Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

S. M. Ahmad ¹, A. M. Dayal ¹, V. M. Padmakumari ¹, K. Gopalan ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, IN

Abstract

Thc ⁸⁷Sr/⁸⁶Sr ratio of seawater has increased significantly over the last 40 Ma. This increase was marked by very high rate of increase in ⁸⁷Sr/⁸⁶Sr and almost zero growth rate. The middle Miocene is one such period when considerable changes in the slope of ⁸⁷Sr/⁸⁶Sr curve have been reported. This study tests and improves on previously published strontium isotopic records for this period.

We report 34 high precision strontium isotopic analyses of well preserved planktonic foraminifera from Ocean Drilling Program (ODP) site 758A in the eastern Indian Ocean. This site was selected because of the high quality magnetostratigraphy, suitable sedimentation rate (0.3 to 1.5 cm/ka) and excellent preservation of calcareous microfossils in sediments. Samples studied represent the period from 15.2 Ma to 10.3 Ma with approximately one sample per 100-200 ka.

The results show that the seawater ⁸⁷Sr/⁸⁶Sr ratio during this period increased from 0.70881 to 0.70892. The rare of increase in ⁸⁷Sr/⁸⁶Sr during 15.2 Ma to 14.3 was -31 x 10^-6/m.y., whereas significantly from 14.3 to 12.8 Ma it was about a factor of two higher (-59x10^-6/ m.y.). High rate of growth in seawater ⁸⁷Sr/⁸⁶Sr during this time interval indicates exceptionally high rates of continental weathering. However, during 12.8 to 10.3 Ma the growth in ⁸⁷Sr/⁸⁶Sr of seawater remained almost constant with an average value of 0.708914. This constant value may he due to decrease in weathering rates of continents as a result ofextensive glaciation. It is suggested that the tectonicuplift and denudation of Himalayas coupled with climatic changes have resulted in the variations of seawater strontium isotopes during this period. The inflection points in the curve may be used as event marker for stratigraphic corretation of marine sediments.

 

Keywords

Marine Geology, Stratigraphy, Strontium Isotopes, Miocene, Planktonic Foraminifera.

Full Text

     

K. Gopalan ¹, J. R. Trivedi ¹, M. N. Balasubrahmanyan ², S. K. Ray ³, C. Anjaneya Sastry ⁴

Affiliations
1 Physical Research Laboratory, Ahmedabad 380009, IN
2 Geological Survey of India, 29, Jawaharlal Nehru Road, Calcutta 700 016, IN
3 Geological Survey of India, Bhutan Circle, Samchi, BT
4 Geological Survey of India, Geochronology and Isotope Geology Division, 29, Jawaharlal Nehru Road, Calcutta 700016, IN

Abstract

Rubidium and strontium determinations are reported for the Udaipur and Saladipura granites from the Khetri copper belt. The whole-rock data define a Rb-Sr isochron corresponding to an age of 1480 ± 40 m.y. and initial Sr ratio of 0.7131 ± 0.006. This age is older than previously believed, but consistent with the generally high age of post-Delhi intrusive rocks so far dated in the region. Biotites associated with these rocks give younger ages around 700 m.y, This indicates mobilisation of Sr isotopes in a later metamorphic event. It is significant that this secondary event is almost synchronous with the intrusion of granites to the west and south of the Aravalli mountains.

Full Text

     

K. Gopalan

Abstract

No Abstract.

Full Text

   

K. Gopalan ¹

Affiliations
1 Physical Research Laboratory, Ahmedabad, IN

Abstract

No Abstract.

Full Text

   

N. Kochhar ¹, K. Pande ², K. Gopalan ²

Affiliations
1 Department of Geology, Punjab University, Chandigarh 160014, IN
2 Physical Research Laboratory, Navrangpura, Ahmedabad 380009, IN

Abstract

Acid volcanics and associated granites from the Tusham Ring Complex in northwestern India yield a well defined Rb-Sr isochron corresponding to an age of 754±20 Ma (λ=1.42 x 10^-11 yr^-1 ) and an initial Sr composition of 0.7131±0.0025. This is in good agreement with the age of the Malani suite of igneous rocks of western Rajasthan.

Full Text

     

K. Gopalan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad, IN

Abstract

No Abstract.

Full Text

     

S. N. Sarkar ¹, J. R. Trivedi ², K. Gopalan ²

Affiliations
1 Indian School of Mines, Dhanbad 826 004, IN
2 Physical Research Laboratory, Ahmedabad 380 009, IN

Abstract

Six whole rock samples of the Tirodi gneiss belonging to the basal part of the Sausar Group rocks in the Dongri Buzurg area yield a good Rb-Sr isochron corresponding to an age of 1525±70 Ma and an initial Sr ratio of 0.7148±0.0033. Mineral isachrons for two of these rocks give concordant but younger ages at 860 Ma in good agreement with the previously reported K/Ar ages on minerals separated from the Sausar Group rocks. The 1525 Ma event is interpreted to mark the main phase of regional metamorphism of the Sausar Group of rocks including the granitization of the basal Tirodi unit. The deposition of the sediments of the Sausar Group and their basement must therefore be even older. The good agreement of the RblSr and K/Ar mineral ages at about 860 Ma indicates the latest thermal overprint on the Sausar - rocks, The regional metamorphism of the Tirodi gneiss at about 1500 Ma ago is also reflected in the Rb/Sr and K/Ar mineral ages of the Amgaon gneiss and Dongargarh granite to the east of the Tirodi gneiss.

Full Text

     

T. R. Venkatesan ¹, K. Pande ¹, K. Gopalan ¹

Affiliations
1 Physical Research Laboratory, Ahmedabad 380 009, IN

Abstract

The feasibility of dating igneous rocks by the ⁴⁰Ar-³⁹Ar method in India using the Apsara nuclear reactor at BARC, Bombay is reported, based on measurements on two tholeiitic basalts and one alkali basalt associated with the Deccan volcanism. The ages of the tholeiitic basalts are 71±4 Ma and 65±4 Ma, respectively and that of the alkali basalt is 682 Ma. Apart from the low neutron flux requiring long irradiation times, the neutron fluence within the sample irradiation volume is somewhat anisotropic. The latter needs to be minimised to achieve the higher precision the method is capable of.

Full Text

     

R. Ramesh ¹, S. K. Bhattacharya ¹, K. Gopalan ²

Affiliations
1 Physical Research Laboratory, Ahmedabad 380 009, IN
2 National Geophysical Research Institute, Hyderabad 500 007, IN

Abstract

Stable isotope ratios of hydrogen, carbon and oxygen in tree cellulose have been recently demonstrated as potential indicators of past climate, However, a number of problems remain to be solved before isotope dendroclimatology can be used for routine reconstruction of palaeoclimate. Here we review the existing Literature, with a special reference to the Indian context and identify key problems that deserve immediate attention.

Full Text

     

M. Santosh ¹, N. G. K. Nair ¹, K. Pande ², K. Gopalan ²

Affiliations
1 Centre for Earth Science Studies, P. B. 2235, Sasthamangalam, Trivandrum 695 010, IN
2 Physical Research Laboratory, Navarangpura, Ahmedabad 380 009, IN

Abstract

Rubidium and strontium determinations are reported for seven whole-rock samples of the Ambalavayal granite of northern Kerala. The data yield a well defined Rb-Sr isochron, corresponding to an age of 595 ± 20 Ma and initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7171 ± 0.0022. The age marks a regime of Late Precambrian-Early Palaeozoic granite magmatism in the southwestern Indian shield. The agreement of the Rb/Sr age with the previously measured K/Ar age of 560± 30 Ma indicates no significant secondary thermal event in the region. The evolved initial Sr ratio suggests either an exclusive origin or significant crustal contamination of magma derived from a deeper source.

Full Text

   

A. B. Chakranarayan ¹, K. B. Powar ¹, K. Pande ², K. Gopalan ³

Affiliations
1 Department of Geology, University of Poona, Pune 411 007, IN
2 Physical Research Laboratory, Ahmedabad 380 009, IN
3 National Geophysical Research Institute, Hyderabad 500 007, IN

Abstract

At Belka Pahar, in the Sirohi district of Rajasthan, metasediments of the Delhi Super Group are associated with a medium-grained, leucocratic, alkali granite. Whole rock isotopic data for this granite define a Rb-Sr isochron corresponding to an age of 860±25 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7059 ±0.0032.

Full Text

     

K. Gopalan ¹

Affiliations
1 National Geophysical Research Institute Hyderabad 500007, IN

Abstract

No Abstract.

Full Text

     

K. Gopalan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad, IN

Abstract

No Abstract.

Full Text

   

Anil Kumar ¹, R. Srinivasan ¹, K. Gopalan ¹, D. J. Patil ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, IN

Abstract

Field observations, trace element and isotopic results are reported for rocks classified as carbonatites from Vinjamur area in the Nellore schist belt. These carbonate rocks occur either inter-bedded with metasedimentary quartzites and pelites or in association with calc-alkaline rather than alkaline volcanic rocks. No fenitised contacts are seen. Their Nb and REE contents are too low for carbonatites. Samples free from baryte veins have less than 150 ppm of Sr with an isotopic ratio greater than 0.7100 δ¹³C_PDB range from -1.58 to - 0.56 ‰ and δ¹⁸O_SMOW from +14.00 to +16‰ , All these features, individually and collectively, show that the carbonate rocks of Vinjamur are most probably sedimentary and not igneous in origin.

Keywords

Carbonatite, Nellore Schist Belt, Andhra Pradesh.

Full Text

     

B. V. Oleinikov ¹, A. F. Korobeinikov ¹, O. V. Koroleva ¹, P. Krishnamurthy ², Anil Kumar ³, K. Gopalan ³

Affiliations
1 Yakut Institute of Geological Sciences, Siberian Branch, Russian Academy of Science, Yakutsk - 677 891, RU
2 Atomic Minerals Division, Hyderabad-500 016, IN
3 National Geophysical Research Institute, Hyderabad-500 007, IN

Abstract

Sixty nine (69) samples of Deccan basalt flows from the Western Ghats representing the Wai, Lonavala and Kalsubai subgroups average 3.7 ppb of gold. Gold content shows limited variation from 3.0 to 5.8 ppb in 62 samples whereas they range from 1.4 to 2.7 ppb in 7 samples. Flows of the Wai subgroup show the maximum variation in gold abundances (1.4 - 5.8 ppb) and which may be related to the presence of primitive uncontaminated basalts (Ambenali)- lowest value-crust-contaminated basalts (Poladpur) and basalts derived from enriched-mantle sources (Mahabaleshwar). Gold appears to be insensitive to crystal fractionation or differentiation as basalts of different types such as the picritic, porphyritic, aphyric and giant plagioclase-bearing types found within a subgroup or formation have similar avcrages (3.6-3.8 ppb).

Deccan Tholciitic basalts contain higher amounts of gold than dome of the occean island (Hawaii) tholciitic basalts island are (Kurile-Kamchatka) tholciites, but contain values similar to those found in the Columbia River basalt province.

Keywords

Gold, Deccan Basalts, Western Ghats, Economic Geology.

Full Text

     

S. Das Sharma ¹, D. J. Patil ¹, R. Murari ¹, K. Gopalan ¹, G. V. Nesterenko ²

Affiliations
1 National Geophysical Research Institute, Uppal Raod, Hyderabad-500007, IN
2 Vernadsky Institute of Geochemistry, Kosygin Street 19, Moscow-117075, RU

Abstract

No Abstract.

Full Text

   

D. Rameshwar Rao ¹, Kewal K. Sharma ¹, K. Gopalan ²

Affiliations
1 Wadia Institute of Himalayan Geology, General Mahadeo Singh Road, Dehradun-248001, IN
2 National Geophysical Research Institute, Hyderabad-500 007, IN

Abstract

The Wangtu Gneissic Complex (WGC) of Himachal Himalaya represents the basement rocks which remained more or less in its original realm. The unique feature of this complex is that the various phases of deformed and undeformed granitoids are exposed in the area. The complex has a peripheral zone of augen gneisses and core of non-foliated granitoids comprising of Coarse Porphyritic Granites (CPG) and Fine Grained Granites (FGG).

In the field the FGG rocks have sharp intrusive contacts with CPG rocks. At places they are entrapped or have protrusions into CPG rocks. Compositionally they represent syenogranites. They are peraluminous to meta-aluminous with Sr initial ratio of 0.705. The major and trace elements of these rocks show unimodal distribution pattern on different variation diagrams. They along with rare earth elements show marked fractionation trends, with a significant negative Eu anomaly. The rocks more or less define an isochron, representing Rb-Sr age of 1895±64 Ma.

The petrochemical studies favour a fractionated rather than multiple intrusions of magma. The studies suggest that fractional crystallisation of parental magma gave rise to early formed CPG rocks and the residual melts resulted in the crystallisation of FGG rocks. The last formed granitoids, the FGG rocks are interpreted as the final residue which crystallised rapidly after build-up and loss of fluids and volatiles which migrated to higher levels to form pegmatite phase in the aureol zone. The Rb-Sr systematics in the present case indicate that the granite have primary crystallisation melts derived from the deep crustal levels around 1895±64 Ma.

Keywords

Granites, Igneous Petrology, Himachal Pradesh.

Full Text

     

D. Rameshwar Rao ¹, Kewal K. Sharma ¹, K. Gopalan ²

Affiliations
1 Wadia Institute of Himalayan Geology, 33 General Mahadeo Singh Road, Dehra Dun - 248001, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

The present paper deals with petrochemistry and geochronology of granite pluton that intrudes the Agglomerate Slate-Panjal Trap sequence in Tangmarg region of Baramulla district, Kashmir Himalaya. This granite body henceforth called 'Tangmarg Granite' contains biotite, amphibole and rare pyroxene as mafic minerals and shows higher abundances of Zr, Nb, Ga, Y and REE, and lower AI, Mg and Ca contents. The geochemical signatures suggest the granite to be chemically distiIict from most of the orogenic granites (M-, I- and S-types) and points to their Anorogenic nature.

The six point whole rock Rb-Sr isochron of the Tangmarg granite gives an age of 161 ± 5 Ma, with Sr ratio of 0.714. The small size of the pluton points to limited melt generated from a partial fusion 'event' of short duration. Such a melt is attributed to pressure release due to reactivation of the faults that were formed during Permo-Triassicrifting event.

Keywords

Geochemistry, Geochronology, Granites, Kashmir Himalaya

Full Text

     

A. K. Choudhary ¹, A. Naik ², D. Mukhopadhyay ³, K. Gopalan ⁴

Affiliations
1 Department of Physics, Samastipur College, Samastipur, Bihar, IN
2 Department of Earth Sciences, Sambalpur University, Orissa, IN
3 Department of Geology, Calcutta University, Calcutta, W.B., IN
4 National Geophysical Research Institute, Hyderabad, A.P., IN

Abstract

The wholerock Rb-Sr isochron age of 2380 &#177; 44 Ma (Sr<SUB>i</SUB> = 0.70492  &#177; 14) obtained for the Sambalpur granodiorite indicates an Early Proterozoic granitic activity in this region younger than Singhbhum Granite (ca. 3.3 to 3.1 Ga). Comparable Rb-Sr ages have also been reported for different granites from Mandhya Pradesh.

Full Text

   

S. Sarangi ¹, K. Gopalan ¹, A. B. Roy ², B. Sreenivas ¹, S. Das Sharma ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN
2 Jai Narayan Vyas University, Jodhpur - 342 00 1, IN

Abstract

Pb-Pb isochron age of carbonate rocks of the Jhamarkotra Formation of the Aravalli Supergroup from the Udaipur sector in Rajasthan has been measured for the first time as 1921±67 Ma (2σ). We consider this as a reset age, as it is indistinguishable from the earlier reported Rb-Sr isochron age of 1900±8O Ma for the Darwal Granite formed synkinematically with the Aravalli deformation. The depositional age of the Jhamarkotra carbonates could be 200-300 Ma older as suggested by the positive δ¹³C anomalies noted in these carbonate rocks.

Keywords

Pb-Pb Isochron Age, Carbonates, Aravalli Supergroup Rajasthan.

Full Text

     

S. M. Ahmad ¹, A. M. Dayal ¹, V. M. Padmakumari ¹, G. Anil Babu ¹, K. Gopalan ¹

Affiliations
1 National Geophysical Research institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

A seawater ⁸⁷Sr/⁸⁶Sr record is generated for early Miocene to early late Miocene based on measurements of 114 samples of cleaned planktonic foraminifera from Ocean Drilling Program site 758 (Hole 758A) in the eastern Indian Ocean. Age model for this Site is based on nannofossil events and interpolated numerical ages are compared with Sr Isotopically-Derived ages, There is a fairly good agreement between the two stratigraphic tools (<±1.5 Ma). ⁸⁷Sr/⁸⁶Sr ratio during ∼23.7 Ma to ∼8.1 Ma increased from 0.70828 to 0.70895, with variations in the rate of ⁸⁷Sr/⁸⁶Sr increase. Seventh-Order polynomial fit curve shows the ⁸⁷Sr/⁸⁶Sr increase rates of 44 ×10^-6m .y.^-1 for -23.7 to ∼19.2 Ma, 41 × 10^-6 m.y.^-1 for -18.2 to - 10.8 Ma, and 25 × 10^-6 m.y.^-1 for ∼10.8 to ∼8.1 Ma. These rates are broadly consistent with the growth rates of ⁸⁷Sr/⁸⁶Sr reported from other DSDP and ODP sites with some minor differences.

A composite Sr isotope curve constructed for the 24-8 Ma is broken into three linear segments with a different linear regression equation. The overall rate of ⁸⁷Sr/⁸⁶Sr increase for ∼24-8 Ma at site 758A (40 x 10^-6 m.y.^-1) is in agreement with the rate of ⁸⁷Sr/⁸⁶increase of the composite curve (42 x 10^-6 m.y.^-1).

Keywords

Sr Isotopes, Miocene, Stratigraphy, Planktonic Foraminifera, Eastern Indian Ocean.

Full Text

     

Anil Kumar ¹, K. Gopalan ¹, K. R. P. Rao ², S. S. Nayak ²

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 Geological Survey of India, Bandlaguda, Hyderabad - 500 068, IN

Abstract

Acid-Leached phlogopite Rb-Sr isochron ages of the Kotakonda and Mudalbid kimberlites in the Narayanpet kimberlite field northwest of the Cuddapah basin, south India are 1085±14 and 1099±12 Ma, respectively. These concordant ages are much younger than the recently reported K-Ar and ⁴⁰Ar-³⁹Ar ages of 1363±48 and 1402±5 Ma, respectively for the former kimberlite, and support our earlier conclusion that the south Indian kimberlites erupted episodically close to 1090 Ma ago. Three lamproites-one each from Ramannapeta at the northeastern margin and Chelima and Zangamarajupalle within the Cuddapah basin-give distinctly different Rb-Sr ages of 1224±14, 1354±17 and 1070±22 Ma, respectively. While the Rb-Sr age of the Ramannapeta lamproite is also younger than an earlier reported K-Ar age of 1381±18 Ma, it is the same as that of many spatially close alkaline complexes like at Elchuru and Kunavaram. The ages for the Chelima and Zangamarajupalle rocks are only tentative, as these rocks contain a large secondary carbonate component with its Sr isotopic composition genetically unrelated to that of their phlogopites.

Keywords

Kimberlites, Lamproites, Narayanpet Kimberlite Field, Rb-Sr Ages, Dharwar Craton.

Full Text

     

Asru K. Chaudhuri ¹, Somnath Dasgupta ², Goutam Bandyopadhyay ², Subir Sarkar ², P. C. Bandyopadhyay ², K. Gopalan ³

Affiliations
1 Geological Studies, Unit, Indian Statistical Institute, Calcutta, IN
2 Department of Geological Sciences, Jadavpur University, Calcutta 700032, IN
3 National Geophysical Research Institute, Hyderabad, IN

Abstract

The Proterozoic sequence around Adilabad, Andhra Pradesh comprises a distinct lithostratigraphic unit of 'Group' status and has been given the name Penganga Group. The Penganga Group has been classified into three Formations which are, in order of younging, Pranhita Sandstone, Chanda Limestone and Sat Nala Shale. The sequence is dominated by limestones and shales which were deposited in relatively deep water environment and, thus, contrast sharply with the conglomerate, sandstone and shallow water limestones of the Pakhal Group. Radiometric dating of fresh glauconites from middle part of the Penganga Group gives an age of 770 ± 30 Ma. Thus, the Penganga Group is the youngest of all other Proterozoic sequences in Pranhita-Godavari Valley.

Full Text

     

V. M. Padmakumari ¹, S. M. Ahmad ¹, A. M. Dayal ¹, R. Soundar Rajan ², K. Gopalan ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007

Abstract

Neodymium isotopic composition (ε_Nd) of authigenic marine sediments is an important tracer of the continental input of Nd to the oceans As the residence time of neodymium in seawater 1s short when compared to the mixing time of the oceans, its isotopic composition shows regional variations In this paper, we present the surface seawater Nd isotopic record from a deep-sea sediment core (SK-72) from the Bay of Bengal Our results support the inference of Vance and Burton (1999) that thoroughly cleaned foraminifera can be used to infer glacial-to-Interglaciacl hanges ln the Nd isotopicc omposition of surface seawater The striking feature of this record is a sharp shift in ε_Nd (˜ 4 epsilon units) from the Last Glacial Maximum (LGM) to Holocene In addition, the variations documented in ε_Nd, show a remarkable correspondence with those of the oxygen isotopic composition of planktonic foraminifera,indicating a process that controls the Nd isotopic compoition, which responds in phase with climate We infer that the shift in ε_Nd from the LGM to Holocene is due to a change in the weathering pattern in the Himalaya-Tibet and the Indo-Burman Ranges glacial erosion during the LGM and a strong monsoonal effect durtng the Holocene.

Keywords

Neodymium Isotopes, LGM, Holocene, Weathering, Monsoon, Riverine Input, Oxygen Isotopes, Planktonic and Benthic Foraminifera.

Full Text

     

J. R. Trivedi ¹, K. Gopalan ¹, K. S. Valdiya ²

Affiliations
1 Physical Research Laboratory, Ahmedabad 380 009, IN
2 Geology Department, Kumaun University, Nainital 263 001, IN

Abstract

Whole-rock Rb-Sr isochron analyses of granitic components of the two prominent tectono-stratigraphic units, the Ramgarh and Almora Groups, in the Kumaun Lesser Himalaya yield two relatively coherent age clusters around 1800 Ma and 550 Ma, respectively. The older age corresponds to the porphyritic granites, quartz-porphyry and augen gneisses from the Ramgarh Nappe, the basal parts of the synformal Almora Nappe and the Askot-Dharamghar Klippen in the inner Lesser Himalaya, whereas the younger age is restricted to the massive Champawat Granodiorite - Almora Granite emplaced in the upper part of the Almora Nappe. The older rock seems to occur also in the ischolar_main zone at the base of the Great Himalaya. The early Palaeozoic granites appear to be very widespread, extending both into the Himachal Pradesh and Nepal sectors. The spatial distribution of the two age components has been used to infer the possible relationships between the two lithotectonic units.

Full Text