Geochemistry and Sr-Nd Isotopes of the Oligo-Miocene Bagh-e-Khoshk Granitoid in SE of the UDMA, Iran: Implications for Petrogenesis and Geodynamic Setting

Abstract

Oligo-Miocene Bagh-e-Khoshk granitoid stock is intruded into the Eocene volcanic rocks in the southeastern part of the Urumieh-Dukhtar Magmatic assemblage in Iran. The granitoids are mainly consisting of diorite, quartz diorite and granodioritic rock types. They are metaluminous to slightly peraluminous, medium to high K calc-alkaline, with SiO₂ ranging from 50.2 to 66 wt.%. The major elements mostly define linear trends and negative slopes with increasing of SiO₂, while K₂O is positively correlated with silica. There is a higher content of Ba, Rb, Nb and Zr elements with increasing SiO₂, whereas Sr shows an opposite behavior. Primordial mantle-normalized multi-element patterns show enrichment in LILE relative to HFSE with distinctive Nb, Ta, Ti negative anomalies. These signatures are typical of subduction related magmas that formed in an active continental margin. The high Ba/La Ba/TiO₂, Ba/Nb and Th/Nb ratios emphasizes the significant involvement of fluids during subduction processes. The chondrite-normalized REE patterns of the Bagh-e-Khoshk granitoids show an enrichment in light REEs ((La/Yb)n = 3.84, 7.41), very slightly HREE fractionation patterns ([Gd/Yb]n=1.26–1.83) and small positive Eu anomalies (Eu_N/Eu_N* = 1.01, 1.44) in diorites. Whole-rock Sm–Nd isotope analysis give εNd values (+2.91 to +3.29) and Sr ratios (0.7046–0.7053). The geochemical characteristics, positive εNd and low Sr ratios of the Baghe-Khoshk granitoids suggest their formation from partial melting of the mantle wedge source, at pressures below the garnet stability field, modified by fluids during subduction processes.