The evolution of continental crust during Hadean and Archean and related geodynamic processes provides important clues to understand the early Earth history. Here we report evidence for Hadean and ...Eoarchean crust from the fringe of the Coorg Block, one of the oldest crustal blocks in Peninsular India. We present geological, petrological, and geochemical data, together with zircon U–Pb ages and Lu–Hf isotopes from a suite of metaigneous (granitoids, diorite, charnockite, metavolcanics) and metasedimentary (quartz mica schist, calcareous schist, ferruginous quartzite and BIF) rocks. Petrological and geochemical studies indicate that the igneous suite formed from subduction-related arc magmatism, and that the sedimentary suite represents an imbricated accretionary package of continental shelf sequence and pelagic components. Mineral thermometry suggests metamorphism under temperatures of 710–730°C and pressures up to 8kbar. Magmatic zircons in the metaigneous suite show oscillatory zoning and high Th/U contents (up to 3.72) and record multiple pulses of magmatism at ca. 3.5, 3.2, 2.7 and 2.5–2.4Ga. The metasedimentary rocks accreted along the margins of the Coorg Block show multiple zircon population with mean 207Pb/206Pb ages at 3.4, 3.2, 3.1, 2.9, 2.7, 2.6, 2.5, 2.2, 2.0, and 1.3Ga, and overprinted by younger thermal event at ca. 0.8–0.7Ga. Zircons in the 3.5Ga dioritic gneiss show positive εHf(t) values ranging from 0.0 to 4.2 and Hf crustal model ages (TDMC) of 3517 to 3658Ma suggesting that the parent magma was derived from Eoarchean juvenile sources. The zircons in the 3.2Ga charnockite display εHf(t) values in the range of −3.0 to 2.9 and Hf crustal model ages (TDMC) of 3345 to 3699Ma. The Neoarchean metagranites, diorites and felsic tuff show both positive and negative εHf(t) values and a range of TDMC values from 2904 to 3609Ma suggesting magma derivation from Meso- to Eoarchean juvenile and reworked components. The TDMC values of the dominant zircon population in the metasedimentary suite range from 3126 to 3786Ma, with the oldest value (4031Ma) recorded by zircon grain in a ferruginous quartzite. The εHf(t) values of detrital zircons also show both positive and negative values, with a dominant crustal source. Our zircon U–Pb and Lu–Hf data suggest vestiges of Neohadean primordial continental crust in Peninsular India with episodic crustal growth during Eoarchean, Mesoarchean and Neoarchean building the continental nuclei. The results contribute to the understanding of crustal evolution in the early history of the Earth.
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•Episodic magmatism during Eoarchean, Mesoarchean and Neoarchean•Magmatic pulses related to subduction tectonics in convergent margins•Evidence for Hadean felsic continental crust•Heterogeneous source reservoirs involving juvenile and reworked components
•Discovery of ophiolite suite from the south-western margin of Dharwar Craton.•Geochemical features indicate suprasubduction zone affinity.•Zircon SHRIMP U–Pb ages reveal Neoarchean magmatism.•New ...model of arc accretion to cratonic margin and continental growth.
The southern margin of the Dharwar Craton in Peninsular India preserves the records of an active convergent margin during the Neoarchean. Here we report the discovery of a relatively well-preserved suprasubduction zone ophiolite suite from the Agali hill in Attappadi, along the western extension of the Bhavani Shear Zone. In the Agali hill, from base to top, the rock sequence includes altered ultramafics with vestiges of dunite, thin layer of cumulate pyroxenite, a thick unit of metagabbro with the upper part grading into anorthositic gabbro and carrying thin layers of hornblendite, capped by metavolcanics (amphibolites) carrying veins and pools of trondhjemite. Fragments of metabasite (dolerite) dykes occur within the gabbroic horizon. Elongate bands of metamorphosed banded iron formation in association with amphibolite occur proximally. The lithological distribution in the area represents a typical ‘Ocean Plate Stratigraphy’ sequence with arc and exhumed sub-arc mantle material toward the north-west, followed by accreted remnants of suprasubduction zone ophiolites, large tracts of TTG gneisses, and amphibolites in association with BIF bands. The central domain is occupied by a granite batholith. Toward the south-east, the dominant lithology grades to a continental shelf sequence represented by metamorphosed psammitic and pelitic rocks (trench) and metacarbonates. The common occurrence of magnesite in association with ultramafic units in the area suggests CO2-induced metasomatism of peridotites in the mantle wedge through fluids released within the subduction zone.
We present major, trace and REE data on the Agali Ophiolite Complex which clearly suggest magma derivation in a suprasubduction setting in the absence of any significant crustal contamination. The internal structure as imaged from CL images of zircons separated from metagabbro, trondhjemite and metagranite show features typical of magmatic crystallization, with the grains mantled by bright structureless thin rims developed during a dominantly dry metamorphic event. The U–Pb concordia ages of 2547±17Ma (MSWD 0.84) and 2547±7.4Ma (MSWD 2.4) obtained from zircons in the metagabbro and trondhjemite are indistinguishable; the zircons in the metagranite also show comparable magmatic age of 2532±8.6Ma (MSWD 2.5) with metamorphic overgrowth at ca. 2470Ma. These ages correlate well with similar age data reported recently from suprasubduction zone and arc-related rocks elsewhere along the southern margin of the Dharwar Craton. We propose a tectonic model that envisages accretion of oceanic arcs and micro-continents onto the margin of the Dharwar Craton during Neoarchean, marking an important event of continental growth, and broadly coinciding with the global crustal growth event at this time.
•The Bababudan Group is an intracontinental rift to passive-margin succession.•The Chitradurga Group is divided into unconformity bound Stages I and II.•Stage I is the sedimentary and volcanic record ...of an extensional back-arc basin.•Stage II is the sedimentary and volcanic record of a transtensional back-arc basin.•Stratigraphic equivalents in the Eastern Dharwar Craton are near-arc basins.
The Dharwar Supergroup comprises the unconformity bound Bababudan and Chitradurga groups. The Bababudan Group, which is best preserved in the Western Dharwar Craton, records a divergent margin comprising a basalt-dominant intracontinental rift sequence, a shale-BIF drift sequence, and a sandstone-shale thermal-subsidence sequence. The rift stage evolved from ∼2765 Ma to ∼2720 Ma, whereas the succession was folded, uplifted and eroded during development of a convergent continental margin from ∼2680 Ma. Ocean opening was to the east or southeast. The Chitradurga Group records a two-stage back-arc basin behind an east-facing continental arc. Stage I evolved between ∼2609 Ma and ∼2582 Ma, and Stage II between ∼2582 Ma and ∼2540 Ma. The two stages are separated by a subaerial unconformity. Stage I comprises siliciclastic fluvial, shallow-marine and deep-marine sedimentary rocks and bimodal volcanic rocks. Stage I is best preserved in the Western Dharwar Craton, but there are equivalents in the Eastern Dharwar Craton. Stage II in the Western Dharwar Craton comprises deep-marine turbidites of siliciclastic and volcaniclastic provenance, and basaltic and felsic volcanic rocks. Volcanic Stage II sequences in the Eastern Dharwar Craton define an arc-adjacent position. Tectonically driven cyclic uplift and erosion were responsible for the mismatch between preserved stratigraphic thickness and time, particularly for Stage II sequences. A remnant of a <2540 Ma late-stage basin in the northeast of the Eastern Dharwar Craton defines a syncollisional tectonic setting. The Dharwar Supergroup was deformed in a SW-verging hinterland magmatic fold/thrust belt from ∼2540 Ma.
•The Bonai Granitic Complex (BGC) from the Singhbhum Craton consists of volumetrically minor high-HREE TTGs, low-HREE TTGs and potassic granites.•The low-HREE Bonai TTG yielded an emplacement age of ...3368 ± 8 Ma (1σ) and the potassic granites were emplaced by 3331 ± 33 Ma (1σ).•A low-HREE TTG from Deonala area yielded a crystallisation age of 3312 ± 8 Ma (1σ).•The trace elemental systematics of the high-HREE TTGs reveal similarity to Icelandite.•The BGC is coeval with granitic magmatism in central Singhbhum and possibly evolved in a stagnant lid tectonic regime.
The Singhbhum Craton, one of five major Archean cratons in the Indian subcontinent, contains abundant well-preserved Paleoarchean supracrustals and granitoids. This study presents zircon U-Pb ages and whole rock geochemistry of tonalite-trondhjemite-granodiorites (TTGs) and granites from the Bonai Granite Complex (BGC) and Older Metamorphic Tonalite Gneiss (OMTG), which are separated from each other by the Jamda-Koira-Noamundi Iron Ore Group (IOG) supracrustals. Emplacement ages obtained in this study indicate that a major episode of TTG magmatism took place in BGC around 3368 ± 8 Ma (1σ), followed by granitic magmatism around 3331 ± 33 Ma (1σ). In contrast, a TTG from the Deo Nala area representing OMTG yielded crystallization age of 3312 ± 8 Ma (1σ). The emplacement and evolution of the BGC were coeval with granitoid magmatism from the central part of the Singhbhum Craton. Whole rock geochemical data identify both high- and low-HREE TTGs in both the BGC and OMTG to the west and east of the IOG basin, respectively. The trace element systematics of high-HREE Bonai TTG are similar to those of Icelandic dacites, suggestive of their derivation from a garnet free, plagioclase rich amphibolite. The low-HREE TTGs of the BGC and OMTG were derived from an amphibolite source with varying amounts of garnet. The potassic granites of the BGC were sourced from the older TTGs which had undergone partial melting at a shallow depth. The evolution of the BGC and OMTG can be attributed to the partial melting under a thickened mafic crust. Dome and keel structures and emplacement ages of granitoids from the west and east of the Jamda-Koira-Noamundi IOG basin, support the origin of these Paleoarchean granitoids in a stagnant lid regime. High geothermal gradients induced by heat supplied by mantle upwelling appear to have induced the melting of the thickened crust, to form the TTG. Delamination induced mafic–ultramafic underplating resulted in melting of early formed TTGs, to form the younger potassic granites of the BGC at ∼ 3.33 Ga.
Sandwiched between the Dharwar Craton in the north and the Neoarchean–Proterozoic crustal blocks to the south, the Coorg Block in southern India is composed dominantly of a suite of arc magmatic ...rocks including charnockites, TTG (tonalite–trondhjemite–granodiorite)-related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block. Coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block. Here we present the petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of all the major lithologies from this block. Computation of metamorphic P–T conditions from mineral chemical data shows consistent granulite-facies P–T conditions of 820–870°C and up to 6kbar. Our geochemical data from major, trace and REE on representative samples of the dominant rock types from the Coorg Block corroborate an arc-related signature, with magma generation in a convergent margin setting. The zircon data yield weighted mean 207Pb/206Pb ages of 3153.4±9 to 3184.0±5.5Ma for syenogranites, 3170.3±6.8Ma for biotite granite, 3275±5.1Ma for trondhjemite, 3133±12 to 3163.8 ±6.9Ma for charnockites, 3156±10 to 3158.3±8.2 for mafic enclaves, 3161±16Ma for diorite and 3173±16Ma for felsic volcanic tuff. An upper intercept age of 3363±59Ma and a lower intercept age of 2896±130Ma on zircons from a charnockite, as well as an evaluation of the Th/U values of the zircon domains against respective 207Pb/206Pb ages suggest that the Mesoarchean magma emplacement which probably ranged from >3.3 to 3.1Ga was immediately followed by metamorphism at ca. 3.0 to 2.9Ga. The ages of magmatic zircons from the charnockites and their mafic granulite enclaves, as well as those from the volcanic tuff and biotite granite, are all remarkably consistent and concordant marking ca. 3.1Ga as the peak of subduction-related crust building in this block, within the tectonic milieu of an active convergent margin. The majority of zircons from the Coorg rocks show Hf isotope features typical of crystallization from magmas derived from juvenile sources. Their Hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8Ga. The crustal blocks in the Southern Granulite Terrane in India preserve strong imprints of major tectonothermal events at 2.5Ga, 2.0Ga, 0.8Ga and 0.55Ga associated with various subduction–accretion–collision or rifting events. However, the Coorg Block is exceptional with our data suggesting that none of the above events affected this block. Importantly, there is also no record in the Coorg Block for the 2.5Ga pervasive regional metamorphism that affected all the other blocks in this region. The geochronological data raise the intriguing possibility that this block is an exotic entity within the dominantly Neoarchean collage in the northern domain of the Southern Granulite Terrane of India. The Mesoarchean arc-related rocks in the Coorg Block suggest that the magma factories and their tectonic architecture in the Early Earth were not markedly different from those associated with the modern-style plate tectonics.
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•Discovery of an ancient microcontinent.•Mesoarchean arc magmatic rocks formed in a subduction zone.•Dominantly juvenile magma sources, with partial recycling of 3.8Ga crust.•An exotic block welded to India at ca. 1.2Ga.
The Sittampundi Anorthosite Complex (SAC) in southern India is one of the well exposed Archean layered anorthosite-gabbro-ultramafic rock associations. Here we present high precision geochemical data ...for the various units of SAC, coupled with zircon U-Pb geochronology and Hf isotopic data for the anorthosite. The zircon ages define two populations, the older yield a concordia age of 2541±13Ma, which is interpreted as the best estimate of the magmatic crystallization age for the Sittampundi anorthosite. A high-grade metamorphic event at 2461±15Ma is suggested by the upper intercept age of the younger zircon population. A Neoproterozoic event at 715±180Ma resulted in Pb loss from some of the metamorphic zircons. The magmatic age of the anorthosite correlates well with the timing of crystallization of the arc-related~2530Ma magmatic charnockites in the adjacent Salem Block, while the metamorphic age is synchronous with the regional metamorphic event. The geochemical data suggest that the rocks were derived from a depleted mantle source. Sub-arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous, aluminous basalt magma. The magma fractionated at depth to produce a variety of high-alumina basalt compositions, from which the anorthositic complex with its chromite-rich and amphibole-rich layers formed as cumulates within the magma chamber of a supra-subduction zone arc. The coherent initial176Hf/177Hf ratios and positive εHf values (1.7 – 4.5) of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source. Our study further confirms that the southern part of the Dharwar Craton was an active convergent margin during the Neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth.
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► First high precision zircon U-Pb ages from the Sittampundi Anorthosite Complex (SAC) suggesting crystallization at 2541±13Ma. ► Geochemical features suggest suprasubduction zone arc magmatism in a convergent margin setting. ► Neoarchean continental growth at the margin of Dharwar Craton in southern India.
The present study provides new petrological and geochemical data of the dredged rocks from submarine volcanoes along the Andaman arc and describes the petrogenetic evolution of the arc system in ...terms of mantle wedge characteristics, nature and quantitative input of subducted slab components, and fractionation processes of precursor magma. The studied rocks include basaltic andesite, andesite, dacite and rhyolite. These volcanic rocks exhibit LILE, LREE enrichments and HFSE depletion, corroborating their generation through subduction processes. High abundances of Th/Nd, La/Sm(N), LREE/HFSE than LILE/HFSE, LILE/LREE suggest a substantial contribution of sediments from the subducting slab over slab-dehydrated aqueous fluids to the mantle wedge. The 87Sr/86Sr-Ba/La mixing model suggests 0.6–0.8% addition of slab fluid (90:10 AOC: sediment fluid) to account for the fluid signature, whereas the 143Nd/144Nd-La/Sm(N) mixing model envisages ∼3–4% addition of sediment melt to the mantle source, reconciling the sediment signature in Andaman submarine volcanic rocks. The presence of N-MORB type mantle is attributed to the absence and/or inefficient convection of asthenospheric material from the Andaman back-arc basin to the mantle wedge. This ineffective convection can be equated with the flat subduction of the Indian Plate, caused by the convergence of the aseismic Ninety East Ridge. The non-modal batch-melting model suggests that 13–24% partial melting of the spinel lherzolite mantle beneath the Andaman submarine volcanic arc formed the parent magma. The crystallization model invokes up to 60–70% of fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, sanidine and magnetite in all the rock types with subordinate proportions of amphibole, biotite, apatite, ilmenite, and sanidine in rhyolites. The basaltic andesites, andesites and dacites do not show upper crustal input, while rhyolites indicate crustal contamination from an upper crust and/or arc crust.
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•First report of Sr-Nd isotopic data for submarine volcanic rocks of Andaman arc.•Mixing model suggests 3–4% sediment melt influx over 0.6–0.8% slab dehydrated fluids.•Partial melting of metasomatized mantle wedge.•20–70% degree of fractionation and generation of B-A-D-R spectrum.
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•First report of whole rock Re-Os isotopic data of Neo-Tethyan mantle rocks from Indian ophiolites.•Sub-chondritic 187Os/188Os indicating primary depleted nature and subsequent melt ...percolation.•Trace and REE chemistry with Re addition corroborate multistage petrogenetic processes.•Recycling of SCLM fragments in depleted mantle linked with the Gondwana reconfiguration.
Naga Hills Ophiolite (NHO) represents a thrusted section of relict Neo-Tethyan oceanic lithosphere comprising distinct crustal and upper mantle lithologies that preserve imprints of different stages of evolution of ancient ocean basins and subsequent accretion onto continental margins during late Cretaceous-Eocene collision involving Indian and Eurasian Plates. This study presents bulk geochemistry and Re-Os isotopic compositions for the mantle peridotites of NHO to address their petrogenetic aspects, implications for Neo-Tethyan upper mantle heterogeneity and thermo-tectonic evolution of Neo-Tethyan oceanic lithosphere through subduction-accretion-collision processes. Geochemical characteristics of the mantle peridotites, as reflected from their chondrite normalized REE patterns, binary relationship between fractionation depletion indices, low Re/Os, suggest that these are mantle residues resulting from ∼ 5–20 % of melt extraction from a spinel peridotite source that experienced decompression melting in a fore-arc extensional setting in response to subduction initiation. Isotopically, these mantle peridotites are subchondritic with 187Os/188Os (0.1218–0.1266) and γOs values of −3.73 to −0.09 comparable with the depleted MORB source. LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns demonstrated by the studied samples with Re addition collectively corroborate multistage petrogenetic processes in compliance with geodynamic transition from extensional to compressional regime in a suprasubduction zone environment and partial melting of a chemically heterogeneous source mantle. This compositional heterogeneity can be equated with mantle depletion by melt extraction and refertilization by influx of slab-dehydrated fluids and percolation of boninitic melts. The non-radiogenic Os isotope signatures, negative γOs values and wide spectrum of melt extraction ages for these mantle peridotites correspond to (i) multiple melt production and melt extraction events prior to the opening of Neo-Tethyan seaway (ii) detachment and incorporation of ancient, depleted SCLM fragments into oceanic mantle concurrent with subduction driven ocean basin closure-congregation of Gondwana Supercontinent and emergence of Neo-Tethyan embryonic ocean in response to disintegration of Gondwana Supercontinent.
•WIOG and Jagannathpur volcanics are coeval basaltic andesites.•WIOG belt evolved in an intracontinental extensional setting.•Depleted mantle derived magma underwent significant crustal ...contamination.•Maximum depositional age from basal quartzite is 3.0 Ga.•No evidence for Mesoarchean convergent margin process in Singhbhum Craton.
The Singhbhum Craton of eastern India, consisting of well-preserved Paleo- to Mesoarchean granitoids, volcanic and sedimentary rocks, is an excellent archive of early earth processes. In this contribution, we investigated the geological evolution of the Noamundi-Jamda-Koira IOG Basin (WIOG) using detrital zircon U-Pb ages, whole rock geochemical and Nd isotopic data on metavolcanic rocks from this belt including the Jagannathpur volcanics. The metavolcanics of the WIOG and Jagannathpur are compositionally basaltic andesites with enriched LREE and negative anomalies of Nb-Ta and Ti. The ratios of crustal contamination sensitive elements such as La/Sm, Sm/Nd, Nb/La, Th/Yb and Nb/Yb are close to that of bulk continental crust. The Nd isotopic composition of these basaltic andesites are broadly chondritic (εNdt = +0.93 to −1.20). The geochemical and isotopic signatures are consistent with significant assimilation of Paleoarchean granitoids of the Singhbhum Craton. The overlapping geochemical and isotopic compositions support the view that Jagannathpur volcanics is an eastward extension of the ∼3.04 Ga WIOG metavolcanics and shouldn’t be considered as a separate stratigraphic unit. The presence of pillowed structures, vesicularity, absence of volcanic breccia, and their association with clastic sediments clearly indicate a subaqueous and shallow marine eruption for the WIOG basaltic andesites. The deposition of clastic sediments before basaltic magmatism points towards lithospheric thinning and basin opening. The detrital zircon ages from the WIOG ranges from 3.51 to 2.96 Ga and the detritus was mainly sourced from the adjacent Singhbhum granite and Bonai granite. The petrogenesis of the WIOG metavolcanics can be attributed to assimilation-fractional crystallisation of a depleted mantle derived tholeiitic mafic-ultramafic magma in an intracontinental extension setting.
We present field, petrographic, mineralogical and whole-rock geochemical data for part of the Neoarchean granite and associated microgranular enclaves (MEs) occur in the Nalgonda region, NE part of ...Eastern Dharwar Craton (EDC), and demonstrate the end-member magma mixing processes in the petrogenesis of the host granite. Extensive occurrence of ME and uni-directional flow band structures (N–S trend) in all the studied outcrops exposed over about 20 km depict that intensive magma mixing–mingling occurred at magma chamber scale. MEs are the portions of intermediate mafic magma that had interacted at two stages with acidic host granite. Mixing at the initial stage promoted efficient thorough mixing which resulted in rapakivi texture and mesocratic enclaves. Wispy filament structures around these enclaves indicate that mafic magma globules were mechanically diluted in the host granite magma by chaotic advection. These filamental magmas were further linearly diluted along with convection-related flow of the host magma. Smaller mafic globules also got linearly stretched along with this flow. Upon solidification of the host magma, these wispy mafic filaments were preserved as flow structures. The second stage of mafic magma mixing did not promote thorough mixing due to the large viscosity contrast with the host granite magma and preserved their physical entity. These enclaves are melanocratic with sharp boundary and devoid of gradational contact and filament structures around it. They characteristically show network of granitic vein injection which often show crenulation folding. These evidences indicate that during the second-stage interaction, the enclave magma was rigid to plastic nature with respect to the host granite magma. Coherent linear Harker variation trend of CaO, MgO,
TiO
2
,
K
2
O/Na
2
O, V, Y and Sc indicates that mixing has promoted to develop chemical gradient between ME and host granite magmas. Mechanical dilution by chaotic advection must have enhanced the chemical diffusion of both magmas. Smooth decreasing Harker trend of Y content in both the magmas indicates that theyhad undergone certain degree of fractional crystallisation. In contrast, elements like Rb, Ba and Sr with large value of diffusion coefficient (
D
) show scattered behaviour in element–element plots suggesting that diffusive fractionation was active during chaotic advection mixing. It is concluded that heterogeneity in compositional variation of Nalgonda granite can be attributed to difference in degree of mechanical dilution of mafic magma, fractional crystallisation and diffusive fractionation. The geochemical evidences indicate the role of subduction in the evolution of these rocks.
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