Petrography, geochemistry and tectonic setting of Tertiary volcanic rocks in the Boshgaz area)northwest of Sarbisheh, Southern Khorasan(

In the Boshgaz area in 50 kilometers northwest of Sarbisheh and eastern margin of Lut block, Tertiary volcanic rocks with basaltic andesite, andesite and dacite composition are cropped out. The main texture of these rocks are porphyry with microlithic-glass groundmass, glomeroporphyritic and vesicular. The main minerals in andesite lavas are plagioclase, pyroxene, amphibole and biotite and in dacites are plagioclase, quartz, amphibole and biotite. Geochemical investigations show that these rocks have medium to high-K calk alkaline nature. Enrichment of Sr, Zr, Cs, Th, K, depletion of Nb, Ti, P, Ba and enrichment of LREE relative to HREE in the studied rocks show that these rocks are related to a subduction zone and active continental margin setting. On the basis of trace elements diagrams, volcanic rocks of the Boshgaz area are formed in an immature arc or early stage of subdution. Mg# values in volcanic rocks of the Boshgaz area are ~48 indicating the role of mantle constituents in their formation. Low ratio of Sm/Yb(0.1), support the existence of phlogopite in the source. The uniform pattern of REEs and medium values of La N /Lu N (8-14)inintermediate and acidic rocks of Boshgaz indicate genetic relationbetween them.