No abstract

The Indo-Myanmar Ranges (IMR) of NE India are host to various ophiolitic rocks, including metamorphosed Alpine-type harzburgite and lherzolite. Compared to abyssal peridotites of normal oceanic lithosphere, these ultramafic rocks are enriched in trace and rare earth elements. Spilitic pillow lavas along with mafic dykes and sills locally intruded into the serpentinized ultramafic rocks and associated pelagic sediments exhibit alkaline compositional affinities. Ophiolite formation and emplacement were by a process analogous to that described for mantle exhumation in hyper-extended continental margin settings and ophiolites in parts of the European Alps, involving very slow passive continental margin rifting accompanied by slow upwelling or extensional unroofing of the subcontinental upper mantle up to the seafloor. Preliminary palaeomagnetic measurements conducted on ultramafic rocks within the IMR ophiolite belt give a virtual geomagnetic pole (VGP) at 478 N, 0458 E for thermal demagnetization (TDM) measurements and 338 N, 0138 E for the alternating field demagnetization (AfD) measurements, requiring an anticlockwise rotation of the ultramafic bodies by 148 during the subduction process. The original trend of the spreading axis of the ophiolites was probably NE-SW, with spreading directed NW-SE. Computation of palaeolatitude of the ultramafic rocks gives an average value of 24.678. Comparison between the palaeolatitude and the present latitude of the sample sites provides a mere latitudinal shift of less than 18.Field studies, combined with an analysis of structural and tectonic features in the IMR, suggest a generalized WNW-ESE (east-west) compression and NNE-SSW (north-south) extension contradictory to the NNE-SSW contraction indicated by seismic data. Area balancing techniques employed along sections orientated perpendicular to regional tectonic strike in the IMR reveal systematic variations in the amount of crustal shortening, with a maximum of approximately 60% recorded in the Nagaland-Manipur segment along 25.6448 N, 93.8268 E -25.0768 N, 95.8978 E. The amount of shortening gradually decreases away from the axis of maximum shortening and on both sides. Calculations of relative plate motion based on rotation vectors given by different workers for various plate pairs represented in the region reveal that the interaction between the Indian and Myanmar plates can ideally produce the structural and tectonic features of this range. Dextral shear coupled to oblique subduction of the Indian Plate below the Myanmar Plate can best explain all of the structural and tectonic features present in the IMR.

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.

The Upper Disang-Lower Barail Transition Sequences of Nagaland in northeast India, Indo-Myanmar Ranges (IMR) are found to preserve reasonably diverse biological sedimentary structures. Three ichnofacies were observed, Skolithos ichnofacies consisting of Bergaueria hemispherica, Gyrochorte comosa, Ophiomorpha nodosa, Skolithos linearis and Thalassinoides paradoxicus; Cruziana ichnofacies – Curvolithus simplex, Scolicia plana, Scolicia strozzii, Scolicia vertebralis, Treptichnus pedum, Taenidium diesingi and Palaeophycus tubularis; and Nereites ichnofacies – Chondrites targionii and Paleomeandron elegans. Presence of Skolithos and Cruziana ichnofacies suggest well oxidising, substrate rich depositional environment in the foreshoreface to offshore setting in general; and an overall possible fluctuation in the depositional bathymetry which is supported by the presence of Nereites ichnofacies in horizons alternating with Skolithos and Cruziana ichnofacies. The depositional system is proposed to be proximal to distal hyperpycnal-delta-fed turbidite system. Observation of sand spherules also suggests possibility of shoreline as one of the depositional environments.