Abstract

The Miocene volcanic rocks of Mesgar area are composed of andesitic basalt  and various pyroclastic rocks including sandy tuff, lithic tuff, volcanic breccia  and  agglomerate  which are calc-alkaline in composition. The major minerals of the andesitic basalt  are plagioclase and  augite  along with some accessory minerals including hematite.  Minor amount  of secondary minerals such as calcite and chlorite are also present as a result of hydrothermal alteration.  In the lava flows plagioclase and augite are mainly found as phenocrysts which are often corroded and sieved-textured.  Various textures including  Glomeroporphyritic texture, cumulophyric texture, trachytic texture, seriate texture  and porphyritic texture  are observed in the andesitic basalt lavas.  According to the chemical analytical data, the andesitic basalt lava flows are calc-alkaline in composition. These rocks show limited changes in silica content ranging between 58.53 and 59.04 wt%.  They are also characterized with LILE and LREE enrichment along with  HFSE depletion which are in consistent with arc volcanism  characteristics.  The REE normalized pattern of the andesitic basalt lavas is relatively smooth with a slight negative Eu anomaly. Their normalized REE pattern show LREE enrichment relative to HREE which might be due to small degree of partial melting or higher abundance of the LREE in the source rocks. The spider diagram of the andesitic basalt is zigzag with peaks for U, Th and Pb and troughs for Nb, P and Ti.  Tectonic setting of the study volcanic  rocks is an active continental margin and their magmas has been originated from a mantle wedge metasomatized by the fluids released from the subducting slab and also has undergone crustal contamination.