Rare earth elements and stable isotope geochemistry of carbonates from the mélange zone of Manipur ophiolitic Complex, Indo-Myanmar Orogenic Belt, Northeast India

Abstract

Carbonates that occur in the melange zone of Manipur ophiolitic complex (MOC), Indo-Myanmar Orogenic Belt, Northeast India contain diverse fauna with dominance of foraminifera assemblages (planktonic and benthic) and calcareous nanofossils. Rare earth element (REE) contents (~51 ppm average value) in the MOC carbonates are high compared with the average value of typical marine carbonate (~28 ppm). The Post-Archean Australian Shale (PAAS)-normalized REE patterns of these carbonates exhibit seawater-like REE patterns with light rare earth elements (LREE) depletion and relative heavy rare earth elements (HREE) enrichment with negative Ce anomalies (Ce/Ce* = 0.28–0.69) and positive Eu anomalies (Eu/Eu* = 1.07–2.08). The carbonates show depleted values of δ13C ‰ (PDB) (0.89–2.74 ‰) which characterize marine precipitates while depletion in δ18O ‰ (PDB), reaching minimum values (−6.29 to −11.40 %). The negative anomalies of Ce in these carbonates suggest that they were deposited under oxygenated environment. The observed shale-normalized positive Eu anomalies and spread in negative δ18O values to a lesser extent of δ13C ‰ (PDB) values of these carbonates suggest their formation were altered by diagenesis in shallow marine environment. Field, petrographical studies in conjunction with REE and stable isotope characteristics suggest that these carbonates form part of the ophiolitic melange zone that emplaced during subduction and obduction processes of the Indian plate and Myanmar plate collision.