A study on Crystal chemistry and zoning types of clinopyroxenes in alkaline volcanic rocks, NW Meshginshahr, Iran

The study area is part of Alborz-Azarbaijan zone exposed in NW Meshginshahr. The volcanic rocks, Eocene in age, include tephritic phonolite and phonolithic Tephrite. They have phenocrysts of clinopyroxene, analcime, plagioclase and alkali feldspar and set in the same minerals in the matrix. Microprobe analyses on polished thin sections were performed by using an electron microprobe analyzer for chemical analyses of the clinopyroxenes. Based on these results, it is concluded that all clinopyroxene phenocrysts are diopsidic in composition and have relatively high Mg-numbers (0.66-0.90), variable Al2O3 (3.39-8.38 wt %), low TiO2 (<2.02 wt %) and Na2O (0.36-1.5 wt %) contents and low AlVI/AlIV ratios (mostly <0.3), suggesting relatively low-pressure crystallization conditions of the magma in the storage region. Also it is revealed they formed at temperature of 1005-1080 ºC and pressure of 4-5.5 kbars. The geochemical studies of clinopyroxenes remarkes they have clinopyroxene characteristics of alkaline rocks related to volcanic arc. Existence of normal and reverse zoning in clinopyroxens deduces differentiation and magma mixing were effective factors on evolution of the area rocks.