Geological significance of the newly discovered Middle Permian ocean island basalt-type gabbros in Ewulang, Nianqing-Sumdo area, Tibet

Numerous research data have been obtained about the Sumdo Palaeo-Tethys, but the reports on the occurrence of Permian Ocean islands are relatively scarce. Here, we present the zircon U – Pb geochronology, geochemistry, and in situ zircon Hf isotopic characteristics of the ocean island basalt (OIB)-type gabbros from Ewulang in Nianqing-Sumdo, Tibet. The zircon U – Pb age obtained from Ewulang OIB-type gabbros is 263 Ma (the Middle Permian), representing the formation age of the gabbros. The geochemical characteristics of the samples show no contamination by crustal material. Meanwhile, all samples are enriched in light rare earths (LREEs) and large-ionic lithophile elements (LILEs), which are similar to the typical OIB. In various geochemical discriminant diagrams, all samples fall into the fields of OIB and related tectonic settings, indicating that the rocks were formed in the oceanic island environment within the oceanic plate. Various geochemical indicators imply that the OIB-type gabbros may originate from mantle plume and the in situ zircon ε Hf ( t ) values of Ewulang gabbros range from (cid:1) 0.34 to + 4.30, suggesting that the magmas derived from an enriched mantle source. Combined with regional data and previous studies, this article further confirms that Sumdo Palaeo-Tethys extends west to Nianqing-Sumdo area and the Tangjia (Zhikong)-Sumdo Palaeo-Tethys was still an ocean basin with a certain scale during the Middle Permian.


| INTRODUCTION
A high/ultrahigh-pressure metamorphic belt has been identified on the basis of the newly discovered Sumdo eclogite in the eastern Lhasa Terrane, the southern Tibetan Plateau (Yang et al., 2006). More subsequent studies have been conducted in this metamorphic belt Chen, Yang, Li, & Xu, 2009;Cheng et al., 2012;Cheng, Liu, Jeffrey, & Lu, 2015;Li et al., 2007Li et al., , 2008; Wang et al., 2018;Yang et al., 2006Yang et al., , 2009. Much has been known about the evolution of the Sumdo Palaeo-Tethys so far. Li et al. (2008) suggested that the opening time of the Sumdo Palaeo-Tethys should be the Late Carboniferous or earlier, according to the age of the protolith of Sumdo eclogite.
Formed by typical MORB-type basalt that underwent high-pressure metamorphism around 261 Ma , the Sumdo eclogite records the subduction metamorphism and exhumation metamorphism of the Sumdo Palaeo-Tethys Zeng, Liu, Gao, Chen, & Xie, 2009). Subsequently, eclogites in other sites among the belt were discovered. For example, the Jilang eclogite formed by deep-subduction of back-arc basin basalts (BABBs) (Cheng et al., 2012), the Bailang eclogite with the geochemical characteristics of oceanic island arc basalts (Cheng et al., 2015), the Xindaduo eclogite featured by subduction metamorphic of oceanic crust (Li et al., 2017) and many others. A large number of chronology data obtained from eclogite in Sumdo area define the time evolution sequence of the Sumdo Palaeo-Tethys approximately (Cheng et al., 2012(Cheng et al., , 2015Li et al., 2008;Zhang, Zhang, Schertl, & Wei, 2019).
It is not hard to find that a large number of current studies are mainly focused on Sumduo Township and its vicinity from previous studies, but there is still little research in adjacent areas. The survey of Tangjia area in Mozhugongka County shows that coinstantaneous ophiolite extends westward to the Tangjia (Zhikong) area and the 'Tangjia (Zhikong)-Sumdo Paleo-Tethys' established Lu, 2019;Xie et al., 2020). Located at continental margins or suture zones, the ophiolites are often considered as the remnants of ancient oceanic crust, providing strong evidence for the reconstruction of ancient oceans (Dai et al., 2013;Dai, Wang, Stern, Yang, & Shen, 2020;Dilek & Furnes, 2014). As the product within the oceanic plate, it is certain that the ocean islands provide strong evidence of the existence of ancient oceans. The reports and researches about ocean islands among Sumdo Palaeo-Tethys, however, are so limited, and with a low level of study . Only Xuelanggou (Chen, 2010), Pairigang (Lu, 2019), and Wenmulang (Wang et al., 2018) ocean islands have been reported so far in this region and to perfect the temporal-spatial evolution framework of the Tangjia-Sumdo Palaeo-Tethys further, more new research data are needed.
In this article, we report the discovery of the typical ocean island basalt (OIB)-type gabbros from Ewulang, Nianqing-Sumdo area. Based on the whole-rock geochemistry, zircon U-Pb age, and in situ zircon Hf isotopic analyses, we discuss the petrogenesis, tectonic setting, and geological significance of the Ewulang OIB-type gabbros.

| GEOLOGIC SETTING
Composed of Songpan-Ganzi, Qiangtang, Lhasa (Gangdese), and Himalayas terranes (from north to south), the Qinghai-Tibet Plateau was formed by the collage of several blocks in different times Pan, Wang, Li, Yin, & Zhu, 2012). Located in the central part of the Qinghai-Tibet Plateau, Lhasa Terrane is an important part of the Qinghai-Tibet Plateau, which is bounded by the Bangong-Nujiang Suture Zone (BNSZ) to the north and the Yarlung Zangbo Suture Zone (IYSZ) to the south (Dewey, Shackleton, Chang, & Sun, 1988;Yin & Harrison, 2000;Figure 1a). The Sumdo ultrahighpressure metamorphic belt within the Lhasa Terrane has attracted extensive attention in the field of geology due to the discovery of the Sumdo eclogite (Yang et al., 2006). It has been shown that there are a large number of eclogites and ophiolites in this tectonic zone, which are the remnants of the Sumdo Palaeo-Tethys oceanic crust (Cheng et al., 2012(Cheng et al., , 2015Huang, Tian, Zhang, Yang, & Chen, 2015;Li et al., 2017;Wang et al., 2018;Yang et al., 2006).   (Ludwig, 2003).
Five gabbro samples were analysed for major and trace elements at ALS Minerals-ALS Chemex in Guangzhou. Major elements were analysed by X-ray fluorescence spectrometry (ME-XRF26D) and rare earth elements were analysed by plasma mass spectrometer (ME-MS81). Integrated Mass/Spectral analysis for trace elements (ME-MS61). Geochemical data processing and map drawing are completed using Geoplot.
In situ zircon Hf isotopic analyses were carried out using a Neptune Plus MC-ICP-MS, coupled with ESI NWR LA system (213 nm) at Beijing Createch Testing Technology, Beijing, China.
The laser beam diameter was 38 μm, the energy density was 8-10 J/cm 2 and the frequency was 8 Hz, the operating voltage of the instrument was 0.89 kV. Details of the process are available from Hou et al. (2007).

| Zircon U-Pb ages
For U-Pb dating, 28 grains of zircons were tested and the results are listed in Table 2. Twenty-five valid data were obtained after removing the spots with low concordance (<90%). In the CL images of zircons

| Crustal contamination
Whether the OIB-type basalt is contaminated is extremely important for understanding the nature of the source region and tectonic evolution (Liu et al., 2013). Some scholars have proposed to use the ratio of Ce and Pb to determine whether the OIB magma is contaminated by crustal materials (Hofmann, 1988;Rudnick & Fountain, 1995). But, F I G U R E 8 Tectonic setting discrimination diagrams for the Ewulang gabbros (a,b, after Pearce, 2008;c, after Mullen, 1983;d, after Meschede, 1986 (Wang et al., 2018). In the diagram of primitive mantlenormalized trace element patterns (Figure 4b), both Pb and K contents are higher than the typical OIBs. Taking into account the phenomenon of mineral alteration, we believe that the enrichment of Pb and K in the samples may be relevant to alteration. Therefore, the determination of the crustal contamination by Ce/Pb in this study may not be applicable. The previous study suggested that when the magma was mixed with lower crustal material, the (Th/Ta) PM value≈1 and the (La/Nb) PM value always greater than 1. Once upper crustal material is mixed, both are greater than 2 (Peng, Mahoney, Hooper, Harris, & Beane, 1994). The (Th/Ta) PM of samples in this study have a large variation range, from 0.65 to 1.56 (with an average of 0.98), but most are less than 1 (only one is greater than 1), which indicates that the Ewulang OIB-type gabbros have not been contaminated by crustal material. In the (Th/Ta) PM -(La/Nb) PM diagram, the results reach the same conclusion (Figure 7a).
The Nb/Th and Ti/Yb are sensitive to whether the mantlederived magma is contaminated by the crustal or not and they are widely used to identify the relative contributions of lithospheric mantle materials and crustal materials . In the diagram of Ti/Yb-Nb/Th (Figure 7b), only one sample in this article shows a trend of weak crustal contamination, most of them are located near the OIB, indicating that the Ewulang gabbro magma is unaffected by crustal material again. Saunders, Storey, Kent, and Norry (1992) believed that basalts affected by crustal contamination have high (Th/Nb) N values (>>1), while the average value of (Th/Nb) N in this study is 1.1. It also shows that there is no contribution from the crustal material. Additionally, there is no older zircon in this sample, showing that there is no contamination of pre-existing rocks (crustal material), which is consistent with the discussion above.

| Tectonic setting
The distribution patterns of rare earth and trace elements ( Figure 4) show that the Ewulang gabbros are enriched in LREEs and incompatible elements and there is no negative Nb-Ta anomaly, indicating that those rocks are not related to subduction.
Besides, the curves of the trace element and rare earth elements are resemblance to the OIBs. High-field-strength elements (HFSE) such as Nb, Ti, Th, Ta, Zr, and Yb, have been used as the most effective discriminating factors for different tectonic environments widely (Dilek & Furnes, 2011;Pearce, 2008;Winchester & Floyd, 1977). Th/Nb(0.13) of the Ewulang gabbros in this study are close to typical OIBs (9.46, 5.83, 12.0, and 0.08, respectively) (Sun & McDonough, 1989), confirming the affinity with OIB once again.
In summary, the geochemical characteristics indicate that the Ewulang gabbros were formed in an oceanic island tectonic setting within the oceanic plate. Unfortunately, there is no limestone and basalt found in Ewulang. However, it is worth noting that a large number of OIB-type basalts and limestone (marble) can be found in other places in our study area. For example, the OIP 1 mb + v in Figure 1  extremely depleted of the above elements so that it cannot be used as the source of OIB (Wang et al., 2018). The content of TiO 2 is also used to determine the source of magma (Zhu, Mo, Wang, Zhao, & Liao, 2008). The Ewulang OIB-type gabbros have high TiO 2 content (all above 2%, with an average of 3.59%), indicating that magma may be related to the deep mantle or it is possible that associated components (e.g., sediments and metasomatic veins) may account for the extremely high content of TiO 2 in this cases (Prytulak & Elliott, 2007). The La/Sm-Sm/Y diagram can reveal the mineral assemblage and the degree of partial melting in the magma source region. The result shows that the magma originated from the garnet lherzolite phase (Figure 9a) and experienced partial melting of 5-10%. Samples of this study fall in the field of primitive garnet lherzolite in the Zr/Nb-Ce/Y diagram ( Figure 9b). The garnet lherzolite magma experienced a low-degree of partial melting will result in significant fractionation of light and heavy rare earth elements (Bloomer, Stern, Fisk, & Geschwind, 1989), which is consistent with the characteristic of Ewulang OIB-type gabbros.
The hypothesis that mantle plumes may be the source of OIBs has been generally accepted by researchers (Huang & Zheng, 2017;Niu, 2009) and has been widely used in interpreting the petrogenesis of OIBs (Chen, Huang, Luo, Yan, & Long, 2011;Harrison, Weis, & Garcia, 2019;Wang et al., 2013Wang et al., , 2018Xia et al., 2012). The (Th/Ta) PM and (La/Nb) PM of Ewulang OIB-type gabbros are below 1 mostly, which may imply that the magma originated from the mantle plume (Neal et al., 2002). The magma originating from the mantle plume has a low La/Ta ratio, generally range between 8 and 15 (Lassiter & DePaolo, 1997) and the La/Ta values of the samples in this study vary from 11.86 to 15.66, which also indicates the magma origin from a mantle plume. The 4Nb (4Nb = 1.74 + log(Nb/Y) À 1.92 Â log(Zr/Y)), proposed by Fitton, Saunders, Norry, Hardarson, and Taylor (1997), was used to distinguish the source of magma, such that magma originated from mantle plume has 4Nb > 0 and N-MORB has 4Nb < 0. The 4Nb varies from À0.06 to +0.29 in this study, four-fifths are greater than 0, showing that the magma of Ewulang OIB-type gabbro origin from the mantle plume.
Zircon Hf isotopic composition is an effective index to explore the attributes of magma source region (Griffin et al., 2000).

| Geological significance
Since the discovery of Sumdo eclogite, a large number of studies have been carried out in the Sumdo area and some direct evidence of the existence of Palaeo-Tethys have been discovered in this area, such as eclogite, ophiolite, OIB, and others ( Figure 10, more details see Xie et al., 2020). Later geological surveys have discovered and identified a mass of ocean islands, seamounts, wedge-top basins, and ophiolite in different regions . This is the case in Tangjia and Nianqing-Sumdo areas, the scope of Sumdo Palaeo-Tethys has been further expanded by those findings. The Ewulang OIB-type gabbros identified in this article provide strong evidence to support the western extension of the ocean basin.  (Chen, 2010). It is speculated that the opening time of the Sumdo Palaeo-Tethys should be the Late Carboniferous or earlier (Li et al., 2008). The Chongni island arc mafic rocks formed by the northward subduction of the oceanic crust indicate that the initial subduction time in Zhikong (Tangjia) will not be later than 270 Ma , which is consistent with the age of the island arc volcanic rocks reported by Chen (2010). The metamorphic age of 260 Ma was obtained from Sumdo eclogites Yang et al., 2009) and 40 Ar-39 Ar age of 220-240 Ma from retrograded eclogites (Li, Xu, Yang, Tang, & Yang, 2011). After that, metamorphic ages of 265 and 230 Ma for Jilang, Bailang eclogites were reported successively (Cheng et al., 2012(Cheng et al., , 2015. The above-mentioned chronology studies illustrate the continued plate subduction.
Based on previous studies, this article believes that the opening

PEER REVIEW
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DATA AVAILABILITY STATEMENT
The data that supports the findings of this study are available in the article.