Significance of aegirine‐bearing metabasic rocks in the metamorphic evolution of the Nagaland Accretionary Prism, northeast India

The ophiolite belt of Nagaland–Manipur states in Northeast India represents a segment of the ocean floor and upper mantle following eastward convergence of the Indian Plate with the Myanmar (Burmese) microplate during the Mesozoic. A variety of high‐pressure metamorphic assemblages have been noted in the metabasics and metacherts from the Phanerozoic ophiolite belt in the central part of the Naga Hills, which constitute the Nagaland Accretionary Prism. The metabasics are represented by very low‐grade assemblages of zeolite, prehnite‐pumpellyite, greenschists, and high‐pressure glaucophane schist and eclogite. We report the occurrence of aegirine‐bearing metabasic rocks, previously not recorded in the region. The metabasics are strongly fractionated and show chemical affinity with low‐K oceanic tholeiite. They are derived from a basic protolith of depleted mantle composition (viz. MORB). Pseudosection modelling reveals that aegirine formed at P–T conditions of c. 1.15 GPa and 490°C in the basic protolith in blueschist facies conditions. Similar P–T conditions have been reported from nearby localities, either as part of retrograde conditions or due to post‐peak cooling changes in the different metamorphic assemblages. However, the P–T observed from our study does not follow the cooling or retrograde path of the reported metabasics. Therefore, we suggest that the aegirine‐bearing metabasics might have formed at an earlier stage in the nascent forearc when temperatures were elevated enough to cause dehydration of the subducting slab to generate Na‐rich fluid fluxes at ambient pressures. The metabasic rocks located in the Purr area of Nagaland are part of the Nagaland Accretionary Prism comprising rocks of ophiolitic affinity, low pressure (LP) to ultra‐high pressure (UHP) metamorphosed crustal rocks along with pelagic sediments, which was obducted onto the continental crust during the India‐Eurasia collisional event. These metabasics exhibit occurrence of pure aegirine, previously not reported from the area, and is different from the commonly found aegirine–omphacite mineral that forms during retrograde metamorphism of eclogites. Mineral chemical studies and P–T modelling reveal that metabasic rock formed at c. 1.15 GPa and 490°C which does not follow the cooling path of the reported eclogites from the area. Thus, we suggest the formation of this aegirine‐bearing metabasic during an earlier stage of subduction due to Na‐metasomatism.