Early growth of the Indian lithosphere: Implications from the assembly of the Dharwar Craton and adjacent granulite blocks, southern India

Highlights

• New insights into the geodynamic evolution of the Biligiri Rangan Block (BRB).

• The BRB charnockites and mafic granulites evolved by Neoarchean arc magmatism.

• Melting of ca. 3400–3200 Ma BRB protolith to form charnockites at ca. 2700–2500 Ma.

• Similar arc magmatic evolution of charnockites in the Nilgiri and Coorg Blocks.

• Closure of the Paleo-Dharwar Ocean and assembly of the Dharwar Craton.

Abstract

This study provides a new perspective into the Archean accretionary tectonic evolution of the Dharwar Craton and the adjacent high-grade granulite blocks of southern India. The necessary constraints for this tectonic assembly were derived using new petrochemical and zircon U-Pb age data from the granulite facies Biligiri Rangan Block (BRB), which occupies a central key position in the Archean collage of southern India. The BRB, composed largely of charnockites and mafic granulites, is a vestige of ca. 3400–3200 Ma cratonic fragment that was amalgamated with the Western Dharwar Craton (WDC) at ca. 2700–2500 Ma. The spatial variations in composition and evolution of the charnockites (i.e. low- and high-Sr/Y variants) are attributed to arc magmatic processes at different crust-mantle depths accompanied by a flat- or shallow-dipping subduction geometry. Different stages in the subduction processes such as exhumation and magma underplating (ca. 2800–2950 Ma), initial arc magmatism (ca. 2700–2650 Ma), and peak magmatism together with metamorphism (ca. 2700–2500 Ma) were identified from zircon age populations of different rock-types. Our new results integrated with all published geological and geophysical data lead to the conclusion that the Kollegal and Mettur Shear Zones on the western and eastern sides of the BRB respectively were sited on an east-dipping Archean suture and a closed back-arc basin. We propose a tectonic model for southern India according to which subduction-accretion of the Coorg Block in the Mesoarchean (ca. 3300–3100 Ma) and of the Nilgiri Block, Biligiri Rangan Block and the Eastern Dharwar Craton in the Neoarchean (ca. 2700–2500 Ma) led to closure of the Paleo-Dharwar Ocean and to terminal collision with the Western Dharwar Craton.

Introduction

Several Archean cratons across the globe are associated with high-grade granulite terranes, where charnockites (pyroxene-bearing granitoids) are the characteristic lithology (Bohlender et al., 1992, Frost et al., 2000, Rajesh and Santosh, 2004, Pouclet et al., 2007, Windley and Garde, 2009, Rajesh et al., 2011, Rajesh, 2012, Shi et al., 2019). Although the origin of charnockites (igneous vs. metamorphic) can be controversial, it is generally accepted that these rocks formed in a dry environment where a H₂O-undersaturated magma with a high CO₂ content was emplaced into the deep crust (Newton et al., 1980, Bohlender et al., 1992, Frost et al., 2000, Rajesh and Santosh, 2004, Rajesh and Santosh, 2012, Pouclet et al., 2007, Rajesh et al., 2011, Rajesh, 2012). It is apparent that the protoliths of charnockites can be generated either by partial melting of a melt-depleted granulite source (Sheraton, 1982, Munksgaard et al., 1992) or by dehydration partial melting of a mantle-derived underplated magma similar to that of a continental flood basalt (Kilpatrick and Ellis, 1992). The magnesian group charnockites probably formed in magmatic arcs, whereas the ferroan and transitional groups charnockites were more likely formed in extensional and continent-continent collisional tectonic settings, respectively (Rajesh, 2007, Frost and Frost, 2008, Mikhalsky and Kamenev, 2013). Correlations of diagnostic geochemical characteristics of charnockites with their times of emplacement and tectonic settings can be a useful proxy to help understand the evolution of high-grade terranes.

The Archean Dharwar Craton in southern India is surrounded by high-grade granulite terranes along its southern margins (Fig. 1), making it ideal for understanding the related tectono-magmatic and geodynamic processes. Like any Archean cratonic fragment, the Dharwar Craton is also made up of TTGs, grantioids, gneisses and greenstone belts. However, there are many controversial problems concerning the evolution of the Dharwar Craton (i.e. plume or arc magmatism?), as well as the tectonic relationships (eastward vs. westward polarity and the timing of subduction-accretion) with adjacent blocks such as the Western and Eastern Dharwar Cratons and the high-grade granulite terranes. The detailed tectonic setting of the Dharwar Craton, the different possible tectonic models and their limitations will be discussed below.

The Dharwar craton is traditionally divided into the Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC) separated by the Chitradurga Shear Zone (Swaminath and Ramakrishnan, 1981, Chadwick et al., 2000, Jayananda et al., 2000). The WDC is mainly composed of TTG (tonalite-trondjhemite-granodiorite) gneiss, popularly known as the Peninsular Gneiss (ca. 3450–3200 Ma), which is closely associated with greenstone belts and calc-alkaline granitoids (i.e., sanukutoids and high-potassic granites) (Jayananda et al., 2013a, Jayananda et al., 2015). Two generations of volcano-sedimentary greenstones comprise an older Sargur Group (ca. 3300 Ma) (Peucat et al., 1995) and a younger Dharwar Supergroup (ca. 2900–2700 Ma) (Kumar et al., 1996), and there are two generations of potassic granites that were intruded into the TTGs at ca. 3300–3000 Ma and ca. 2640–2600 Ma (Jayananda et al., 2006, Chadwick et al., 2007, Chardon et al., 2011, Peucat et al., 2013, Jayananda et al., 2015). The Sargur Group includes ultramafic-mafic rocks that vary in composition from komatiite to high-Mg basalt (ca. 3380–3150 Ma), felsic volcanic rocks (3298 ± 6 Ma), and interbedded quartzites, carbonates, and BIF (ca. 3600–3230 Ma). The Dharwar Supergroup is divisible into the lower Bababudan and upper Chitradurga Groups. The Bababuden Group contains oligomict conglomerates, basaltic flows (ca. 2911–2848 Ma) with interbedded volcanic tuffs (2720 ± 7 Ma) and meta-sediments such as quartzites, phyllites, carbonates, and BIF (Kumar et al., 1996, Trendall et al., 1997a). In contrast, the Chitradurga Group consists of basal polymict conglomerates, basaltic flows (ca. 2747 ± 15 Ma), intermediate to felsic volcanic rocks (ca. 2677–2576 Ma), and meta-sediments such as carbonates (2639 ± 39 Ma), greywackes, argillites, and BIF (Kumar et al., 1996, Nutman et al., 1996, Trendall et al., 1997b, Jayananda et al., 2013b).

The Eastern Dharwar Craton (EDC) is divisible into an older Central Dharwar Province and a younger Eastern Dharwar Province (e.g. Peucat et al., 2013). Peucat et al., 2013, Jayananda et al., 2013a, Jayananda et al., 2018 defined the Central Dharwar Province (CDP) as a transitional lithospheric domain extending from the Chitradurga Shear Zone northwards to the western margin of the Kolar-Kadiri-Hungund (KKH) belt. The KKH belt comprises older migmatitic TTG gneisses (ca. 3375–2960 Ma), meta-volcanic greenstones (ca. 2700 Ma), younger banded tonalitic to granodioritic gneisses (ca. 2700–2560 Ma), and late granitoids. A 500 km-long NS-trending Closepet granite batholith (ca. 2528–2513 Ma) (Friend and Nutman, 1991, Jayananda et al., 1995) has been considered as a part of the Eastern Dharwar Craton, the western boundary of which is sited on a 200 km-wide lithologic transitional zone extending to the Peninsular Gneisses of the Western Dharwar Craton (e.g. Meert et al., 2010). The volcanic rocks in the Central Dharwar Province comprise different greenstone belts with ultramafic rocks (komatiite), metabasalts (ca. 2746–2707 Ma), amphibolites (2740 ± 90 Ma), rhyolites (ca. 2691–2658 Ma), and pyroclastics (2707 ± 18 Ma) (Nutman et al., 1996, Balakrishnan et al., 1999, Naqvi et al., 2002, Jayananda et al., 2018). Peucat et al. (2013) considered that the evolution of the Eastern Dharwar Province involved tonalitic magmatism (ca 2.7–2.6 Ga), syntectonic juvenile felsic magmatism (ca. 2.55–2.53 Ga), and granulite facies metamorphism (ca 2.52–2.51 Ga). The volcanic greenstones in the Eastern Dharwar Province include minor komatiites, major basalts (ca. 2700–2600 Ma) and felsic volcanic rocks (ca. 2700–2680 Ma and 2587–2545 Ma), and interbedded pelites such as greywacke, argillite, and BIF (ca. 2719–2698 Ma) (Balakrishnan et al., 1990, Rogers et al., 2007, Khanna et al., 2014, Khanna et al., 2016, Jayananda et al., 2013b).

Several granulite facies, discrete crustal blocks that crop out to the south of the Archean TTG-granite-greenstone Dharwar Craton include the Coorg Block, Nilgiri Block, Biligiri Rangan Block, Shevaroy Block, and the Madras Block, which are delimited by the Fermor line (Fermor, 1936, Drury et al., 1984) (Fig. 1). The E-W trending Palghat Cauvery Shear System separates these Archean granulite terranes to the north from the Southern Granulite Terrain (SGT). However, Ghosh et al. (2004) suggested that the Karur–Kamban–Painavu–Trichur Shear Zone (KKPTSZ) (Fig. 1) marks the continuation of the Dharwar Craton into the SGT, which they considered to be a Neoarchean suture that separates older metasediments (>2.9 Ga) in the Northern Madurai Block from younger (<2.0 Ga) paragneisses in the Southern Madurai Block.

The tectonic evolution of the Western Dharwar Craton in relation to that of the Eastern Dharwar Craton has proven to be highly controversial. A plume-related origin was suggested for the TTG rocks in the Western Dharwar Craton (Choukroune et al., 1994, Choukroune et al., 1997). However, Jayananda et al., 2015, Jayananda et al., 2018 proposed a combined mantle plume-arc model in which the Craton evolved by the multiple accretion of oceanic plateaus derived from mantle plumes, and the TTGs formed during a second stage of melting at the base of the arc/plateau crust; these events eventually led to closure of the intervening ocean at ca. 3450–3150 Ma. According to these models, the Western Dharwar Craton was cratonized at ca. 3230–3100 Ma. The accretionary tectonic models concern the polarity and timing of accretion between the eastern and western parts of the overall Dharwar Craton, and its tectonic relationships with adjoining high-grade granulite terranes/blocks. A westward subduction model was mainly based on the ideas: (1) accretion of the Western Dharwar Craton with the Eastern Dharwar Craton was preceded by WNW-directed subduction of oceanic lithosphere beneath the overriding Western Dharwar Craton on the site of the Chitradurga Shear Zone at ca. 2750–2510 Ma (Chadwick et al., 2000); (2) accretion of microcontinents and oceanic plateaus through progressive westward subduction of the intervening oceanic crust was successively beneath the eastern margins of the Western Dharwar Craton, and the Central Dharwar and Eastern Dharwar Provinces, leading to their final amalgamation at ca. 2740–2560 Ma (Jayananda et al., 2013a, Jayananda et al., 2018). Both these models have serious limitations, mainly because there is a lack of expected Neoarchean arc magmatism in the overlying Western Dharwar Craton. The westward subduction model proposed for the 2740–2560 Ma amalgamation of the Western and Eastern Dharwar Cratons was based only on the chemical signature of magmatic arc rocks in and around the Chitradurga Shear Zone, but no convincing evidence for a significant late Archean magmatic arc has been reported from the Western Dharwar Craton. Therefore, the model of late Archean westward subduction beneath the Western Dharwar Craton and consequent amalgamation with the Eastern Dharwar Craton is untenable.

The Biligiri Rangan Block (BRB) (Fig. 1, Fig. 2) is separated from the Western Dharwar Craton by the NNE/SSW-trending Kollegal Shear Zone (a southward extension of the Chitradurga Shear Zone) and that joins with the Moyar Shear Zone in the south, where it delineates the Nilgiri Block. To the east, the NE/SW-trending Mettur Shear Zone separates the BRB from the Shevaroy Block. Because the BRB occupies a key position among the assembly of the Dharwar Craton and high-grade granulite blocks (Fig. 1), it represents a crucial area for resolution of the problem of regional tectonic framework, subduction polarity and metamorphic evolution of the blocks in the assembly. Therefore, the present study examines the limited published results from the BRB and addresses the following existing data gaps in order to provide a better understanding of its tectonic evolution.

The BRB was formerly considered to be a deeply buried, southerly extension of the Dharwar Craton (Pichamuthu, 1960, Ramakrishnan and Vishwanathan, 1983). Later studies of the crustal evolution of the BRB were mostly based on strain fabric analysis (Chardon et al., 2008), and on limited studies of lithologies (Janardhan et al., 1994, Peucat et al., 2013, Basavarajappa and Srikantappa, 2014, Ratheesh-Kumar et al., 2016), and hence they failed to explain the spatial variations of data necessary for regional tectonic interpretations, and that has created significant ambiguities in understanding the evolution of the Dharwar Craton. Peucat et al. (2013) broadly divided the lithologies in the BR-Hill, namely charnockites, enderbites, mafic granulites, and meta-sediments, into three age groups including >3300 Ma, ca. 3000 Ma, and ca. 2540 Ma based on their Nd model ages. Of these groups, the first two exhibit a typical TTG affinity, whereas the youngest has transitional TTG characteristics related to a crustal remelting process. The zircon U-Pb age data of their pyroxene bearing granitoid samples show two prominent age groups with inherited magmatic zircons that yield older ages between ca. 3000 and 3400 Ma, and younger granulite facies ages between 2510 and 2600 Ma, and yet another group of ca. 2648 Ma magmatic zircons were attributed to a specific thermal event. Accordingly, Peucat et al. (2013) interpreted the BRB largely as a central transition zone, i.e., between the deeper levels of the eastern fringe of the Western Dharwar Craton and the Central Dharwar Province. Based on fabric strain analysis Chardon et al. (2008) opined that the BRB was underlain by more attenuated lithospheric mantle than the WDC, and the resulting enhanced heat exchanges between the mantle and crust were responsible for upward flushing of CO₂-rich fluids that ultimately resulted in the charnockitization. However, Ratheesh-Kumar et al. (2016) reported that the BRB was a discrete crustal block that evolved by arc magmatism, which was the result of eastward subduction of the Western Dharwar oceanic crust beneath the BRB in the Neoarchean (ca. 2500–2700 Ma). This model challenges the ideas of a southward deeper level of erosion of the WDC (Peucat et al., 2013), of lithospheric attenuation-related CO₂-rich fluid flushing and charnockitization (Chardon et al., 2008), and of westward subduction of the Eastern Dharwar oceanic crust beneath the WDC (Chadwick et al., 2000, Chardon et al., 2008, Jayananda et al., 2013a). A summary of the available age data on different lithologies from the BRB is presented in Table 1.

This study presents new compositional and age data of the major lithologies from the BRB, which provide crucial evidence for the major tectono-magmatic events and mode of crustal evolution of this block in relation to the Dharwar Craton and the neighboring high-grade granulite blocks. We integrate our results with published data from the Nilgiri and Coorg high-grade granulite blocks, which enable us to propose a new model for the amalgamation of the microcontinental blocks to form the tectonic ensemble of the Dharwar Craton at different times in the Archean. The advantage of this approach lies in the fact that it encompasses a more comprehensive account of the crustal evolution and accretionary tectonics of the different neighboring tectonic blocks that were closely associated with the Dharwar Craton and accordingly it resolves many of the existing ambiguities of different interpretations.