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Integrated petrological, geochemical, isotopic and thermobarometric study of metasedimentary rocks from the amphibolite‐ to granulite‐facies transition zone of the Eastern Dharwar Craton (EDC), South India, has provided new insight into the evolution of the lower continental crust in this region. Phase equilibrium modelling of metapelites and metagreywackes suggests that they reached peak metamorphism at ~800–850°C and 6–7 kbar (corresponding to a paleodepth of ~20 km), with minor retrograde change occurring at ~700°C and 3–5 kbar during exhumation. U–Pb ages of conventionally separated zircon from metapelite samples range from 2.5 to 3.4 Ga, whereas garnet‐hosted zircons yield younger ages of 2.5–2.7 Ga. Zircon Th/U ratios and Hf isotopes reveal several significant pulses of zircon growth at 3.0, 2.95 and 2.7 Ga. Hf isotope data suggest the evolution of juvenile magma at around 3.2 Ga, while Hf model ages show that the crust building process also involved the recycling of pre‐existing Mesoarchean crust. Our study confirms the presence of a Paleoarchean component in the EDC lower crust, as well as older metamorphic events in the terrain and the gradational distribution of the metamorphic rocks. [ABSTRACT FROM AUTHOR]

The Sargur Group has been considered to be the oldest group (>3.0 Ga) in the Archaean sequence of the Dharwar Craton in south India, whereas the rocks of the Dharwar Supergroup are younger (between 3.0 and 2.55 Ga). The supracrustal units of the Sargur Group were deposited during the Archaean period. The Banavara quartzite forms part of the supracrustal Sargur Group and contains significant amounts of chromian spinel (Cr-spinel). Here, U–Pb and Hf isotopes of detrital zircons are integrated with compositional data and X-ray refinement parameters for Cr-spinels to decipher the provenance of the metasediments. Zircons show an age spectrum from 3.15 to 2.50 Ga, and juvenile Hf isotopic compositions (ϵHf = +0.8 to +6.4) with model ages between 3.3 and 3.0 Ga. Major- and trace-element contents of the Cr-spinels do not resemble those in the Sargur ultramafic rocks, but resemble well-characterized Archaean anorthosite-hosted chromites. Cr-spinel trace-element signatures indicate that they have undergone secondary alteration or metamorphism. X-ray refinement parameters for the Cr-spinels also resemble the anorthosite-hosted chromites. We conclude that the detrital minerals were probably derived from gneissic and anorthositic rocks of the Western Dharwar Craton, and that the Sargur Group sequences have experienced a younger (2.5 Ga) metamorphic overprint. [ABSTRACT FROM AUTHOR]

We present a detailed investigation of the micrometer- to nanometer-scale textural-chemical features in partially serpentinized dunites from the lower ultramafic unit of the Mesoarchean Nuasahi Massif, eastern India; these data are used to interpret the evolution in fluid chemistry at the reaction interface during progressive hydration of olivine. In the first stage of serpentinization, a mixed layer of Mg-rich lizardite + brucite + magnetite formed within fractures producing a typical mesh texture, followed by homogeneous brucite-free lizardite at the inner interface of the mixed layer. A unique feature of the serpentinization in the Nuasahi dunites, which to our knowledge has not been previously described, is the formation of a pseudomorphic rim of Mg-rich olivine (Fo98) directly in contact with the primary olivine (Fo92). The textural-chemical relations, such as (1) sharp but irregular textural-chemical reaction interfaces, (2) propagation of replacement fronts along fractured pathways, allowing transfer of aqueous species towards the center of olivine crystals, and (3) the presence of micro- to nano-porosity within product phases, indicate that each replacement process was governed by an interface-coupled dissolution-precipitation mechanism. Additionally, Mg-rich chromite, distributed within the samples, was partially pseudomorphically replaced by an Fe-rich chromite rim and/or isolated magnetite. Mass-balance calculations for different replacement reactions suggest that lizardite formation reactions may be coupled with chromite replacement reactions, allowing a micrometer-scale transfer of aqueous species (e.g. Al3+, Cr3+) between chromite and olivine reaction interfaces. The lizardite formation reactions have produced hierarchical patterns of reaction-induced fractures within primary olivine crystals and the replacement front has progressed from the outer rim towards the center of the olivine grain. The sequential formation of different pseudomorph phases during progressive hydration of primary olivine reflects a micrometer-scale variation in silica and/or water activity in the precipitating solution across the reaction interface, and may also be associated with a decrease in temperature. The Fe2+Mg-1 chemical exchange potential of the equilibrating system at the reaction interface also plays an important role, controlling the Fe/Mg ratio of the secondary minerals and the molar proportions of magnetite. The pattern of fluid-mobile element mobilization (e.g. Li, B) during replacement suggests that serpentinization of the Nuasahi dunite occurred during interaction with a B-rich solution, possibly in an ocean-floor setting. [ABSTRACT FROM AUTHOR]

Paleogeographic reconstructions of India and Madagascar before their late Cretaceous rifting juxtapose the Antongil Block of Madagascar against the Deccan Traps of India, indicating that the Western Dharwar Craton extends below the Deccan lavas. Some recent studies have suggested that the South Maharashtra Shear Zone along the northern Konkan coast of India limits the northern extent of the Western Dharwar Craton, implying that the craton does not extend below the Deccan Traps, raising a question mark on paleogeographic reconstructions of India and Madagascar. The continuity of the Western Dharwar Craton north of the South Maharashtra Shear Zone below the Deccan Traps--or its lack thereof--is critical for validating tectonic models correlating Madagascar with India. In this study, zircons in tonalitic basement xenoliths hosted in Deccan Trap dykes were dated in situ, using the U-Pb isotope system. The data furnish U-Pb ages that define three populations at 2527 ± 6, 2456 ± 6, and 2379 ± 9Ma. The 2527 ± 6Ma ages correspond to the igneous crystallization of the tonalites, whereas the 2456 ± 6 and 2379 ± 9 Ma ages date metamorphic overprints. The results help to establish for the first time that the basement is a part of the Neoarchean granitoid suite of the Western Dharwar Craton, which extends northward up to at least Talvade in central and Kihim beach in the western Deccan. By implication, the South Maharashtra Shear Zone cannot be the northern limit of the Western Dharwar Craton. The granitoids are correlated with the Neoarchean felsic intrusions (2.57-2.49) of the Masaola suite in the Antongil Block ofMadagascar, supporting the existence of a Neoarchean Greater Dharwar Craton comprising the Western Dharwar Craton and the Antongil-Masora Block. [ABSTRACT FROM AUTHOR]

The combination of U Pb zircon ages with Hf-Sr-Pb isotopes of different intrusive and extrusive felsic and sedimentary rocks provides constraints on the petrogenetic evolution of the continental crust in the western Dharwar Craton, India. The oldest detrital zircon preserved formed at ∼3.6 Ga and represents a relic of the oldest felsic crustal material in the region. The dominant granitoid units of the western Dharwar Craton contain zircon grains with magmatic ages between 3.4 Ga and 3.0 Ga that indicate the formation of major felsic continental crust during this interval. Trace element abundances of the granitoids indicate that the oldest members of the intermediate to felsic suite derived by partial melting of mafic material at ∼3.6–3.4 Ga. The initial bulk rock Hf isotope compositions of these granitoids are consistent with their formation by melting of even older mafic material that was slightly enriched relative to the depleted mantle composition. This mafic and slightly enriched material formed by mantle melting at ∼ < 3.8 Ga. The Hf isotope compositions of individual zircon grains, obtained by two different analytical techniques (in-situ and complete dissolution followed by chromatographic separation) give evidence for the presence of such older mafic material (<3.8 Ga) that formed the immediate precursor of their granitoid host rocks. Such a mafic source for the granitoids is consistent with Pb Sr isotope systematics of these that shows no indication of Eoarchean enriched/evolved material in the western Dharwar Craton. The mafic source material of the granitoids thus represents an intermediate stage of crust formation that started after 3.8 Ga with the formation of mafic crust by mantle melting. The combined geochronological and isotopic constraints suggest that the Mesoarchean felsic crust of the Dharwar Craton formed by differentiation of melts derived from an amphibolite/eclogite source rock and included increasing contributions of reprocessed crustal material with time from ∼3.6 to 3.0 Ga. The major interval of growth of felsic continental crust was from 3.4 to 3.0 Ga. The younger generation of granitoids formed mostly by reworking of older intermediate to felsic crust. These different felsic magmatic bodies with distinct petrogeneses and sources, that include the depleted mantle, older mafic crust and the evolved continental crust, became essential elements of the stable continental crust of the western Dharwar Craton, the majority of which was generated from 3.4 to 3.0 Ga. • Major crust formation, stabilization in the western Dharwar Craton at 3.4–3.0 Ga • Pb Sr isotopes show no indication of evolved continental crust before 3.6 Ga. • Old mafic source material represents an intermediate stage of crust formation. • Younger granitoids formed dominantly by reworking of older felsic crust. • ɛHf i measured in-situ (zircon) and by isotope dilution (whole rock and zircon). [ABSTRACT FROM AUTHOR]

Both primary mantle differentiation and early extraction of oceanic and continental crust can have resulted in a chemically heterogeneous Archean mantle. Evidence for heterogeneity of the Archean mantle is preserved in the correlated Hf and Nd isotopes in mafic (basaltic) magmatic rocks and their 'decoupling' in ultramafic rocks, especially in komatiites, from the Archean Dharwar Craton, India. The komatiites and komatiitic basalts from the western Dharwar Craton were emplaced at ~3.25 Ga and have large spread in initial radiogenic isotope compositions with initial ɛ Hf (ranging from +0.9 to +21.3) and initial ɛ Nd from +0.2 to +7.4 that do not correlate. In contrast, the ɛ Hfi and ɛ Ndi in contemporaneous mafic (ɛ Hfi = +5.9 ± 11.2; ɛ Ndi = +3.2 ± 7.6) and felsic (ɛ Hfi = +4.3 ± 4.6; ɛ Ndi = +2.7 ± 4.2) rocks in the same area correlate, as is typical of rocks that are the product of magmatic processes involving melting and magmatic differentiation. The high initial ɛ Hf and the lack of correlation ('decoupling') of ɛ Hf and ɛ Nd in the komatiites and komatiitic basalts indicate that the source had superchondritic Lu/Hf which is attributed to early mantle differentiation at great depth, possibly in the perovskite stability field. This finding indicates that the primordial mantle was mineralogically and chemically layered and this layering was the result of fractionation processes very early in Earth's history, probably during magma ocean solidification. During crystallization of a primordial magma ocean, fractionation of Mg-perovskite with minor Ca-perovskite component could have resulted in a melt with high Lu/Hf and nearly unfractionated Sm/Nd. The correlation of the initial Hf isotope composition of the komatiites and komatiitic basalts with their MgO content, and thus melting temperatures, indicates the presence of a highly fractionated component derived from the deep mantle. The coupling of ɛ Hf -ɛ Nd in mafic to felsic igneous rocks and the decoupling in contemporaneous ultramafic rocks is thus the result of mantle material originating from different depths involving different ancient fractionated reservoirs. The existence of a chemical and isotopically heterogeneous Archean mantle that supplied material to form the continental crust has to be accommodated in crustal growth and preservation models that commonly assume a nearly homogeneous and gradually depleting mantle reservoir as the source of the continental crust. • Hafnium-Nd isotope decoupling seen in komatiites in the western Dharwar Craton. • Uranium-Pb zircon age of 3.25 Ga from felsic volcanic rocks. • The extent of decoupling increases with increasing Mg content. • Early differentiation of source by perovskite fractionation in the deep mantle. • Mafic rocks after 3 Ga show enriched Hf-Nd isotope signatures. [ABSTRACT FROM AUTHOR]

Combined geochronological and (isotope) geochemical investigations of tonalite-trondhjemite-granodiorite (TTG) suites, which dominate Archean cratons, provide constraints on the sources and petrogenetic processes that gave rise to the Earth's early continental crust. In situ U-Pb and bulk Lu-Hf analyses of single zircon grains from TTGs of the Paleoarchean Bastar Craton in central India date the emplacement of the igneous rock suite and trace the chemical evolution of its source rocks. Complementary whole rock Lu-Hf and major and trace element data constrain the petrogenesis of the TTGs. The rocks are variably enriched in fluid mobile elements, particularly K and Pb, and are characteristically depleted in heavy rare earth elements. Calculated initial Hf isotopic compositions of zircon and whole rocks cluster around the chondritic value at ca. 3.55 to 3.45 Ga. The high precision Hf-isotope data reveal a trend of increasing radiogenic 176Hf/177Hf with age, the slope of which implies a 176Lu/177Hf = 0.02, and thus derivation from a mafic protolith. Geochemical modeling indicates that the granitoids can be directly produced by melting of a hydrous basalt in the garnet stability field, followed by minor fractional crystallization. Variations in the modal abundance of garnet suggest the source rocks melted at different depths. Rocks with moderately to strongly depleted HREE patterns require >10% garnet in the residue, consistent with derivation from a garnet-amphibolite or garnet-pyroxenite source. Modeling of the Lu-Hf systematics further highlights the important role of melting as opposed to fractional crystallization for reproducing the primary chemical characteristics of TTGs. The global Hf-isotope record of Archean mantle-derived rocks is considerably more radiogenic than that of the bulk silicate Earth, providing unambiguous evidence that parts of the mantle were significantly depleted in incompatible elements since at least the early Archean. The differences between Hf isotopic compositions of the mafic (greenstone belts) and felsic (gneissic terranes) rocks can be accounted for by crustal residence times of several 100 million years for the protoliths of the felsic magmatic suites. Consequently, TTGs integrate the geologic history of their precursor(s), complicating their isotopic record. Therefore, the near-chondritic Hf isotopic composition of TTGs is not direct evidence for their derivation from a primitive mantle, but rather a consequence of their specific petrogenesis involving an aged, mafic precursor. This relationship introduces uncertainties into models of crustal growth and geodynamics that are based on the felsic record, especially where the whole rock context is absent. In contrast, high-precision Hf-isotope analyses of single zircon grains, combined with whole rock isotope and elemental constraints, can be used to reliably identify the sources and processes involved in the generation of Earth's oldest continental crust. • First Lu-Hf isotopic constraints on TTGs from the Paleoarchean Bastar Craton, India • Bulk Lu-Hf analyses of single zircon grains • Geochemical modeling suggests melting of a garnet-pyroxenite source produced TTGs • Lu-Hf systematics may reveal clues about Archean crustal residence times [ABSTRACT FROM AUTHOR]

• Barite and apatite preserve initial 87Sr/86Sr at the time of formation. • Initial 87Sr/86Sr of apatite measured in situ with good internal precision. • Differentiation of crust initiated at 3.6 Ga. • Not much older felsic/evolved crust during the early Archaean. Among the rocky planets of the solar system only the Earth has "granitic" continental crust. The timing and processes involved in the formation of Earth's first extensive crust is still enigmatic. The chemical and isotope compositions of ancient crustal rocks preserve a record of their genesis. The Rb-Sr system proves to be an efficient proxy for the reconstruction of crust-mantle evolution since it can bring together information from seawater as preserved in chemical sedimentary rocks and information from magmatic rocks that can trace the time and extent of crust formation and concomitant mantle depletion during the Archaean eon. The Dharwar Craton in India preserves a suite of metamorphosed igneous and sedimentary rocks that record its early crustal evolution. To overcome the susceptibility for resetting and the difficulty in determining initial 87Sr/86Sr, the minerals barite and apatite are used to obtain precise and accurate 87Sr/86Sr, because these minerals preferentially incorporate Sr and exclude Rb and preserve the initial Sr isotope compositions at the time of their formation. Initial 87Sr/86Sr of apatite were obtained in situ using Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry. The robustness and only minor dispersion of the isotope ratios demonstrate the capability of matrix apatites in preserving initial Sr ratios. The least radiogenic value is used as the best estimate for the initial 87Sr/86Sr. The 87Sr/86Sr ratios of apatite from igneous rocks that formed from 3.5 Ga to 2.6 Ga constrain the Rb/Sr of the source over the whole time-span. A comparison of the Sr isotopes between seawater-derived barite and initial Sr isotope ratios in apatite from igneous rocks reveals that significant mafic to intermediate crust had formed by 3.2 Ga. Studying the entire Archaean time window, a dominantly mafic crust was the main source for the granitoid rocks in the Dharwar Craton from 3.5 to 3.1 Ga, whereas the rocks from 2.9 to 2.7 Ga were extracted dominantly from the depleted mantle. [ABSTRACT FROM AUTHOR]