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Andesite; dacite; Barren island; Narcondam island; magma mixing.

Andesite and dacite from Barren and Narcondam volcanic islands of Andaman subduction zone are composed of plagioclase, orthopyroxene, clinopyroxene, olivine, titanomagnetite, magnesio-hornblende and rare quartz grains. In this study, we use the results of mineral chemical analyses of the calc-alkaline rock suite of rocks as proxies for magma mixing and mingling processes. Plagioclase, the most dominant mineral, shows zoning which includes oscillatory, patchy, multiple and repetitive zonation and ‘fritted’ or ‘sieve’ textures. Zoning patterns in plagioclase phenocrysts and abrupt fluctuations in An content record different melt conditions in a dynamic magma chamber. ‘Fritted’ zones (An₅₅) are frequently overgrown by thin calcic (An₇₂) plagioclase rims over well-developed dissolution surfaces. These features have probably resulted from mixing of a more silicic magma with the host andesite. Olivine and orthopyroxene with reaction and overgrowth rims (corona) suggest magma mixing processes. We conclude that hybrid magma formed from the mixing of mafic and felsic magma by two-stage processes initial intrusion of hotter mafic melt (andesitic) followed by cooler acidic melt at later stage.

Dwijesh Ray¹ S Rajan¹ Rasik Ravindra¹ Ashim Jana¹

1. National Centre for Antarctic and Ocean Research, Goa 403 804, India.

Published

February 2011

Manuscript received

14 December 2009

Manuscript revised

25 August 2010

Accepted

25 August 2010