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

Abstract

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 SiO 2 /(Zr-TiO 2 ) and (Zr-TiO 2 )/(Nb/Y) Harker-type diagrams shows similarities to subalkaline andesitic basalts. An additional classification of the basalts on TAS (Na 2 O + K 2 O/SiO 2 ) and AFM (FeO-MgO-Na 2 O + K 2 O) 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 are similar to REE patterns of mixed hydrothermal fluids and seawater; alkaline altered BIFs whose (La/Yb)N ratio is >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.

Geochemistry of the Krivoy Rog Banded Iron Formation, Ukraine, and the impact of peak episodes of increased global magmatic activity on the trace element composition of Precambrian seawater

“…This suggests that REY redistribution in some samples rather occurred during upgrade of the BIF protore by supergene weathering processes in the upper BIF horizons. This is supported by studies showing Fe enrichment in the upper Krivoy Rog BIF (Mboudou et al, 2012) and strong LREY enrichment relative to MREY and HREY in supergene iron ores in general (Gutzmer et al, 2008). However, such postdepositional alterations of the BIF's REY budget are only shown by a very small subset of the Krivoy Rog sample set studied here.…”

“…Despite the general robustness of REY against diagenetic and regional low to high grade metamorphism (Bau, 1993), the unusual REY SN patterns of a few BIFs samples in sequence 6 cannot be explained by syn-depositional processes such as contamination with detritus and, therefore, must have been produced by post-depositional fluid-rock interactions during diagenesis, metamorphism and/or upgrading of the BIF to Fe-ore by supergene processes. Beletsev et al, 1989 andMboudou et al (2012) reported greenschist-facies metamorphic conditions in the lower part of the Saxagan Group, but amphibolite-facies metamorphic conditions in the upper BIF stratigraphy. However, no significant mineralogical differences exist between the different BIF horizons we studied and only index minerals typical of low-grade metamorphic conditions are observed (Table 1 and Table 1 in the Appendix).…”

“…reach a thickness of up to 1400 m and indicate a cycle of transgression-regression periods of the Krivoy Rog sea during the deposition of the Krivoy Rog Supergroup (Kulik, 1991). The Fe content of the lower sequences is less than 40%, while upper Saxagan Group BIF horizons may contain up to 65% Fe (Mboudou et al, 2012), probably triggered by post-depositional supergene Fe-ore upgrade. Mboudou et al (2012) and Belevtsev and Belyaev (1989) report greenschist facies metamorphic conditions in the lower part of the Saxagan BIF stratigraphy which increase to amphibolite facies in the upper part of the sequence and are probably related to a Kulik and Korzhnev, 1997). regional granitoid emplacement at 2000 ± 100 Ma (Shcherbak et al, 1984).…”

“…The Fe content of the lower sequences is less than 40%, while upper Saxagan Group BIF horizons may contain up to 65% Fe (Mboudou et al, 2012), probably triggered by post-depositional supergene Fe-ore upgrade. Mboudou et al (2012) and Belevtsev and Belyaev (1989) report greenschist facies metamorphic conditions in the lower part of the Saxagan BIF stratigraphy which increase to amphibolite facies in the upper part of the sequence and are probably related to a Kulik and Korzhnev, 1997). regional granitoid emplacement at 2000 ± 100 Ma (Shcherbak et al, 1984). However, all BIF samples we studied are mainly composed of magnetite and quartz with minor siderite, hematite and low-grade metamorphic trace minerals such as riebeckite, cummingtonite, stilpnomelane and chlorite minerals.…”

“…For most Krivoy Rog BIF samples, Hf and Th concentrations are at or below the lower limit of determination. Therefore, the following section is based on Zr and REY concentration data of Krivoy Rog BIFs and schists (this study), Archean TTGs and amphibolites of the ambient Aulian Gneiss Complex (Samsonov and Chernyshev, 1996), Belozerka Group metasediments (Bibikova et al, 2010), and New Krivoy Rog Group metavolcanic rocks (Mboudou, 2001;Mboudou et al, 2012). Fig.…”