Geochemical Characterization of Novokrivoyrog Metavolcanics: Tectonic Implications and Relationship with the Early Proterozoic Banded Iron Formation (BIF) of Krivoy Rog in Ukraine

摘要

The geochemical characterization of Novokrivoyrog metavolcanics (2.2 Ga) and Krivoy Rog iron ores (1.8 Ga) in Ukraine represent an important tool for the understanding of their genesis and tectono-magmatic evolution. The petrological classification of the metavolcanics on SiO2/(Zr-TiO2) and (Zr-TiO2)/(Nb/Y) Harker-type diagrams shows similarities to subalkaline andesitic basalts. An additional classification of the basalts on TAS (Na2O + K2O/SiO2) and AFM (FeO-MgO-Na2O + K2O) diagrams exhibits a variable magmatic character from calc-alkaline to tholeiitic. The distribution of High Field Strength Elements, (HSFE: Ti, Zr, Y, Hf, Nb), V, Cr, and Rare Earth Elements (REE) in most of the rocks is close to calc-alkaline basalts (CAB) and can be compared to Precambrian mid-ocean ridge basalts (MORB) where high thermal (>250°C) basaltic alteration is intensive under pH conditions between 2 and 4. These contributed to the deposition of the Krivoy Rog BIFs. Indeed REE distribution patterns of the BIFs suggest that they can be subdivided into shales and shaly BIFs (rich in LREEs since their detrital and clastic inputs are much higher) with (La/Yb)N > 1 as indication of clastic inputs;chert and cherty BIFs showing positive Eu anomaly with (La/Yb)N 1 emphasizes post-depositional effects related to the enrichment of light REEs over heavy REEs with a positive Eu anomaly. The distribution of REE patterns of Krivoy Rog BIFs can finally be compared to Precambrian iron formations of mixed submarine hydrothermal fluids and seawater origin which correspond to the MORB signature of the Novokrivoyrog metavolcanics.