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. Moeen ¹

Affiliations
1 Department of Geology, Andhra University, Visakhapatnam 530 003, IN

Abstract

The fluid inclusions in Vinjamuru baryte can be broadly classified into primary and pseudo-secondary types. Primary inclusions are represented by monophase carbonic and mixed carbonic-aqueous types. CO₂ densities lie in the range of 0.78-0.88 g/cm³. The pseudo-secondary inclusions are represented by aqueous bi-phase type, coexisting with vapor-rich inclusions. Their Th overlaps in the range of 220-230°C suggestive of entrapment from boiling fluids. The primary carbonic inclusions appear to be volcanic entrapments and the fluids in pseudo-secondary inclusions were entrapped from hydrothermal solutions.

Keywords

Fluid Inclusions, Baryte, Vinjamuru, Nellore Schist Belt.

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D. V. Subba Rao ¹, S. Nirmal Charan ¹, S. Moeen ¹

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

Abstract

Sapphirine is found in the pelitic gneisses of Kosavankovilpatti near Usilampatti in Tamil Nadu. It is intimately associated with melt products and the associated minerals are garnet, biotite, orthopyroxene, plagioclase, mesoperthite and K-feldspars. The sapphirine-bearing rock is characterised by (i) enrichment of MgO, depletion of SiO₂ (ii) absence of coronas and symplectites, (iii) presence ofmesoperthites (iv) high X_Mg values of the associated minerals and (v) high P-T conditions (855±20°C and 8.0±O.3 Kb) in contrast to the adjacent sapphiri ne absent rocks (750-800° and P of 6.5-7.5 Kb). This occurrence of prismatic sapphirines is in contrast with the reported sapphirine occurrences where it occurs as symplectites and coronas defining a decompression path (T= 700°C and p 5-7 Kb) in the adjacent Kodaikanal massif. It is suggested that the sapphirine being reported is of prograde origin.

Keywords

Petrology, Precambrian, Sapphirine, Metamorphism and Geothermobarometry.

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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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