Abstract: Palaeoproterozoic mafic magma intruded the Gwalior Group of sediments in the form of gabbroic sills. Huge sills extending more than 120 m in depths are exposed in quarries. They are coarse to medium grained, massive and have chilled margins. They predominantly consist of plagioclase feldspar, pyroxene with accessory quartz, iron oxides and apatite. Ophitic to sub-ophitic, porphyritic and intergrowth textures are common. These are tholeiitic, sub-alkaline magma types which have been generated by varying degrees of partial melting and have experienced fractionation of dominantly pyroxene + olivine ± plagioclase. They are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and exhibit negative Nb and P anomalies and positive Pb anomalies. A mineral–whole-rock Sm–Nd isochron corresponds to an age of 2104 ± 23 Ma with an initial 143 Nd/ 144 Nd = 0.509938 ± 0.000023 and ε Nd t = −0.9 ( t = 2000). Model ages (2.6–1.7 Ga) indicate that their mantle sources had a protracted evolution. Mantle source melting may have been triggered by mantle plume in an extensional intra-continental rift tectonic setting, facilitating emplacement of a Large Igneous Province (LIP) in the northern Indian Shield. Thus, Gwalior mafic magmatism represented by the studied sills is the manifestation of one such LIP in the northern Indian Shield. These magmatic activities are contemporaneous with the widespread mafic magmatic activities concomitant with the development of supra-crustal basins on Archaean cratons worldwide and the Indian Shield in particular, where upwelling plumes triggered large-scale crustal extension, breaking-up of supercontinents and emplacement of LIPs.

Abstract A mafic magmatic sequence of the Bhandara–Balaghat Granulite (BBG) Belt is represented by gabbroic rocks containing orthopyroxene (Opx)–clinopyroxene (Cpx)–plagioclase (Pl)–hornblende±quartz±garnet and showing tholeiitic affinity. These rocks are divided into two groups: (I) garnet-bearing; and (II) garnet-free. The garnet-bearing group is characterized by nearly flat REE patterns. In the multi-element plots, Sr, Zr and Ti show negative anomalies, indicating plagioclase, Ti-magnetite and apatite fractionation. The garnet-free rocks are geochemically subdivided into two subgroups: IIa and IIb. Subgroup IIa is marked by flat REE patterns; the LREE shows 20–30 times chondrite abundances and small positive Eu anomalies. Multi-element patterns show negative anomalies of Nb, P and Ti. Subgroup IIb is characterized by slightly enriched patterns; the LREE shows 10–60 times chondrite abundances. The REE patterns for the Subgroup IIb show moderately to highly fractionated LREE with flat HREE. Multi-element plots show negative anomalies in Nb, Ti and Zr. The Nd–Ce relationship suggests that mafic granulites of the BBGs are derived from higher degrees (Group I, c. 15–30%; Subgroup IIa, c. 20–40%; and Subgroup IIb, c. 18–35%) of partial melting of variably enriched mantle sources, followed by the evolution of the parental melt by fractional crystallization of Opx–Cpx–Pl. The geochemical signatures also suggest that the magma was further modified by crustal contamination during the course of its evolution. The Nd ( T DM ) model ages, which vary from 3.2 to 1.6 Ga, suggest a long-term evolution of the mafic granulites, possibly starting with overprinting of the isotope composition of their mantle source by crustal isotope signatures as a consequence of crustal recycling; evolving by emplacement and crystallization of the protolith at 2.7 Ga, as well as through later tectonotermal events up to granulite-facies metamorphism and exhumation of the BBG Belt during the collision of the Archaean Bundelkhand and Bastar cratons, and the formation of the Central Indian Tectonic Zone (CITZ) at 1.5 Ga.

Abstract Field and geochemical studies combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating set important constraints on the timing and petrogenesis of volcanic rocks of the Neoarchaean Kadiri greenstone belt and the mechanism of crust formation in the eastern Dharwar craton (EDC). The volcanic rocks are divided into three suites: tholeiitic basalts, calc-alkaline high-Mg# andesites and dominant dacites–rhyolites. The basalts (pillowed in places) show flat rare earth element (REE) and primordial mantle-normalized trace element patterns, but have minor negative Nb and Ta anomalies. They are interpreted as mantle plume-related oceanic plateau basalts whose source contained minor continental crustal input. The andesites are characterized by high Mg# (0.66–0.52), Cr and Ni, with depletion of high-field strength elements (HFSE) and enrichment of light REE (LREE) and large-ion lithophile elements (LILE). They were probably derived from a metasomatized mantle wedge overlying a subducted slab in a continental margin subduction zone. The dacites–rhyolites are silicic rocks (SiO 2 =61–72 wt%) with low Cr and Ni, K 2 O/Na 2 O mostly 0.5–1.1, highly fractionated REE patterns, enrichments of LILE and distinctly negative HFSE anomalies. One rhyolite sample yielded a zircon U–Pb age of 2353±32 Ma. This suite is similar to potassic adakites and is explained as the product of deep melting of thickened crust in the arc with a significant older crustal component. Collision between a continental margin arc with an oceanic plateau followed by slab break-off, upwelling of hot asthenosphere and extensive crustal reworking in a sustained compressional regime is proposed for the geodynamic evolution of the area. This is in corroboration with the scenario of EDC as a Neoarchaean hot orogen as suggested recently by some workers. Supplementary material: Details of whole-rock major and trace element determination, Nd isotope analysis and zircon U–Pb dating and trace element analysis, the geographical coordinates of the samples and the values of the international rock standards analysed are available at http://www.geolsoc.org.uk/SUP18660