Titanian andradite and hydroandradite containing up to ~ 10 wt% TiO2 were identified in rodingite as well as the host serpentinite in a sample obtained from the Nomo unit, Nagasaki Metamorphic Rocks (western Kyushu, Japan), representing a Cretaceous subduction complex. The rodingite consists of garnet (solid solution among three end–member compositions: grossular, andradite, and titanian hydrogarnet), diopside and chlorite, along with minor amounts of perovskite, ilmenite, and titanite. This assemblage was formed by metasomatism under conditions of epidote–blueschist facies metamorphism at 0.6–0.8 GPa and ~ 400°C. X–ray single crystal diffraction and chemical analyses determined the formula Ca3.00(Fe3+0.64Ti4+0.24Mg0.03V0.02Sc0.01Al0.06)2(Si0.90Al0.03□0.07)3O11.35
(OH)0.65, in which Fe3+ preferentially occupies the Y–sites and Ti is solely in the tetravalent state. We propose a new type of coupled substitution, Fe3+ + H+ + □ = Ti4+ + □, in the titanian andradite, indicating that it does not belong to the solid solution in the andradite–morimotoite–schorlomite system of the International Mineralogical Association (IMA) classification scheme. The Fe ions are ferric (Fe2+ is estimated to be 3% or less), showing that the formation of the titanian andradite occurred under oxidizing conditions. In the reaction zone, the titanian andradite overgrowth on the ilmenite and the partial replacement of the perovskite by titanian andradite show the incorporation of TiO2 into the garnet via reaction with TiO2–bearing minerals under relatively oxidizing conditions. In addition, the presence of andradite coronas around the magnetite rim formed from the alteration of primary chromite provides evidence of Ti mobility in the serpentinite. These reaction relations highlight the titanian andradite formation process and allow us to propose associated reactions based on the singular value decomposition method.