The NQUB is located on the northeastern margin of the Qinghai–Tibetan Plateau within Qinghai Province, China. It extends along the northern margin of the Qaidam Basin in a WNW–ESE orientation. To the north, it is bordered by the Qilian terrane, while the Qaidam terrane lies to the south. To the east, it connects with the Qinling orogenic belt (Figure 1). To the westernmost of NQUB, it cut by the Altun strike-slip fault. Approximately 300 km north of this belt is the Northern Qilian suture zone, which formed during the Caledonian Orogeny and is characterized by ophiolitic and low-temperature–high-pressure metamorphic rocks (Zhang et al., 2012). From west to east, the NQUB contains the Lvliangshan–Yuka, Xitieshan, and Dulan UHP metamorphic units. The rocks exposed within this belt were derived primarily from sedimentary protoliths and include paragneiss and granitic gneiss. In addition, eclogite, mafic granulite, and lens-shaped occurrences of garnet-bearing olivine gabbro occur in this region (Song et al., 2013).

The Oulongbuluke micro-continent is located between the Qilian terrane and the NQUB. This micro-continent has Neoproterozoic to Paleoproterozoic crystalline basement, with geological affinities to the South China Block. It has been proposed that the Oulongbuluke micro-continent separated from the South China Block at ca. 780 Ma, during subduction in the NQUB and leading to the migration of the micro-continent to the southern margin of the Qilian terrane. During the Early Paleozoic, a continental arc developed on the fragmented continental basement, transforming the Oulongbuluke micro-continent into an active continental margin (Xiao et al., 2009; Tung et al., 2012; Tung et al., 2013; Yan et al., 2015). Following closure of the North Qaidam Ocean, the Qaidam Block collided with the southern Oulongbuluke micro-continent within the southern Qilian terrane. This collisional event ultimately led to the formation of the NQUB.

The Tanjianshan Group consists of lower Paleozoic strata that are widespread in the NQUB, which crop out in a NW–SE orientation. The group is in fault contact with the Archean Dawan Group and is in contact with strata of different ages via either faults or angular unconformities. The lower part of the Tanjianshan Group consists of intermediate–mafic submarine volcanic rocks, while the upper part consists of clastic and carbonate rocks. Based on fossil assemblages, isotopic dating, and tectonothermal events, Gao et al. (2011) suggested that the Tanjianshan Group formed in an island arc and represents the products of different stages of oceanic–continental subduction during the early Paleozoic (510–460 Ma; i.e., Cambrian–Ordovician).

Our study area is located in the Dachaidan region to the west of the NQUB. The main faults within the belt are NW–SE-trending. In this region, the ophiolite unit is well-exposed and the different rock types are in tectonic contact. The lower stratigraphic units are strongly serpentinized and metamorphosed peridotites, while the middle units are metamorphosed cumulate gabbros (pyroxenites), with locally developed igneous cumulate textures. These textures are characterized by alternating light-colored bands with relatively high-An plagioclase and dark-colored bands with relatively high contents of mafic minerals. The upper units consist of volcanic rocks (i.e., amphibolites) of the Tanjianshan Group, which are intercalated with abundant pyroxenite (Zhu et al., 2014). In addition, this region contains outcrops of Precambrian metamorphosed basement (i.e., the Archean Dawan Group). The protoliths of these rocks include volcanic and clastic rocks, which have underwent medium-to high-grade metamorphism. Geochronological studies indicate that the age of the granitic gneiss in this region is ca. 952 Ma, which is production of crustal melting (Lin et al., 2006). There are outcrop two types of eclogite: (1) lens-shaped bodies within the granitic gneiss and (2) in association with the metamorphosed crustal rocks, which have either lens-shaped or layered structures. Ren et al. (2019) discovered eclogites with mid-ocean ridge basalt (MORB) characteristics in the Yuka–Luofengpo area, which have ages of 1,280–1,070 Ma. These eclogites are considered to be remnants of the Mesoproterozoic Mirovoi Ocean surrounding Rodinia superterrane, which underwent two utral high-pressure metamorphism during the Neoproterozoic (amphibolite facies) and Early Paleozoic (UHP metamorphism).

The gabbros have underwent variable degrees of metamorphism and have been transformed into amphibolite with a gray–green color, granular crystalline texture, and blocky fabric (Figures 2A, B). The main minerals include pyroxene (40 vol.%), clinopyroxene (30 vol.%), and hornblende (30 vol.%) (Figures 2C–F).

Zircon U–Pb isotopic dating was conducted on gabbro sample DC-399 and DC-400 from the Dachaidan ophiolite. Forty zircon grains were selected for analysis, and their CL images are predominantly gray–black, with some appearing white. The zircon shape with elongate or short prismatic, and irregular shapes (Figure 3A), with length:width ratios of 1:1 to 2:1. Individual zircon U–Pb ages for each grain are presented in Supplementary Table S1. The U and Th contents of the 40 zircon grains, they are 165–2037 and 143–3,225 ppm, respectively. The Th/U ratios range from 0.57 to 2.19, which are typical of magmatic zircons (Hoskin and Black, 2000). The ²⁰⁶Pb/²³⁸U ages of the 40 zircon grains in the gabbro exhibit minimal variation and vary from 502 to 520 Ma. The weighted-mean ages are 510.0 ± 2.8 Ma and 510.0 ± 2.9 Ma, which represents the crystallization age of the gabbro. This indicates that the ophiolite in this region formed during the middle Cambrian. On a chondrite-normalized rare earth element (REE) diagram, the zircon grains exhibit light REE depletion, heavy REE enrichment ([Gd/Yb]_N = 0.029–0.103), and distinct positive Ce and negative Eu anomalies (Figures 3C, E).

Apart from sample DC-399, which has a LOI value of 5.01 wt.%, most of the gabbro samples from the Dachaidan ophiolite have low LOI values (<3 wt.%), indicative of minimal weathering. The gabbros have relatively low and uniform SiO₂ (47.15–49.63 wt.%) and TiO₂ (0.26–0.75 wt.%) contents. The samples have elevated contents of Al₂O₃ (14.84–16.25 wt.%) and MgO (6.35–9.04 wt.%), resulting in Mg^# (Mg^# = molar MgO/[MgO + Fe₂O₃ ^T] × 100) values of 55–74. These elevated MgO contents and Mg^# values are indicative of a mafic parental magma. The total alkali contents (K₂O + Na₂O) of the gabbros range from 2.52 to 3.00 wt.% (Supplementary Table S2). On the Zr/TiO₂–Nb/Y diagram used for classifying igneous rocks (Figure 4), the samples plot into the gabbro field.

In a chondrite-normalized trace element diagram (Figure 5A), the gabbros are characterized by depletions of light REEs to flat REE patterns (La/Yb)_N ratios are 0.51–1.06, indicative of moderate enrichment in light REE. In addition, there are obvious positive Eu anomalies, with Eu/*Eu values (2Eu_N/[Sm_N + Gd_N]) of 1.06–1.40. The gabbros have relatively low and uniform total REE contents (ΣREE = 8.35–28.1 ppm; average = 15.82 ppm). This average is less than that of normal-type MORBs (N-MORBs; ΣREE = 39.1 ppm; Sun and McDonough, 1989). On a primitive-mantle-normalized trace element diagram (Figure 5B), the gabbros exhibit depletions in Th and U, and positive Sr anomalies. These trace element features, and the positive Eu anomalies and lower ΣREE contents than N-MORBs, can be attributed to plagioclase accumulation in the gabbros. The results are listed in Supplementary Table S2.

Whole-rock measured and initial Sr–Nd isotope ratios for the gabbros in the Dachaidan area are listed in Supplementary Table S3 and shown in Figure 6. The gabbros exhibit significant variations in ((⁸⁷Sr/⁸⁶Sr)_i from 0.704586 to 0.707441, which are much higher than depleted mantle source, may have been affected by alteration. The initial Nd isotope ratios are also indicative of a depleted mantle source, with ε_Nd(t) = +4.68 to +6.59 (Supplementary Table S3). The initial Sr–Nd isotopic compositions of the gabbros are similar to those reported for basalts from the Lajishan oceanic plateau ((⁸⁷Sr/⁸⁶Sr)_i values (0.70434–0.70704), ε_Nd(t) values (0.9–8.9)), as well as the Yuka boninites ((⁸⁷Sr/⁸⁶Sr)_i values (0.7054∼0.7065), and the ε_Nd (t) values (4.1∼6.1)) and magnesian andesites (⁸⁷Sr/⁸⁶Sr)_i values (0.7059∼0.7067)), and ε_Nd (t) values (2.5∼5.3), and Proto-Tethyan oceanic ophiolites in the region (Figure 6A). The age-corrected Pb isotope ratios of these volcanic rocks are variable, with ²⁰⁶Pb/²⁰⁴Pb(t) = 18.085–18.253, ²⁰⁷Pb/²⁰⁴Pb(t) = 15.595–15.614, and ²⁰⁸Pb/²⁰⁴Pb(t) = 37.880–38.148. In plots of ²⁰⁷, ²⁰⁸Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb (Figures 6C, D), all the samples plot above the Northern Hemisphere Reference Line (NHRL) and are similar to the isotopic compositions of typical Indian MORBs.

The zircon Lu–Hf isotope data for gabbro sample DC-399 and DC-400 are listed in Supplementary Table S4. The initial Hf isotope ratios of the Dachaidan gabbros vary from 0.282678 to 0.282837. The weighted-mean is 0.282751, with a corresponding ε_Hf(t) value of +10.0. All data plot above the chondritic uniform reservoir evolution line, indicative of a depleted mantle source (Figure 6B). Single-stage Hf isotope model ages (t_DM1) vary from 812 to 661 Ma, while two-stage Hf isotope model ages (t_DM2) range from 977 to 731 Ma.

We analyzed a relatively fresh clinopyroxene phenocryst (in sample DC-400) from a Dachaidan gabbro. Cation contents were calculated based on six oxygen atoms (Supplementary Table S5). In back-scattered electron images of clinopyroxene within the gabbro, most clinopyroxene grains do not exhibit compositional zoning. They occur as large phenocrysts (2–10 mm, but typically 3–6 mm), microphenocrysts (0.5–1.0 mm), and microcrysts within the matrix (Figures 5–8). The SiO₂ contents are 48.31–51.20 wt.%, MgO contents are 13.21–15.00 wt.%, Mg^# values are 63–70, CaO contents are 11.60–12.32 wt.%, FeO contents are 13.38–16.45 wt.%, and TiO₂ contents are 0.10–0.42 wt.%. Based on the Wo–En–Fs ternary diagram, the pyroxene can be classified as augite.

Chondrite-normalized REE patterns and a primitive-mantle-normalized trace element diagram for the clinopyroxenes are shown in Figures 8A, B. All the clinopyroxenes have similar REE and trace element patterns, suggestive of a common origin. The ΣREE contents are relatively low (13.50–173.88 ppm), with (La/Sm)_N = 0.21–0.92 and (La/Yb)_N = 0.19–1.29 (Supplementary Table S5). These values are indicative of significant depletion of light REEs as compared with middle and heavy REEs (Figure 8A), and are associated with significant negative Eu anomalies (Eu/Eu^* = 0.15–0.77). Compared with gabbro sample DC-400, the clinopyroxenes exhibit clear depletions in Sr, Eu, Nb, Ta, Zr, and other REEs (Figure 8B).

Microscopic observations indicate that metamorphic processes have affected the Dachaidan gabbros, resulting in well-developed amphibolite-facies mineral assemblages, particularly the presence of hornblende (Figures 2C, D). Although alteration and metamorphic processes often mobilize large-ion lithophile elements (e.g., K, Rb, Sr, Ba, Cs, and Pb), REEs and high-field-strength elements (e.g., Nb, Ta, Zr, Hf, Th, and Ti) tend to remain relatively immobile (Hajash, 1984; Zhu et al., 2008; Escuder et al., 2010). This study predominantly relies on these less-mobile elements for the following discussion.

The gabbros have relatively high initial Sr isotope ratios of 0.704586–0.707434, with an average of 0.705737 (Supplementary Table S3). This is broadly equivalent to the Sr isotope ratios of MORBs (0.70229–0.70613; Saunders et al., 1988), but higher than the Sr isotope ratios of ocean island basalts (OIBs), as typified by Hawaiian volcanic rocks with ⁸⁷Sr/⁸⁶Sr = 0.70317–0.70412 (Stille et al., 1983). The elevated initial Sr isotope ratios may be due to contamination by crustal materials or subsequent seawater alteration. The gabbros in this region have only experienced minor crustal contamination, indicating seawater alteration is the likely cause of the elevated initial Sr isotope ratios.

Ophiolites are remnants of oceanic lithosphere and commonly underwent metasomatism by mantle fluids and subsequent seawater alteration during their formation. Strontium is a relatively mobile element and is susceptible to various post-magmatic alteration processes. In a ε_Nd(t)–⁸⁷Sr/⁸⁶Sr diagram, ε_Nd(t) is almost constant with increasing ⁸⁷Sr/⁸⁶Sr (Figure 6A), which can be interpreted as the result of fluid metasomatism or seawater alteration (McCulloch et al., 1980). Neodymium is immobile element, and the Nd isotope ratio of weakly altered seafloor rocks remains largely unchanged until the water/rock ratio reaches 10⁵ (McCulloch et al., 1981). In addition, zircon is a stable mineral that is hard to altered during weathering and metasomatic reaction, and consequently preserves its elemental and isotopic composition while its formation (Li et al., 2009).

The La/Yb ratio can be an indicator of the depth of source melting. A lower La/Yb ratio suggests shallow melting, with spinel being the primary residual phase in the source. Conversely, a higher La/Yb ratio indicates deeper melting, with garnet being the main residual phase in the source (Yang et al., 2007). The relatively low La/Sm, Sm/Yb, and Dy/Yb ratios of the studied samples suggest that partial melting occurred in a spinel lherzolite source and similar to the IBM forearc ophiolite mantle source (Figure 9; Zhang et al., 2008).

Lead is a fluid-mobile element and is added to the mantle in the form of slab-derived fluids or sediment melts, which modifies the Pb isotopes in the mantle (Pearce and Peate, 1995). The addition of a small number of subducted sediments will change the Pb isotopic composition from that of depleted mantle to that of the subducted sediments (Kersting and Arculus, 1994). Therefore, Pb isotope data is a sensitive indicator for the involvement of subduction components in the mantle source. As such, we undertook three-component mixing in Nd–Pb isotope space (Figures 6E, F) to further constrain the nature of the subduction components. The modeling results suggest that less than 10% of a subduction component with variable amounts of slab-derived fluid and sediment melt could have generated the source of the parental melts of the Dachaidan gabbros. Previous studies have found that ophiolite altered by ocean floor alteration exhibits significant positive Eu anomalies (Philpotts et al., 1969; Sun and Nesbitt, 1978), therefore, we believe that the mantle source of Dachaidan gabbros has been mixed with slab-derived fluid, leading to Eu positive anomalies.

ε_Nd(t) values for the Dachaidan gabbros range from +4.7 to +6.6, indicative of derivation from a depleted mantle source. In general, if zircon ε_Hf(t) > 0, the source is depleted mantle or newly formed young crust derived from depleted mantle. For the Dachaidan gabbros, the zircon ε_Hf(t) values of +7.6 to +11.4 indicate an isotopically depleted mantle source.

The Dachaidan gabbros have moderate MgO contents (6.35–9.04 wt.%) and Mg^# values (55–74), with low contents of Cr (22–272 ppm) and Ni (65–94 ppm), and do not contain olivine. This suggests olivine fractionation occurred during magma evolution. As Mg^# decreases, Cr and Ni contents, and CaO/Al₂O₃ ratios also decrease, indicative of fractionation involving olivine and clinopyroxene (Figures 10A–D). Mg^# values are used as an indicator of fractional crystallization, with Mg^# >60 indicative of a (near-) primary magma (Langmuir et al., 1977). Samples DC-400, -401, and −402 have relatively high Mg^# values of 65–74, similar to primary magmas. Conversely, samples DC-396 and -399 have Mg^# = 55–59, indicative of fractional crystallization.

The composition of clinopyroxene can provide important information on the magma evolution, including the type of parental magma, and the temperature and pressure of crystallization (Leterrier et al., 1982). Igneous thermobarometers can provide important insights into magma formation, and thus these were applied to the Dachaidan gabbros (Putirka, 2008).

The clinopyroxene in the gabbro is euhedral–subhedral and has relatively uniform major and trace element compositions. This suggests the clinopyroxenes crystallized in a relatively closed magmatic system (Carracedo, 1999). By calculating the equilibrium constant K_D (Fe–Mg)^cpx/liq, the state of equilibrium between clinopyroxene and the host rock can be determined. Based on experimental results, when K_D (Fe–Mg)^cpx/liq is 0.2–0.4 the clinopyroxene and host magma are in equilibrium (Liotard et al., 1988). We used the formula of Wood (1997) (K_D [Fe–Mg]^cpx/liq = 0.109 + 0.186 × Mg^# _cpx) to calculate the equilibrium constant for the studied clinopyroxene. The range of K_D (Fe–Mg)^cpx/liq values for the clinopyroxenes is 0.226–0.239, indicative of equilibrium between clinopyroxene and the host magma. Therefore, the temperature and pressure calculated from clinopyroxenes in the Dachaidan gabbros can constrain the physicochemical conditions during magma formation.

We used the clinopyroxene–melt equilibrium thermobarometer of Putirka (2008) to calculate the temperatures and pressures (Supplementary Table S5). The crystallization temperatures range from 1,326°C to 1,363°C, with an average of 1,341°C. The crystallization pressures vary from 1.27 to 1.64 GPa, with an average of 1.47 GPa.

To gain further insights into the gabbroic magmas, a comparison was made with MORB samples from the Mid-Atlantic Ridge and East Pacific Rise. The P–T data calculated using the clinopyroxene thermobarometer plot within the anhydrous melting field for mid-ocean ridges (Figure 11), indicating the Dachaidan gabbros formed in a high-temperature, moderate-pressure, anhydrous melting environment. The P–T conditions of melting for the Dachaidan gabbros are similar to those of MORBs.

In summary, the primary magmas of the Dachaidan gabbros were derived from depleted mantle, and partial melting occurred in spinel-facies mantle. There was then some fractional crystallization of minerals such as olivine and clinopyroxene, which ultimately resulted in the formation of the Mg-rich gabbros.

In the context of ophiolite formation, some studies have proposed that only a small proportion of ophiolites are the product of mid-ocean ridge spreading, and most form in supra-subduction zones (Pearce et al., 1984). Gabbros in ophiolites are derived from the mantle by partial melting and/or underwent subsequent fractional crystallization. Therefore, gabbros in ophiolites can constrain the geochemical characteristics of ophiolites in orogenic belts and provide insights into the tectonic setting of ophiolite formation.

Island arc basalts (IABs) and depleted MORBs typically have Ta and Nb contents of <0.7 and <12 ppm, respectively, with Nb/La <1, Hf/Ta >5, La/Ta >15, and Ti/Y <350. In contrast, within-plate basalts (WPBs), transitional MORBs (T-MORBs), and enriched MORBs (E-MORBs) have opposite features (Condie, 1989). The Dachaidan gabbros have low Ta (0.04–0.23 ppm) and Nb (0.59–2.79 ppm) contents, with Nb/La = 0.80–2.97, Hf/Ta = 4.91–7.95, La/Ta = 7–24, and Ti/Y = 195–260. Although some samples have low La/Ta ratios, the elemental abundances and ratios of the Dachaidan gabbros are similar to those IABs or depleted MORBs. In Figure 12, the Dachaidan gabbros plot near the N-MORB trend and similar to the IBM forearc basalts mantle source.

In the 2Nb–Zr/4–Y tectonic discrimination diagram (Figure 13A), the samples plot near the N-MORB and volcanic arc basalt fields, similar to Yuka MORB-like gabbros and basalts, and Lashuixia gabbros. In the V–Ti/1,000 diagram (Figure 13B), the samples plot in the IAB field, indicative of a subduction-related origin. In summary, the Dachaidan ophiolite exhibits the geochemical features of both N-MORBs and IABs. Volcanic rocks with such characteristics are typically associated with forearc basins above subduction zones (Xu et al., 2003; Tian et al., 2008; Reagan et al., 2010). Forearc basin volcanic rocks typically include basalts and high-Mg andesites. Previous studies in the Yuka–Kaipinggou area have identified basalt, IAB, and magnesian andesite (Zhu, 2011; Zhu, 2021; Tuo, 2019), similar to the IBM forearc basalts (Ishizuka et al., 2014). Implying a possible forearc tectonic origin for the Dachaidan gabbro. In the V–Ti/1,000 diagram (Figure 13B), all samples plot within the Izu–Bonin–Mariana Forearc basalts and boninite fields. In addition to the above evidence, the TiO₂, MgO, and FeO^T contents of the samples in this study are similar to those of IBM basalt (TiO₂ from 0.67 to 1.64, MgO from 4.62 to 9.76 and FeO^T content of 8.88–14.16) (Figures 13C, D).

Due to the resistance of clinopyroxene to alteration during low-grade metamorphism and late-stage magmatic processes, its composition is useful for determining the tectonic setting (Leterrier et al., 1982). The SiO₂, TiO₂, and Al₂O₃ contents of clinopyroxene are controlled mainly by its composition rather than the physicochemical conditions of crystallization (Brown et al., 1967). In the TiO₂–SiO₂/100–Na₂O ternary diagram, the Dachaidan gabbros plot in the field for island arc tholeiites (IATs) and forearc basaltic andesites (BA-A). In a Ti–Al (atomic proportions) diagram (Figure 14B), most clinopyroxene plots in the fields for IATs, BA-A, and basaltic andesite (BA). The clinopyroxene in the Dachaidan gabbros has lower Ti contents and Ti/V ratios than MORB and BABB, similar to IATs and low-Ti basaltic andesites (Figure 14C), indicative of crystallization from a high-Mg magma. This is consistent with the high Mg^# values of the gabbro samples. Melting of a mantle wedge during subduction of hot and young oceanic lithosphere likely led to ophiolite formation under high-temperature and medium-pressure conditions. Both the whole-rock and clinopyroxene geochemical data confirm that the Dachaidan gabbros formed in a forearc setting. Previous studies have suggested that forearc tholeiites form during subduction initiation and the transition to a continental setting (Xiao et al., 2016). Therefore, the Dachaidan gabbros were the products of intra-oceanic subduction at the northern margin of the North Qaidam Ocean.

The NQUB contains granitic gneiss, pelitic gneiss, and sporadic occurrences of eclogite and garnet clinopyroxenite. It also contains island arc ophiolitic mélange, and collisional and post-collisional granites (Wu et al., 2004; Wu et al., 2007; Wu et al., 2008; Wu et al., 2014; Zhu et al., 2010; Zhu et al., 2012; Zhu et al., 2014; Song et al., 2015; Zhao et al., 2017). The rock assemblages associated with the eclogites suggest that the UHP metamorphic zone in the NQUB is typical of an early Paleozoic continental orogenic belt (Ren et al., 2019). Some eclogites have geochemical characteristics typical of oceanic basalts, indicative of derivation from either middle Proterozoic oceanic crust, as is the case for the Shaliuhe section in the South Dulan belt. This section consists of blueschist garnet, phengite–garnet, and epidote–garnet eclogites with the protolith ages from 1,280 to 1,070 Ma, the protolith may be Mesoproterozoic oceanic crust and a peak metamorphic age of 439–430 Ma (Ren et al., 2019). Alternatively, these rocks may have been derived from early Paleozoic oceanic crust, as exemplified by the blueschist garnet eclogites from the Shaliuhe area, the protolith of these eclogites ages formed at 517–514 Ma and peak metamorphic ages of 457–440 Ma (Zhang et al., 2009a; Zhang et al., 2009b; Zhang et al., 2016). These findings provide insights into the presence of an early Cambrian Proto-Tethyan oceanic basin. In summary, the geological investigations of the NQUB revealed a complex tectonic history involving continental collision and the presence of an early Cambrian Proto-Tethyan oceanic basin.

Song et al. (2014b) investigated the northern Qilian region and NQUB, where oceanic-type eclogites with a peak metamorphic age of 489–464 Ma were previously identified (Song et al., 2006). This age is similar to the metamorphic age of oceanic eclogites in the NQUB, leading to the conclusion that the northern Qilian region and NQUB are part of the same geological system. The continental-type, UHP metamorphic belt in the NQUB formed by northward subduction that caused the Qilian–NQUB block to undergo continental subduction. The UHP metamorphic rocks in the belt are considered to have formed by either exhumation along the subduction path or exhumation through bottom-driven uplift (Song et al., 2006; Song et al., 2014b; Yin et al., 2007). MORB-type ophiolites in the NQUB include the Dulan–Shaliuhe section dated at 520 Ma (Wang et al., 2020) and the Amunike ophiolite dated at 485.6 Ma (Li et al., 2018). In addition, some mafic igneous rocks within the intercalated island arc mélange of the NQUB had diverse tectonic settings, such as island arc, back-arc basin, ocean island, and mid-ocean ridge environments. These include the 521 Ma Xitieshan OIB-type basalts (Zhu et al., 2012), 480 Ma Tuomoerite N-MORB-type basalts (Wang et al., 2022), and 542–460 Ma island arc volcanic rocks (Yuan et al., 2002; Shi et al., 2003; Wang et al., 2023; Wang et al., 2005; Zhu et al., 2010, Zhu et al., 2012; Zhu et al., 2014; Gao et al., 2011). These features are similar to those of early Paleozoic oceanic eclogites in the Dulan region, which protolith age is 510 ± 19 Ma (Zhang et al., 2009a; Zhang et al., 2009b; Zhang G. B. et al., 2016). Consequently, we infer that there was an independent ocean (i.e., the North Qaidam Ocean) in the region during the early Paleozoic, and that the northern Qilian region and NQUB should not be considered to have been a single entity.

Geochronological studies have revealed that the eclogites in the Xitieshan experienced peak metamorphism at 458–420 Ma (Chen et al., 2012; Liu et al., 2012; Zhang et al., 2013b; Zhang L. et al., 2016). The subordinate gneisses in the Xitieshan underwent prograde metamorphism from 460 to 455 Ma, followed by retrograde metamorphism at 425–422 Ma (Zhang et al., 2012). The metamorphic ages of quartz diorites and eclogites intercalated with the subordinate gneisses in the Dulan region are 458–421 Ma (Yang et al., 2005; Chen et al., 2008). Blueschist garnet eclogites in the Shaliuhe area have peak metamorphic ages of 457–440 Ma (Zhang et al., 2009a; Zhang et al., 2009b; Zhang G. B. et al., 2016). All these ages indicate that subduction in the North Qaidam Ocean started at ca. 460 Ma and persisted until ca. 421 Ma. Given the oceanic nature of these rocks, this study suggests the Dachaidan gabbros were formed in an intra-oceanic forearc environment. The zircon U–Pb age of 510 Ma indicates subduction in the North Qaidam Ocean started prior to 510 Ma.

We propose the early Paleozoic tectonic evolution of the NQUB was as follows. During 510–460 Ma, the North Qaidam Ocean existed between the Qaidam Block and Oulongbuluke micro-continent. Subsequently, the ocean experienced northward subduction (Figure 15A), which led to the development of a forearc ophiolite in the Dachaidan region, a back-arc basin ophiolite in the Tuomoerrite Basin, and other related features. The present-day NQUB ophiolite is considered to comprise remnants of the oceanic crust.