Late Devonian magmatic event in the South Beishan orogenic belt, Gansu: Constraints from zircon U-Pb chronology, geochemistry and Sr-Nd-Hf isotopes

Abstract

北山造山带处于中亚造山带南缘，研究其岩浆事件对于推演中亚造山带地质构造演化具有重要意义。在甘肃北山南带的双鹰山-花牛山岛弧带中，分布有大量花岗质岩体。本文选取甘肃北山南带双峰山南复式岩体中的花岗闪长岩和二长花岗岩，开展LA-ICP-MS锆石年代学、Hf同位素、全岩地球化学和Sr-Nd同位素研究。研究结果表明该复式岩体成岩时代、地球化学与同位素特征基本一致，具体如下：（1）锆石年代学结果显示二长花岗岩年龄为367.5±1.7Ma、368.0±1.8Ma，花岗闪长岩年龄为366.3±2.2Ma，均为晚泥盆世；（2）复式岩体SiO₂含量为64.39%~74.95%、碱含量为5.01%~9.21%，属准铝质（A/CNK=0.88~1.00），P₂O₅含量低（0.02%~0.17%），且与SiO₂呈负相关，具有典型的I型花岗岩特征；（3）样品总体富集轻稀土元素（（La/Yb）_N=6.23~23.0），具有较强烈的Eu负异常（δEu=0.31~0.55），富集Rb、Th、U等大离子亲石元素，亏损Nb、Ti、P等高场强元素；（4）锆石ε_Hf（t）值为-3.1~+6.0，t_DM2为928~1327Ma，全岩（⁸⁷Sr/⁸⁶Sr）_i在0.705607~0.708523之间，ε_Nd（t）在-4.1~-1.9之间。结合区域构造环境，实验数据表明，双峰山南晚泥盆世复式岩体是在活动大陆边缘的环境下，由幔源岩浆底侵加热下地壳使之部分熔融，并与之混合形成的，代表了晚泥盆世柳园洋向北俯冲的构造事件。;Beishan orogenic belt (BOB) is located on the southern margin of the Central Asian Orogenic Belt (CAOB). The study of magmatic events in BOB is of great significance for understanding the tectonic evolution of the CAOB. A large number of granitiod plutons are distributed in the south of BOB, where the Shuangyingshan and Huaniushan arcs are located. In this study, we focus on granodiorite and monzogranite from the south Shuangfengshan complex in the southern BOB. We present whole rock geochemistry, Sr-Nd isotopic composition, in situ U-Pb dating and Hf isotopic composition of zircons from this complex, from which the following results are obtained:(1) The granodiorites and monzogranites have similar zircon U-Pb chronology, whole rock geochemistry and isotopic composition. (2) Zircon LA-ICP-MS U-Pb dating yields ages of 367.5±1.7Ma and 368.0±1.8Ma for the monzogranite, and 366.3±2.2Ma for the granodiorite, suggesting they were formed in the Late Devonian. (3) The SiO₂ contents from samples collected in the complex are 64.39%~74.95%, with alkali contents of 5.01%~9.21%, suggesting it belongs to metaluminous (A/CNK=0.88~1.00); the measured samples have low P₂O₅ contents (0.02%~0.17%), which are negatively correlated to SiO₂. These features indicated they are of typical I-type granite characteristics. (4) For the REE compositions, this complex is characterized by enriched light rare earth elements ((La/Yb)_N=6.23~23.0), negative Eu anomaly (δEu=0.31~0.55), enriched large ion lithophile elements, such as Rb, Th, U, and depleted high field strength elements such as Nb, Ti, and P. (5) The complex has ε_Hf(t) values of -3.1~+6.0, two stage model ages (t_DM2) of 928~1327Ma, (⁸⁷Sr/⁸⁶Sr)_i values of 0.705607~0.708523, and ε_Nd(t) values of -4.1~-1.9. Combined with the regional geology and tectonic environment, these data reveals that the south Shuangfengshan complex was formed in an active continental margin in Late Devonian, and during its evolution, the lower crust was partially melted by the underplating of mantle-derived magma and mixing with mantle-derived materials, which represented a northward subduction of the Liuyuan Ocean plate in the Late Devonian.