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In this paper, detailed petrology, mineral chemistry and geochemistry of lamprophyres from the Rongjeng–Nongchram area, Meghalaya, northeast India, is being reported. These dykes are intruded into the Proterozoic Assam Meghalaya Gneissic Complex and Neoproterozoic porphyritic granites with limited field exposures in two separate clusters controlled by lineaments. These lamprophyres are alkaline, potassic to ultra-potassic, and metaluminous and have been classified into camptonite and sannite from their petrography and geochemical affinity. Resorption texture of biotite in camptonite and compositional zoning in clinopyroxene of sannite indicate their crystallization under in-equilibrium conditions and subsequent interaction with changing composition of the magma(s). Detail geochemistry indicates fractionation as the imperative mechanism during the evolution of the magma(s) and shows heterogeneity at the mantle source and evidence for minimal crustal contamination. Various tectonic discrimination diagrams indicate the emplacement of the lamprophyres in a post-collisional and within plate setting, signifying a possible plume mechanism involved in their emplacement. The affinity of the lamprophyres towards an OIB setting was recorded from the chondrite-normalized REE and primitive mantle-normalized multielement patterns. The geochemistry of the lamprophyres points towards the presence of possible enriched metasomatic pods in the mantle, which might have melted due to the heat from the mantle plume and triggered the emplacement of the lamprophyres in the Rongjeng–Nongchram area. Research highlights: New lamprophyre bodies from Garo Hills, Meghalaya Plateau, NE India, are being reported with detailed petrology, geochemistry, and mineral chemistry. The lamprophyres are of alkaline affinity. The Kerguelen plume played a vital role in the generation of these lamprophyres. There are geochemical signals of the presence of enriched metasomatic pods in the mantle, which were melted due to plume interaction. The study also indicates a distinctive mantle source heterogeneity across the Meghalaya Plateau. [ABSTRACT FROM AUTHOR]

Along the tropical coast, there is cyclic variation in wind, wave currents, water and sediment influx, and the coasts experience a monsoonal storm surge that reaches nearly 2–3 times higher than the rest of the year, and foreshores undergo a complex evolution during an annual cycle. Hence, knowledge of foreshore evolution is critical in shoreline management. In this paper, study results on meso-scale foreshore evolution in a low-energy headland bay beach, situated in the Central West Coast of India near Manki, are discussed based on their seasonal and decadal changes in sandy shoreline configuration, wave refraction, sediment movement and depositional process. Decadal changes in the shoreline configuration indicated headland-side erosion and bay growth whereas the southern end grown seawards between the period 1979 and 2001–2002, while the beaches adjacent to the creek present in the area showed accretion and farther south of the creek showed minor erosion, and the trend continued through 2015. Wave refraction patterns for the waves approaching from northwest (NW), west and southwest (SW) for the wave periods 6, 8, 10 and 12 s indicated (a) wave convergence at the central part of the Manki beach; for the waves approaching from NW for wave periods 6 s; (b) wave divergence for the waves approaching from west with wave periods 6, 8, 10 and 12 s and (c) wave convergence at the bay side for the SW as well as NW approach of waves for wave period 10 s. Seasonal beach modification, in general, indicated post-monsoonal to pre-monsoonal (September to April) accretion and May to September erosion. Foreshore is gentle and dissipative (<6°). Sediments in all the seasons are bimodal (at 2ϕ and 2.752ϕ) in nature, and show dominantly saltation to suspension mode of transport. A parabolic shoreline prediction model indicated negative sediment budget in the central and northern parts of the beach (near the headland) and positive sediment budget at the southern end of the beach. Integration of the foreshore profile, sediment movement pattern, results of parabolic shoreline prediction model and decadal changes in shoreline configuration indicated that sediment budget is a critical issue in headland-bounded beaches, and utmost care is needed in the management of such shore line. Research highlights: This paper uses seasonal variations in the foreshore morphology, beach volume, sediment characteristics and wave refraction pattern to understand the sediment movement within the beach. Based on Hsu et al.'s (2004) model shoreline position is predicted. For a finite sediment input beach, shoreline management strategy is suggested. [ABSTRACT FROM AUTHOR]

The heavy mineral assemblage (HMA) of sandstones belonging to Mesoproterozoic Kaimur Group in the eastern Son valley, Central India, was investigated. The present communication is aimed at assessing mineralogical maturity, quantifying the contribution of heavy fraction to the bulk rock and its REE budget, sediment recycling and provenance. The transparent heavies record a relative decreasing trend upwards from 63 to 36.4% and on the contrary, opaque heavies show gradual increasing trend upwards from 36.4 to 65.4%. Tourmaline in the transparent heavy fraction reveals an overall decreasing trend of 59–22.8%. Whereas, zircon registers an increasing trend of 3.3–9.5% stratigraphically upwards. The contribution of opaque detrital clasts of ferruginous material composed of hematite and magnetite increases in the Upper Kaimur Sandstone. Tourmaline and zircon grains are characterised by etched, pitted and grooved surfaces. Zircon is also present as tiny inclusion within tourmaline and zircon itself. Heavy fraction of Upper Kaimur sandstone from the lower stratigraphic horizon exhibit lower ∑REE as compared to higher levels. The heavy fraction exhibit LREE/HREE < 1 as compared to bulk rock which has 8–10 times higher ratio. Heavy fraction exhibits enrichment in HREE and [Gd/Yb]N = 0.36–0.43, stratigraphically upwards, which is consistent with increasing Zr values suggesting zircon to be the major contributor to the REE budget of Upper Kaimur Sandstones. The percentage contribution of the zircon to the REE budget of bulk rock could have been ~50–60%. ZTR index (99–100) reveals mineralogically mature nature of Kaimur sandstones. Dominance of zircon amongst the heavies and Th/Sc vs. Zr/Sc plot shows the effect of zircon addition and sediment recycling. The plausible explanation for dominance of opaque minerals in HMA in the Upper Kaimur reflects their contribution from Paleoproterozoic metasedimentary rocks of Mahakoshal Group. HMA of Kaimur sandstones suggests derivation from granitic source. Nb/Yb vs. U/Yb diagram indicates that zircon was derived from magmatic arc source, which is in conformity with inference that Bundelkhand Granite Complex and Chhotanagpur Granite complex dominantly contributed to the Lower and Upper Kaimur Groups, respectively. Research highlights: The paper deals with the study of heavy mineral assemblage of the Kaimur Group sandstones, Vindhyan Supergroup, Son Valley, Central India. Provenance, sorting and recycling of the sediments were inferred. The ferruginous clasts found in the Dhandraul Quartzite/Sandstone also have been studied to understand their mineralogy and texture. Tourmaline and zircon dominate the Lower and Upper Kaimur Sandstones, respectively. The provenance of the Lower Kaimur sandstones is Bundelkhand granite, and Upper Kaimur sandstone is Chhotanagpur Gneissic Complex. The dominance of zircon in HMA and Th/Sc vs. Zr/Sc plot shows the significant effect of zircon addition and sediment recycling. The occurrence of ferruginous detrital clasts in Upper Kaimur sandstones can be attributed to increasing detrital input from adjoining Mahakoshal Group that has the presence of BIFs/BHQ/BHJ in the Agori Formation. The heavy fractions from sandstones of Mangesar Formation and Dhandraul Quartzite from different horizons exhibit an increase in Zr and HREE stratigraphically upwards. This suggests remarkable enrichment of HREE, low LREE/HREE, [La/Yb]N <1 and [Gd/Yb]N <1. HREE enrichment is proportional to the Zr concentration signifying the dominant role of zircon amongst the heavies in managing the ΣREE budget of the bulk rock. The percentage contribution of the zircon is calculated to be ~50–60%. [ABSTRACT FROM AUTHOR]

Evidence of frictional melting in the form of pseudotachylite is innumerably found along the Gangavalli fault zone (GFZ), southern India. Such catastrophic deformation influences the strength of rock masses across the fault zone. In this paper, we have investigated the strength variation in fault zone rocks during post-seismic deformation. The experimental study suggests that the average uniaxial compressive strength (UCS) for pseudotachylite hosting charnockite (i.e., Pt-charnockite) is almost double (45.58 MPa) as compared to the pseudotachylite devoid charnockite (i.e., D-charnockite) of 24 MPa. The melt associate seismic deformation shows high stress drop due to melt lubrication at the fault interface and simultaneously produced numerous fractures in the rock. We have determined the stability and denseness of both rock types using acoustic emission (AE) technique that remarks a distinct difference in AE signatures. The analysis indicates a more stable and dense Mogi type-III signature for post-seismic deformed rocks that are assisted by pseudotachylite melt. However, negligible AE energy is observed in the D-charnockite. We proposed that the melt-assisted seismic deformations are more inhibitable to reactivate the slip surfaces than the conventional earthquake-deformed rock having no melting phenomenon and therefore, pseudotachylites enhance the strength of fault zone rocks during post-seismic deformation. [ABSTRACT FROM AUTHOR]

The paper documents occurrence, origin and significance of glauconite-rich intervals at certain specific stratigraphic levels within Early to Middle Eocene Sylhet Formation of Assam and Assam–Arakan Basin. The microfacies with which glauconite is associated are fossiliferous silty shale, glauconitic sandstone, lime mudstone–wackestone and wackestone–packstone. The glauconites occur in three forms, viz., glauconite pellets, infillings within bioclasts and cements (i.e., non-pellet stringers and blebs), representing a complete spectrum of maturity with presence of four types, nascent, slightly evolved, evolved and highly evolved varieties. The elliptical, immatured pellets occur in glauconitic sandstone and exhibit yellowish-green to grass green colour, while relatively matured pellets are dark brownish green to brown, irregular, broken and mostly associated with shale. The Sylhet glauconites indicate deposition in a tide-affected marginal marine to inner-shelf carbonate ramp with low sedimentation rate, and normal marine salinity and represent an overall fining-up 2nd order TST. High-resolution sequence stratigraphic analysis allows to subdivide the 2nd order TST into 3rd order systems tracts: (i) a basal TST, represented by shale/limestone, (ii) an HST incorporating sandstone, limestone and shale, and (iii) a TST at the top. The intervening 3rd order HST is further subdivided into 4th order parasequence sets separated by marine flooding surfaces. Electrolog correlation reveals that the ubiquitous presence of glauconite-rich horizons corresponds to marine flooding events (MFE) of 3rd and 4th order sea-level changes. The glauconites associated with MFE result in a condensed section with minimum sediment supply and provide a key parameter to build the sequence stratigraphic architecture. The occurrences of green clays may possibly owe their origin due to migration of K+/Fe2+ ions from seawater or surrounding sediments to a given reducing microsystem centered on organic debris. The sources of K and Fe are seawater and detrital minerals, respectively, and are achieved by the diffusion-nucleation-crystal growth process. [ABSTRACT FROM AUTHOR]

The occurrence of workable beach placer deposits of garnet and ilmenite along the coastal region of southern Tamil Nadu is known for long. Though the adjoining rivers like Kallar, Vaippar and Vembar are generally thought to be the major contributors of placer minerals, especially garnet, to their respective beaches, none of the river basins has yet been explored. The present paper discusses the distribution, resource potential and geomorphic controls of garnet placer in the Vaippar basin using ArcGIS. Analysis of sediment samples collected from the Vaippar River and its tributaries shows that the concentration of garnet is up to 20 wt.% and predominantly localized in river alluvium, coastal alluvium and pediment. A distribution map of garnet for the entire basin (5508 km2) portrays three zones of enrichment: zone I: 0.06–1.0 million tonnes (Mt); zone II: 1.0–1.1 Mt; zone III: 1.1–1.21 Mt. It is estimated that in zone III (942 km2) for every 100 km2, about 17 km2 area has an average garnet reserve of 1 Mt and this could be profitably mined. [ABSTRACT FROM AUTHOR]

The polymetallic mineral district in the Singhbhum shear zone (SSZ), eastern India, hosts some of the most important Palaeoproterozoic uranium, copper and apatite–magnetite deposits in India. Although the shear zone had been the locale of profuse hydrothermal alteration and there is general consensus about hydrothermal mineralisation, the alteration types vis-à-vis ore mineralisation are not amply clear. In this paper, we describe ore mineralogy and multiple alteration types vis-à-vis uranium-rare earth element (U-REE) mineralisation in the Narwapahar uranium deposit located in the SSZ. Uranium mineralisation is hosted by intensely deformed and metamorphosed tourmalinite, albite schist and chlorite schist, which are mineralogically heterogeneous showing extreme variations in constituent minerals and their modal abundances in the outcrop, hand specimen and microscopic scales often grading to nearly monomineralic rocks. Thorium-poor uraninite is an ubiquitous ore mineral of uranium. High uranium content is also noted in ilmenorutile and some unidentified U silicates. Allanite is the most abundant REE-bearing mineral whereas monazite and xenotime are subordinate. Low-Ti magnetite is nearly ubiquitous in the U ores. Based on locally preserved cross-cutting hydrothermal veins and frozen-in mineral replacement textures, the sequential metasomatic events deciphered from the Narwapahar deposit are B (boron) → Na–Fe (±Ca)→ K–Fe (±Ca)→ K→ H+. Uranium mineralisation is associated with B, Na–Fe and H+ metasomatism and perhaps with K–Fe metasomatism too whereas REE mineralisation is associated with K–Fe metasomatism and resultant hydrothermally altered rocks. The alteration types, especially the well-known pervasive regional-scale alkali metasomatism, the alteration paragenesis as described here and metal association (Fe–Cu–U–REE–Co–Ni–Mo–Au) are akin to Fe oxide (Cu–U–REE) (IOCG)-type mineralisation. The presence of a substantial amount of uranium in ilmenorutile and very fine-grained inclusion of uraninite in magnetite may potentially affect the recovery of uranium during beneficiation leading to low leachability of some uranium ores. The close association of REE-bearing minerals with the U ores requires focused study to examine the potential of U ores as a source of by-product REEs. [ABSTRACT FROM AUTHOR]

Anthropogenic activities such as gold and coal mining, oil and gas production, filling of artificial water reservoirs, harnessing of geothermal energy, etc., have induced/triggered earthquakes. Koyna dam, located in the Deccan volcanic province of India, was impounded in 1962, and soon after tremors were reported from the vicinity of Koyna dam. The largest triggered earthquake of magnitude (M) 6.3 occurred on 10 December 1967. The triggered earthquakes have continued until now and the region has experienced 22 events of an M~5, over 200 events of an M~4 and several thousand smaller events. The reservoir-triggered seismicity has continued until 2021, which is unique. Some of the early investigations, leading to drilling of a 3 km deep pilot borehole at Koyna for near-field studies of earthquakes, and a few recent results are summarised in this paper. Research highlights: Koyna, located near the west coast of India, in the Deccan volcanic province is one of the most prominent sites of artificial water reservoir-triggered seismicity (RTS). Triggered earthquakes started soon after the impoundment of Shivaji Sagar lake created by Koyna dam in 1962 and have continued until now. Unlike other RTS sites where triggered earthquakes did not continue for long, Koyna is still active, and it is estimated that RTS may continue for another one to two decades. Detailed field work and analyses of earthquake data demonstrated that Koyna is a very suitable site for near-field studies of earthquakes: an observation that is much needed to validate earthquake genesis models. International Continental Drilling Programme sponsored workshops and participation of experts from all over the world helped in deciding to dig a 3 km deep pilot borehole. The borehole was completed in June 2017. Recent research studies further demonstrated that the Koyna region is under critical stress. Studies under progress would help in designing the proposed ~7 km deep borehole laboratory. [ABSTRACT FROM AUTHOR]

In slope stability analysis, identification of the mechanism behind slope failure is the key factor during the study. Rock mass characterization and kinematics analysis, like geotechnical investigation methods, are not infallible all the time for the weathered slopes. The presence of different grades of weathered mass on the slope profile acts independently to influence the overall failure pattern. Numerical analysis by finite element method (FEM) is evolved as a primary slope stability analysis tool by overcoming the limitations of the stratified weathered materials on the slope. The paper focuses on the stability analysis of railway-cut laterite slopes of the Eastern Ghats of India. The medium to high-grade metamorphism, followed by physio-chemical weathering, favours the region for more prone to failure. The Force Equilibrium Model and Moment Equilibrium Model have been considered to determine the possibility of independent failure events for the probability analysis. Monte Carlo's probability simulation method has delivered a better result for the weathered slopes by taking many variables for the studied region. A compression study has been made between the outcomes of slope stability analysis methods and the probability analysis models, which provides a comprehensive list of factors to understand the failure mechanism of weathered slopes. [ABSTRACT FROM AUTHOR]

The author presents a number of comments on the article "Volatile Displacement of Meghalaya Coals: A Pointer to Explore Low Sulphur Coals," which was published in the April 2007 issue. He argues that the paper is full of common mistakes, wrong computations, and long-drawn conclusions. In methodology, he notes that the author of the said paper indicates about drawing of regression curves which is wrong.