Late Paleozoic–Early Mesozoic granitoids in the Khangay–Khentey basin, Central Mongolia: Implication for the tectonic evolution of the Mongol–Okhotsk Ocean margin

The Mongol–Okhotsk Belt is the youngest segment of the Central Asian Orogenic Belt, which is the venue of the massive juvenile crust emplacement, and it is formation and evolutions are still pending problems. This paper presents the first up-to-date U-Pb zircon ages, Hf-in-zircon isotope, geochemical and whole-rock Nd isotope data from igneous rocks of the Khangay–Khentey basin, Central Mongolia. The U-Pb zircon ages indicate three groups of magmatism at ~296 Ma, ~280 Ma, and ~230 Ma. The ~296 Ma Group I magmatic rocks are characterized by negative eHf(t) and eNd(t) values and old two-stage Hf and Nd model ages suggesting their derivation by the melting of the crustal source. The ~280 Ma Group II rocks A2-type monzonites, granitoids, and rhyolites show positive eHf(t) and eNd(t) values and Neoproterozoic Hf and Nd model ages. The geochemical and isotope data suggest that Group II granitoids and rhyolites were derived by the underplating-related melting of a juvenile crustal source. The ~230 Ma Group III rock assemblage includes granitoids and volcanic rocks. The I-type calc-alkaline granitoids are enriched in K, Rb, U, and Th. The geochemical characteristics suggest that they have formed by the melting of a hornblende-bearing crustal source with the participation of fluids separated from the subducting slab. The positive eHf(t) and eNd(t) Group III rocks suggest partial melting of a depleted lower crustal material with the contribution of ancient crustal material. The Group I granitoids possess coherent/coupled Nd-Hf isotopic compositions supporting their origin from the ancient crust. However, the Nd-Hf isotopic compositions of the Group II and III igneous rocks are decoupled. Such isotope decoupling could have resulted from the involvement of sediments in magma generation and the partial melting of decoupled Nd-Hf-bearing or juvenile lower crust. The small number of Group I samples, and their heterogeneous composition make differentiation of their tectonic setting problematic. Group II samples could have formed in a setting of local extension environment, whereas Group III granitoids were probably emplaced at an Andean-type active continental margin during the subduction of the Mongol–Okhotsk Ocean beneath the Central Mongolia–Erguna Block.