Summary:
The Paleoarchean (~3.37–3.50Ga) Algoma-type Tomka Banded Iron Formation (BIF), from the Daitari Greenstone Belt, India, experienced only greenschist-facies metamorphism, in contrast to Eo- to Paleoarchean amphibolite-facies BIFs.
The Palaeoarchean Daitari Greenstone Belt (DGB) is part of the Singhbhum Craton, located in northeastern India. It is surrounded by the Singhbhum Granitoid complex with an age ranging from 3.5 to 3.1Ga (Dey et al., 2019; Goswami et al., 1995; Hofmann et al., 2022; Mishra et al., 1999; Upadhyay et al., 2019). The Daitari Greenstone Belt is a pile of volcano-sedimentary strata with a thickness of ca. 7km. It belongs to the Badampahar Group and is unconformably overlain by the Mahagiri Quartzite with a maximum depositional age of 2868±28Ma (Jodder et al., 2025).
The Badampahar Group comprises five main formations. The ca. 3km thick Kalisagar Formation is the lowermost unit and comprises mainly submarine (ultra)mafic volcanics, including pillow basalts, which are intercalated with rare bedded chert units. The Kalisagar Formation is overlain by the ca. 1.5km thick 3507±5Ma (Jodder et al., 2023) old Talpada Formation, consisting of felsic volcanic and volcaniclastic rocks. Interbedded chert horizons are common towards the top of this formation. The ~500-m-thick Sindurimundi Formation lies erosively on top of the Talpada Formation. It mainly comprises clastic sedimentary rocks such as conglomerates, sand-, siltstones and shales. Uranium-Pb dating of detrital zircons of this unit implies a maximum deposition age of 3502±2Ma (Jodder et al., 2023). In sharp contact with the underlying Sindurimundi Formation lies the ca. 700m thick Tomka Formation, the topic of this study. The Tomka Formation comprises orthochemical sedimentary rocks, such as banded black and white chert, as well as the banded iron formation.
The BIF consists of finely laminated hematite and magnetite intercalated with jasper and chert bands (Jodder, 2021). The uppermost unit of the Badampahar Group is the almost 1km thick Talangi Formation, which is in sharp contact with the underlying Tomka Formation. It consists mainly of (ultra)mafic volcanic rocks erupted in a sub-aquatic setting and some thin interbedded cherts. The time of deposition of the Tomka Formation is constrained by the underlying shale from the Sindurimundi Formation by detrital zircons, implying a maximum deposition age of ca. 3.50Ga and the timing of felsic intrusive rocks at 3.38 to 3.35Ga, dated via magmatic zircons (Jodder et al., 2023).
The highest metamorphic grade experienced by the meta-volcano-sedimentary rocks of the DGB, including the Tomka Formation, is greenschist facies. This is indicated by Raman spectroscopy and chlorite thermometry, as well as by the presence of chlorite in the metapelitic rocks of the Sindurimundi Formation (Jodder, 2021; Mukhopadhyay et al., 2008). This makes it possibly the oldest sub-amphibolite facies BIF known so far. The Badampahar Group experienced a regional deformation and metamorphic event at 3.28Ga (Nelson et al., 2014). The Singhbhum Craton finally stabilised at around 3.1Ga (Hofmann et al., 2022), and a large igneous province was emplaced around 2.8Ga (Kumar et al., 2017).