Cold subduction of the Neotethys: the metamorphic record from finely banded lawsonite and epidote blueschists and associated metabasalts of the Nagaland Ophiolite Complex, India

Abstract

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 mélange, 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 ± epidote ± 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(H₂O), T–M(O₂) (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⁻¹ 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 terrane/the Karakoram microcontinent were also active towards east between Indian and Burmese plates.