Formation of magmatic titanite and titanite–ilmenite phase relations during granite alteration in the Tribeč Mountains, Western Carpathians, Slovakia

In this study, formation of titanite, in Carboniferous I-type granites/granitoids from the Tribeč Mountains (Slovakia), and its subsequent alteration products are described. Titanite occurs in early magmatic differentiates of I-type granites/granitoids. Calculation of model mineral equilibria in the system K2O–CaO–FeO–Al2O3–TiO2–SiO2–H2O–O2 (KCFATSHO) indicates 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 magma crystallisation. Evidence for this reaction can be found in low temperature inclusions of Ti-rich magnetite and biotite inclusions in the titanite, as well as in the formation of titanite not only interstitially, but also locally in quartz. The titanite alteration process has lead to the complete replacement of titanite by ilmenite, quartz, REE-bearing epidote, and allanite while preserving the characteristic euhedral, diamond shape of the former titanite. Completely altered titanite grains become optically opaque due to the presence of ilmenite. The formation of REE-bearing epidote and allanite is probably due to the release of REE from the titanite during alteration. This can be explained by model hydration reactions such as titanite + anorthite + annite + H2O = ilmenite + clinozoisite + muscovite + quartz. Calculation of mineral equilibria involving magmatic precursor minerals, such as plagioclase and biotite, indicate that the formation of ilmenite from titanite requires an influx of H2O and/or an increase in fO2. Titanite with the highest degree of ilmenite replacement occurs in a strongly mylonitized granite, in which the rock-forming minerals have been replaced by sericite, epidote, and chlorite. This breakdown of titanite to ilmenite is a common phenomenon in some granites from the southwest area of the Tribeč Mountains, indicating widespread late-stage fluid activity, associated with high fO2 during the subsolidus cooling.