The Early Silurian Gabbro in the Eastern Kunlun Orogenic Belt, Northeast Tibet: Constraints on the Proto-Tethyan Ocean Closure

: The early Paleozoic is a crucial period in the formation and evolution of the Eastern Kunlun Orogenic Belt (EKOB), and is of great significance for understanding the evolutionary history of the Proto-Tethyan Ocean. This paper presents new petrography, geochemistry, zircon U– Pb dating, and Lu–Hf isotopic research on the Yuejingshan gabbro from the eastern segment of the EKOB. Zircon U–Pb data suggests that the gabbro formed in the Early Silurian (435 ± 2 Ma). All samples have relatively low TiO 2 contents (0.45–2.97%), widely varying MgO (6.58–8.41%) and Mg # (58–65) contents, and are rich in large ion lithophile elements (LILE such as Rb, Ba, Th, and U) and light rare earth elements (LREE). This indicates that it has a similar geochemical composition to island arc basalt. The major element features indicate that the formation of this gabbro underwent fractional crystallization of clinopyroxene, olivine, and plagioclase. The depletion of high field strength elements (HFSE, such as Nb, Ta, and Ti), and a slightly positive Hf isotope (with εHf (t) ranging from 1.13 to 2.45) may be related to the partial melting of spinel-bearing peridotite, led by slab fluid metasomatism. The gabbro likely represents magmatic records of the latest period of the early Paleozoic oceanic crust subduction in the Eastern Kunlun. Therefore, the final closure of the Proto-Tethyan Ocean and the beginning of collisional orogeny occurred before the Early Silurian.


Introduction
The Eastern Kunlun Orogenic Belt (EKOB) is located in the northeast margin of the Qinghai-Tibetan Plateau, which extends for more than 1200 km and connects the Qaidam Block to the north with the Bayan Har-Songpanganzi Terrane (BH-SG) to the south [1,2]. Two significant stages of orogeny occurred in the EKOB, which are closely related to the subduction of the Proto-Tethyan Ocean during the Neoproterozoic-early Paleozoic and the Paleo-Tethyan Ocean during the Late Carboniferous-Triassic, respectively. Furthermore, these two complete tectonic cycles have established the present tectonic framework of the Eastern Kunlun region.
It is generally accepted that the Proto-Tethyan Ocean had opened and consistently expanded into the Eastern Kunlun region during the Early Cambrian; the subduction of the oceanic crust lasted until the beginning of the Early Silurian [3][4][5][6]. However, precise evidence to constrain the tectonic evolution in the Eastern Kunlun region from compression to extension during the Late Silurian is  [26][27][28][29].
The East Kunlun consists of the North Kunlun Terrane (NKT) and the South Kunlun Terrane (SKT), which are divided by the CEKF [17,[30][31][32]. The NKT is located between the southern margin of the Qaidam Basin and the CEKF. It is generally accepted that the EKOB experienced four orogenic cycles: Neo-Archean to Proterozoic (summarized as the Precambrian cycle by Mo [33]), the early Paleozoic cycle (the eclogite blocks that developed in the EKOB are most related to this cycle by Bian [2], Dong [8,10], and Chen [34]), the late Paleozoic-early Mesozoic cycle by Bian, Luo [23], Gu [30], and the Mesozoic to Cenozoic cycle by Gu [30], and Xu [35]. Zircon U-Pb chronology indicates that geotectonic-thermal events distribute through a wide range of 2500-1680 Ma in the source area of the Qaidam-East Kunlun basement, according to the Neo-Archean to Paleo-Proterozoic cycle [36,37]. Moreover, the crystalline basement was further solidified in the Neoproterozoic, and early magmatic activities occurred 1050-750 Ma according to geochronological data of the basalt in the lower Wanbaogou Group and Sanchakou granodiorite gneiss of the Qingshuiquan area [31]. In the process of the early Paleozoic cycle, the peripheral ocean around Qaidam experienced an important tectonic transition during which the stable Qaidam continental margin changed to an active continental margin, accompanied by extensive magmatism. Furthermore, during this cycle the tectonic evolution of the Proto-Tethys Ocean transformed from subduction-collision [31,38] to postcollision-extension. The third cycle began with the occurrence of the Late Devonian and contributed over eighty igneous rocks all over the EKOB, related to the tectonic evolution of the Paleo-Tethys Ocean [39]. After the Late Triassic, the EKOB entered the Neo-Tethys tectonic evolution stage.
The EKOB is considered to be a hard or rigid block of crystalline basement, consisting of advanced metamorphic rocks of the Paleoproterozoic Jinshuikou Group [25]. The basement is exposed on the western and southern margins of the block. The exposed strata include the Paleoproterozoic Jinshuikou Group, the Mesoproterozoic Xiaomiao Group, the Neoproterozoic QiuGuerDonggou Group, the Cambrian-early Silurian Qimantage Group, and the late Devonian Maoniushan Group. Granites are mainly Silurian-Devonian and late Permian-late Triassic in age. The SKT is located between the CEKF and the SEKF, with an E-W orientation. The block contains few intrusive rocks, including late Paleozoic-early Mesozoic granitic rocks and a small number of early Paleozoic intrusive rocks. Early Paleozoic strata are strongly deformed and were metamorphosed to granulite facies.
The Yuejinshan gabbro assumes a veinlike form, intrudes into the Jinshuikou rock group, and the boundary with the surrounding rock mostly appears as an undulating curve ( Figure 2). Phenomena of contact thermal metamorphism at the boundary are obvious, and hornfelization may occasionally be found (Figure 3a). Except for the rocks at the contact boundary, which show a medium-fine grain texture (Figure 3b), all rocks in the center have a coarse grain texture (Figure 3c). Observed under a microscope, the main minerals of the rocks are plagioclase, pyroxene, and biotite.
Pyroxene and plagioclase have similar granularity, and are interwoven to form the gabbro texture. The pseudomorph left after pyroxene alteration and the hornblende reaction rim on the periphery of pyroxene can be seen locally (Figure 3d).

Major and Trace Elements
For geochemical analysis, after removal of altered surfaces, whole-rock samples were crushed and milled to ~75 μm. After acid digestion of samples in Teflon bombs, X-ray fluorescence (XRF; PW1401/10) with fused-glass disks and ICP-MS (Agilent 7500a with a shield torch) were used to measure the compositions of major and trace elements, respectively. These measurements were conducted at the Testing Center of Jilin University, Changchun, China. The analytical results for BHVO-1 (basalt), BCR-2 (basalt), and AGV-1 (andesite) standards indicate that the analytical precision for the detection of major elements exceeds 5%, generally exceeds 5% (~ >10 ppm) for trace elements, and is often better than 10% (~ <10 ppm). The analytical results of major and trace elements are listed in Table 2.

Zircon U-Pb Dating
Zircon crystals were extracted from whole-rock samples by a combination of magnetic and heavy liquid separation, and finally, by handpicking under a binocular microscope at the Langfang Regional Geological Survey, Hebei Province, China. All samples were examined by Olympus BX53M optical microscopy (Olympus Ltd., Tokyo, Japan), using both transmission and reflection light microscopes. To identify their internal structures, cathodoluminescence (CL) images were obtained by a JEOL scanning electron microscope (JEOL Ltd., Tokyo, Japan). Grains of transparent, euhedral, unfractured, and inclusion-free zircons were chosen for isotopic analysis. Zircon U-Pb dating and trace element analysis were conducted by Sample Solution Analysis Laboratories Ltd. The analysis used a beam diameter of 30 μm and an ablation depth of 20-40 μm. Helium was used as carrier gas to transport the ablated material from the standard laser-ablation cell. Zircon 91500 was used as the external standard for age calibration [40], and Nist 610 and GJ-1 were used as external standards to calculate concentrations [41]. 29 Si was used as the internal standard element. Analytical methods are available in the literature [42]. Laser sampling was performed using a GeolasPro laser ablation system that consists of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system (MicroLas Ltd., Frankfurt, Germany). An Agilent Technologies Ltd. (Santa Clara, CA, USA) 7700e ICP-MS instrument was used to acquire ion-signal intensities. Helium was applied as carrier gas. Argon was used as make-up gas and mixed with the carrier gas via T-connector before entering the ICP. This laser ablation system includes a "wire" signal smoothing device [43]. The spot size and frequency of the laser were set to 32 μm and 5 Hz, respectively. Common Pb was corrected using the method proposed by Andersen [44]. The ICP-MS-DATECAL program was used to calculate isotopic ratios and element contents [45,46]. The age calculations and Concordia plots were made using Isoplot version 4.15 [47] and listed Table 2.

Zircon Hf Isotopic Analysis
Zircon Hf isotopic analysis was conducted using a NewWave UP213 LA-MC-ICP-MS at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China. The instrument conditions and data acquisition methods followed Wu et al. [48]. During Hf isotope analysis, a laser repetition rate of 7 Hz was used, and the spot size was 45 μm. The original counting rates of 172 Yb, 173 Yb, 175 Lu, 176 (Hf + Yb + Lu), 177 Hf, 178 Hf, 179 Hf, 180 HF, and 182 W were collected, and the isobaric interference correction of 176 Hu and 176 Yb on 176 HF was determined. The zircon Hf isotopic data obtained in this study are presented in Table 3.

Whole-Rock Geochemistry
The test results for the major and trace elements of the whole rock and the related parameters are listed in Table 1. Diagrams related to the main, rare earth, and trace elements are all drawn using GEOKIT software [49].

Zircon Lu-Hf Isotopic Compositions
The results of Hf isotope analysis are presented in Table 3 and Figure 6c. The spot locations on the zircon grains are indicated with blue circles in Figure 7. All tested spots showed a low ratio of 176 Lu/ 177 Hf (0.000358-0.001035, Table 3), which indicates no significant radioactivity. Hf accumulated after zircon had formed. The measured 176 Hf/ 177 Hf ratio may represent the composition for the zircons forming stage [63]. The 176 Hf/ 177 Hf ratios of 16 measuring spots are relatively concentrated and distributed in the range 0.282534-0.282570. The εHf(t) values are positive, ranging from 1.13 to 2.45 (with an average of 1.81, Figure 6d). In addition, the 16-point two-stage model ages (TDM2) range from 1.16 to 1.23 Ga, which is much older than the crystallization age (435 ± 2 Ma). This indicates an origin from the enriched mantle source area or contamination by crustal materials during the magmatic evolution process. This composition suggests Ordovician-Silurian reworking of a Mesoproterozoic juvenile crust. The Neoproterozoic globular granitic gneiss in the vicinity of these gabbros in the Nomuhong area also proved to have originated from Mesoproterozoic ancient crustal material [64].

Fractional Crystallization and Crustal Contamination
The contents of Mg # , Cr, and Ni in the Yuejinshan gabbro samples float slightly. The contents of Mg # range from 58 to 65, those of Cr from 89 to 157 ppm, and those of Ni from 13.0 to 25 ppm. These characteristics indicate that it is not obvious that fractional crystallization of this gabbro occurred before the final emplacement. The binary diagram, with Mg # as abscissa, shows that only Cr and Ni are very weakly positively correlated with Mg # (Figure 8), indicating that fractional crystallization of clinopyroxene and olivine did not occur or is not obvious in the formation of the magma [65][66][67]. P2O5, FeOT, and TiO2 exhibit significant negative correlations with Mg # , indicating that the magma possibly experienced fractional crystallization of apatite and iron-titanium oxide. The ratio (2CaO + Na2O)/TiO2 shows clear negative correlation with Al2O3/TiO2 (Figure 9), and part of the samples display an obvious Eu negative anomaly. This suggests that the magma underwent fractional crystallization of plagioclase [68]. In summary, the Yuejinshan gabbros magma likely experienced mainly fractional crystallization of apatite, iron, titanium oxide, and plagioclase during its formation.  Studies have shown that crustal contamination and fractional crystallization often occur simultaneously in the process of magmatic evolution and emplacement [69][70][71]. Crustal contamination commonly increases LILE, K2O, and Na2O, while decreasing P2O5 and TiO2 [71,72]. The binary diagram, constructed with Mg # as the abscissa, shows that as Mg # decreases, P2O5 and TiO2 gradually increase (Figure 8). This is inconsistent with the characteristics of mafic magma, formed under the involvement of crustal contamination. The Yuejinshan gabbros have relatively lower Nb/La and Nb/Ce ratios (ranging from 0.12 to 0.31 and from 0.07 to 0.29, respectively), which are lower than the Nb/La and Nb/Ce ratios of the primitive mantle, the average crust, and the average lower crust (1.02, 0.4; 0.69, 0.33; and 0.83, 0.39, respectively) [54]. This suggests that mafic magma may not form by mixing a certain proportion of crustal and mantle material. In addition, a few inherited zircon cores in the CL image analysis and slightly positive εHf(t) values (1.13-2.45) were found in the gabbros, indicating that a small amount of crustal contamination occurred once. Therefore, it may be concluded that the Yuejinshan gabbros were contaminated to a lesser extent by the crust during their formation process.

Nature of the Mantle Source
Mafic rocks, e.g., gabbro, usually derive from the lithospheric mantle or asthenospheric mantle [72,73]. Compared with the primitive mantle, rocks derived from the lithospheric mantle are generally enriched in LILE and LREE and depleted in HFSE (e.g., Nb, Ta, and Ti). However, materials from the asthenospheric mantle are normally enriched in LILE and HFSE and possess isotopic compositions characterized by the depleted mantle. The gabbro samples from Yuejinshan all have relatively low TiO2 (ranging from 0.45% to 2.97%), are rich in LILE (i.e., Rb, Ba, Th, and U), and show obvious depletion of HFSE (i.e., Nb, Ta, and Ti) compared with the primitive mantle (Figure 5b). Moreover, they show a slightly positive Hf isotope composition (εHf(t) values between 1.13 and 2.45); Hf-tDM (in scope from 0.95 to 1.00 Ga) is much earlier than its crystallization age (~435 Ma), indicating that the gabbros are a product of reworked Mesoproterozoic (1.16-1.23 Ga) juvenile crust of the lithospheric mantle. The Yuejinshan gabbros have a relatively low Ce/Y ratio (ranging from 0.62 to 1.67, with an average value of 1.10), indicating that it may derive from the spinel-garnet facies-stable area with a depth ranging from 60 to 80 km [74]. The low (Tb/Yb)N ratio (ranging from 1.22 to 1.35, with an average of 1.29) of the rocks further confirms that the mafic rocks were derived from the partial melting of spinel-bearing peridotite [75].
Compared with Huxiaoqing mafic rocks, Yuejinshan gabbro samples have a relatively lower Th/Yb ratio (ranging from 0.27 to 2.00). The points plotted in Th/Yb-Nb/Yb diagrams all deviate clearly from the MORB-OIB evolution line (Figure 10), suggesting that its formation is significantly affected by the subduction component [76]. The transformation of the mantle source area through plate subduction is mainly realized through interaction between fluid or melt derived from altered oceanic crust or subduction zone sediments and mantle peridotite [77][78][79]. Mafic magma ultimately formed by the interaction between mantle peridotite and slab melt, which generally contains relatively high Na2O, P2O5, and TiO2 levels, and shows positive anomalies of the elements Nb, Ta, and Ti compared with the primitive mantle [77,80]. The Yuejinshan gabbro samples have lower TiO2 and P2O5 contents, and show obvious negative anomalies of elements Nb, Ta, and Ti compared with the primitive mantle. Basically, the interaction between mantle peridotite and slab fluid can be excluded. Trace element ratios (Ba/Th, Th/Nb, Ba/La, and Th/Yb) may identify hydrous fluid or subduction zone sediments. All Yuejinshan gabbro samples have relatively concentrated Th/Nb and Th/Yb ratios (in the ranges of 0.29-3.20 and 0.27-3.02, respectively), and a wider variation range of Ba/Th and Ba/La ratios (in the ranges of 23.73-356.16 and 10.06-27.26, respectively) ( Figure 11). These data imply that a large amount of hydrous fluid entered the mantle source area [78,79,81]. Moreover, most rock samples have a wider variation range of Sr/Nd ratios and relatively stable Th/Yb ratios (in the ranges of 2.00-43.96 and 0.27-3.02, respectively), suggesting that the hydrous fluid mainly originated from the altered oceanic crust [82].  In summary, it may be concluded that the genesis of Yuejinshan gabbros is related to the partial melting of spinel-bearing peridotite replaced by slab hydrous fluid. Moreover, their formation underwent fractional crystallization of clinopyroxene, olivine, and plagioclase.

Implications for the Early Silurian Tectonic Nature of the EKOB
Compared with the primitive mantle, Yuejinshan gabbro samples are rich in LILE (e.g., Rb, Ba, Th, and U) and LREE, but are clearly depleted in HFSE (e.g., Nb, Ta, and Ti). These characteristics are similar to the features of island arc basalt and the Early Ordovician Eastern Kunlun mafic rocks with island arc characteristics ( Figure 12). However, they obviously differ from the mafic rocks related to the postcollision extension during the Early Devonian in Eastern Kunlun ( Figure 5), indicating that it is closely related to subduction. Based on the studies of Qingshuiquan, Kekesha, and Nuomuhong ophiolites from the Early Cambrian to the Early Ordovician, the existence of an early Paleozoic ocean was identified [3,84,85] (Figure 13a). Many high-temperature peraluminousstrong peraluminous granites (the saturation temperatures of most zircons exceed 800 °C), magnesium-rich diorite (Mg # all exceed 45), and a small number of HFSE-enriched gabbros (such as Nb, Ta, and Ti) were identified to belong to the age from the Late Silurian to the Early Devonian [10,57,86,87]. This indicates that the East Kunlun region had begun postcollision extension during this period. However, there is still much controversy over the final closure of the early Paleozoic ocean basin in East Kunlun and when collision orogeny began. Mo et al. [33] suggested that the emergence of Late Ordovician amphibole schist and its associated gabbro (Ar-Ar age is 445 ± 2 Ma) in the southwest of Tumuluk, Qimantag, could be regarded as a sign of the end of subduction and the beginning of collision. However, Ren et al. [88] and Gao et al. [89] considered that in the Late Ordovician-Early Silurian, the oceanic basin had still not been closed, the East Kunlun region was in a stage of partial back arc extension, and the final closing time of the ocean basin was in the Middle Silurian [34]. The findings on Wanbaogou rapakivi granite reported by Wang et al. [90] suggested that the continental collision had ended and had entered the postorogenic extensional stage during the Late Ordovician. The Yuejinshan gabbros reported in this paper may provide crucial petrologic evidence for further investigation of the specific tectonic evolution history of the East Kunlun region from the Late Ordovician to the Early Silurian. This evidence may be used to determine the conversion time between the oceanic crust subduction and collision orogeny during the early Paleozoic in the East Kunlun region.
Moreover, previous analysis on its petrogenesis also confirmed that the subduction components (especially from hydrous fluid) contributed more to the mantle source area. These characteristics suggest that the Yuejinshan gabbros most likely formed in a subduction-related environment. However, two studies have also shown that mafic rocks, which formed during the postcollision extension period, tend to have a similar geochemical composition to island arc basalt because they inherited mantle source areas modified by paleoslab subduction [91,92] (Figure 13b). The crystallization age of Yuejinshan gabbros belongs to the Early Silurian (435 ± 2 Ma), which is clearly earlier than the metamorphic age (428 Ma) of the collision-related eclogite facies [26] and the peak metamorphic age (427 Ma) [34] of medium pressure (epidotization) amphibolite facies in the EKOB. This indicates that it could not have formed during the postcollision extensional stage. Ren et al. [88] and Liu et al. [10] reported that Qingshuiquan diabase (438 ± 1 Ma) and Huxiaoqin mafic rocks (438 ± 2 Ma), respectively, also possess Early Silurian subduction magmatic characteristics and are situated in the middle Kunlun suture zone. Both are similar to the Yuejinshan gabbros reported in this paper in terms of age, spatial distribution, and geochemical composition (Figures 1 and 10). These similarities indicate that these three sets of mafic rocks could have formed in an identical tectonic setting.
Subduction-related intrusive rocks with ages of 515-436 Ma (e.g., Qingshuiquan, Kekesha, and Huxiaoqing) are found in the Northern and Middle Kunlun Areas. These include calc-alkaline gabbro, diabase, diorite, quartz diorite, and granodiorite, which have been reported in the southern part of the Qingshuiquan-Nuomuhong ophiolites [10,88,93,94]. Additionally, granulite with a metamorphic age of 508 ± 8 Ma, as well as gneisses and amphibolites with ages of 517-482 Ma have been reported, thus confirming the connections of the tectonic thermal events to oceanic crust subduction [95,96]. In addition, based on the existing regional data, it can be inferred that the magmatic rocks between 515 and 436 Ma are dominated by diorite, basalt, andesite, and gabbro with island arc magmatic rocks [3,84,85]. However, the magmatic rocks between 430 Ma and 391 Ma are mainly Atype granites, peraluminous-strong peraluminous granites [57], magnesia-rich diorite, and gabbro ( Figure 13c). The magmatic rocks of the two periods are obviously different, which may correspond to oceanic crust subduction and postcollision, respectively, indicating extension of two different tectonic evolution processes. All previous research implied that the ocean during the early Paleozoic opened at least at the early Cambrian and the subduction lasted from the Ordovician to the early Silurian [35,96,97]. Figure 13. Summary of early Paleozoic magmatic, metamorphic, and tectonic evolution of the Eastern Kunlun Orogen. a, from the Early Cambrian to the Early Ordovician, the existence of an early Paleozoic ocean was identified; b, two studies [91,92] have also shown that mafic rocks, which formed during the postcollision extension period, tend to have a similar geochemical composition to island arc basalt because they inherited mantle source areas modified by paleoslab subduction; c, the magmatic rocks between 430 Ma and 391 Ma are mainly A-type granites, peraluminous-strong peraluminous granites [57], magnesia-rich diorite, and gabbro. Data originate from the literature [1,14,15].

Conclusions
(1) LA-ICP-MS zircon U-Pb dating showed the age of the Yuejingshan gabbros at ~435 Ma.
(2) Yuejingshan gabbro formed from the partially melting of spinel-bearing peridotite interpreted by plate fluid, and was subjected to fractional crystallization of olivine, plagioclase, and clinopyroxene.
(3) The geochronology and geochemical characteristics of the Yuejingshan gabbros indicate that the EKOB was in a postcollisional extensional environment during the Early Silurian, which was triggered by slab break-off.
(4) The gabbro reflects the magmatic record of the latest period of the early Paleozoic oceanic crust subduction in the Eastern Kunlun. The region witnessed the final closure of the Proto-Tethyan Ocean and the beginning of collisional orogeny, which occurred before the Early Silurian.