Geochronological and geochemical insights into episyenite metasomatic overprint in the Proterozoic Suomenniemi rapakivi granite complex (Finland)

Abstract

Rapakivi granites from the late Paleoproterozoic Suomenniemi Complex (south-eastern Finland) experienced pervasive localised metasomatism via interaction of high-temperature, oxidising peralkaline fluids and granite along dyke-like zones, resulting in the formation of episyenite bodies. To constrain the timing and conditions of episyenite formation, we combine in-situ UPb zircon, UPb and LuHf apatite, and UPb titanite geochronology with petrography and mineral chemistry from four episyenite bodies. Zircon yields UPb ages coeval with crystallisation of the ca. 1644 Ma granite, preserving the igneous protolith age, whereas UPb ages from hydrothermally modified zircon (1635.1 ± 2.7 Ma; 1627.8 ± 6.3 Ma) and apatite (1611.6 ± 9.5 Ma; 1630 ± 19 Ma), along with hydrothermal titanite (1603 ± 43 Ma; 1623 ± 30 Ma) record younger ages, reflecting variable degrees of disturbance of the UPb system and indicating that episyenite formation post-dated granite emplacement. Apatite LuHf ages (1648 ± 30 Ma; 1643 ± 41 Ma) and some titanite UPb ages (1646 ± 12 Ma; 1642 ± 62 Ma) overlap with the host granite but their large uncertainties preclude confident attribution to either magmatic crystallisation or subsequent metasomatism. Collectively, these data indicate that episyenitisation postdated granite emplacement and was likely driven by regional thermal perturbations linked to the emplacement of the ∼1630 Ma Wiborg Batholith.

Petrographic and mineral chemistry analyses of zircon, apatite, and titanite indicate that hydrothermal alteration of the protolith was highly variable, both between episyenite bodies and even over small metre-scale intervals within individual bodies. This is evident in zircon, where different areas of high intensity hydrothermal alteration under high temperature and oxidising conditions resulted dissolution and reprecipitation of zircon. Apatite in the host granite was likely recrystallised during episyenitisation, as evidenced by textural modification and younger UPb ages. Titanite is an accessory mineral found exclusively within the episyenite bodies and is absent in the surrounding granite protolith. It commonly occurs as inclusions in mafic mineral aggregates or as rims around FeTi oxides (magnetite, ilmenite), suggesting a hydrothermal origin associated with episyenitisation and crystallisation under oxidising conditions. Variations in the major (Al, Fe, Ti, F) and trace element (light REE, Nb) chemistry of titanite and Zr-in-titanite thermometry (660–875 °C) further indicate differences in hydrothermal fluid composition and temperatures during titanite formation across different episyenite bodies. These findings, along with the overlapping ages of hydrothermal titanite and the host protolith, suggest that the episyenites in the Suomenniemi Complex formed by fluid-rock interaction, either shortly after consolidation of the main granite sequence of the complex and/or during subsequent regional rapakivi granite magmatism. Thus, resolving the timing of episyenite formation is crucial for understanding fluid-assisted alteration of granitic systems, identifying potential fluid sources, and assessing their geochemical evolution and metallogenic potential.