## 5 Discussion  
### Petrogenesis  
The Early Devonian volcanic rocks from the Haerdaban area in the northern margin of YB are characterized by high SiO\({}_{2}\) (70.16 to 77.52 wt.%), Na\({}_{2}\)O+K\({}_{2}\)O (5.10 to 9.56 wt.%), and Zr (156-411 ppm) contents, as well as low concentrations of Al\({}_{2}\)O\({}_{3}\) (11.27 to 16.36 wt.%), CaO (0.21 to 2.57 wt.%), and MgO (0.1 to 1.22 wt.%). On the primitive mantle-normalized spider diagram (Figure 7b), these samples show the strong depletion of Ba, Sr, P, and Ti, and the enrichment of Th, U, and K indicates that they have experienced a notably fractional crystallization. The high differentiation index (DI = 82.6-96.8) and the Zr/Hf ratios (27.1-36.5) suggest the moderately evolved granites attribute [52; 53; 54]. Whole-rock Zr saturation temperatures (T\({}_{\text{Zr}}\)) were calculated and the results have a range from 783 \({}^{\circ}\)C to 952 \({}^{\circ}\)C (average T\({}_{\text{Zr}}\) = 860 \({}^{\circ}\)C). It is similar to those of typical A-type granites (800-900 \({}^{\circ}\)C) [55], but lower than those plume-related A-type granites (930-1050 \({}^{\circ}\)C) [56] and higher than I-type granites and S-type granites [57]. These geochemical signatures, including high SiO\({}_{2}\) and Zr contents, as well as the high zircon saturation temperatures, are consistent with their classification as A-type granite [52; 54; 58]. In the discrimination diagrams (Figure 8a,b), the rocks fall into the field of A-type granites. Sometimes, the high-Si A-type granites may show the similar geochemical characteristics with the high fractionated I-type granites [58]. The high Fe\({}^{\#}\) values (Fe\({}^{\#}\) = FeO\({}^{\text{T}}\)/(FeO\({}^{\text{T}}\) + MgO), from 0.67 to 0.93) and high Zr + Nb + Ce + Y (average 456 ppm) content are obviously different from those high fractionated I-type granites [48; 59; 60]. Figure 7: (**a**) Chondrite-normalized REE patterns and (**b**) primitive mantle-normalized trace element patterns of the Early Devonian volcanic rocks from Haerdaban area in the northern margin of Yili Block. Normalizing values are from [50]. Values of the upper continental crust (UCC) and lower continental crust (LCC) are from [51]. The pathogenesis of A-type alkaline magmas is still a matter of debate. Commonly, the A-type granites can be produced by the fractionation of mantle-derived magma, crust anatexis, or mixing of mantle-derived magma and crustal rock [58; 61; 62]. Fractionation of mantle-derived magma or re-melting of under-plating basaltic rocks generally produces small amounts of silicic magma [63]. Therefore, a significant quantity of mafic magma and mafic residues were required in such a model [64; 65]. The metaluminous to weakly peraluminous composition of the samples are inconsistent with the peralkaline attribute of fractionation mantle-derived magma (Figure 6d) [53; 57]. The lack of mafic rocks, high SiO\({}_{2}\) content with low Ni and Co of samples is indicative of the minor contribution from mantle-derived material in generation. Therefore, we can predict that the volcanic rocks did not directly generate by the fractionation mantle-derived magmas or mixing origin. The low Al\({}_{2}\)O\({}_{3}\)/TiO\({}_{2}\) ratios (36-39) and high CaO/Na\({}_{2}\)O ratios (0.07-1.37), as well as the negative whole-rock \(\varepsilon_{\rm Nd}\)(t) values (\(-1.8\) to \(-4.0\)) indicate a lower crustal source [66]. [PERSON] [67] proposed that the pure crust can melt and form the granite induced by long-lived heat accumulation with no or minimal basalt flux and the heat source chiefly from the radioactive and basal heat flux on the basis of heat budget calculation. In the heat balance models, more than 20 Ma is necessary to accumulate heat and induce the crust melting at a thick crust [67]. As shown in previous studies, there is little or no magmatic event from Early Silurian to Late Silurian (440 Ma to 420 Ma) in the northern margin of YB [9; 18], representing a long-time relatively stable condition.