EGU24-15695, updated on 09 Mar 2024
EGU General Assembly 2024
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

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

Monika Chaubey and A. Krishnakanta Singh
Monika Chaubey and A. Krishnakanta Singh
  • Wadia Institute of Himalayan Geology, Geochemistry, Dehradun, India (

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.

How to cite: Chaubey, M. and Singh, A. K.: Geochemical Insights into Neo-Tethyan Ophiolites of Indo-Myanmar Orogenic Belt, Northeast India: Multistage Melting and Supra-Subduction Zone Modification of Upper Mantle Rocks., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15695,, 2024.