Browsing by Author "Anant Shastry"

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Fe-Ti-enriched mafic rocks from south Andaman ophiolite suite: Implication of late stage liquid immiscibility
(2001) Anant Shastry; Rajesh K. Srivastava; R. Chandra; George A. Jenner
Andaman ophiolite suite (AOS) is amongst one of the ophiolite complexes of the world, which comprises complete dismembered ophiolitic sequence. It consists of ultra-mafic and mafic cumulates, sheeted dykes, pillow lavas, felsic rocks (mainly plagiogranites), and sedimentaries, including ribbon chert. In the present communication, a mafic rock that is highly enriched in FeO-TiO2 and encountered in the ophiolites of south Andaman is presented. Fe-Ti-enriched mafic rock is not yet reported from the south AOS. Genetically this rock may be associated with leucocratic rocks of AOS because it is experimentally proved that Fe-enriched basaltic melt and plagiogranite melt can be generated by late stage liquid immiscibility. Similar to most of the other plagiogranite occurrences of the world, Andaman plagiogranites are also supposed to be end-products of magmatic differentiation of a mafic magma. But the discovery of Fe-Ti-enriched mafic rocks supports liquid immiscibility process.
Geochemical characteristics and genesis of oceanic plagiogranites associated with South Andaman ophiolite suite, India: A late stage silicate liquid immiscible product
(2002) Anant Shastry; Rajesh K. Srivastava; R. Chandra; George A. Jenner
Plagiogranite is a leucocratic low-K member of the Andaman Ophiolite Suite (AOS) that occurs as bands (dykes?) of irregular shapes and sizes, particularly in the cumulate sequence of the suite. It is medium-to coarse-grained and commonly shows hypidiomorphic granular texture. At places granophyric texture (intergrowth between feldspar and quartz) is also seen. The main mineral constituents are plagioclase (mainly albite), quartz and minor mafic minerals. Mineralogically and chemically these rocks are classified as low-K dacite (trondhjemite/tonalite type). Low Rb concentrations (< 5) and low Rb/Sr ratios (< 0.015), observed in studied plagiogranites and associated Fe-Ti enriched rock, are characteristic of oceanic derivatives. All elements, plotted on the primordial mantle normalized multi-element spidergrams show enriched patterns. Variations in the concentrations of Nb, Sr, Zr, and Y are limited but other elements show wide variations. Strong negative anomalies for Nb are also observed. These rocks show characters analogous to those of orogenic granites, emplaced in volcanic are settings. On the basis of observed petrological and geochemical characteristics and their spatial association with Fe-Ti enriched mafic rocks, it is concluded that the Andaman plagiogranites are the integral part of AOS having crystallized from late stage immiscible silicate liquid.
High-Ti type N-MORB parentage of basalts from the south Andaman ophiolite suite, India
(Indian Academy of Sciences, 2004) Rajesh K. Srivastava; R. Chandra; Anant Shastry
A complete dismembered sequence of ophiolite is well exposed in the south Andaman region that mainly comprises ultramafic cumulates, serpentinite mafic plutonic and dyke rocks, pillow lava, radiolarian chert, and plagiogranite. Pillow lavas of basaltic composition occupy a major part of the Andaman ophiolite suite (AOS). These basalts are well exposed all along the east coast of southern part of the south AOS. Although these basalts are altered due to low-grade metamorphism and late hydrothermal processes, their igneous textures are still preserved. These basalts are mostly either aphyric or phyric in nature. Aphyric type exhibits intersertal or variolitic textures, whereas phyric variety shows porphyritic or sub-ophitic textures. The content of alkalies and silica classify these basalts as sub-alkaline basalts and alkaline basalts. A few samples show basaltic andesite, trachybasalt, or basanitic chemical composition. High-field strength element (HFSE) geochemistry suggests that studied basalt samples are probably derived from similar parental magmas. Al2O3/TiO 2 and CaO/TiO2 ratios classify these basalts as high-Ti type basalt. On the basis of these ratios and many discriminant functions and diagrams, it is suggested that the studied basalts, associated with Andaman ophiolite suite, were derived from magma similar to N-MORB and emplaced in the mid-oceanic ridge tectonic setting. © Printed in India.
Trace and rare earth elements geochemistry of alkaline rocks of Sarnu-Dandali, Barmer, Rajasthan
(1996) Anant Shastry; Santosh Kumar
The alkaline rocks belonging to ijolite series and carbonatites constitute the Sarnu-Dandali igneous complex (68.5 Ma) associated with Precambrian acid volcanics. Trace elements including REE of ten selected Late Cretaceous rock-types viz. melteigite, melanephelinite, phonolite, carbonatite, basalt and Precambrian rhyolites of SDIC have been examined and discussed. The data of alkaline rocks suggest their derivation from an alkaline (ijolitic) magma formed by the partial melting of mantle source. Ba-Sr-REE enriched carbonatites possibly represent an immiscible split from nephelinite. The basalt (Hawaiite) represents a subalkaline magmatic pulse unrelated to alkaline suite. The associated Precambrian rhyolites have formed by a combination of magmatic processes such as partial melting, fractional crystallization and/or magma-mixing.