Author
Abstract
Tertiary andesitic basalts of the Sirjan area, Urmieh-Dokhtar Magmatic Arc, Iran, contain plagioclase phenocrysts enclosed in a matrix of amphibole, clinopyroxene and rare olivine crystals. Textural and compositional evidence suggests two significantly different types of plagioclase phenocrysts occur in the andesitic basalts. The most common type of plagioclase phenocrysts have zoning patterns that display abrupt fluctuations in An content (more than 20 mol %) that correspond to well-developed dissolution surfaces. The less common type of plagioclase phenocrysts is characterised by a core with sieve texture, which is overgrown by oscillatory zoned rims. Changes in temperature, composition and H2O content of the surrounding melt caused the development of resorption zones in the engulfed plagioclases. In addition to the petrographical and mineral chemical evidences, Crystal Size Distribution (CSD) measurements show a kinked plot that reflects the effects of degassing and reveals mixing of two different phases of nucleation and growth. It is assumed that plagioclase phenocrysts originally crystallizing from the host magma were interrupted by mixing with a volatile-rich magma possessing low phenocryst content. The gradual loss of volatiles from plagioclase-saturated intrusive magma could be responsible for the sieve-textured core.
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