Mineral Chemistry, Trace Elements, Isotopic Analysis and Zircon U-Pb Dating in the Hesar Pluton, Northern UDMA, Iran: Implications for Pre-Collisional Magma Mixing

Abstract

The Hesar pluton in the northern Urumieh–Dokhtar magmatic arc hosts numerous mafic-microgranular enclaves (MMEs). Whole rock geochemistry, mineral chemistry, zircon U-Pb and Sr-Nd isotopes were measured. It is suggested that the rocks are metaluminous (A/CNK = 1.32–1.45), subduction-related I-type calc-alkaline gabbro to diorite with similar mineral assemblages and geochemical signatures. The host rocks yielded an U-Pb crystallization age of 37.3 ± 0.4 Ma for gabbro-diorite. MMEs have relatively low SiO₂ contents (52.9–56.6 wt%) and high Mg^# (49.8–58.7), probably reflecting a mantle-derived origin. Chondrite- and mantle-normalized trace element patterns are characterized by LREE and LILE enrichment, HREE and HFSE depletion with slight negative Eu anomalies (Eu/Eu* = 0.86–1.03). The host rocks yield (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.70492–0.70510, positive ε_Nd(t) values of +1.55–+2.06 and T_DM2 of 707–736 Ma, which is consistent with the associated mafic microgranular enclaves ((⁸⁷Sr/⁸⁶Sr)_i = 0.705014, ε_Nd(t) = +1.75, T_DM2 = 729 Ma). All data suggest magma-mixing for enclave and host rock formation, showing a complete equilibration between mixed-mafic and felsic magmas, followed by rapid diffusion. The T_DM1(Nd) and T_DM2(Nd) model ages and U-Pb dating indicate that the host pluton was produced by partial melting of the lower continental crust and subsequent mixing with injected lithospheric mantle-derived magmas in a pre-collisional setting of Arabian–Eurasian plates. Clinopyroxene composition indicates a crystallization temperature of ∼1000°C and a depth of ∼9 km.