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 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...

Petrogenesis and deformation history of the lawsonite‐bearing blueschist facies metabasalts of the Diamante‐Terranova oceanic unit (southern Italy)

“…The obtained peak conditions are remarkably in line with estimates proposed for the lawsonite‐blueschists from the Northern Qilian belt (displaying an extremely similar pumpellyite‐bearing paragenesis; Zhang, Wang, & Song, ), as well as with those for other low‐ T lawsonite‐blueschists worldwide (e.g. Alpine Corsica Tenda unit, Molli et al., ; Nagaland Ophiolite Complex, Ao & Bhowmik, ; Sivrihisar Massif, Davis & Whitney, ; Sarikaya sliver, NW Turkey, Topuz, Okay, Altherr, Satir, & Schwarz, ). Even more significantly, the proposed peak conditions are consistent with previous estimates for the DIATU from Liberi et al.…”

Tectonics and Evolution of the Trans-Himalayan Mountains and Nagaland Ophiolite Belt

“…2B) (Ao and Bhowmik, 2014). Metabasalts in all the three metamorphic facies have recorded prograde metamorphic overprints directly on primary igneous textures and igneous augite.…”

“…The blueschist facies body of finely banded lawsonite blueschist and epidote blueschist facies metabasalts and marble has recorded peak P-T conditions at ~11.5 kb, ~340 o C and a single cycle, near-complete, hairpin, clockwise P-T loop ( Fig. 2B; Ao and Bhowmik, 2014). Apparent low thermal gradient of 8 o C km -1 , corresponding to a maximum burial depth of 40 km and the hairpin P-T trajectory, were considered as robust evidence for a cold and mature stage of an intra-oceanic subduction zone setting for the Nagaland blueschists (Ao and Bhowmik, 2014).…”

“…2B) and post-peak decompression (11 kb, 540°C) metamorphic stages, and proposed retrograde origin for garnetiferous blueschists in outer shells of eclogitic cores. Ao and Bhowmik (2014) and Bhowmik and Ao (2016) proposed a two-fold classification of the NOC to decipher the anatomy of accretionary wedge and signatures of suture between the Indian and Asian/ Burmese plates (Fig. 2B): (i) westernmost part of the Nagaland Accretionary Complex (NAC) of a collage of metasedimentary rocks, metabasalts, metagabbros and serpentinites with a range of metamorphic conditions from greenschist through pumpellyite-diopside, blueschist to eclogite facies (Fig.…”

“…2B; Ao and Bhowmik, 2014). Apparent low thermal gradient of 8 o C km -1 , corresponding to a maximum burial depth of 40 km and the hairpin P-T trajectory, were considered as robust evidence for a cold and mature stage of an intra-oceanic subduction zone setting for the Nagaland blueschists (Ao and Bhowmik, 2014).…”