Rodingite Occurrence in Ciletuh Ophiolitic Mélange Complex (Indonesia): Evidence of Fluid-Rock Interaction in Subducted Oceanic Crust

The presence of ophiolitic mélanges in the Indonesian archipelago has been acknowledged as pivotal for the paleo-tectonic reconstruction of the Southeast Asia Region (southwestern Sundaland). However, the occurrence of rodingite as an integral component of ophiolite in Indonesia has not been thoroughly documented. The rodingites within the Ciletuh Ophiolitic Mélanges in Western Java are notably fresh and widespread, providing a valuable opportunity to initiate comprehensive research on rodingites, particularly focusing on the interplay of metasomatism between serpentinite and meta-gabbro. These metasomatic processes can provide valuable insights into reconstructing the evolution of P-T conditions and elemental transfer involving fluids in a subduction zone. This study aims to focus on field occurrences, petrography, whole-rock geochemistry, with the application of P-T pseudosection modeling to describe their metamorphic conditions.

Rodingitized gabbro in Ciletuh occur as dykes intersecting serpentinized peridotite with sharp and diffuse contact. These dykes are controlled by faults in direction opposite perpendicular or diagonal to the shearing direction of serpentinites. Sheared-foliated rodingite and serpentinite are common, usually forming boudinages. Rodingites are classified into six types which are: (1) Fine to medium grain clinopyroxene rich, (2) Fine to medium grain hydrogarnet rich, (3) Coarse grain, (4) Schistose, (5) Diffused serpentinite-rodingite at peripheral dyke, and (6) Diffused serpentinite-rodingite embedded within serpentinite.  Type (1) to (3) are dykes which mostly composed by garnetized plagioclase (47-49%) and clinopyroxene (29-32%), orthopyroxene (20%), olivine (<3%), epidote (<5%), zoisite (<3%) with minor spinel and magnetite (2-4%). Type (4) is comprised of foliated mineralogical domains such as carbonate-rich and chlorite-rich domain. Types (5) and (6) are characterized with diffused contact of rodingite (hydrogarnets) and chloritized serpentinite. Chlorite reaction zones can be encountered at the contacts between rodingite and serpentinite, typically indicated by the presence of chloritized groundmass, chloritized serpentine mesh, chlorite and hydrogarnet veinlets, as well as partial clinopyroxene rim around hydrogarnet.

The general whole rock geochemical characteristics of rodingitized gabbro are indicated by low SiO₂ content (34-42 wt%), high CaO (3-16 wt%), high MgO (17-32 wt%), medium Al₂O₃ (5-10 wt%), and FeOt (5-18 wt%), with high LOI (5-10 wt%). Significant whole rock geochemical variations characterize the serpentinite-rodingite reaction zones in both the sharp dykes (Type 1-3) and diffused (Type 5-6) contact. Serpentinite near the reaction zone exhibits enrichment in Al₂O₃, CaO, but depletion in MgO, FeO, and SiO₂. Conversely, rodingitized gabbro experiences enrichment in MgO, slight enrichment in FeO, and depletion in CaO and Al₂O₃.  

P-T psedosection models show that rodingitization occured concurrently with serpentinization during medium to low-grade metamorphism at temperatures ranging from 200 to 490˚C. This process involved diffusional metasomatism with fluids derived from serpentinization and metamorphism of gabbro (e.g., Ca, Mg, and Al), considering the addition of H₂O and other potential fluids during the exhumation of ophiolitic mélanges in the subduction slab.