Petrology and monazite dating of the Fe-rich gneisses from Kokava (Veporic Unit, Western Carpathians, Slovakia): Devonian sediments supplied from Gondwanan-sources metamorphosed in the Variscan times

Abstract

The Fe-rich gneisses from vicinity of Kokava nad Rimavicou (Central Slovakia) were studied by electron microprobe to resolve the origin of the iron ore bodies. Whole-rock chemistry of the ironstones shows the lack of CaO, MgO and partly elevated P 2 O 5 contents. This, together with the mineral assemblage of almandine + magnetite + grunerite + ilmenite + quartz + apatite + allanite + zircon + monazite suggests an origin by metamorphism of sedimentary protolith and disproves the older idea about the formation as a skarn. The Fe-Ti oxides thermometry suggests temperatures of 360-420 °C, and the average oxygen fugacity of ∆log fO 2 normalized to the FMαQ of +0.13. The reconstructed oxygen fugacity for temperatures 500-600 °C (amphibolite facies) yields an interval of c. +1 to +3 ∆log fO 2 (FMαQ). The presence of detrital zircons, and monazites dated in this study, reveals a participation of lithologies of the pan-African orogen. The source rocks of the Devonian ironstones sedimented probably as oolitic chamosite in lagoons. The dated detrital monazite cores (612-400 Ma) show an affinity to source rocks formed in northern peri-Gondwana due to rifting and opening of the Rheic Ocean and separation of Avalonia and Armorica microcontinents. The majority of monazite data from the rims of detrital grains and from unzoned metamorphic grains bear an evidence of Meso-Variscan metamorphism with ages clustering between 360 and 320 Ma (342 ± 4Ma -weighted mean ± 2σ). The younger population with elevated Th contents (18.0-20.5 wt. %) giving the ages of 310-240 Ma (279 ± 2 Ma -weighted mean) is connected with the collapse of Variscan orogen and/or the onset of the early Alpine continental rifting. The Alpine ages are very rare, but a few monazites and one Th-U-Si inclusion in quartz yielded Cretaceous ages (115-85 Ma).