Petrology and geochemistry of Oligocene rhyolite lavas, from south of Qezel Ozan River: post- collision acidic volcanism

Cenozoic acidic volcanic rocks at the south of Qezel Ozan River are located within the Western Alborz-Azarbaijan magmatic zone and northern part of the Iranian-Turkish Plateau. The Oligocene acidic lavas in north part of Zanjan show rhyolitic to dacitic composition.  Hyaloporphyritic to hyalomicrolithic porphyritic textures are the main texture in these rocks. They were erupted along the main faults in this area. Feldspars, biotites and hornblendes are the major phenocrysts which are embedded in a glass matrix or micro phenocrysts of felsic and mafic minerals. By Geochemical studies it is indicated that these rocks have high-K calc-alkaline to shoshonitic nature and classified as meta-aluminous and I-type acidic rocks. In the chondrite normalized rare earth elements diagram, these rocks demonstrate LREE enrichment and high LREE/HREE ratio. Enrichment in LREE and depletion in HREE is characteristic of the calc-alkaline rocks in active continental margins. Furthermore, these rocks show enrichment in LILEs and negative anomalies of HFSEs (Ti, Nb and Ta) which is the feature of magmatic rocks associated with Post-COLG subduction zones. The geochemical evidences suggest that the parental acidic magma is resulted from partial melting of lower crust as a result of pressure reduction during the local tension mechanism.