Bio: Tapas Bhattacharyya is a academic researcher from University of Calcutta. The author has contributed to research in topic(s): Gneiss & Volcanic rock. The author has an hindex of 5, co-authored 10 publication(s) receiving 147 citation(s).
01 Jan 1994-Precambrian Research
Abstract: This paper reviews the state of structural and geochronological knowledge of Precambrian cratonic rocks in part of central Rajasthan, India, and contributes new observations and isotopic data on the structural features and age of granitic rocks in the area. The granitic rocks comprise plutons of two suites (BGC and Sendra suites), which intrude ortho- and paragneisses the Archean Banded Gneiss Complex (BGC) and its inliers, as well as the Proterozoic Delhi Supergroup. Nearly all the plutons show a foliation and a lineation, which are mostly the result of solid state deformation, although a magmatic component of strain is apparent in some of them. The intensity of these structural fabrics varies from weak to extremely strong. Samples of granitic rocks from the BGC suite yield ages from SmNd isotopic data which scatter around a 2.83 Ga reference isochron [ϵNd(t) of ∼+4.1 ], with depleted mantle model ages ranging from 2.6 to 3.0 Ga. Scatter of the data suggests that the BGC is a composite group of gneisses, and confirms the antiquity of the BGC within this part of the Aravalli-Delhi orogenic belt. RbSr isotopic data of the BGC suite lie on a 1.7 Ga reference isochron, indicating a thermal event at that time. Data from plutons of the Sendra suite, on the other hand, show some scatter but define a model II isochron with slope equivalent to an age of 0.966±250 Ga (initial 87 Sr 86 Sr of 0.70390±0.01889, MSWD-311). The scatter of data could reflect open system behavior during a subsequent thermal event (Malani volcanics? ), although independent evidence for such is lacking in the study area. Ages calculated from our RbSr data are within the error of published results from previous workers. Structural observations on granitic rocks within the BGC inlier at Ajmer suggest that the inlier could represent a remobilized diapiric ridge emplaced into the Delhi Supergroup during the 1.7 Ga thermotectonic event contemporaneously with formation of D1 structures in the wall rock, although the radiometric age of the latter structures is not well-constrained. Microtexture, mineralogy, and field relations of granitic rocks of the Sendra suite point to those plutons being emplaced into the Delhi Supergroup during the 0.966 Ga thermotectonic event late syntectonic relative to D2 indicating that D2 is ∼ 0.96–1.0 Ga. D3 represents retrograde deformation synchronous with cooling of the regional thermal framework and local pluton cooling, probably between ∼ 0.96 and 0.90 Ga. Further geochronological, geochemical and structural work on the plutons and wall rock of both suites is necessary to establish a rigorous thermotectonic chronology by which to accurately model the crustal evolution of this part of central Rajasthan.
01 Jan 2015-Precambrian Research
Abstract: In significant contrast to other cratonic blocks of India, the Singhbhum cratonic successions record continuous depositional record from the Palaeoarchaean to Mesoproterozoic. Although the sedimentary facies characteristics and mode of stratigraphic sequence building of the Dhanjori and Chaibasa Formations are well known, sedimentary geochemistry, provenance and tectonic milieu of deposition of these two formations are hitherto unknown. The current manuscript presents geochemical and Sm–Nd isotopic data from the Dhanjori and Chaibasa Formations for the first time and combine previous sedimentological data with the goal to expand the framework for understanding the depositional and tectonic setting of these two formations. The Sm–Nd isotopic data for the Chaibasa clastics is unambiguous with respect to provenance. Average ɛ Nd ( t = 2.2 Ga) = −0.8 ± 1.0 and average Nd model age (TDM) = 2.51 ± 0.08 Ga with average 147 Sm/ 144 Nd ratios = 0.1114 ± 0.0041 for phyllites and quartzites indicate an extremely homogeneous source signature consistent with a late Archaean “juvenile” crustal provenance, possibly a dominantly upper crustal provenance. The Sm–Nd isotopic data from the older Dhanjori Formation also indicate broadly similar provenance as comparable lithologies in the younger Chaibasa Formation. Our Sm–Nd isotopic data is entirely consistent with the previous sedimentological data and confirms a terrestrial, rift-dominated tectonic setting for the Dhanjori Formation (proximal sources, poorly mixed provenance) and a marginal marine to offshore setting for the more homogeneous Nd isotopic signature of the Chaibasa Formation (distal sources, well mixed provenance).
01 Jan 2012-Journal of Structural Geology
Abstract: The structural geometry in the Barr conglomerate and the neighboring rocks in the western flank of the Meso- to Neoproterozoic South Delhi Fold Belt indicate superposed deformation, with structures developed by horizontal dextral simple shear deformation superimposed on earlier structures formed during approximately ESE-WNW compression and subvertical maximum elongation. The first deformation produced NNE-SSW trending subvertical schistosity, and associated steeply plunging isoclinal folds, mineral lineation and pebble elongation lineation having almost downdip alignment on the schistosity surface. The second deformation produced dextral folds both on bedding and schistosity surfaces and modified the shape of the pebbles already deformed by the first deformation. On vertical sections perpendicular to the schistosity trace the pebbles show sub-ellipsoidal shape with their mean elongation direction parallel to the schistosity trace. On the horizontal section the pebbles often show asymmetrical shape and asymmetrical deflection of the schistosity surface around the pebbles. The mean elongation direction makes a small angle (2°–8°) in the counter-clockwise sense with the schistosity surface. This obliquity is due to modification of post-first-deformation sub-ellipsoidal shape by later horizontal simple shear using the schistosity surface as the movement plane. Analytical expressions are derived for the modification of an original ellipse by simple shear parallel to the long axis of the ellipse. A family of curves has been generated to depict the change in axial ratio and orientation of long axis for different initial axial ratios, and for different values of shear strain (γ). Using these curves it has been possible to factorize the total strain into two components representing the earlier compression and the later simple shear. It is noted that the computed earlier shortening strain ellipsoids are consistently in the flattening field close to the line of pure oblate ellipsoids in the Flinn plot. There is no systematic spatial control on the variation in the values of shear strain (γ), strain ellipsoid shape parameter (k), and intensity of distortion (d) as one proceeds south to north along the strike of the conglomerate.
16 Feb 2010-Journal of The Geological Society of India
Abstract: The structural geometry of the Anasagar gneiss dome in the axial zone of the South Delhi Fold Belt is controlled by polyphase folding. It is classified as a thrust-related gneiss dome and not as a metamorphic core complex. Four phases of deformation have affected both the gneiss and the enveloping supracrustal rocks. D 2 and D 3 deformations probably represent early and late stages of a progressive deformation episode in a simple shear regime combined with compression. The contact between the gneiss and the supracrustal rocks is a dislocation plane (thrust) with top-to-east sense of movement which is consistent with the vergence of the D 2 folds. The thrust had a ramp-and-flat geometry at depth. At the present level of exposure it is a footwall flat (that is, parallel to the gneissosity in the footwall), but it truncates the bedding of the hanging wall at some places and is parallel at others. The thrusting was probably broadly coeval with the D 2 folds and the thrust plane is locally folded by D 2 . D 2 and D 3 folds have similar style and orientation as the first and second phases respectively of major folds in the Delhi Supergroup of the South Delhi Fold Belt and these are mutually correlatable. It is suggested that D 1 may be Pre-Delhi in age. Available geochronological data indicate that the emplacement of the Anasagar gneiss predated the formation of volcanic rocks in the Delhi Supergroup and also predated the main crust forming event in the fold belt. The Anasagar gneiss and its enveloping supracrustal rocks are probably older than the Delhi Supergroup.
30 Jan 2014-Journal of The Geological Society of India
Abstract: The variant rock types of an Alkaline-Carbonatite Complex (ACC) comprising alkali pyroxenite, nepheline syenite, phoscorite, carbonatite, syenitic fenite and glimmerite along with REE and Nb-mineralization are found at different centres along WNW-ESE trending South Purulia Shear Zone (SPSZ) in parts of Singhbhum Crustal Province. The ACC occurs as intrusions within the Mesoproterozoic Singhbhum Group of rocks. Alkali pyroxenite comprises of aegirine augite, magnesiotaramite, magnesiokatophorite as major constituents. Pyrochlore and eucolite are ubiquitous in nepheline syenite. Phoscorite contains fluorapatite, dahllite, collophane, magnetite, hematite, goethite, phlogopite, calcite, sphene, monazite, pyrochlore, chlorite and quartz. Coarse fluorapatite shows overgrowth of secondary apatite (dahllite). Secondary apatite is derived from primary fluorapatite by solution and reprecipitation. The primary fluorapatite released REE to crystallize monazite grains girdling around primary apatite. Carbonatite is composed dominantly of Srcalcite along with dolomite, tetraferriphlogopite, phlogopitic biotite, aegirine augite, richterite, fluorapatite, altered magnetite, sphene and monazite. The minerals comprising of the carbonatite indicate middle stage of carbonatite development. Fenite is mineralogically syenite. Glimmerite contains 50–60% tetraferriphlogopite. An alkali trend in the evolution of amphiboles (magnesiotaramite-magnesiokatophorite-richterite) and chinopyroxenes (aegirine augite, aegirine) during the crystallization of the suite of rocks is noted. Monazite is the source of REE in phoscorite and carbonatite. Fluorapatite has low contents of REE, PbO, ThO2 and UO2. Pyrochlore reflects Nb-mineralization in nepheline syenite and it is enriched in Na2O, CaO, TiO2, PbO and UO2. Pyrochlore containing UO2 (6.605%) and PbO (0.914%) in nepheline syenite has been chemically dated at 948 ± 24 Ma by EPMA.
01 Jan 1996-The Journal of Geology
Abstract: The end-Paleozoic Pangea appears to have contained three continents that had grown in the Precambrian and remained intact until Mesozoic rifting: Ur, formed at ~3 Ga and accreted to most of East Antarctica in the middle Proterozoic to form East Gondwana; Arctica, an approximately 2.5-2 Ga continent that contained Archean terranes of the Canadian and Siberian shields and Greenland; and Atlantica, formed at ~2 Ga of cratons of ~2 Ga age that now occur in West Africa and eastern South America. Arctica grew at ~1.5 Ga by accretion of most of East Antarctica plus Baltica to form the continent of Nena. Collision of Nena, Ur, and Atlantica, plus minor plates, formed the supercontinent of Rodina at ~1 Ga. Rifting of Rodinia between 1 and 0.5 Ga formed three continents: East Gondwana; Atlantica (which became the nucleus for West Gondwana); and Laurasia (which contained North America, Greenland, Baltica, and Siberia). Gondwana formed at ~0.5 Ga by amalgamation of its eastern and western parts. Various plates accret...
01 Jan 2002-Journal of Petrology
Abstract: compositions ( Nd 750 = +5·46 to −0·87; ISr 750 = 0·7021– The Seychelles islands consist of undeformed and unmetamorphosed, 0·7061) that can be modelled as basaltic magmas derived from metaluminous monzogranites and granodiorites of Neoproterozoic depleted mantle, variably contaminated (0–15%) by Archaean age (>750 Ma). Subsolvus, and lesser hypersolvus granitoids are silicic crust. All petrologic, petrographic, geochemical, isotopic and crosscut by coeval dolerite dykes, dominantly of olivine tholeiite chronologic data for Neoproterozoic magmatic rocks of the Seychelles, composition. Field relations suggest that mixing between granitoid coupled with palaeomagnetic data indicating its position at the and doleritic magmas generated a variety of minor intermediate margin of the Rodinia supercontinent at >750 Ma, are at least rocks that occur as irregular masses and enclaves; their compositions consistent with, if not suggestive of, a continental or Andean-type plot as linear arrays between those of dolerites and granitoids. Two arc setting. We argue, therefore, that the conventionally accepted groups of granitoids can be distinguished based on colour, chemistry notion of an extensional (i.e. rift or plume) setting for Seychelles and isotopic signature. Mahe Group granitoids are grey, with magmatism is vulnerable. relatively low incompatible element concentrations and primitive isotopic signatures that cluster at Nd 750 = +2·85 ± 0·17 and ISr 750 = 0·7031 ± 0·0008 (some samples with impossibly low ISr 180 ppm,
29 Jul 1996-Chemical Geology
Abstract: In order to establish the stabilization age of the Aravalli Craton of northwestern India, we have used an ion microprobe to measure 207 Pb 206 Pb ages of zircons from six samples collected from the southern Aravalli Mountains. Our analyses gave precise results for four granitoids whose minimum crystallization ages ranged between 2562 ± 6 and 2440 ± 8 Ma (1 σ). A single sample of gneiss gave an age of 2502 ± 4 Ma, which is unexpectedly younger than an age of 3281 Ma which we previously obtained from another gneissic sample. We interpret the 2502 Ma age as the time of emplacement of the sample's igneous protolith, suggesting that the Mewar Gneiss contains multiple metamorphic components of differing ages. Based on our data, we conclude that the southern segment of the Aravalli Craton had broadly stabilized by ∼ 2.5 Ga. This stabilized landmass formed the basement on which the younger Aravalli Supergroup was unconformably deposited. This study represents the first application of a small geometry ion microprobe for routine geochronology.
01 May 1996-Geological Magazine
Abstract: Single grain 207Pb/206Pb zircon ages were determined for granitoids and gneisses which constitute the Archaean basement rocks of the Aravalli craton of Rajasthan, northwestern Indian Shield. The protolith ages for two gneisses, collected from east of Udaipur, are ˜3230 Ma and 2887 Ma respectively. The granitoids display an intrusive relationship with the gneisses and yielded ages ranging between 2666 Ma and 2620 Ma. These ages provide the basis for a geochronological model of evolution of the oldest basement of the Aravalli craton.
01 Jan 1940-Nature
Abstract: THIS book, which from its first appearance has been the standard text-book of Indian students, has now been brought up to date by the addition of recent advances in Indian geology. The new geological map is an added attraction. Geology of India By D. N. Wadia. Second edition. Pp. xx + 460 + 20 plates. (London: Macmillan and Co., Ltd., 1939.) 24s. net.
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