Conversion of titanite to rutile during the albitization of granitoids from the Sakar Batholith, SE Bulgaria: experimental studies

Petrological studies of the Sakar granitoid batholith (SE Bulgaria) have revealed that rutile-rich aggregates were formed from the titanite during albitization and desilification of the granitoid at ∼600 °C and ∼300 MPa. Titanite conversion to rutile has been tested experimentally. Four experiments involving fluid-aided alteration of titanite in a granitic system were conducted using cold-seal pressure vessels on a hydrothermal line at 280 MPa and 600 °C. The starting material included titanite, fluorapatite and a powdered, unaltered, natural granite from the Sakar Batholith. Four Na-bearing aqueous fluids (NaCl + H2O, NaF + H2O, Na2Si2O5 + H2O, and 2 M NaOH) were used. Rutile was formed from titanite in experiments involving NaCl + H2O (RT-1) and NaF + H2O (RT-2). The mineral assemblage formed in RT-1 consisted of rutile, Ti-Th-U oxides, and monazite, whereas in RT-2 rutile, fluorite, lorenzenite, albitized plagioclase, REE-enriched rims on fluorapatite, and a glass formed. Rutile did not form in experiments involving Na2Si2O5 + H2O or 2 M NaOH. The comparison between natural observation and the experimental results supports the premise that a NaCl + H2O fluid could have been responsible for the conversion of titanite to rutile during albitization, which further supports the role of NaCl-bearing fluids during the albitization of granitoids in general at mid- to upper-crustal pressures in the presence of alkali-bearing fluids. This implies that the geochemical relationship between rutile and titanite is more a function of the chemistry of the fluid and host rock, especially if they are Na-rich, rather than the P-T conditions. Subsequently rutile forming from titanite can be used as a geochronometer for dating the age of the process responsible both for the formation of the rutile and metasomatism of the rock. Detrital rutile is commonly thought to mostly originate from medium- to high-pressure igneous and metamorphic source rocks. Formation of rutile during albitization and desilification of granitoids under relatively high-temperature hydrothermal conditions in the upper crust has implications regarding rutile provenance studies of areas sourced by regionally albitized quartzofeldspathic rocks.