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

A. K. Banerjee ¹, S. K. Mitra ²

Affiliations
1 Rajasthan Circle-I, Western Region, Geological Survey of India, Jaipur, IN
2 Technical Co-Ordination Cell, Western Region, Geological Survey of India, Jaipur, IN

Abstract

The varied lithological units of the Precambrian complex, represented by the Pre-Aravallis, Aravallis and Delhis, exhibit a set of complicated structural and metamorphic features that were the result of overprinting of at least four successive cycles of non-affine deformative movements, followed by later minor orogenic impulses spread over a long span of time. The Delhis are characterised by two sets of major folding movements giving rise to NNE-SSW (F₃) and WNW-ESE (F₄) structural trends, while the Aravallis are associated with two completely separate structural trends i.e. E-W (F₁) and NW-SE (F₂), which indicates that the tectonicity varied in space and also in time during the Delhi and Aravalli times. The fold-pattern in the Pre-Aravallis is the same as is noticed in the Aravallis.

The main metamorphic episodes in the Pre-Aravallis, Aravallis and Delhis, which were, sensu stricto, regional in nature, were coeval with the earlier phases of the major folding movements and were followed by dislocation and diaphthoresis in zones of shearing, thereby causing retrograde metamorphism in limited locales. The grade of regional metamorphism, in general, varies between lower greenschist facies to Upper amphibolite facies. The presence of granulites, at places, however, suggests higher temperature and pressure conditions prevailing at least locally. The thermal metamorphic effects in contact with basic and ultrabasic intrusives and with effusive acidic phases are very local.

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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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