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

Sumana Dasgupta ¹, Jyotisankar Ray ¹, Abhijit Mazumder ¹, Niroj K. Sarkar ², Sumita Das ¹, Chandrani Dasgupta ¹

Affiliations
1 Department of Geology, Calcutta University, 35, Ballygunge Circular Road, Calcutta 700 019, IN
2 59/3, Tatipara Lane, Howrah, West Bengal, IN

Abstract

The porphyritic granite bodies occurring around Raghunathpur have been studied petrographically to find out correlation characteristics among several mineralogical parameters with the help of systematic grid-sampling and modal analyses. Q-A-P diagram clearly suggests that the porphyritic granite bodies belong either to tondite or quartz-rich granitoid. The quartz-rich granitoids of porphyritic granite bodies are found to have distinct spatial control. Average mineralogical composition of porphyritic granite bodies and nature of correlation amongst significant parameters have been worked out following statistical procedures. The correlations among different mineralogical parameters however, yield poor values indicating the effects of assirnilation/granitisationlmagma mixing. A fair degree of silica enrichment, imposed on the crystallising magma parental to the porphyritic granite, has been suggested for its genesis, which is also corroborated by textural evidences.

Keywords

Porphyritic Granite Bodies, Correlation Characteristics, Mineralogy, Petrogenesis, Purulia, West Bengal.

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Kaushik K. Ghosh ¹, Jyotisankar Ray ¹, Keya Nandy ¹

Affiliations
1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Calcutta 700 019, IN

Abstract

The intrusive suite comprises gabbro, dolerite (with hornblende-bearing variant), diorite and granitoids with both porphyritic and non-porphyritic types. A calc-alkaline character, on the acid intrusives is evident. Genetic link amongst the gabbro-dolerite-diorite intrusive members is suggested. The granitoid intrusives, on the other hand. appear to have no direct genetic lineage.

Keywords

Igneous Petrology, Central Indian Suture, Madhya Pradesh.

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Jyotisankar Ray ¹

Affiliations
1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Raod, Calcutta-700 019, IN

Abstract

The chemistry of the mafic chain silicate minerals in the Torappadi layered ultramafic-mafic complex occurring in the southern granulite terrain of the Indian shield has been presented. The pyroxene compositions are found to be sympathetically related to the whole-rock chemistry; further the orthopyroxenes are discriminated to be metamorphic in terms of chemical parameter. All amphiboles of the complex are 'calcic' and their compositions spread from magnesio-hornblende to edenitic homblende/tschermakitic hornblende depending upon the rock-chemistry. Pyroxene geothermometry (assuming complete disordering) using different methods reveals a comparatively large range for a particular rock (up to 181°C). Amphibole geothermometry, on the other hand, gives consistent temperature values (716°-804°C) which has been taken as the range of temperature of equilibration of the whole complex. Estimation of pressure with amphibole-compositions yields a mean value of 4.16 kb. It has been suggested that Torappadi rocks correspond to shallow level - low pressure granulite equivalent to a depth of ca. 10- 17 km.

Keywords

Granulites, Geothermobarometry, Torappadi, Tamil Nadu.

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Sampa Hazra ¹, Jyotisankar Ray ¹, Abhishek Saha ¹

Affiliations
1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata - 700 019, IN

Abstract

The Proterozoic mafic suite of rocks within the Khasi greenstone belt of the Meghalaya plateau, north eastern India, occurs as intrusives into weakly metamorphosed Shillong Group of rocks. This paper highlights field, petrographic and mineral-chemical studies of mafic suite of rocks of the Khasi greenstone occurring near the type area Laitlyngkot in east Khasi Hills district of Meghalaya. Field relations and petrographic studies bring out presence of two broad lithotypes viz. 'massive' and 'foliated' which are integral parts of the Khasi greenstone. Chemical characters of constituent pyroxene grains indicate that they belong to augite field. Amphiboles are 'calcic' in nature and may be further discriminated as magnesio-hornblende, ferro-hornblende, tschermakitic hornblende, ferro-tschermakitic hornblende, tschermakite and ferro-tschermakite. Composition of plagioclase suggests initial magmatic crystallization which was partly re-equilibrated by later metamorphic adjustment. Chemistry of accessory mineral phases like opaque oxides, sphene and biotite is also presented. Use of relevant thermobarometric methods involving pyroxene, amphibole and plagioclase yields a temperature of ∼1200°C to ∼510°C at a pressure of ∼10 Kb to ∼1 Kb. It signifies initiation of magmatic intrusion corresponding to the peak of ideal igneous temperature followed by substantial drop of temperature corresponding to lower range of magmatic crystallization at hydrous condition or later metamorphic reconstitution. Chondrite normalized REE patterns of the mafic suite of rocks indicate a common parent magma with variable degree of differentiation. Importance of PGE-studies to constrain petrogenesis of these rocks has also been discussed.

Keywords

Mineral Chemistry, Thermobarometry, Cooling History, Re-Equilibration, Khasi Greenstone.

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Jyotisankar Ray ¹

Affiliations
1 Dept. of Geology, Univ. of Calcutta, 35 Ballygunge Circular Road, Kolkata - 700 019

Abstract

No Abstract.

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Biswajit Ghosh ¹, John Mahoney ², Jyotisankar Ray ³

Affiliations
1 Geological Survey of India, Op WB-SK-AN, ER, Kolkata - 462 016, IN
2 Department of Geology and Geophysics, University of Hawaii, SOEST, 1680 East-West Road, Honolulu, HI96822, US
3 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata - 700 019, IN

Abstract

The ophiolite assemblage around Mayodia, Dibang Valley, Arunachal Pradesh falls in the eastern extension of the Indus suture belt and is represented by peridotite tectonite hornblendite (dyke) and amphibolite. The entire succession is overlain by metabasalt carapace interlayered with metapelitic pelagic sedimentary rocks. The basal peridotite may be classified as wehrlite. The hornblendite typically occurs as intrusive within the peridotite tectonite and is represented by cumulus primary amphibole. The amphibolite is characterized by well developed gneissose banding. The pillow lava is represented by actinolite-Chlorite-Albite-Epidote schist. The evolutionary trend of the ophiolite suite has been assessed based on major, trace and rare earth element data which favours partial melting of a depleted mantle source. Tectonic discrimination diagrams for the amphibolite and metabasalt clearly indicate their MORB affinity. Such ophiolite assemblage has developed as a result of collision of India and Asia that started with the closing of Tethyan ocean during Mesozoic and Early Tertiary.

Keywords

Mayodia Ophiolite, Petrology, Geochemistry, Tectonic Setting, Northeastern Himalaya, Arunachal Pradesh.

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Sweta Bose ¹, Indrani Chattapodhyay ¹, Jyotisankar Ray ²

Affiliations
1 Department of Geology, University of Calcutta 35, Ballygunge Circular Road, Kolkata - 700 019, IN
2 Present Address University of Hawaii at Manoa, Department of Geology and Geophysics, SOEST, 1680 East-West Road, Hawaii - 96822, US

Abstract

The Dalmg Group of rocks ot Darjeehng-Sikkim Himalaya is often traversed by Lingtse granite occurring as thrust sheets In the piesent contnbuhon, a cohesive petrogenetic model of the Lingtse granite (in Linkey Barapathing stietch of East Sikkim) has been proposed based on critical evaluation of its mode of occurrence, petrographic analyses, chemistry and one-atmosphere experimental studies The investigated Lingtse granite, m general, maintains a very close spatial association with the adjoining schistose country rocks and the planar elements like schistosity and crenulation cleavage are found to be continuous from the country rocks to the Lmgtse granites The poor t orrelation amongst several mineralogical parameters is suggestive of open system melt-rock interation A closei scan of several chemical variation diagiams for the investigated Lingtse granites reveals a wide scatter of the data-plots which may be indicative ot metasomatism/hybridization and other processes related to open system Interestingly, presence of nebeckite in Lingtse granite is experimentally verified to be the product of alkali (soda) - silica metasomatism of the chlonte-biotite muscovite schist country rock The A type characteristics of Lmgtse granite is suggestive of syn-or-post collisional tectonic setting of the host terrain.

Keywords

Lingtse granite, Petrology, Geostatisties, Metasomatism, Experimental studies, East Sikkim.

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Srinanda Chaudhuri ¹, Rajarshi Saha ¹, Jyotisankar Ray ¹, S. K. Bhaduri ²

Affiliations
1 Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata - 700 019, IN
2 Geological Survey of India, Eastern Region, Salt Lake City, Kolkata - 700 091, IN

Abstract

The petrological details of the ultramafic-mafic-alkaline complex related to Sylhet Trap occurring near to Mawpyut (25°25'N:92°10'E) of Jaintia hills district Meghalaya, northeastern India, are poorly known. Field investigations indicate that the Mawpyut body occurs as a pluton distinctly intrusive into adjacent low grade metasedimentary Shillong Group of rocks. This body reveals development of two broad lithotypes namely ultramafic (olivine clinopyroxenite, clinopyroxenite and plagioclase bearing ultramafic) and mafic (mostly gabbro, orthopyroxene gabbro, olivine gabbronorite, mela gabbro and mela-gabbro-norite) with minor presence of later syenitic veins. Though, in general, the pluton shows mineralogical variations, the field boundaries among those petrographic types are not discernible. Careful consideration of major and trace element chemistry of the constituent lithomembers clearly suggest progressive insitu fractionation of a common parent magma.

Keywords

Sylhet Traps, Mawpyut Complex, Major and Trace Element Characters, Differentiation History, Meghalaya.

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Abhishek Saha ¹, Sohini Ganguly ¹, Jyotisankar Ray ¹, Avik Dhang ²

Affiliations
1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata 700 019, IN
2 Aurum S.P.R.L., 59, Avenue Colnol Muzemba, Lubumbashi, CD

Abstract

A new occurrence of (syenite-hosted) Vanadium bearing titaniferous magnetite ore body has been reported from Ganjang (26°09'35" N: 93°20' E), Karbi-Anglong, Northeastern India. The magnetite ore bodies have lumpy and sporadic occurrences within the host syenite pluton intrusive into gneissic country rocks. Ore microscopic studies reveal that magnetite is often associated with haematite and ilmenite depicting different textural patterns. Critical consideration of several elemental patterns suggests magmatic differentiation to be main ore-forming process. The ore body is suggested to have been formed as late stage segregation from a differentiating alkaline magma in a fluid enriched milieu.

Keywords

Vanadium Bearing Titaniferous Magnetite, Karbi-Anglong, Widmanstatten Texture, Magmatic Differentiation, Northeastern India.

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Sampa Hazra ¹, Pritam Saha ¹, Jyotisankar Ray ¹, Avijit Podder ¹

Affiliations
1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata - 700 019, IN

Abstract

The porphyritic granite body (∼600 Ma) around Mylliem (25°32'N:91°52'E), east Khasi hills district, Meghalaya occurs as a distinct intrusive body into the host rock of low-grade meta-sediments belonging to Proterozoic Shillong Group. The porphyritic granite body manifests prominent lath-shaped feldspar phenocrysts giving rise to dominant porphyritic texture. Further, this porphyritic granite body is characterized by primary foliation (defined by parallely arranged feldspar laths), tongues and appophyses and xenoliths of older metamorphics. Petrographically, the Mylliem porphyritic granite is grey leucocratic, coarse grained, phanerocrystalline with modal variants ranging from granite to granodiorite tending to tonalite. Simple statistical studies based on correlation coefficient values involving modal variables of the Mylliem porphyritic granite bodies appear significant in majority of the cases indicating magmatic crystallization. Use of relevant thermometric method indicates temperature of equilibration of the body in the range of 369°C to 507°C. The crystallization of the Mylliem porphyritic granite pluton initiated at an average lithostatic pressure in the tune of ∼11 Kb followed by dominant P_H2O controlled milieu. Spatially projected mineralogical parameters suggest a general trend of the magmatic cooling of the body from margin inward with sudden influx of volatile matters that occasionally offsets mineralogical trends.

Keywords

Mylliem Porphyritic Granite, Mineral Chemistry, Magmatic Crystallization, Statistical Studies, Correlation Coefficients, Meghalaya.

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Sarmistha Mukhopadhyay ¹, Jyotisankar Ray ¹, Basab Chattopadhyay ², Shyamal Sengupta ², Biswajit Ghosh ¹, Subrata Mukhopadhyay ¹

Affiliations
1 Department of Geology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata - 700 019, IN
2 Geological Survey of India, Central Petrological Laboratories, 15 A&B Kyd Street, Kolkata - 700 016, IN

Abstract

The Elagiri complex (12°31' N: 78°35' E) represents one of the important silica - oversaturated syenite plutons of the Southern Granulite Terrane of the Indian shield. This article for the first time reports the mineral chemistry of the Elagiri complex and brings out important petrogenetic significance. The litho-members of Elagiri complex are intrusive into high grade country rocks viz. granite gneiss, amphibolite and pyroxene granulite. The country rocks are foliated bearing evidences of multiple folding and deformation. On the other hand, the constituent litho members of the Elagiri complex (syenites, gabbro and later intrusives marked by lamprophyre and carbonatite) show preservation of igneous layering in terms of discernible parallelism of the constituent minerals. The Elagiri complex shows presence of sharp contacts among litho members and marked absence of chilled facies peripheral to the margin. Electron microprobe data have been critically used to systematize the constituent mineral-phases of the different lithomembers of the complex. Geothermobarometric data indicate a temperature of equilibration in the range of ∼700° to 500° C at ∼2.0 to 5.2 kb which corresponds to shallow level (cf. 18.2 km) equilibration-depth of the complex. Field observations and mineral chemistry data suggest that liquid immiscibility plays an important role during the evolution of the Elagiri complex.

Keywords

Mineral Chemistry, Pyroxene Thermometry, Equilibration Depth, Liquid Immiscibility, Elagiri Complex, Indian Shield.

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Piyali Sengupta ¹, Jyotisankar Ray ¹

Affiliations
1 Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata - 700 019, IN

Abstract

Deccan volcanism with a tremendous burst of volcanic activity marks a unique episode in Indian geological history and covers nearly two third of Peninsular India. Occurrences of mafic sill in the continental basalts are rather rare throughout the flood basalt provinces and only few sporadic reports have been described from different Continental Flood Basalts of the world. In the present article, petrology of mafic sill from the Narshingpur-Lakhnadon section of Eastern Deccan province of India has been presented. The mafic sill in the field is found to occur in a relatively deep valley amidst Gondwana rocks, which occur as the basement of the extrusion. The sill is spatially associated with three initial flows viz. flow I, II and III of adjacent Narshingpur-Harrai-Amarwara section. The sill in its central part is a medium grained rock and petrographically corresponds to dolerite containing augite, plagioclase and rare olivine grains; the chilled facies of the sill is characterized by phenocrysts of olivine, plagioclase and augite that are set in groundmass consisting predominantly of plagioclase, olivine and glass. Mineral chemistry indicates that olivine phenocrystal phase is magnesian (Fo₆₁). Plagioclase phenocrystal composition ranges from An ₅₁ to An ₇₁ whereas the same variation of the groundmass plagioclase composition corresponds to An ₃₁ to An ₆₂. The overlap in the compositions for groundmass and phenocrystal plagioclase may be explained due to fluctuating PH₂O condition. The pyroxene compositions (both groundmass and phenocryst) in majority of the cases are clubbed well within the augite field, however, in a few cases, groundmass compositions are found to fall in the sub-calcic augite and pigeonite field. Some zoned pyroxene phenocrysts, characteristically display different types of zoning patterns. Opaque minerals in the mafic sill are found to be magnetite and ilmenite and this coexisting iron-oxide composition helps to constrain the prevalent fO₂ condition in the parent magma. The geochemistry of the mafic sill and associated basaltic lava flows indicates close genetic link amongst them. Critical consideration of trace elements indicates a distinct enriched mantle source (EM-I/EM-II/HIMU) for the parental magma. Trace element modeling indicates that equilibrium batch- melting of plume source followed by fractionation of olivine, clinopyroxene and plagioclase and subsequent heterogeneous mixing of melt and settled crystals can very well explain the genesis of the mafic sill and the associated basaltic flows.

Keywords

Mafic Sill, Pyroxene Zoning, Thermometry, Ree Geochemistry, Eastern Deccan Province.

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