Rodingites from the Xigaze ophiolite, southern Tibet - new insights into the processes of rodingitization

The Xigaze ophiolite represents one of the most prominent occurrences of Neo-Tethyan ocean remnants in the central part of the Yarlung-Zangbo suture zone, southern Tibet, which marks the collision of the Indian and Eurasian plates during the Jurassic and Cretaceous. Abundant veins and bands of rodingite, which are spatially related to gabbro and diabase dykes and intermingled with serpentinized mantle peridotite, were found in the ophiolite sequence, extending from Jiding to Luqu, Bairang and Baigang. The gabbro as well as diabase formed in a back-arc basin that developed by subduction of the Tethys ocean in response to the convergence of India and Eurasia. Both rodingite and gabbro/diabase show the same geochronological and geochemical behavior, thus demonstrating the genetic relationship of these lithological types with oceanic lithosphere. Rodingitization can be seen as a Ca-involving metasomatic process that is genetically linked to serpentinization; the Xigaze rodingites typically show a gain in Ca and a loss of Na and K. The mineral assemblages of the rodingites are highly variable and can be divided into two groups. Group I rodingite, where prehnite is absent, consists of the subgroups clinozoisite rodingite, garnet-diopside rodingite and garnet-vesuvianite rodingite. The grossular-rich (hydro) garnet may contain up to 13.7 wt% TiO2, essentially as morimotoite component (23 mol%). In group II rodingite, prehnite is present and vesuvianite absent; subgroups are prehnite rodingite and prehnite-garnet rodingite. Group I rodingites mainly occur in deformed schistose serpentinites and have experienced a more intense rodingitization in a SiO2 undersaturated environment than those of group II. Group II rodingites occur along extensional fractures in serpentinized peridotite. Their setting is SiO2 saturated and reflects a later metasomatic fluid/rock interaction that occurred at shallow crustal levels. Group I rodingites presumably formed within the oceanic crust and were later involved in a subduction-zone environment at high pressures in the early stages of the geodynamic evolution of the Xigaze ophiolite, with a final late-stage imprint at shallow crustal levels. Group II rodingites are interpreted to have formed during late-stage ductile and brittle deformation events at shallow crustal levels, when the uplift process of the mantle section was nearly completed. Rodingites may thus represent an important chronological marker within ophiolites, which may help to evaluate the sequence of processes such as early magmatic intrusions, rodingitization, serpentinization, late basic intrusions, shallow crustal level alterations.