Petrography, geochemistry and dating for the Takht-e baz granitoid, north-western of Birjand, east of Iran

The Takht- e baz granitoid in the northwest of Birjand is cropped out in the vicinity of pyroclastic units composing of tuff، marly tuff and breccia. Petrography and major elements analysis show that this intrusion is alkali granite to granite in nature. Common textures in these rocks are granular، graphic، myrmekite، granophyric and perthitic. Potasium feldspars (perthitic orthoclase and microcline)، quartz and sodic plagioclases are essential minerals. Biotite as the only mafic mineral and zircon، titanite and Fe-oxide are the accessory minerals of these rocks. Biotite is altered to chlorite، and feldspars to sericite، calcite and clay minerals. Age dating with zircon uranium-lead method indicated that the alkali granite is 55. 7 ± 0. 6 Ma (early Eocene) old. Chemically، the rock suite is characterized by high total alkali، Fe/Mg، Ga/Al، Hf، REE (except for Eu)، SiO2 and Zr and low contents of Ba، CaO، MgO، P2O5 and Sr. These features along with various geochemical discrimination diagrams suggest that the Takht-e baz granitoid is post-collisional A-type (A2-type). This suite may be derived from melting of continental crust in an extensional setting after collision of the Lut and the Afgan continental blocks in the east of Iran.