The West Junggar Faults System is located between the Chenggis-Junggar Fault, the Tianshan Fault, and the Irtysh Strike-slip Fault (Figure 1B). The region is characterized by large-scale left or right lateral strike-slip faults (Chen et al., 2009; Chen et al., 2015; Han et al., 2019; Ding et al., 2020). The left-lateral strike-slip faults consist of three large-scale faults: the Darabut Fault, the Karamay Fault, and the Baerluke Fault. And a series of smaller-scale secondary faults. The right-lateral strike-slip faults are primarily composed of large-scale faults, including the Zalulshan-Labashan Fault and the Abbey Lake Fault (Figure 2).

Sedimentary strata have been widely distributed in the West Junggar area since the Phanerozoic era, with the Paleozoic era being the most developed (He et al., 2004; Figure 2). The sedimentary strata can be divided into two parts: one is the Early Paleozoic metamorphic stratigraphy, which is dominated by flysch formation and ophiolite mélange, with strong metamorphism and deformation; the other is the Late Paleozoic pyroclastic sedimentary formation, which consists of giant-thick marine-continental slope-face sedimentary rocks (Zhu et al., 2013).

Regional ophiolite mélanges such as Darabut, Karamay, and Baerluke are distributed along the main fault zones (Figure 2). The rock compositions include serpentinite peridotite, dunite, serpentine, pyroxenite, gabbro, diabase, basalt, oceanic plagiogranite, and siliceous rocks.

The Late Paleozoic magmatic activity in the region was intense, with abundant rock types, divided into large granitic basement and small and medium-sized rock strains (Figure 2).

The granite basement comprises the Miaoergou, Kurumusu, Karamay, and Hatu batholiths. The rocks consist mainly of A-type granites, such as alkaline and monzonitic granite. These rocks are characterized by high silica, quasi-alumina-weak per aluminum, high-K calc-alkaline, low magnesium, and ferrum, which belong to the post-collisional orogenic stage of the Late Carboniferous-Early Permian (Han et al., 2006; Su et al., 2006; Zhou et al., 2008).

The small and medium-sized rock strains are dispersed, and the rocks consist mainly of granodiorite, biotite granite, quartz diorite, and diorite porphyrite.

These rocks may have formed during the Middle-Late Carboniferous subduction of the mid-ocean ridge or in the island arc environment, and some of the island arc granites have obvious adakite characteristics (Geng et al., 2009; Zhang et al., 2006; Tang et al., 2009; Tang et al., 2010a; Tang et al., 2010b; Shen et al., 2009).

The West Junggar area is characterized by higher (¹⁴³ Nd/¹⁴⁴ Nd)i values and positive ε _Nd (t) values, with an overall range of εNd(t) values of 2.29–8.75, and an overall range of crustal model ages of 345–851 Ma (Xu et al., 2022a). The basement of the West Junggar region is dominated by juvenile crust (Figure 3). The Nd isotope maps show the distribution of isotope provinces, which are divided into three-high, three-medium, and one-low, with material sources from Paleoproterozoic, middle-late Neoproterozoic, and early Neoproterozoic material respectively.

The isotopic ratio range for granites (⁸⁷Sr/⁸⁶Sr)_i in the West Junggar region is considerably wide, spanning from 0.697,397 to 0.708,336, which is coupled with the Nd isotopic signatures (Xu et al., 2022a). The Sr isotopic characteristics also suggest that the crust of the West Junggar area mainly originated from the juvenile crust, with minimal ancient crustal material. The Sr isotope maps show the distribution characteristics, with ultra-low-value, low-value, and medium-low-value provinces (Figure 4).

The ratio range for (²⁰⁶Pb/²⁰⁴Pb)_i of the West Junggar area spans from 17.4975 to 19.0352 (Xu et al., 2022a), indicating that the crust of the study region did not mixed the ancient crust during the Late Paleozoic magmatic intrusion process. The Pb isotope maps show the distribution characteristics, with high-value, medium-value, and low-value provinces (Figure 5).

The West Junggar region is characterized by higher ¹⁷⁶Hf/¹⁷⁷Hf ratios and positive ε _Hf (t) values, with individual sample values displaying a broad range of variability. The overall mean range of εNd(t) values and crustal model ages are 6.67–16.82 and 251–893 Ma respectively The minimum value of ε _Hf (t) ranges from 2.79 to 14.49, and the maximum value of crustal model age is 425–1,148 Ma. The crust of the West Junggar area is mainly juvenile crust, which can be divided into three levels of Hf isotope provinces (Figure 6): High ε _Hf value province, Medium ε _Hf value province, and Low ε _Hf value provinces. The high ε _Hf values provinces are mainly distributed in the numerous rock masses proximal to the Darabut Fault and the partial rock mass of the north-central segment in the Mayile Fault. The Medium ε _Hf value province is mainly distributed in the area around the Tiechanggou batholith-western Miaoergou batholith-Labahedong batholith. The northwest of Karamay, south of Tiechanggou batholith, near Tuoli County, and the Jietebudiao batholith exhibit relatively low ε _Hf values.

Geophysical exploration combined with surface geological surveys has been one of the most important means of probing the composition of the lithosphere. Xu et al. (2022a) concluded that there is a certain degree of correspondence between the aeromagnetic anomalous area and the isotope anomalous area, as evidenced by comparative analysis of regional aeromagnetic maps, gravity maps, and Sr-Nd-Pb contour maps. The Hf isotopic signatures of the rocks can indicate the properties of the deep source region. The Hf-Nd isotopic decoupling due to multiple factors can be excluded by calculating the average Hf isotope values and analyzing them compared to Nd isotopes. The whole-rock Sr-Nd-Pb isotope results of magma-mixed granites reflect the process of magma mixing, which cannot completely exclude the interference of ancient materials. Since Hf isotope values can be measured from single zircons and have a wide range of values in magmatic samples, it is possible to explore the presence of ancient material through Hf isotope signatures. The Sr-Nd-Pb isotope results generally indicate the absence of an ancient crystalline basement probably in the West Junggar region. To investigate the properties of the deep material source area, mean crustal model age maps were produced using the Hf isotopes’ arithmetic mean value and compared with the Nd isotope contour map. Additionally, the maximum value of the crustal model age map is produced using the minimum value of the Hf isotopes, which aids in the analysis of the composition of the deep ancient materials in the West Junggar area.

The age range of ancient crustal materials in the West Junggar area is extensive. (1) The maximum crustal model ages in some areas are about 400–550 Ma, which is near the Hongshan batholith, the Miaoergou batholith, and the Dulunhedong batholith near the Darabut fault, which indicates that the deep oldest crustal materials have been formed in the Early Paleozoic. (2) The maximum crustal model ages of large rock bases such as Miaoergou batholith, Hatu batholith, Tiechanggou batholith, Akbastao batholith, Hatu batholith, and Kurumusu batholith are ∼550–700 Ma, indicating that the deep oldest crustal material may have formed in the late Neoproterozoic. (3) The maximum crustal model ages in the southern part of Karamay, the northern part of Hongshan batholith, the southern and the western part of the Tuoli, and the Jietebudiao batholith is ∼700–1,000 Ma, indicating that the oldest crustal material at depth may have formed in the Early Neoproterozoic (Figure 7).

In this paper, we produced a Pb-Nd-Hf isotope typical profile (The location of the profile is shown in Figure 2). To discuss the boundaries of the regional tectonics and the deep materials and the properties of the deep crust, we compared and analyzed the profile map between the magnetotelluric sounding (Xu et al., 2016), (²⁰⁶Pb/²⁰⁴Pb)_i, Hf crustal model maximum age and Nd model age comprehensively (Figure 8).

Although there are some differences in resistivity between the two sides of the Baerluk fault, Nd-Pb-Hf isotope values are more consistent, both indicating the age of deep crust material is relatively older. Near the Mayile Fault, the value of (²⁰⁶Pb/²⁰⁴Pb)_i shows a decreasing trend, and the Nd-Hf mode age becomes younger, indicating that the age of deep material is gradually becoming younger.

In the region between the Mayile Fault and the Darabut Fault, the variations of Nd-Pb-Hf isotopic and the deep materials are relatively consistent, and have good correspondence. There are obvious electrical differences on both sides of the Darabut Fault, the Hatu Fault, and the Mayile Fault, which are also the inflection points of Nd-Pb-Hf isotope changes. It can be concluded that there are highly comparable in some features of the geology, geophysics, and geochemistry of the West Junggar area through the above joint sections comparative study. It shows that the Isotopic mapping can provide sufficient evidence of deep material for the deep lithosphere study.

Previous studies have significant controversial on the crustal growth of the CAOB during the Phanerozoic (Yang et al., 2017; Huang et al., 2018; Wang T. et al., 2018; Lv et al., 2017). Xu et al. (2022a) concluded that the mantle contribution values of West Junggar are all greater than 50%, concentrating on 75%–95% by performing Sr-Nd-Pb Isotopic mapping in the West Junggar area. It is indicated that the crustal growth volume of the West Junggar area is huge in the Late Paleozoic, the whole is greater than 50%.

In this paper, we map the scatter plot of Hf isotopes in the West Junggar area to discuss the evolution of deep crust during the Late Paleozoic (Figure 9). During the Early Carboniferous (∼360–320 Ma), the Hf isotope values of the West Junggar area exhibited a gradual increase trend, indicating that the contribution degree of mantle source material to the crust is gradually increasing. During the Late Carboniferous-Early Permian (∼320–294 Ma), in the West Junggar region, the Hf isotopic signature indicates that the predominant magmatism and the most significant crustal growth (the majority of the crustal growth volume was greater than 75%) due to the contribution of a considerable amount of mantle source material. Since the Early Permian (∼294 Ma), there has been a notable decrease in the contribution of the juvenile mantle source material to the West Junggar region. Although the isotopic values of intermediate-acid magmatism in the area still exhibit a relatively younger signature, the contribution of the juvenile material shows a gradual weakening trend compared with the previous peak of magmatism. During the Middle Permian to the Early Triassic, most regions of West Junggar experienced a quiescence period concerning hypogene magmatic activity, there were nevertheless significant deep magmatic intrusions in the southwestern Jietebudiao area (Xu et al., 2020). Furthermore, the contribution of juvenile material exhibits an upward trend.

The geotectonic evolution of the West Junggar area is controlled by the opening and closure of the Junggar Ocean closely. Existing evidence indicates that the primary evolutionary phase of the Junggar Ocean has been ongoing since the early Paleozoic.

Among them, the West Junggar Tangbala ophiolite mélanges (limited ocean basin) with the blueschist metamorphism (458–470 Ma; Zhang, 1997), the Hongguleleng ophiolite mélanges (Zhang and Huang, 1992; Zhang and Guo, 2010), and the Taerbahatai Kujibai ophiolite mélanges (Zhu and Xu, 2006) were formed in the Ordovician (He et al., 2007). The Karamay ophiolite mélanges may form in the Cambrian-Ordovician (He et al., 2007; Zhu et al., 2007). The Darabut ophiolite mélanges was formed in the Early Devonian (Feng et al., 1989; Zhang and Huang, 1992). These Early Paleozoic ophiolitic mélanges suggest that the West Junggar area was in the stage of oceanic basin evolution before the Early Carboniferous. So, the West Junggar area experienced the following four periods of tectonic evolution during the Late Paleozoic (Figure 10):

(1) The Early Carboniferous Period (360–320 Ma). The Junggar Ocean underwent intra-oceanic subduction (Yang et al., 2017) and formed an intra-oceanic arc represented by the Bogutu Island Arc (Tong et al., 2014), which is similar to the multi-oceanic island form of the West Pacific tectonic domain. The arc is dominated by I-type granitoids (including adakites) and deposits a set of huge deep-sea and semi-deep-sea sedimentary constructions (An and Zhu, 2009). A large amount of juvenile crust was continuously added under the oceanic crust and islands of the West Junggar area as the subduction progressed. The Hf isotopes overall exhibited a trend of gradual increase, and the crust underwent lateral accretion.