## 5 Discussion
### Tectonic Setting and Implications  
The Cambrian to Early Ordovicican strata distributed in the northern margin of YB reveal a passive continental margin environment [72; 27]. The Middle to Late Ordovician arc-related plutons were reported in the northern margin of YB and indicated that the Junggar Ocean is likely to have begun southwards subduction at least in the Middle Ordovician [4; 5; 18; 72]. The Middle Devonian to Carboniferous arc-related volcanic and intrusive rocks produced by the southward subduction of the Junggar Ocean combined with Late Paleozoic accretionary complexes in the northern margin of YB indicate an active continental margin [73; 74; 75; 37]. The Early Devonian volcanic rocks from Haerdaban area show relative depletion of Nb, Ta, and Ti, and enrichment of LLEs, which are similar to the arc magmas typically generated in subduction-related settings [76; 77; 78]. In the Nb versus Y and Rb versus Y + Nb tectonic discrimination diagrams, these samples plot in the volcanic arc granite (VAG) and within plate granite (WPG) fields (Figure 9). Besides, the samples have relatively high HREE concentrations and low Ti contents, unlikely formed in the rift- or plume-related settings. The Middle-Late Devonian arc-related magmatism are exposed along the northern margin of YB, which were generated in a continental arc setting, indicating the continuous southward subduction of Junggar oceanic crust (or North Tianshan oceanic crust) beneath the Northwestern Chinese Tianshan [11]. This is also supported by the sedimentary evidence. Our investigation found that the Late Paleozoic sedimentary strata in the Biezhentao mountain area, including Lower Devonian Dahalajunshan Formation, Middle Devonian Hanjiga Formation, and Upper Devonian Tuosikuertawu Formation, which are characterized by near-source deposition and contain amounts of arc-related volcanic debris, suggest an active continental margin [79]. Additionally, the Devonian Tulasu Formation in the Tulasu basin also reflected an active margin [14]. Thus, we infer that the northern margin of the YB is an active continental margin in the Early Devonian. Figure 9: Tectonic discrimination diagrams for the Early Devonian volcanic rocks in the Haerdaban area. (**a**) Nb versus Y and (**b**) Rb versus Y + Nb (reference area after [76; 77]). Syn-COLG: syn-collision granites; WPG: within plate granites; VAG: volcanic arc granites; and ORG: ocean ridge granites. The Early Devonian granites (our unpublished data), Early Devonian volcanic rocks (this study), and previous reported magmatism [11; 13; 19] have similar geochemical characteristics (such as enrichment in LILEs relative to HFSEs, and depletion of Nb, Ti, and P) to that of continental arc magma, which can be related to the southward subduction of the Junggar Ocean. The Early-Middle Devonian magmatism suggests a relatively extensive magmatism in the northern margin of YB [11], which is the eastward counterpart of southern limb of the DVB. The DVB and BY magmatic belts were superimposed together in the northern margin of YB, which are different from the spatial-temporal distributions in the western and northern margins, implying a continuous arc-related magmatism. The \(\varepsilon_{\text{Nd}}\)(t) and \(\varepsilon_{\text{Hf}}\)(t) values of these Early Devonian to Permian arc magmatic rocks increase steadily, which are different from that of magmatic rocks generated during the slab retreating (Figure 10a,b) [80; 81; 82]. Therefore, the \(\
eg\)E-W linear trending of arc magmatism in the northern margin of YB combined with the geochemical and Nd-Hf isotope data in this study and previous studies indicate that they were generated in a relatively steady-stated subduction setting without significant slab retreat or advance. The Early Devonian volcanic rocks in this study formed in a continental arc setting related to the southward subduction of Junggar oceanic crust (Figure 11). Figure 11: Tectonic model of the north margin of Yili Block during the Early-Middle Devonian.