Petrology and geochemistry of acidic volcanic rocks in the north of Soleiman Bolaghi (southwest Hashtjin, north of Zanjan) with considering perlitization

The studied area is located in the SE of Mianeh and north of Zanjan، and it is a part of Western Alborz- Azarbaijan structural zone. This area consists of volcanic rocks including rhyolite، rhyodacite، dacite and pumiceous tuff with Oligocene age and andesitic lithic tuff with Eocene age. Acidic volcanic rocks crop out in the form of domes، lava flows and tuffs. Perlites crop out in lower part of the rhyolitic- rhyodacitic domes. All acidic rocks have shoshonitic nature and classified as peraluminous rocks. These rocks are characterized by LREE- rich patterns with high LREE/HREE ratio and distinctive negative Eu anomaly. There are distinctive positive anomalies of Rb، Th and Pb on the spider diagrams which is related to continental crust role in genesis and evolution of the parent magma. On the tectonic setting discrimination diagrams، these rocks are plotted within the Post-COLG setting. The overal petrographic، petrologic and geochemical evidences demonstrate that during the final stages of the continental collision، the thickened sub- continental lithosphere in the Alborz was delaminated and replaced by hot asthenosphere mantle. Rapid upwelling of hot asthenosphere gave rise to the partial melting of asthenosphere which is produced the Oligocene basaltic rocks near the studied area. Emplacement of basaltic magma in the upper part of the continental crust led to the partial melting of the crust and generation of the parent magma of the acidic rocks. Acidic lavas cooled rapidly and formed the glassy rocks such as obsidian which changed to perlite in contact with meteoric and warm waters.