Time and place of progressive regional metamorphism: A discussion

That in areas of regional metamorphism thermal fronts surging from the depths invade the geosynclinal prism in the waning stage of its deformation and bring about re- and neocrystallization oftener than not outdating the penetrative movements, have been demonstrated time and again from metamorphic terrains of all ages and continents. In the present issue of this journal Dr. K.Naha describes a case from a polymetamorphic complex of Eastern Indian Precambrians, where the metamorphism is pictured to have taken place syn- to posttectonically with reference to the folding movements, regional thrusting following subsequently. It is pointed out that compelling evidence, leading to the conclusion that folding and metamorphism on the one hand and thrusting on the other belong to one and the same cycle of orogeny in his area, is yet to be presented. Such a proof, the onus of which lies withNaha, is indispensable before this unique sequence of regional metamorphism prior to diastrophic paroxysm may be claimed to have been established.

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Zusammenfassung Studien in den verschiedensten Gebieten mit Regionalmetamorphose zeigen immer wieder, daß die metamorphe Kristallisation mit den Durchbewegungen beginnt und sie meist überdauert.Naha (in diesem Heft) beschreibt aus einem polymetamorphen Komplex einen Fall von — bezogen auf die Faltung — synbis posttektonischer Metamorphose, welcher eine regionale Überschiebung folgt. Damit ist aber noch nicht bewiesen, daß alle diese Vorgänge ein und demselben orogenen Zyklus angehören. Eine solche Beweisführung ist unerläßlich, um eine Metamorphose, die vor der Hauptphase der Tektonik stattfindet, glaubhaft erscheinen zu lassen.

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Résumé Des recherches dans les schistes lustrés de la région du Simplon indiquent que de grandes déformations ont suivi la formation des nappes penniques. En même temps que ces déformations, apparaït une métamorphose (recristallisation et néocristallisation) qui dure plus longtemps que les processus tectoniques. Ceci confirme les observations faites dans d'autres régions alpines.Remarques critiques sur le travail deNaha (paru dans ce tome).

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Time and place of progressive regional metamorphism: A repley to N. D. Chatterjee

Misreading of some published papers has led Dr. N. D.Chatterjee to erroneous conclusions about the local geology in the Ghatsila area in Singhbhum, eastern India, and his doubts regarding the metamorphic history deduced by the author here are therefore unjustified. The stratigraphy and structural geometry in this terrain are not at all debatable, and the evidences for a progressive metamorphism broadly contemporaneous with folding and preceding thrusting are incontrovertible. To take any particular metamorphic terrain, whether Simplon or Singhbhum, as the prototype of regional metamorphism is as yet premature.     

Precambrian tectonic evolution of eastern India: A model of converging microplates

The Precambrian formations of the Singhbhum and Chotanagpur region of the Indian Peninsular Shield are tectonically classified and their implications in the context of plate tectonics are reviewed on the basis of the stratigraphic, structural, petrologic, geochemical, geophysical and geochronologic data that have accumulated through extensive research in the region in recent years. It is shown that the essential elements in tectonic settings, geological facies and structural and metamorphic characters of the Singhbhum orogenic belt and the reactivated Chotanagpur plateau are elegantly interpretable in terms of interaction of two converging microplates, named here as the Singhbhum and Chotanagpur plates. A detailed correlation of the tectonic evolution with the different stages of a proposed model of plate motions is attempted in the paper.The study reveals three cycles of plate motions with intervening periods of “quiescence”. During the first cycle (2000-1600 Ma), the Singhbhum plate moved northward and collided with the Chotanagpur plate: this led to the tectonic emplacement of the Dalma ophiolite belt and development of the F₁ folds and thrusts and M₁ metamorphism. During the second cycle (1550-1170 Ma), a clockwise rotation of the Singhbhum plate towards the NE generated the F₂ folds and a transcurrent sinistral shear zone. Obduction of the continental lithosphere of this plate occurred during the third cycle (1000-850 Ma) as a result of its continued impingement on the Chotanagpur plate in the NNW direction; this is documented by the evolution of the F₃ folds, M₃ metamorphism and the Singhbhum thrust zone. The “quiescence” periods allowed time for isostatic readjustments, viz., uplifts, intrusions of basic dyke swarms, erosion and paralic sedimentation.     

Evolution of microstructures in Precambrian shear zones: An example from eastern India

Shear zones are areas of intense deformation in localized zones which can be used as natural laboratories for studying deformation characteristics. Metre to-micro scale structures that develop in response to a progressive simple shear in a shear zone are characterized by a protracted history of deformation and are immensely useful in delineating the history of progressive deformation. To decipher these localized zones of deformation and to establish the continuous non-coaxial character of deformation, detail microstructural studies are very useful. Singhbhum shear zone (SSZ), a regional Precambrian tectonic dislocation zone in eastern India, depicting a top-to-south thrust movement of the hanging wall provides a scope for studying microstructural characteristics developed in response to a progressive shear at mid-crustal level. SSZ is characterized by intense stretching lineation, isoclinal folds, shear planes, superposed schistosity and deformed quartz veins. Quasi-plastic (QP) deformation mechanisms were predominantly active in the SSZ. The overprinting relationship between the earlier and later schistosity with a consistent sense of shear indicates that earlier schistosity is transposed to later schistosity through the intermediate stages of crenulation cleavage during a progressive non-coaxial deformation. The recrystallization of quartz in mylonitic quartzite suggests protracted history of deformation. The analysis of the character of quartz grains of both the porphyroclasts and recrystallized grains suggests that strain was partitioned between the most intensely deformed central part of the shear zone and the shear-related deformation zone outside the central part of the shear zone.     

Precambrian mafic magmatism in the Singhbhum craton,eastern India

The Singhbhum craton has a chequred history of mafic magmatism spanning from early Archaean to Proterozoic. However, lack of adequate isotopic age data put constraints on accurately establishing the history of spatial growth of the craton in which mafic magmatism played a very significant role. Mafic magmatism in the craton spreads from ca.3.3 Ga (oldest “enclaves” of orthoamphibolites) to about 0.1 Ga (‘Newer dolerite’ dyke swarms). Nearly contemporaneous amphibolite and intimately associated tonalitic orthogneiss may represent Archaean bimodal magmatism. The metabasic enclaves are appreciably enriched and do not fulfill the geochemical characteristics of worldwide known early Archaean (>3.0 Ga) mafic magmatism. The enclaves reveal compositional spectrum from siliceous high-magnesian basalt (SHMB) to andesite. However, the occurrence of minor depleted boninitic type within the assemblage has so far been overlooked. High magnesian basalt with boninitic character of Mesoarchaean age is also reported in association with supracrustals from southern fringe of the granitoid cratonic nucleus. The subcontinental lithospheric mantle (SCLM) below the craton is conjectured to have initiated during the early Archaean. Significantly, recurrence of depleted magma types in the craton is observed during the whole span of mafic igneous activity which has been vaguely related to “mantle heterogeneity”, although the alternative model of sequential mantle melting is also being explored. The Singhbhum craton includes the Banded Iron Formation (BIF) associated mafic lavas, MORB-like basic and komatiitic ultrabasic bimodal volcanism — documented as Dalma volcanics, Dhanjori lavas, and the Proterozoic Newer dolerite dykes. Three different types of REE fractionation patterns are observed in the BIF-associated mafic lavas. These are the REE unfractionated type is more depleted than N-MORB and some lavas with boninitic type of REE distribution. MORB-like basic and komatiitic ultrabasic (Dalma volcanics) are emplaced within the Proterozoic Singhbhum Basin (PSB). The vista of magmatism in the basin was controlled by a miniature spreading centre represented by the mid-basinal Dalma volcanic ridge. The volcano-sedimentary basinal domain of Dhanjori emerged at the interface of two subprovinces (viz. the mobile volcano-sedimentary belt of PSB and rigid granite platform) under unique stress environment related to extensional tectonic regime. Trace element distribution in Dhanjori lavas is remarkably similar to that in PSB minor intrusions and lavas (except a Ta spike in the latter). The Proterozoic Newer dolerite dykes within Singhbhum nucleus manifest an unusually wide spam of intrusive activity (ca 2100 Ma to 1100 Ma) and unexpectedly uniform mantle melting behaviour.     

Nd isotopic ages of crust formation and metamorphism in the Precambrian of eastern and southern Mexico

Sm-Nd ages for garnets in the three Precambrian exposures of eastern and southern Mexico demonstrate that they belong to the Grenville tectonothermal event. The Sm-Nd garnet ages of 0.95 Ga for the Oaxacan Complex and 0.90 Ga for the Huiznopala Gneiss, Molango and the Novillo Gneiss, Ciudad Victoria, are postdated 75 Ma by Rb-Sr ages on biotites. Both sets of data document a cooling history following Grenville metamorphism at or before 1.0 Ga ago. Our garnet data are consistent with a blocking temperature for Sm-Nd in that mineral around 600° C suggested by Humphries and Cliff (1982).The three Precambrian occurrences have Nd chemical ages of separation from depleted mantle (T_DM) grouped in the range 1.40–1.60 Ga. This may result from derivation of the rocks from actual crustal protoliths which had been separated from the mantle 0.5 Ga before the Grenville Orogeny. It is much more likely, however, that crustal materials of 1.7 Ga or older age were mixed with mantle-derived products during Grenville events to produce intermediate T_DM ages and _Nd values around zero 1.0 Ga ago.     

Nitrogen isotope fractionations during progressive metamorphism: A case study from the Paleozoic Cooma metasedimentary complex, southeastern Australia

The well-studied Paleozoic Cooma metamorphic complex in southeastern Australia is characterized by a uniform siliciclastic protolith, of uniform age, with a continuous range of metamorphic grade from subgreenschist- to upper amphibolite-facies, and migmatite-grade in an annular pattern around the Cooma granodiorite. Those conditions are optimal for investigating variations of N concentrations and δ¹⁵N values during progressive metamorphism. Nitrogen concentrations decrease and δ¹⁵N increases with increasing metamorphic grade (sub-chlorite zone: 120 ppm N, δ¹⁵N = 2.3‰; chlorite zone: 110 ppm N, δ¹⁵N = 3.0‰; biotite and andalusite zone: 85 ppm N, δ¹⁵N = 3.8 ‰; sillimanite and migmatite zones: 40 ppm N, δ¹⁵N = 10.7‰). Covariation of K and N contents is consistent with N substituting for K as NH₄⁺ in micas. Observed trends of increasing δ¹⁵N values with decreasing nitrogen concentrations can be explained by a continuous release of nitrogen depleted in ¹⁵N with progressive metamorphism, which causes an enrichment of ¹⁵N in the residual nitrogen of the rock. Equilibrium models for Rayleigh distillation and batch volatilisation for data of the greenschist and amphibolite facies metasedimentary rocks can be explained by N₂-NH₄⁺ exchange at temperatures of 300-600 °C, whereas observed large fractionations for the upper amphibolite-facies and melt products in the migmatite-grade samples may be interpreted as NH₃-NH₄⁺ exchanges at temperature of 650-730 °C. Lower values in the highest grade zones may also stem in part from input of ¹⁵N-depleted fluids from the granodiorite.The magnitude of isotope fractionation of nitrogen is about 1-2‰ during progressive metamorphism of metasedimentary rocks from sub-chlorite zone to biotite-andalusite zone, which is consistent with previous studies. Consequently, the large spread of δ¹⁵N values in Archean greenschist-facies metasedimentary rocks of −6‰ to 30‰ can be accounted for by variable mixtures of mantle plume-dominated volatiles with a δ¹⁵N of −5‰, and a ¹⁵N-enriched marine sedimentary kerogen component inherited from a CI chondrite veneer having δ¹⁵N of 30‰ to 42‰.     

High-grade regional metamorphism of ultramafic and mafic rocks from the Archaean Sargur terrane,Karnataka, South India

Deformed and metamorphic ultramafic to mafic rocks emplaced into the Archaean Sargur supracrustal series (>3.0 Ga) in Karnataka, southern India, represent layered igneous bodies. The terrane has been affected by several episodes of deformation and metamorphism in the time span from 3.4 to 2.5 Ga ago.During the regional metamorphism about 2.5 Ga ago the igneous bodies re-equilibrated partly or completely at conditions of the upper amphibolite to granulite facies. The development of sagvandites with enstatite + magnesite and anthophyllite + magnesite-bearing assemblages, and of mafic garnet—pyroxene charnockites indicates the presence of CO₂-rich intergranular fluids (X_CO2 ? 0.5) in these rocks during metamorphism.The physical conditions of metamorphism have been estimated by applying methods of geothermobarometry to the recrystallized ultramafic assemblages with olivine, pyroxenes and spinel and to the charnockitic assemblages with garnet, pyroxenes, plagioclase and quartz. A best temperature estimate of 700 ± 30°C was derived with the geothermometers of Evans and Frost (Ol—Spi), Fabriès (Ol—Spi), Wells (Opx—Cpx), Powell (Opx—Cpx), Ellis and Green (Gra—Cpx), Lal and Raith (Gra—Opx), and Danckwerth and Newton (Al₂O₃-content in opx). A mean pressure estimate of 8.6 ± 0.8 kbar has been obtained with the models of Perkins and Newton (Gar—Opx/Cpx—Plag—Qtz). The P—T data indicate a minimum crustal thickness of about 35 km at c. 2.5 Ga in this part of the South Indian Archaean craton.     

The Precambrian of Finnish Lapland: Evolution and regime of metamorphism

The Precambrian crust of northern Finland has been the subject of detailed geochemical and petrological studies. The geochemical data and the present-day geotectonic setting suggest the existence of an old island arc system with the West Inari schist zone (Tana River belt) as an outer tholeiitic arc, the granulite complex as a calcalkaline inner arc, and a stable continental block (granite gneiss complex + Kola nucleus) in the northeast.The entire terrain was affected by intense deformation and polyphase high-grade metamorphism during the Karelian orogeny (2200–1900 m.y.). The regional and temporal development of the mineral assemblages is discussed. The conditions of the postkinematic stage of metamorphism have been evaluated from mineral stability data, with methods of geothermometry and geobarometry, and by the study of fluid inclusions. Metamorphic grade was of amphibolite fades (650–750° C, 5–8 kbars, XH₂O 1) in the Tana River belt and the granite gneiss complex, but of hornblende-granulite facies (700–800° C, 6–8 kbars, XH₂O0.3) in the granulite complex.

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Zusammenfassung Stoffbestand und metamorphe Entwicklung der präkambrischen Kruste Nordfinnlands werden auf der Basis detaillierter geochemischer und petrologischer Untersuchungen diskutiert.Die geochemischen Daten der Gesteine und die geotektonischen Verbandsverhältnisse weisen auf die Existenz eines alten Inselbogensystems hin. Die West Inari Schist Zone (Tana River Belt) kann als äußerer tholeiitischer Bogen, der Granulit-Komplex als innerer kalk-alkalischer Bogen und der Granitgneis-Komplex (mit Kola-Nukleus) im NE als stabiler kontinentaler Block aufgefaßt werden.Der gesamte Bereich wurde während der karelischen Orogenese (2200-1900 Ma) intensiv deformiert und von einer mehrphasigen hochgradigen Metamorphose erfaßt. Die regionale und zeitliche Entwicklung der Mineralparagenesen wird diskutiert. Die Bedingungen des postkinematischen Metamorphosestadiums werden anhand von Mineralstabilitätsdaten, mit geothermometrischen und geobarometrischen Methoden und durch die Untersuchung der Flüssigkeitseinschlüsse abgeleitet. Die Gesteinsserien des Tana River Belt und des Granitgneis-Komplexes liegen in Amphibolitfazies vor (650–750° C, 5–8 kb, XH₂O 1), im Granulit-Komplex dagegen in Hornblende-Granulitafzies (700–800° C, 6–8 kb, XH₂O<0.3).

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Résumé L'origine et l'évolution métamorphique de la croûte précambrienne de Laponie sont discutées sur la base d'études géochimiques et pétrologiques.Les données géochimiques et l'aspect actuel de la disposition géotectonique indiquent l'existence d'un ancien système d'arcs insulaires, ou la «West Inari schist zone» (ceinture du Tana), représenterait un arc volcanique extérieur d'affinité tholéiitique, la ceinture des granulites étant un arc intérieur d'affinité calcoalcaline, et le «granite gneiss complex» (nucléus de l'Inarijärvi et le craton de la Péninsule de Kola), un vieux bloc continental nord-est.Ce terrain a été affecté en entier par une déformation intense et par un métamorphisme polyphase de haut degré pendant l'orogenèse carélienne (2200–1900 Ma). L'évolution régionale et temporelle des paragénèses est discutée. Les conditions de la phase postdéformative du métamorphisme ont été évaluées par application des données sur la stabilité des minéraux, par l'utilisation de géothermomètres et géobaromètres et par l'étude des inclusions fluides. Les séries de la ceinture du Tana et du nucléus de l'Inarijärvi sont caractérisées par le faciès amphibolites (650–750° C, 5–8 kb, ), tandis que la ceinture des granulites s'est développée en faciès granulite à hornblende (700–800° C, 6–8 kb, XCO₂<0.7).

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A record of pre-Variscan Barrovian regional metamorphism in the eastern part of the Slavonian Mountains (NE Croatia)

Summary Petrological investigations and monazite dating are carried out on medium-grade metamorphic rocks (micaschist, gneiss and amphibolite) from the Kutjevačka Rijeka transect in the Slavonian Mts., Tisia Unit (NE Croatia). Field, mesoscopic and microstructural observations, as well as the preserved mineral chemistry, point to a single metamorphic event during peak assemblage growth reaching amphibolite facies conditions of ca. 600–650 °C and 8–11 kbar. Th, U and Pb contents of yttrium-rich accessory monazites indicate a pre-Variscan, i.e. Ordovician-Silurian age (444 ± 19 and 428 ± 25 Ma) for the medium-grade metamorphism of garnet-bearing micaschist.     

Structural pattern in the Precambrian rocks of Sonua-Lotapahar region,North Singhbhum,eastern India

In the western part of the North Singhbhum fold belt near Lotapahar and Sonua the remobilized basement block of Chakradharpur Gneiss is overlain by a metasedimentary assemblage consisting of quartz arenite, conglomerate, slate-phyllite, greywacke with volcanogenic material, volcaniclastic rocks and chert. The rock assemblage suggests an association of volcanism, turbidite deposition and debris flow in the basin. The grade of metamorphism is very low, the common metamorphic minerals being muscovite, chlorite, biotite and stilpnomelane. Three phases of deformation have affected the rocks. The principal D1 structure is a penetrative planar fabric, parallel to or at low angle to bedding. No D1 major fold is observed and the regional importance of this deformation is uncertain. The D2 deformation has given rise to a number of northerly plunging major folds on E-W axial planes. These have nearly reclined geometry and theL ₂lineation is mostly downdip on theS ₂surface, though some variation in pitch is observed. The morphology of D2 planar fabric varies from slaty cleavage/schistosity to crenulation cleavage and solution cleavage. D3 deformation is weak and has given rise to puckers and broad warps on schistosity and bedding. The D2 major folds south of Lotapahar are second order folds in the core of the Ongarbira syncline whose easterly closure is exposed east of the mapped area. Photogeological study suggests that the easterly and westerly closing folds together form a large synclinal sheath fold. There is a continuity of structures from north to south and no mylonite belt is present, though there is attenuation and disruption along the fold limbs. Therefore, the Singhbhum shear zone cannot be extended westwards in the present area. There is no evidence that in this area a discontinuity surface separates two orogenic belts of Archaean and Proterozoic age.     

Loss of metals from pelites during regional metamorphism

In aluminous metapelites the ratio H₂O⁺/K₂O decreases with increasing metamorphic grade and degree of reaction. This ratio is a very practical indicator for the progress of the mineral reconstitution during progressive metamorphism. With decreasing values of the ratio H₂O⁺/ K₂O the Cu concentration and the following element ratios also decrease either continuously or in stepwise fashion: Tl/K₂O, Ba/K₂O, Pb/K₂O, Bi/K₂O, Hg/K₂O, Sr/Na₂O, Zn/(Fe²⁺+Mg), Cd/(Fe²⁺+Mg); Rb/K₂O remains approximately constant. In the aluminous metapelites of the Damara Orogen in Namibia the following losses occur between the biotite isograd and anatexis: 61% Cu, 20% Tl, 34% Ba, 59% Pb, 86% Bi, 46% Hg, 30% Sr, 25% Zn, 31% Cd. Thus the potential of regional metamorphism to form hydrothermal deposits in the low grade environment should not be neglected.     

Juxtaposition of India and Antarctica During the Precambrian: Inferences from Geochemistry of Mafic Dykes

There are several geological, geochemical and geophysical evidences, which corroborate reconstruction of Gondwanaland and juxtaposition of India and Antarctica. Petrology of the Precambrian mafic dykes of East Antarctica and Central-East India also support juxtaposition of India and Antarctica. Mafic dykes of different generations are emplaced in the Archaean granite gneisses of these regions. These dykes appear to be an important tool to support juxtaposition of India and Antarctica. Geological and petrological data of the Central-East India Precambrian mafic dykes suggest four episodes of mafic magmatism in the region - three tholeiitic and one noritic (?). Similarly, East Antarctica also comprises four dyke suites, emplaced during three distinct periods. These suites are 2.4 Ga meta-tholeiites, 2.4 Ga high-Mg tholeiites, 1.8 Ga dolerites and 1.2–1.4 Ga dolerites. Geochemical compositions of these mafic dykes are compared and they show good relationships with each other. Similarities in petrological and geochemical characteristics of Precambrian mafic dykes of East Antarctica and Central-East India strongly support juxtaposition of these two continents.     

Isotopic-geochemical study of zircons from metabasites of the Kontokki dike complex: Age of regional metamorphism in the Kostomuksha structure

Results of local isotopic-geochemical and chemical examination of zircons from metabasites of the Kontokki dike complex in the Kostomuksha structure, western Karelia, Russia, make it possible to interpret the concordant U-Pb zircon age of 2674 ± 13 Ma as the boundary between regional amphibolite-facies metamorphism and accompanying metasomatism. Zircons from the metabasites arte heterogeneous and consist of central parts with relict magmatic cores, metamorphic intermediate zones (which are pale in CL), and younger metasomatically altered zones (dark in BSE images), which development along boundaries between zones and lengthwise arrays of cracks permeable to fluids. The dark altered zones are characterized by high (for zircons) concentrations of LREE, MREE, Th, U, Ca, Sr, Ba, Fe, and Al. The REE distribution in the zircons was proved to be much less susceptible to overprinted metasomatic processes than the U-Pb system of the same zircons. Characteristics of the REE distribution in the zircons makes these zircons comparable with metasomatic zircons. Genetically, the metasomatic processes that affected the geochemistry of the zircons could be related to synmetamorphic granitoid intrusions (Bibikova et al., 2005).     

Trend of regional metamorphism: an example from “Delhi System” of rocks occurring around Bairawas,North-Eastern Rajasthan,India

Metamorphic pelitic assemblages, containing andalusite, garnet and staurolite, occur in the pre-Cambrian Delhi System of rocks in North-Eastern Rajasthan, India. Some of these assemblages have been studied petrographically and chemically in a selected area around Bairawas. It is suggested that metamorphism has been achieved in the area under conditions of relatively low pressure and high temperature. The assemblages do not strictly correspond to any known trend of regional metamorphism and can be best described as a type intermediate between Barrovian- and Abukuma-types. The possible paragenetic history of the mineral assemblages and the trend of metamorphism have been discussed.

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Zusammenfassung Metamorphe pelitische Serien, die Andalusit, Granat und Staurolith enthalten, kommen im präkambrischen Delhi-System im nordöstlichen Rajasthan vor. Einige dieser Serien eines ausgewählten Gebietes bei Bairawas wurden petrographisch und chemisch untersucht. Es ist anzunehmen, daß Metamorphose in dem Gebiet unter relativ niedrigen Druck- und hohen Temperaturbedingungen zustande gekommen ist. Die Kriterien entsprechen keinem eindeutigen Trend der regionalen Metamorphose, sie kann am besten als ein Typ beschrieben werden, der zwischen Barrovian und Abukuma einzuordnen ist. Die mögliche paragenetische Geschichte der Mineralassoziation und der Trend der Metamorphose werden besprochen.

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Résumé Dans le «Delhi System» d'âge pré-Cambrien situé au Nord-Est du Rajasthan (Inde) se trouve un assemblage métamorphique et pelitique contenant de l'andalousite, du grenat et de la staurotide. Une partie de cet assemblage a été étudié au point de vue pétrographique et chimique dans la région de Bairawas. L'auteur est d'avis que le métamorphisme s'est achevé dans la région sous des conditions relatives de haute température et de basse pression. Cet assemblage ne correspond à aucun métamorphisme régional connu, et on peut l'identifier comme intermédiare entre les types «Barrovian» et « Abukuma ». L'auteur i discuté la direction du métamorphisme ainsi que l'histoire paragénétique probable des minéraux de cet assemblage.

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, , , - Rajasthan. Bairawas . , . . Barrovian Abukuma. .

     

Petrography and metamorphism of the lower Gondwana (Permian) coal of Rangit Valley, eastern Himalays, India

A triangular outlier of coal-bearing lower Gondwana rocks comprises a tectonic window within the Precambrian metamorphic terrain of the Rangit Valley, in the eastern Himalays of India. Due to a series of tectonic events which took place during the Mesozoic and Tertiary the succession has become reversed and the coal has undergone severe physical, chemical, microstructural and optical changes. Studies reveal that (1) the rank of the coal prior to tectonism was low, (2) the coal underwent oxidation in the peat forming stage and during subsequent orogenic stages but effect of oxidation on the properties of coal was insignificant, (3) the coal reached its present abnormally high rank mainly due to heating under tectonic pressure and (4) the heat which was generated due to tectonic pressure, affected all the coal seams of this belt.     

Precambrian crustal evolution of Peninsular India: A 3.0 billion year odyssey

The Precambrian geologic history of Peninsular India covers nearly 3.0 billion years of time. India is presently attached to the Eurasian continent although it remains (for now) a separate plate. It comprises several cratonic nuclei namely, Aravalli–Bundelkhand, Eastern Dharwar, Western Dharwar, Bastar and Singhbhum Cratons along with the Southern Granulite Province. Cratonization of India was polyphase, but a stable configuration between the major elements was largely complete by 2.5 Ga. Each of the major cratons was intruded by various age granitoids, mafic dykes and ultramafic bodies throughout the Proterozoic. The Vindhyan, Chhattisgarh, Cuddapah, Pranhita–Godavari, Indravati, Bhima–Kaladgi, Kurnool and Marwar basins are the major Meso to Neoproterozoic sedimentary repositories. In this paper we review the major tectonic and igneous events that led to the formation of Peninsular India and provide an up to date geochronologic summary of the Precambrian. India is thought to have played a role in a number of supercontinental cycles including (from oldest to youngest) Ur, Columbia, Rodinia, Gondwana and Pangea. This paper gives an overview of the deep history of Peninsular India as an introduction to this special TOIS volume.     

Sargipali galenas: Unusual lead-isotope data from eastern India

The Sargipali sulphide deposit, hosted by a sequence of metasediments of the Precambrian Gangpur Series, represents the stratiform and stratabound Pb-Cu mineralization. Lead-isotope analyses obtained from six galena samples of different forms and stratigraphical levels, yielded a model Pb age of 1665 Ma, which predates the episodes of deformation and metamorphism ( 850 Ma) as well as syn-to late-tectonic magmatism. The model Pb age, on the contrary, shows a relationship to the closing phase of the Gangpur sedimentation (1700 Ma) in a shallow-water marine environment. The notable paucity of the interlayered volcanic rock in the deposit and its Pb-rich nature possibly point to its being the sedimentary exhalative (SEDEX) type. Documentary evidence for the Pb component from the geological environment of the upper continental crust is revealed by an exceptionally high (= ²³⁸U/²⁰⁴Pb) and the slightly above average W(= ²³²Th/²³⁸U) values. These indicate that the source rock was initially rich in U-Th-Pb concentration. For the Sargipali ore Pb, the parent is assumed to be late Archean-lower Proterozoic soda granite.     

An integrated hydrogeological study to support sustainable development and management of groundwater resources: a case study from the Precambrian Crystalline Province,India

The rapid expansion of agriculture, industries and urbanization has triggered unplanned groundwater development leading to severe stress on groundwater resources in crystalline rocks of India. With depleting resources from shallow aquifers, end users have developed resources from deeper aquifers, which have proved to be counterproductive economically and ecologically. An integrated hydrogeological study has been undertaken in the semi-arid Madharam watershed (95 km²) in Telangana State, which is underlain by granites. The results reveal two aquifer systems: a weathered zone (maximum 30 m depth) and a fractured zone (30–85 m depth). The weathered zone is unsaturated to its maximum extent, forcing users to tap groundwater from deeper aquifers. Higher orders of transmissivity, specific yield and infiltration rates are observed in the recharge zone, while moderate orders are observed in an intermediate zone, and lower orders in the discharge zone. This is due to the large weathering-zone thickness and a higher sand content in the recharge zone than in the discharge zone, where the weathered residuum contains more clay. The NO₃ ^? concentration is high in shallow irrigation wells, and F^? is high in deeper wells. Positive correlation is observed between F^? and depth in the recharge zone and its proximity. Nearly 50 % of groundwater samples are unfit for human consumption and the majority of irrigation-well samples are classed as medium to high risk for plant growth. Both supply-side and demand-side measures are recommended for sustainable development and management of this groundwater resource. The findings can be up-scaled to other similar environments.