f Petrographical and Geochemical Evidences for Magma Mixing in the Volcanic Rocks in the Sahand Volcano

Nowadays there are numerous evidences that indicate some magmas with intermediate composition have derived from contamination and mixing of magmas that have different composition. Petrographic studies on some tuffs and volcanic rocks in the Sahand area have indicated evidences for magma mixing phenomenon. This evidences are included: peresence of two (or three) type glass with different chemical composition, existence of basic micropillows in acidic matrix, existence of sieve texture in plagioclase phenocryst, corrosion and resorption of crystals, existence of zonation specially in plagioclase phenocryst and existence of quartz xenocrysts with pyroxene reaction rim. Also it is necessary to mention, although according to major and trace elements variation diagrams, the fractionation crystallization has been effective for evalution of alkali olivine basalt magma, but magma mixing process and contamination of magma with crustal materials have been also responsible for forming of rocks, specially, rocks with intermediate to acidic composition and made some anomalies in variation diagrams. Key words: Sahand, Plagioclase, Micropillow, Xenocryst.