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The age of emplacement and geochemistry of the lamprophyres are of tectonic significance as they have potential to unravel global scale geodynamic processes. We present petrology, U-Pb SHRIMP apatite and titanite ages and bulk-rock Sr-Nd isotope data for an unmetamorphosed and undeformed lamprophyre dyke from the Simdega area of the Chhotanagpur Gneissic Complex (CGC) which is a component of the E -W trending Central Indian Tectonic Zone (CITZ), India. The CITZ is a major intercontinental suture which separates the northern Indian and the southern Indian blocks whose polarity of their subduction is a contentious issue. The lamprophyre exhibits a strong porphyritic-panidiomorphic texture imparted by the megacrysts/phenocrysts of mica and amphibole with feldspar, apatite, titanite, zircon and opaques confined to the groundmass. Based on combined mineralogy and geochemistry, the lamprophyre is classified to be of calc-alkaline variety (minette) with shoshonitic affinities. Mg# (70.7–78.2) contents highlight the primitive melt character whereas incompatible trace element ratios exclude crustal contamination and are indistinguishable from those of the subduction-related global as well as Eastern Dharwar craton (southern India) calc-alkaline lamprophyres. U-Pb dating of apatite gave an emplacement age of 944 ± 82 Ma which is indistinguishable, within the error limits, from the U-Pb titanite age of 942.1 ± 5 Ma demonstrating a Neoproterozoic magmatic emplacement age of the lamprophyre synchronous with the Rodinia assembly. Bulk-rock 87Sr/86Sr initial (0.707239 and 0.708910) and ɛNd initial (-8.9 to -8.3) highlights the involvement of an enriched mantle source. Calculated Paleoproterozoic model ages (Nd depleted mantle) of 2.1 Ga of the lamprophyre are indistinguishable from those of the co-spatial amphibolite dykes. Petrogenetic modeling involving rare earth elements reveals that the derivation of the lamprophyre magma from 2 to 3% partial melting of a mixed garnet (70%) and spinel (30%) lherzolitic mantle source with minor phlogopite. Our study highlights that the western part of the CGC was less affected, relative to the eastern part, by the M3 regional amphibolite grade metamorphic event (ca. 920-880 Ma) and also supports the geodynamic models involving northward-directed subduction of the Southern Indian block under the Northern Indian block.- [ABSTRACT FROM AUTHOR]

Metamorphosed mafic dykes are significant stratigraphic markers which also represent a shift from extensional to compressional geodynamic processes. We present petrology, bulk-rock geochemistry, and Sr-Nd isotope data on nineteen E-W to ENE-WSW trending mafic dykes metamorphosed under amphibolite facies in the Simdega area in the south western domain of the Chhotanagpur Gneissic Complex (CGC). The CGC is widely regarded as an eastern extension of the Central Indian Tectonic Zone (CITZ) which separates the Northern Indian and the Southern Indian blocks. Calcic amphiboles and plagioclases are the dominant minerals whereas clinopyroxene (diopside), biotite, chlorite, quartz, apatite, titanite, ilmenite and magnetite are present in minor amounts. Relict igneous textures are also present in some of the samples. Conventional thermobarometry and pseudosection modelling gave consistent peak temperatures (619 ± 13 °C) and pressures (5.8 ± 0.1 kbar) corresponding to the amphibolite facies. Major oxide geochemistry of the studied samples show differentiation trends and display arc-tholeiitic, high Fe-tholeiitic, high Mg-tholeiitic and calc-alkaline characters. Based on the LREE and HREE fractionation (La N /Yb N , La N /Sm N and Gd N /Yb N) the studied samples can be classified into two broad groups: Group-I and Group II. In terms of the 87Sr/86Sr initial and 143Nd/144Nd initial contents, both the Group I and II samples are confined to an array of continental flood basalts and EM-1 and EM-2 reservoirs. The T DM (depleted mantle) model age of ca. 1.9 Ga suggest the source enrichment of the dykes during the Paleoproterozoic. Trace element ratios of both groups of the amphibolite dykes reveal that the (i) crustal contamination to be minimum, (ii) subduction-related fluid enrichment influenced their source regions, and (iii) mixed arc-, OIB-, and oceanic plateau- type geochemical signatures to be their characteristic features. Petrogenetic modelling involving REE suggest variable degrees of partial melting ranging from 5 to 10% for most of the samples of G-I and 1–3% for G-II dykes from a mixed source comprising predominantly of spinel lherzolite and minor amounts of garnet lherzolite. A three-stage model is proposed for the evolution of the CGC amphibolite dykes which involves: (i) sub-continental lithopheric mantle (SCLM) source enrichment due to the fluids derived from the subduction zone prior to ca. 1.9 Ga, (ii) emplacement of the mafic dykes in a post-collisional intra-continental extensional tectonic setting due interaction between the mantle plume and SCLM during the Tonian period at ca. 0.95 Ga and (iii) subsequent amphibolite facies thermal event at 0.92–0.88 Ga, associated with the amalgamation of the Rodinia supercontinent, that metamorphosed the mafic dykes as well as the much of the south-western part of CGC. Our findings support models proposing a northward subduction of the southern Indian block beneath the northern Indian block along the CITZ as well as bring out the role of a mantle plume in the generation of the Precambrian mafic dykes in the CGC. • Emplacement in a post-collisional intra-continental extensional tectonic setting • Trace elements show arc- as well as plume-derived components. • Nd isotope model ages highlight ancient (1.9 Ga) subduction event in their genesis. • Amphibolite grade metamorphism linked to Rodinia amalgamation [ABSTRACT FROM AUTHOR]

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 ∗ 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. • Olivine compositions of a picrite dyke from western Dharwar craton is presented. • Olivine grains are magmatic and derived from a fractionated melt. • The source is pyroxenite and peridotite (olivine poor, pyroxene rich) mixed type. • The Neoarchean convergence may be the cause of this pyroxene-rich source. • The Al-in-olivine thermometry suggests a mantle plume induced melting. [ABSTRACT FROM AUTHOR]