Geochemistry and tectonic setting of high silica adakitic domes of Ahmad Abad Khartouran (South East of Shahrood)

In the Ahmadabad Khartouran region located at 175 Km southeast of Shahrood, a considerable number of adakitic domes crop out, which are intruded into the Paleocene- Eocene volcanosedimentary rocks. Lithological compositions of these domes include andesite, trachyandesite and dacite. Pyroxene (augite), green hornblende and plagioclase are typical mafic and felsic rock forming minerals. With respect to low HREE and high LREE along with other characteristic such as silica content (58.91- 63.41), Na2O more than 3%, Al2O3 more than16%, Yb less than 1.8 ppm, Y less than 18 ppm and K2O/Na2O ratio between 0.98- 2.3, these rocks can be classified as high silica adakite. Enrichment of LREE relative to the HREE and depletion of Nb, Ti, and high concentration of Rb, Ba, K and Th, imply crustal contamination of the mentioned adakitic domes. Enclaves with different sizes and compositions were seen in these domes which indicate contamination and magma mixing with continental crust. Petrographic and geochemical evidence show that the magma forming these rocks originated melting of subducted metamorphosed Neotethys oceanic slab (Sabzevar – Darouneh branch) at the Peak-T conditions of amphibolite facies.