• Arijit Ray

      Articles written in Journal of Earth System Science

    • Primary volcanic structures from a type section of Deccan Trap flows around Narsingpur-Harrai-Amarwara, central India: Implications for cooling history

      Piyali Sengupta Arijit Ray

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      Field investigations of the Deccan Trap lava sequence along a 70 km traverse in the Narsingpur-Harrai-Amarwara area of central India indicate twenty lava flows comprising a total thickness of around 480 m. Primary volcanic structures like vesicles and cooling joints are conspicuous in this volcanic succession and are used to divide individual flows into three well-defined zones namely the lower colonnade zone, entablature zone, and the upper colonnade zone. The variable nature of these structural zones is used for identification and correlation of lava flows in the field. For twenty lava flows, the thicknesses of upper colonnade zones of eight flows are ∼5 m while those of eight other flows are ∼8 m each. The thicknesses of upper colonnade zones of remaining four flows could not be measured in the field. Using the thicknesses of these upper colonnade zones and standard temperature-flow thickness-cooling time profiles for lava pile, the total cooling time of these sixteen Deccan Trap lava flows has been estimated at 12 to 15 years.

    • Petrology, geochemistry of hornblende gabbro and associated dolerite dyke of Paharpur, Puruliya, West Bengal: Implication for petrogenetic process and tectonic setting

      Aditi Mandal Arijit Ray Mayukhee Debnath Sankar Prasad Paul

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      Paharpur gabbroic intrusive is an arcuate body running east–west paralleling the foliation of Chhotanagpur Granite Gneiss which acts as country rock. The main gabbroic body is intruded by a number of dolerite dykes running north–south. It is composed of clinopyroxene (Wo48En40Fs12–Wo51En40Fs09, mg no. 72–82), plagioclase (An52–An90), hornblende (magnesian hornblende to ferro-tschermackite), orthopyroxene (En76–En79) and ilmenite. Hornblende occurs as large poikilitic grain and constitutes around 60% of the rock. Both gabbro and associated dolerite dykes, show relatively primitive character (mg no. 65–73). Primitive mantle-normalized and MORB-normalized spider diagrams indicate enrichment in Rb, Ba, Th, La, Sr and depletion in Nb, Zr, Y, Ti and Nd. The LILE enrichment and Nb, Ti, Zr, Y depletion suggest arc like geochemical signature for the gabbroic and doleritic rocks of Paharpur. Flat to slightly LREE fractionated pattern and variable degree of REE enrichment is observed. An early stage fractionation of clinopyroxene, plagioclase, orthopyroxene, ilmenite and late stage reaction of cumulate pile and evolved melt/hydrous fluid is suggested for magmatic evolution of gabbro. Associated dolerite dykes, which are geochemically similar to the gabbro, have tholeiitic with boninitic character. The mineralogical and chemical compositions of intrusive rocks also have some similarity with mafic rocks of ophiolite complex of subduction zone.

    • Mineralogical and chemical character- istics of newer Dolerite Dyke around Keonjhar, Orissa: Implication for hydrothermal activity in subduction zone setting

      Piyali Sengupta Arijit Ray Sayantani Pramanik

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      The newer dolerite dykes around Keonjhar within the Singbhum Granite occur in NE–SW, NW–SE and NNE–SSW trends. The mafic dykes of the present study exhibit several mineralogical changes like clouding of plagioclase feldspars, bastitisation of orthopyroxene, and development of fibrous amphibole (tremolite–actinolite) from clinopyroxene, which are all considered products of hydrothermal alterations. This alteration involves addition and subtraction of certain elements. Graphical analyses with alteration index and elemental abundances show that elements like Rb, Ba, Th, La and K have been added during the alteration process, whereas elements like Sc, Cr, Co, Ni, Si, Al, Fe, Mg and Ca have been removed. It is observed that in spite of such chemical alteration, correlation between major and trace elements, characteristic of petrogenetic process, is still preserved. This might reflect systematic alteration (addition or subtraction) of elements without disturbing the original element to element correlation. It has also been established by earlier workers that the evolution of newer dolerite had occurred in an arc-back arc setting which may also be true for newer dolerites of the present study. This is evident from plots of pyroxene composition and whole rock composition of newer dolerite samples in different tectonic discrimination diagrams using immobile elements. The newer dolerite dykes of the Keonjhar area may thus be considered to represent an example of hydrothermal activity on mafic rocks in an arc setting.

    • Petrological and geochemical studies of ultramafic–mafic rocks from the North Puruliya Shear Zone (eastern India)

      Aditi Mandal Arijit Ray

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      Ultramafic and mafic rocks occur within a linear belt, trending nearly E–W along North Puruliya Shear Zone of the Chhotanagpur Gneissic Complex (CGC). These rocks are classified as gabbro, norite, gabbronorite, dolerite, diorite, olivine-websterite and lherzolite. Mafic rocks (Group 1) often occur in association with ultramafic variants (Group 2) and sometimes in isolation. A genetic link has been established between these mafic and ultramafic rocks using disposition of ultramafic and mafic rocks in the outcrop, systematic variation in modal mineralogy, co-linearity of plots in biaxial chemical variation diagram. Chemical composition of biotite and clinopyroxene reveal calc-alkaline nature and arc signature in these mafic–ultramafic rocks and whole rock geochemical characters indicate similarity with arc magma in subduction zone setting. The high values of Mg no. (47–81) and Al2O3 (5.5–17.9) of mafic rocks indicate primitive, aluminous nature of the parental melt and presence of amphibole and biotite indicate its hydrous nature. The parent mafic melt evolved through fractionation of olivine, spinel, clinopyroxene and plagioclase. The crystal cumulates gave rise to the ultramafic rocks and the associated mafic rocks formed from residual melt. Crustal contamination played an important role in magmatic evolution as evident from variation in abundance of Rb in different lithomembers. Mafic–ultramafic rocks of the present study have been compared with intra-cratonic layered complexes, mafic–ultramafic rocks of high grade terrain, Alaskan type ultramafic–mafic complex and ophiolites. It is observed that the ultramafic–mafic rocks of present study have similarity with Alaskan type complex.

    • Geochemical characterisation of the Neoarchaean newer dolerite dykes of the Bahalda region, Singhbhum craton, Odisha, India: Implication for petrogenesis

      PIYALI DASGUPTA ARIJIT RAY TUSHAR MOULI CHAKRABORTI

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      The mafic dyke swarm, newer dolerite dykes (NDDs) intrudes the Archaean Singbhum granite of the Singhbhum craton, eastern India. The present investigation focuses on the petrography and geochemistry of 19 NNE–SSW to NE–SW trending NDDs in two sectors in the northern and south-western part of Bahalda town, Odisha, Singhbhum. Chondrite normalised rare earth element (REE) patterns show light REE (LREE) enrichment among majority of the 13 dykes while the remaining six dykes show a flat REE pattern. Critical analyses of some important trace element ratios like Ba/La, La/Sm, Nb/Y, Ba/Y, Sm/La, Th/La, La/Sm, Nb/Zr, Th/Zr, Hf/Sm, Ta/La and Gd/Yb indicate that the dolerite dykes originated from a heterogeneous spinel peridotite mantle source which was modified by fluids and melts in an arc/back arc setting. REE modelling of these dolerite dykes were attempted on LREE-enriched representative of NDD which shows that these dykes might have been generated by 5–25% partial melting of a modified spinel peridotite source which subsequently suffered around 30% fractional crystallisation of olivine, orthopyroxene and clinopyroxene. The reported age of $\sim 2.75–2.8$ Ma seems to be applicable for these dykes and this magmatism appears to be contemporaneous with major scale anorogenic granitic activity in the Singhbhum craton marking a major event of magmatic activity in eastern India.

    • Reappraisal of the ‘early proterozoic gabbro-anorthosite suite’ rocks from the eastern Singhbhum craton, India: Insights from field features, petrography–mineralogy and geochemistry

      TUSHAR MOULI CHAKRABORTI ARIJIT RAY GAUTAM KUMAR DEB

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      The present study focuses on six mafic–ultramafic units from eastern Indian Singhbhum craton: (i) gabbroic rocks of Galudih, (ii) dolerite from North of Bisoi, (iii) Bangriposi wehrlite (BW), (iv) dolerite from North of Kuliana, (v) Kuliana-layered gabbro (KLG) and (vi) dolerite of Jashipur.These rocks had been grouped earlier as a co-genetic unit titled ‘early proterozoic gabbro-anorthosite suite’. Dolerites of Jashipur, North of Bisoi and North of Kuliana are found as undeformed intrusives into the Mesoarchaean Mayurbhanj granite (MBG) unit and show hydrothermal alteration features and variable LREE enrichment. Multiply-deformed gabbroic rocks of Galudih also intrude the MBGs with similar hydrothermal alteration features and comparatively lower REE abundances. KLG and BW are found as dismembered rocks, emplaced within a multiply-deformed metasedimentary assemblage of Mid-Proterozoic North Singhbhum mobile belt. Through a critical assessment of their field features, petrography, mineralogy and geochemistry, the earlier grouping was found to be invalid. Instead, these rock units should be categorised into three groups: Group I (Galudih), Group II (Jashipur, North of Bisoi, North of Kuliana) and Group III (KLG and BW). Group I rocks are the oldest among them, but their stratigraphic affinity remains unclear. Group II mafic rocks possibly belong to the Proterozoic newer dolerite dykes, whereas Group III represents fragments of oceanic crust and mantle.

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