Geochemistry and tectonic setting of the Neoproterozoic granitoidic gneisses from Saghand region, Central Iran

The Neoproterozoic basement granitoidic gneisses are located in the Saghand region in central Iran. Petrographically, S-types granitic gneiss samples show chiefly of K-feldspar, plagioclase, quartz, muscovite, biotite, garnet fibrolite and which display a number of subtypes in terms of structure, texture and mineralogy. Geochemically, I- and S-types granitic gneisses of the Saghand region depict calc–alkalic characteristics. The S–type gneisses are strongly peraluminous, and the I–type gneisses, with relatively medium to low FeOt, TiO2, and CaO/Na2O, Al2O3/TiO2, and Rb/Sr values. The S–type gneisses show strongly negative Sr, Nb and positive Th and Pb anomalies in the primitive mantle– normalized spider diagram. The REE in the basement S–type gneisses are moderate to strongly fractionated with (La/Lu)N =11 to 128, which indicates heterogeneity in the sources for investigated rocks and display small to significant negative Eu anomalies (Eu/Eu* = 0.02–0.11) in the rocks, which is attributed to fractionation of plagioclase. The lesser to moderate fractionated REE patterns (La/Lu)N = 1.43 to 29.02 of I–type gneisses indicate that they derived from varying degrees of partial melting of a tonalite–granodiorite source. Enriched LREE, Y and Yb are attributed to free garnet in the residue, indicating a shallow source for the plutons. The S-type gneisses samples indicate syn–collision and volcanic arc granite (VAG), whereas Itype gneisses are categorized mostly as post–collision granites. The calc–alkaline nature of these rocks suggests that these rocks plot in the fields of island arc and rocks were generated in a thicker crust, probably shortly after the continental margin arc. The S- and I- type granitoidic gneisses may have been emplaced in a syn- to postcollision uplift tectonic setting, consistent with an origin of water saturated magmas with heterogeneous composition derived from different sources of crust.