GEOCHEMISTRY OF SUMIAN BASALTIC ANDESITES OF CENTRAL KARELIA

Abstract

A large Paleoproterozoic province of rocks of basaltic andesite composition with a high magnesium content (MgO = 5–10 wt %) was formed within the Karelian craton 2.55–2.4 Ga ago. The representative Sumian sequences were described in Central Karelia near Krasnaya Rechka and Koikary villages, Semch River, and Kumsa structure. Massive, amygdaloidal, variolitic, and pillow lava flows 9–45 m thick dominate the sections with reconstructed thicknesses up to 1500–1650 m. Tuffs compose rare interbeds. Based on variations of the SiO 2,TiO2, and MgO contents in rocks, up to seven volcanic stages can be distinguished. The volcanic rocks are similar to boninites by their concentrations of SiO2(54.62 ± 3.14wt %), Ni (135 ± 70 ppm), Cr (295 ± 134ppm),but significantly differ from them by Mg# (#Mg = 50.05 ± 6.22); concentrations of TiO2(0.84 ± 0.26wt %), Hf(<4.2 ppm), Ta (<0.7 ppm), Zr (<120 ppm); and topology of REE spectra with (La/Sm)n= 1.6–5.2 and (Ce/Yb)n= 4.1–18.6. Model calculations testify to a shallow origin of initial magmas, which is typical of volcanic rocks of subduction zones. Geochemical characteristics of Sumian volcanic associations allow us to correlate them with basaltic andesites of active continental margins of the Andean type.