In this study, we have deduced the thermal history of the subducting Neotethys from its eastern margin, using a suite of partially hydrated metabasalts from a segment of the Nagaland Ophiolite Complex (NOC), India. Located along the eastern extension of the Indus-Tsangpo suture zone (ITSZ), the N-S-trending NOC lies between the Indian and Burmese plates. The metabasalts, encased within a serpentinitic m elange, preserve a tectonically disturbed metamorphic sequence, which from west to east is greenschist (GS), pumpellyite-diopside (PD) and blueschist (BS) facies. Metabasalts in all the three metamorphic facies record prograde metamorphic overprints directly on primary igneous textures and igneous augite. In the BS facies unit, the metabasalts interbedded with marble show centimetre-to metre-scale interlayering of lawsonite blueschist (LBS) and epidote blueschist (EBS). Prograde HP/LT metamorphism stabilized lawsonite + omphacite (X Jd = 0.50-0.56 to 0.26-0.37) + jadeite (X Jd = 0.67-0.79) + augite + ferroglaucophane + high-Si phengite (Si = 3.6-3.65 atoms per formula unit, a.p.f.u.) + chlorite + titanite + quartz in LBS and lawsonite + glaucophane/ferroglaucophane AE epidote AE omphacite (X Jd = 0.34) + chlorite + phengite (Si = 3.5 a.p.f.u.) + titanite + quartz in EBS at the metamorphic peak. Retrograde alteration, which was pervasive in the EBS, produced a sequence of mineral assemblages from omphacite and lawsonite-absent, epidote + glaucophane/ferroglaucophane + chlorite + phengite + titanite + quartz through albite + chlorite + glaucophane to lawsonite + albite + high-Si phengite (Si = 3.6-3.7 a.p.f.u.) + glaucophane + epidote + quartz. In the PD facies metabasalts, the peak mineral assemblage, pumpellyite + chlorite + titanite + phengitic white mica (Si = 3.4-3.5 a.p.f.u.) + diopside appeared in the basaltic groundmass from reacting titaniferous augite and low-Si phengite, with prehnite additionally producing pumpellyite in early vein domains. In the GS facies metabasalts, incomplete hydration of augite produced albite + epidote + actinolite + chlorite + titanite + phengite + augite mineral assemblage. Based on calculated T-M(H 2 O), T-M(O 2 ) (where M represents oxide mol.%) and P-T pseudosections, peak P-T conditions of LBS are estimated at~11.5 kbar and~340°C, EBS at~10 kbar, 325°C and PD facies at~6 kbar, 335°C. Reconstructed metamorphic reaction pathways integrated with the results of P-T pseudosection modelling define a near-complete, hairpin, clockwise P-T loop for the BS and a prograde P-T path with a steep dP/dT for the PD facies rocks. Apparent low thermal gradient of 8°C km À1 corresponding to a maximum burial depth of 40 km and the hairpin P-T trajectory together suggest a cold and mature stage of an intra-oceanic subduction zone setting for the Nagaland blueschists. The metamorphic constraints established above when combined with petrological findings from the ophiolitic massifs along the whole ITSZ suggest that intra-oceanic subduction systems within the Neotethys between India and the Lhasa t...

Results of biostratigraphic and geochronological investigations in eastern Nagaland and Manipur, NE India, provide new constraints on the tectonic evolution of the western margin of the Burma microplate. U/Pb zircon ages indicate that the Naga Hills ophiolite developed in a suprasubduction zone setting as part of an intraoceanic island arc developed during late Early Cretaceous (mid‐Aptian) time and is younger than similar rocks exposed along the Indus‐Yarlung Tsangpo suture zone. Radiolarian microfossils provide Jurassic and Cretaceous age constraints for Tethyan ocean floor sediments that were subducted beneath the forming ophiolite. Timing of the emplacement of these rocks onto the passive margin of eastern India is constrained by Paleocene/Eocene radiolarians in sediments over which the ophiolitic assemblage has been thrust. Previously undated schists and gneisses in the Naga Metamorphics are of Early Ordovician age, and their sedimentary protolith was most likely derived from sources in the south of Western Australian and East Antarctica. After Barrovian‐style metamorphism, these rocks were uplifted and eroded becoming an important source of detritus shed into the Eocene Phokphur Formation. This unit also contains abundant clasts sourced from the disrupted basement of the Naga Hills ophiolite, which it overlies. It also contains Permo‐Triassic‐aged detritus eroded off an enigmatic source that was possibly a continental convergent margin arc system somewhere along the northern margin of Gondwana.

High-P metamorphic rocks that are formed at the onset of oceanic subduction usually record a single cycle of subduction and exhumation along counterclockwise (CCW) P-T paths. Conceptual and thermo-mechanical models, however, predict multiple burial-exhumation cycles, but direct observations of these from natural rocks are rare. In this study, we provide a new insight into this complexity of subduction channel dynamics from a fragment of Middle-Late Jurassic Neo-Tethys in the Nagaland Ophiolite Complex, northeastern India. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of a medium-grade amphibolite tectonic unit within a serpentinite m elange, we establish two overprinting metamorphic cycles (M 1 -M 2 ). These cycles with CCW P-T trajectories are part of a single tectonothermal event. We relate the M 1 metamorphic sequence to prograde burial and heating through greenschist and epidote blueschist facies to peak metamorphism, transitional between amphibolite and hornblende-eclogite facies at 13.8 AE 2.6 kbar, 625 AE 45°C (error 2r values) and subsequent cooling and partial exhumation to greenschist facies. The M 2 metamorphic cycle reflects epidote blueschist facies prograde re-burial of the partially exhumed M 1 cycle rocks to peak metamorphism at 14.4 AE 2 kbar, 540 AE 35°C and their final exhumation to greenschist facies along a relatively cooler exhumation path. We interpret the M 1 metamorphism as the first evidence for initiation of subduction of the Neo-Tethys from the eastern segment of the Indus-Tsangpo suture zone. Reburial and final exhumation during M 2 are explained in terms of material transport in a large-scale convective circulation system in the subduction channel as the latter evolves from a warm nascent to a cold and more mature stage of subduction. This Neo-Tethys example suggests that multiple burial and exhumation cycles involving the first subducted oceanic crust may be more common than presently known.

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