Tectonic slices of greenschist – epidote blueschist – epidote amphibole schist – garnet epidote amphibole schist from the Nagaland ophiolite complex, NE India: a look into their metamorphic and tectonic evolution

ABSTRACT

Tectonic slices of greenschist – epidote blueschist – epidote amphibole schist – garnet epidote amphibole schist are exposed in the northern part of the NOC. Metamorphic P-T conditions and P-T paths of the garnet epidote amphibole schist and epidote blueschist were determined with the help of pseudosection calculations in the MnNCKFMASHTO system using the software Perple_X. The garnet epidote amphibole schist records a pre-peak metamorphic stage at ~12 kbar and ~360°C in the lawsonite blueschist facies field. Lawsonite and glaucophane are not preserved in the rock and are attributed to progressive metamorphic reactions during prograde metamorphism. The peak metamorphic stage is constrained at ~14 kbar and ~570°C in the hornblende eclogite facies field. Post-peak metamorphism shows slight cooling and decompression from ~550°C and ~13 kbar, possibly down to ~350°C and ~7 kbar in the greenschist facies field. These three metamorphic stages reveal a clockwise metamorphic P-T path of evolution for the garnet epidote amphibole schist. The geotherm at metamorphic peak P-T condition is ~12.5°C/km, which is hotter by ~350°C compared to thermal models of mature subduction zones but shows good agreement with models of warm subduction zones. Therefore, the low geothermal gradient of ~8°C/km during the prograde metamorphism possibly reflects the subduction of cold Neo-Tethyan ocean crust when the temperature conditions of the subduction zone were still warm. The epidote blueschist shows peak metamorphism at ~10 kbar and 380°C with a clockwise P-T path. It indicates metamorphism at a more mature stage of subduction. Buoyancy-driven exhumation along the subduction channel due to the serpentinization of the upper mantle wedge played a vital role in the detachment and exhumation of the tectonic slivers of metamorphic rocks from different depth sections in the northern margin of the NOC.

KEYWORDS:

• P-T pseudosection
• warm subduction
• clockwise P-T path
• Nagaland ophiolite complex

Acknowledgments

The scientists and staff at the XRF lab WIHG Dehradun and EPMA-National Facility at IIT Kharagpur are thanked for their help during the analyses. This manuscript has benefitted from constructive comments and detailed reviews by two anonymous reviewers and competent editorial handling by Robert J. Stren. AA is also thankful to Watinaro Imsong for the help in digitising and preparing the geological map and cross-section of the NOC. AA acknowledges the support and local hospitality extended by the Villagers and Church elders at Wui, Kenjong and Chipur villages during the fieldwork. DST-FIST program awarded to the Centre is also gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Additional supporting information may be found in the online version of the article at the publisher’s website.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2263769.

Additional information

Funding

This work was supported and funded by DST-INSPIRE Faculty Project [DST/INSPIRE/04/2016/002214] awarded to A.A.