Major-trace element and Sr-Nd isotope compositions of mafic dykes of the Singhbhum Craton : insights into evolution of the lithospheric mantle

The SinghbhumCraton in eastern India is host to at least seven sets of mafic dyke swarms. Four previously dated swarms (studied here) include the NNE-trending Keshargaria (ca. 2.80 Ga) and Ghatgaon (ca. 2.76 - 2.75 Ga) swarms, the ENE-trending Kaptipada swarm (ca. 2.26 Ga), and the ESE-trending Pipilia swarm (ca.1.76 Ga). The dykes range in composition from basalt to andesite and have transitional tholeiitic to calc-alkaline affinities. They show intra- and inter-swarm geochemical and Sr-Nd isotopic heterogeneities and have SiO2 content ranging from 47 to 60 wt.%. The chondrite normalized REE patterns show enrichment in LREE and the Primitive- Mantle normalized multi-element patterns show elevated U, Th, Cs, Rb, K, and Pb; and depletion in Nb, Ta, and Ti. These characteristics indicate involvement of crustal component in the petrogenesis of these dykes. The dykes of different swarms have variable 87Sr/86Sri and εNd(i) values,which define a crust-like isotopic growth trajectory with time from a common chondritic to depleted source that was enriched contemporaneously with the formation of the crustal rocks of the Singhbhum Craton. The isotope data indicate involvement of older enriched crustal material in the petrogenesis of these dykes. Variable but mostly high (compared to similarly evolved magmas) Ni (40 - 590 ppm), Cr (40 - 1110 ppm), and V (120 - 434 ppm) contents particularly of the most primitive dykes indicate that parental melts were in equilibrium with mantle peridotite and experienced only minor fractional crystallization of olivine, pyroxene, and magnetite. The Sr-Nd isotope ratios do not show any correlation with differentiation indices which indicates that the melts were not modified significantly by crustal assimilation during ascent and emplacement. The crust-like secular trend of the Sr and Nd isotopic compositions suggests that the enriched crustal materialwas incubated in themantle (i.e.,metasomatized lithospheric mantle) for a long time and this source was periodically tapped leading tomultiple dykeemplacement events over at least 1 Gyr. The recycled crustal material played a role in metasomatizing the subcontinental lithospheric mantle prior to ca. 2.80 Ga. Mantle plume activity triggered melting of the metasomatized lithospheric mantle many times, leading to the emplacement of mafic dykes of different generations across the craton

GIS and geophysics: an integrative and inter-evaluating approach towards groundwater assessment

Efforts have been made to explore groundwater assessment possibilities employing geospatial and geophysical techniques as well as statistically authenticate the connection between these two techniques. The study was carried out at Chotanagpur Granite-Gneiss Complex (CGGC), Ranchi, Jharkhand, India. The analytical hierarchy process (AHP) was used for Multi-Criteria Decision Making (MCDM) for various thematic factors such as land use land cover, drainage density, lineament density, lithology, rainfall, soil texture, and slope based on their interaction with atmospheric water and contribution towards groundwater replenishment. These remotely-sensed raw datasets were enriched and overlaid in the Geographic Information System (GIS) atmosphere using the weighted overlay analysis tool, which yielded the groundwater potential index (GWPI) map. Based on the estimated GWPI, the watershed area has been demarcated into five distinct groundwater potential classes in terms of poor, low, moderate, good, and very good. The inverted resistivity tomography models revealed exploitable groundwater zone lies at five sites at depth ranges between 60–70, 70–100, and 140–170 m underlie in the saturated weathered/fractured rocks where characteristic resistivities range between 80 and 800 Ω.m. In the target area, i.e. the IIAB campus, five different zones have been demarcated for groundwater exploitation in terms of low, moderate, and good considering groundwater exploitable depth and their connectivity to the near-surface recharge source. Geophysical findings were also corroborated with the existing borehole datasets for their authenticity and helped to generate conceptual geological models. The accuracy assessment combining geospatial and geophysical results validated their inter-relationship as per the generated error matrix with the overall accuracy of 87.5%

Evidence of crustal reworking in the Mesoarchean: Insights from geochemical, U-Pb zircon and Nd isotopic study of a 3.08-3.12 Ga ferro-potassic granite-gneiss from north-eastern margin of Singhbhum Craton, India

Evidences of Mesoarchean crustal reworking are not very common in geological records, being only found from Precambrian terrains like Kaapvaal and Pilbara Cratons where it is preserved as anorogenic/post-collisional granitic activity of 3-3.2 Ga age. The present study focuses on a granitoid unit of similar age, found near the eastern margin of the Precambrian Singhbhum Craton of eastern India. This ellipsoidal, N-S trending, well-foliated granite-gneiss body is found within the polydeformed, metasedimenatry rocks of Singhbhum Group, belonging to the North Singhbhum Mobile Belt. LA-ICP MS U-Pb analysis of zircon grains from one sample give crystallization ages ranging from 3079.4 +/- 6.8 Ma to 3115 +/- 10 Ma. These rocks hereby dubbed as ``Bangriposi Granite Gneiss'', are composed of quartz, alkali feldspar, ferroan biotite (Fe/Fe + Mg: 0.7-0.9), titanite, illmenite, hastingsite, apatite, and numerous U-Th-REE- bearing accessory phases. They have high SiO2 (67-77 wt%) and Na2O + K2O (8.19-9.01 wt%), low CaO (0.35-1.5 wt%), MgO (0.05-036 wt%) and shows enrichment of Nb, Rb, Zr, Y, Th, and REEs, and depletion of Cr, Ni, U, with high FeOt/FeOt, + MgO(similar to 0.9), Ga/Al (2.04-3.51), (La/Yb)(N) (6.5-13), and low Eu/Eu* (0.11-0.47). Geochemically and mineralogically they are categorized as metaluminous to weakly peraluminous (A/CNK: 0.9-1.1) ferro-potassic alkali feldspar granites. These rocks show unequivocal affinities towards `A-type'' granites and from geochemical evidences it is suggested that the parent magma was produced in response to crustal anatexis under low foe with P-T estimates of -900 degrees C and 7-8 kbar. Geochemical modelling has revealed that the probable source was lower crustal amphibolites belonging to the Paleoarchean Older Metamorphic Group, which suffered low degrees (5-15%) of melting. Negative CNd values ( -0.5 to -1.5) are also in favour of reworking of older crust and their Nd isotopic signature bears similarities with other coeval anatectic granites. Bangriposi Granite-Gneiss, along with Mayurbhanj Granite and Bonai Granite, represent a major phase of Mesoarchean anorogenic/post-collisional granitic activity in Singhbhum Craton, indicating the onset of its stabilization. Similar Mesoarchean crustally reworked felsic units are encountered in Pilbara and Kaapvaal cratons suggesting a possibility of correlation with the hypothesized ``Vaalbara'' supercontinent. (C) 2019 Elsevier B.V. All rights reserved

Integrated subsurface investigation for magmatic sulfide mineralization in Betul Fold Belt, central India

Magnetotelluric (MT), Electrical Resistivity Tomography (ERT), Time-Domain Induced Polarization (TDIP), Geochemical and Geological studies followed by drilling and down-hole logging were undertaken with in the Betul Fold Belt (BFB) in Central India, to demarcate zones of magmatic Ni-Cu-PGE sulfide mineralization. The BFB is predominantly composed of circular to elongate gabbro bodies of the Padhar Mafic-Ultramafic Complex, intruded into a sequence of bimodal volcanic rocks and quartzites. Near-surface samples of ultramafic rocks were subjected to precise geochemical analysis and scanned by an electron microscopy with an energy dispersive spectrometer (SEM-EDS). This work indicated the presence of pyrite, pyrrhotite, chalcopyrite, pentlandite, and minor amounts of W-Cd bearing boweiite and palladenite assemblage. These minerals are regarded as favorable to the occurrence of Ni-Cu-PGE sulfide mineralization. MT data derived from two profiles were analyzed and modeled using 2D and 3D inversion algorithms. The robust conductivity anomalies obtained from the MT model have been interpreted coupled with electrical tomography, geology, and geochemistry data. The near-surface shallow depth conductors observed in the ERT sections are interpreted as the sulfide mineralized zones. They corroborate the MT results. These conductive zones reflect the occurrence of the magmatic Ni-Cu-PGE bearing sulfide mineralization associated with rocks of the mantle-derived Padhar Mafic Ultramafic Complex. This geophysical data, in conjunction with petrological and geochemical analysis of drill core samples have allowed the identification of the origin and paragenesis of sulphide mineralization in the study area. Geochemical studies suggest that the parental magma was generated from a subduction modified, metasomatized and an enriched mantle source that was subsequently emplaced in a magmatic continental arc setting. The interpreted conductors, observed at shallow depths (similar to 200-300 m), have been generated by secondary hydrothermal fluid circulation leading to vein formation in the host Padhar Mafic-Ultramafic Complex. The MT and electrical tomography models delineate the geological boundaries of the sulfide-bearing mineralized deposits in the BFB