Source characterisation of the residual lherzolite and harzburgite ultramafics of Manipur Ophiolite Belt in the Indo–Myanmar Hill ranges, NE India

Abstract

An attempt for the calculation of primordial mineralogy of the source rock of the ultramafics of Manipur ophiolite, north‐east India is made with the use of an established schematic ternary phase diagram of the anhydrous system Forsterite‐Diopside‐Enstatite at 20 kb pressure for a hypothetical upper mantle rock (X). For the case of batch partial melting, if the proportions of the phases entering the melt (Pα) are different from their initial proportion in the rock (X0α), then the weight fraction of the melt (F) at which each phase in question is consumed is given by F = X0α/Pα. From the phase diagram, the bulk composition of the source rock X can be expressed as 0.4 Di, 0.2 En, and 0.4 Fo (these are the X0α values). The proportion (Pα) in which the various phases entered the melt is initially given by the proportion in the eutectic composition E, that is, 0.7 Di, 0.2 En, 0.1 Fo. With the value of Fcpx = 0.4/0.7 = 0.57, it is found that the diopside is the only phase to be completely consumed into the partial melt, which occurs at 57% partial melting. The least serpentinized group of ultramafics of the Ophiolite Belt in Manipur is considered to be the residue of those which have suffered the least percent of partial melting (about 5%), and the average proportions of the three main phases of such rocks are taken as yardsticks for necessary calculation of the primordial mineralogy of the upper source rock. From the computed value of Fcpx (Mo) = 0.14, it is inferred that the diopside in the original source rock of Manipur Ophiolite must be completely melted at about 15% partial melting as against 57% of the hypothetical mantle rock discussed in the phase diagram. Thus, at about 15% partial melting of the source rock of Manipur Ophiolite, the levels of melting of the three main phases, as derived from the phase diagram, is calculated to be, Diopside = 100%, Enstatite = 57%, and Olivine = 14%. From these values of levels of melting, the different degrees of melting of the three main phases have been computed separately for 5%, 10%, 15%, and 20% bulk partial melting. From the modal proportions of the main phases of the least serpentinized ultramafics of Manipur Ophiolite (considered to be a residue of 5% partial melting), the mineralogical proportions of the original source rock are computed and these are found to correspond to Spinel Lherzolite composition.