Magmatic Interaction as Recorded in Texture andComposltlon of Plagioclase Phenocrysts from the SirjanArea, Urumieh-Dokhtar Magmatic Arc, Iran

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 evidencesuggests two significantly different types of plagioclase phenocrysts occur in theandesitic basalts. the most common type of plagioclase phenocrysts have zoningpatterns that display abrupt fluctuations in an content (more than 20 mol %) thatcorrespond to well-developed dissolution surfaces. the less common type ofplagioclase phenocrysts is characterised by a core with sieve texture, which isovergrown by oscillatory zoned rims. changes in temperature, composition andh2o content of the surrounding melt caused the development of resorption zonesin the engulfed plagioclases. in addition to the petrographical and mineralchemical evidences, crystal size distribution (csd) measurements show akinked plot that reflects the effects of degassing and reveals mixing of twodifferent phases of nucleation and growth. it is assumed that plagioclasephenocrysts originally crystallizing from the host magma were interrupted bymixing with a volatile-rich magma possessing low phenocryst content. thegradual loss of volatiles from plagioclase-saturated intrusive magma could beresponsible for the sieve-textured core.