The Petrology and Petrogenesis of Tertiary Volcanic Rocks of Jasb Area

The Jasb area is located about 230 Km South of Tehran (10 Km of the N-NE of Delijan) in UrmiyehDokhtar magmatic belt. The scattered volcanic rocks belong to Eocene , Miocene and Pliocene, and they can be classified into two groups: intermediate to basic lava flow and pyroclastic rocks. The volcanic rocks’ type variations are rhyodacite, dacite –andesite, andesite and basaltic andesite. The petrographic studies revealed that the textures of volcanic rocks are mainly porphyritic.The andesite rocks belong to Eocene and Miocene and these rocks are located in the middle of the area that was investigated in this study. Some kinds of this lava are riched in Al2O3, Sr. and Ba. The subvolcanicVolcanic domes of Pliocene that locally cut the upper red formation (Miocene) deposits. The petrographic study shows that they are rhyodacite and that there are euhedral phenocrysts of plagioclase and amphibole in these kinds of rocks . The high viscosity magma had formed the rhyodacite domes. The magmatic series of volcani c rocks except basaltic andesite types that are toleitic are calk alkaline. The change in abundan ce of minerals such as pyroxene , magnetite, amphibole, biotite, plagioclase and feldspar in the volcanic rocks, indicates the successive process of differentiation in intermediate magma; the same change created variation in lithology . Pyroclastic rocks belong to Eocene and have acidic composition (i.e., dacite lithic- crystal tuff); they were formed by explosive volcanic eruptions and were deposited in shallow sea environment. The pattern of Rb/Yb+Nb diagram of the volcanic rocks in the study area shows the similarities with magmatism of Volcanic Arc regions. Based on the analyses results, the geochemical diagrams were drawn and interpreted. Defining the lithology of volcanics , the tectonomagmatic setting condition, magmatic series, and geochemical variations are the results of geochemical and petrological studies of volcanics in the area.