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In the Singhbhum Craton of the Indian shield, the Remal granite-gneiss preserves felsic magmatic fabrics onto which a low-temperature segregation layering has been superposed. Planar, sub-horizontal to gently dipping layers (Sign1) comprise K-feldspar megacrysts, plagioclase and quartz, with the base of each layer defined by segregations of biotite. Sign2 consists of trough cross-bedded layers composed of K-feldspar phenocrysts, plagioclase and quartz with biotite schlieren defining the base of each layer. Microstructural features such as concentrically arranged mineral inclusions in K-feldspar phenocrysts and graphic intergrowth textures testify to the magmatic origin of these fabrics, with insignificant subsequent metamorphic reconstitution. The tectonic fabric S1 has developed sub-parallel to localized greenschist-facies mylonite bands, and is defined by weakly aligned flakes of biotite. Crystallographic preferred orientations away from the mylonitized domains show a strong alignment of K-feldspar, quartz and biotite parallel to the magmatic fabric due to efficient segregation during magmatic flow. Quartz crystallographic preferred orientations within the mylonitized domains show a strong preferred orientation and dextral asymmetry. Temperature constraints from synkinematic chlorites along with estimates of deformation temperature from quartz crystallographic preferred orientations indicate that mylonitization occurred at the lower limits of quartz crystal plasticity. The results of combined thermodynamic and multiphysics modelling studies show that felsic magmas can undergo significant convective motion for a wide range of crystallinities and water contents before solidification. Additionally, segregation layering resembling a gneissosity can develop at low temperatures owing to localized mylonitization and concomitant dissolution–precipitation of biotite. [ABSTRACT FROM AUTHOR]

We examined the microstructures of three ductile shear bands of granitic composition that deformed under mid-to-upper greenschist facies. These shear bands, from the Bundelkhand Craton, India, are 0.8–4.4 mm thick with 20–30 μm median grain sizes and share sharp contacts with the host granite. Comminution of the rheologically stronger feldspars increased permeability, promoted fluid influx, and allowed strain localisation in the incipient shear bands. The major components, quartz and feldspars, deformed via phase-specific crystal plastic deformation mechanisms. Quartz in the monomineralic bands deformed predominantly via dislocation creep, while the feldspars deformed via diffusion creep. Finer quartz grains, which occur at plagioclase triple junctions, resulted from myrmekitisation – also evident from the anti-clustered distribution of quartz-plagioclase phase boundaries and CPO inheritance. The weak CPOs and high mean misorientation angles of the fine plagioclase and K-feldspar grains and yet a high shear strain (γ = 16–37) suggest a significant contribution of grain boundary sliding (GBS) in strain localisation inside the ductile shear bands. Since intracrystalline slip alone cannot explain the high ductile strain in the shear bands, we suggest that GBS controlled the deformation and maturation of the shear bands and promoted superplasticity to accommodate further shearing. • Thin granitic shear bands show large shear strain accommodation. • Feldspar and quartz dynamic recrystallisation reduced the grain sizes. • Myrmekitisation caused phase mixing of plagioclase and quartz grains. • Fine quartz and feldspar grains show weak CPOs and high mean misorientation. • The enormous shear strain is mostly accommodated by GBS. [ABSTRACT FROM AUTHOR]

• The metamafic volcanics and metasediments from the Sargur Greenstone Belt recorded the 2.50–2.44 Ga regional metamorphism. • The garnet-bearing amphibolite and garnet-staurolite-kyanite mica schist yield an amphibolite-facies metamorphism with peak P–T conditions of 681–769 °C and 7.5–9.3 kbar. • The ca. 2.50–2.44 Ga regional metamorphism was possibly resulted from accretion-collision of different Archean blocks in the WDC. We report petrological and geochronological data for metamafic volcanics and metasediments from the Sargur Greenstone Belt, Western Dharwar Craton (WDC), southern India and discuss the end-Archean metamorphic P–T conditions of the Sargur Greenstone Belt, as well as geotectonic implications of the end-Archean metamorphism in the WDC. The LA-ICP MS zircon U-Pb dating yields a metamorphic age of 2500 ± 20 Ma for the garnet-bearing amphibolite. The garnet-staurolite-kyanite mica schist sample has metamorphic zircon and monazite U-Pb ages of 2443 ± 7 Ma and 2468 ± 10 Ma, respectively, consistent with metamorphic zircon U-Pb ages of 2459 ± 17 Ma and 2455 ± 18 Ma yield by the amphibole-garnet schist samples. The peak mineral assemblage of the garnet-bearing amphibolite is garnet + clinopyroxene + amphibole + plagioclase + quartz + rutile + ilmenite, and that of the metapelitic schist is garnet + staurolite + kyanite + biotite + muscovite + quartz + plagioclase. Phase equilibrium modelling, together with geothermobarometers yield P–T conditions of 7.9–8.9 kbar/681–769 °C and 7.5–9.3 kbar/683–694 °C for the garnet-bearing amphibolite and garnet-staurolite-kyanite mica schist, respectively, corresponding to a medium geothermal gradient of 20–25 °C/km. These data indicate that the Sargur Greenstone Belt has experienced a typical Barrovian-type (amphibolite-facies) metamorphism at ca. 2.50–2.44 Ga. Combined with the previous studies, it is suggested that the Sargur Greenstone Belt was most likely to be formed during 3.35–3.12 Ga and was reworked through four episodes of magmatic-metamorphic events occurred at ca. 3.1 Ga, ca. 3.0 Ga, ca. 2.8–2.6 Ga and ca. 2.5–2.4 Ga. The prominent ca. 2.5–2.4 Ga regional metamorphism is the last stage of the Neoarchean tectono-thermal event in the WDC and thus most possibly indicate accretion-collision of different Archean blocks at ca. 2.5–2.4 Ga and lead to the final cratonization (stabilization) of the whole Dharwar Craton. [ABSTRACT FROM AUTHOR]

• SIMS U-Pb zircon age of 2522 ± 12 Ma for anorthosite from the Sittampundi Complex. • Plagioclase 87Sr/86Sr ratios and chromite γ Os values argue for depleted mantle source. • Parental magma of anorthosite was contaminated by ancient mafic lower crust. • Anorthositic-gabbroic intrusions likely represent the lowermost crust of the Dharwar Craton. • Magmas underplating facilitated the Dharwar cratonization. Mantle-derived magmas at the base of the lower crust exerted a key control on late Archean cratonization in many continents. Since well-preserved, complete lower crustal section is rarely exposed, direct studies on the genetic link between mantle-derived magmas and cratonic lower crustal stabilization are inadequate. Cratonic lower crustal section is well-preserved in the southern margin of the Dharwar Craton (southern India), with a number of late Archean anorthositic-gabbroic complexes. Among these complexes, the Sittampundi anorthosite complex (SAC) consists of white- and dark-anorthosite (>60 vol%), gabbros, and ultramafic rocks. In this study, SIMS zircon U-Pb dating of the anorthosite revealed a minimum emplacement age of 2522 ± 12 Ma, similar to the chromite Os model ages (2528–2563 Ma) of the anorthosite-hosted chromitite. In-situ plagioclase (87Sr/86Sr) i ratios (0.70079–0.70100) of the dark anorthosite and the chromite γ Os (T) values (−0.2 to −0.4) of the chromitite suggest that the SAC was derived from a depleted mantle source. From the dark to white anorthosite, the (87Sr/86Sr) i ratios increase while the An contents decrease, suggesting crustal assimilation occurred during fractionation. Similarly, the mantle-like zircon δ18O values and relatively-wide ε Hf (T) (−2.1 to +8.4) range of the SAC anorthosite suggest that the parental magma had assimilated the ancient mafic lower crust. Emplacement ages of the SAC and published ages of the mafic/felsic granulites and charnockites altogether indicate that the anorthosites were formed during the Dharwar cratonization, and that the mantle-derived magma underplating may have led to extensive lower crustal melting. We argued that during underplating, high-density olivine-pyroxene cumulates (from fractionation of the mantle-derived magma) and partial-melting residues (in the overlying lower crust) mostly sank back to the underlying mantle. In contrast, the lower-density plagioclase and minor amphibole remained in the lower crust to form anorthositic-gabbroic sills. The magmas underplating and subsequent lower-crustal melting have likely made the cratonic lower crust more refractory and buoyant, which facilitated cratonization. [ABSTRACT FROM AUTHOR]