Subduction-related rodingites from East Othris, Greece: Mineral reactions and physicochemical conditions of formation

Abstract The partly to pervasively metasomatized doleritic and gabbroic dykes or small to medium sized bodies found in East Othris, within Mid to Late Jurassic serpentinized peridotites of ophiolitic units and ophiolitic mélange formations are classified as rodingites and can be divided into two types. Type 1 rodingites are mainly characterized by the frequent occurrence of prehnite, while Type 2 rodingites include mostly garnets and vesuvianite. Isocon analysis showed that rodingitization essentially occurred with mass and volume preservation. Desilification, depletion of alkalies, as well as Ca enrichment was more intense for the Type 2 rodingites. Al, Fe and Mg remained rather immobile, while Ti, Y, Zr and REE were variably depleted. Rodingitization took place in an intraoceanic subduction system. It occurred in three successive stages during the exhumation of the mafic–ultramafic mantle wedge rocks in a fore-arc setting within a serpentinitic subduction channel, which developed close to the slab. The incorporation of the mafic rocks to the subduction channel probably resulted after entraining a directed mantle flow towards the slab. The first stage of rodingitization formed mainly grossular, hydrogrossular, Ti- and Cr-bearing hydrogarnets and calcite under relatively acidic and mildly oxidizing physicochemical conditions, with increased CO2/H2O ratio. During the second and more extensive rodingitization stage, alkaline and reducing conditions prevailed and CO2/H2O ratio was decreased. The modeling of the mineral reactions of this stage, using the software winTWQ v. 2.34 in the CFMASH system, reveals that in Type 1 rocks prehnite replaced most of the initial garnet, while Type 2 rocks continued to be rodingitized, mostly forming grossular and/or hydrogrossular and chlorite. Hydrogrossular, instead of grossular, was crystallized from hydrous fluids under high silica activity. Type 2 rodingites underwent further rodingitization during the third stage, due to infiltration of Ca-rich hydrothermal fluids of oceanic and/or subducted slab origin, at lower temperatures and depths. This stage is characterized by the appearance of hydroandradite and vesuvianite, under alkaline and oxidizing conditions, due to very low CO2/H2O ratio and relatively high fO2. All three rodingitization stages are estimated to have occurred under relatively moderate temperature and pressure (~ 300 to 400 °C; ~ 3–6 kbar respectively). Locally, Type 2 rodingites show derodingitization of variable extent, forming high-variance assemblages mostly consisting of chlorite ± pumpellyite. Some chlorite marginal zones in rodingite dykes may also have been developed by Mg-rich diffusional fluid flow, during this derodingitization process.