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

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Petrography and Mineral Chemistry of Neovolcanics Occurring Between Pacific and Nazca Plate Boundaries


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1 Department of Geology, University of Delhi, Delhi - 110 007, India
     

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Mid-Oceanic Ridge Basalt (MORB) samples collected from southern East Pacific Rise (SEPR) have been investigated. These highly phyric plagioclase basalts (HPPB) and moderately phyric plagioclase basalts (MOPB) show rare cumulate and vitrophyric textures with plagioclase (>10% as phenocryst) and abundant glass (>72%). Electron Probe Micro Analysis (EPMA) showed large compositional variations in the megacrysts as well as microcrysts of plagioclase (An62 to An82), olivine (Fo78 to Fo87), pyroxene (ferroaugite to augite) and iron oxides, mostly titaniferous magnetite. Olivine grains show high Mg# (>80%) and distinctly low in NiO (0.01-0.2%). Ferroan trevorite (NiO = 16.22 and FeO(t) = 83.06) a characteristic meteoritic mineral has been identified from the olivine megacrysts of MORB, possibly attributed to Ni-enrichment, resulted from heterogeneity of the lower mantle. Wide range of An composition in plagioclase is indicative of large pressure range of crystal nucleation under decompression at a depth of ∼70 km (An82) up to the ocean spreading centre. Absence of zoning observed in all the minerals present in the MORB samples, possibly attributed to unmixing and dominant fractionation process.

Keywords

Mid Ocean Ridge Basalt, Magmatism, Megacrysts and Microcryst, Ferroan Trevorite.
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  • Petrography and Mineral Chemistry of Neovolcanics Occurring Between Pacific and Nazca Plate Boundaries

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Authors

S. K. Pandey
Department of Geology, University of Delhi, Delhi - 110 007, India
J. P. Shrivastava
Department of Geology, University of Delhi, Delhi - 110 007, India
G. S. Roonwal
Department of Geology, University of Delhi, Delhi - 110 007, India

Abstract


Mid-Oceanic Ridge Basalt (MORB) samples collected from southern East Pacific Rise (SEPR) have been investigated. These highly phyric plagioclase basalts (HPPB) and moderately phyric plagioclase basalts (MOPB) show rare cumulate and vitrophyric textures with plagioclase (>10% as phenocryst) and abundant glass (>72%). Electron Probe Micro Analysis (EPMA) showed large compositional variations in the megacrysts as well as microcrysts of plagioclase (An62 to An82), olivine (Fo78 to Fo87), pyroxene (ferroaugite to augite) and iron oxides, mostly titaniferous magnetite. Olivine grains show high Mg# (>80%) and distinctly low in NiO (0.01-0.2%). Ferroan trevorite (NiO = 16.22 and FeO(t) = 83.06) a characteristic meteoritic mineral has been identified from the olivine megacrysts of MORB, possibly attributed to Ni-enrichment, resulted from heterogeneity of the lower mantle. Wide range of An composition in plagioclase is indicative of large pressure range of crystal nucleation under decompression at a depth of ∼70 km (An82) up to the ocean spreading centre. Absence of zoning observed in all the minerals present in the MORB samples, possibly attributed to unmixing and dominant fractionation process.

Keywords


Mid Ocean Ridge Basalt, Magmatism, Megacrysts and Microcryst, Ferroan Trevorite.

References