Ashwini Kumar Choudhary

Neoarchaean crustal growth by combined arc–plume action: evidence from the Kadiri Greenstone Belt, eastern Dharwar craton, India

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 (SiO2=61–72 wt%) with low Cr and Ni, K2O/Na2O 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

The diversification of granitoids and plate tectonic implications at the Archaean–Proterozoic boundary in the Bundelkhand Craton, Central India

Abstract The Bundelkhand Craton in Central India holds a large Archaean granitoid complex consisting of cores of TTG (tonalite–trondhjemite–granodiorite) gneisses of island arc or oceanic origin surrounded by abundant younger high-K calc-alkaline granitoids. Major and trace element groupings and ion probe U–Pb zircon datings of the groups show a time gap of 130 Ma between the main formation episodes of the TTGs (3.5/3.3–2.7 Ga) and the emplacement of the first high-K granitoids (2.57–2.54 Ga). Based on their geochemical diversity, the high-K calc-alkaline granitoids can be divided into low-silica high-magnesium (LSHM) granitoids such as sanukitoids and Closepet-type granitoids, and high-silica low-magnesium (HSLM) monzogranites with low-HREE and low-Eu subgroups. The former group points to mantle or mixed mantle and crustal sources, and the latter to pure crustal sources. All the varieties of the high-K granitoids formed within a narrow time span, which indicates large-scale partial melting and fluid activity in the mantle and crust, possibly resulting from a slab breakoff or delamination at the margin of an Archaean TTG continent. Supplementary material: Major and trace element concentrations and U-Pb results of granitoids from the Bundelkhand Craton are available at https://doi.org/10.6084/m9.figshare.c.3576377

Geochemical characterization and petrogenesis of mafic granulites from the Central Indian Tectonic Zone (CITZ)

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 (TDM) 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.

Effect of Working Pressure on Structural, Electrical and Optical Properties of CIGS Thin Film Deposited by PLD

CuIn0.8Ga0.2Se2 (CIGS) thin films have been successfully deposited on soda lime glass substrate at different working (Ar gas) pressures by Pulsed laser deposition (PLD). The effect of working pressure on the structural, electrical and optical properties of CIGS thin films has been investigated. All deposited CIGS thin films are found to be polycrystalline in nature with preferred orientation along (112). Crystallinity of CIGS thin films has been improved with decreasing of working pressures in PLD chamber. Stoichiometry of CIGS thin films has been maintained for all working pressures. Optical transmittance and electrical sheet resistance of CIGS thin films have been found to decrease with decrease of working pressure. The results of the present study signify that desired structural, electrical and optical properties of CIGS thin films can be obtained by changing the processing parameter in PLD method.Keywords: CIGS, XRD, Stoichiometry, UV-Vis-NIR spectrophotometer.

Thickness dependent structural, optical and electrical properties of CuIn0.8Ga0.2Se2 thin films deposited by pulsed laser deposition

CuIn0.8Ga0.2Se2 (CIGS) polycrystalline thin films have been deposited on soda lime glass substrate at different deposition time by pulsed laser deposition. The effect of thickness on structural, surface morphological, optical and electrical properties of thin films were investigated by X-ray diffractometer (XRD), field emission scanning electron microscope (FE-SEM), atomic force microscopy (AFM), UV-Vis-NIR spectrophotometer and electrical measurement unit. XRD study reveals that all deposited films are polycrystalline in nature and have tetragonal phase of CIGS. Crystallinity of CIGS films has been found to improve with increase in thickness of CIGS films as evidenced by sharp XRD peaks for (112) orientation. Grain size and rms surface roughness of CIGS films have been found to be increased with increase in thickness. All the deposited CIGS films exhibit direct band gap semiconducting behaviour with ∼106 cm−1 absorption co-efficient. Optical band gap and resistivity of CIGS films have been found to decre...