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 (t_DM2) 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.

东昆仑造山带位于青藏高原东北部，以显生宙（Phanerozoic）花岗岩广泛分布为特征。然而，原特提斯（Proto-Tethys）的晚期演化仍存争议。本研究对达格勒地区孤山花岗岩（含碱性长石花岗岩与正长花岗岩）开展了详细野外观察、岩相学、地球化学、锆石U-Pb定年及Hf同位素分析，旨在明确其形成时代与成因，并探讨东昆仑造山带晚加里东期的构造背景。激光剥蚀电感耦合等离子体质谱锆石数据显示，孤山花岗岩形成年龄介于423–422 Ma，归属于晚志留世。地球化学分析表明，该花岗岩具有高K₂O、FeO/MgO、1000Ga/Al及w(Zr+Nb+Ce+Y)值，低Al₂O₃与CaO含量，富集大离子亲石元素（Large Ion Lithophile Elements，LILE）和稀土元素（Rare Earth Elements，REE），且Sr、Ba、Eu显著亏损。正长花岗岩的锆石饱和温度为814–847°C（平均830°C），碱性长石花岗岩为794–849°C（平均820°C），属于高钾钙碱性系列的A2型花岗岩（A2-type granite）。锆石Hf同位素分析显示，εHf(t)值介于-0.2至2.6之间，以正值为主；两阶段模式年龄（t_DM2）为1262–1080 Ma，提示花岗岩源区主要为新生玄武质地壳，且受古老硅铝质地壳混染。微量元素及其特征比值的构造判别图解表明，孤山花岗岩形成于晚志留世东昆仑造山旋回的造山后伸展阶段。据此推测，原特提斯洋（Proto-Tethys Ocean）的最终闭合时间不晚于晚志留世，而非早泥盆世。东昆仑达格勒沟地区孤山正长花岗岩年龄为419.0±5.6 Ma，碱性长石花岗岩为423±5.2 Ma；该花岗岩具有典型A型花岗岩特征，属A2型。高Rb/Sr、Rb/Nb比值及锆石εHf(t)值进一步表明，其源区为新生玄武质地壳，且受古老铝硅质地壳混染。综上，该区域原特提斯洋的闭合时间为晚志留世。