Mineral chemistry, Thermobarometry, Geochemistry and tectonic setting of Tertiary andesitic lavas in the Shourestan area (west of Sarbisheh), Southern Khorasan

In the Shourestan area, 14 kilometers west of Sarbisheh city, in South Khorasan province, volcanic rocks with pyroxene-andesite composition belonging to Eocene-Oligocene are exposed. The constituent minerals of these rocks include plagioclase and pyroxene. The composition of plagioclases have range from Ab32, An68 to Ab58, An42 and are andesine-labradorite type. Clinopyroxene and orthopyroxene have diopside-like augite and enstatite composition, respectively. The crystallization temperature for clinopyroxene and orthopyroxene were about 1175 and 1200 °C respectively and the pressure (for both types) was 2 to 5 kb. The geochemical data of whole rocks show that the andesitic lavas of Shourestan have high potassium calc-alkaline nature and the amount of Mg# in them varies from 40.97 to 60.97, which indicates the role of mantle components in their formation. These rocks show signs of differentiation including LREE/HREE ((La/Yb)N) between 9.95 to 12.42, LREE/MREE ((La/Sm)N) between 3.53 to 6.55, MREE/HREE ((Sm/Yb)N) between 1.89 to 2.99. High ratios of Zr/Nb (9.81-22.10), Th/Nb (0.68-1.79), Th/Ta (7.29-24), and Nb/Ta (9.69-15.66) along with the pattern of LIL elements, support the possibility of different degrees of crustal contamination-assimilation of magma during its ascent to the earth's surface. The studied rocks have low ratios of Ce/Y (2.44-3.48), (Tb/Yb)N (1.17-1.39), Sm/Yb (1.92-2.78), and relatively flat MREE-HREE pattern that confirms the melting of the subcontinental lithospheric mantle in the field of spinel stability and at a depth of fewer than 75 kilometers.