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

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% similar to 74. 95% , with alkali contents of 5. 01% similar to 9. 21% , suggesting it belongs to metaluminous (A/CNK = 0. 88 similar to 1. 00) ; the measured samples have low P2O5, contents (0. 02% similar to 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 similar to 23. 0) , negative Eu anomaly (delta Eu = 0. 31 similar to 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 epsilon(Hf) (t) values of -3. 1 similar to + 6. 0, two stage model ages (t(DM2)) of 928 similar to 1327Ma, (Sr-87/Sr-86), values of 0. 705607 similar to 0. 708523 , and epsilon(Nd) ( t) values of -4. 1 similar to - 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.