Acta Geodynamica et Geomaterialia

Title: CRYSTAL SIZE DISTRIBUTION OF AMPHIBOLE AND PLAGIOCLASE FROM ZANBIL ADAKITIC DACITES, URMIA- IRAN: EVIDENCE FOR MAGMA MIXING AND TEXTURAL COARSENING
Authors: Modjarrad Monir and Shaykhbaglou Saber
DOI: 10.13168/AGG.2015.0040
Journal: Acta Geodynamica et Geomaterialia, Vol. 13, No. 1 (181), Prague 2016
Full Text: PDF file (6.1 MB)
Keywords: crystal size distribution (CSD), residence time, magma mixing, textural coarsening Zanbil, Iran
Abstract: In this contribution the crystal size distribution (CSD) of amphibole and plagioclase from the Zanbil adakitic dacites of Iran were measured for a series of 20 samples spanning most of the exposed dome. Growth histories, residence time and textural features were studied at the rocks. All amphiboles and plagioclases show non-linear concave up CSD trends, forming a population density with two distinct parts. It can be modeled by mixing two crystal populations, here named fine grains, with intercept values of approximately 12 cm-4 for amphiboles and 7 cm-4 for plagioclase and phenocrysts, with intercept values of 8 cm-4 for amphibole and 4 cm-4 for plagioclase, each with overlapping linear CSDs. The plagioclase CSDs characterized with an individual flat part as a result of textural coarsening. The latest data show that plagioclase nucleation sites are lower than amphiboles, but growth was noticeable. The magmas bearing the fine grains and phenocrysts started to crystallize at 15-20 and 30-38 years, respectively, before each eruption, assuming a growth rate of 10-10 cm/s. Phenocrysts crystallized during longer residence times. Subsequently, new magma containing few or no crystals was introduced and the fine grains crystallized from the mixed magma. Eruption followed 15-20 years after mixing. Such a model suggests that some porphyritic volcanic rocks erupted from shallow magma chambers that were never fully emptied.