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

Abstract

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 (TMax ~ 335°C at ~6 kbar) to lawsonite blueschist (TMax ~ 340°C at ~11.5 kbar) to epidote eclogite (TMax ~ 550–660°C at ~24 kbar in Mokie to TMax ~ 630°C at ~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°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 M1 with TMax ~ 625°C at ~13.8 kbar and garnetiferous EBS facies M2 with TMax ~ 540°C at ~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 M2 cycle of metamorphism only. We relate this combined M1–M2 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.