Tectonic framework of the high-pressure metamorphic rocks of the Nagaland Ophiolite Complex, North-east India, and its geodynamic significance: A review

In this study, we made a review of the published metamorphic and geochronological data of an array of high-pressure metamorphic rocks from the Nagaland Ophiolite Complex, North-east India, and present them in the tectonic framework of two temporally separate subduction systems within the eastern arm of the Neo-Tethys. The metamorphic rocks are studied in terms of three structural units. The structurally lowermost Mokie-Satuza-Thewati (MST) unit shows a metamorphic zonation from greenschist through pumpellyite-diopside (T-Max similar to 335 degrees C at similar to 6 kbar) to lawsonite blueschist (T-Max similar to 340 degrees C at similar to 11.5 kbar) to epidote eclogite (T-Max similar to 550-660 degrees C at similar to 24 kbar in Mokie to T-Max similar to 630 degrees C at similar to 26-28 kbar in Thewati localities) facies structurally upward. Both the blueschists and eclogites record a clockwise (CW) P-T path of evolution. We relate the low apparent thermal gradient at the metamorphic peak (7-9 degrees C/km), the steep dP/dT slope of the metamorphic field gradient, the general CW metamorphic P-T paths, and the published U-Pb zircon dates in the eclogites (between ca. 205 and ca. 172 Ma) and radiolarian biostratigraphy data in the MST metamorphic sequence with the development of a Late Triassic to Middle Jurassic-aged cold mature stage of an intra-oceanic subduction system within the Neo-Tethys. This is considered to be one of the oldest in the Yarlung-Tsangpo Suture Zone. The metamorphosed uppermost part of the structurally intermediate unit, named the Moya-Salumi (MS) sequence, consists of a structurally upper hornblende-eclogite (Hbl-EC) to epidote amphibolite facies subunit and a lower epidote blueschist (EBS) facies subunit. The eclogite subunit is polyphase metamorphosed (Hbl-EC facies M-1 with T-Max similar to 625 degrees C at similar to 13.8 kbar and garnetiferous EBS facies M-2 with T-Max similar to 540 degrees C at similar to 14.4 kbar) with both the events recording counterclockwise P-T paths. The monocyclic EBS facies subunit, on the other hand, has a memory of the M-2 cycle of metamorphism only. We relate this combined M-1-M-2 cycles of metamorphism in the MS sequence to a second oceanic subduction beneath the continental Naga Metamorphics, the latter constituting the structurally uppermost unit. The metamorphic pulses together reflect progressive cooling of the second subduction channel from an initially warm stage during subduction infancy to a later cold mature stage of subduction. The different metamorphic rocks in the MST and MS units reveal architectures of two temporally separate accretionary complexes. Their present occurrence as a tectonic collage is likely to be the product of the Eocene collision of the Indian and the Myanmar plates.

Automated summary

This study reviewed metamorphic and geochronological data of high-pressure metamorphic rocks from the Nagaland Ophiolite Complex in North-east India and placed them in the tectonic framework of two temporally separate subduction systems. It analyzed metamorphic zonation, facies, and evolutionary paths, linking them to Late Triassic to Middle Jurassic-aged subduction systems. The study also identified two distinct accretionary complexes within the area, likely formed during the Eocene collision of the Indian and Myanmar plates.