The Kynsijärvi quartz alkali feldspar syenite, Koillismaa, eastern Finland—silicic magmatism associated with 2.44 Ga continental rifting

Abstract

The Kynsijärvi quartz alkali feldspar syenite (KS) is a small A-type silicic pluton closely associated with the ∼2440 Ma Koillismaa complex, one of the Paleoproterozoic mafic layered intrusion complexes of central and northern Finland and Kola Peninsula. In the Koillismaa area, these mafic and felsic intrusions were emplaced into a late Archean (2900–2600 Ma) basement and stratigraphically underlie supracrustal rocks of the Paleoproterozoic Kuusamo schist belt. The KS is a medium-grained hypersolvus rock with mesoperthitic alkali feldspar, quartz, and ferro-edenite as the major rock forming minerals. Magnetite, titanite, zircon, fluorite, and secondary stilpnomelane are found in accessory amounts. The rock shows extensive subsolidus reactions, for example well-developed intergranular albite rims around alkali feldspar crystals. The KS is characterized by relatively high SiO 2 (70.1–71.7 wt.%), total alkalies (∼10 wt.%), Ba (>700 ppm), Zr (>500 ppm), Ga (>30 ppm), and Nb (>40 ppm). Chondrite-normalized REE patterns are moderately enriched in the LREE [(La/Yb) N∼12.65] and show a negative Eu-anomaly (Eu/Eu*∼0.3). Four zircon fractions from the KS yield a U–Pb upper intercept age of 2442±3 Ma. The KS was probably emplaced in an extensional environment, the extension being caused by a mantle plume, which also generated the mafic layered complexes in the region. Overall, the 2.44 Ga mafic and felsic plutonic rocks mark a ∼400-km-long zone of early Paleoproterozoic rifting of the Fennoscandian Archean craton.