Geochemical characteristics of mafic intrusive rocks from the Naga Hills Ophiolite, north‐east India: Constraints on petrogenesis and tectonic setting

Abstract

The Naga Hills Ophiolite (NHO) of Late Jurassic to Eocene age is one of the well‐exposed domains of Tethyan ophiolites, located in the northern margin of the Indo‐Myanmar Orogenic Belt, north‐east India. It is composed of suites of tectonite peridotite and cumulative ultramafic, mafic intrusive‐extrusive rocks, felsic intrusive, radiolarian, and marine pelagic sediments. The mafic intrusives of the NHO exhibit rhythmic to cryptic layering and isotropic in nature. Based on their mineralogy, the mafic intrusives are identified as gabbro, gabbronorite, hornblende gabbro, and olivine gabbro. The gabbros are adcumulate and mesocumulate and essentially composed of plagioclase feldspar, clinopyroxene, orthopyroxene, hornblende, with minor olivine, serpentine, chlorite, and magnetite. These mafic intrusive rocks show affinity to tholeiitic parental magma and fractionated trends in the multi‐elemental diagram, with negative anomalies at Zr, P, Ce and positive anomalies at Sr, Nb, and Ti suggesting that the magmatism was influenced by subduction zone components. The majority of the samples represent light rare earth elements (LREE) depletion (LaN/SmN = 0.28–0.86), except two samples that display LREE enrichment (LaN/SmN = 2.11–3.03), with negligible negative Eu anomaly. Petrogenetic modelling suggests 5%–20% partial melting of a depleted mid‐ocean ridge basalt‐type mantle source, within the spinel lherzolite stability field, as well as low degree partial melting (<10%) of an enriched mantle source probably metasomatized by sediment‐melt from a subducted slab. Therefore, based on mineralogical and geochemical data, we propose that the mafic intrusive of the NHO were generated in the intra‐oceanic arc system within the Neotethys Ocean with the active participation of possible wet basaltic melt from spinel‐lherzolite source‐mantle, subsequently modified by melts derived from sediments in a subduction zone.