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

Abstract

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