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

Abstract

AbstractHesar pluton in northern Urumieh‐Dokhtar‐Magmatic‐Arc hosts numerous Mafic‐Micro‐granular Enclaves (MMEs). Whole rock geochemistry, mineral chemistry, zircon U‐Pb and Sr‐Nd isotope 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 U‐Pb crystallization age of 37.3±0.4Ma for gabbro‐diorite. MMEs have relatively low SiO2 contents (52.9‐56.6wt.%) and high Mg# (49.8‐58.7), probably reflecting a mantle‐derived origin. Chondrite‐ and mantle‐normalized trace element patterns are characterized by LREEs and LILEs enrichment, HREEs and HFSEs 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 ɛNd(t) values of +1.55‐+2.06, and TDM2 of 707‐736 Ma, which is consistent with associated mafic microgranular enclave (87Sr/86Sr)i=0.705014, ɛNd(t)=+1.75, TDM2=729Ma). All data suggest a magma‐mixing for enclaves and host rocks formation, showing a complete equilibration between mixed‐mafic and felsic magmas, followed by rapid diffusion. The TDM1(Nd) and TDM2(Nd) modal ages and U‐Pb dating indicate that 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 ∼9km.