Geochemical studies and petrogenesis of ~2.21–2.22 Ga Kunigal mafic dyke swarm (trending N-S to NNW-SSE) from eastern Dharwar craton,India: implications for Paleoproterozoic large igneous provinces and supercraton superia

The Archean eastern Dharwar craton is transacted by at least four major Proterozoic mafic dyke swarms. We present geochemical data for the ~2.21–2.22 Ga N-S to NNW-SSE trending Kunigal mafic dyke swarm of the eastern Dharwar craton to address its petrogenesis and formation of large igneous province as well as spatial link to supercontinent history. It has a strike span of about 200 km; one dyke of this swarm runs ~300 km along the western margin of the Closepet granite. Texture and mineral compositions classify them as dolerite and olivine dolerite. They show compositions of high-iron tholeiites, high-magnesian tholeiites or picrites. Geochemical characteristics of the sampled dykes suggest their co-genetic nature and show variation from primitive (Mg#; as high as ~76) to evolved (differentiated) nature. Although geochemical characteristics indicate possibility of minor crustal contamination, they show their derivation from an uncontaminated mantle melt. These mafic dykes are probably evolved from a sub-alkaline basaltic magma generated by ~20 % batch melting of a depleted lherzolite mantle source and about 15–30 % olivine fractionation. Paleoproterozoic (~2.21–2.22 Ga) mafic magmatism is recognized globally as dyke swarms or gabbroic sill complexes in the Superior, Slave, North Atlantic, Fennoscandian and Pilbara cratons. Possible Paleoproterozoic Dharwar–Superior–North-Atlantic–Slave correlations are constrained with implications for the configuration of supercraton Superia.     

Precise U–Pb dating of Paleoproterozoic mafic dyke swarms of the Dharwar craton,India: Implications for the existence of the Neoarchean supercraton Sclavia

We report seven high precision U–Pb age determinations for mafic dykes from a number of major Precambrian swarms located in the Dharwar craton, south India. These new age results define two previously unrecognized widespread Paleoproterozoic dyking events at 2221–2209 and 2181–2177 Ma, and confirm a third at 2369–2365 Ma. Three parallel E–W trending mafic dykes from the petrographically and geochemically variable Bangalore dyke swarm, the most prominent swarm in the Dharwar craton, yield indistinguishable U–Pb baddeleyite ages of 2365.4 ± 1.0, 2365.9 ± 1.5 and 2368.6 ± 1.3 Ma, indicating rapid emplacement in less than five million years. A compilation of Paleoproterozoic U–Pb ages for mafic magmatic events worldwide indicates that the 2369–2365 Ma Bangalore dyke swarm represents a previously unrecognized pulse of mafic magmatism on Earth.     

Pb–Pb zircon ages of Archaean metasediments and gneisses from the Dharwar craton,southern India: Implications for the antiquity of the eastern Dharwar craton

²⁰⁷Pb–²⁰⁶Pb ages of zircons in samples of metasediments as well as ortho- and para-gneisses from both the western and the eastern parts of the Dharwar craton have been determined using an ion microprobe. Detrital zircons in metasedimentary rocks from both yielded ages ranging from 3.2 to 3.5 Ga. Zircons from orthogneisses from the two parts also yielded similar ages. Imprints of younger events have been discerned in the ages of overgrowths on older zircon cores in samples collected throughout the craton. Our data show that the evolution of the southwestern part of eastern Dharwar craton involved a significant amount of older crust (>3.0 Ga). This would suggest that crust formation in both the western and eastern parts of the Dharwar craton took place over similar time interval starting in the Mesoarchaean at ca. 3.5 Ga and continuing until 2.5 Ga. Our data coupled with geological features and geodynamic setting of the Dharwar craton tend to suggest that the eastern Dharwar craton and the western Dharwar craton formed part of a single terrane.     

Petrogenesis of 3.15 Ga old Banasandra komatiites from the Dharwar craton,India: Implications for early mantle heterogeneity

Spinifex-textured.magnesian(MgO 25 wt.%) komatiites from Mesoarchean Banasandra greenstone belt of the Sargur Group in the Dharwar craton,India were analysed for major and trace elements and~(147,146)Sm-~(143,142)Nd systematics to constrain age,petrogenesis and to understand the evolution of Archean mantle.Major and trace element ratios such as CaO/Al_2O_3.Al_2O_3/TiO_2,Gd/Yb,La/Nb and Nb/Y suggest aluminium undepleted to enriched compositional range for these komatiites.The depth of melting is estimated to be varying from 120 to 240 km and trace-element modelling indicates that the mantle source would have undergone multiple episodes of melting prior to the generation of magmas parental to these komatiites.Ten samples of these komatiites together with the published results of four samples from the same belt yield ~(147)Sm-~(143)Nd isochron age of ca.3.14 Ga with an initial ε_(Nd)(f) value of+3.5.High precision measurements of ~(142)Nd/~(144)Nd ratios were carried out for six komatiite samples along with standards AMES and La Jolla.All results are within uncertainties of the terrestrial samples.The absence of~(142)Nd/~(144)Nd anomaly indicates that the source of these komatiites formed after the extinction of ~(146)Sm,i.e.4.3 Ga ago.In order to evolve to the high ε_(Nd)(t) value of +3.5 by 3.14 Ga the time-integrated ratio of~(147)Sm/~(144)Nd should be 0.2178 at the minimum.This is higher than the ratios estimated,so far,for mantle during that time.These results indicate at least two events of mantle differentiation starting with the chondritic composition of the mantle.The first event occurred very early at ~4.53 Ga to create a global early depleted reservoir with superchondritic Sm/Nd ratio.The source of Isua greenstone rocks with positive ~(142)Nd anomaly was depleted during a second differentiation within the life time of ~(146)Sm,i.e.prior to 4.46 Ga.The source mantle of the Banasandra komatiite was a result of a differentiation event that occurred after the extinction of the ~(146)Sm,i.e.at 4.3 Ga and prior to 3.14 Ga.Banasandra komatiites therefore provide evidence for preservation of heterogeneities generated during mantle differentiation at4.3 Ga.     

Major and trace element geochemistry of olivine from an ENE-trending picrite dyke,genetically linked to the ca. 1.89–1.88 Ga Hampi swarm,western Dharwar craton: Implications for plume induced melting of a mixed peridotite-pyroxenite source

The presence of recycled crust in the lithospheric mantle of the Dharwar craton has been investigated using trace element geochemistry of olivine grains from an ENE-trending Paleoproterozoic picrite dyke (associated with the ca. 1.89–1.88 Ga Hampi dyke swarm) emplaced in the western Dharwar craton. Olivine grains are purely magmatic, formed as early phenocrysts in a fractionated basaltic melt. They exhibit enrichment in NiO contents (0.32–0.43 wt%) and depletion in Ca (1366–2105 ppm), Mn (1578–2663 ppm) and 100 1 Mn/Fe (1.28–1.48). Further, the compiled whole-rock geochemical data of the picrite dyke and associated dyke swarm illustrates relatively low CaO/MgO (0.55–1.78), intermediate FeO/MnO (47–54), negative to positive PX# (?0.34 to +1.86), and high values of FC3MS (0.24–0.90) and FCKANTMS (0.19–1.11). These chemical markers are not consistent with the derivation of the primary melt from a pure peridotite or a pyroxenite source; therefore, contribution from a mixed type of source having both peridotite and pyroxenite end members (pyroxene rich and olivine poor lithology) is suggested. The amount of pyroxenite and recycled crust varies from 46% to 86% and 14% to 44%, respectively. The Al-in-olivine based thermometer estimates the maximum crystallization temperature as 1407 °C, which is 137 °C higher than the average temperature of MORB and accordant with several well-established plume-induced large igneous provinces (LIPs) worldwide. Therefore, it is suggested that the studied picrite dyke is derived from a primary melt generated by plume-induced melting of a peridotite-pyroxenite mixed source. The ca. 1.89–1.88 Ga Hampi dyke swarm, being genetically linked with the studied dyke, could also be derived from this same source. Further, the recycled crust in the subcontinental lithospheric mantle of the western Dharwar craton may have generated the pyroxene rich mafic source during the Neoarchean convergence between eastern and western Dharwar craton.     

Geochemistry of an ENE–WSW to NE–SW trending ∼2.37 Ga mafic dyke swarm of the eastern Dharwar craton,India: Does it represent a single magmatic event?

A vast tract of ENE–WSW to NE–SW trending mafic dyke swarm transects Archaean basement rocks within the eastern Dharwar craton. Petrographic data reveal their dolerite/olivine dolerite or gabbro/olivine gabbro composition. Geochemical characteristics, particularly HFSEs, indicate that not all these dykes are co-genetic but are probably derived from more than one magma batch and different crystallization trends. In most samples the La^N/Lu^N ratio is at ∼2, whereas others have a La^N/Lu^N ratio >2 and show higher concentrations of high-field strength elements (HFSEs) than the former group. As a consequence, we assume that the ENE–WSW to NE–SE trending mafic dykes of the eastern Dharwar craton do not represent one single magmatic event but were emplaced in two different episodes; one of them dated at about 2.37 Ga and another probably at about 1.89 Ga. Trace element modelling also supports this inference: older mafic dykes are derived from a melt generated through ∼25% melting of a depleted mantle, whereas the younger set of dykes shows its derivation through a lower degree of melting (∼15%) of a comparatively enriched mantle source.     

Geochronological and geochemical constraints on the origin of the 304 ± 5 Ma Karamay A-type granites from West Junggar,Northwest China: implications for understanding the Central Asian Orogenic Belt

U–Pb zircon geochronological, geochemical, and whole-rock Sr–Nd isotopic analyses are reported for a suite of Karamay A-type granites from the Central Asian Orogenic Belt (CAOB) in the western Junggar region of northern Xinjiang, Northwest China, with the aim of investigating the sources and petrogenesis of A-type granites. The Karamay pluton includes monzogranite and syenogranite. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating yielded a concordant weighted mean ²⁰⁶Pb/²³⁸U age of 304 ± 5 Ma (n = 11), defining a late Carboniferous magmatic event. Geochemically, the rock suite is characterized by high SiO₂, FeOt/MgO, total alkalies (K₂O + Na₂O), Zr, Nb, Y, Ta, Ga/Al, and rare earth elements (REEs) (except for Eu), and low contents of MgO, CaO, and P₂O₅, with negative Ba, Sr, P, Eu, and Ti anomalies. These features indicate an A-type affinity for the Karamay granitic intrusions. Isotopically, they display consistently depleted Sr–Nd isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr = 0.7014–0.7022, ?_Nd(t) = +5.6–+7.0). Geochronological, geochemical, and isotopic data suggest that the Karamay A-type granites were derived from remelting juvenile lower crust, followed by fractional crystallization. The Karamay A-type granites as well as widespread late Carboniferous magmatism in the western Junggar region of the southwestern CAOB may have been related to ridge subduction and a resultant slab window. This further demonstrates the importance of the late Palaeozoic granitic magmatism in terms of vertical crustal growth in northern Xinjiang.     

Rb–Sr and Sm–Nd isotope systematics and geochemical studies on metavolcanic rocks from Peddavura greenstone belt: Evidence for presence of Mesoarchean continental crust in easternmost part of Dharwar Craton,India

Linear, north–south trending Peddavura greenstone belt occurs in easternmost part of the Dharwar Craton. It consists of pillowed basalts, basaltic andesites, andesites (BBA) and rhyolites interlayered with ferruginous chert that were formed under submarine condition. Rhyolites were divided into type-I and II based on their REE abundances and HREE fractionation. Rb–Sr and Sm–Nd isotope studies were carried out on the rock types to understand the evolution of the Dharwar Craton. Due to source heterogeneity Sm–Nd isotope system has not yielded any precise age. Rb–Sr whole-rock isochron age of 2551 ± 19 (MSWD = 1.16) Ma for BBA group could represent time of seafloor metamorphism after the formation of basaltic rocks. Magmas representing BBA group of samples do not show evidence for crustal contamination while magmas representing type-II rhyolites had undergone variable extents of assimilation of Mesoarchean continental crust (>3.3 Ga) as evident from their initial ε _Nd isotope values. Trace element and Nd isotope characteristics of type I rhyolites are consistent with model of generation of their magmas by partial melting of mixed sources consisting of basalt and oceanic sediments with continental crustal components. Thus this study shows evidence for presence of Mesoarchean continental crust in Peddavura area in eastern part of Dharwar Craton.