Geochemical Insights into Neo-Tethyan Ophiolites of Indo-Myanmar Orogenic Belt, Northeast India: Multistage Melting and Supra-Subduction Zone Modification of Upper Mantle Rocks.

Upper mantle rocks from tectonically emplaced fragments of the oceanic lithosphere are generally considered to represent variably depleted residues of the oceanic upper mantle remaining after mantle melting and crust-mantle segregation. Geochemical data of these mantle rocks and their compositions of mineral phases are pondered as a powerful petrogenetic indicator and their chemical compositions are dependent on the conditions and degree of partial melting and melt-rock interactions and also contribute to our understanding of the original tectonic setting of lithosphere generation. The Neo-Tethyan ophiolites of the Indo-Myanmar Orogenic Belt (IMOB), northeast India which lie along the southern extension of the Indus-Tsangpo Suture Zone (ITSZ) are investigated through the mantle-derived peridotite sequence. The lithology of the IMOB comprises ultramafic tectonic (dunite–harzburgite–lherzolite), ultramafic–mafic cumulates (pyroxenite–gabbro), mafic intrusives, volcanic, and volcano-clastics dominated by basalt, spilite, and marine sediments. A wide range of chemical compositions is observed in the mantle sequence of the IMOB ophiolites. 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. Conversely, the harzburgites and dunites 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. The dunite is composed of a very refractory olivine-spinel assemblage (Fo: 92.1-93.6; Cr#: 71-83), corroborating a boninitic parentage, with influence from melt-rock interactions. The NMO also hosts both refractory grade high-Al chromitites (0.46 < Cr# < 0.53) and metallurgical grade high-Cr chromitites (0.71 < Cr# < 0.79). The high-Al chromitites originated from MORB-like melts whereas high-Cr chromitites were crystallized from a boninitic melt. The available isotopic ages reveal two episodes of the IMOB ophiolites formation at 148 Ma (K–Ar ages) and 118-117 Ma (U–Pb age). Upper mantle rocks show lower concentration of PGE (Rh < 2 ppb; Pd < 25 ppb; Re < 16 ppb; Pt = < 10 ppb; Au < 28 ppb; Os < 9 ppb; Ir < 3 ppb; Ru = 5-11 ppb). And the total PGE content (60-190 ppb) of the high-Al chromitites is less as compared with the total PGE content (118-2341 ppb) in high-Cr chromitites. The occurrence of both MOR and SSZ types of melting regimes indicates that the peridotites along with chromitites in the IMOB ophiolites formed at different stages of the pre-subduction period and subduction, respectively. Thus, we argue that the upper mantle of the NMO of the IMOB has been modified by a substantial amount of supra-subduction zone components after initially being formed in a mid-ocean ridge tectonic environment supporting multistage melting and melt-rock reaction processes.