Geochemistry and tectonic setting of Tertiary volcanic rocks from Garejgan area (southwest of Birjand)


1 Department of Geology, Faculty of Sciences, University of Birjand, Birjand, Iran

2 بیرجند- دانشگاه بیرجند- دانشکده علوم گروهزمین شناسی


The study area is located in 60 km to the southwest of Birjand, and northeastern margin of the Lut block. Tertiary volcanic domes of dacite, rhyodacite, trachyandesite along with tuff and breccia are cropped out in this area. The main textures of lavas are porphyritic with microcrystalline groundmass, poikilitic and subophitic. Common phenocrysts in dacite and rhyodacite include plagioclase, quartz, hornblende and biotite; and phenocrysts of trachyandesite are consisting of plagioclase, hornblende, pyroxene, biotite and sanidine. There are also some small bodies of Plio-Quaternary olivine basalt in this area that olivine, pyroxene and plagioclase are main phenocrysts. Common alterations in Tertiary volcanic rocks are silicic, argillic, carbonatizatian and opacitization. Magma series of these rocks are calcalkaline. The trace and REE patterns normalized to primary mantle and chondrite show these acidic and intermediate volcanic rocks are co-magmatic and their enrichment of LREE and LILE and depletion of HREE and HFSE (Nb and Ti), indicating that they are related to subduction zone and active continental margin. Geochemical characteristics such as high values of SiO2 (62.61 wt%), Al2O3 (16.29 wt%), Sr (806 ppm) associated with low Yb (1.7 ppm), Y (15.27 ppm) and no Eu anomaly show that these rocks have adakitic nature. This magmatism can be originated from a garnet amphibolite source that is related to a post collisional tectonic setting.