Petrogenesis of mantle peridotite and cumulate peridotite rocks from the Nagaland Ophiolite Complex, NE India

Abstract

Here, we report new geochemical data from mantle peridotite (spinel lherzolite, dunite, and harzburgite) and cumulate peridotite (websterite) rocks from the northernmost portion of the Nagaland Ophiolite Complex. Based on detailed textural, mineral (major element) chemistry, and whole-rock (major and trace element) chemistry, we reveal the origin of these rocks in different tectonic settings and their modification via the melt–mantle interaction process. The spinel lherzolite rocks with low Cr# (0.16–0.25) chrome spinel are mantle residues formed after low degrees (~10–15%) of partial melting in a mid-oceanic ridge setting, which has been subsequently modified in a subduction zone setting. However, the dunites and harzburgite samples are interpreted to be depleted mantle residue formed in a supra-subduction zone (SSZ) setting. Additionally, the dunites are inferred to be of replacive origin representing former harzburgites. Based on Cr# of chrome spinel, the dunites are classified into low Cr# (0.53–0.57), medium Cr# (0.73–0.79), and high Cr# (0.89–0.90) dunites. High Cr# dunite with low-TiO2 (0.01–0.06 wt%) possibly formed by interaction with a boninitic melt, whereas low-medium Cr# dunite with high TiO2 (0.14–0.34 wt%) formed by interaction with a high-Ti melt in an SSZ setting. Harzburgite samples with their highly resorbed orthopyroxene grain boundaries contain chrome spinels (Cr# = 0.84–0.88; TiO2 = 0.00–0.06 wt%) similar to the high Cr# dunite samples and are also interpreted to be formed by interaction with a boninitic melt. The websterite samples with their low Mg# (0.79) olivine and low Ni (345–413 ppm) content are interpreted to be of cumulate origin formed in an SSZ setting. However, the interstitial chrome spinel blebs with low Mg# (0.18–0.19), high Cr# (0.60–0.61), Fe3+ (0.20–0.21), and very high TiO2 (2.03–2.04 wt%) are interpreted to be a result of post-cumulus interaction of interstitial chrome spinel with high-Ti intercumulus melt. These mantle peridotites and cumulates of different tectonic origins were obducted and juxtaposed during the final stages of the India-Asia collision.