Petrogenesis of mantle peridotites in Neo‐Tethyan ophiolites from the Eastern Himalaya and Indo‐Myanmar Orogenic Belt in the geo‐tectonic framework of Southeast Asia

Abstract

The Neo‐Tethyan ophiolites of the Tuting‐Tidding Suture Zone (TTSZ), Eastern Himalaya (viz., Tidding Ophiolite Complex and Mayodia Ophiolite Complex) and Indo‐Myanmar Orogenic Belt (IMOB, i.e., Nagaland Ophiolite Complex and Manipur Ophiolite Complex) which lie along the southern extension of the Indus‐Tsangpo Suture Zone have been collectively investigated through the mantle‐derived peridotite sequence. The peridotites (harzburgite and dunites) in the Eastern Himalaya ophiolites are refractory, constrained by the high Fo olivine content (~95), high Cr# [Cr/(Cr + Al)] (0.90–0.99), as well as by the parental melt composition of Cr‐spinels (Al2O3 melt = 3.05–7.55 wt%, FeO/MgO = 0.69–6.46). They have slightly “U‐shaped” chondrite‐normalized Rare Earth Elements (REE) patterns with slight enrichment in Light Rare Earth Elements (LREE) relative to the patterns expected for residues of partial melting, thereby indicating a reaction with the LREE‐enriched melt. These peridotites represent residual portions of a depleted/enriched mantle that underwent partial melting up to 23% in the nascent forearc of an intra‐oceanic subduction zone, and later metasomatized by high‐temperature silicate melts and low‐temperature hydrous fluids. They are composed of a very refractory olivine‐spinel assemblage (Fo: 91.68–96.44; Cr#: 0.90–0.99), corroborating a boninitic parentage, with influence from melt‐rock interactions. In the case of the IMOB ophiolites, a wide range of chemical compositions is observed in the mantle sequence. Lherzolites display low Cr# (0.12–0.26) and TiO2 (<0.11) associated with high Mg# [Mg/(Mg + Fet)] (0.69–0.76) in the Cr‐spinels present in them. They represent the residual product of a fertile mantle that underwent low‐degree partial melting (2%–10%) in a divergent mid‐ocean ridge (MOR) tectonic setting. On the other hand, the harzburgites and dunites of the IMOB have high Cr# (0.84–0.90) and low TiO2 (<0.06 wt%) Cr‐spinels and exhibit slightly U‐shaped REE distributions indicating their derivation from a highly depleted mantle source, which experienced higher degree partial melting (17%–24%) similar to those of supra‐subduction zone (SSZ) harzburgites and dunites of the TTSZ peridotites. The occurrence of both MOR and SSZ types of melting regimes indicates that the peridotites in the IMOB ophiolites formed at two major different stages of the pre‐subduction and subduction events, respectively. However, the peridotites of Eastern Himalaya ophiolites were formed only during subduction tectonics. Thus, we argue that the mantle peridotites in the ophiolites of northeast India evolved from lherzolite through clinopyroxene‐harzburgite and harzburgite then to highly refractory dunite, supporting multistage melting and melt‐rock reaction processes during their generation.