Metasomatic alkali-feldspar syenites (episyenites) of the Proterozoic Suomenniemi rapakivi granite complex, southeastern Finland

Abstract

Peralkaline to marginally metaluminous alkali-feldspar syenites and quartz alkali-feldspar syenites are hosted by subalkaline, ferroan rapakivi granites in the 1644Ma Suomenniemi complex of southeastern Finland. These alkali syenites form NW-oriented dikes and small (<10m in diameter) bodies that are distinguished from the surrounding granites by their color (violet-red), general lack of quartz, as well as pronounced interstitial character of mafic minerals. Microtextures of the syenites imply pervasive alkali metasomatism and growth of secondary sodic and oxidized ferromagnesian minerals. Both subsolvus (~Ab99 and Or90–100Ab0–10) and hypersolvus (Or40–60Ab40–60) feldspar assemblages are present and display red luminescence characteristic of alkali feldspar recrystallized in the presence of an oxidizing fluid. In the marginally metaluminous syenites, primary magmatic hastingsite has been metasomatized to ferro-actinolite or decomposed to ferro-ferri-hornblende and magnetite. In some of the peralkaline syenites, primary hastingsite was replaced by magnetite and feldspars and has been overgrown by aegirine-augite and riebeckite. Sodic clinopyroxene (sodic augite–aegirine) is the most common and, in many cases, the only ferromagnesian silicate in these syenites. Three peralkaline alkali-feldspar syenites analyzed for zircon U-Pb and O isotopic compositions by single-grain SIMS have zircon 207Pb/206Pb ages of 1645±5, 1642±4 and 1644±4Ma, and zircon δ18OVSMOW values of 8.04±0.18, 8.19±0.17 and 8.26±0.17‰. Whole-rock Nd isotope data imply an overall εNd(1644Ma) value of ca. −1.5 for the syenites. These ages and isotopic fingerprints are, within error, identical to those of the subalkaline granites of the complex. We propose that the Suomenniemi alkali-feldspar syenites are episyenites, formed as the result of pervasive local metasomatism of the subalkaline granites caused by high-temperature oxidizing peralkaline fluids. The process led to major geochemical changes, e.g., addition of Na, Al and Fe3+, depletion of Si and Fe2+, and partial to complete recrystallization of the granites along fluid pathways.