Geochemical and isotopic (Nd and Sr) constraints on elucidating the origin of intrusions from northwest Saveh, Central Iran

Three intrusive granitoid bodies from northwest Saveh, central Iran, are embedded in volcanic sedimentary rocks of the Eocene,forming isolated small outcrops: Khalkhab quartz monzodioritic units (SiO2: ~52-57 wt %) to the northwest, Neshveh granodioriticunits (SiO2: ~62-71 wt %) to the northeast, and Selijerd granodioritic units (SiO2: ~63-69 wt %) to the southeast. The Khalkhab unit iscomposed of quartz monzogabbro and quartz monzodiorite with medium- to coarse-grained textures. The Neshveh unit is composedmainly of granodiorite with subordinate granite of medium grain size and a porphyritic texture with plagioclase megacryst. The Selijerdunit ranges from granodiorite to tonalite with a medium- to coarse-grained granophyric texture. The rocks studied display a relativelyhigh Na2O content,with a molecular A/CNK ratio less than 1.1, Na2O/ K2O ratio of ~2.06 and calc-alkaline affinity. They containmodal clinopyroxene, hornblende, magnetite and titanite, suggesting I-type characteristics for these rocks and formation in an activecontinental margin. Isotopic data (87Sr/86Sr39Ma= 0.704536-0.704860; ε(Nd)39Ma= 2.2-3.9) from northwest Saveh intrusive rocks areplotted to the left of bulk silicate Earth. These ratios, together with geochemical data, suggest that the parent magmas of the rocksstudied might be generated by crystal fractionation of arc basalts in crustal magma chambers, coupled with some lower crustalassimilation prior to silica enrichment, to form quartz monzogabbros. Consequently, granodiorites formed dominantly by crystalfractionation from evolved parental magmas that ascended into the upper crustal magma chambers.