GEOCHEMISTRY AND PETROGENESIS OF MIDDLE EOCENE ALISHAHI DACITIC-RHYODACITIC COLUMNAR JOINTS, SE OF KERMAN, IRAN

The Middle Eocene Alishahi dacitic-rhyodacitic columnar joints are located in the southeastern part of the Urumieh-Dokhtar volcanic belt in the Dehaj-Sarduieyeh volcanic-sedimentary sub-belt of Central Iran. The columnar joints occur as unequal, 5 to 6 sided prisms and show entablature and colonnade sections along with striaes. Although the dominant texture of these rocks is porphyritic, hyaloporphyritic, glomeroporphyritic, flow and sieve textures are detectable in many samples. Perlitic cracks are also visible in thin sections. The main phenocrysts are plagioclase along with a few microphenocrysts of sanidine and orthoclase within a crystallite bearing glassy matrix. The accessory minerals are hornblende, pyroxene, biotite, apatite, sphene and some opaque minerals. The matrix exhibits some degree of devitrification which has resulted a change into a mixture of quartz and alkali feldspars. In general, the columnar joints are affected by secondary alterations such as sericitization, saussuritization and to some extent, kaolinitization. Based on the geochemical studies, it is suggested that Alishahi dacitic-rhyodacitic columnar joints belong to calc-alkaline series and show enrichment in lithophile elements with large ionic radius and depletion in high field strength elements. These observations with chondrite normalized rare elements patterns suggest that these rocks have formed in a continental arc tectonic setting.