Geochemical, U–Pb geochronological, and Hf isotopic characterisation and geological significance of late Caledonian granites in Gushan area of Dagelegou, East Kunlun, China

The East Kunlun orogenic belt, located in the northeastern part of the Qinghai–Tibet Plateau, is distinguished by the widespread distribution of Phanerozoic granites. However, the late evolution of the proto-Tethys remains controversial. In this study, detailed field observations, petrography, geochemistry, zircon U–Pb age, and Hf isotope analyses were performed on the Gushan granites (alkali-feldspar granite and syenogranite) from the Dagele area. This research establishes the formation age and petrogenesis of the granites and discusses the late Caledonian tectonic background of the East Kunlun Orogenic Belt. The laser ablation inductively coupled plasma mass spectrometry zircon data indicate that the Gushan granites range in age from 423 to 422 Ma, assigning them to the late Silurian. Geochemical analyses reveal that the Gushan granites exhibit high K2O, FeO/MgO, 1000Ga/Al, and w(Zr + Nb + Ce + Y) values, low Al2O3 and CaO, enrichment in LILE and REE, and pronounced depletion in Sr, Ba, and Eu. The zircon saturation temperatures for syenogranite range from 814 to 847 °C (average temperature 830 °C), while those for alkali-feldspar granite range from 794–849 °C (average temperature 820 °C), categorising them as A2-type granite of the high potassium calc-alkaline series. The εHf(t) values range from −0.2 to 2.6, predominantly positive, as demonstrated by the Hf isotope analysis of zircon. A two-stage model age (tDM2) spans 1262–1080 Ma, suggesting the granite source is primarily a newly formed basaltic crust with contamination from an ancient silicon–aluminium crust. Tectonic discrimination diagrams of trace elements and their characteristic ratios indicate that the Gushan granites formed during the post-orogenic extension stage of the late Silurian East Kunlun orogenic cycle. Accordingly, it is proposed that the latest closure of the Proto-Tethys Ocean occurred no later than the late Silurian, as opposed to the Early Devonian. The age of the Gushan syenogranite in the Dagelegou area of the East Kunlun is 419.0 ± 5.6 Ma and that of the alkali-feldspar granite 423 ± 5.2 Ma. The Gushan granites have typical A-type granite characteristics and belong to A2 type. The high Rb/Sr and Rb/Nb ratios, and zircon εHf(t) values suggest that the source area of the granites is a new basaltic crust with an ancient aluminosilicate crustal contamination. In this area, the closure time for the Proto-Tethys Ocean is late Silurian.