The Source and Tectonic Characteristics of the Boroujerd Granitoid Mass (western Iran)

Boroujerd granitoid mass belongs to Sanandaj – Sirjan zone (SSZ). It consists of granodiorites, quartz diorites and monzogranites. The country rocks into which the Boroujerd granitoid was emplaced consist of greenschist rocks of the regional metamorphism.The statistical technique of discriminant analysis, using major element differences alone, shows that this mass has characteristics of orogenic granitoids (R>0). The Boroujerd granitoid has geochemical characteristics typical of arc intrusives e.g. I-type, high-K calc-alkaline series, metaluminous to weakly peraluminous, a greater enrichment in LILEs (Cs, K, Rb, U, Th) compared to HFSEs (Nb, Ta), depletion in Sr, Ba, P and Ti relative to other trace elements. High Th/Yb (>5) ratios correlated to high values (>10, up to ca. 100) for La/Yb and plot as volcanic arc granites on various discriminant diagrams. This granitoid is a typical representative of a volcanic arc environment, spatially related to an active continental margin. Crustal contamination processes provide a further complication in the interpretation of the Boroujerd rocks. The rarity of primitive magma compositions in arcs, particularly continental margin arcs, points to the important and consistent role of such processes. Probably, Boroujerd granitoid is the result of the subduction of Neo- Tethyan oceanic plate below the Iranian microcontinent.