Archean granitoids of India: windows into early Earth tectonics – an introduction

Abstract Granitoids form the dominant component of Archean cratons. They are generated by partial melting of diverse crustal and mantle sources and subsequent differentiation of the primary magmas, and are formed through a variety of geodynamic processes. Granitoids, therefore, are important archives for early Earth lithospheric evolution. Peninsular India comprises five cratonic blocks bordered by mobile belts. The cratons that stabilized during the Paleoarchean–Mesoarchean (Singhbhum and Western Dharwar) recorded mostly diapirism or sagduction tectonics. Conversely, cratons that stabilized during the late Neoarchean (Eastern Dharwar, Bundelkhand, Bastar and Aravalli) show evidence consistent with terrane accretion–collision in a convergent setting. Thus, the Indian cratons provide testimony to a transition from a dominantly pre-plate tectonic regime in the Paleoarchean–Mesoarchean to a plate-tectonic-like regime in the late Neoarchean. Despite this diversity, all five cratons had a similar petrological evolution with a long period (250–850 myr) of episodic tonalite–trondhjemite–granodiorite (TTG) magmatism followed by a shorter period (30–100 myr) of granitoid diversification (sanukitoid, K-rich anatectic granite and A-type granite) with signatures of input from both mantle and crust. The contributions of this Special Publication cover diverse granitoid-related themes, highlighting the potential of Indian cratons in addressing global issues of Archean crustal evolution.

Granitoids constitute a major part of the Archean cratons, and originate from partial melting of a variety of crustal and mantle sources in a wide range of tectonic settings (Martin et al. 2005;Halla et al. 2017;Moyen 2020). Granitoids, therefore, act as windows into the early Earth's geodynamic processes and the secular changes thereof. The Indian cratons ( Fig. 1) are rich archives Archean crustal growth, and comparable, in many respects, to other well-studied cratons such as Superior, Pilbara, Yilgarn, Kaapvaal, North China and Karelia (Champion & Sheraton 1997;Zhao et al. 2003;Holtta et al. 2012;Percival et al. 2012;Dey 2013;Champion & Smithies 2019;Chaudhuri 2020;Moyen 2020). During the last few decades, a significant amount of structural, geochemical, isotope and geochronological data have been gathered on the diverse types of Archean granitoids occurring in the Indian cratons. Many of these granitoids are also linked to important mineralizations. However, the information is fragmentary, locality specific and often not readily accessible to international readers.
The contributions of this Special Publication synthesize the available information on Archean granitoids from each Indian craton and highlight their potential in understanding evolution of early Earth. The papers discuss a broad spectrum of themes related to granitoid typology, emplacement mechanism, petrogenesis, phase-equilibria modelling, temporal distribution, tectonic setting, and their roles in fluid evolution, metal delivery and mineralizations. The volume presents a broader picture incorporating regional-to craton-scale comparisons, implications for Archean geodynamic processes and secular changes thereof. We hope that this volume will generate interest among researchers, teachers, students and exploration geologists interested in early Earth processes, and foster future international collaborative research.

Indian cratons
Peninsular India comprises five cratonic blocks (Dharwar, Singhbhum, Bastar, Bundelkhand and Aravalli) bordered by mobile belts (Eastern Ghats Mobile Belt, Southern Granulite Terrain and Central Indian Tectonic Zone) ( Fig. 1) (Jain et al. 2020;Jayananda et al. 2020b). The Indian cratons have distinct evolutionary histories marked by multiple cycles of basin formation, sedimentation, magmatism, deformation, metamorphism and mineralization (Sarkar & Gupta 2012;Jain et al. 2020;Jayananda et al. 2020b). We provide a short description of granitoid formation events within the Indian cratons for the international readership interested in Indian Archean geology. For a more detailed description, readers are referred to the individual papers in this volume.
The Aravalli Craton is composed of Archean gneisses and granitoids which form the basement for the Proterozoic sediments of the adjacent Proterozoic Aravalli-Delhi Fold belt (ADFB) (Fig. 2a) Fareeduddin & Banerjee 2020;Jain et al. 2020). The basement, conventionally known as the Banded Gneissic Complex (BGC), constitutes an ensemble of amphibolitesfacies polyphase (c. 3.31 and 2.56-2.55 Ga) tonalite-trondhjemite-granodiorite (TTG) gneisses and migmatites containing enclaves of metasedimentary and mafic-ultramafic meta-igneous rocks (Kaur et al. 2019;Ahmad et al. 2020). Some workers consider the enclaves as dismembered bodies of Archean greenstone belts (Sinha-Roy 1985). Some of the gneisses also show a transitional TTG character . During 2.55-2.49 Ga, sanukitoids and K-rich anatectic granites intruded the BGC, marking the stabilization of the craton (Kaur et al. 2019;Ahmad et al. 2020). The ADFB comprises c. 1.8-1.0 Ga sedimentary and volcanic rocks which have undergone polyphase deformation and metamorphism, and intrusion of Proterozoic granitoids (Fareeduddin & Banerjee 2020). The relationship between the BGC and the ADFB has been debated for decades, although the recognition of unconformities and, in places, palaeosols (Roy & Jakhar 2002) suggests that the BGC formed the basement for the supracrustal rocks of the ADFB.
The Bundelkhand Craton is exposed over a triangular area of c. 26 000 km 2 (Fig. 2b). TTG magma emplaced episodically within the craton over a period from 3.55 to 2.70 Ga (Fig. 3) (Joshi et al. 2016;Kaur et al. 2016;Singh et al. 2020). During 2.58-2.50 Ga, diverse types of crust-and mantlederived granitoids intruded the craton, including sanukitoids, Closepet-type granitoids, anatectic K-rich granites and A-type granites, marking the stabilization of the craton. These granitoids indicate widespread crust-mantle interaction and reworking of older crust (Joshi et al. 2016;Kaur et al. 2016). Greenstone belts containing mafic-ultramafic and felsic volcanic rocks, banded iron formations (BIFs) and quartzites, which range in age from the Mesoarchean to the Neoarchean, occur within the granitoids (Singh & Slabunov 2016). The volcanic rock associations and their geochemical signatures have been interpreted to reflect a range of tectonic settings, including plume, arc and mid-oceanic ridge settings (Singh & Slabunov 2016;Raza & Mondal 2019).
A vast tract of granitoids is well exposed in the craton containing 3.56-3.50 and c. 3.0 Ga TTG gneisses, c. 3.51 Ga transitional TTGs, and 2.50-2.48 Ga K-rich anatectic and A-type granites (Sarkar et al. 1990(Sarkar et al. , 1993Narayana et al. 2000;Mondal et al. 2020;Santosh et al. 2020) (Fig. 3). Interestingly, Rajesh et al. (2009) reported a 3.58 Ga granite from the Bastar Craton, which is the oldest known K-rich granite in the world. Several generations of Paleoarchean-Paleoproterozoic supracrustal/greenstone belts are exposed within the craton hosting important Fe and Au resources. The craton stabilized during the early Paleoproterozoic concurrently with intrusion of voluminous, K-rich granitoids (although some are sodic), which contain the Malanjkhand Cu (+Mo) deposit and rare-metal (Nb, Ta, Be, Sn and Li) pegmatites (Ramesh Babu 1999;Pandit et al. 2020).
The Dharwar Craton, occupying an area of c. 350 000 km 2 , is divided into western and eastern blocks (Fig. 2e). The western block or Western Dharwar Craton (WDC) is composed of 3.42-3.2 Ga TTG gneisses, 3.35-3.15 Ga transitional TTGs and 3.20-3.00 Ga K-rich anatectic granites (Jayananda et al. 2020b, c;Ranjan et al. 2020a) (Fig. 3). The granitoids are interleaved with the Paleoarchean-Mesoarchean greenstone belts of the Sargur Group. The craton stabilized at c. 3.0 Ga. Subsequently, the younger (2.9-2.6 Ga) Dharwar Supergroup greenstone successions were formed, which host Fe, Mn and Cu deposits (Bhaskar Rao et al. 2020). At 2.6 Ga, K-rich granites intruded the WDC, marking a second stage of cratonization. On the other hand, the eastern block or Eastern Dharwar Craton (EDC) is composed mainly of 2.7-2.5 Ga linear north-south-trending granitoid-greenstone belts with small remnants of Paleoarchean-Mesoarchean (3.3-3.0 Ga) TTG gneisses. The Neoarchean granitoids of the EDC have a wide range of compositions, including TTG, transitional TTG, sanukitoid (+Closepet-type granitoid), K-rich anatectic granite and A-type granite (+syenites) (Fig. 3) (Moyen et al. 2003;Dey et al. 2017a;Jayananda et al. 2020a). Some researchers consider the EDC as a Neoarchean hot orogen characterized by a lateral constrictional flow of soft and ductile lower crust against the older and rigid WDC in a convergent set up (Chardon et al. 2011). The EDC stabilized at 2.5 Ga, concomitant with the formation of granulites in the lower crust and gold mineralization within the granite-greenstone belts (Peucat et al. 2013;Pandit et al. 2020).

Towards a unifying scenario?
Recent petrological and isotope studies suggest that each Indian craton has a unique evolution pattern org/OnegeologyGlobal/). All maps are at the same scale. Topography from Natural Earth (https://www. naturalearthdata.com). When applicable, the units have been reinterpreted based on Jain et al. (2020). The map from onePortal is compiled (by the Geological Survey of India) at a scale of 1:2 500 000, and is therefore not very detailed, more detailed maps of each craton appear in the relevant chapters in this volume. In these maps, some earliest Proterozoic (c. 2.5 Ga) units are depicted as Archean, even though they are technically on the Proterozoic side of the border, when they clearly belong to the Archean evolution of their region. . For example, a zircon U-Pb age v. ε Hf (t) plot shows that crust formation in the Singhbhum Craton started in the Hadean which was reworked up to 3.6 Ga. During 3.50-3.25 Ga, the craton underwent continuous addition of juvenile crust and its immediate reworking. The WDC evolved through juvenile magma input until 3.25 Ga, followed by significant reworking of older crust during 3.25-3.00 Ga. The craton recorded another event of juvenile crust formation and reworking of older crust during late Neoarchean (2.7-2.5 Ga). The detrital zircon archive in the EDC reflects the 3.6-3.2 Ga addition of juvenile crust, followed by continuous reworking of older crust up to 2.5 Ga. The craton hosts predominantly Neoarchean rocks which show a wide range of zircon ε Hf (t) values, suggesting both juvenile crustal addition and reworking of older crust. The 3.3 Ga rocks of the Aravalli Craton shows near-chondritic zircon ε Hf (t) values, whereas the 2.55-2.50 Ga rocks predominantly record the reworking of significant older crust with the addition of some juvenile crust. The Buldelkhand Craton shows the addition of juvenile crust at 3.58 Ga, followed by reworking of older crust during 3.55-3.10 Ga. No data are available for Neoarchean rocks of the craton. The Bastar Craton also lacks data on Paleoarchean-Mesoarchean rocks. Limited data suggest 2.8-2.7 Ga reworking of older crust, succeeded by both juvenile magma input and crustal reworking at c. 2.5 Ga. In general, crustal formation in the Indian cratons started with a relatively restricted range of zircon ε Hf (t) values, eventually showing a wider span at the end stage of cratonization (Fig. 4) indicating effective crustmantle interaction, a flare-up of juvenile magmatism and the reworking of older crust.
Notably, the Singhbhum Craton and the WDC stabilized relatively early (at c. 3.25 and c. 3.00 Ga, respectively), and display diapirism or sagduction tectonics characterized by the sinking of the denser greenstone belts and the rising of granitoid domes (Bouhallier et al. 1995;Prabhakar & Bhattacharya 2013). The presence of Mesoarchean-early Neoarchean terrestrial to marginal-marine mature clastic sediments, deposited over the cratonic basement, mark the attainment of early crustal stability and the subaerial emergence of the two cratons (Srinivasan & Ojakangas 1986;Mukhopadhyay et al. 2016). The Eastern Dharwar (EDC), Bastar, Bundelkhand and Aravalli cratons, on the other hand, have significant Neoarchean crust (granitoid-greenstone belts) and more diversified granitoid types (Ahmad  (Fig. 3). The regional structural pattern, rock association and geochemical signatures of the greenstone belt volcanic rocks and the adjacent granitoids suggest accretion-collision of diverse plume-and arc-related terranes in convergent settings, which has been particularly well documented in the EDC (Dey et al. 2017a;Manikyamba et al. 2017;Chadwick et al. 2000;Mohan et al. 2020;Santosh et al. 2020;Singh et al. 2020).
In conclusion, Indian cratons provide testimony to the transition from a Paleoarchean-Mesoarchean dominantly pre-plate tectonic regimecharacterized by diapirism or sagductionto a plate tectonic-like regime during the late Neoarchean (2.7-2.5 Ga). Despite this fact, a common feature of all Archean cratons of India (and worldwide: Laurent et al. 2014;Cawood et al. 2018) is the succession of a relatively long period (250-850 myr) of episodic TTG magmatism, followed by a shorter period (30-100 myr) of granitoid diversification (the appearance of one or more of the following additional granitoid types: sanukitoid, K-rich anatectic granite and A-type granite) (see Fig. 3) with signatures of input from both mantle and crust. The granitoid diversification was a reflection of crust-mantle interaction (Laurent et al. 2014;Halla et al. 2017) and, possibly, the emergence of a variety of geodynamic processes facilitating such interaction (e.g. subduction, slab break-off, flake tectonics, crustal thickening and/or delamination), a signature of plate tectonics, at the last stage of the evolutionary history of a craton. The diversification is less prominent within the Paleoarchean-Mesoarchean Singhbhum and Western Dharwar cratons (less K-rich granites, negligible sanukitoid: Fig. 3), which can be construed as failed/incomplete attempts to develop plate tectonics. Conversely, the cratons that stabilized during the Neoarchean have more voluminous and diversified types of granitoids, which, together with regional structural and metamorphic patterns, signify a more evolved and stabilized plate-tectonic-like regime.

The contributions
This Special Publication starts with a chapter by Moyen (2020), who discusses the wide range of sources, melting conditions and subsequent modifications (e.g. fractionation, assimilation and mixing) leading to the origin of granitoid magmas in the Archean. The author, while stressing the difficulty in pigeonholing the granitoid magmas into discrete groups, classified the granitoids into two broad types: (i) crust-derived 'C-type' and (ii) enriched mantle-derived 'M-type'. The C-type granitoids are derived from a wide range of sources (mafic to felsic lithologies and metasediments), and include the TTGs, transitional TTGs, peraluminous potassic granites, common biotite granites and alkaline granites, including syenites. The M-type class comprises the sanukitoids and high-K calc-alkaline Closepettype granitoids, and also shows wide-ranging chemical compositions grading into the C-types through hybrid granitoids, depending on the nature of the enrichment in the source mantle, magma mixing and crustal contamination. The author notes that on a global scale, as well as on a regional craton scale, the granitoids display a compositional diversity reflecting the evolving Earth gradually transitioning from a non-plate-tectonic regime to a plate-tectonic regime.
The Aravalli Craton also has a preserved long record of crust formation and crustal reworking from the Paleoarchean to the Paleoproterozoic. Ahmad et al. (2020) examine the geochemical and geochronological data of Archean granitoids of the craton and divide the granitoids into three groups -TTGs, transitional TTGs and sanukitoidswhich intruded episodically over a period from 3.31 to 2.55 Ga. The authors explain the TTGs as a product of the melting of a subducted oceanic plateau at different depths. In contrast, the transitional TTGs are considered to have formed by the reworking of a heterogeneous crust consisting of mafic to felsic rocks. The sanukitoids (2.55 Ga) are interpreted to be a product of the melting of a metasomatized mantle wedge in a subduction setting. Singh et al. (2020) present petrological and geochemical data of the Neoarchean TTGs and sanukitoids of the Bundelkhand Craton. A synthesis of the available geochemical and geochronological data suggests the emplacement of first-generation TTGs (both low and high pressure) at 3.5-3.2 Ga followed by second-generation TTGs (low pressure) at 2.69-2.65 Ga, sanukitoids at 2.58-2.54 Ga and high-K anatectic granites at 2.54-2.50 Ga. The authors present a tectonic model of the secular crustal evolution of the Bundelkhand Craton in which both the Paleoarchean and Neoarchean TTGs, showing a juvenile character, were formed in an intra-oceanic arc setting. At the end of the Neoarchean (2.58-2.50 Ga), the area converted into an active continental arc characterized by terrane accretion and, finally, continental collision. At this stage, the high-K anatectic granites formed through the reworking of older felsic crust, whereas the sanukitoids originated from the interaction between metasomatized mantle melts with anatectic granite melts.
An exceptionally long and almost complete record of crust formation from the Paleoarchean to the Paleoproterozoic is preserved in the Singhbhum Craton Chaudhuri 2020). Dey et al. (2020) presented new geochemical and zircon U-Pb and Hf isotope data from 3.32-3.25 Ga TTGs from the northern part of the Singhbhum Craton, and interpreted them as being the product of the reworking of mafic rocks under low-to medium-pressure conditions. The rocks are coeval with the late K-rich granitoids of the craton. This fact suggests that, contrary to popular understanding, both K-rich and Na-rich granitoids can form at the terminal stage of cratonization through the reworking of heterogeneous crust. Dey et al. (2020) suggest that the craton nucleated in an oceanic plateau. The gradual change from sodic to K-rich granitoid magmatism within the craton over the period 3.53-3.25 Ga is explained through episodic crustal reworking due to recurrent plume-related maficultramafic magmatism. Mondal et al. (2020) synthesize available geochemical and geochronological data on Bastar granitoids, discuss their petrogenesis and suggest possible tectonic implications. Scanty geochronological data indicate the existence of 3.56 and 3.50 Ga gneisses that are divided into TTGs (both high-and low-heavy REEs (HREEs)) and transitional (enriched) TTGs. Mondal et al. (2020) propose melting at variable depths within a thick oceanic plateau to explain the occurrence of both low-and high-HREE TTGs. The reworking of the gneissic crust at 2.5-2.48 Ga produced K-rich granites.
Archean cratons generally show a transition from initial sodic granitoids (TTG) to late potassic granitoids, implying an increasing importance of the reworking of more felsic crust. The geodynamic implication of this change is controversial . Jayananda et al. (2020c) describe three stages of granitoid magmatism in the southeastern part of the WDC and explain the mechanism of episodic crustal reworking leading to cratonization. The first stage is represented by 3.2 Ga granodiorites, transitional in composition between TTG and high-K granites, which intruded a 3.45-3.23 Ga TTG basement. Subsequently, mostly high-K granitoids (granodiorite, granite and quartz monzonite) formed by the mixing of two magma components: a dominant felsic magma derived by the partial melting of the TTG basement; and a minor mantle-derived, fractionated intermediate magma. Jayananda et al. (2020c) interpret the 3.0 Ga crustal reworking and attendant cratonization in terms of a mantle-plumerelated thermal anomaly. The WDC witnessed another event of crustal reworking and the generation of high-K granites at 2.6 Ga.
The EDC is a classic granitoid-greenstone terrain and an ideal region to address important issues regarding Neoarchean crustal growth. Mohan et al. (2020) review the elemental and isotope geochemistry, geochronology, and petrogenesis of 2.7-2.5 Ga granitoids of the EDC. The authors divided the granitoids into four major types, namely: TTGs; sanukitoids; biotite and two-mica granites; and hybrid granite. The melting of hydrous basalt, metasomatized mantle and felsic crust has been proposed for the origin of the TTGs, sanukitoids and biotite granite (and two-mica granite), respectively. The hybrid granites are explained as the product of the interaction between two or more such types of granitoid magmas. Mohan et al. (2020) argue that terrane accretion followed by continental collision and crustal thickening was responsible for the close assembly of geochemically diverse sources. The interplay between mantle melting and the reworking of older crust in a collisional zone led to the formation of a spectrum of granitoids.
The quantity of fluids in the lithosphere significantly influences the partial melting and magma generation processes. Nicoli (2020) uses phase equilibria modelling to estimate the volume and flux of water within the Archean crust of the Dharwar Craton during the burial of supracrustal rocks into the TTG basement. The calculations indicate that rapid burial of the water-saturated supracrustal keel will expel water into the surrounding basement through devolatilization of hydrous minerals. The process results in water-fluxed partial melting of the basement, in addition to water-absent partial melting and the production of Neoarchean syn-to post-tectonic anatectic granites.
Working on the 2.51 Ga Closepet granite in the EDC, Bhattacharya (2020) shows that field and microstructural information, coupled with wholerock and mineral chemical data, are valuable tools in understanding the ascent rate, deformation condition and emplacement mechanism of granitoid magmas. The Closepet granite has mineralogicalchemical variations between two end members: phenocryst-rich granodiorites and more evolved silica-rich granites. Bhattacharya (2020) interestingly concludes that the mineralogical-chemical variations are results of synmagmatic deformationdriven ascent of magma at variable rates (at a maximum in crystal-poor magma).
Late-magmatic fluids often play an important role in magma evolution and leave metasomatic imprints on granitoid plutons. Słaby et al. (2020) present whole-rock and mineral (feldspar and apatite) chemical analyses of the Closepet granite to investigate the role of fluids. By applying a new statistical tool (polytopic vector analysis) on the data, the authors identify intense fluid-rock/mineral interactions that are likely to involve four components (magmatic and fluid-rich) in a hybrid magma. A dynamic tectonic setting (possibly related to a mantle plume) characterized by heat influx, and redistribution and interaction between mantle-and crust-derived fluids, is proposed. Mamtani et al. (2020) elucidate the utility of anisotropy of magnetic susceptibility (AMS) in unravelling the deformation history and emplacement mechanism of granitoid plutons, especially for those granitoids lacking visible foliations/lineations. The authors discuss three Precambrian case studies: the Godhra Granite (Aravalli-Delhi Fold Belt), the Chakradharpur Granitoid (North Singhbhum Mobile Belt) and the Mulund Granite (Dharwar Craton). It is explained that the AMS study, in combination with field and microstructural data, can be effectively applied to recognize regional superposed deformation, to understand the link between granite emplacement, fabric development and regional deformation, and even to quantify the vorticity of the magma flow.
Granitoids are important sources of metals, as well as hydrothermal fluids, and are genetically connected to a wide variety of metal deposits. Pandit et al. (2020) evaluate the ore-forming potential of granitoids associated with two different cases of mineralization: the Malanjkhand Cu (+Mo) deposit located within the 2.48 Ga Malanjkhand Granite, Bastar Craton; and the Neoarchean greenstonebelt-hosted Au mineralizations occurring near the 2.56-2.51 Ga granitoids, EDC. The authors show that the apatite and biotite chemistry of the Malanjkhand Granite, in combination with whole-rock geochemistry, can be used to characterize the water and halogen content of the magma, the evolution path of the magmatic-hydrothermal fluid, and removal of metals and consequent mineralization. Further, based on fluid-inclusion studies, Pandit et al. (2020) suggest that granitoid-derived oxidized, late magmatic fluids might have contributed to the gold mineralization, in addition to metamorphogenic fluids.

Future directions
Vast tracts of Archean granitoid-greenstone belts are well exposed within the Indian cratons. The existing maps, mostly the result of commendable efforts by the Geological Survey of India (GSI), need to be updated by the application of modern concepts of granite typology. Academic institutions can play a significant role in the remapping of critical areas, which will also help in mineral exploration modelling. Also, geochronological and isotope geochemical data are still inadequate, especially for the granitoids of the Bastar, Bundelkhand and Aravalli cratons, to put the different juvenile crustal addition and crustal reworking events, and the secular changes in crustal evolution, within a global perspective.
With the advent of new, more affordable and faster analytical techniques, the next decade will see a flurry of data, particularly zircon U-Pb dates and Hf isotopes. However, the large amount of data should be utilized and appreciated effectively by being compiled, archived and integrated against a geological background to see the larger picture.
In particular, the data should be rooted in a spatial/ geological framework (e.g. maps) in order to better unravel patterns, trends and associations. This development will significantly improve our understanding of the antiquity and ancestry (in supercontinent or supercraton) of the Indian cratons. The recent discovery of Hadean xenocrystic zircons from the Singhbhum Granite (Chaudhuri et al. 2018) underlines the necessity of undertaking more such types of studies of Indian cratons in order to unravel very early Earth history. The Indian cratons offer ample scope for research over a broad spectrum of granitoid-related studies covering emplacement mechanism and processes, deformation and associated tectonics, fluid-rock interactions and attendant mineralizations, mineralogy and textural analysis (using techniques such as cathodoluminescence), exploration targeting, melting and formation of migmatites, and phase equilibria and experimental studies. This research will a play critical role in understanding the 'most common' but enigmatic granite systems better.
Acknowledgements The present understanding on the fascinating and complex geology of the Indian cratons is a result of the toiling and arduous work of generations of geologists, often in inhospitable terrains, starting from the early nineteenth century. This volume is a tribute to their contributions. The editors thank the authors for submitting their work to the special volume. The manuscripts were painstakingly reviewed by Anupam Chattopadhyay, Asko Kapyaho, Federico Farina, Jaana Halla, J.L. Vigneresse, John Ridley, Perttu Mikkola, Roberto Dall'Agnoll, W.J. Collins, Yamuna Singh, and 12 other reviewers who wished to be anonymous. The very efficient and prompt support from the editorial staffs of the Geological Society of London, especially Bethan Phillips, Angharad Hills, Samuel Lickiss and Tamzin Anderson, is thankfully acknowledged. We also thank the Geological Society of London for providing the platform to publish this special volume. Aniruddha Mitra is thanked for the help in preparation of Figure 4. Author contributions SD: conceptualization (lead), data curation (lead), formal analysis (equal), project administration (equal), visualization (equal), writingoriginal draft (lead), writingreview & editing (equal); JF-M: conceptualization (equal), formal analysis (equal), methodology (equal), project administration (equal), validation (lead), writingoriginal draft (supporting), writingreview & editing (equal).