Bundelkhand Massif Research Articles

Electrical Moho and upper mantle metasomatism in the Archaean Bundelkhand craton of the Indian peninsular shield

The Bundelkhand Craton (BKC), one of the Archaean nuclei in the north central part of the Indian shield, has a lesser surface exposure compared to other cratons. Recent magnetotelluric (MT) study in the Central Ganga Basin (CGB) showed NNE-SSW extension of the cratonic block as a transverse ridge and delineated a shear zone within the buried section of the BKC. In the present study, we extend the MT profile to further south with 19 broadband MT sites in the exposed BKC to complement the lithospheric electrical resistivity architecture. Two-dimensional (2-D) geoelectric structure down to 100 km is obtained from inverse modelling of the distortion corrected MT impedance tensors together with the 31 sites in the CGB. The resulting geoelectric model reveals a flat and sharp resistivity contrast at 40–42 km depth, the electrical Moho (e-Moho), separating highly resistive Bundelkhand massif and moderately conductive upper mantle. The 2-D model also brings out two shear zones represented by near-vertical moderately conductive zones within the massif, indicating zones of amalgamation of different fragments of the pre-Bundelkhand craton. In contrast to other major cratons of the Indian shield, the present MT study has delineated a moderately conductive upper mantle beneath the BKC. In view of the estimated low mantle heat flow along with relatively flat topography, we infer metasomatism of the mantle lithosphere by percolation of fluids. We hypothesize that infiltration of fraction of water (H2O) in nominally anhydrous mantle minerals as hydrogen (H) bonded to structural oxygen (O) originated from the subducted oceanic slab has altered the mantle rheology and metasomatized the lithosphere.

GIS based integration of multi-spectral remote sensing and morphostructural analysis for delineating potential areas of hydrocarbon exploration in Western Vindhyan Basin, India

The Chambal Valley forms the western part of the Proterozoic Vindhyan Basin that has been less studied and poorly explored from the hydrocarbon perspective than its eastern counterpart—the Son Valley Vindhyans. In the present manuscript, attempts have been made to use remote sensing and GIS (geographic information system) based analysis to focus on the northern part of Chambal Valley to decipher its tectonics and structural configuration, its tectonic relation with the Son Valley Vindhyans, Bundelkhand Massif and Ganga Basin. This study also assumes significance as it attempts to delineate potential lead areas for hydrocarbon exploration in Chambal Valley. As part of the study, a detailed spectral analysis based on image data sets is carried out to identify areas of microseeps as proxies of potential hydrocarbon accumulations. Also, a neotectonics-based geomorphic analysis is carried out to delineate a fault network and geomorphic highs that act as aliases to subsurface faults and structural highs. The results are subsequently validated by available geophysical data and extensive fieldwork, and suggest four structural trends in the Chambal Valley as striking NE-SW, E-W, NW-SE and N–S, of which the NW-SE trend is the oldest, followed by the NE-SW, E-W and N–S trends. A GIS-based analysis of the E-W trends indicates that these are probably manifestations of the westward continuity of E-W-trending shear zones developed in Bundelkhand massif within the basement of the Chambal Valley. Further remote sensing studies of the shear zone network exposed in the Bundelkhand massif indicate the pattern to be replicated on a larger scale in the morphostructural fault layout in the adjacent Ganga Basin, implying these shear zones may be continuing in the basement of Ganga Basin and controlling structures developed later, and consequently the different petroleum system elements in the basin. A dip data based structural analysis of the Chambal Valley describes a regional synform attributed with an almost vertical NW-SE-trending axial trace. This axial trace may be in continuity with the NE-SW-trending synformal axis of the Son Valley with its trajectory sympathetic to the southern boundary of the Bundelkhand massif and the Central Indian Tectonic Zone (CITZ) – Aravalli Delhi Fold Belt (ADFB) geometry. Finally, based on the integration of the results of spectral and morphostructural analysis, a number of potential areas of hydrocarbon exploration are demarcated in the study area.

Groundwater quality assessment using water quality index (WQI) under GIS framework

Groundwater is an important source for drinking water supply in hard rock terrain of Bundelkhand massif particularly in District Mahoba, Uttar Pradesh, India. An attempt has been made in this work to understand the suitability of groundwater for human consumption. The parameters like pH, electrical conductivity, total dissolved solids, alkalinity, total hardness, calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, fluoride, nitrate, copper, manganese, silver, zinc, iron and nickel were analysed to estimate the groundwater quality. The water quality index (WQI) has been applied to categorize the water quality viz: excellent, good, poor, etc. which is quite useful to infer the quality of water to the people and policy makers in the concerned area. The WQI in the study area ranges from 4.75 to 115.93. The overall WQI in the study area indicates that the groundwater is safe and potable except few localized pockets in Charkhari and Jaitpur Blocks. The Hill-Piper Trilinear diagram reveals that the groundwater of the study area falls under Na+-Cl−, mixed Ca2+-Mg2+-Cl− and Ca2+-{text{HCO}}_{3}^{ - } types. The granite-gneiss contains orthoclase feldspar and biotite minerals which after weathering yields bicarbonate and chloride rich groundwater. The correlation matrix has been created and analysed to observe their significant impetus on the assessment of groundwater quality. The current study suggests that the groundwater of the area under deteriorated water quality needs treatment before consumption and also to be protected from the perils of geogenic/anthropogenic contamination.

Crustal structure of the Indian plate underneath the alluvial plains of the central Ganga basin by broadband magnetotellurics

The Ganga basin consists of thick Cenozoic sediments that mask the tectonic fabric of the underlying Indian plate. Despite the availability of fairly good information on the intra-basin structure, especially at the foothills of the Himalaya due to hydrocarbon exploration activities, very little is known about the basement and the crustal/lithospheric structure although composition and thermomechanical properties of the underthrusting Indian plate are expected to have a significant role in controlling the deformation and seismicity along the Himalayan arc. Here, we present a two-dimensional geoelectric model of the crustal structure along a 285-km-long profile across the central Ganga basin. The results were obtained by inversion of decomposed TE- and TM-mode impedance tensors derived from broadband magnetotelluric data recorded at 45 sites along the profile. The results reveal three main features: (i) the highly resistive Bundelkhand massif (BKC) extends northward beneath the sediments up to Lucknow; (ii) the massif is constituted of two blocks separated by a 10-km-wide moderately resistive nearly vertical zone; and (iii) a super-deep sedimentary basin constituted of highly conductive sedimentary succession exists in the northern segment of the profile. We report the presence of an electrical Moho at 40–45 km depth for the buried BKC and a crustal shear zone within the craton masked by the alluvial sediments. These results shall facilitate in furthering our understanding of the evolution of the BKC as well as the lithospheric/flexural deformation of the Indian continental lithosphere.

Hydrochemical characteristics and spatial analysis of groundwater quality in parts of Bundelkhand Massif, India

The tribulations of water quality have become more serious than the quantity, as the environmental evils are getting more severe day by day in different parts of the world. Large number of components like soil, geology, sewage disposal, effluents and other environmental conditions in which the water tends to reside or move and interact with ground and biological characteristics, greatly persuade the groundwater quality. Therefore, hydrochemical study has been carried out graphically and spatially in GIS environment in part of Bundelkhand Massif. The hydrochemical study exposes the water quality by measuring the concentration of parameters and comparing them with the drinking water and irrigation standards. Groundwater samples have been collected and analysed for physiochemical characteristics in order to understand the hydrochemistry of the water. The results revealed that ground water is alkaline in nature and total hardness observed in all samples falls under moderately hard to very hard category. At some places higher concentration of Cl− could be dangerous from health point of view. Major hydrochemical facies were identified using Piper Trilinear diagram and Durov diagrams, etc. Analysis of different determinations such as sodium adsorption ratio, residual sodium carbonate and per cent sodium revealed that most of the samples are unsuitable for irrigation. It was also observed that the quality of groundwater was not suitable for drinking purpose in industrial and irrigation area. In the area, few sampling sites showed unsuitability because of influences of urban and industrial waste discharge, aquifer material mineralogy, other anthropogenic activities and increased human interventions.

A Petrological and Geochemical Account of Subsurface Noritic Intrusion in the Western Part of Bundelkhand Massif, Shivpuri District, M.P.

ABSTRACT Subsurface exploration for uranium in the northwestern part of Bundelkhand massif, near Khor area, Shivpuri dist., M.P., resulted in intercepting a substantial thickness of mafic rock within Bundelkhand granitoid. Intercepts of this mafic rock at various levels in the boreholes, indicate that the rock mainly occurs as dyke-like intrusion and fracture-fills within Bundelkhand granite. It is essentially composed of hypersthene and plagioclase, with or without olivine, leading to the characterisation as hypersthene microdolerite, noritic dolerite and norite (±olivine), depending on the grainsize and variation from intergranular to ophitic texture. Chemically, the rock is characterised by av. 49.09% SiO2, 2.46%TiO2, 2.33 Fe2O3, 9.45% FeO, 5.75% MgO, 8.37% CaO and 0.96% K2O. The normative composition ranges from 3.53% quartz, 46.86% plagioclase, 12.58% diopside, 19.24% hypersthene. The olivine normative samples show av. 5.65% olivine. Geochemical plots indicate an intra-plate affinity along with oceanic signature, while presence of mineralogical and normative olivine, together with the REE pattern, point towards a lower crustal or mantle source. The mineralogical and normative presence of either quartz or olivine in these mafic rocks implies that it has an intermediate character between the tholeiitic dolerite dykes and the komatiite-type ultramafics reported from Bundelkhand craton. The complex geochemical signature of the rocks also reveals that both intra-continental as well as a mixture of oceanic-to upper mantle signatures are evident. The present study is a first time report of the occurrence of this hitherto unknown noritic body at depth within the Bundelkhand granite, which has no visible surface expression. The findings may strengthen the existing concept of a continuum between Rajasthan craton in the west and Bundelkhand craton in the east, as a single proto-continent.

N–S crustal shear system in the Bundelkhand massif: A unique crustal evolution signature in the northern Indian peninsula

The Bundelkhand massif, located in the northern part of the Indian shield, is a poly-deformed and poly-metamorphic terrain. This paper reports a new shear system developed throughout the massif in the form of N–S trending quartz veins that are sometimes quartzo-feldspathic and rarely granitic in composition. The veins are vertical and commonly occur in conjugate sets. This tectono-magmatic event appears to represent the youngest shear system of the massif as it cross-cuts all the earlier shear systems (E–W, NE–SE and NW–SE). Emplacement of this N–S vein system may have taken place due to extensional processes that developed some cracks along which siliceous magma was vertically emplaced. The complete absence of signature of the N–S event from the surrounding sedimentary cover of Vindhyan Supergroup, Bijawar and Gwalior Groups suggests that this shear system is pre-tectonic to the nearly E–W trending passive basins developed at the margins of the Bundelkhand craton. Further, several workers have considered the Bundelkhand massif as a part of the Aravalli craton. However, due to the absence of N–S, as well as the other (i.e., E–W, NW–SE and NW–SE), tectonic fabrics of the Bundelkhand massif in other cratons of the Peninsular India, and vice versa, makes the Bundelkhand block a separate and unique craton of its own and is not part of the Aravalli craton.

Thick sedimentary sequence around Bahraich in the northern part of the central Ganga foreland basin

We present the results of a magnetotelluric study along a 285km long profile between Hamirpur and Rupadia (Nepal border) across the central Ganga basin. The electrical resistivity image obtained by combining 1-D Occam inversion models for 39 sites reveals a significant contrast in the subsurface structure from south to north along the profile. At the southern end, the Bundelkhand massif is delineated as a high resistivity block buried beneath 250–300m thick sediments. The thickness of sediments gradually increases to about 500–600m at Kanpur, and to about 1.2km at Lucknow. Here, the basement depth increases to more than 2.5km within a profile distance of 20km, which could be attributed to the Lucknow fault. The underlying rocks also have moderate resistivity and possibly represent the Vindhyans. The sedimentary sequence at the northern end of the profile around Bahraich is more than 9km thick. Integrating the resistivity image with a published seismic velocity structure from the region and the lithology from the 3927m deep Matera-I well reveals that the top 4km succession is constituted of highly conductive Oligocene and younger rocks of the Matera Formation and the Siwaliks, and recent sediments whereas the underlying >5km section is composed of sedimentary rocks of the Bahraich Group overlying the Archean basement. The high conductivity of sediments in conjunction with the low seismic velocity and large Vp/Vs obtained by receiver function analysis implies poor consolidation of sediments and thus high seismic hazard potential. The present results have implications for hydrocarbon exploration, hazard potential scenario of the central Ganga basin, and flexural strength of the Indian Plate.

High grade metamorphism in the Bundelkhand massif and its implications on Mesoarchean crustal evolution in central India

The Bundelkhand Gneissic Complex (BnGC) in the central part of the Bundelkhand massif preserves a supracrustal unit which includes pelitic (garnet–cordierite–sillimanite gneiss, garnet–sillimanite gneiss, biotite gneiss and garnet–biotite gneiss) and mafic (hornblende–biotite gneiss and garnetiferous amphibolite) rocks. Granulite facies metamorphism of the complex initiated with breaking down of biotite to produce garnet and cordierite in the pelitic gneisses. Geothermobarometric calculations indicate metamorphic conditions of 720°C/6.2 kbar, followed by a retrograde (687°C/4.9 kbar) to very late retrograde stages of metamorphism (579°C/4.4 kbar) which is supported by the formation of late cordierite around garnet. The P–T conditions and textural relations of the garnet–cordierite-bearing gneiss suggest a retrograde cooling path of metamorphism.

Diverse mode of phosphatic rocks in the environs of Proterozoic Bijawar and Sonrai basins and its relevance to uranium mineralisation — A case study from central India

The Bundelkhand massif comprising a variety of Archean-Paleoproterozoic granitoids along with low grade and high-grade metamorphites and located in the centre of the Indian Plate, underwent extension during Paleoproterozoic period, resulting in the formation of homotaxial intracratonic Bijawar and Sonrai basins in the south and Gwalior basin in the northern margin. The Bijawar and Sonrai basins are typified by their characteristic sediments and basic volcanic rocks. A feature common to both the basins, is the overwhelming occurrence of phosphatic rocks across stratigraphy and lithotype in the Bijawar basin and its confinement to the basal part of the sedimentary column in Sonrai basin. Most of these rocks are primarily of marine origin, and later subjected to periods of repeated phosphatic redistribution. Multiple episodes of such phosphatisation culminates in the proliferation and enrichment of phosphate in the upper Bijawar rocks of Bijawar basin (phosphatic breccia of Hirapur-Mardeora) and lower Bijawar rocks of Sonrai basin (phosphatic breccia of Lalitpur). Apart from these established phosphatic rocks in both the basins, quartz reefs occurring in the basement as well as the lower Bijawar Malhera Chert Breccia Formation in Bijawar basin at places are endowed with anomalously high phosphate content. The phosphatic component in all the lithotypes is in the form of apatite varying in form from microcrystalline to well formed coarser crystal aggregate comprising cement, veins and botroidal encrustations. Irrespective of its spatial, temporal and paragenetic position, it invariably registers weak to moderate radioactivity, due to the presence of uranium within it, as is evident from microprobe data. Although intra-grain and inter-grain distribution of uranium is found to be random and erratic, in general, it is observed that uranium tends to be enriched in the later generation phosphates, due to secondary process of dissolution and reprecipitation. The present paper, with fresh inputs from petrological, geochemical, minerochemical and isotope data pertaining to apatite from all these diverse units, not only explores the already established association of uranium and phosphate in these basins but also provides new insight to the phosphatic quartz reef within the basement and the phosphatised arenaceous sediments of the lower Bijawar Formation.

Proterozoic crustal scale shearing in the Bundelkhand massif with special reference to quartz reefs

Quartz reefs that constitute the most spectacular feature of the Bundelkhand massif of Central India are as yet riddled with controversy. We consider the NE-SW trending quartz reefs and the NE-SW trending brittle-ductile shear system developed outside the quartz reefs to constitute a single tectonic fabric of the massif. The study of structural features of the quartz reefs and the associated NE-SW trending shear zones coupled with the mineralogical and structural features suggests that the quartz reefs represent strike-slip dominated vertical to subvertical shear zones with dominantly sinistral sense of shear. The loci of the quartz reefs constitute the large scale fractures developed as a consequence of extensional processes in this part of Central India. These fractures were the sites for later emplacement of siliceous magmatic melt, commonly known as quartz reef that was also responsible for the mineralization of some base metals as well as pyrophyllite and diaspore. The lithotectonic system of these quartz reefs thus developed was later on subjected to strike-slip movements. A generalized model of large scale extensional tectonics during the Proterozoic times has thus been suggested for the development of the NE-SW tectonic fabric of the Bundelkhand massif.

Platinum group elements geochemistry of ultramafic and associated rocks from Pindar in Madawara Igneous Complex, Bundelkhand massif, central India

Ultramafic rocks comprising dunite, harzburgite, lherzolite, olivine webserite and websterite occur as intrusives in the form of small hillocks at Pindar into the granite–gneisses of Bundelkhand Gneissic Complex (BnGC). The peridotites are dominated by olivine cumulates where chromite and precious metal-bearing sulphides crystallized along with pyroxenes, subsequent to crystallization of olivine into the interstitial spaces of cumulates during cooling. Ultramafic rocks of Pindar are characterized by high MgO (up to 46.0 wt%) and FeO (up to 5.8 wt%); low SiO2 (40.8 to 48.0 wt%), TiO2 (0.2 to 0.5 wt%), Al2O3 (~3.2 wt% av.), CaO (~2.7 wt% av.) and Cu (11 to 73 μg/g). Cr and Ni values range from 2297 to 3150 μg/g and 2434 to 2767 μg/g, respectively. Distribution of Ir (up to 20 ng/g), Ru (27 to 90 ng/g), Rh (3 to 14 ng/g), Pt (18 to 72 ng/g), Pd (10 to 27 ng/g) and Au (22 to 57 ng/g) indicate platinum group element (PGE) and associated gold mineralization in these ultramafic rocks. A mineral phase representing sperrylite (PtAs2) was also identified within the sulphides in scanning electron microscopy with energy dispersive spectrometer (SEM–EDS) studies. The primitive mantle-normalized siderophile elements pattern shows platinum group element PGE (PPGE) enrichment (Rh, Pt, Pd). Discrimination diagrams of Pd/Ir vs. Ni/Cu, Pd/Pt vs. Ni/Cu, Cu/Pd vs. Pd, and Cu vs. Pd for the peridotites of Pindar attribute to affinity towards komatiite magma, derived from high degree of partial melting of prolonged depleted mantle, and the sulphur saturation condition incurred during the crystallization of chromite which was favourable for PGE mineralization.

Interpretation of aero-magnetic data and satellite imagery to delineate structure — a case study for uranium exploration from Gwalior Basin, India

The Paleo-Meso Proterozoic Gwalior basin (E - W), lying to NW fringe of Bundelkhand massif is represented by litho-package of lower arenaceous Par Formation and upper chemogenic Morar Formation. It is bounded by Indo-Gangetic alluvium in north and east, Kaimur sediments in west and Bundelkhand granitoids in south. Gwalior Basin has been the exploration target for uranium mineralization right from early 60’s. Surface radioactivity anomalies due to uranium has been reported in both Par and Morar Formations of Gwalior Group and Vindhyan sediments. Besides presence of syngenetic uranium in the system, presence of post-depositional faults and fractures are the favorable factors. Aeromagnetic survey was carried out by AMD in 2002 with N-S lines of 500 m interval covering 9406 line km. The data with sampling interval of 0.1 sec was corrected for spikes, diurnal variation, IGRF, heading and lag. Final processed images are prepared after suitable leveling and gridding. First vertical derivative of TMI-RTP and tilt-angle derivative images are used to map the litho-contacts, lineaments and structural features. Numerous NE-SW trending low amplitude and NW-SE trending high amplitude magnetic linears corroborate with quartz reefs and basic dykes respectively. Besides, E-W to WNW-ESE and ENE-WSW trending fractures are also evident from the processed image maps. Further, the Euler’s depth solution of gridded aeromagnetic data calculated for structural indices of 0 and 1 are very consistent in locating the position of the causative sources. Based on the amplitude and textural character of processed aeromagnetic data, alteration zone is delineated well within the Morar Formation. Enhanced Thematic Mapper (ETM+) image with 30m resolution was merged with IRS PAN 1D (5.8 m resolution) for better spatial/radiometric resolution to extract litho-contacts and lineament patterns. Merged PAN band-4 after linear contrast and edge enhancement techniques deciphered detailed lineament pattern, which corroborate the magnetic data. Merged ETM+ (RGB 751) and PC (PC1-PC2-PC5) images depict litho-logical contrast. Integration of aeromagnetic and satellite imagery data helped in understanding the structural fabric of the Gwalior Basin and to identify favorable loci of uranium mineralization.

Geothermal Investigations in the Upper Vindhyan Sedimentary Rocks of Shivpuri Area, Central India

Abstract Heat flow has been determined by combining temperature measurements in 7 boreholes with thermal conductivity measurements in the Upper Vindhyan sedimentary rocks of Shivpuri area, central India. The boreholes are distributed at 5 sites within an area of 15 × 10 km2; their depths range from 174 to 268 m. Geothermal gradients estimated from borehole temperature profiles vary from 8.0-12.7 mK m−1 in the sandstone-rich formations to 25.5-27.5 mK m-1 in the shale-rich formations, consistent with the contrast in thermal conductivities of the two rock types. Heat flow in the area ranges between 45 and 61 mW m−2, with a mean of 52±6 mW m−2. The heat flow values are similar to the >50 mW m−2 heat flow observed in other parts of the northern Indian shield. The heat flow determinations represent the steady-state heat flow because, the thermal transients associated with the initial rifting, convergence and sedimentation in the basin as well as the more recent Deccan volcanism that affected the region to the south of the basin would have decayed, and therefore, the heat flow is in equilibrium with the radiogenic heat production of the crust and the heat flow from the mantle. The present study reports the heat flow measurements from the western part of the Vindhyan basin and provides heat flow information for the Bundhelkhand craton for the first time. Radioelement (Th, U and K) abundances have been measured both in the sedimentary rocks exposed in the area as well as in the underlying basement granite-gneiss of Bundelkhand massif exposed in the adjacent area. Radioactive heat production, estimated from those abundances, indicate mean values of 0.3 µW m−3 for sandstone with inter-bands of shale and siltstone, 0.25 µW m−3 for sandstone with inter-bands siltstone, 0.6 µW m−3 for quartzose sandstone, and 2.7 µW m−3 for the basement granitoids. With a total sedimentary thickness not exceeding a few hundred metres in the area, the heat production of the sedimentary cover would be insignificant. The radioactive heat contribution from the basement granitoids in the upper crust is expected to be large, and together with the heat flow component from the mantle, would control the crustal thermal structure in the region.

Peperite Occurrence and its Implications on Origin and Temporal Development of the Proterozoic Dhala Basin, Mohar Area, Shivpuri District, Madhya Pradesh

Abstract In the western part of Bundelkhand massif, a caldera with intra-caldera sediments, known as Dhala Formation, occurs as an outlier in and around Mohar village of Shivpuri district, Madhya Pradesh. For the first time, occurrence of peperite is being reported from the basal part of the Dhala sediment. Two types of peperites have been recognized: blocky and fluidal or globular with variable morphology. In peperitic zones, features like soft sediment deformations, presence of sediment into the rhyolite along cracks, vesiculation of the sediments and other evidences suggestive of sediment fluidization are some definite characteristics of interaction of hot magma with wet sediments forming peperite. The occurrence of peperites reflects the contemporaniety of deposition of the Dhala sediments and volcanism, which is well in accordance to the volcanic origin of Dhala structure. Further, the nature of unconformity between the Dhala and overlying Kaimur which is characterized by merely a few centimeter thick pebbly/conglomeratic bed does not appear to represent a large hiatus as expected between the Semri and Kaimur of Vindhyan Supergroup. So, the contemporaniety of the Dhala Formation (at least the lower part) as reflected by occurrence of peperites, coupled with the available age of the rhyolite and the nature of the unconformity between the Dhala and overlying Kaimur provide convincing evidence to correlate the Dhala Formation with the Lower part of the Kaimur and unlikely with the Semri Group or Bijawar as proposed earlier.

Conjugate faulting along pre-existing fractures in Bundelkhand granite, Hirapur, central India

Re-examination of the outcrop of conjugate of strike-slip faults mapped by Roday et al. (1989) near forest rest house at Hirapur reveals that the main dextral strike-slip fault that strikes N35°E and is a manifestation of the earliest NE-SW trending subhorizontal σ1 that produced extensional reef system in the Bundelkhand massif. Although the change in the stress system though 90° rotation of the principal compressive stress σ1 and σ3 (with σ2 maintaining near vertically) is correct, another point of interest is that the σ1 for the system of faults bisects the obtuse angle between the two sets and not an acute one as required by the brittle failure criterion. The sinistral strike-slip faults were probably formed by rejuvenation of the initial dextral strike-slip faults that were generated when the maximum principal compressive stress was oriented NS. The reversal of fault displacement is seen on all scales in the Bundelkhand massif. The dextral strike-slip fault related to the late stress system was preferentially produced along pre-existing tensile fractures that were generated under NE-SW directed subhorizontal σ1. Some of these fractures were converted into sinistral strike-slip faults under NS directed maximum principal compression acting subhorizontally.

Magnetic susceptibility mapping of felsic magmatic lithounits in the central part of Bundelkhand Massif, central India

Late Archaean to Palaeoproterozoic felsic magmatic lithounits exposed in the central part of the Bundelkhand massif have been mapped and their redox series (magnetite vs ilmenite series) evaluated based on magnetic susceptibility (MS) data. The central part of Bundelkhand massif comprises of multiple felsic magmatic pulses (∼2600–2200 Ma), commonly represented by coarse grained granite (CGG-grey granite, CPG-pink granite), medium grained pink granite (MPG), fine grained pink granite (FPG), grey and pink rhyolites and granite porphyry (GP). However, the pink colour of these felsic rocks is the result of hydrothermal fluid-flushing leading to potassic alteration of grey granites. MS values of CGG vary from 0.058 to 14.75×10−3 SI with an average of 6.35×10−3 SI, which mostly represent oxidized type, magnetite series (73%) granites involving infracrustal (igneous) source materials. CPG (av. MS=3.95×10−3 SI) is indeed a pink variety of CGG, the original oxidizing nature of which must have been similar to the bulk of CGG, but has been moderately to strongly reduced because of distinctly more porphyritic nature together with partial assimilation of metapelitic (supracrustal) materials, surmicaceous enclaves, carbonaceous material included in the source materials, and to some extent, induced by hydrothermal and later deformational processes. MPG (av. MS= 1.15×10−3 SI) as lensoidal stock-like bodies intrudes the CPG and represent both magnetite series (18%) and ilmenite series (82%) granites, which are probably formed by heterogeneous (mixed) source rocks. GP (av. MS=6.26×10−3 SI) occur as dykes (mostly trending NE-SW) intrudes the MPG, CPG and migmatites and bears the nature similar to oxidized type, magnetite series granite. FPG (av. MS= 0.666×10−3 SI) trending NE-SW occur as lensoid bodies including a large outcrop, is intrusive into both CPG and MPG, and is moderately to very strongly reduced type, ilmenite series granites, which may be derived by the melting of metapelitic crustal sources. FPG hosting microgranular (mafic magmatic) enclaves commonly exhibit high MS values (7.31–10.22×10−3 SI), which appear induced by the mixing and mingling of interacting felsic and mafic magmas prevailed in an open system. Grey (av. MS=10.30×10−3 SI) and pink (av. MS=6.72×10−3 SI) rhyolites represent oxidized type, magnetite series granites, which may have been derived from infracrustal (magmatic) protoliths. Granite series evaluation of felsic magmatic rocks of central part of Bundelkhand massif strongly suggests their varied redox conditions (differential oxygen fugacity) mostly intrinsic to magma source regions and partially modified by hydrothermal and tectonic processes acting upon them.

The Wide Spread 2 Ga Dyke Activity in the Indian Shield -Evidences from Bundelkhand Mafic Dyke Swarm, Central India and Their Tectonic Implications

The mafic dyke swarms are important feature of the Proterozoic and in parts of some stabilised cratonic areas. The early Proterozoic Bundelkhand massif of Central India is extensively intruded by suites of NW-SE and NE-SW trending mafic and ultramafic dykes. These dykes are mostly dolerites with subordinate pyroxenite, or lamproites, moreover, geochemical signatures of the two compositional types are different for the NW-SE and NE-SW trending suites. 40Ar/ 39Ar age determinations of the dolerite dykes suggest two phases of dyke activity at c.2150Ma and c.2000 Ma in this region. The dolerites are typically tholeiites and quartz normative types represented by Group I and Group II, whilst the ultramafics are komatiite or basaltic komatiite in composition and show an olivine-normative character. Rare earth element (REE) patterns show some enrichment of LREE and exhibit both positive and negative Eu anomalies. Most of the tholeiites display incompatible elements patterns indicative of an enriched mantle source, whilst those of the ultramafics indicate a depleted source. The 2 Ga event is a global event and well documented in various parts of Singhbhum, Aravalli terrane, Tamilnadu, Andhra Pradesh and Kerala regions of Indian Peninsular Shield and many parts of globe. The genesis of these dyke swarms clearly constitutes a major thermal event affecting the Earth's mantle during that period.

Ion microprobe 207Pb/ 206Pb ages of zircons from the Bundelkhand massif, northern India: implications for crustal evolution of the Bundelkhand–Aravalli protocontinent

207Pb/ 206Pb ages of zircon from gneissic rocks and granitoids representing all major lithological units of the Bundelkhand massif, northern India, have been determined using an ion microprobe. The presence of early Archaean granitic crust in this region is established from these ages. Ages for the Kuraicha and the Mahoba gneisses and associated metabasic enclaves suggest the oldest partial and metamorphic melting event in this region occurred at ≈3.3 Ga. The presence of younger gneisses with ages of ≈2.7 and ≈2.5 Ga suggests the existence of multiple protolith age components within the massif. Three major granitoid suites (hornblende-, biotite-, leuco-granitoids) were emplaced at ≈2.5 Ga within a time span of a few tens of millions of years. Felsic volcanism also occurred around this time. An age of 2.49 Ga for the youngest phase of leucogranitoid marks the time of overall stabilization of this massif. The geochronological history of the Bundelkhand massif appears to be remarkably similar to that of the basement complex of the Aravalli–Delhi orogenic belt, another prominent Archaean crustal block to the west of this massif. Geochemical features of the different rock types in these two regions are also very similar. We propose that these two apparently discrete blocks evolved as a single crustal unit with stabilization taking place at ≈2.5 Ga.

Bundelkhand mafic dykes, Central Indian Shield: Implications for the role of sediment subduction in Proterozoic crustal evolution

Abstract The Archean to Paleo–Proterozoic Bundelkhand massif basement of the central Indian shield has been dissected by numerous mafic dykes of Proterozoic age. These dykes are low‐Ti tholeiites, ranging in composition from subalkaline basalt through basaltic‐andesite to dacite. They are enriched in light rare earth elements (LREE), large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE: Nb, P and Ti). Negative Sr anomaly is conspicuous. Nb/La ratios of the dykes are much lower compared with the primitive mantle, not much different from the average crustal values, but quite similar to those of continental and subduction related basaltic rocks. Bulk contamination of the mantle derived magma by crustal material is inadequate to explain the observed geochemical characteristics; instead contamination of the mantle/lithospheric source(s) via subduction of sediment is a better proposition. Thus, in addition to generating juvenile crust along the former island arcs, subduction processes appear to have influence on the development of enriched mantle/lithospheric source(s). The Bundelkhand massif basement is inferred to represent subduction related juvenile crust, that experienced lithospheric extension and rifting possibly in response to mantle plume activities. The latter probably supplied the required heat, material (fluids) and extensional environment to trigger melting in the refractory lithospheric source(s) and emplacement of the mafic dykes. Proterozoic mafic magmatic rocks from Bundelkhand, Aravalli, Singhbhum and Bastar regions of the Indian shield and those from the Garhwal region of the Lesser Himalaya display remarkably similar enriched incompatible trace elements characteristics, although limited chemical variations are observed in all these rocks. This may indicate the existence of a large magmatic province, different parts of which might have experienced similar petrogenetic processes and were probably derived from mantle/lithospheric source(s) with similar trace element characteristics. The minor, less enriched to depleted components of the Jharol Group of the Aravalli terrane and those from the Singhbhum terrane may represent protracted phases of rifting, that probably caused thinning and mobilization of the lithosphere, facilitating the eruption/emplacement of the asthenospheric melts (with N‐ to T‐types mid‐oceanic ridge basalts signatures) and deposition of deep water facies sediments in the younger developing oceanic basins. In contrast, Bundelkhand region did not experience such protracted rifting, although dyke swarms were emplaced and shallow water Bijawar Group and Vindhyan Supergroup sediments were deposited in continental rift basins. All these discrete Proterozoic terranes appear to have experienced similar petrogenetic processes, tectonomagmatic and possibly temporal evolution involving subduction processes, influencing the lithospheric source characteristics, followed by probably mantle plume induced ensialic rifting through to the development of oceanic basins in the Indian shield regions and their extension in the Lesser Himalaya.