Geochemical and isotopic constraints on the petrogenesis and tectonic setting of basalts in the Nagaland Ophiolite Complex, Northeast India

Abstract

The Nagaland Ophiolite Complex (NOC) is exposed at the eastern margin of the Indo-Burma Range, which represents the suture zone between the Indian and Burmese Plates. The NOC that contains a complete mantle and crustal lithological assemblage can provide important information regarding the tectonic evolution of the Neo-Tethys Ocean. The studied basaltic rocks from the NOC can be classified into two groups according to their geochemical results. The Group I rocks are geochemically comparable with the E-MORB, which have slight LREE enrichment ((La/Yb)_N = 1.45–1.96). The Group II rocks share geochemical characteristics with typical N-MORB and exhibit LREE depletion, with (La/Yb)_N of 0.47–0.69. All samples showed slightly positive or insignificant anomalies in high-field-strength elements. The Group I and Group II rocks were derived from the partial melting of spinel-facies mantle peridotite, and the degrees of partial melting of the former (approximately 5 %) are lower than those of the latter (approximately 10 %). Both groups of basaltic rocks are characterized by high positive ε_Nd(t) values (+7.0 − +7.6 and + 5.4 − +8.3, respectively). Our study revealed that the Group I rocks were predominantly derived from a MORB-like asthenospheric source that was variably enriched by OIB-type components, whereas the Group II rocks were derived from a depleted asthenospheric mantle source without the contribution of OIB-type components. The heterogeneous asthenospheric mantle sources contributed to the geochemical complexities of the basalts from the NOC. These results, combined with regional geological records, suggest that the basalts from the NOC were mainly generated in a mid-ocean ridge setting during the evolution of the Neo-Tethys Ocean.