近日，长安大学栾燕课题组的最新研究探究了扬子板块西北部新元古代构造演化，研究提供了来自汉南地块碧鸡沟中酸性岩浆作用的制约证据。相关论文于2024年8月18日发表在《地球化学学报》杂志上。

本文报道了扬子板块西北缘碧鸡沟正长岩脉和围岩花岗岩的锆石U-Pb年龄、Lu-Hf同位素组成和整体岩石地球化学数据。激光烧蚀电感耦合等离子体质谱(LA-ICP-MS)锆石U-Pb定年结果表明，碧鸡沟正长岩脉和围岩花岗岩形成于770±3.5Ma(MSWD=0.17,n=28)和810±4Ma(MSWD=0.84,n=26)。

正长岩脉的锆石εHf(t)值在+1.52至+5.33(平均+3.05)之间，钾含量高，Nb-Ta负异常，Pb正异常，表明其可能来源于幔源玄武岩浆，并受到俯冲洋板物质的变质作用所改造。围岩花岗岩锆石εHf(t)年龄在+0.71至+5.71(平均±3.06)之间，T_DM2年龄在1344至1659Ma(平均1519Ma)之间，表明花岗岩是由幼年地壳部分熔融形成的。碧鸡沟正长岩和花岗岩的地球化学特征表，明它们形成于大陆边缘弧环境。

因此，结合前人研究，本研究人员认为，在824-720Ma期间扬子板块西北缘存在一个大的俯冲体系，汉南地块的岩浆活动可分为两期；(1)早期岩浆活动(824-790 Ma)源于大陆弧背景下早期或已存在的花岗岩部分熔融的产物，如碧鸡沟、洪庙镇和黄观镇花岗岩；(2)后期岩浆活动(789-718 Ma)，形成于与俯冲有关的弧后伸展环境，包括碧鸡沟正长岩、五都门、二里坝和祖师店花岗岩类。824-720Ma期间扬子板块西北缘的长期俯冲体系表明，扬子板块早期位于罗迪尼亚超大陆的边缘。

据悉，碧鸡沟岩体是中国南方扬子地块西北缘汉南地块中规模最大、分异最优的含铁-钛层状岩体之一。矿区除暴露出与成矿作用有关的基性-超基性岩石外，还出露中-酸性侵入岩，这对了解扬子板块新元古代构造演化具有重要意义，但已往的研究对这些中-酸性岩石的关注较少。碧箕沟基性—超基性层状岩体被花岗岩包围，并被正长岩脉切割。

附：英文原文

Title: Neoproterozoic tectonic evolution of the northwestern Yangtze Block: Constraints from the Bijigou intermediate-acid magmatism in the Hannan massif

Author: Luan, Yan, Ren, Yunxiu, Wang, Ruiting, Sun, Xiaohui

Issue&Volume: 2024-08-18

Abstract: The Bijigou intrusion is one of the largest and most well-differentiated Fe–Ti oxide-bearing layered intrusions in the Hannan massif located in the northwestern margin of the Yangtze Block, South China. Besides the mineralization-related mafic-ultramafic rocks, the intermediate-acid intrusive rocks are also exposed in the mining area, which is of great significance for the understanding the Neoproterozoic tectonic evolution of the Yangtze Block, but studies on these intermediate-acid rocks are scarce. The Bijigou mafic-ultramafic layered intrusion is surrounded by granite and cut by syenite veins. Here, we report new zircon U-Pb ages, Lu-Hf isotope composition and bulk rock geochemical data of the Bijigou syenite vein and wall-rock granite in the northwestern margin of the Yangtze Block. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating results show that the Bijigou syenite vein and wall-rock granite formed at 770±3.5 Ma (MSWD=0.17, n=28) and 810±4 Ma (MSWD=0.84, n=26), respectively. The zircon εHf(t) values of the syenite veins range from+1.52 to+5.33 (average of+3.05), combined with its high potassium contents, negative Nb–Ta anomalies and positive Pb anomalies, suggesting that they may have originated from mantle-derived basaltic magma, which was modified by materials from subducting oceanic slab. The zircon εHf(t) and TDM2 ages of the wall-rock granite range from+0.71 to+5.71 (average+3.06) and 1344 to 1659 Ma (average 1519 Ma), respectively, indicating that the granite was produced by partial melting of juvenile crust. The geochemical characteristics of the Bijigou syenite and granite indicate that they were formed in a continental margin arc setting. Thus, combined with previous studies, it suggests that there was a major subduction system along the northwestern margin of the Yangtze Block during 824–720 Ma, and the magmatism in the Hannan massif was divided into two episodes: (1) early magmatism (824–790 Ma), such as the Bijigou, Hongmiaozhen and Huangguanzhen granitoids, was derived from partial melting of the juvenile or pre-existing crust in a continental arc setting; (2) later magmatism (789–718 Ma), including Bijigou syenite, Wudumen, Erliba and Zushidian granitoids, formed in a subduction-related back-arc extensional environment. The long-term subduction system along the northwestern margin of the Yangtze Block during 824–720 Ma suggests that the Yangtze Block was previously located at the periphery of the Rodinia supercontinent.

DOI: 10.1007/s11631-024-00731-0

Source: https://link.springer.com/article/10.1007/s11631-024-00731-0