Geochemistry and petrogenesis of tonalite from Iju area, northwest of Shahr-e Babak (Kerman province), with emphasis on adakitic magmatism


School of Geology, College of Sciences, University of Tehran, Tehran, Iran


The Iju porphyry stock as a part of the Urumieh-Dokhtar magmatic belt is situated 72 km northwest of Shahr-e Babak city and 147 km west-northwest of Sarcheshmeh porphyry copper deposit. The stock, intruded as a hybabyssal Miocene granitoid (9.25±0.5 Ma) the Eocene volcanic-volcanoclastic rocks and induced mineralization and alteration in the region. The existing phenocrysts in the Iju intrusive rocks including tonalite and quartz diorite with plagioclase, alkali feldspar, biotite and hornblende, which reside in a groundmass consisting of quartz and minor potassium feldspar and secondary minerals. The dominant texture in the rocks is porphyry. Based on petrography and geochemical analyses, the tonalitic rocks in the Iju region are classified as calc-alkaline and arc-related continental margin magmatism. High ratios of Ba/Nb (average 101 ppm) and Ba/Ta (average 1326 ppm) as well as the distribution of mantle-normalized trace elements (enrichment in LREE and LILE relative to HREE and HFSE, negative anomalies in Nb and Ta) indicate formation of the Iju granitoid is related to subduction. Evaluation of geochemical evidence including the amounts of SiO2>56%, Al2O3≥15%, MgO<3%, Na2O>3%, Sr>300 ppm, Y<18 ppm, Yb<1.8 ppm, Sr/Yb>20, La/Yb>20, high enrichment in LREE and low enrichment in HREE show that the parent magma of the Iju tonalite has high silica adakitic affinity. The lack of negative anomalies in Eu and Ba in the rock samples point to the lack of influence of partial melting in the lower crust for this unit formation. Thus, the adakitic magma developed as the result of melting of subducting oceanic crust in the mantle and at the final stages of subduction or post-subduction.