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Volume 32 Issue 6
Dec.  2021
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Chakravadhanula Manikyamba, Sohini Ganguly, Arijit Pahari. Geochemical Features of Bellara Trap Volcanic Rocks of Chitradurga Greenstone Belt, Western Dharwar Craton, India: Insights into MORB-BABB Association from a Neoarchean Back-Arc Basin. Journal of Earth Science, 2021, 32(6): 1528-1544. doi: 10.1007/s12583-021-1472-5
Citation: Chakravadhanula Manikyamba, Sohini Ganguly, Arijit Pahari. Geochemical Features of Bellara Trap Volcanic Rocks of Chitradurga Greenstone Belt, Western Dharwar Craton, India: Insights into MORB-BABB Association from a Neoarchean Back-Arc Basin. Journal of Earth Science, 2021, 32(6): 1528-1544. doi: 10.1007/s12583-021-1472-5

Geochemical Features of Bellara Trap Volcanic Rocks of Chitradurga Greenstone Belt, Western Dharwar Craton, India: Insights into MORB-BABB Association from a Neoarchean Back-Arc Basin

doi: 10.1007/s12583-021-1472-5
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  • This study presents a comprehensive account of the petrogenetic and geodynamic evolution of the Bellara Trap volcanic rocks from the Ingaldhal Formation, Chitradurga Group, western Dharwar Craton (WDC). Geochemical attributes of these rocks are consistent with two groups with distinct evolutionary trends: one comprising tholeiitic, MORB (mid-ocean ridge basalt) type basalts (BTB) and the other corresponding to calc-alkaline andesites (BTA). Basalts are essentially composed of clinopyroxene and plagioclase whereas the andesites are porphyritic with phenocrysts of plagioclase, clinopyroxene and polycrystalline quartz embedded in a groundmass of K-feldspar, quartz and opaques. Primary igneous mineralogy is overprinted by greenschist facies metamorphism resulting in chlorite-actinolite-plagioclase assemblage. The BTB samples reflect nearly flat REE patterns with weak LREE enrichment in contrast to pronounced LREE enhancement over HREE discernible for BTA. Tectonically, the BTB samples correspond to an active mid-oceanic ridge-rift setting with a MORB composition, whereas a back-arc basin (BAB) regime is corroborated for the BTA samples fractionating from back-arc basin basalts. Geochemical imprints of subduction input are more pronounced in BTA compared to BTB as mirrored by their elevated abundances of incompatible fluid mobile elements like Ba, Th, U and LREE. The BTB is endowed with an N-to E-MORB signature attributable to minor contributions from subduction-related components at the inception of a back-arc basin in the vicinity of an active subduction system. The BTA derived through differentiation of a basaltic magma with BABB (back-arc basin basalt) affinity compositionally akin to a heterogeneous source mantle carrying depleted MORB-type and enriched arc-type components inducted with progressive subduction. The BABB-type andesites and MORB-type basalts from Bellara Traps record a compositional heterogeneity of mantle in an intraoceanic arc-back arc system. Mantle processes invoke a BABB-MORB spectrum with a MORB-like endmember and an arc-like endmember associated with a juvenile back-arc basin. This study infers a Neoarchean analogue of Mariana-type back-arc rift setting proximal to the arc with a gradual transition from anhydrous to hydrous melting processes synchronized with MORB-mantle and arc-mantle interaction during initiation of a nascent back arc adjacent to the arc. The MORB-BABB compositional spectrum for the Bellara Traps conforms to a Neoarchean back-arc basin that evolved under an extensional tectonic regime associated with incipient stages of back-arc rifting and incorporation of subduction-derived components in the mantle output. This study complies with Neoarchean intraoceanic accretionary cycle plate tectonics in WDC.
  • Electronic Supplementary Material: Supplementary material Table S1 is available in the online version of this article at https://doi.org/10.1007/s12583-021-1472-5.
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Geochemical Features of Bellara Trap Volcanic Rocks of Chitradurga Greenstone Belt, Western Dharwar Craton, India: Insights into MORB-BABB Association from a Neoarchean Back-Arc Basin

doi: 10.1007/s12583-021-1472-5

Abstract: This study presents a comprehensive account of the petrogenetic and geodynamic evolution of the Bellara Trap volcanic rocks from the Ingaldhal Formation, Chitradurga Group, western Dharwar Craton (WDC). Geochemical attributes of these rocks are consistent with two groups with distinct evolutionary trends: one comprising tholeiitic, MORB (mid-ocean ridge basalt) type basalts (BTB) and the other corresponding to calc-alkaline andesites (BTA). Basalts are essentially composed of clinopyroxene and plagioclase whereas the andesites are porphyritic with phenocrysts of plagioclase, clinopyroxene and polycrystalline quartz embedded in a groundmass of K-feldspar, quartz and opaques. Primary igneous mineralogy is overprinted by greenschist facies metamorphism resulting in chlorite-actinolite-plagioclase assemblage. The BTB samples reflect nearly flat REE patterns with weak LREE enrichment in contrast to pronounced LREE enhancement over HREE discernible for BTA. Tectonically, the BTB samples correspond to an active mid-oceanic ridge-rift setting with a MORB composition, whereas a back-arc basin (BAB) regime is corroborated for the BTA samples fractionating from back-arc basin basalts. Geochemical imprints of subduction input are more pronounced in BTA compared to BTB as mirrored by their elevated abundances of incompatible fluid mobile elements like Ba, Th, U and LREE. The BTB is endowed with an N-to E-MORB signature attributable to minor contributions from subduction-related components at the inception of a back-arc basin in the vicinity of an active subduction system. The BTA derived through differentiation of a basaltic magma with BABB (back-arc basin basalt) affinity compositionally akin to a heterogeneous source mantle carrying depleted MORB-type and enriched arc-type components inducted with progressive subduction. The BABB-type andesites and MORB-type basalts from Bellara Traps record a compositional heterogeneity of mantle in an intraoceanic arc-back arc system. Mantle processes invoke a BABB-MORB spectrum with a MORB-like endmember and an arc-like endmember associated with a juvenile back-arc basin. This study infers a Neoarchean analogue of Mariana-type back-arc rift setting proximal to the arc with a gradual transition from anhydrous to hydrous melting processes synchronized with MORB-mantle and arc-mantle interaction during initiation of a nascent back arc adjacent to the arc. The MORB-BABB compositional spectrum for the Bellara Traps conforms to a Neoarchean back-arc basin that evolved under an extensional tectonic regime associated with incipient stages of back-arc rifting and incorporation of subduction-derived components in the mantle output. This study complies with Neoarchean intraoceanic accretionary cycle plate tectonics in WDC.

Electronic Supplementary Material: Supplementary material Table S1 is available in the online version of this article at https://doi.org/10.1007/s12583-021-1472-5.
Chakravadhanula Manikyamba, Sohini Ganguly, Arijit Pahari. Geochemical Features of Bellara Trap Volcanic Rocks of Chitradurga Greenstone Belt, Western Dharwar Craton, India: Insights into MORB-BABB Association from a Neoarchean Back-Arc Basin. Journal of Earth Science, 2021, 32(6): 1528-1544. doi: 10.1007/s12583-021-1472-5
Citation: Chakravadhanula Manikyamba, Sohini Ganguly, Arijit Pahari. Geochemical Features of Bellara Trap Volcanic Rocks of Chitradurga Greenstone Belt, Western Dharwar Craton, India: Insights into MORB-BABB Association from a Neoarchean Back-Arc Basin. Journal of Earth Science, 2021, 32(6): 1528-1544. doi: 10.1007/s12583-021-1472-5
  • Precambrian and Phanerozoic intraoceanic subduction systems are potential contributors to growth and recycling of the Earth's crust through a dynamic interplay between the hydrosphere, lithosphere and asthenosphere. Paleo-arc sections preserved in supracrustal assemblages of all ancient cratonic nuclei of the world are indicative of lateral crustal growth through accretion of juvenile crustal materials at ocean-ocean and ocean- continent convergent plate margins (Windley et al., 2021; Moyen and Laurent, 2018; Smithies et al., 2018; Manikyamba et al., 2017; Cawood et al., 2016; Xiao and Santosh, 2014; Condie and Kroner, 2013; Polat, 2013; Santosh, 2013). The along-arc and across-arc compositional variations of mantle outflux at subduction zones are explicitly mediated by two kinds of inputs into the arc system i.e., (ⅰ) the mantle and (ⅱ) the subduction components. The former invokes refertilization of the sub-arc mantle by enriched asthenospheric components from upwelling plumes and recycling of pre-existing continental/oceanic crust via lithospheric delamination. The subduction input can be translated in terms of variable influx of (ⅰ) fluid and melt derived by dehydration and melting of subducted oceanic slab respectively (ⅱ) altered basaltic oceanic crust, and (ⅲ) subducted sediment veneer carrying continent- and ocean-derived components. These inputs trigger elemental cycling at ocean-crust-mantle interface, metasomatism, hybridization and melting of hydrated mantle wedge which eventually contributes to the chemical composition of arc magmas (Zheng, 2019; Ancellin et al., 2017; Pearce and Stern, 2006; McCulloch and Gamble, 1991; Defant and Drummond, 1990). The principal proponents for back arc basin (BAB) magmatism include: (1) the composition of mantle asthenosphere flowing inward and its response to anhydrous melting conditions; (2) the migration of subduction-derived fluids and melts into the arc-basin system; (3) the nature and extent of interaction between the depleted MORB-type and enriched arc-type mantle components; and (4) the fluid-fluxed partial melting of metasomatized mantle wedge ensued by crustal assimilation and crystal fractionation of resultant magma. Back arc basins, though generally envisaged to be analogous to mid-oceanic ridge-rift systems, are however characterized by distinct petrogenetic trends governed by incipient to matured stages of back-arc rifting and relative proximity to the adjacent arc regime. Back arc basin (BAB) magmatism constitutes an integral component of the global subduction factory and manifests adiabatic decompression melting of asthenospheric mantle upwelled to shallow depths in response to extension of overriding plate and thinning of lithosphere triggered by subduction retreat and slab roll-back (Magni, 2019; Masuda and Fryer, 2015; Stephenson and Schellart, 2010; Sdrolias and Muller, 2006).

    The geodynamic evolution of Archean cratons invokes episodic crust generation and terrane accretion processes at accretionary and collisional orogenic systems including amalgamation of oceanic plateaus, oceanic and continental fragments from convergent plate margins (Manikyamba and Ganguly, 2020; Manikyamba et al., 2017; Cawood et al., 2016, 2009; Kusky et al., 2016; Deng et al., 2014; Furnes et al., 2014; Wang et al., 2013; Zhai and Santosh, 2011). Therefore, every Archean craton across the globe comprises a composite mosaic of microcontinental blocks that preserve distinct imprints of mantle plume activities, subduction processes, parallel arc amalgamation and plume-arc interactions (Wyman, 2018; Condie and Aster, 2013). The predominant manifestations of mantle plume and convergent margin magmatic processes are observed in the komatiite-tholeiitic basalt and calc-alkaline basalt-andesite-dacite-rhyolite assemblages of volcanic supracrustals of Archean greenstone belts respectively (Manikyamba et al., 2014a, b, 2009; Manikyamba and Kerrich, 2012; Polat and Kerrich, 2006; Smithies et al., 2005; Hollings and Kerrich, 2000; Kusky and Polat, 1999; Kerrich et al., 1998). Typical MORB-type mantle signatures are rarely documented from Archean greenstone terranes. However, the contributory role of plume versus subduction tectonics in Archean continent generation and crustal growth has been made complicated by input from periodically rejuvenated sub-cratonic lithospheric mantle (SCLM), thermal erosion and delamination of Archean cratonic mantle roots (ACMR), recycling and reworking of older crust (Wang et al., 2017; Arndt et al., 2009; Foley, 2008; Foley et al., 2003).

    The Dharwar Craton is a potential archive of polychronous accretionary and collisional orogens that can be equated with the ancient orogenic records of worldwide Archean cratons in terms of global tectonic evolution. Integrated geological, geochemical and geochronological evidence from the Precambrian granite-greenstone terranes of the Dharwar Craton provides key perspectives testifying thermo-tectonic evolution of the Earth from pre-3.0 Ga accretionary orogen style plate tectonics marked by intra-oceanic arc-back arc systems to post 2.7–2.5 Ga modern style plate tectonics involving both accretionary orogen style and Wilson Cycle style tectonics (Windley et al., 2021). In this paper, we present new petrological and geochemical data for volcanic rocks of the Bellara Traps of the Ingaldhal Formation of the Chitradurga Group, the western Dharwar Craton to (ⅰ) first-time elucidate MORB-BABB signatures from Neoarchean volcanic supracrustals of the Dharwar Supergroup, (ⅱ) address their petrogenesis and tectonic affiliation, and (ⅲ) correlate the observations towards augmentation of the tectonomagmatic evolution of the WDC.

  • The Dharwar Craton (Fig. 1a) comprises a complex collage of Archean protocontinental blocks accreted along paleo-oceanic sutures that collectively preserve discrete thermo-tectonic records of accretionary to collisional orogenic processes, vertical and lateral growth of crust and episodic assembly and dispersal of continents with emergence and closure of paleo-oceans (Pahari et al., 2020, 2019; Manikyamba et al., 2015, 2014a, b; Chardon et al., 2011, 1998, 1996; Jayananda et al., 2008, 2006, 2000; Naqvi, 1983). The major lithological associations of the Dharwar Craton constitute (ⅰ) 3.6–2.56 Ga basement gneisses including 3.45–2.96 Ga TTG, 3.23–3.2 Ga diapiric trondhjemites and 2.7–2.67 Ga younger TTGs; (ⅱ) 3.38–3.15 Ga slivers and enclaves of older Sargur Group greenstones; volcano-sedimentary sequences of 2.9–2.85 Ga Bababudan Group and 2.74 Ga Chaitradurga Group; and (ⅲ) 3.15–2.61 Ga calc-alkaline potassic granitoids and 2.58–2.52 Ga sanukitoids (Jayananda et al., 2020, 2013a, b; Pahari et al., 2020; Chadwick et al., 2000, 1981). Recent geological, geophysical and geochronological studies propounded that the Dharwar Craton evolved through congregation of western, central and eastern microcontinental blocks with distinct thermal, metamorphic and tectonic records. The TTG basement gneisses, older and younger supracrustals of volcano-sedimentary sequences and syn- to post-collisional granitoid plutons exposed in different areas of western (WDC), central (CDC) and eastern (EDC) sectors of the Dharwar Craton yield important insights into the differentiation of primitive mantle, lithospheric evolution and mineralization in diverse tectonic environments. The geochemical attributes of Meso- to Neoarchean greenstone belt assemblages from WDC and CDC have invoked magmatism from chemically heterogeneous, hydrated, ancient mantle plumes, plume-lithosphere interaction, oceanic and continental convergent margin processes. However, the volcanic supracrustals from EDC predominantly substantiate composite arc back arc magmatism placing compelling evidence for incipient to mature stages of intraoceanic subduction with back arc rifting proximal and distal to subduction realm (Li et al., 2018; Santosh and Li, 2018; Radhakrishna and Curtis, 1999; Peucat et al., 1995; Janardhan et al., 1994; Swami Nath and Ramakrishnan, 1981).

    Figure 1.  (a) Inset map showing different cratons of India (after Sharma, 2009); BuC. Bundelkhand Craton; SiC. Singhbhum Craton; BaC. Bastar Craton; DhC. Dharwar Craton; (b) geological map of Dharwar Craton (after Jayananda et al., 2013b); EDC. eastern Dharwar Craton; WDC. western Dharwar Craton; TTG. trondhjemite, tonalite, granodiorite; (c) geological map of Chitradurga schist belt (modified after Sheshadri et al., 1981); BIF. banded iron formation; and (d) structural domain sketch (not to scale) of studied area and associated gold mines with sample location (GSI unpublished report).

    The Sargur Group (3.3–3.1 Ga) supracrustals reported from WDC predominantly comprise komatiite to high-Mg volcanic rocks with minor basaltic to felsic rocks interlayered with ferruginous quartzite-pelite-carbonate of shallow shelf assemblages (Jayananda et al., 2016, 2008; Lancaster et al., 2015; Nutman et al., 1992; Taylor et al., 1984). The Dharwar Supergroup in WDC is further subdivided into the lower Bababudan Group and the upper Chitradurga Group. The 2 910–2 720 Ma Bababudan Group comprises thin bands of iron stone, sheets of ultramafic rocks (talc-tremolite-chlorite-carbonate schist), amygdular metabasalt, cross bedded quartzite and Neralakatte oligomictic conglomerate lying unconformably over the 3.6–3.23 Ga peninsular gneissic complex (PGC). The Chitradurga Group unconformably overlies the Bababudan Group and PGC with prominent unconformity constitutes the major part of Dharwar Supergroup. The Chitradurga schist belt (CSB; Fig. 1b) is exposed along the middle of the Karnataka State with a NNW-SSE trend. It extends from Gadag in the north to Srirangapatna in the south for a length of 460 km. The average width of the belt is 15 km and it attains maximum width of 40 km near Chitradurga. The PGC flanks the CSB to the east and west and also occurs as diapiric domes within the belt. The PGC encloses several lenticular bodies of ultramafic rocks adjoining and parallel to the CSB. The western arm extends for a short distance up to Mayakonda and the eastern arm continuous up to Gadag in the north.

    The Chitradurga Group is conventionally classified into three major formations (Table 1). The Vanivilas Formation exposed along the western margin of the belt includes chlorite schist, quartzite, carbonates, Fe-Mn formations and the Talya conglomerate. These lithounits are well exposed around the Srirankatte gneissic dome. The Ingaldhal Formation comprising basic to acidic lavas, pyroclastics, banded iron formation and fine grained clastics (argillites) is exposed mainly south and east in the Chitradurga greenstone belt; the Hiriyur Formation is represented by the greywacke-argillitic suite interspersed with BIF, metabasalt and polymictic conglomerate. The SHRIMP U-Pb zircon age of 2 614±10 Ma obtained for the Chitradurga granite indicates the youngest intrusive event in this belt (Jayananda et al., 2013a, 2006). In Chitradurga schist belt, known gold mineralization is confined to the eastern section and the well-known mineralized corridor comprising Bellara, Ajjanahalli, Anesidri, G. R. Halli, C. K. Halli, Hionnemaradi and Ramajogihalli deposits/ prospects is bounded by two regional faults (Subrahmanyam et al., 2001). The Ingaldhal Formation represents several individual volcanic cycles which are exposed at Haradihalli, Mallappanhalli and Bellara areas within the Chitradurga greenstone belt. Thin flows of volcanic rocks, interbedded with greywacke-phyllites at Bellara Village, are referred as Bellara Traps. Lens shaped outcrops of traps, occurring between Bellara and Bukkapatna, exhibit intermediate to basic composition. The Bellara Trap is represented by intermediate to basic lava flows, locally pillowed and intruded by a large number of basic dykes (Sheshadri et al., 1981). Due to structural complexities, their stratigraphic relation to the Ingaldhal Formation is uncertain. The geochemistry of the Bellara Traps is also unknown compared to the well-studied volcanic sections of Ingaldhal, Jogimaradi and Maradihalli. Bellara Traps are well known for lode gold mineralization hosted in metabasalts occurring to the south of Ajjanahalli. Pillow structures have been recognized in some of the flows in the vicinity of the abandoned gold mines at Bellara (13°36′34″N; 76°41′44″E). Two auriferous reefs were located (Hill reef and Tank reef) at Bellara with a strike length of 2 km. Though the overall grade of the ore is very poor, according to the available literature during 1944–1954, 62 kg of gold has been extracted from a rich ore of 400 t. The Tank reef explored > 600 m strike and 125 m depth whereas the Hill reef was explored > 135 m strike and 70 m depth (Radhakrishna and Curtis, 1999). Later on, Bharat Gold Mines Limited (BGML) conducted exploration studies and found that the overall grade of the ore is very poor.

    Dharwar Supergroup Chitradurga Group Basic dyke gabbro and dolerite
    Younger granites (Chitradurga, Hosadurga, and Jampalnaikan kote)
    Hiriyur Formation Basic dykes (gabbro and dolerite);
    younger granite (Chitradurga, Hosadurga and J. N. Kote);
    greywacke—argillite suit+basic to intermediate volcanics, banded ferruginous chert and polymict conglomerates (Aimangala and Holelkere);
    K. M. Kere and G. R. Halli conglomerates
    --------------Disconformity-----------------------
    Ingaldhal Formation Basic, intermediate/acid lavas/pyritiferous chert/argillite, chloritic phyllite, banded ferrugionous chert, limestone and dolomite
    Vanivilas Formation Chlorite-biotite+garnet
    phyllite/quartizite and quartz schist, Talya conglomerate
    -------------------Unconformity-----------------------
    Bababudan Group Amygdular metabasalt closely interbedded with cross bedded and ripple marked quartzite, ultramafite (talc-tremolite chlorite and serpentinite) and thin beds of iron stones, oligomictic conglomerate (Neralakatte)
    ----------------------------Unconformity--------------------------------------
    PGC Peninsular gnessic complex
    Sargur 3 300 Ma Gattihosahalli belt
    fuchsite quartzite with barites, aluminous quartz schist
    meta-ultramafite, amphibolite

    Table 1.  Stratigraphic succession of Chitradurga schist belt (modified after Beeraiah et al., 2010; Swami Nath and Ramakrishnan, 1981)

  • The volcanic lithounits exposed near the Bellara Village are resembling massive melanocratic intermediate variety of tuffaceous rocks. These volcanic rocks rarely show flow structures. We have not collected the sample from the outcrop due to thick vegetation cover and accessibility problem. Drilling core samples (BGM) have been collected from the BGML (Bharat Gold Mine Limited) core library for the present study. These drill holes are mainly scout drill hole for gold exploration with an interval of 100 m on surface and 60 m in depth. We have collected 38 samples from three bore holes and depth of these core samples is 60 m.

    After the petrographic screening, rock samples with minimal weathering effects were selected for bulk chemical analyses and were finely powdered to 240 mesh size with the help of jaw crusher and pulverizer. For major element analyses, pressed pellets were prepared by using collapsible aluminium cups. These cups were filled with boric acid and about 1 g of the finely powdered rock sample was kept on the top of the boric acid and pressed under a hydraulic press at 20 tons pressure to get a pellet. The sample pellets were analyzed using a Philips MagiX PRO model PW-2440 wavelength dispersive X-ray fluorescence spectrometer (XRF) coupled with automatic sample changer PW 2540. The international geological reference standards that were used for the major element analyses by XRF are JB-2 and BHVO-1. Trace elements including REE were determined by HR-ICP-MS (Nu Instruments, ATTOM High Resolution ICP-MS, UK), following the procedure of closed vessel acid digestion technique. The 50 mg representative sample powders were dissolved in savillex® vessels containing 10 mL acid mixture of HF : HNO3 in 7 : 3 proportion, and kept on hot plate at 150 ℃ for 48 h. After complete digestion, 2–3 drops of perchloric acid (HClO4) was added and the entire mixture was evaporated to complete dryness. Freshly prepared 20 mL of 1 : 1 HNO3 was added and kept on hot plate at ~80 ℃ for 10–15 min. After obtaining clear solution, 5 mL of Rh (1 ppm concentration) was added as an internal standard and made into 250 mL. Five milliliter of this solution was further diluted to 50 mL to achieve suitable TDS (total dissolved solids). Certified reference materials of United States Geological Survey (USGS; BHVO-1) was run as standards during the analysis for accuracy of the data. The quantitative measurements were performed using the instrument software (Attolab v.1). Precision and accuracy obtained for international reference materials are found to be < 5% RSD for the majority of the trace elements (Manikyamba et al., 2014a). Major and trace element data for representative samples of the Bellara volcanic rocks are summarized in Table S1.

  • The essential mineralogy of the Bellara volcanic rocks is marked by clinopyroxene and plagioclase with quartz, chlorite, actinolite, and opaques occurring as accessory phases (Fig. 2a). The petrographic characters suggest that the pristine mantle- derived mineral composition of the Bellara Trap volcanic rocks has been overprinted by chlorite-actinolite amphibole-plagioclase assemblage thereby conforming green schist to lower amphibolite facies metamorphism (Fig. 2b). Andesites show porphyritic texture with large lath shaped amphiboles and groundmass is mainly quartz, K-feldspar and opaques (Fig. 2c). The groundmass is aligned parallelly or subparallely indicating the flow direction in some sections whereas crude parallelism is observed in the phenocrysts which developed nematoblastic texture. Secondary opaque minerals are present as accessories. A primary intergranular igneous texture (Fig. 2d) is rarely preserved in few sections. Due to recrystallization, in some samples schistose nematoblastic texture has been developed. As these rocks are metamorphosed to green schist facies metamorphism, "meta" is implicit for all the studied samples.

    Figure 2.  Photomicrographs of Bellara Trap basalts showing (a) the represented minerals, clinopyroxene, plagioclase, and the intergranular texture; (b) greenschist facies metamorphism indicated by chlorite-actinolite-plagioclase assemblage; (c) andesites exhibiting porphyritic texture with amphibole phenocryst; and (d) intergranular texture observed in some sections.

  • The Bellara volcanic rocks exhibit a distinct bimodal compositional spectrum and are primarily classified as basalts and andesites on Nb/Y vs. SiO2 diagram of Winchester and Floyd (1977) (Fig. 3a). In terms of Fe2O3/MgO vs. SiO2 (Fig. 3b) and Zr vs. Y variations (Fig. 3c; after Ross and Bédard, 2009; Miyashiro, 1984), the Bellara Trap basalts (BTB) show a tholeiitic trend, while the Bellara Trap andesites (BTA) reflect a calc-alkaline composition of parental magma. Major oxide compositions for the basalts are characterized by lower SiO2 contents (44.81 wt.%–49.56 wt.%), comparable TiO2 (0.74 wt.%–1.54 wt.%), Al2O3 (10.63 wt.%– 14.57 wt.%), Fe2O3 (11.05 wt.%–15.1 wt.%) and relatively higher CaO (11.54 wt.%–14.86 wt.%) and MgO (7.1 wt.%–11.0 wt.%) as compared to the andesite samples marked by 52.01 wt.%–61.83 wt.% SiO2, 0.64 wt.%–0.95 wt.% TiO2, 11.01 wt.%–19.1 wt.% Al2O3, 7.5 wt.%–15.4 wt.% Fe2O3, 5.61 wt.%–7.58 wt.% CaO and 1.96 wt.%–3.94 wt.% MgO. On MgO vs. different major elements and their ratios, these basalts and andesites are forming as two clusters and exhibiting magmatic trend (Fig. 4).

    Figure 3.  (a) Nb/Y vs. SiO2 discrimination diagram indicating the basalt and andesitic characteristics of the studied samples (after Winchester and Floyd, 1977); (b), (c) Y vs. Zr and SiO2 vs. FeO*/MgO discrimination diagrams (after Ross and Bédard, 2009; Miyashiro, 1974) in which the Bellara Trap basalts (BTB) show a tholeiitic trend, while the Bellara Trap andesites (BTA) display calc-alkaline composition. Symbols are the same in all the binary and ternary figures.

    Figure 4.  Diagrams of MgO vs. various major elements and their ratios showing the magmatic trend of BTB and BTA.

    The concentrations of Ni, Co and Cr are higher in BTB (Ni: 83.73 ppm–170.59 ppm; Co: 12.08 ppm–68.31 ppm, Cr: 233.47 ppm–328.87 ppm) than that for BTA (Ni: 25.17 ppm–42.31 ppm; Co: 18.7 ppm–30.9 ppm, Cr: 20.58 ppm–31.2 ppm). Chondrite normalized REE profile (Figs. 5a and 5c) for the BTB samples exhibits flat patterns analogous to MORB and is devoid of any prominent fractionation trends as mirrored by LREE/MREE (La/SmN: 0.84–0.99), MREE/HREE (Gd/YbN: 1.1–1.2) and LREE/HREE (La/YbN: 0.96–1.25) ratios. In stark contrast to this, the BTA samples augment pronounced LREE/MREE (La/SmN: 3.2–4.79), MREE/HREE (Gd/YbN: 1.53–2.4) and LREE/HREE (La/YbN: 6.88–17.63) fractionations with negative Eu anomalies when normalized with chondrite (Fig. 5c). Mantle normalized trace element abundance patterns for BTB (Figs. 5b and 5d) depict LREE enrichment (Ce, Nd, Sm) with marked depletions in HFSE (Th, Nb, Zr and Hf). The BTA samples are characterized by enrichment in Th and U, negative anomalies at Nb and Ti and peaks at Zr and Hf (Fig. 5d).

    Figure 5.  (a), (c) Chondrite normalized rare earth element (REE) patterns of the basalts exhibiting flat patterns analogous to MORB whereas andesites showing fractionated patterns with Eu anomalies; (b), (d) mantle normalized trace element patterns for BTB depicting LREE enrichment with marked depletions in HFSE whereas the BTA samples are characterized by negative anomalies at Nb and Ti with peaks at Zr and Hf. Normalizing factors are from Sun and McDonough (1989).

  • The pristine mantle signatures of Archean volcanic rocks are inevitably overprinted by ocean floor hydrothermal alteration, greenschist to amphibolite facies metamorphism and polyphase deformation. The behavior of fluid-mobile trace elements in response to these secondary processes posits serious concern for interpreting the petrogenetic and tectonic features of the rocks. Thus, it is essential to screen the elements for appropriate evaluation of magma chemistry (Manikyamba et al., 2009; Polat et al., 2002). Extensive studies have shown that the major elements Al, Ti, Fe, and P, the HFSE (Th, Nb, Ta, Zr, Hf), the REE (except Ce, Eu), transition metals (Cr, Ni, Sc, V), and Y in mantle-derived rocks behave isochemically and are relatively immobile during alteration and metamorphism. Among the REE, resistance to alteration increases in the order LREE < MREE < HREE. The large ion lithophile elements (LILE), such as Cs, Na, K, Rb, Ba, Ca, Pb and Sr, are typically mobile during alteration (Kerrich and Manikyamba, 2012; Polat and Hofmann, 2003). In terms of Ce/Ce* ratios, the studied basalt samples correspond to the range 0.9–1.1 specified for alteration resistant rocks (Table S1; Polat and Hofmann, 2003). Being the most immobile element, Zr has shown good correlation with Yb, Nb, Th and La whereas Sr and Rb has shown scatter (Fig. 6). Furthermore, in the mafic-felsic weathering diagram (Fig. 7a; Ohta and Arai, 2007), the studied samples are concentrated at the fresh basalt and andesite region indicating the unaltered nature of these rocks. Besides, the weathering index (W) is used as a proxy to evaluate the effect of weathering. There is no significant correlation between La/Sm and Eu/Eu* and the weathering index (W) endorsing primary magmatic nature of the studied rocks (Figs. 7b, 7c; O'Neil et al., 2011). Therefore, HFSE and REE compositions and their ratios have been considered for petrogenetic and tectonic interpretations in the present study.

    Figure 6.  Diagrams of Zr versus representative LREEs (La and Ce), HFSE (Nb), and LILEs (K2O, Sr and Rb) exhibiting positive correlations between Zr and La, Sm, Nb, for both the BTB and BTA, which indicate that the primary igneous compositions of LREEs and HFSEs are generally preserved, whereas least correlation among K2O, Sr and Rb suggests the mobile nature of LILE.

    Figure 7.  (a) MFW ternary diagram indicating the least altered nature of the studied samples (after Ohta and Arai, 2007). Values plotted in the MFW diagram are: exp(M), exp(F), exp(W), re-totaled to100. Values for M, F and W calculation are normalized to 100 wt.%. M= -0.395·ln (SiO2)+0.206·ln (TiO2)–0.316·ln (Al2O3)+ 0.160·ln (Fe2O3)+0.246·ln (MgO)+0.368·ln (CaO)+0.073·ln (Na2O)–0.342·ln (K2O)+2.266; F=0.191·ln (SiO2)–0.397·ln (TiO2)+0.020·ln (Al2O3)–0.375·ln (Fe2O3)– 0.243·ln (MgO)+0.079·ln (CaO)+0.392·ln (Na2O)+0.333·ln(K2O)–0.892; W=0.203·ln (SiO2)+0.191·ln (TiO2)+0.296·ln (Al2O3)+0.215·ln (Fe2O3)–0.002·ln (MgO)– 0.448·ln (CaO)–0.464·ln (Na2O)+0.008·ln(K2O)–1.374. (b), (c) La/Sm and Eu/Eu* vs. weathering index (W) showing no correlation endorsing primary magmatic nature of the studied rocks (O'Neil et al., 2011).

  • The mantle outflux and juvenile crust generation at oceanic spreading centres and marginal back arc basins are primarily controlled by anhydrous decompression melting of a depleted asthenospheric mantle upwelling beneath diverging oceanic plates in an extensional tectonic regime. However, magmatism in active subduction systems is consistent with hydrous melting of a fertile mantle wedge metasomatized by fluids and melts flushed into the mantle wedge from the subducting oceanic slab (Niu and O'Hara, 2009; Pearce and Stern, 2006; Martinez and Taylor, 2003; Elliott et al., 1997; Hofmann, 1997; Hawkesworth et al., 1993). In this study, REE signatures for the tholeiitic BTB, as defined by mild LREE enrichment over MREE and HREE strongly compete with an N-MORB to E-MORB compositional spectrum. On the contrary, LREE/MREE and LREE/HREE ratios for the calc-alkaline BTA samples reflect pronounced fractionation trends substantiating a subduction imprint in the precursor melts. These observations comply with a heterogeneous mantle source with depletion- enrichment history. Trace element compositions for BTB, normalized to primordial mantle compositions, yield negative anomalies at Nb, Zr, Hf with respect to higher abundances of LILE and LREE, while the BTA samples show Zr-Hf peaks with Nb-Ti troughs. Mantle-normalized negative Nb-Ta-Ti anomalies coupled with LILE-LREE enhancement could be ascribed to (ⅰ) precursor parent melt extraction from a metasomatically enriched mantle modified by subduction input or (ⅱ) contamination of parent magma by crustal component. However, negative Nb-Ta anomalies coupled with negative Zr-Hf anomalies discard the possibility of continental input. Higher abundances of LREE corroborate contributions from upper continental crust (felsic to intermediate in composition), whereas relatively lower LREE concentrations attest to input from lower continental crust (more mafic in compositions). The mantle- normalized positive Zr-Hf anomalies in conjunction with distinct Nb troughs for the Bellara Trap andesites collectively augment the role of lower crust that had been recycled and reworked by paleo-subduction events. The ratios of Zr/Hf and Zr/Sm for BTB (26.82–37.79 and 10.81–26.36) and BTA (48.80–57.55 and 61.49–105.30) compared to Zr/Hf=36 and Zr/Sm=25 of primitive mantle reflect compositional heterogeneity in the source mantle and refertilization of an erstwhile refractory mantle by subduction-derived fluid components. Trace element abundances suggest that the BTB and BTA samples are uniformly enriched in Th and U over Nb. The arc signatures are more prominent in BTA with elevated abundances of Rb (avg. 58 ppm), Sr (avg. 174 ppm) and Zr (avg. 327 ppm) compared with BTB (Rb: avg. 8.85 ppm; Sr: avg. 150 ppm; Zr: avg. 42 ppm) which may posit an important implication on the thermo-tectonic evolution of a back-arc basin. In Ti/1 000 vs. V (Fig. 8a) and FeO*/MgO vs. TiO2 (Fig. 8b) diagrams, they are occupying the field of MORB and BABB. They are enriched in LILEs and depleted in HFSEs (Figs. 5b, 5d), which is a typical feature of volcanic arc basalts (VAB; Stern, 2002; Peate et al., 1997). The LILE/HFSE and LREE/HFSE ratios (e.g., Ba/Nb, Th/Nb, and La/Nb) are very useful ratios to distinguish different types of basalt that are generation from various tectonic settings (Pearce et al., 1995). The LILE/HFSE and LREE/HFSE ratios of the Bellara samples are higher than MORB, but lower than that of VAB. Therefore, the studied samples exhibit the geochemical characteristics of both MORB and VAB. Furthermore, in Nb/Th vs. Nb (Fig. 8c) diagram, the BTB are occupying the BABB field whereas BTA is showing a trend towards arc basalt and transitional between those of MORB and arc basalts i.e., transitional between those of MORBs and Mariana arc basalts (He et al., 2021; Pearce et al., 2005; Elliott et al., 1997). The Th and Nb contents in the parental magmas change with crustal contamination and/or incorporation of subducted sediments/fluids. Thus the Th/Yb vs. Nb/Yb diagram can divulge the relative input of crustal materials vs. subduction derived material into the magmatic system (Li et al., 2013; Pearce, 2008). The ratios of Nb/Yb and Th/Yb have been used as tracers to identify the subduction input in the source and the studied samples follow MORB-OIB array indicating subduction zone signatures in which BTB occupy the Mariana BABB field and BTA are close to Mariana IAB with prominent calc-alkaline affinity (Fig. 8d; Pearce, 2008; Pearce and Stern, 2006).

    Figure 8.  (a) Ti/1 000 vs. V (after Shervais, 1982), and (b) FeO*/MgO vs. TiO2 (after Shuto et al., 2006) discrimination plots indicating the MORB and BABB nature of the studied volcanic rocks; (c) Nb versus Nb/Th diagram (after He et al., 2021) showing the BABB nature of basalts; and (d) Nb/Yb vs. Th/Yb diagram (after Pearce, 2008) showing that the BTB are occupying Mariana BABB field whereas the BTA are close to Mariana IAB. Typical arc and back arc basin basalts (BABB) from the Mariana arc-back arc system as well as N-MORB, E-MORB, and OIB are plotted for comparison (Pearce et al., 2005; Elliott et al., 1997; Sun and McDonough, 1989); Achaean (A) crust data are from Rudnick and Fountain (1995); Mariana Trough BABB and Mariana IAB data are from Pearce et al. (2005); LCC, MCC, and UCC are from Rudnick and Gao (2003). The arrows in the bottom right are subduction component (S), crustal contaminant component (C), within plate (W), and fractional crystallization vectors (f). N-MORB. Normal mid-ocean ridge basalt; E-MORB. enriched-MORB; IAB. island arc basalts; OIB. ocean island basalts.

    Oceanic basalts having relatively high Ce/Nb corresponds to a depleted MORB mantle (DMM) source, while those with low Ce/Nb and high Th/Nb are interpreted to be derived from mantle carrying a residual slab component (RSC) recycled by paleo-subduction events and a subduction-derived component (SDC) complementary to RSC respectively (Saunders et al., 1988). On the Ce/Nb vs. Th/Nb plot (Fig. 9a), the BTB samples correspond to the fields of Mariana back-arc, whereas the BTA samples show affinity towards the Mariana trough, suggesting their derivation from a mixture of DMM and SDC. Likewise, the Nb/Lapm=avg. 0.84 observed in the Bellara Trap volcanic rocks is relatively higher than that of typical arc basalts (Nb/Lapm=0.27; Hawkesworth et al., 1991) thereby corroborating an heterogeneous mantle that experienced an interaction between DMM and SDC. The DMM component refers to a refractory mantle asthenosphere melting under anhydrous conditions at the inception of the back-arc rift while the SDC signature implicates a gradual progression to hydrous melting of the back-arc mantle metasomatized by the migration and influx of dehydrated components from the proximal arc regime. The high Th/Yb ratios compared to Ta/Yb for the BTA corroborate influx of subduction-derived components into the mantle source (Table S1; Belova et al., 2010; Saccani et al., 2010, 2008; Yaliniz, 2010; Dilek et al., 2008; Godard et al., 2006; Beccaluva et al., 2005). The melting style can be illustrated by the model based on the relationship between Cr and Y which are not affected by the processes that produce upper mantle inhomogeneities (Pearce et al., 1984). Figure 9b describes the asthenosphere composition and melting trend which reflect the primary melt composition. The lines proceeding downward from the melting trends represent the crystallization vectors ofthese melts. The steeper segment links to the olivine+Cr spinel± clinopyroxene crystallization, while the gentler, to the olivine+ Cr-spinel+clinopyroxene+plagioclase crystallization. The combination of these trends signifies a "petrogenetic path" for different types of basalts (Pearce et al., 1984; Pearce, 1982). Figure 9b depicts three petrogenetic paths: A, B, and C. Path C reflects the crystallization of N-MORB basalts, path B, the crystallization of the suprasubduction melt, and path A, the crystallization of boninitic melts. It is observed that the BTB are following the trend of N-MORB and formed by 10%–20% partial melting, whereas BTA samples are in the Island arc (IAT) field and indicating higher degree (> 30%) partial melting of mantle material. Furthermore, Yb versus TiO2 diagram (after Pearce and Stern, 2006) indicating the melting region is spinel lherzolite in which the BTA corresponds to > 30% melting for the generation of precursor melts, while the array for BTB samples corroborates 10% to 20% melting (Fig. 9c). Subduction-derived volcanic rocks have La/Nb ratio > 1.4, whereas oceanic plateau basalts (OPB) or Ocean Island Basalts (OIB) sourced from plumes and basalts from MOR mantle are characterized by La/Nb < 1.4 (Condie, 2003; Rudnick, 1995; Floyd et al., 1991; Bougault et al., 1980). In consonance with this conjecture, the studied BTB samples having La/Nb (0.82–1.28) conform to MORB-type source mantle, while the BTA samples with La/Nb in the range of 1.43–2.64 display arc affinities. The gradual transition from depleted MORB to enriched arc-like geochemical affinities influenced by adjacent subduction feedback is concurrent with the evolutionary stages of a juvenile back arc basin (Manikyamba et al., 2015, 2014a, b). Trace element chemistry of modern subduction zone magmas suggests that the Ba/Nb ratio is an important tracer of the fluid/water content in the mantle source, while Ba/La constrains the total slab-derived input into the sub-arc mantle wedge. The elevated Ba/Th values indicate contributions from low- temperatures fluids released by dehydration of subducted oceanic crust and sediments, while increase in Th/Nb reflects input from sediment melts from subducted slab (Hergt and Woodhead, 2007; Pearce and Stern, 2006). The ratios of Ba/Nb, Ba/Th, Ba/La, Nb/Yb, Nb/U for BTA and BTB (Table S1) are in compliance with (ⅰ) gradual increase of subduction input during juvenile stage of back-arc rifting proximal to the arc thereby augmenting a MORB-BABB mantle domain and (ⅱ) metasomatism of a refractory mantle through migration of slab-dehydrated fluids and selective mobilization of incompatible fluid mobile elements like Ba, U, Th, etc. It has been envisaged that the geodynamic conditions prevailing in an active subduction system are controlled by the (a) velocity of the subducted slab roll-back and (b) the oceanward velocity of the overriding plate. Dewey (1980) advocated three families of arc systems based on these two parameters. If a > b, then extensional arcs are formed with back-arc extensional basins, e.g., Mariana and Tonga arcs in eastern Indonesia; if a≈b, well-developed subduction complexes are formed without back-arc extensions and are called neutral arcs, e.g., Alaskan, Aleutian and Sumatran arcs; and if a < b, then compressional arcs are formed as in the Peruvian Andes, Cordillera. This study infers that the Bellara Trap volcanics are products of Neoarchean extensional back-arc basin magmatism analogous to the evolution of back-arc basalts from modern arc-back arc systems of Mariana, Scotia, Izu-Bonin, Manus and Lau-Tonga that are compositionally modified by variable subduction input (Pearce and Stern, 2006). The Ba/Yb ratio is used to infer the characteristics of MORB in terms of subduction input. On the Nb/Yb vs. Ba/Yb diagram, the studied BTB correspond to BABB region and proximal to arc compared to the BTA samples that are enriched in Ba and Nb indicating the enriched mantle characteristics (Fig. 9c). The BTB samples originate from a transitional N- to E-type MORB mantle with mild LREE enrichment attributable to minor subduction input at initiation of back-arc rifting proximal to an arc. The BTA samples are fractionated derivatives of subduction- modified back-arc basin basalts generated during juvenile stage of back-arc basin development proximal to the associated arc and thus inherit imprints of a heterogeneous MORB-type depleted and arc-type enriched metasomatized mantle.

    Figure 9.  (a) Th/Nb vs. Ce/Nb discrimination diagram (after Saunders et al., 1988) illustrating mixing between the mantle components: DMM (depleted MORB mantle), SDC (subduction zone component) and the RSC (residual slab component). The BTB samples occupy the Mariana back-arc basin field (BABB; Pearce et al., 2005) whereas the BTA samples are close to Mariana arc field. (b) Diagram of Y vs. Cr (after Pearce et al., 1984) showing the partial melting and fractional crystallization trends in which the BTB are following the trend of N-MORB formed by 10%–20% partial melting whereas BTA are in the Island arc (IAT) field indicating higher degree (> 30%) of partial melting of mantle material. Fields are from El Bahariya (2018). (c) The Yb vs. TiO2 plot in which the BTA corresponds to ~30% melting of a spinel lherzolite mantle for the generation of precursor melts, while the array for BTB samples corroborate 5% to 20% melting (after Pearce and Stern, 2006); PUM. primitive upper mantle; DMM. depleted MOR mantle. (d) Nb/Yb vs. Ba/Yb diagram (Pearce and Stern, 2006) indicating that the BTBs correspond to BABB region and proximal to arc compared to the BTA samples that are enriched in Ba and Nb indicating the enriched mantle characteristics.

  • The Chitradurga schist belt (CSB) sutured the western and central blocks of the Dharwar Craton and preserves conspicuous record of subduction-accretion-collision tectonics that potentially influenced Meso–Neoarchean crustal growth and continent generation processes in this region (Manikyamba et al., 2017; Hokada et al., 2013; Jayananda et al., 2013a). The geodynamic evolution of the Dharwar Supergroup supracrustals of WDC, CDC and EDC complies with episodic Neoarchean accretionary and collisional orogenic processes at multiple convergence systems along the proto-continental margins. In this study, the geochemical attributes of Bellara Trap volcanic rocks are characterized by distinct tholeiitic MORB-type and tholeiitic to calc-alkaline BABB-type lineage comprehensively reflect the development of an embryonic Neoarchean intraoceanic back arc basin concurrent with progressive subduction activities proximal to it. The genesis of Bellara Trap volcanic rocks can be translated in terms of: (ⅰ) extraction of first batch of MORB-type melt through decompression melting of a depleted mantle asthenosphere in response to oceanic plate spreading during slab roll-back and initiation of back-arc rift; and (ⅱ) gradual refertilization of the back-arc mantle by subduction input from proximal arc resulting into the second batch melts of BABB composition. The MORB-BABB chemistry for the studied BTB and BTA respectively accounts for a pervasive geochemical inhomogeneity of mantle marked by a geochemical progression from depleted to enriched conditions (Brandl et al., 2017; Inglis et al., 2017; Masuda and Fryer, 2015; Ohta et al., 1997). Mid oceanic ridge basalts (MORB) are characterized by a conspicuous depletion in incompatible trace element concentrations owing to their generation via anhydrous melting of a refractory mantle asthenosphere rising beneath a diverging oceanic lithosphere; while the enhanced LILE-LREE contents with relative HFSE depletion for island arc basalts (IAB) are ascribed to hydrous melting of a metasomatized sub-arc mantle wedge flushed by fluids and melts from the subducting slab (Keller et al., 2008; Pearce and Stern, 2006; Elliott et al., 1997; Tatsumi and Eggins, 1995). The MORBs and BABBs are produced at oceanic spreading centres under similar geodynamic conditions. However, the LILE enriched-HFSE depleted, hydrated compositions of magmas generated in a back-arc basin environment are controlled by the subducted slab-mantle wedge system adjacent to it (Manikyamba et al., 2009; Kelly et al., 2006; Langmuir et al., 2006; Martinez and Taylor, 2003; Smith et al., 2001; Gribble et al., 1996; Stolper and Newman, 1994; Fryer et al., 1990). The geochemical characteristics of BTA with arc affinity deviated from typical MORB melts are coherent with the magmas influenced by subduction processes owing to the spatial proximity between arc and nascent back-arc systems. The MORB-type chemistry of BABB is retained in mature back-arc system where the melting domain is spatially separated from subduction-arc regime (Pearce and Stern, 2006; Stern et al., 1990). Thus, in this study, the BTB with MORB signatures was derived from an anhydrous depleted asthenospheric mantle melting under decompression at back-arc spreading centre. The interaction between MORB-type and arc-type mantle components driven by induced convection of the erstwhile refractory asthenosphere into the proximal sub-arc mantle eventually metasomatized the juvenile back arc mantle that upon fluid-fluxed partial melting produced melts parental to the BTA (Gribble et al., 1998; Pearce and Peate, 1995; Gamble et al., 1994; Woodhead et al., 1993; McCulloch and Gamble, 1991).

    The petrogenetic and tectonic implications for the BTB and BTA are in compliance with: (ⅰ) back arc basin initiation in proximity to an active subduction system, (ⅱ) prominent signatures of subduction input and compositional heterogeneity of juvenile back arc mantle triggered by mixing between MOR and arc mantle components, (ⅲ) refertilization of depleted back-arc mantle by influx of slab-derived fluids, and (ⅳ) geochemical progression from anhydrous, depleted to hydrous, enriched mantle domain with a transition from decompression to fluid-fluxed melting processes (Lee and Wada, 2017; Keller et al., 2008; Pearce and Stern, 2006; Wiens et al., 2006). The geochemical signatures of BTB and BTA are analogous to magmas produced in intraoceanic arc-back arc system from different parts of the world (Deng et al., 2020; Manikyamba et al., 2020, 2015, 2014a, b; Hergt and Woodhead, 2007; Hochstaedter et al., 2001; Gribble et al., 1998; Maillet et al., 1995). The Mariana, Tonga, East Scotia, Lau and Manus back arc basins exemplify modern-day back-arc spreading systems formed close to the arc with the basalts recording geochemical fluctuations from depleted MORB-type and enriched arc-type mantle domains. Petrological and geochemical characteristics of BTB and BTA reveal a Neoarchean analogue for Mariana-type subduction of old, cold, thick and dense paleo- oceanic lithosphere with slab-roll back and back-arc rifting. The BTA derived from tholeiitic to calc-alkaline basaltic melts generated during initial stage of back-arc rifting proximal to an intraoceanic arc and inherited their BABB signatures via subduction- driven metasomatic overprint of a depleted MORB-type asthenospheric mantle by an arc-type mantle. The N- to E-MORB signature of BTB reflects a depleted mantle heritage with diminished subduction input associated with back-arc spreading proximal to the arc. Thus, the Neoarchean Bellara Trap volcanics from the Ingaldhal Formation of Chitradurga Group, WDC are consistent with evidence of back arc rifting, magmatism and heterogeneous interaction of arc-type and MORB-type mantle. Such processes are evident from 3.3–2.7 Ga supracrustals of the west Pilbara Block, Australia, Meen-Dempster greenstone belt, Uchi sub province, Superior Province of Canada, from western Greenland, Kadiri-Jonnagiri and Veligallu greenstone belts, eastern Dharwar Craton, India and Early Proterozoic Pechenga- Varzuga belt, Kola Peninsula, Russia. This substantiates the validation of uniformitarian consensus of Phanerozoic style Pacific type arc-back arc tectonics throughout the Archean (Deng et al., 2020; Sotiriou et al., 2020; Polat et al., 2016; Khanna et al., 2015; Manikyamba et al., 2015, 2014a, b; Appel et al., 2009; Jenner et al., 2009; Hollings et al., 2000; Krapez and Eisenlohr, 1998; Sharkov and Smolkin, 1997).

  • (1) The metavolcanic rocks from Bellara Traps are geochemically classified as tholeiitic MORB type basalts and calc-alkaline andesites with distinct petrogenetic and geodynamic implications.

    (2) This paper accounts for the first report of a Neoarchean BABB-MORB association from the Chitradurga greenstone belt of the WDC.

    (3) The N- to E-MORB type geochemical attributes of BTB are in compliance with active back-arc rifting proximal to the arc.

    (4) The BTA are fractionated derivatives of BABB transitional between MORB-mantle and arc-mantle.

    (5) The subduction imprints translated in terms of LREE and fluid mobile element (FME) abundances are more pronounced in BTA relative to BTB samples.

    (6) MORB-BABB association reflects a Neoarchean analogue of Mariana-type arc-back arc system with distinct signatures of subduction input during incipient stages of back arc basin evolution.

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