MANIPUR OPHIOLITIC COMPLEX AS A PROSPECTIVE HOST FOR MICRODIAMONDS IN EASTERN HIMALAYA, INDIA

A recent trend in studying subduction involves the use of microdiamonds as a tracer for subduction dynamics as well as characterizing the geochemical environment of the involved mantle.

In the Himalayan belt, ophiolite-hosted diamonds have been discovered in Luobusa Ophiolite, S. Tibet- associated with Indus-Yarlung Ophiolitic belt (IYOB). Geochemical characters of these diamonds are discussed by many previous workers. Till date, no discovery of microdiamonds has been reported from Manipur Ophiolitic Complex (MOC), E. Himalaya, India- which is genetically linked to the IYOB.

We studied data of chromite analyses from MOC, Luobusa, Nidar-Karzog - which are all part of IYOB. Diamond hosting chromites from Luobusa ophiolite show high Cr ratio (0.72-0.81) and moderately high Mg ratio (0.50-0.76). In MOC, deformed chromites from podiform chromitites show very high Cr ratio (0.76-0.81) and moderately high Mg ratio (0.49-0.71); very similar to that of Nidar-Karzog. Chromites of MOC and Nidar-Karzog fall in fields of ophiolitic chromitite of Boninitic affinity; showing close resemblance to the geochemistry of diamond hosting chromites of Luobusa. The distribution of platinum group elements in MOC chromitite also resemble that of Luobusa. The calculated parental melt composition for all studied rocks resemble that of subduction related Boninitic magma. Similarity of calculated parental melt with that of Oman Chromitite suggests parental melt for Tethyan ophiolites were similar in nature. Hence, we come to preliminary conclusion that these kinds of microdiamonds may be expected in the MOC. This is further supported by the recent discovery of Manganilmenite from MOC - a diamond indicator mineral. Presence of C bearing inclusion have been reported from Nidar chromitites. Geochemical similarity of Nidar and Luobusa chromitites, as revealed in this study, indicate Nidar is also a potential microdiamond host- which is confirmed by recent discovery of in situ diamond inclusions in peridotites of Nidar ophiolite. Further, studying the documented isotopic character of microdiamonds from Luobusa Ophiolite, their origin seems to be subducted Tethyan sediments.

The possibility of expecting microdiamonds in MOC poses constraints on geodynamic and mantle evolutions of E. Himalaya.