Geochronologic, geochemical, and isotopic constraints on petrogenesis of the dioritic rocks associated with Fe skarn in the Bisheh area, Eastern Iran

Abstract

There are several intermediate (SiO2 = 57.4–61.2 wt.%) subvolcanic bodies in the Bisheh area of eastern Iran. Petrochemical studies show that these bodies are calc-alkaline and metaluminous (A/NK ≥ 1.68, A/CNK ≤ 0.99) diorite porphyries. They are enriched in large-ion lithophile elements (LILE) and have negative anomalies of Nb, Ti, Ta, and P. Chondrite-normalized REE patterns exhibit light-rare-earth-elements (LREE) enrichment, with mildly fractionated REE patterns ((La/Yb)N < 10). Their Nb/Yb versus Th/Yb ratios are similar to rocks formed in active continental margins. Their isotopic (initial 87Sr/86Sr ratios is 0.70642 and initial eNd values is −1.49) and other geochemical data suggest that the Bisheh diorite porphyries formed by melting of an enriched phlogopite-bearing mantle source combined with subsequent crustal contamination. High values of Rb, Ba, and Th support magma contamination in the upper crust during magma evolution. Zr-U-Pb age dating for two diorite porphyries yield middle Eocene (Bartonian) ages of 39.25 ± 0.43 and 39.16 ± 0.41 Ma. These bodies have intruded into Paleocene limestone and caused metasomatism with iron oxide skarn formations.