Petrography and Geochemistry of Tertiary Volcanic Rocks in the North and West of Birjand and Identification of Their Enclaves

The Tertiary volcanic rocks in the north and west of Birjand are dacite; andesite, basaltic andesite, trachyandesite and rhyolite.They belong to calc- alkaline from magmatic series. Mineralogcal, petrographcal and geochemical evidence show that they have formed in active continental margins. Most andesites and dacites are appeared as volcanic domes or subvolcanic dykes that injected older units contianing Late Cretaceous ophiolite melange and Late Cretaceous and Paleocene flysch. One of the most important characteristic of these rocks is the presence of enclaves with various compositions. These enclaves are xenolithic types including pelitic hornfels, amphibolitic, tonalitic gneiss and autolithic types with andesite (diorite) or dacite (tonalite) compositions. The xenoliths are fragments of older flysch and ophiolite melange units that fall to ascending magma and at least bearing two type metamorphism.The first type of metamorphism is dynamothermal that can be related to emplacement of ophiolite upon continental crust. Along with this metamorphism, flysch units change to slate, phyllite and micaschist; basic units change to amphibolite and acidic unit change to granitic geniss. Second type is contact metamorphism resulting from temperature of lava that caused these enclaves metamorphosed to medium or high grade.