Petrogenesis of Adakitic Intrusion Bodies of the Ophiolitic Belt, NESabzevar

Some syntectonic tonalitic and trondhjemitic intrusion bodies have been injected into the Soltan Abad (NE Sabzevar) metamorphic complex. The geochemical investigations of these intrusion bodies demonstrated their very obvious adakitic characteristics (very high ratio of Sr/Y and very low contents of Y and Yb). According to the field and microscopic evidences and the geochemical characteristics of the whole intrusion bodies, the creation of such adakitic properties originates from the influence of both partial melting and magmatic differentiation processes. The probable scenario is the partial melting of subducted oceanic lithosphere in a hot subduction zone and the generation of adakitic intermediate melt, then amphibole fractionation and enhancing of adakitic indicators of final acidic melt. The resultant residual of partial melting is garnethornblenditic blocks that observed adjacent to some intrusion bodies. The primary adakitic intermediate melt accurs as amphibole-rich diorite in the study area. The differentiated cumulate phase and final fractionated melt constitute the hornblenditic dykes and hololococratic tonalitictrondhjemitic bodies, respectively. The creation of hot thermal regime and partial melting of subducting oceanic lithosphere have been affected by the factors such as subduction velocity variations, mid oceanic ridge subduction or the slab break-off of the subducting oceanic plate and upwelling of the hot asthenosphere.