The Bükk Mts. in NE Hungary exposes Paleozoic and Mesozoic successions containing volcanic formations both in the oceanic crust-derived Szarvaskő Unit and in the continental crust-derived Paraautochthonous Unit. The rocks of this latter unit were subject of multiple metamorphic, also metasomatic alterations and deformation events obscuring and overprinting original petrographic and geochemical characteristics and producing a complex structure in which stratigraphic relationships are not always possible to be identified. This situation was leading to various stratigraphic hypotheses. This study aims to provide a basis for distinguishing metavolcanic formations using trace element geochemical data combined with quantitative mineralogical data based on XRD and EPMA. Our data were obtained from a wide range of samples collected in the Bükk Mts. Mineralogy and major element geochemistry reflect regional Alpine metamorphism and local alteration processes but high field strength elements remained relatively stable during most of these processes except local HFSE enrichment. Zr/TiO2, Nb/Y and Nb/Ta ratios were effectively used for classifying the rocks into three formations: Bagolyhegy Metarhyolite, Szentistvánhegy Metavolcanics and Szinva Metabasalt. Bagolyhegy Metarhyolite is a unique volcanic formation formed from highly differentiated and HFSE-depleted magma, probably in a single volcanic centre of uncertain age. Szentistvánhegy Metavolcanics comprises heterogeneous rocks of a calc-alkaline arc-type suite with wide distribution in a Ladinian chronostratigraphic horizon. Szinva Metabasalt represents within-plate-type alkaline lava flows and adjacent volcaniclastic, mostly peperitic rocks embedded in Carnian platform and basin facies limestone formations. Like the sedimentary formations of the Bükk Mts, rocks of magmatic origin can be correlated with the formations of the South Alpine and Dinaric successions with the significant difference that abundant metavolcanics are not accompanied by intrusive bodies.
The foliated low-grade metamorphic rocks of the Triassic Bagolyhegy Metarhyolite Formation, mainly of pyroclastic origin, host post-metamorphic quartz-albite veins containing abundant tourmaline and occasionally rutile/ilmenite. The study of the Ti-oxide-mineralized veins with SEM-EDX revealed an unusual mineral assemblage comprising fine-grained Nb–Ta-bearing oxides (columbite-tantalite series, fluorcalciomicrolite and other Nb–Ti–Y–Fe-REE-oxide minerals) intergrown with Nb-rich polymorphs of TiO2 (anatase, rutile), ilmenite and zircon enriched with hafnium. This high field strength elements (HFSE)-bearing paragenesis is unexpected in this lithology, and was not described from any formation in the Paleozoic-Mesozoic rock suite of the Bükk Mountains (NE Hungary) before. The host metavolcanics are significantly depleted in all HFSE compared to the typical concentrations in felsic volcanics and the mineralized quartz-albite veins have even lower Ti–Nb–Ta concentration than the host rock, so the mineralization does not mean any enrichment. From proximal outcrops of the Triassic Szentistvánhegy Metavolcanics, potassic metasomatized lenses with albite-quartz vein fillings containing rutile/ilmenite are known. We studied them for comparison, but they only contain REE mineralization (allanite-monazite-xenotime); the Nb–Ta-content of Ti-oxide minerals is undetectably low. LA-ICP-MS measurements for U–Pb dating of Hf-rich zircon of the Nb–Ta-rich mineral assemblage gave 71.5 ± 5.9 Ma as lower intercept age while dating of allanite of the REE mineralized quartz-albite veins gave 113 ± 11 Ma as lower intercept age. The REE-bearing vein fillings formed during a separate mineralization phase in the Early Cretaceous, while the Nb–Ta mineralization was formed by post-metamorphic alkaline fluids in the Late Cretaceous., controlled by fault zones and fractures.
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