As a major tectonic unit along the southwestern rim of the Central Qilian Belt, the Lajishan region plays an important role in the amalgamation of continental plates in the Qilian Orogenic Belt. In this study, zircon laser ablation inductively coupled plasma mass spectrometry for U–Pb dating and Hf isotope analysis, as well as geochemical analysis of intermediate-acid intrusive rocks from the Lajishan area were conducted. Results show that formation ages of the intermediate-acid intrusive rocks are 438–452 Ma. Whole-rock geochemistry results show that the diorite and monzodiorite are metaluminous, low- to medium-K series, and characterized by obvious Nb, Ta, and Ti depletions as well as large-ion lithophile element and light rare earth element enrichments. The ε_Hf(t) values of zircons from monodiorite range from +2.71 to +10.59 and the two-stage model ages range from 751 to 1,253 Ma. A large transversal span of ε_Hf(t) values suggests that the source of depleted mantle was mixed with a small amount of older crustal material. The diorite and monzodiorite show a covariant relationship between ε_Hf(t) values and Th/U ratios, which imply the existence of source heterogeneity. The diorites and monzodiorites were most likely produced in an arc setting. The granites are low K in composition and metaluminous to weakly peraluminous. In view of obvious Nb, Ta, and Ti depletions as well as large-ion lithophile element and light rare earth element enrichments, parent magmas of the granites are suggested to have formed with the participation of crustal source materials. The ε_Hf(t) values of zircons from granite range from +5.82 to +8.90, and the two-stage model ages range from 861 to 1,057 Ma, indicating that magmas of the granites were mainly derived from partial melting of depleted mantle or juvenile thickened lower crust. The granites were most likely produced in an arc setting and share a similar source with the diorite and monzodiorite. Correlation between major oxides and SiO₂ show that during the formation of the three kinds of rocks, crystallization and differentiation both played important roles. Combined with published data from the Central Qilian Belt and its vicinity, our results conclude that the northward subduction of oceanic crust contributed to the formation of an Early Palaeozoic subduction–accretion system in the Lajishan region. This model reveals a tectonic–magmatic thermal event and provides important insights into the tectonic evolution process in the Qilian region, as well as the Central Orogenic Belt during the Early Palaeozoic.

中文翻译：

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中祁连拉脊山地区中酸性侵入岩的侵位年龄和成因：锆石U-Pb年龄、Hf同位素和全岩地球化学的约束

拉脊山地区作为中祁连带西南缘的主要构造单元，在祁连造山带大陆板块的合并中发挥着重要作用。本研究采用锆石激光烧蚀电感耦合等离子体质谱仪进行 U-Pb 定年和 Hf 同位素分析，以及对拉脊山地区中酸性侵入岩的地球化学分析。结果表明，中酸性侵入岩的形成年龄为438～452 Ma。全岩地球化学结果表明，闪长岩和二闪长岩为金属铝质、中低K系列，具有明显的Nb、Ta、Ti贫化以及大离子亲石元素和轻稀土元素富集的特征。的ε_的Hf （t）的单闪长岩中锆石的值范围为 +2.71 至 +10.59，两阶段模型年龄范围为 751 至 1,253 Ma。ε _Hf (t)值的大横向跨度表明耗竭地幔的来源与少量较旧的地壳物质混合。闪长岩和二闪长岩显示出ε _Hf (t)之间的协变关系值和 Th/U 比值，这意味着存在来源异质性。闪长岩和二长闪长岩很可能是在弧形环境中产生的。花岗岩的组成为低 K，金属铝至弱过铝。鉴于明显的Nb、Ta、Ti贫化以及大离子亲石元素和轻稀土元素的富集，认为花岗岩母岩浆是在地壳源物质的参与下形成的。的ε_的Hf （t）的花岗岩锆石的值范围为+5.82～+8.90，两阶段模型年龄范围为861～1057 Ma，表明花岗岩的岩浆主要来源于贫化地幔或幼年增厚下地壳的部分熔融。花岗岩很可能是在弧形环境中生产的，与闪长岩和二闪长岩的来源相似。主要氧化物与SiO ₂之间的相关性表明，在三种岩石的形成过程中，结晶和分异都起到了重要作用。结合中祁连带及其附近已发表的数据，我们的结果得出结论，大洋地壳向北俯冲促成了拉积山地区早古生代俯冲-增生系统的形成。该模型揭示了构造-岩浆热事件，并为了解祁连地区以及早古生代中央造山带的构造演化过程提供了重要见解。