petrology and tectono-magmatic environment of the volcanic rocks of west torud – iran

Middle-upper eocene volcanic and volcano-sedimentary rocks of the torud region have been formed by the sequences of basicintermediatelavas, pyroclastic rocks, and sedimentary layers (e.g., siltstone, sandstone, and nummulite-bearing limestone) within ashallow marine basin. according to microscopic studies, the volcanic rocks of the region include basalt, basaltic andesite,trachyandesite, andesite, and dacite. these rocks have originated from the differential crystallization processes and occasionally calcalkalinecontamination geochemical properties. generally, they contain olivine, clinopyroxene, and plagioclase ± amphiboleminerals. porphyritic to megaporphyritic textures with microlithic and flow matrixes are observed in these rocks. studying the mainand rare elements of these rocks indicates that reducing mgo content is accompanied by an increase in al2o3, k2o, na2o, and sio2and a decrease in fe2o3 and cao concentrations. these rocks are mainly enriched in lil and lree elements but depleted of hfselements. the prominent features of these rocks are the presence of positive anomalies in the k, sr, rb, and ba elements, thedepletion of some samples of nb and ta, and their depletion of ti and p. this result reveals the crustal contamination of the mantlemafic magma constructing these rocks. according to the geochemical data, magmatic pollution has not been an effective process inthe formation of these rocks. in addition, the relatively higher levels of cr, ni, and mgo in the alkali basalts of the region indicatethat these rocks are originated from partial melting (5 to 10%) of a spinel-garnet peridotite. overall, they have no subductiondependentrock characteristics and mainly represent characteristics of alkali basaltic magmas of the preliminary back-arc basin(bab). these features, attributed to their calc-alkaline nature, represent the formation of these rocks in a tectonic back-arc setting inthe middle-upper eocene.