Petrology and tectonic setting of volcanic and sub-volcanic rocks in the of Khousf (Southwest of Birjand)

Abstract Abstract The study area is located in 10 km east of Khousf (southwest of Birjand).This area is situated in the northwest margin of suture zone and it’s boundary with Lut block. The area comprises outcrops of Eocene- Oligocene volcanics with dacite, rhyodacite, andesite, trachy andesite, microdiorite, basaltic andesite composition and pyroclastic rocks (tuff, bereccia and agglomerate) which cross- cut of the upper Cretaceous ophiolitic melange and Eocene flysch. The common texture in these rocks is porphyritic with microgranular matrix, glomerophorphyritic and poikilitic Plagioclase, amphibole, biotite and pyroxene are the main phenocrysts in the andesites. Plagioclase, biotite, amphibole and quartz are the main minerals in dacitic rocks. There is also fine crystals of sanidine in rhyodacite and trachy andesites.. Presence of amphibolittic and pelitic xenolites are very common in these rocks. Signs of magma contamination and mixing, such as sieve texture, zoning in minerals, marginization, compaction, corrosion and dusty margins are observed in these units. These rocks belong to the medium to high potassium calc-alkaline series. The trace and REE patterns normalized to primary mantle and chondrite show these volcanics are co- magmatic and high proportion of LREE / HREE in the calc-alkaline rocks represents a subduction-related environment and active continental margin. Geochemical evidence show that these volanics have adakitic nature. Based on petrogenetic diagrams the mentioned adakites resulted from crystallization of melts originated from partial melting metasomatized mantle wedge above subduction zone and or garnet amphibolitic source resulting of metamorphism of thicked lower crust.