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

S. H. Jafri ¹, S. Moeen ¹, V. Divakara Rao ¹

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

Abstract

Geochemistry of a Neoproterozoic/Phanerozoic basalt dyke of komatiitic affinity, in the Precambrian metamorphosed gneissic terrain of Schirmacher Oasis, Queen Maud Land, East Antarctica is reported here. The dyke is composed of olivine (Fo_87-90) which is largely pristine, clinopyroxene, magnesiochromite and devitrified glass. Clinopyroxene exhibits microspinifex texture. It has moderately high MgO (14.10-15.10 %), Cr (569-813 ppm), Ni (457-629 ppm) and low K₂O (0.46-0.68 9%) and TiO₂, (0.93-1.00 %) contents and high CaO/AI₂O₃ (1.11-1.22) ratios. The dyke shbws Al-depleted (low Al₂O₃, TiO₂, = 8.36-8.72, depleted HREE Gd_N = Yb_N = 1.66-1.96) signature. suggesting that the source magma was generated by low degree of partial melting of the mantle with garnet in the residue. LREE enrichment (La_N/Yb_N = 3.01-3.51) and negative Nb anomalies in the dyke samples further suggest crustal contamination of the source magma during ascent through the continental crust.

Keywords

Geochemistry, Basalt Dyke, Komatiitic Basalt, Mantle Plurne, Neoproterozoic/phanerozoic, East Antarctica.

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S. H. Jafri ¹, Ramavati Saxena ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

Geochemical data is presented of an anorthosite dyke from Nuggihalli made of calcic plagioclase (An_70-80), amphibole, pyroxene and accessories like magnetite, sphene and apatite. The chemical data shows high Mg/Mg+Fe⁺⁺ ratio (Avg.=0.48) and CaO/Na₂O ratio (Avg.=8.70) with, <5 ppm Rb and 56-90 ppm Sr., indicating a substantial fractionation of ferromagnesium phase from a highly calcic ultrabasic parent magma.

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S. H. Jafri ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

Hagiastrids in chert (Radiolarian chert) associated with Port Blair Series of South Andaman is reported. A rich diversified and fairly well preserved assemblage of Hagiastrids representing 18 species are noticed. Comparison of these Hagiastrids assemblage of the chert from South Andaman with similar Hagiastrids from DSDP (Deep Sea DriHing Project) data suggest an early Cretaceous age for the former. Presence of chert as an inlier within the predominantly sedimentary sequence of Port Blair Series of Eocene age suggests major uplift/tectonic movement in Andarnan-Nicobar basin during Eocene time.

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M. Raza ¹, S. H. Jafri ², S. H. Alvi ¹, M. Shamim Khan ¹

Affiliations
1 Department of Geology, Aligarh Muslim University, Aligarh-202002, IN
2 National Geophysical Research Institute, Uppal Road, Hyderabad - 500007, IN

Abstract

Geochemical data on Proterozoic volcanic rocks of Indian shield overwhelmingly suggest that tlhey were generated in two different tectonic settings. The mafic volcanic suites of Aravalli (Aravalli craton) and Bijawar (Rundelkhand craton) belts and those of lesser Himalayan region appear to be lift related continentat type. On the other hand Delhi volcanics (Aravalli craton), Dongargarh volcanics (Bhandara craton) and Dbanjori-Jagannathpur volcanics (Singhbhum craton) exliibit the characteristic signature of subduction zone magmatism implying the destruction of oceanic lithosphere by processes akin to those of modern plate tectonics. Geochemical and geological evidences suggest the occurrence of a nearly Proterozoic ocean in Central India located between Dharwar-Singbhum-Malwar rotocontinent in the south and Bundelkhand protocontinent in the north. During middle Proterozoic, the oceanic lithosphere subducted towards south along the northern margin of southern protocontinent. The lack of complete calc-alkaline series in the subduction - generated volcanic suites suggests that are volcanism along this margin could not acquire maturity and that the period of subduction was short lived. The collision of the two protocontinents led to the closure of ocean and induced the Satpura - Delhi orogeny at about 1500 m.y. ago-The ophiolitic seqeunce of Phulad (Delhi belt), Dalma (Singhbhum belt) and probably Tal (Bijawar belt) are the possible evidences of closure.

Keywords

Geodynamics, Indian Shield, Proterozoic Volcanism, Crustal Evolulion.

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S. H. Jafri ¹, D. V. Subba Rao ¹, S. M. Ahmad ¹, Ramavati Mathur ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500007, IN

Abstract

Spinifex textured ultramafic rocks occur at Bhaktarahalli in Nuggihalli schist belt and as an enclave within the Peninsular Gneiss near Dodda Gudda, Holenarasipur belt in Hassan District. They have Komatiitic chemistry pointing to ultramafic volcanism in the Dharwar Craton during the Archaean Era.

Keywords

Petrology, Geochemistry, Spinifex Texture, Komatiite, Karnataka.

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S. H. Jafri ¹, V. Shailaja ¹, S. L. Ramesh ¹

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

Abstract

Sitagota mafic volcanics, belonging to the Khairagarh Group of Dongargarh Supergroup, are composed of numerous basaltic flows. One of these basaltic flow is characterized by the occurrence of several morphologies of quench crystals of pyroxene and plagioclase. Plagioclase shows a spherulitic pattern, while pyroxene shows elongate parallel chain, dendritic and spherulitic habits. Most of these crystals are similar to those reported from submarme basalts, lunar basalts, spinifex textured rocks and experimentally produced quench textures. The occurrence of quench crystals, in this basaltic flow suggests that these forms were produced by rapid cooling rates of the basaltic magma, approximately at 2-50°C/hr. Further, the delicate nature of these quench plagioclase and pyroxene crystals, which are confined to the upper part of the flow, suggests in situ crystallization.

Keywords

Quench Crystals, Petrography, Cooling Rate, Sitagota Mafic Volcanics, Dongargarh Supergroup, Central India.

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S. H. Jafri ¹, S. Moeen ¹, N. Charan ¹, B. L. Narayana ¹, V. Divakara Rao ¹

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

Abstract

Presence of several morphologies of quench olivine crystals, which provide important clues on cooling rate(s) of magma, are reported here in a basaltic dyke, from the Precambrian gneissic terrain of Schirmacher Oasis, Queen Maud Land, East Antarctica. The dyke, composed of plagioclase, clinopyroxene, olivine, magnetite and glass, shows intergranular texture. It is alkali basalt in composition with low silica (SiO₂ = 46.20-47.20) and negative Nb anomaly. The morphologies of quench olivine crystals suggcst rapid cooling of the basaltic magma approximately at 15-80°C/hour. Further, the delicate nature of these quench olivine crystals, confined mostly to the glassy margins of the dyke, also suggest in situ crystallisation.

Keywords

Basaltic Dyke, Quench Olivine Crystals, Petrography, Geochemistry, Schirmacher Oasis, East Antarctica.

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