M. E. A. Mondal

Evolution of continental crust of the Aravalli craton, NW India, during the Neoarchaean–Palaeoproterozoic: evidence from geochemistry of granitoids

Neoarchaean–Palaeoproterozoic granitoids of the Aravalli craton, represented by four plutons with different ages, viz. Gingla (2.6–2.4 Ga), Ahar River (2562 Ma), Untala (2505 Ma), and Berach (2440 Ma) granitoids, are classified into three suites: TTG-like, Sanukitoid, and High-K Granitoid suite, all exhibiting negative Nb and Ti anomalies. The TTG-like suite is characterized by high contents of SiO2, Na2O, and LREEs, high (La/Yb)N, low contents of K2O, MgO, Cr, and Ni, and low (Dy/Yb)N, suggesting that this suite formed by partial melting of a subducted basaltic slab without interacting with a mantle wedge. In contrast, the calc-alkaline Sanukitoid suite is marked by a high content of LILEs and mantle-compatible elements, which indicate that this suite formed by partial melting of a slab-fluid metasomatized mantle wedge in a subduction-related arc environment. On the other hand, the High-K Granitoid suite is characterized by high contents of SiO2 and K2O, and low contents of Na2O, MgO, Cr, and Ni with variable Eu anomaly, along with high (La/Sm)N and (La/Yb)N, and low (Dy/Yb)N and Nb/Th. Some high-K granitoids also exhibit A-type characteristics. These features indicate that the High-K Granitoid suite formed by melting of crustal rocks. Early Neoarchaean continental crust formation reflected a slab-melting-dominated magmatic process as evidenced by the TTG-like suite, whereas Palaeoproterozoic petrogenesis was governed by the interaction of slab melt with mantle wedge as demonstrated by the Sanukitoid suite. The High-K Granitoid suite formed during the waning stages of subduction. This study reveals that granitic rocks of the Aravalli craton evolved from slab melting in the Neoarchaean to melting of mantle wedge in the Palaeoproterozoic. Melting of older crust led to the formation of the High-K Granitoid suite.

Petrological and Geochemical Characteristics of Archean Gneisses and Granitoids from Bastar Craton, Central India – Implication for Subduction Related Magmatism

The gneisses and granitoids of Bastar craton (with rock suites up to 3.5 Ga) show calc-alkaline trondhjemitic characteristics. The rocks are enriched in both LILE and HFSE than primordial mantle. They have also relatively higher abundances of LILE and strong depletion at P and Ti in the multielement diagram. The depletion of Ti and P indicates retention of these elements by titanite and/or apatite during partial melting. It is proposed that subduction of an oceanic slab and its consequent melting led to the formation of the protoliths of the gneisses without much interaction with the mantle wedge. The granitoids represent temporally distinct suites formed in response to further melting of slab at greater depth and interaction of magma with the mantle wedge during their transport to the crust.

Geodynamic evolution and crustal growth of the central Indian Shield: Evidence from geochemistry of gneisses and granitoids

The rare earth element patterns of the gneisses of Bastar and Bundelkhand are marked by LREE enrichment and HREE depletion with or without Eu anomaly. The spidergram patterns for the gneisses are characterized by marked enrichment in LILE with negative anomalies for Ba, P and Ti. The geochemical characteristics exhibited by the gneisses are generally interpreted as melts generated by partial melting of a subducting slab. The style of subduction was flat subduction, which was most common in the Archean. The rare earth patterns and the multi-element diagrams with marked enrichment in LILE and negative anomalies for Ba, P and Ti of the granitoids of both the cratons indicate interaction between slab derived melts and the mantle wedge. The subduction angle was high in the Proterozoic. Considering the age of emplacement of the gneisses and granitoids that differs by ∼ 1 Ga, it can be assumed that these are linked to two independent subduction events: one during Archaean (flat subduction) that generated the precursor melts for the gneisses and the other during the Proterozoic (high angle subduction) that produced the melts for the granitoids. The high values of Mg #, Ni, Cr, Sr and low values of SiO2 in the granitoids of Bastar and Bundelkhand cratons compared to the gneisses of both the cratons indicate melt-mantle interaction in the generation of the granitoids. The low values of Mg#, Ni, Cr, Sr and high values of SiO2 in the gneisses in turn overrules such melt-mantle interaction.

Stability assessment of Himalayan road cut slopes along National Highway 58, India

Himalaya is one of the most tectonically and seismically active mountain chains in the world having complex geological and geotechnical conditions. The Himalayan region experiences frequent slope failure posed due to various natural and anthropogenic causes. Slope instability issues have consequent effects on the socio-economic development of the people and the region in a large scale. In the present study, stability analysis of vulnerable road cut slopes along NH-58 from Rishikesh to Devprayag in the Lesser Himalayas has been conducted. Critical slopes were identified by considering the geological and the geotechnical complexities within the region. Rock mass characterisation techniques have been employed for slope stability assessment. Rock mass rating (RMR), slope mass rating (SMR) and continuous slope mass rating (CSMR) methods have been applied to evaluate different stability levels of rock mass along the highway. Spatial variation of stability classes using RMR, SMR and CSMR techniques has been analysed on geographic information system (GIS) tool. Kinematic analysis technique was also employed to identify the different modes of structurally controlled failures in jointed rock mass. Accordingly remedial measures have been suggested to improve slope stability.

SMR geomechanics and kinematic analysis near Rasulpur, Fatehpur Sikri, Uttar Pradesh

Slope stability of mine slopes is often associated with safety and economics during excavation. Sandstone is excavated from Rasulpur area of Fatehpur Sikri in Uttar Pradesh for the purpose of crushed, decorative and dimension stones. In the present paper an attempt has been made to characterize the rock slope faces into different stability classes. Characterization is based on geological and geotechnical parameters recorded on the outcrop during field investigation and supplemented by geomechanical properties by the laboratory test for strength of the rock intact. SMR Geomechanics classification is used to identify the stability class and remedial measures are also suggested to reduce any possible hazard. Kinematic analysis of slope was also investigated to determine the probability of any possible structurally controlled failure. On the basis of SMR Geomechanics calculations slope under investigation lies under good stability class i.e. 2a and 2b. Installation of nets during excavation can be done and for better safety spot and systematic rock bolting can be done. Kinematic study reveals that toppling failures may occur, special care must be given to the joint set which can trigger toppling failure.

Rock mass rating and Kinematic analysis for slope stability investigation of Utari dam, Lalitpur district, Uttar Pradesh

Rock mass characterization of Utari dam in Lalitpur district of Uttar Pradesh was done to identify different stability classes of rock mass. For better stability of Utari dam, foundation conditions were carefully studied by detailed field investigations of the site supplemented by laboratory tests. During feasibility and preliminary stages, rock mass characterization of slopes was conducted to identify the vulnerable zones of failure. Rock mass characterization was done by compilation of information obtained from intact rock as well as from rock mass to determine its grade and long term slope stability of the site. On the basis of Rock Mass Rating (RMR) and Geological Strength Index (GSI) slope stability is identified which lies under good quality rock mass. Kinematic analysis was conducted to find out the probability for different types of structurally controlled slope failure. Microscopic analyses were conducted to identify the degree of chemical alteration of feldspar. Clay formation by sericitization along joint planes is harmful for the stability of dam structure. Remedial measures must be taken to reduce the extent of chemical alteration. Granitoids at dam site forms a compact and stable foundation consisting of four sets of joints in which two sets were prominent which are dipping on the upstream side of the dam which reveals good condition on the dam site as leakage from reservoir will be minimum and least up-thrust on the dam structure.

Petrogenesis of mafic magmatic enclaves of the Bundelkhand granitoids near Orchha, Central Indian shield: evidence for rapid crystallization

Abstract An attempt has been made to understand the origin and emplacement of the widespread mafic magmatic enclaves (MMEs) in the Neoarchean–Palaeoproterozoic Bundelkhand granitoids in the central Indian shield. These MMEs are very fine grained in texture, elliptical and ovoidal in shape and have a very sharp contact with the host granitoids. The MMEs exhibit sub-ophitic texture, acicular apatite and overgrowth of orthopyroxene over olivine crystals pointing towards a rapid crystallization of the MMEs magma in the granitoid magma. The host granitoids are calc-alkaline while the MMEs are tholeiitic, indicating contrasting geochemical composition. Low concentration of Rb, Sr, Ba and K in MMEs points away from the magma mixing with the granitoid magma. Both MMEs and host granitoids are metaluminous and are formed in a subduction zone environment. Although both MMEs and the granitoids were formed at the same time as the mafic magma was injected into the granitoid magma which was still crystallizing (semimolten stage), negligible to no mixing took place between the two contrasting magmas. We propose that the MMEs in the Bundelkhand granitoids are the result of rapid crystallization of the mafic magma in the cooler felsic one.

Geochemical and Nd Isotopic Studies of the Neoarchaean-Palaeoproterozoic Granitoids of the Aravalli Craton, NW India: Evidence for Heterogeneous Crustal Evolution Processes

Whole-rock Sm–Nd isotope and elemental geochemistry of the Neoarchaean to Palaeoproterozoic granitoids (NPG) of the Aravalli Craton, northwestern India have been presented to decipher its source(s) and its implications for crustal evolution processes. Based on petrography and geochemistry, we have classified the NPG into two suites viz.: (i) high-Mg granitoids (HMG); and (ii) K-rich granitoids (HKG). Both the suites show negative Nb and Ti anomalies, and magnesian characteristics indicating their origin in an arc setting. The HMG suite is calc-alkaline in nature exhibiting low and restricted SiO2-content (avg. 66.6 wt%); high contents of MgO (avg. 1.83 wt%), Ni, Cr and large ion lithophile elements (LILE; Sr, Ba, K) along with high (La/Yb)N ratios; low Sr/Y ratios and negative Eu anomaly (avg. 0.8). These features point towards a sanukitoid-type magmatism for the origin of the HMG suite. Further, negative eNd(t) values (−1.6 to −5.5) attest this mechanism involving a LILE-enriched mantle source. On the other hand, the HKG suite exhibits high contents of SiO2 (avg. 75 wt%), K2O (avg. 5 wt%), Th and Pb, high (La/Yb)N ratios and lower contents of MgO, Na2O, Cr, and Ni along with variable eNd(t) values and older Nd model ages (avg. 2.69 Ga). Altogether, these characteristics suggest that the HKG suite probably formed by the re-melting of heterogeneous older crust.

Archean TTG Magmatism in the Aravalli Craton, NW India: Petrogenetic and Geodynamic Constraints

The Archean Banded Gneissic Complex (BGC) of the Aravalli Craton (NW India) is volumetrically most important basement complex scattered along the northwestern margin of the Indian craton. It is mainly comprised of grey gneisses, undeformed granitoids and volcano-sedimentary sequence. Amongst various lithocomponents, the grey gneisses represent the oldest component of the BGC. Rock association comprising tonalite, trondhjemite and granodiorite (TTG) form a major component of the grey gneisses. Geochemically, the TTG are essentially sodic in nature (avg. K2O/Na2O = 0.31) and characterized by high Al2O3 (avg. 15.5 wt%), Na2O (avg. 5.3 wt%) and Sr (avg. 413 ppm), and low Y (avg. 10 ppm) contents. They are characterized by enrichment of large ion lithophile elements (LILE) and light rare earth element (LREE) contents. On chondrite- and primitive mantle normalized trace element diagrams, the TTG exhibit: (i) highly fractionated rare earth element (REE) patterns; and (ii) negative anomalies of Nb and Ti. These geochemical characteristics of the TTG suggest that they were not produced from partial melting of mid-oceanic ridge basalts/normal oceanic crust. Instead we propose that the TTG rocks of the Aravalli Craton were formed by partial melting of an enriched source (oceanic plateau) in a subduction setting; with melting taking place at variable depths.

Geochemical Constraints on the Petrogenesis of the Metasedimentary Rocks Forming the Basement of the Shillong Plateau, Northeast India

Granite gneisses, cordierite bearing granulitic gneisses (pelitic gneisses) and quartz-sillimanite schists (pelitic schists) comprise the Precambrian metamorphic basement complex of the Shillong plateau. Pelitic gneisses and pelitic schists were geochemically studied to determine the paleoweathering conditions and provenance characteristics of their precursor sediments. The chemical indices of alteration and weathering (CIA and CIW) values of the pelitic gneisses vary from 62 to 84 and from 74 to 96 respectively, which indicate a moderate to extremely weathered precursor that was formed probably in a warm and humid climate. The pelitic schists are characterized by CIA and CIW values that range from 99.2 to 99.6 indicating the schists probably had extremely weathered precursor(s).Chondrite normalized rare earth elements (REE) patterns of the rock suites display highly fractionated REE patterns along with prominent negative Eu anomaly [pelitic gneisses: LaN/YbN = 19.7 − 81.7, Eu/Eu* = 0.17 − 0.68; pelitic schist: LaN/YbN = 27.4 − 43.7, Eu/Eu* = 0.15 − 0.32]. Post-Archean Australian Shale (PAAS) normalized multi-element diagram for the pelitic gneisses exhibits enrichment of Nb, Th, Ce, La, Ba, K, Rb with strong depletion of Zr and Sr., suggesting precursor sediments were rich in clay fractions. The pelitic schists however exhibit a highly fractionated trend with enrichment of Zr, Th, Ce and La and depletion of P, Nb, Ti, Y, Sr, Ba, K and Rb relative to PAAS. The geochemical characteristics of both the rock types thus suggest that the precursor sediments were derived from felsic sources; however, those for the schists were derived from a recycled felsic source.