Singhbhum Craton Archean Earth

1. 1
CHAPTER 1:
Introduction
1.1 Motivation
The Earth is a unique planet in our solar system; oxygen rich atmosphere, presence of
surficial liquid water, bimodal distribution of crust, and the operation of plate tectonics are
some of the features that makes our planet special. We know that thick, felsic rocks form the
continental crust and thin, mafic rocks forms the oceanic crust. Crust along with the
underlying anhydrous, non-convecting part of mantle forms the lithosphere or tectonic plate.
Plate tectonics is the process of horizontal movement of these rigid plates with deformation
mostly limited to its boundaries as they move past each other (Cawood et al., 2018). The
Earth’s crust is being recycled through subduction and erosion and, new crust is being formed
continuously at mid oceanic ridges, collisional belts and arcs. The emergence of continental
crust has utmost implications on several features of our planet, such as the evolution of
terrestrial animals including humans.
Multiple studies have estimated that around 70 percent of the present day exposed
felsic crust had formed by the end of the Archean (Belousova et al., 2010; Dhuime et al.,
2012; Pujol et al., 2013). Using combined zircon hafnium and oxygen isotopic composition, it
was estimated that the growth of continental crust was linear till 3.0 Ga at a rate of 3.0 km3
/yr
and later reduced to 0.8 km3
/yr (Dhuime et al., 2012). The Argon isotopic compositions of
inclusions in hydrothermal quartz also support the formation of large volumes of continental
crust (>70 %) during the Archean (Pujol et al., 2013). Though the computed volumes of
Archean continental crust are high, they form only <5 % of the present-day exposed crust
(Champion and Smithies, 2007). Apart from being archives of early earth processes, Archean
terrains host several major and valuable deposits of gold, copper, nickel, iron, zinc and rare
earth elements, making them of significant economic interest (Groves and Barley, 1994).
Our understanding on the origin and evolution of the early continental crust remains
poor, mainly due to the dearth of well-preserved Archean rocks. The nature of the primary
felsic crust, its source and mechanism(s) of formation have direct implications on the tectonic
processes operated in the Archean Earth. Therefore, understanding the formation of the
Archean continental crust and its evolution can help to constrain the Archean geodynamic
processes.

2. 2
1.2 Archean geodynamics: Uniformitarian, non-uniformitarian or a dual tectonic
regime?
The initiation of plate tectonics is one of the most controversial topics in the Earth
sciences (Harrison, 2009; Cawood et al., 2018). The temporal distribution of plate tectonic
indicators is significantly heterogenous (Fig. 1.1). The popular tectonic models proposed for
the Archean Earth can be broadly classified into two end member models, i.e., mobile lid and
stagnant lid tectonics. The mobile lid, or the uniformitarian school of thought claims that plate
tectonics has always been operational in the Earth’s history, though it could be different from
the Present day (Kusky et al., 2013; Polat et al., 2014; Keller and Schoene, 2018). Most of
these studies rely on field observations, rock associations, structural, geochemical and
isotopic evidences (Windley et al., 2021).
Figure 1.1: The initiation of plate tectonics based on different studies (modified after
Korenaga, 2013).
The uniformitarian theory is driven by the similarity in rock associations, their
geochemistry and field relations of Archean rocks with that of present-day accretionary belts
(Windley et al., 2021). On the whole, the composition of continental crust is andesitic, similar
to the rocks formed in present-day subduction zones (Taylor, 1967). The above observation
highlights the significance of arc magmatism, and thus, the role of plate tectonics in the
growth of continental crust (Tatsumi, 2005). The Hadean Jack hill zircons from Narryer

3. 3
Terrane of Western Australia, display negative to positive ɛHf ratios, hinting towards the
emergence of Hadean continents and associated depleted mantle (Harrison et al., 2005).
Boninite- like rocks in the Eoarchean Isua Greenstone Belt of the West Greenland, is
considered to have formed in an intra-oceanic setting and supports the operation of plate
tectonics in the early Earth (Polat et al., 2002).
The mafic volcanic rocks from the Eoarchean Nuvvuagittuq Greenstone Belt of
Canada possess trace element patterns akin to Izu-Bonin-Mariana forearc rocks (Turner et al.,
2014). Similarly, the zircon hafnium isotopic systematics and titanium isotopes of granitoids
from the Acasta Gneiss Complex preserve evidence for a 3.8 Ga shift towards plate tectonic
regime (Aarons et al., 2020; Bauer et al., 2020). The Eoarchean Itsaq Gneiss Complex,
Greenland and Paleo to Mesoarchean granitoids of the Kaapvaal Craton records evidence for
accretionary tectonics in the form of field, rock assemblage, structural and isotopic evidences
(Moyen et al., 2006; Schoene and Bowring, 2010; Nutman et al., 2014). Undisputable
evidences for the operation of plate tectonics, such as paired metamorphic belt(s), melanges,
ophiolites, forearc basins, blue schist and high-pressure rocks are more common from the
Neoarchean and beyond (Condie and Kroner, 2008). The absence of Archean high pressure
metamorphic rocks and ophiolites can be due to their chemical instability and weatherability.
The geochemical record of continental basalt since 4 Ga doesn’t reveal variation in water
content, slab derived elements and degree of melting (Keller and Schoene, 2018). This
observation is strongly consistent with the operation of plate tectonics since the Hadean
(Keller and Schoene, 2018).
The non-uniformitarian view proposes a stagnant lid regime in which mantle plume
driven tectonic processes dominated the Hadean- Archean Earth (Bedard, 2018). When
compared to the present-day mantle potential temperature (Tp ~1350 °C), the Archean mantle
is estimated to be hotter by ~300 °C (Herzberg et al., 2010). The higher temperatures result in
weaker lithosphere, making subduction less viable (van Hunen and Moyen, 2012). The
Archean sub continental lithospheric mantle (SCLM) is strongly depleted and different than
the modern counterparts, hinting towards the existence of a different tectonic regime in the
Archean (Griffin et al., 2003). The poor preservation of subduction generated magmatism
clearly demonstrates that subduction-accretion alone cannot explain the growth of continental
crust (Hawkesworth et al., 2009).

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Multiple tools such as field observations, whole rock geochemistry, zircon U-Pb-Hf-O
systematics, paleomagnetism, experimental petrology, numerical and phase equilibria
modelling, mineral inclusions and statistical geochemistry have been used to precisely
understand when the stagnant to mobile lid transition took place. The global zircon hafnium
and oxygen isotopes reveal a decrease in net crustal growth and an increase in crustal
reworking since 3 Ga, which is attributed to the initiation of plate tectonics (Dhuime et al.,
2012). The geochemistry of igneous rocks points towards a significant shift in key elemental
ratios (Eu/Eu*, Sr/Y, La/Yb, Na2O/K2O) by the end of the Archean and is linked to the
widespread operation of plate tectonics (Keller and Schoene, 2012). The appearance of
eclogitic inclusions in the post 3.0 Ga diamonds can be linked to the deep burial of oceanic
crust which could possibly be due to subduction (Shirey and Richardson, 2011). The bulk
composition of the upper continental crust records a mafic to felsic compositional change
around 3.0 Ga and is attributed to the operation of plate tectonics (Tang et al., 2016).
Consolidating the solid arguments from the above end member theories, it would be
apt to consider that the Archean earth evolved under a dual tectonic regime. Moyen and
Laurent (2018) noted that Archean basalts have geochemical systematics intermediate to arc,
mid oceanic ridge and ocean island basalts, possibly due to the absence of a global depleted or
enriched mantle reservoir (s), and true Archean subductions were rare. One should also keep
in mind that localised evidences of subduction, doesn’t necessarily mean the widespread
operation of plate tectonics on a global scale (Lenardic, 2018). For example, the surface
features of Latona Corona in the Venus, display evidence for subduction, but we know that
plate tectonics doesn’t operate on the Venus (Sandwell and Schubert, 1992). Understanding
the thermomechanical and physical properties of the Archean crust through computational
modelling can provide critical insights into the nature of tectonics and stability of the Archean
crust (Sizova et al., 2010; Gerya, 2014; Johnson et al., 2014; Capitanio et al., 2020).
Mantle potential temperature (TP) is one of the most crucial parameters that
determines the lithospheric strength. When (TP) is 250 K higher than the present-day value,
horizontal plate movements wouldn’t be possible (Sizova et al., 2010). The higher mantle
potential results in the formation of a magnesium rich mafic-ultramafic crust (Herzberg et al.,
2010). The base of this thick crust (45km or more) would be gravitationally unstable due to
density contrast with the underlying mantle and as a result drips down into the mantle
(Johnson et al., 2014). The dripping crust results in an asthenospheric upwelling resulting in
remelting of the ultramafic crust to form Mg- poor basalts that can source tonalite-

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trondhjemite -granodiorites (Johnson et al., 2014). Recent studies based on convection
modelling such as Capitanio et al. (2019), suggest that the end member ‘pseudo plate tectonic’
and ‘stagnant lid’ regime can co-exist together. The pseudo plate tectonic regime involves
‘proto-plates’ that are mobile and rigid enough to sustain under-thrusting and compression,
leading to subduction like features (Capitanio et al., 2019).
While studying the evolution of the early Earth, we should also keep in mind about the
role of bolide impact on the evolution of all the planets in our solar system. The studies on
bolide impacts suggest that the Hadean and Archean Earth was susceptible to intense and
large meteor bombardments, commonly referred as the Late Heavy Bombardments or LHB
(Gomes et al., 2005; Bottke et al., 2012). It has been demonstrated that meteoroid impacts can
trigger volcanism similar to that of the present-day flood basalt provinces (Elkins-Tanton and
Hager, 2005). The bolide impact models can explain the Archean continental growth,
undepleted mantle, crustal recycling and initiation of plate tectonics (Hansen, 2018).
However, most of our understanding on the tectonics operated in other planets is in
developing stage and future studies can provide some critical constraints on our understanding
about the early Earth.
1.3 The early Archean continental crust and Tonalite Trondhjemite Granodiorite record
Among various rocks, grey gneisses are the most abundant rock types found in the
Archean cratons. Grey gneisses can be divided into Tonalite-Trondhjemite-Granodiorite
(TTG) and granites. TTGs are defined by a set of geochemical features: silica rich (SiO2= 68
to 74 wt.%), sodic (K2O/Na2O< 0.5), ferromagnesian poor (Fe2O3+MgO+MnO+TiO2< 5
wt.%), mildly peraluminous (A/CNK= 1 to 1.1) with Mg numbers exceeding 40 (Moyen,
2011; Laurent et al., 2014). These rocks display strongly fractionated rare earth element
pattern with high Sr/Y ratios due to the presence of residual garnet in their source. On
primitive mantle normalised variation diagrams, TTGs are similar to calc-alkaline arc
granitoids with anomalies of Nb, Ta, Ti and Pb. Despite the similarity in geochemical
anomalies, arc granitoids are potassic and metaluminous when compared to the TTGs.
TTGs are considered as juvenile continental crust that records the mafic to felsic
transition of the crust. The potassic granites can either be formed by extensive reworking of
the older felsic crust or by fractionation of mantellic magmas (Laurent et al., 2014).
Therefore, TTGs can help in understanding the mechanisms of continental crust formation
and also the tectonic regime prevalent in the early Earth. Eo- to Paleoarchean TTGs are

6. 6
reported from many cratons including the Slave, Kaapvaal, Pilbara, Amazonian and Indian
cratons (Bundelkhand, Bastar, Dharwar and Singhbhum). I will briefly describe the tectonic
models proposed for the Eo- and Paleoarchean evolution of several Archean cratons, giving
due importance to their salient field, geochemical and isotopic evidences.
The Acasta Gneiss Complex from the Slave Craton contains the oldest terrestrial rocks
having crystallisation age up to 4.03 Ga and contains extensively preserved felsic Eoarchean
rocks (Iizuka et al., 2007; Reimink et al., 2016). The whole rock major and trace elemental
systematics of 4.02 Ga tonalitic gneiss shows remarkable similarity with Icelandites (Reimink
et al., 2014). The 3.6 Ga TTGs from the same area, display high La/Yb, Sr/Y and Nb/Ta ratios
requiring burial of its source up to 50 to 60 km, which is most likely possible in subduction
like setting (Reimink et al., 2016). The above inference is supported by the modern mantle
like ɛ182
W and zircon Hf systematics of the 3.6 Ga rocks (Willbold et al., 2015; Reimink et
al., 2019). The evolution of the Acasta Gneiss Complex was explained by: (i) the formation of
4.02 Ga gneisses in an oceanic plateau setting, (ii) the formation younger TTGs and granites
in a convergent margin setting (Reimink et al., 2016).
Figure 1.2: The distribution of Archean cratons around the globe (Reference: Figure 1, Lee et
al., 2011). (1) Slave, (2) Wyoming, (3) Superior, (4) Greenland, (5) Fennoscandian, (6)
Siberian, (7): North China, (8) west Australia, (9) Indian, (10) Tarim, (11) Tanzanian, (12)
Kaapvaal, (13) Congo, (14) west African, (15) Amazonia and (16) Colorado Plateau.

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The Eoarchean Itsaq Gneiss Complex of the South Greenland, North Atlantic Craton
is another well preserved fragment of early Archean crust that extends over 3000 km2
(Nutman et al., 1996). The zircon ages from these rocks suggest their emplacement between
3890 to 3660 Ma (Nutman et al., 2014). The TTGs are strongly deformed to form
orthogneisses and are associated with slightly older gabbros, basalts and andesites with
subduction signatures. The supracrustals which forms a minor component in this terrane,
occur as slivers and enclaves. The geochronological studies indicate that some of the TTGs
formed before terrane amalgamation and during the tectonic stacking. The presence of 3660
Ma granulite supports tectonic thickening of the crust as a result of horizontal crustal
movement (Friend and Nutman, 2005).
The Pilbara Craton of the Western Australia is one of the best-preserved granite
greenstone terrains containing abundant Paleo- Mesoarchean rocks (Smithies et al., 2009).
Presence of dome and keel structure, ensialic and weekly deformed nature of greenstones and
absence of microcontinent juxtaposition and terrain accretion, makes the Pilbara Craton
different from other terrains (Kranendonk et al., 2004). The dome and basin structures are in
sharp contrast with the linear arc batholiths found in convergent margins. The Paleoarchean
granitoid magmatism can be broadly grouped into two phases: 3.5 to 3.42 Ga and 3.32 to 3.34
Ga (Champion and Smithies et al., 2019). The geochemistry of 3.3 Ga TTGs and granites
from the Pilbara Craton suggest their formation by partial melting of an older mafic crust at
depths less than 60km (Collins, 1993). Diapirism resulting from partial convective overturns,
which is driven by density contrast between granitic and mafic-ultramafic magma, has been
recorded in the Pilbara Craton (Collins, 1998; Kranendonk et al., 2004). The results from
geodynamic modelling demonstrates that the low Mg basaltic rocks which are the source for
TTGs are formed by melting of the Mg-rich, dense ultramafic primary crust which collapses
due to gravitational instability (Johnson et al., 2014). The Paleoarchean TTGs of this craton
was formed by partial melting of basalts and basaltic andesites of the 3.5 Ga Coucal
Formation (Smithies et al., 2009; Johnson et al., 2017). The 3.5 to 3.24 Ga Pilbara TTGs are
dominantly low to medium pressure types formed by partial melting at depths < 45 km
(Johnson et al., 2017; Champion and Smithies, 2019).
The Barberton Granitoid Greenstone Terrain (BGGT) of the Kaapvaal Craton in the
South Africa, is well known for their well exposed Paleoarchean rocks. The major magmatic
events in the BGGT occurred in four discrete events, i.e., 3.55-3.49 Ga, 3.49-3.42 Ga, 3.25-
3.22 and 3.10-3.07 Ga (cf. Moyen et al., 2007; 2019 for review). These granitoids, commonly

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referred to as ‘grey gneisses’, contain both TTGs and granites. They vary from strongly
deformed to undeformed, and display great temporal, spatial and compositional variability
(Moyen et al., 2019). The geothermal gradient estimated from 3.2Ga garnet albite bearing
amphibolite from this terrain varies between 12-15 °C/km and is similar to that present day
subduction zones (Moyen et al., 2006). The tectonic evolution of the BGGT can be explained
by: (a) the formation of 3.55 Ga TTGs under shallow depth of melting possibly related to a
mafic plateau. (b) The younger TTGs (>3.28 Ga) were formed by lower crustal melting and
delamination of a thickened oceanic plateau. (c) The 3.2 Ga TTGs formed by subduction
accretion as evident from the high-pressure signature of TTGs (15-20 kbar) and the
tectonically juxtaposed stratigraphic units (Moyen et al., 2019).
1.4 The Archean crustal record from the Indian Shield
The Indian shield is a collage of five Archean cratons, namely Aravalli, Bastar,
Bundelkhand, Dharwar and Singhbhum cratons, which are separated from each other by
Proterozoic mobile belts (Ramakrishnan and Vaidyanathan, 2010). Granitoids as old as 3.5
Ga are reported from different Indian cratons, but they form volumetrically minor component.
Except Dharwar and to some extent Singhbhum, the understanding on early Archean
evolution of the other Indian cratons are extremely limited by the lack of robust geochemical
and isotopic data. Most of these cratons have widespread Neoarchean granitoids that are
compositionally different from Paleoarchean TTGs and granites. The Neoarchean granitoids
across the globe, display great compositional diversity and are classified into; TTG,
sanukitoids, biotite and two-mica granites and hybrid granites.
The Dharwar Craton of the southern India contains granitoids emplaced ca 3.43 Ga,
3.36 Ga, 3.29 Ga, and 2.66 Ga (Ranjan et al., 2020). The trace elemental systematics of the
3.35-3.28 Ga TTGs suggest their formation by shallow melting in a thickened island arc/
oceanic plateau. The later 3.23-3.2 Ga TTGs possess high pressure signatures and were
formed by melting at the base of this arc like crust/ oceanic plateau under 14 to 18 kbar
(Jayananda et al., 2015). The zircon Hf systematics of these granitoids indicate rapid
recycling of juvenile basalts similar to that in subduction zones (Ranjan et al., 2020). Further
studies, by including spatially associated (Paleoarchean?) Sargur Group supracrustals may
provide additional constraints on the evolution of the Dharwar Craton.

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Figure 1.3: A schematic map of Indian subcontinent demarcating different Archean cratons
(Reference: Figure 1, Dey and Moyen, 2020). (a) Aravalli Craton (b) Bundelkhand Craton (c)
Singhbhum Craton (d) Bastar Craton and (e) Dharwar Craton.
The Bundelkhand Craton of central India records evidence for granitoid magmatism as
early as 3.55 Ga (Kaur et al., 2014). The 3.55 Ga event was followed by episodic granitic
magmatism at 3.39 Ga, 3.28 Ga, 3.20 Ga (Kaur et al., 2016). The zircon Hf isotopic
compositions reveal a long-lived mafic source that was buried to depths of garnet and rutile
stability (Kaur et al., 2016). The 3.39 to 3.20 Ga TTGs formed with contributions from older
TTG as evident in the form of inherited older zircons (Kaur et al., 2016). The Paleoarchean
components are intruded by the voluminous and compositionally diverse Neoarchean
granitoids (Singh et al., 2020). Similarly, Bastar Craton, lying towards the southeast of
Bundelkhand Craton has sporadic occurrence of Paleoarchean granitoids. Two episodes of
granitoids are recognised, i.e., 3.56 Ga and 3.50 Ga (Sarkar et al., 1993; Ghosh, 2004).
However, these units are yet to be well-characterised using whole rock geochemistry and
zircon hafnium isotopic systematics. Likewise, the Aravalli Craton of the western India has
3.31 Ga TTGs with sub-chondritic Hf isotopic compositions (Kaur et al., 2019).

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1.5 Objectives of the study
The Singhbhum Craton in the eastern India is an Archean craton with abundant rocks
spanning from Paleoarchean to Paleoproterozoic (Chaudhuri, 2020). Recent studies have
demonstrated the presence of an Eoarchean and Hadean crust in the form of detrital and
xenocrystic zircons (Chaudhuri, 2018; Sreenivas et al., 2019; Ranjan et al., 2020). Hence, the
Singhbhum Craton would be an ideal terrain to understand early Archean geological
processes. The available emplacement ages indicate that the major phase of granitoid
magmatism was between 3.45 and 3.25 Ga (Upadhyay et al., 2019; Chaudhuri, 2020). The
objectives of this study are to understand the petrogenesis and geodynamic setting(s) of the
abundant Paleoarchean TTGs and granites in the Singhbhum Craton. The results from this
study will provide new constraints on Archean continental crust generation and geodynamics.
The proposed problem will be addressed using field, petrography, whole rock geochemistry
and zircon U-Pb isotopes. The results would be verified with available age and isotopic data
from the Singhbhum Craton.