Leila Dargahi; Mohsen Arvin; Sara Dargahi
The Alishahi rhyodacitic-dacitic columnar joints are outcropped in the Middle-Upper Eocene Razak complex in the southeastern of Urumieh-Dokhtar volcanic belt in the Dehaj-Sarduieh volcano-sedimentary ...
The Alishahi rhyodacitic-dacitic columnar joints are outcropped in the Middle-Upper Eocene Razak complex in the southeastern of Urumieh-Dokhtar volcanic belt in the Dehaj-Sarduieh volcano-sedimentary belt. The complex consists of intermittent of pyroclastic rocks, andesite, rhyolite, and rhyodacite-dacitic lava flows. The latter partially shows 5 to 6 unequal sided columnar structure in which entablature and colonnade sections along with stria are clearly visible. The predominant textures are phyritic, hyaloporphiritic, glomeroporphyritic, flow and sieve textures along with perlitic cracks. Mineralogically, the Alishahi rhyodacitic-dacitic rocks consist of plagioclase phenocrysts together with rare microphenocrysts of sanidine, orthoclase, hornblende, pyroxene, biotite and accessory minerals including apatite, titanite and opaques are setting in a cryptocrystalline glassy matrix. The matrix is partly in the verge of devitrification process and conversion to mixture of quartz and alkali feldspars. Geochemically, the Alishahi columnar joints are rhyodacitic-dacitic in composition with calc-alkaline nature. Negative Eu anomaly and decrease in Sr content along with increase in Si amount reveal the significance of plagioclase as a differentiated phase. Enrichment in large lithophile elements and depletion in high field strength elements normalized to primitive mantle such as Ti, Ta, Nb along with their chondrite normalized rare earth elements patterns are pointing to their magma formation in a volcanic arc setting in an active continental margin. The depletion in Nb and Ta could be related to their low solubility in aqueous fluids and melts formed under relatively low pressures in the shallow part of the subduction zone.