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Chakraborty, S. K.
- Petrology of Early Palaeozoic Lamprophyres from Schirmacher Oasis, East Antarctica
Authors
1 Antarctica Division, Geological Survey of India, NH 5P, N.I.T., Faridabad- 121001, IN
2 Central Petrological Laboratory, Geological Survey of India, 15 A & D Kyd Street, Calcutta-700016, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 56, No 6 (2000), Pagination: 593-604Abstract
Lamprophyre dykes are conspicuous within the Proterozoic high-grade metamorphic rocks of Schirmacher Oasis. These dykes have been classified chemically into alkaline and calc-alkaline types on the basis of K2O/Na2O ratio and CIPW norm. The close association, age and geochemical similarities of both the-lamprophyres and melasyenite in thc area indicale their consanguinity and derivation from same source region. These lamprophyres have a high concentration of incornpatible elements like Ba, Rb, Sr and Zr. These rocks also have a high Mg# and depict a negative Ta-Nb-Ti anomaly. The chondrite normalised REE patterns show fractionated trends with large variation in LREE and small variation in HREE concentration in both the rock groups. The calc-alkaline lamprophyre is more enriched in LREE than the alkaline lamprophyre.
It is proposed that metasomatised phlogopite-bearing enriched mantle source has undergone partial melting to generate lamprophyric magma. This magma is generated in a continental within-plate tectonic setting. The collision of East and West Gondwana during Pan-African orogeny, resultant overthickening of continental lithosphere and generation of widespread magmatic activity in the region due to partial melting has culminated in the intrusion of lamprophyres around 450 Ma ago.
Keywords
Lamprophyre, Melasyenite, Schirmacher Oasis, East Antarctica, Pan-African Orogeny.- Depositional Architecture of Mio-Pliocene Sequences in Offshore Bengal Basin
Authors
1 Reliance Industries Ltd , Mumbai, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 67, No Spl Iss 5 (2006), Pagination: 575-583Abstract
Bengal Basin located along the northeast coast of India has a thick pile of MIO-Pliocene sediments fed by a number of large rivers like Ganga, Brahmaputra, Mahanadi and smaller rivers like Subarnarekha, Baitaram and Brahmani. The basin covers an area of 40,000 sq km in the Bay of Bengal. The sediments in the present day shelf area exhibit a complex deposition history with unique juxtaposition of shallow and deep water sediments in the form of deltas, distributary channels, Pro-Delta sediments and incised canyons, filled up with channel-levee complex deposits.A detailed study was carried out on the morphology and architecture of the different depositional elements and units utilizing different vintages of 2D/3D seismic and data from drilled wells Geo-Body mapping and attribute studies reveal finer details of the complex system Special seismic attributes such as spectrally decomposed amplitude, waveform classifier and 'sweetness' (amplitude by square ischolar_main of frequency) were quiet helpful in understanding the morphological details of the system. The following depositional elements have been brought out
•Delta-Distributary channel complex associated with incised valley and tidal inlets.
•Shelfal Canyon cut and fill sequences.
o Simple canyon fills Simple "V" shaped cuts with fills in the form of low to moderate sinuous.
channels.
o Complex Canyon cut and fills Numerous, repeated canyon cuts caused by Mass-Wasting with intervening fills in the form of deep water to Sub-Aerial channels and deltaic progradational deposits.
•Deep-Water Channel Complex Fed by large canyon up-dip. These depositional elements do not form part of the canyon fill, but occur down-dip.
The sand and si1t lithological distribution in the above depositional complexes form the predominant hydrocarbon reservoir units. The numerous reservoir bodies within the above units of varying lateral and vertical extents, are expected to hold the future exploration potential in this frontier basin.
Keywords
Morphology, Architecture, Bengal Basin.- Alkali Basalt of Kachchh: its Implication in the Tectonic Framework of Mesozoic of Western India
Authors
1 Geochronology and Isotope Geology Division, Geological Survey of India, 15 A & B, Kyd Street, Kolkata - 700 016, IN
2 Geological Survey of India, Operation Gujarat, Gandhinagar - 382 093, IN
3 Central Petrological Laboratories, Geological Survey of India, 15 A & B, Kyd Street, Kolkata - 700 016, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 66, No 5 (2005), Pagination: 599-608Abstract
Field evidences in Kachchh show that alkali basalts occur as flows over Mesozoic sediments. The top parts of Mesozoic Bhuj Sandstone often carry scoria and lapilli related to alkali basalt magmatism indicating that alkali magmatism and sedimentation went side by side. Tholente flows occur as blanket over both Mesozoic sediments and alkali basalt bodies.Evidences are provided to show that alkali basalt magmatism in Kachchh was not only an earlier episode than tholente emplacement, it also predates the closing of Kachchh Mesozoic basin and seems to support a protracted period of Deccan magmatism (70 - 60 Ma). The study also shows that huge concentration of alkali basalt in the western Kachchh mainland is associated to a zone of transtension related to the rifting of Kachchh basin. Alkali basalt bodies and whole of Mesozoic sequence were affected by reverse faults related to closure of Kachchh basin with development of series of hanging wall anticlinal and footwall synclinal structures.
Keywords
Alkali Basalt, Tholente, Flow, Plug, Releasing Bend, Kachchh.- Chemistry of Blotites from Pelitic Schists and Granitic Rocks of McCluskieganj, District Palamau, Bihar
Authors
1 Department of Geology, Patna University, Patna, IN