Geochemistry and Petrogenesis of the Adakites in the Southern Domains of the Northern Sabzevar Ophiolites; With Emphasis on Sr-Nd-Pb Isotopes Results

The studied volcanic rocks are situated in NW of the Sabzevar, mainly at southern domains of the Northern Sabzevar ophiolitic belt. Abundant rhyolitic domes (e. g., Sarough, Zard-Kuhi, Kuh-e-Sefid Mehr and Nahr domes) in the southern margin of the Davarzan-Sabzevar ophiolitic belt crosscut the above-mentioned ophiolitic rocks. The Geochemical signatures of these volcanic rocks including enrichment in LREEs and LILEs relative to HREEs and HFSEs, depletion in TNT (Ta-Nb-Ti) elements, metaluminous to peraluminous and medium to high K-calc-alkaline characteristics, and their positions in discrimination tectonomagmatic diagrams, displayed that these rocks derived from a subduction-related environment. These adakitic domes are characterized by adakitic signatures (e. g., 69. 2 wt. % < SiO2، 14. 7 wt. % < Al2O3، MgO < 0. 35 wt. %، Sr > 273 ppm, enrichment in LREEs and LILEs, depletion in Y and HREE (Y < 9. 7 and Yb < 0. 86(and high ratio of Sr/Y > 58 and La/Yb > 12. 1 and belong to high silica adakites (SiO2 ≥ 69. 2 wt. %, MgO = 0. 19-0. 31 wt. %، CaO+Na2O < 7. 62 wt. % and Sr = 273-936). Nd-Sr-Pb isotopes characteristics of the adakitic rocks show similarities to a MORB-type and/or PREMA mantle source, highlighting that their magma (s) is originated from a depleted MORB-type mantle source associated with minor contamination with crustal materials. The geochemical and isotopes characteristics of the Sabzevar adakitic rocks display derivation from partial melting of subducted oceanic crust (garnet amphibolite) associated with subsequent fractional crystallization.