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（East Kunlun orogenic belt）地处青藏高原东北部，以广泛分布的显生宙（Phanerozoic）花岗岩为典型特征，但原特提斯洋（Proto-Tethys）的晚期演化机制仍存争议。
本研究针对达戈勒地区的姑山花岗岩（涵盖碱长花岗岩alkali-feldspar granite与正长花岗岩syenogranite）开展了系统的野外地质调查、岩石学、地球化学、锆石U-Pb定年及Hf同位素分析，明确了该类花岗岩的形成时代与成因机理，并探讨了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法（laser ablation inductively coupled plasma mass spectrometry）的锆石测年数据显示，姑山花岗岩的形成时代介于423~422 Ma，归属晚志留世。地球化学分析结果表明，姑山花岗岩具有高K₂O、FeO/MgO比值、1000Ga/Al比值及w(Zr+Nb+Ce+Y)值，低Al₂O₃与CaO含量，富集大离子亲石元素（LILE）和稀土元素（REE），并表现出显著的Sr、Ba及Eu亏损。
正长花岗岩的锆石饱和温度介于814~847 ℃（平均830 ℃），碱长花岗岩的锆石饱和温度为794~849 ℃（平均820 ℃），据此将其归属于高钾钙碱性系列的A2型花岗岩（A2-type granite）。
锆石Hf同位素分析结果显示，其εHf(t)值范围为-0.2~2.6，以正值为主；两阶段模式年龄（t_DM2）为1262~1080 Ma，指示该花岗岩的源区主要为新生玄武质地壳，并受到古老硅铝地壳的混染作用。
微量元素构造判别图解及其特征比值表明，姑山花岗岩形成于晚志留世东昆仑造山旋回的后造山伸展阶段。据此提出，原特提斯洋的最终闭合时代不晚于晚志留世，而非早泥盆世。
东昆仑达戈勒沟地区的姑山正长花岗岩形成时代为419.0±5.6 Ma，碱长花岗岩为423±5.2 Ma。姑山花岗岩具备典型的A型花岗岩（A-type granite）特征，归属于A2型。较高的Rb/Sr、Rb/Nb比值及锆石εHf(t)值进一步佐证，其源区为新生玄武质地壳并遭受了古老硅铝地壳的混染。本研究区原特提斯洋的闭合时代为晚志留世。