Role of magma mixing in generating of the Gheshlagh–Aftabrow intrusions, SW Buin Zahra, Iran: Evidence for a juvenile origin from geochemical and Sr–Nd isotopic data

Abstract

In this paper, we investigate host granodiorites and associated mafic microgranular enclaves (MMEs), including gabbroic to quartz diorite enclaves from the Middle Eocene Gheshlagh–Aftabrow granitoid, to assess magma mixing and crust–mantle interaction in the Urumieh–Dokhtar magmatic arc. The host granodiorites yield zircon U–Pb ages of ca. 40 Ma. Geochemical and bulk rock Nd–Sr isotopic modelling, together with the widespread occurrence of mafic microgranular enclaves with ellipsoidal and spherical shapes and chilled margins, K‐feldspar and plagioclase megacrysts in the enclaves, and the presence of acicular apatite, ocellar quartz, plagioclase with oscillatory zoning and resorption surfaces, suggests that the mafic microgranular enclaves are globules of a more mafic magma derived from the mantle that was injected into and mixed/mingled with the magma derived from partial melting of a lower crust. Geochemically, the granitoid rocks are high‐K calc‐alkaline and metaluminous (A/CNK = 0.87–0.92) and belong to I‐type suite. These granitoid host rocks have much higher SiO2 (65.4–67.3 wt%) and relatively low Fe2O3 (1.8–2.1 wt%) and MgO (1–1.58 wt%), so compared with the host rocks, the mafic microgranular enclaves have much lower SiO2 contents (50.5–55 wt%), considerably higher Fe2O3 (3.4–4.2 wt%) and MgO (3.9–4.8 wt%). Both the host intrusion and enclaves are enriched in LREE relative to HREEs (LREE/HREE = 3.1–4.1; (La/Yb)N = 2.1–4.1) and show slightly negative Eu anomalies (δEu = 0.75–0.90) and nearly flat HREE patterns ((Gd/Yb)N = 0.86–1.26). Analyses of Sr–Nd isotopes in the host granodiorites and associated enclaves show (87Sr/86Sr)i host = 0.705070–0.705174, εNd(t) host = 0.21–2.31, TDM1 host = 782–983 Ma, (87Sr/86Sr)i enclave = 0.704997–0.705170, εNd(t) enclave = 2.15–2.17, and TDM1 enclave = 874–980 Ma, indicating of juvenile nature for these granites. This characteristic together with similar trace‐element compositions indicates intense magma mixing and a high degree of geochemical equilibration between the host granodiorites and their enclaves.A binary mixing model based on Sr–Nd isotopic compositions of the GAG indicates that rocks were mostly generated by mixing of 7–12% melts produced by partial melting of meta‐igneous old lower crust rocks with 88–93% contribution of mantle‐derived juvenile basaltic magmas.