Metamorphic evolution of the Ilomantsi greenstone belt in the Archaean Karelia Province, eastern Finland

Abstract

The Ilomantsi greenstone belt is a Neoarchaean, c. 2.75–2.70 Ga volcanic–sedimentary complex in which metamorphic grade increases from staurolite grade in the SW of the belt to sillimanite grade in the NE. In the staurolite zone, prograde garnet zoning indicates pressure and temperature increases from 480–500°C at 2–4 kbar to 560–570°C at 6–7 kbar. Within the sillimanite zone temperatures peaked at 660–670°C at pressures of around 6 kbar. The U–Pb age determinations on monazite from the sillimanite zone yielded both Archaean and Proterozoic ages. One sample contains an exclusively Archaean monazite population of 2620±24 Ma, while another sample has two generations of monazite, with ages of 2664±33 Ma and 1837±13 Ma. The monazite data confirm that the Ilomantsi greenstone belt was metamorphosed simultaneously with the surrounding Neoarchaean migmatite complexes. The apparent clockwise PT path and medium P/T-type metamorphism are consistent with collisional tectonic settings, but the two distinct metamorphic events recorded by monazite indicate that a second, Palaeoproterozoic thermal event caused recrystallization and new mineral growth, in line with previous evidence from other isotopic systems. Accordingly, great care is necessary in defining metamorphic evolutionary P–T–t paths in rocks with complex mineral assemblages, to ensure correct identification of truly coeval mineral assemblages.

Archaean granitoids: an overview and significance from a tectonic perspective

“…Several contributions from Karelia and the Indian Shield (Dey et al 2016;Heilimo et al 2016;Joshi et al 2016;Mikkola et al 2016;Ramiz & Mondal 2016) find evidence for a long-or short-lived enriched mantle source below the cratons (western Karelia, Bundkelkhand and eastern Dharwar), as well as for subsequent or simultaneous mantle and crust melting. Metamorphic studies of Hölttä et al (2016) provide further support for a collisional tectonic setting in the Karelia Province.…”

“…The last paper, by Hölttä et al (2016), describes the metamorphic evolution of the approximately 2.75 -2.70 Ga Ilomantsi greenstone belt in the central Karelia Province, Fennoscandian Shield (Fig. 1) by providing monazite U -Pb ages and pressure -temperature (P-T ) paths for metamorphism.…”

“…The paper of Hölttä et al (2016) brings metamorphic evidence on the tectonic setting of high-K granitoids from the Ilomantsi greenstone belt in the central Karelia Province by identifying metamorphic zonation that can be explained by regional heat advection in a collisional setting associated with late orogenic high-K magmatism. The Ilomantsi belt differs from tholeiitic greenstone belts in its restricted age (2.75-2.73 Ga), the mainly felsic and intermediate compositions of the volcanic rocks and the scantiness of basalts and ultramafic rocks, and their arc-type geochemistry and association with abundant coeval sanukitoids.…”

Summanen structure: Further geological and geophysical evidence of a meteorite impact event in Central Finland

“…Sample details are available in the Table A1. Clasts of breccia and shatter cone‐carrying rocks have undergone a pre‐impact regional Svekofennian metamorphism, generally in greenschist facies (Hölttä & Heilimo, 2017). The studied samples are moderately altered, related to regional metamorphism or syn−/post‐impact hydrothermal processes.…”

Evolution of the Gállojávri ultramafic intrusion from U-Pb zircon ages and Rb-Sr, Sm-Nd and Lu-Hf isotope systematics

“…This finding is not a complete surprise. In Finland, the Archaean basement of the western Karelian province contains compositionally diverse TTGGs formed between 3.5 and 2.8 Ga (Hölttä et al, 2012;Mutanen and Huhma, 2003). If the wall-rock reservoir had a composition similar to the c. 3.5 Ga Siurua tonalitic gneiss (Mutanen and Huhma, 2003), with an εNd (2.05 Ga) ~ -23 (Fig.…”