Geochemical and geochronological study of rodingites from the northeast India ophiolites: Petrogenetic significance and timing of rodingitization

Abstract

The Indo‐Myanmar Range is thought to represent a relict eastward‐dipping subduction zone that runs from the eastern edge of the Himalayan Range. In the Indo‐Myanmar Ophiolite (IMO) Belt rodingite is present as discontinuous lenses, dikes, and small‐ to medium‐sized blocks in the serpentinized zones of ultramafic rocks. Petrological and geochronological studies of rodingite from the IMO Belt have been carried out to characterize and elucidate the timing and genesis of the rodingite. Petrography and mineral chemistry indicate that rodingites are composed of grossular garnet, vesuvianite, diopside, tremolite, actinolite, chlorite, albite, prehnite, apatite, and sphene (titanite). The investigated rodingite samples show variability in mineralogical assemblage and geochemical composition. Two mineral assemblages have been identified: (a) grossular, diopside, vesuvianite, apatite, and titanite, observed in veins and dikes; (b) chlorite, epidote, prehnite, diopside, apatite, anatase, and sphene, observed in the samples exposed as blocks. Rodingites are classified into different categories based on their occurrence, and mineralogical and chemical composition. Subtype 1B rodingite is enriched in Al2O3 and incompatible trace elements, most likely formed during seawater infiltration as it contains minerals typically formed during ocean floor alteration. Subtypes 1A and Type 2 rodingite are highly enriched in CaO, Fe2O3T, MgO, and compatible trace elements. The formation of these types of rodingites probably followed serpentinization of the oceanic lithosphere as they contain minerals characteristic of high‐Ca, low‐Si fluid infiltration. These variations might have resulted due to infiltration of a metasomatic fluid that interacted with two different protoliths or infiltration of different fluids that may have interacted (at different times) with the same protolith. Complex variations in mineral assemblages and chemical compositions of Subtype 1 and Type 2 rodingites are the result of compositional variations in their protoliths and the degree of rodingitization. Our data indicate that the T of formation for the investigated rodingites varied from ~435 to ~200°C, corresponding to greenschist facies. The T estimates deduced from the mineralogy of rodingites are consistent with the ambient T during cooling of an oceanic lithospheric slab. Development of a low‐ to medium‐T oceanic environment may be a result of exothermic reactions concurrent to serpentinization. The rodingitization in the study area most likely happened within a rift‐produced oceanic setting during a late period of cooling between 114 and 137 Ma as evidenced by zircon geochronological data.