On the factors controlling the incorporation of aluminium within titanites: a case study from medium pressure calc-silicate granulites in parts of the East Indian shield

Abstract

High-aluminous, fluoro-titanites (~ 6.8–11.5 wt% Al₂O₃, up to ~ 3.8 wt% F) from a suite of calc-silicate granulites in the Chotanagpur Granite Gneiss Complex, East Indian shield, were examined to investigate the controls on Al incorporation in titanite. The studied high aluminous titanites have the third highest Al content (X_Al= up to ~ 0.46), reported from low to medium-pressure rocks till date. These titanites develop in three different associations (association 1, 2 and 3) along with the F-bearing hydrous minerals like amphibole or vesuvianite. These three associations occur as veins and patches close to the pegmatitic veins that intruded the granulite facies calc-silicate rocks. The titanite in the host calc-silicate rock (association 4), away from the pegmatite veins, preserves an anhydrous assemblage: garnet-clinopyroxene-plagioclase and low Al titanite (Al₂O₃ = 3.4–3.8 wt%, F ~ 0.8 wt%). Integrating field features, petrography and textural modeling, it is suggested that infiltration of F-bearing aqueous fluid, presumably derived from the pegmatites, into the host calc-silicate rock was responsible for the partial destabilization of the anhydrous assemblage 4, and formation of the Al-F rich titanite bearing assemblages 1–3. The published information and close proximity of the association 1–4 outcrops suggest that the infiltration-driven growth of Al-F-rich titanite occurred virtually under isothermal-isobaric conditions (5.5–6.5 kbar and 650–750 °C). The titanite in associations 1–3 show a positive correlation between Al₂O₃/TiO₂ and F/OH indicating the substitution Ti^+ 4 +O²⁻ = Al^+ 3+ (F + OH)⁻. Based on the findings of the present study, combined with the published information on titanite chemistry, it is argued that the f_F2 present in the system plays a dominant role, if not the most important, in regulating the extent of Al substitution in titanites, in addition to pressure, temperature or coexisting mineral assemblage.