Significance of aegirine‐bearing metabasic rocks in the metamorphic evolution of the Nagaland Accretionary Prism, northeast India

Abstract

The ophiolite belt of Nagaland–Manipur states in Northeast India represents a segment of the ocean floor and upper mantle following eastward convergence of the Indian Plate with the Myanmar (Burmese) microplate during the Mesozoic. A variety of high‐pressure metamorphic assemblages have been noted in the metabasics and metacherts from the Phanerozoic ophiolite belt in the central part of the Naga Hills, which constitute the Nagaland Accretionary Prism. The metabasics are represented by very low‐grade assemblages of zeolite, prehnite‐pumpellyite, greenschists, and high‐pressure glaucophane schist and eclogite. We report the occurrence of aegirine‐bearing metabasic rocks, previously not recorded in the region. The metabasics are strongly fractionated and show chemical affinity with low‐K oceanic tholeiite. They are derived from a basic protolith of depleted mantle composition (viz. MORB). Pseudosection modelling reveals that aegirine formed at P–T conditions of c. 1.15 GPa and 490°C in the basic protolith in blueschist facies conditions. Similar P–T conditions have been reported from nearby localities, either as part of retrograde conditions or due to post‐peak cooling changes in the different metamorphic assemblages. However, the P–T observed from our study does not follow the cooling or retrograde path of the reported metabasics. Therefore, we suggest that the aegirine‐bearing metabasics might have formed at an earlier stage in the nascent forearc when temperatures were elevated enough to cause dehydration of the subducting slab to generate Na‐rich fluid fluxes at ambient pressures.

Interaction of seawater with (ultra)mafic alkaline rocks—Alternative process for the formation of aegirine

“…It mainly occurs in differentiated alkaline rocks (alkaline granites/rhyolites, foid syenites, phonolites, leucocratic foidites and related pegmatoid rocks) in the association with sodic amphibole, alkali feldspar and nepheline or quartz (e.g., Yagi 1966;Larsen 1976;Pazdernik 1997;Piilonen et al 1998;Baudouin et al 2016). Aegirine was also frequently reported in highpressure low-temperature metamorphic rocks such as greenschist, blueschist and epidoteamphibolites (e.g., Banno and Yamada 2012;Flores et al 2015;Ghose et al 2021), as a product of sodic metasomatism -fenitization (e.g., Sutherland 1969;Cooper et al 2016;Weidendorfer et al 2016), as authigenic phase resulting from diagenesis/anchimetamorphism This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.…”

Introduction, Part 2—Geodynamic evolution of Eastern Himalaya and Indo‐Myanmar orogenic belts: Advances through interdisciplinary studies

“…Geological map of Eastern Himalaya, Indo‐Myanmar orogenic belt, northern, Central Myanmar, and adjacent areas (modified after Mitchell et al, 2007; Searle et al, 2007; Y. R. Singh, Singh, Singh, et al, 2022). Locations of study area presented in the issue include 1/S2: Kundu and Hazarika (2022); 2/S2: Y. R. Singh, Singh, Singh, et al (2022); 3/S2: Rawat and Luirei (2022); 4/S2: Kayal et al (2022); 5/S2: Choudhury et al (2022); 6/S2: N. M. Sharma et al (2022); 7/S2: Goswami, Gogoi, et al (2022); 8/S2: Bikramaditya et al (2022); 9/S2: Goswami, Kalita, et al (2022); 10/S2: Joshi (2022); 11/S2: Jha and Sharma (2022); 12/S2: Shukla et al (2022); 13/S2: Khonglah et al (2022); 14/S2: Ali and Duarah (2022); 15/S2: Mohanty et al (2022); 16/S2: Chanu et al (2022); 17/S2: Majumdar, Gogoi, and Ghatak (2022); Majumdar, Gogoi, Ghatak, Saikia, et al (2022); 18/S2: Ding et al (2022); 19/S2: Ghose et al (2022); 20/S2: Hussain and Dey (2022); 21/S2: Ozukum et al (2022); 22/S2: Jamir et al (2022); 23/S2: Chaubey et al (2022); 24/S2: Bora, Borah, et al (2022); Bora, Mukherjee, et al (2022); 25/S2: Khuman and Ibotombi (2022); 26/S2: Y. R. Singh, Singh, Singh, et al (2022); 27/S2: Thokchom and Kshetrimayum (2021); 28/S2: Barman et al (2022); 29/S2: V. Sharma and Biswas (2022)…”

“…Ghose et al (2022) report the occurrence of pure aegirinebearing metabasic rocks from the Naga Hills of north-east India with a detailed account of their mineralogy and mineral chemistry to assess the P-T conditions of their formation. They have considered that the studied metabasics are strongly fractionated and show chemical affinity with low-K oceanic tholeiite and were derived from a basic protolith of depleted mantle composition (viz.…”

Geochemical characteristics of mafic intrusive rocks from the Naga Hills Ophiolite, north‐east India: Constraints on petrogenesis and tectonic setting

“…The Ophiolite belonging to Naga Hills occurs along a narrow (5-15 km wide), NNE-trending 200 km linear belt in the northern sector of IMOB (Ghose et al, 2010(Ghose et al, , 2021Ghose & Singh, 1980, 1981. It is tectonically dismembered, but all the units of a complete ophiolite are present, except the sheeted dykes (Agrawal & Ghose, 1986;Agrawal & Kacker, 1980;Chattopadhyay et al, 1983;Ghose & Agarwal, 1989;Ghose et al, 2010;Venkataramana et al, 1986).…”