V. P. Chekulaev, Yu. S. Egorova, N. A. Arestova

Magmatic system evolution in the Karelian province of the Fennoscandian Shield during the Archean–Proterozoic transition

Abstract. The authors compared the Neoarchean and Early Paleoproterozoic (Sumian) igneous rock compositions in the Karelian province of the Fennoscandian Shield. The Neoarchean basite-ultrabasite magmatism was not widely spread, but granitoid magmatism was critical for crust formation. At that time, the first subalkaline and alkaline rocks appeared in the Shield: sanukitoids (2.75–2.71 Ga), granites and pegmatites (2.7–2.65 Ga), and carbonatites (2.61 Ga). After a magmatic activity hiatus of about 100 Ma, the Proterozoic began with large-scale ultrabasite-basite magmatism manifested as layered intrusions, drusites, and volcanic complexes (2.5–2.45 Ga). Sumian basites are enriched in light rare earth elements and have negative εNd(t) values ranging from −3.4 to −0.5. The Neoarchean metakomatiites and metabasalts include primitive varieties as well as those enriched in light rare earth elements; they both have positive εNd(t) values ranging from +0.5 to +2.6. The Neoarchean rocks of intermediate-felsic composition present metavolcanites and various granitoids — from tonalite-trondhjemite-granodiorite (TTG) to sanukitoids and plagiomicrocline granites. TTG, sanukitoids, and metavolcanics (2.76– 2.70 Ga) are enriched in K, Ba, Sr, and light rare earth elements, and are characterized by εNd(t) values from –1 to +2.5. Plagiomicrocline granites (2.72–2.65 Ga) vary in rare and rare earth element contents, as well as εNd(t) values from –5 to +2.2. The Sumian intermediate-felsic rocks have two groups. Metavolcanites with SiO2 <65 %, which have no intrusive analogs, are more likely to be derivatives of the Sumian mafic volcanites. The rocks with SiO2 >65 % are represented by potassic metarhyolites and granites (2.45–2.40 Ga) with high contents of Ti, Zr, Y, as well as Ba and Rb; they predominantly have negative εNd(t) values from +0.5 to –3.

Keywords: Neoarchean, Paleoproterozoic, igneous rock geochemistry, metavolcanite, granite, TTG, Sm–Nd