern Finland, in the Suomussalmi district, within the Archean Karelian Craton. The intrusive complex covers an area of 2km2 and is cut by both Neoarchean leucogranite and Paleoproterozoic dolerite dykes. Three zircon fractions define a concordant U–Pb age of 2741±2Ma for the quartz alkali feldspar syenite. The syenitic complex also contains carbonatite patches and angular fragments of ultrabasic rock less than 50cm across. However, the paucity of outcrop has prevented direct observation of the relationship between carbonatites and other rock types. The quartz alkali feldspar syenite is not alkaline sensu stricto, but has alkaline affinities, with major element compositions in the range SiO2=62.6–70.3wt.%, K2O=1.09–5.73wt.%, Na2O=4.41–7.19wt.% and MgO=0.65–2.06wt.%. The ultrabasic fragments are interpreted as cumulates containing clinopyroxene (generally altered to actinolite), apatite, allanite, epidote, and albite; these minerals occur in varying proportions, leading to considerable whole-rock compositional heterogeneity, with major elements in the range SiO2=31.4–48.9wt.%, CaO=16.35–30.74wt.% and MgO=2.00–14.07wt.%, as well as notable enrichment in P2O5 (3.2–13.5wt.%). The carbonatites are calcitecarbonatites displaying mantle-like stable isotope values with δ18OVSMOW from 5.2 to 6.9‰ and δ13CVPDB values from −3.8 to −5.5‰, also trace element characteristics are consistent with a magmatic origin. Based on Sm–Nd model ages, the carbonatites are approximately coeval with the host quartz alkali feldspar syenite and thus possibly among the oldest known carbonatites in the world. All three rock types are interpreted to have originated from metasomatized mantle and the intrusive complex consists of two or more silicate magma pulses. Positive εNd values and their small scatter from −0.2 to 2.0 at 2741Ma suggest only limited assimilation of older crust or Paleoproterozoic disturbance of Sm–Nd systematics.