Showing papers in "Journal of The Geological Society of India in 1982"

Sapphirine, Kornerupine, and Sillimanite + Orthopyroxene in the Charnockitic Region of South India

Abstract: Sapphirine, kornerupine, and the associations sillimanite-orthopyroxene and sillimanite-gedrite are reported from 23 localities (or areas of closely spaced localities) in Fermor's (1936) charnockitic region and in transitional zones to Fermor's non-charnockitic region in south India (Andhra Pradesh, Tamil Nadu, and southern Karnataka). The rocks containing these minerals and mineral associations are believed to have a pelitic sedimentary precursor. They are in general closely associated with khondalite (garnet-sillimanite gneiss), quartzite, and calc-silicate rocks, and less commonly with metabasites of presumed metavolcanic origin. However, the sapphirine occurrences in the Sittampundi anorthosite complex, where sapphirine and sillimanite-gedrite are found in reaction zones between anorthosite and chromite layers, are an exception. Assemblages of sapphirine with garnet and sillimanite are not rare, but sapphirine in direct contact with quartz, which has not previously been reported from south India, was found at Paderu, Vishakhapatnam District, A. P. Kornerupine occurs with sillimanite at three localities but with orthopyroxene only at Ganguvarpatti, a new locality for kornerupine. Temperatures and pressures calculated from pyroxenes, garnets, and feldspars in associated rocks suggest that sapphirine, kornerupine, and sillimanite-orthopyroxene crystallized at 800-850°C and 6-10 kbar in the charnockitic region and sapphirine at 780°C in a transition zone between the granulite and amphibolite-facies near Sargur. Water partial pressure during the metamorphism was significantly less than total pressure. Regional differences in pressure are reflected in the distribution of mineral assemblages: sillimanite-orthopyroxene is found in Madras and Andhra Pradesh, but is conspicuously absent in the Palni Hills, where spinel-cordierite is an important assemblage. Sapphirine and sillimanite-orthopyroxene (and sillimanite-gedrite) are restricted to rocks relatively rich in Al 2 O 3 with high Mg/Fe ratios relative to khondalite. Kornerupine appears to be confined to rocks containing boron. These special rock compositions (except Sittampundi) are believed to reflect unusual compositions of the precursor pelitic sediments rather than changes in chemistry during metamorphism.

Zircon Geochronology of Berach Granite of Chittorgarh, Rajasthan

Abstract: Discordant ages of zircons from Berach granite suggest that this granite crystallized around 2610 m.y. ago and lost lead during an episodic event around 710 m.y. The 710 m.y. date probably reflects a response to a regional thermal event also recorded in the 730 m.y. zircon age for the Mt. Abu 'Erinpura type' granite.

Angular Unconformity Between Sargur and Dharwar Supracrustals in Sigegudda, Karnataka Craton, South India

Abstract: The major angular unconformity between the Peninsular Gneiss and Dharwar Supergroup in Sigegudda belt, Karnataka, is now 'discovered' to separate the Sargur from Dharwar Supracrustals. It is the most critical area known so far in the craton to exemplify the Sargur-Dharwar relationship. In conformity with modern stratigraphic practice, the unconformity marked by supermature conglomerates incontrovertibly separates two major rock sequences in early Precambrian geological record of Karnataka Craton.

Age Data on Pegmatites Of South Kerala and their Tectonic Significance

Abstract: K/Ar mica age of chrysoberyl-bearing pegmatites from four localities of Trivandrum district, Kerala ranges from 445 to 474 m.y., with a mean around 460 m.y. There is a close agreement between the ages of the concordant and cross-cutting pegmatites, between muscovite and biotite ages, as also between various size fractions. Chrysoberyl occurs within feldspars in association with quartz and muscovite. Since the formation of chrysoberyl-quartz association requires a higher pressure or temperature, the reported age would indicate the time of emplacement of the chrysoberyl-bearing pegmatites at the closing phase of a major tectonomagmatic event. It is interesting to note that these pegmatites of south-western India are younger compared to those of other localities viz., Rajasthan, Bihar. Nellore and Mysore.

Palaeogeographic Reconstruction of the Jaffna Peninsula, Sri Lanka

Abstract: The study of the tonal variations of aerial photographs showing the vegetational cover in the Jaffna Peninsula of the north of Sri Lanka clearly points to the progressive formation of spits and lagoons. The contrasting tonal variations of the vegetation found in the emerging lands of different ages can be used to some extent effectively in the delineation of lands in a chronological sequence as exemplified by studies in the Karativu island.

Alteration of Beach Sand Ilmenite from Manavalakurichi, Tamil Nadu, India

Abstract: The study shows that the alteration of ilmenite has taken place according to the two stage model proposed by Grey and Reid (1975). Identification of the intermediate altered compound, pseudorutile, was found difficult in earlier studies by optical and X-ray diffraction methods, because of the occurrence of this phase in altered ilmenite in very fine grain size (30 A), its poor crystallinity and coincidence of many of the diffraction lines with those of other phases. The present study has shown that the Mossbauer technique is more suitable in the identification and estimation of this phase. The relative amount of pseudorutile increases progressively in the magnetic fractions in the following order: 0.30 - 0.35 amps (5%), 0.25 - 0.30 amps (40%), 0-0.25 amps (75%) and 0.35 - 0.50amps (95%), indicating that the magnetic susceptibility of the grains is increasing initially with progressive pseudorutile formation, and decreasing in the later stages of alteration. Reflectivity and hardness are also increasing up to the stage of pseudorutile formation and decreasing in the later stages.

Iron Formations in Archaean Granulite-Gneiss Belts With Special Reference to Southern India

Abstract: In Archaean granulite-gneiss belts there are banded iron formations (BIF) less than 20m thick which are weakly banded magnetite quartzites occurring in a supracrustal sequence of quartzites, mica schists, marbles and metavolcanic amphibolites. This sequence forms stratigraphic markers within voluminous intrusive tonalitic to trondbjemitic gneisses. These rocks are highly folded and metamorphosed to high amphibolite or granulite facies at 7-12 kb pressure in the lower Archaean crust. Such iron formations are well represented in Tamil Nadu, Southern Karnataka, Kerala and Andhra Pradesh in South India; in Hebei Province in China; Kola Peninsula, Stanovoy Range, and Ukraine in USSR; Kambui schists of Sierra Leone; Nimba series of Liberia; Limpopo belt of southern Africa; Imataca complex of Venezuela; Uivak gneisses of Labrador; and Amitsoq gneisses of West Greenland. They are obviously different in type and environment from the Proterozoic Superior type of BIF; it is shown here how they are different from the Archaean Algoma type of BIF. They were deposited in a quartzite-pelite-carbonate association and they lack the silicate-carbopate-sulphide facies of the Algoma-type which, in contrast, was deposited with a greywacke-flyschconglomerate- shale association in subsiding basins (Greenstone belts). Because they are dissimilar to the Algoma-type of BIF in both their sedimentary facies and tectonic environment (the granulite-gneiss belts are not highly metamorphosed greenstone belts), they should be given a separate status and we tentatively suggest they be termed the Tamil Nadu type.

A Note on the Tectonic Framework and Geologic Set-Up of the Pangong-Chushul Sector, Ladakh Himalaya

Abstract: The Pangong Tso-Spanggur Tso and the Shyok-Chushul-Dungti are the two tectonic zones in the area north of the Ladakh Range The former zone has an association of metapelites, metacarbonates and metavolcanics in a graben type basin bounded by granitoids The latter zone comprises Orbilina limestone, argillite, lava flows and grey-purple conglomerates in another graben type basin bounded on the NE by Pangong Granitoid and on the sw by the Ladakh Granitoid belts These, along with the Indus Tectonic zone represent NW-SE trending parallel tectonic zones in the Trans-Himalayan Zone The Pangong Tso-Spanggur Tso appear to be part of the larger Permian basin of Tibet The Orbiloina limestone-volcanics-conglomerates or the Shyok-Chushul Tectonic Zone are comparable with the Sangeluma belt or the Indus Tectonic Zone These are independent zones but comtemporaneous in development

Textural Analysis and Mineralogy of the Black Sand Deposits of Visakhapatnam-Bhimunipatnam Coast, Andhra Pradesh, India

Abstract: Textural analysis reveals that the black sand samples are well sorted. negatively skewed, mostly mesokurtic and unimodal in distribution with the Mean Size of the grains falling mostly within the fine sand. The opaques and non-opaque heavies in a weighted average sample constitute 78.1% and represent the full range of minerals found in the rocks of the Eastern Ghats. The marked preference of a particular heavy mineral to a certain sieve fraction can be related to the grain size of the heavy mineral in the source rocks. The occurrence of thin layers of black sand in the stream channels inland. the non-disappearance of any unstable heavy mineral in the deposits, suggest that the black sands are derived from the highland area.

Mode of Occurrence and Origin of Manganese Ores of Shimoga District, Karnataka

Abstract: Four types of manganese ores are distinguished: 1. Reworked metasedimcntary ore in Precambrian banded manganese formation; 2. Sedimentary oolitic/pisolitic ore associated with claystone; 3. Cavity filling/replacement type ore in weathered Precambrian phyllitic schists and laterites; 4. Bouldery-pebbly float ore in soils and laterite. The reworked metasedimentary ore is low in silica (∼6%), high in Fe content (∼26%) and its Mn/Fe ratio varies between 0.75 and 1.2. The oolitic/pisolitic ore on the other hand is high in silica (∼14%), low in Fe ( ∼17%) and Mn/Fe ratio of 1.7 and 2.1. Supergene processes, during Tertiary-Quaternary (7) have contributed significant quantities of sedimentary oolitic/pisolitic ore and minor amount of cavity filling/replacement type ore. The surficial exposures of reworked metasedimentary and oolitic/pisolitic ores have been subjected to mechanical disintegration and detrital accumulation with repeated breaks leading to extensive development or float ore in both laterite and soil profile.

Mesozoic Ammonites from the Spong Valley, Zanskar, N. W. India

Abstract: A Callovian-Oxfordian sphaeroceratid, (?) Prograyiceras, proves for the first time, the presence of Jurassic flysch in the allochthonous Spongtang Klippe. A berriasellid, Kilianella, similarly proves the presence of probable Valanginian in the autochthonous shelf sequence of the Zanskar range, and supports the presence of the (upper) Spiti Shale hitherto recognized in the area on lithological grounds.

Terraces, in the Himalayan Tributaries of Ganges in Uttar Pradesh

Abstract: The Ganga system in Garhwal Himalaya comprises of Alaknanda followed by Bhagirathi, Bhalangana, Mandakini, Pindar, Atta Gad and Jalkhur with their terraces. Alaknanda is the trunk stream and the others are tributaries. The Alaknanda and its tributaries include terraces of three domains: glacial, fluvio-glacial and fluvial deposits in three distinct environments in Pleistocene, late-Pleistocene and Holocene. Stratigraphic succession of terraces and sequential stages of valley development in the Ganga system of Garhwal Himalaya has been worked out. There was continuous decrease in rate of uplift of Himalaya from early to late Holocene, as evidenced by the conspicuous divergent disposition of older terraces and convergence of younger terraces all along the length of the Alaknanda river.

Sivaganga Formation, its Sedimentology, Micropalaeontology and Sedimentation History

Abstract: The textural study of the sand grains of Sivaganga Formation indicates that sedimentation commenced in a fluvial environment passing through the lacustrine-marsh conditions and ending with the influence of tidal estuary. The clayey sediments in the upper part have yielded an arenaceous foraminiferal assemblage having a low dominance and diversity value belonging to the Neocomian-Aptian (Early Cretaceous) age. The sedimentary basin was formed as a result of downwarping of the crystalline basement along the prominent Eastern Ghat trend during Late Jurassic and Early Cretaceous. The basin established its fragmentary connections with the open sea situated at some distance towards the east and southeast.

The Early Precambrian Crust with Reference to the Indian Shield: An Essay

Abstract: The existence of several greenstone generations and of continuous transitions between gneiss-greenstone terrains and granulite terrains in the Indian Shield allow a unique insight into the structure and evolution of the Archaean crust. The long standing controversy on granite-greenstone relations reflects contrasting basic assumptions, methods and criteria. Many of the problems reflect the uncertain significance of Rb-Sr ages, which often represent thermal resetting events. Correlations between volcanic units in separate greenstone belts and between shallow and deep crustal zones should be soon enabled by U-Pb and Sm-Nd isotopes. The concept of greenstone belts as primary basins is not applicable to the oldest supracrustals whose remnant enclaves and xenoliths are distributed on all scales throughout the Peninsular Gneiss. The oldest directly identifiable rocks in south India consist of ultramafic-mafic-anorthosite-pelite-chert assemblages of the Sargur Group intruded by ca 3.3 Gyr gneiss. While sialic nuclei may have been present, a pre-existence of regional sialic crust is inconsistent with the petrological, geochemical and isotopic characteristics of the dominant tonalite-trondhjemite components of the Peninsular Gneiss. The gneisses are extensively imprinted by a ca 3.0 Gyr thermal event which reset Rb-Sr systematics, but left least-reset isotopic 'windows' such as around Hassan. The age of the discrete depositories in which the Dharwar Supergroup volcano-sedimentary successions accumulated remains to be clarified. These rocks were possibly deposited in rifted zones floored by Peninsular Gneiss blocks, older greenstones and simatic gaps. The formation of charnockites and the generation of K-rich adamellites about 2.7-2.6 Gyr ago may have been genetically linked and appears to have been essentially imprinted on the older gneiss-greenstone system. Repeated thermal rises at infracrustal levels during the Proterozoic resulted in isotopic resetting of Rb-Sr systems in the Eastern Ghats granulite-gneiss belt. The temporally unique nature of the Archaean Era and major unresolved questions are pointed out.

Geochemistry of Some Thermal and Cold Waters of Godavari Valley

Abstract: Results of chemical analyses of fifteen thermal and cold waters of Godavari valley are reported. Agnigundala (Bhadrachalam) thermal water is of Na-Cl type and the rest are of Na-Ca-HCO 3 -SO 4 type. It is likely that the Agnigundala geothermal system is associated with Archaean gneisses and is characterised with a reservoir temperature around 170°C. Other warm waters of the valley show low concentrations of dissolved solids and are similar to those which are associated with sedimentary rocks. It appears that the groundwater after movement to appropriate shallow depth, got heated up to moderate temperature (around 80-90°C). Thereby the basic chemical character of the water has not changed much due to movement, both downward and upward, through the same type of rocks.

Bolaspidella from the Cambrian of Kashmir and its Stratigraphic Significance

Abstract: New species of the trilobite genus Bolaspidella viz., B. himalayensis, B. costalus, B. magamensis and Bolaspidella sp. have been recorded from the top part of the Middle Cambrian of Kashmir. Bolaspidella bearing beds constitute a well marked zone which may be extending into the base of Upper Cambrian. This is the first record of Bolaspidella from Asia and the stratigraphic position of this genus corresponds exactly to that known from North America.

Deformation Structures in the Delhi Group of Rocks around Nibhor, Rajasthan

Abstract: The metamorphic rocks occurring around Nibhor, northwestern part of the Alwar district, Rajasthan, bear evidence of three phases of folding and one period of regional metamorphism of green-schist facies. The first deformation phase, F 1 produced the principal, foliation S 1 and the later deformation phases, F 2 and F 3 , produced crenulation cleavages, S 2 and S 3 respectively. Kinking by the rotation mechanism emerges as the dominant structure supported by kinks with tabular dilatant zones between parted laminae. Some kinks may also have developed by simple shear mechanism.

The chromites of byrapur, karnataka

Abstract: Chemical study on clean chromites reveals enrichment of chromium and iron due to alteration. Physico-chemical properties show that alteration has enhanced the cell-edge, reflectivity and specific gravity but reduced the microhardness of the mineral. The reemplacement and subsequent instability of tectonic conditions might be the ultimate cause for the shift of Byrapur chromites from aluminian to ferrian field of composition.

Further Evidence of Biogenic Nature of Dodguni Microbiota, Dharwar Craton, India

Abstract: Evidence of life from Dharwar greenstone belts has been reported by previous workers. Biologic origin of these forms, however, has been doubted. Filamentous bodies described from the Dodguni cherts have been considered to be mineralogic artifacts, The present study adduces fresh evidences gathered from examination of thin sections under scanning electron microscope, microchemical tests, and organic geochemical studies, in support of biogenic origin of these microfossils.

Rare Minerals in Rajpura-Dariba Ores-Some Further Comments

Abstract: Thirteen rare sulfides, sulfo-salts and native metals: geocronite, owyheeite, bournonite, polybasite/pearceite, native arsenic, enargite, electrum, aurostibite, pyrargyrite, argentite, argentopyrite, gudmundite, stannite (?l) have been identified in the Rajpura-Dariba ores on the basis of their optical properties including quantitative spectral reflectance. The localisation of these rare minerals in remobilised veins of massive chalcopyrite or galena-tenantite along with their crystallisation history suggests that most of the rare minerals formed by mobilization of the metals involved during regional metamorphism under amphibolite facies conditions.

Grain Characteristics of Zircons of Beach Sands from Southwest Coast of India

Abstract: The mineral beach sands of the southwest coast of India, particularly of Chavara and Manavalakurichi areas, contain 4-6% zircon. Although the general setting of the zircon-bearing sands in these two areas is similar, the zircon grain characteristics show some differences. The Manavalakurichi zircon is coarser than the Chavara zircon and in the former, the grain-size has in general a positive correlation with the aspect ratio, whilst in the latter, the correlation is not straightforward. Zoning is more prominent in the Chavara zircon than in the Manavalakurichi zircon. In contrast to the Chavara zircon, the Manavalakurichi zircon shows statistically significant preferred association of idiomorphic grains with microfractures. The level of inclusions (ilmenite, rutile, monazite, quartz, apatite and fluid), either in single phase or in polyphase, is higher in the Manavalakurichi zircon than in the Chavara zircon. The source rock (s) of these zircons may not have been much dissimilar.

The Khondalites of Orissa, India - A Case Confusing Terminology

Abstract: The paper traces the progressive metamorphosis of the term 'Khondalite' from 1902 to 1975 from a specific rock name to the collective group name of a suite of high grade metamorphic and metasomatic rocks and presents examples of the extent of compositional variations of these rocks from Dhenkanal, Angul and Tikarpara areas of Orissa. It is recommended that prevalent usage of the term 'Khondalite' as a mapping unit should be discontinued by general consensus, since the term can now be used only in the sense of a 'group'.

Chromian Edenite from the Sinduvalli Ultramafic Body Karnataka, South India

Abstract: The occurrence of chromian edenite, confined to the degraded chromitite layers of the Sinduvalli ultramafic body is reported. Field, petrographic and chemical studies suggest that its development has occurred during the prograde metamorphism of the layered body. The enrichment of silicate phases in chrome-amphibole and chloritesis attributed to re-equilibration of chromite during the metamorphic episodes affecting the area and the formation of 'ferritchromit'.

Lead Age Measurements on Galenas from Peninsular India

Abstract: Measurements of lead isotope ratios on some Indian galenas and calculation of single-stage model ages were presented in a previous paper. Techniques have been improved since then, and more samples from some of these areas and several new samples have been measured. The present measurements cover a wide age-span, Eocene to Archaean (∼3300 m.y.) and the Pb-isotope abundances are not inconsistent with a 2-stage evolution of lead which show an episodic change in μ-value at ∼ 3000m.y. Model ages and μ-values have been calculated using single and double-stage models and an attempt made to interpret these values.

Theralites Around Pikkili, Dharmapuri District, Tamil Nadu

Abstract: Theralites occur as autoliths in nepheline syenites around Pikkili (Δ 2670') in Dharmapuri District, Tamil Nadu. The theralites were emplaced first, followed by the intrusion of different varieties of nepheline syenites.

Dehydroxylation and Dehydration as Parameters For Characterisation of Clays - Part II: Kaolinites, Pyrophyllite and Talc

Abstract: A sophisticated technique of modified TG, DTG has been described which can detect clay species of kandite, pyrophyllite and talc groups by employing a simple home-made TG Instrument. Many new parameters like half weight loss, differential phase lag and half weight loss temperature are obtained along with other thermal parameters and their rationale are discussed in relation to species level characterisation. The parameteral derivation and its relationship to surface morphology of clays has been described. This highly sensitive method has special application in developing countries and could be adopted in clay based industries for material quality control.

Anorthositic Enclaves in the Gneisses Around Holenarasipur Schist Belt, Hassan District, Karnataka

Abstract: Two occurrences of anorthosite enclaves in the 3.4 b.y. old gneisses surrounding the Holenarasipur schist belt are reported. The Mandagere enclave has An 92 whereas, Arkalgud enclave has An 83 . Both the bodies are metamorphosed to higher amphibolite facies and exhibit granoblastic texture. They are associated with ultramafic enclaves. These occurrences along with the lunar type anorthosites of Holenarasipur Group greenstone belts are significant in early crustal evolution models.

Petrology of a Scapolite-Bearing Rock From Karera, District Bhilwara, Rajasthan

Abstract: The scapolite-garnet-epidote-hornblende-pyroxene-calcite ± plagioclase rock of the Aravalli Group (Early Precambrian) is found as thin bands within the high-grade paragneisses of the underlying Banded Gneissic Complex around Karera in northwest peninsular India. The mineralogy and chemical composition of the rock imply that it is derived from impure calcareous sediments. Significant amounts of chlorine are present in both scapolite and hornblende. In epidote and garnet ferric iron is an important component and its presence together with hematite indicates high fo 2 during recrystallization of the rock. The partitioning data of Na/(Na+Ca) between scapolite (Me 71.3 ) and the coexisting plagioclase (An 85-90 ) and its reference to the experimentally based diagram yield recrystallization temperature of 650°-700° C, conforming to those computed from other mineral equilibria of the enclosing paragneisses. The scapolite is considered to develop as a result of extreme diminution of PH 2 0, presumably by incoming of chlorine from an evaporite component.

Carbonatite-Alkaline Complex of Mundwara

Abstract: Two types of carbonatite dykes - alvikite and beforsite are associated with (the Mundwara alkaline igneous complex. Petrochemical and mineralogical studies indicate enrichment of alkalies and niobium. Pyrochlore is noted in beforsites along with dolomite, apatite, barite and flourite, whereas calcite, ankerite, heamatite, aegirine, apatite and barite are noted in the alvikites. The presence of both metasomatic as well as intrusive syenites are noticed. Higher content of Nb is noted in the syenites containing sphene, aegirine and magnetite. The presence of annular fractures around the plutons and the arcuate shatter zone with breccia on the outer margins of the Mer pluton indicate upwarping and later intrusion of carbonatites through the Erinpura granite. High P 2 O 5 and Nb content of the soil samples around Mer Pluton suggest possible existence of a carbonatite plug below.

Structural Studies Around Kathar, Udaipur Rajasthan

Abstract: The deceptively simple map pattern of quartzite, marble, and phyllite and mica schist, with nearly NS 'generalized strike' and steep dips of beds around Kathar in the Udaipur district, Rajasthan, conceals folds of two generations in large scale and structures of two additional phases in smaller scales. The folds of the first set (F 1 ) are isoclinal with a pervasive axial plane cleavage (S 1 ), which has largely blurred stratification in the more schistose rocks. These are buckle folds modified extensively by flattening. The S 1 cleavage and F 1 axial planes have been involved in tight to isoclinal upright folding on nearly NS-striking axial planes (F 2 ). These folds have extensively reoriented the F 1 axes and axial planes, but because of the style and orientation of the folds of the two systems, both the sets of axial planes have subparallel strikes in a major part of the area. Hook-shaped folds with double closures and orthogonal relation of stratification and S 1 cleavage point to the presence of F 1 folds. The F 2 folds are also flattened parallel folds formed by a maximum compressive strain in a horizontal EW direction. Rare F 3 folds with subhorizontal axial planes are gravity-induced. The F 4 kinks with WNW-striking axial planes may represent structures formed by longitudinal shortening in the last stage of deformation.