Zircon U–Pb age, whole-rock geochemistry and Nd–Sr–Pb isotope constraints on petrogenesis of the Eocene Zajkan gabbro–monzogranite intrusion, Tarom-Hashtjin magmatic belt, NW Iran

Abstract

The late Eocene high-K to shoshonitic Zajkan intrusion, situated in the Tarom-Hashtjin magmatic belt (THMB), NW Iran, ranges in composition from gabbro to pyroxene quartz monzodiorite, pyroxene quartz monzonite and monzogranite, and is classified as an I-type metaluminous granitoid. On the primitive mantle-normalised spider diagram, these rocks demonstrate negative high field strength element (HFSE) anomalies, large-ion lithophile element (LILE) enrichment, and positive Pb anomaly, indicating a subduction-related signature. Furthermore, the granitoids are characterised by coherent chondrite-normalised rare earth element patterns with high light rare earth elements/heavy rare earth elements (LREE/HREE) and moderate to weakly negative Eu anomalies (Eu/Eu* = 0.6–0.86). LA-ICP-MS zircon U–Pb dating demonstrates that the Zajkan intrusion crystallised at ca 36.5 Ma. Relatively low to moderate 87Sr/86Sr ratios (0.7046–0.7061) and positive εNd(t) values (1.52–1.88) of the Zajkan samples are consistent with an enriched lithospheric mantle source that was metasomatised by an earlier subducted slab. Geochemical features show that the parental magma of the Zajkan intrusion likely resulted from a partial melting of metasomatised amphibole-bearing mantle source. Sr–Nd isotope modelling is consistent with the Zajkan intrusion formed from a mix of ∼80% of a mantle-derived magma and ∼20% of a juvenile lower crustal-derived magma. Partial melting of the metasomatised mantle may have resulted from late Eocene lithospheric thinning and hot asthenospheric upwelling through slab roll-back or lithospheric delamination. KEY POINTS The late Eocene high-K to shoshonitic Zajkan intrusion is located within the Tarom-Hashtjin magmatic belt, NW Iran. U–Pb zircon dating revealed an age of ca 36.5 Ma for emplacement of the Zajkan intrusion. Sr–Nd isotope modelling is consistent with the Zajkan intrusion formed from a mix of ∼80% of mantle-derived magma and ∼20% of juvenile lower crustal-derived magma.