苏州大学孙志远小组对北祁连山造山带北大坂花岗岩进行了锆石U-Pb年代学、地球化学和Sr-Nb-Pb同位素特征研究，以探究其岩石成因和构造意义。相关论文于2024年4月5日发表于国际顶尖学术期刊《地球化学学报》杂志上。

研究表明，锆石U-Pb测年结果显示北大坂花岗岩年龄为468±10Ma，大部分样品含角闪孔，富钾且A/CNK值在0.7-0.9之间，说明奥陶纪中期北大坂花岗岩为富钾、含金属铝的钙碱性花岗岩类。地球化学特征表明，北大坂花岗岩是形成于岛弧背景下的过渡型I/S型花岗岩。初始(⁸⁷Sr/⁸⁶Sr)_i值范围为0.70545-0.71082(平均0.70842)，εNd(t)值为-10.9到-6.7 (平均--8.8)，两期Nd模式年龄(T_DM2)为1.74-2.08Ga，表明北大坂花岗岩源自古元古代的地壳物质。

此外，初始Pb同位素组成(²⁰⁶Pb/²⁰⁴Pb= 19.14-20.26；²⁰⁷Pb/²⁰⁴Pb = 15.71 -15.77；²⁰⁸Pb/²⁰⁴Pb=37.70-38.26)，地球化学特征，如高 Th/Ta(17.43-30.12)和Rb/Nb(6.01-15.49)值，表明北大坂花岗岩岩浆源涉及再循环的火成岩和地壳成分。结合前人已发表的其他早古生代火成岩的年代学和地球化学资料，研究人员认为北祁连洋从岛弧到初始碰撞再到最终闭合的构造转变的时间，可以限定在奥陶纪中-晚期(ca. 468–450Ma).

据研究人员介绍，北祁连造山带（NQOB）的构造演化和地壳增生过程一直存在争议，原因是缺乏完整的限制条件，特别是缺乏从弧形到初始碰撞的构造转换的认识。本文通过对北大坂花岗岩锆石U-Pb年代学、全岩地球化学和Sr-Nd-Pb同位素地球化学的研究，为NQOB的地球动力学演化提供了重要信息。

附：英文原文

Title: Zircon U–Pb geochronologic, geochemical and Sr–Nd–Pb isotope characteristics of the Beidaban granites in the North Qilian Orogenic Belt: Petrogenesis and tectonic implications

Author: Yang, Tao, Sun, Zhi-yuan, Wang, Ming-liang, Zhu, Xiao-qiang, Zhao, Jing-yu

Issue&Volume: 2024-04-05

Abstract: The tectonic evolution and crustal accretion process of the North Qilian Orogenic Belt (NQOB) are still under debate because of a lack of integrated constraints, especially the identification of the tectonic transition from arc to initial collision. Here we present results from zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Pb isotope geochemistry of the Beidaban granites to provide crucial information for geodynamic evolution of NQOB. Zircon U–Pb dating yields an age of 468±10 Ma for the Beidaban granites and most of the Beidaban samples contain amphibole, are potassium-rich, and have A/CNK values ranging from 0.7 to 0.9, illustrating that the Middle Ordovician Beidaban granites are K-rich, metaluminous, calc-alkaline granitoid. The geochemical characteristics indicate that the Beidaban granites are transitional I/S-type granitoids that formed in an arc setting. The isotopic compositions of initial (⁸⁷Sr/⁸⁶Sr)_i values ranging from 0.70545 to 0.71082 (0.70842 on average) and εNd(t) values ranging from -10.9 to -6.7 (-8.8 on average) with two-stage Nd model ages (TDM2) of 1.74–2.08 Ga suggest that the Beidaban granites originated from Paleoproterozoic crustal materials. In addition, the initial Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb=19.14–20.26; ²⁰⁷Pb/²⁰⁴Pb=15.71–15.77; ²⁰⁸Pb/²⁰⁴Pb=37.70–38.26) and geochemical features, such as high Th/Ta (17.43–30.12) and Rb/Nb (6.01–15.49) values, suggest that the Beidaban granite magma source involved recycled crustal components with igneous rocks. Based on these results in combination with previously published geochronological and geochemical data from other early Paleozoic igneous rocks, we suggest that the timing of the tectonic transition from arc to the initial collision to the final closure of the North Qilian Ocean can be constrained to the Middle-Late Ordovician (ca. 468–450 Ma).

DOI: 10.1007/s11631-024-00684-4

Source: https://link.springer.com/article/10.1007/s11631-024-00684-4