PETROGRAPHICAL AND GEOCHEMICAL EVIDENCES FOR MAGMA MIXING IN THE

VOLCANIC ROCKS OF THE SAHAND VOLCANO (NORTH-WEST IRAN)

Pirmohammadi F.

Geology Department, Faculty of science, University of Tabriz, Iran, petrofarhad@yahoo.com

Abstract

Nowadays, there are numerous evidences indicating that some magma with intermediate composition have derived from contamination and mixture of magmas with different compositions. Petrographic studies of some tuffs and volcanic rocks in the Sahand area have shown evidences for the magma mixing phenomenon. These evidences are as follows: presence of two (or three) type glass with different chemical composition, existence of basic micropillows in acidic matrix, sieve texture in plagioclase phenocryst, corrosion and resorption of crystals, existence of zonation specially in plagioclase phenocryst and that of quartz xenocrysts with pyroxene reaction rim. According to major and trace element variation diagrams, it is also necessary to mention that the fractionation crystallization has been effective for evaluation of alkali olivine basalt magma. However, the magma mixing process and magma contamination with crust materials have also been responsible for forming the rocks, especially, rocks with intermediate to acid composition, and some anomalies in variation diagrams.

Key words: Sahand, plagioclase, micropillow, xenocryst.