Permo‐Carboniferous crustal accretion in the northern Beishan Orogen, Inner Mongolia: Constraints from Sr–Nd–Hf isotopes and zircon U–Pb geochronology of diorites and granitoids

Abstract

To research the Late Palaeozoic accretionary orogeny and vertical crustal accretion in the northern Beishan Orogen, the zircon U–Pb age, zircon Hf, and whole‐rock Sr–Nd isotopic composition from the Permo‐Carboniferous granitoids and diorites at Hazhu were recorded. Zircon laser ablation inductively coupled plasma mass spectrometry U–Pb dating shows that the diorite (323.9 ± 1.4 Ma) and quartz monzodiorite (322.6 ± 1.4 Ma) were emplaced during the Early Carboniferous; the tonalite (317.8 ± 1.4 Ma), granodiorite (318.4 ± 1.3 Ma), and monzogranite (316.0 ± 1.3 Ma) were emplaced during the Late Carboniferous; and the monzogranite (257.8 ± 1.1 Ma) was emplaced during the Late Permian. Compared with the Permian granitoids in this area, the mafic microgranular enclave contents in the Carboniferous granitoids are lower, with relatively high (87Sr/86Sr)i values (0.700812–0.706410), low εNd(T) values (0.95–3.72), and old Nd isotopic model age TDM (730–1,228 Ma). However, these Carboniferous granitoids still exhibit partial melting of juvenile crust. The εHf(T) values of Carboniferous granitoids are high (4.3–14.0) but lower than those of the Permian monzogranite (8.1–12.3). The Hf model age (392–781 Ma) is older than that of the Permian monzogranite (408–585 Ma). Recent regional geological data suggested that the Permo‐Carboniferous granitoids at Hazhu are all of juvenile crustal origin, exhibiting considerable mantle source components. Carboniferous granitoids and diorites have obvious arc magmatic properties, which are formed of juvenile crust during subduction accretion orogeny; Permian granitoids show the addition of more mantle‐derived components, which are formed by mantle‐derived magma underplating in an extensional environment and demonstrate the vertical growth of the crust. Our results provide new evidence for the time limit of the Late Palaeozoic accretion and orogeny in the northern Beishan Orogen and even for the vertical accretion of Phanerozoic crust along the southern margin of the Central Asian Orogenic Belt.