A study of different zoning types in clinopyroxenes of sodic and potassic alkaline volcanic rocks from North Lahrud, NW Iran

Abstract

Clinopyroxene phenocrysts and microphenocrysts in potassic and sodic Eosen alkaline volcanic rocks from the northern Lahrud (NW Iran) record various stages in the crystallization and evolution history of the alkaline melt as well as its origin. The rock series hosting the clinopyroxene phenocrysts is phonolitic tephrite and tephritic phonolite composition. Theses rocks generally show porphyritic texture and have a variable phenocryst-rich nature (25–50%), with phenocryst assemblages characterized by Cpx ± An ± Pl. The studied clinopyroxenes have relatively high Mg-numbers (0.66-0.90), variable Al₂O₃ (3.77-7.31 wt%), low TiO₂ (<2.02 wt%) and Na₂O (<1.23 wt%) contents and low Al^VI/Al^IV ratios (mostly <0.25), suggesting relatively low-pressure crystallizing conditions of the magma in the magma chamber. The calculated pressures for the clinopyroxenes in these rocks vary in the range of 4–5.5 kbars. Oscillatory reverse zoning of clinopyroxenes related to the different crystallization paths under a variable oxygen fugacity and different oxidation conditions and sudden pressure differentiation and magma mixing processes. Normal zoning related to the differentiation and fractional crystallization of the magma. The resorption texture of core parts of some clinopyroxenes are attributed to the changes of crystallization pressure though such textures have been ascribed to magma mixing.