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

Partha Pratim Chakraborty ¹, Tapan Pal ¹, Tanay Dutta Gupta ¹, Kalyan Surya Gupta ¹

Affiliations
1 Geological Survey of India, Calcutta, IN

Abstract

Detailed facies analysis in Eocene Mithakhari Group reveals eight different lithofacies of wide ranging paleogeographic significance. These facies are i) disorganised matrix-supported conglomerate, ii) graded matrix-supported conglomerate, iii) graded pebbly sandstone, iv) massive and thick-bedded sandstone, v) plane laminated and cross-stratified sandstone, vi) interbedded sandstone and mudstone, vii) massive to faintly laminated shale and viii) interbedded shale and coal; which are grouped into-five different facies associations (FA). viz. Subaerial alluvial plain (FAl), Shallow water, wave dominated shelf (FA2), Delta slope (FA3), Prodelta slope (FA4) and Submarine fan (FA5).

Lying unconformably on oceanic basement (ophiolite slices?) these sediments constitute short truncated successions. Litholog measurement in three isolated sections viz. Kaushalyanagar. Sagwannala and Rangat-Nimbutala reveal widely varying facies succession pattern. Frequent facies change, predominance of massflow deposits, signatures of synsedimentary basinal disturbance and wide paleogeographic variation indicate sedimentation in small isolated basins in an immature trench-slope setting. Profusion of progradational depositional cycles and evidences of emergence in the studied sections provide indication of frontal accretion and tectonic shoaling in the filling history of these basins.

Keywords

Sedimentology, Depositional Environment, Accretion, Eocene, Middle Andaman.

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Tapan Pal ¹, Hi-Soo Moon ², Sachinath Mitra ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta- 700032, IN
2 Department of Geology, Yonsei University, Seoul, KR

Abstract

⁵⁷Fe Mossbauer spectroscopic investigation of natural chromites from two chromite deposits of India (Sukinda and Byrapur) documents partly inverse spinel structure arising out of oxidation. The spectral fitting was based on allowing a disordering distribution of Fe²⁺ and Fe³⁺ ions at tetrahedral (A) and octahedral (B) sites. Mossbauer investigation of the samples taken from the physico-chemically distinct two horizons of Sukinda viz. brown are and grey ores, and Byrapur area revealed three types of iron ion distribution as: Fe²⁺(A), Fe³⁺(A) and Fe²⁺(B) (GC-group); (b) Fe³⁺(A1), Fe³⁺(A2) and Fe²⁺(B) (BC-2 group), and (c) Fe³⁺(A1), Fe³⁺(A2) and Fe³⁺(B) (BC-1 group).

The distribution pattern of iron cations at A and B sites was linked to the degree of oxidation. The stages of oxidation could be modelled from normal to inverse fonn. A model suggesting 'electron localisation' at the B-sites makes the intermediate stage. Iron site occupancy determined by Mossbauer spectroscopy of the presently studied samples indicates that these fall under three groups of oxidation stages. An early stage of oxidation is shown by samples of group GC, intermediate stage by group BC-2 and final stage by BC-1 group of chromite samples. The imprint of progressive oxidation manifested by Fe cation site occupancy has been correlated with the Fe²⁺/Σ:Fe ratios, obtained for each group of samples.

Keywords

Chromites, Mineralogy, Sukinda, Orissa, Byrapur, Karnataka, Mossbauer Spectroscopy.

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Tapan Pal ¹, Biswajit Ghosh ², Sujit Chakraborty ³

Affiliations
1 Petrology Division, ER, Geological Survey of India, Kolkata, IN
2 Project: Andaman and Nicobar, Op: WSA, ER, Geological Survey of India, Kolkata, IN
3 Monitoring Division, CHQ, Geological Survey of India, Kolkata, IN

Abstract

The chromite bearing ultramafic rocks of Roro-Jojohatu areas in the Singhbhum Craton within the metasediments of the Iron Ore Group (IOG) is a part of Archaean greenstone terrain of Eastern India. In this area since all the primary silicates are altered, chromite has been used for host rock petrogenesis. Chromite occurs in different forms viz. (a) mainly cumulus phases in chromitite layers, (b) intercumulus phases forming banded/net textured chromite and (c) fine disseminations. Chemically all the chromites are aluminian chromite. The massive chromite and banded/net-textured chromites show high Cr-number (79-83) and intermediate Mg-number (51-60), but disseminated chromites have consistently low Cr-number (76-79) and low Mg-number (43-52). The plots of TiO₂ versus Cr-number, Al₂O₃ versus TiO₂, and Mg-number versus Cr-number reflect boninitic parental melt for the chromites. Al₂O₃ contents in liquid ranging from 9.67-11.03wt% and TiO₂ contents in liquid 0.42-0.75wt% also correspond for boninitic parentage. The FeO/MgO ratio in liquid ranging from 1.15 to 1.29 for massive chromites and 0.86-1.23 for banded/net-textured chromite, also suggest for boninitic source. Al₂O₃ versus TiO₂ plots indicate that chromites plot in the field of supra-subduction zone peridotites. Boninitic magma was produced from hydrous mantle melting in supra-subduction zone. At supra-subduction setting an oxidizing hydrous fluid derived from subducting slab facilitated the formation of chromitite layers at high fO₂ conditions.

Keywords

Chromites, Boninite, Supra-Subduction Zone, Roro-Jojohatu, Singhbhum Craton.

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S. Viswanathan ¹, Tapan Pal ²

Affiliations
1 IIT - Campus, Powai, Mumbai, IN
2 Geological Survey of India, Kolkata, IN

Abstract

No Abstract.

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Tapan Pal ¹, P. C. Bandopadhyay ¹, S. K. Mitra ¹, S. Raghav ¹

Affiliations
1 Geological Survey of India, Salt Lake, Kolkata - 700 091, IN

Abstract

The 2005 eruption of Barren Volcano was explosive and pulsative Strombolian type (15-30 seconds interval). A thick column of grey smoke along with red-hot pyroclasts was found ejecting out with rumbling sound from the crater. The eruption column of ash particles attained a height of more than 300 m, whereas larger pyroclasts were ejecting out to about 100 m above the crater. The assorted mixture of air fall tephra in the form of blocks, cinders, lapilli and spatter along with ash formed two cinder cones. Pyroclasts are of basaltic composition and show vitrophyric texture with phenocrysts of plagioclase, olivine and diopside set in a glassy groundmass. EPMA of the mineral phases shows that plagioclase phenocrysts are represented by anorthite (An 94.5) and bytownite (An8 1.6-89.0),whereas in the groundmass, feldspar microlites are labradorite (An57.3). Compositionally olivines are forsteritic (Fo72-79), and the clinopyroxenes as phenocrysts are diopside and clinopyroxenes in groundmass as pigeonite. Major oxide analyses of the bulk pyroclastics show that the basalts are high alumina basalt of Island Arc Tholeiitic (IAT) affinity Mineralogical as well as chemical similarities of these basalts with lavas of earlier eruptions from Barren Volcano suggests no major change in magmatic evolution with time in the inner arc setting of Andaman subduction complex.

Keywords

Pyroclast, Ash cloud, Strombolian type, Cinder cone, High alumina basalt, Vitrophyric, Island arc tholeiite, Barren Island, Andamans.

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Tapan Pal ¹, Partha Pratim Chakraborty ², R. N. Ghosh ¹

Affiliations
1 Geological Survey of India, Opn: WB-SK-AN, ER, Geological Survey of India, 5th floor, North Building, Bhubignan Bhawan, DK-6 Block, Salt Lake, Kolkata - 700 091, IN
2 Geology Department, Indian School of Mines, Dhanbad, IN

Abstract

Chromite placers occurring in Rutland coast of Andaman and Nicobar Islands, India, show PGE incidence. Serpentinised dunite containing thin stringers/lenses of chromite occurring adjacent to placer was possibly the source rock of the chromite placers. Chemical analysis of these chromite placers show PGE elements incidence in the range: Ru <10-56 ppb, Ir <10-12 ppb, Rh <10-14 ppb, Pd <10-136 ppb, Pt <10-28 ppb. Normalization of these PGE values against CI-chondrite indicates enrichment of Ir and Ru and depletion of Rh and Pt, a typical character of ophiolitic chromite. Except few Pd values, the PGE values depict highly fractionated PGE distribution pattern. High Nig-olivine (Fo_92-90)and highly fractionated PGE pattern may suggest derivation of the source rock from boninitic melt. Ru-Ir-Os bearing sulphides or alloys were crystallised at early magmatic stage, Pt, Pd were remobilized during hydrothermal activity and concentrated in serpentine.

Keywords

Chromite Placers, PGE Incidence, S-Undersaturation, Boninite, Rultand, Andamans.

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Tapan pal ¹, T. Dutta Gupta ¹, S. C. Das Gupta ¹

Affiliations
1 Project: Andaman, Op: WSA, ER, GSI, GSI Complex, Karunamoyee, DK Block, Salt Lake, Kolkata - 700 091, IN

Abstract

Vitric tuff is being reported from Archipelago Group of South Andaman. The tuff is composed of delicately shaped glass shards with variable amount of microlites and broken phenocrysts of quartz, plagioclase and mica. Major oxide analyses show dacitic composition of the tuff. Petrographic and XRD studies show that the glass is altered to clinoptilolite and analcime. The sedimentary structures in tuff beds suggest these as products of ash turbidite.

Keywords

Vitric tuff (dacitic), Mio-Pliocene, South Andaman.

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S. Nag ¹, R. K. Gaur ¹, Tapan Pal ²

Affiliations
1 Geological Survey of India, Lucknow, IN
2 Geological Survey of India, Calcutta, IN

Abstract

Late Cretaceous-Tertiary sediments together with the underlying Sylhet Trap and Precambrian granites/gneisses are exposed along the southern fringe of Meghalaya plateau. This huge pile of sediments is dissected by E-W and NNE-SSW striking faults. Several workers postulated a direct correlation between tectonism and sedimentation, interpreting the E-W striking Dauki fault as a 'growth fault'. The present work suggests that the sedimentation of this pile is controlled by basin transgression and regression and not by the Dauki or other faults. Dauki fjult being post-Kopili (post-Eocene) in age, could not have acted as a growth fault during the deposition of these Late Cretaceous-Palaeogene sediments.

In the plate tectonic version of Greater India in Gondwanaland, a broadly coeval lithofacies and biofacies assemblages existed in Late Cretaceous-Palaeogene sequences of the southern Tibetan plateau and southern Meghalaya plateau. While the sedimentation in Tibet during Palacogene is dominated by carbonate rocks - except at the KIT boundary (Jidula Formation), the sedimentation in southern Meghalaya is represented by arenaceous and calcareous rocks.

Keywords

Sedimentology, Lithofacies, Structure, Cretaceous, Tertiary, Faunal Assemblage, Dauki Fault, Tibetan Plateau, Meghalaya Plateau, Plate Tectonics.

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