RNDr. Igor BROSKA, DrSc.

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Geochemical and mineralogical characterization of the Fe-Ti oxide paragenesis in the magmatic and hydrothermal systems of the Western Carpathians

Distribution and geochemistry of granite Fe-Ti accessory mineral phases

Magnetite is present in the both Meso- and Late-Variscan I-type granitoid suites. I-type granitoids are metaluminous to slightly peraluminous biotite (leuco)tonalites to granodiorites, rarely biotite to muscovite-biotite granites, locally with pink K-feldspar phenocrysts. They are characterized by increase Ca, Mn, Fe, Ba, Sr, P, the presence of plagioclase with An30 in cores, biotite with Fe/(Fe+Mg) below 0.5, and TiO2 contents of approximately 2 - 3 wt %. Apatite, magnetite, titanite, allanite-(Ce), epidote and zircon with high S subtypes (S12, S13, S25) are characteristic accessory minerals. On the basis of recent data, their origin is connected with lower crustal continental melting with a contribution of infracrustal or mantle material. Locally quite abundant microgranular mafic enclaves in these granites are evidence for such processes. Typical occurrences: Tríbeč, Mts., Malá Fatra Mts. in Veporic hills (sihla type), Veľká Fatra Mts. (Smrekovica type) and Čierna Hora Mts. (Sokol and Sopotnica valleys). Affinity to I-type granites show also Modra massif (Malé Karpaty) and High and Low Tatra granitoids .

Fe-Ti evolution in granitoides Magnetite is presented in granite in several generations. In the Tribeč I-type Late-Variscan granite suite magmatic magnetite mostly form inclusions in titanite or in other rock- forming minerals. Calculation of model mineral equilibria in the system K2O-CaO-FeO-Al2O3-TiO2-SiO2-H2O-O2 (KCFATSHO) indicate that titanite forms in granites/granitoids as a consequence of a reaction between titanomagnetite, biotite, and anorthite in a fluid-rich environment under relatively oxidizing conditions during the later stages of crystallisation: titanite + anorthite + annite + H2O = ilmenite + clinozoisite + muscovite + quartz During intraoxide re-equilibration titanium in titanomagnetite is removed by the reaction to ilmenite. This process is modelled by the reaction: 6 Ulvöspinel = 6 Ilmenite + 2 Magnetite + O2 During granite evolution oxidatiom regime increases and newly-formed stoichiometric magnetite can precipitate along with other accessory mineral phases. Magnetite is commonly altered and hematite origin on the rim of its grains. Partial replacement of titanite by ilmenite is seen in some tonalites in Tribeč Mts., which have experienced metamorphism. This process took place under reducing conditions since formation of ilmenite would involve Fe2+ as opposed to Fe3+.