The affinity of microcontinents in northern East Gondwana in the Silurian: Hainan Island response to the closure of the Proto-Tethys Ocean

Abstract

During the existence of Proto-Tethys Ocean (550–430 Ma), microcontinents in northern East Gondwana merged with the northern margin of India-Australia, completing the assembly of Gondwana. Ongoing controversy surrounds the disappearance of the Proto-Tethys Ocean, the dynamic mechanisms of suturing and the palaeogeographic relationships among microcontinents in northern East Gondwana, contributing to the uncertainty about the tectonic evolution of the region. This paper concerns the lower Silurian Zusailing Formation in the Hainan Island and focuses on the affinity between Hainan Island and various microcontinents in northern East Gondwana during the early Silurian. We use detrital zircon geochronology to reconstruct the closure process of the Proto-Tethys Ocean and show that the detrital zircon U–Pb age groups of the lower Silurian Zusailing Formation are 2800–2200, 2100–1350, 1250–950, 600–480 and 480–430 Ma, with a significant age peak of ca. 449 Ma. Furthermore, the analysis of detrital zircon geochemistry and europium anomalies shows that the Hainan Island crust continued to thicken during 600–434 Ma. Comparing the age spectrum of early Palaeozoic detrital zircons from Hainan Island and various microcontinents in northern East Gondwana, as well as the affinity among them during the Silurian, we conclude that the closure of the eastern Proto-Tethys Ocean evolved from unidirectional subduction (600–480 Ma) to bidirectional subduction (480–430 Ma).

1. Introduction

The tectonic evolution of the Tethys Ocean is a crucial stage in the evolution of the Earth, which can be divided into three stages: the Proto-Tethys stage (Cambrian–Silurian), the Palaeo-Tethys stage (Devonian–mid-Triassic) and the Neo-Tethys stage (since the Cretaceous) (Lu, 2004; Cawood et al. 2007; Sorkhabi & Heydari, 2008; Xu et al. 2012; Deng et al. 2016, 2019). The Proto-Tethys stage, which marks the incipient phase of the entire Tethys tectonic evolution, is crucial for understanding the history of the Gondwana. During the Proto-Tethys stage, various microcontinents in the northern East Gondwana (north of India and Australia) were gradually assembled, including the South China Block, Tarim Block, South Qiangtang Terranes, North Qiangtang Terranes, Indochina Block and Sibumasu Block (Fig. 1, Nie, 2016; Zhao, 2019; Zhang et al. 2022). According to palaeomagnetic, fossil and detrital zircon data during the early Palaeozoic, the South China Block, Tarim Block, North Qiangtang Terrane and Indochina Block were located close to each other. They are collectively referred to as the North Vietnam-South China and these microcontinents divide the Palaeo-Tethys Ocean into several narrow basins (Duan et al. 2011; McKenzie et al. 2011; Metcalfe, 2013; Xu et al. 2013; Zhou et al. 2015 a; Chen et al. 2016; Lee et al. 2016; Han et al. 2016; Wang et al. 2016; Zhao et al. 2017, 2018; Liu, 2020; Wu et al. 2023). Therefore, the closure of the Proto-Tethys Ocean has stages, and its sutures are characterized by multiple zones, such as Truong Son, Changning–Menglian and Jinshajiang–Diancangshan–Ailaoshan structural zones (Fig. 1, Zhao, 2019; Zhou et al. 2021 a, b; Peng et al. 2022).

Figure 1. Schematic diagram of principal tectonic fragments and sutures in Southeast Asia (after Metcalfe, 2011, 2013, 2017).

Due to the lack of preserved records of late Palaeozoic magmatic activities near the original Tethys Ocean suture zone, as well as the thermal structural events associated with the closure of the ancient Tethys Ocean and the Himalayan Orogeny, the early Palaeozoic tectonic evolution and palaeogeographic positioning of the North Vietnam-South China with respect to the northern margin of India-Australia have not been well constrained. There are two dynamic models (the passive continental margin and the Andean-type active continental margin) to explain these events. The former is controlled by the break-up of the Rodinia supercontinent and the assembly of the Gondwana during the Pan-African Orogeny (Miller et al. 2001; Xu et al. 2005; Yang et al. 2012c; Cai et al. 2013; Liu et al. 2016). The latter is controlled by the subduction of the Proto-Tethys Ocean underneath Gondwana and adjacent microcontinents after the Pan-African Orogeny (Cawood et al. 2007; Dong et al. 2010; Zhu et al. 2012; Wang et al. 2013 a; Li et al. 2016). In addition to the uncertainty of dynamic mechanisms, there is also controversy over the independence of, and affinity among, microcontinents in northern East Gondwana, including the reconstruction of the tectonic evolution and assembly of the microcontinents in the region during early Palaeozoic (north of India and Australia). Some believe that these microcontinents have an affinity with Gondwana (Dopieralska et al. 2012; Usuki et al. 2013; Burrett et al. 2014; Metcalfe, 2017), while others posit that they evolved as independent blocks in the Proto-Tethys Ocean and have the characteristics of the Pan-Cathaysian landmass groups (Zhang, 1994; Lu, 2001; Replumaz & Tapponnier, 2003; Zhang et al. 2005; Cocks & Torsvik, 2013).

During the early Silurian, Hainan Island was located in the northern region of East Gondwana (Cawood et al. 2007; Dong et al. 2010; Metcalfe, 2013; Xu et al. 2014; Zhou et al. 2015 a). Due to the tectonic position of Hainan Island, the lower Silurian Zusailing Formation contains important information about the evolution of Proto-Tethys tectonics, which is an important clue to reveal the affinity between North Vietnam-South China and the northern margin of India-Australia. By studying the U-Pb ages of detrital zircons and geochemical data from the Zusailing Formation in early Silurian of Hainan Island, this article explores the tectonic events associated with Hainan Island during the late Early Palaeozoic. It reconstructs the main provenance areas during early Silurian and analyses the source-to-sink system, thereby determining the affinity among the various microcontinents in northern East Gondwana. These findings provide sedimentological evidence for the tectonic evolution stages of the early Palaeozoic in East Gondwana.

2. Geological setting and sample description

Hainan Island is located at the junction of the South China Block and the Indochina Block, which is located in the Palaeo-Tethys tectonic domain (Fig. 1) (Wen et al. 2013; Zhou et al. 2015 b; Zhang et al. 2017; Li et al. 2018; Zhao et al. 2020; Gao et al. 2022 b). The Hainan Island region experienced multiple periods and different forms of tectonic movements. The superposition of the effects of these tectonic movements has made the internal structural features of the Hainan Island complex. In the Mesoproterozoic-Neoproterozoic, the Jinning Orogeny formed the basement of Hainan Island. In the Palaeozoic, the Caledonian and Hercynian Orogeny laid the basic structural framework of Hainan Island, accompanied by large-scale magmatic activity and migmatization. In the Mesozoic, the Indosinian Orogeny mainly developed ductile deformation, controlling the eruption of a large number of intermediate to acidic magmas. The Cenozoic orogeny was dominated by fault block activity and large-scale basic magmatic eruption (Tang, 2010; Du et al. 2013; Zhou et al. 2015 b; He et al. 2016; Yan et al. 2017; Wang et al. 2019). There are faults with a northeast trend in the region, from west to east the Gezhen and Baisha faults, and there are four nearly east-west oriented faults, namely the Wangwu–Wenjiao, Changjiang–Qionghai, Jianfeng–Diaoluo and Jiusuo–Lingshui faults from south to north (Fig. 2a) (Chao et al. 2016; Zhang et al. 2019; He et al. 2020; Liu et al. 2022 a).

Figure 2. (a) Geological sketch of Hainan Island (according to the Bureau of Geology and Mineral Resources of Guangdong Province (BGMRGP) 1988; Gao et al. 2022b; Kong et al. 2022); GZF: Gezhen fault; BSF: Baisha fault; WWF: Wangwu-Wenjiao fault; CQF: Changjiang-Qionghai fault; JDF: Jianfeng-Diaoluo fault; JLF: Jiusuo-Lingshui fault; (b) Stratigraphic column of the lower Silurian Zusailing Formation; (c) sandstone representative photograph (The length of pencil is 17.5 cm); (d) slate representative photograph (The length of hammer is 30 cm); (e) sericite phyllite representative photograph (The length of pencil is 17.5 cm).

Existing research indicates that except for Jurassic rocks, strata from the Mesoproterozoic to the Quaternary system are exposed in Hainan Island, most of which are Palaeozoic (Wang et al. 1992). However, Palaeozoic rocks are in a fragmented state on Hainan Island, dispersed in different regions and with poor correlation between units. The early Palaeozoic strata are widely distributed in this area, mainly consisting of low-grade metamorphic, littoral to neritic facies sandstone and siltstone with carbonate rocks. Cambrian rocks are only preserved in the Sanya and Wanning areas, whereas Ordovician and Silurian rocks are widely distributed in Wanning and Qionghai in the east; Danzhou, Chengmai and Danzhou in the north; Changjiang in the west and Ledong and Nanhao in the south. Upper Palaeozoic rocks are mainly distributed in the north of the Jiusuo–Lingshui fault, including Devonian limestone, sandstone, siltstone and shale, as well as Carboniferous shales, metamorphic volcanic rocks and Permian limestone and sandstones (Hu et al. 2002; Long et al. 2007). This makes it extremely difficult to reconstruct Palaeozoic sedimentary environments and tectonic histories (Fig. 2a). The Silurian strata in the Hainan Island are entirely lower Silurian, despite disputes over the ages of these rocks (Li et al. 2007, 2009; Chen et al. 2012; Zhang et al. 2013). The widely accepted stratigraphic framework is Kongliecun Formation (S₁k), Dagancun Formation (S₁d), Kaoqinshan Formation (S₁kq), Zusailing Formation (S₂z). Some Tetracoralla fossils, such as Amplexoides and Tryplasma, are present in the upper part of the limestone in the S₁k (Wang et al. 1992). The strata record changes in sea level, neritic sedimentation with abundant volcanic activity and the end of early Palaeozoic transgression in the Hainan Island (Yao et al. 1999). The S₁d is a set of clastic rocks and carbonate sedimentary deposits of a littoral–neritic shelf (Zeng et al. 2003, 2004). The S₁kq consists of fine siliciclastic rocks and carbonate rocks, with a rhythmic structure similar to flysch. It contains fossils such as Nalivkinia, Xinanospirifer and Striispirifer, indicating a sedimentary environment in the upper part of the subtidal zone (Zeng et al. 2003, 2004). Although the Zusailing Formation (S₂z) is rich in fossils, the fossil preservation is poor, indicating high-energy shallow water environment and the sedimentary facies evolved from neritic shelf to foreshore or nearshore facies (Li et al. 2007, 2009). The section is upwards coarsening from S₁kq to the lower part of Zusailing Formation, reflecting regression. There is evidence of an intracontinental orogeny in the early Palaeozoic of South China. The strong folding and deformation of the pre-Devonian system caused an angular unconformity below the overlying strata. This tectonic event also produced widespread magmatism and resulted in the formation of the Wuyi–Yunkai Orogen (Faure et al. 2009; Liu et al. 2010 a; Charvet, 2013; Zhang et al. 2015). Therefore, the study area recorded an unconformity contact between top of Silurian Zusailing Formation and the Middle Devonian (Zhao et al. 2020; Yao et al. 2021), which marks the end of marine sedimentation and the beginning of long-term weathering and erosion in Hainan Island.

The lower Silurian Zusailing Formation studied in this paper is exposed in the Nanbing–Nanhao Highway section of Baoting County (109°27′58″, 18°36′24″). It is a set of clastic rocks formed in littoral to neritic shelf environments (Fig. 2b), and it mainly consists of sandstone, siltstone, slate, and greywacke rocks (Fig. 2c-e). The lower part consists of grey-yellow quartz sandstone and siltstone that were deposited in the littoral environment. There is also grey-black thin-layered slate and yellow-black, thin-layered sericite phyllite with black carbonaceous grains (Fig. 2b). The upper part mainly consists of greyish-yellow thin-layered siltstone that was deposited in the neritic environment, as well as medium-grained, yellowish-brown, quartz sandstone in thin to medium beds and slate in the middle to upper part (Fig. 2b), which has undergone sericitization. The group yield a large number of fossils, including brachiopods (Xinanospirifer flabellum, Xinanospirifer cf. amanuensis, Xinanospirifer sp., Rhynchonellida gen. sp. indet. Spirifeda gen. sp. indet), corals (Favocites sp. Tetracoralla gen. sp. indet.), trilobites (Latiproetus cf. latilimbatus), gastropods (Homotoma sp.), Sinacanthus (Sinacanthus wuchangensis) and fragmented crinoid fossils (Pentagonocyclicus sp., Cyclocyclicus sp.) (Li et al. 2009; Zhang et al. 2013).

We collected four samples BT-2, BT-3, BT-5 and BT-7 of medium-grained lithic quartz sandstone in the upper part of the section on the Nanbing–Nanhao Highway in Baoting County. The lithological characteristics of the samples are similar, all being sediments of the neritic shelf origin. Photomicrographs of the samples reveal some degree of alteration (sericitization), a medium-grained sandy texture, argillaceous cement and pore cementation, with a content of debris (87–90%) and interstitial fillings (10–13%). The grain size of the detrital particles ranges from 0.05mm to 0.5mm, and it is concentrated in 0.2 mm to 0.3 mm. The detrital particles are mostly granular, moderately sorted and subangular. The sandstone grains are mostly quartz (about 75%), with hypidiomorphic texture predominantly in the form of single crystal quartz, with a small amount of polycrystalline quartz and flint. The single crystal quartz exhibits undulose extinction. Lithic clasts (about 20%), include felsic volcanic rock, quartz sandstone, quartzite and schist (Fig. 3). Feldspar makes up about 5% of the grains, and these have undergone complete sericite alteration, with only pseudomorphs remaining. The interstitial fillings are mainly clay and calcareous mixed detrital constituents, and almost all of them have undergone sericite alteration. Sericite is identified as an interstitial phase, which is a scale aggregate with bright interference colour. Despite the weathering of the samples due to the hot and humid modern climate, the original rocks are identified as medium-grained lithic-rich quartz sandstones.

Figure 3. Photomicrographs of Lithic clasts (BT-4: medium-grained lithic quartz sandstone) (Lv: Volcanic rock fragments; Ls: Sedimentary rock fragments; Lm: Metamorphic rock fragments).

3. Analytical methods

Hebei Langfang Yuneng Mineral Sorting Company was responsible for the crushing treatment of samples. Zircon grains were separated from clastic rocks through heavy liquid and magnetic mineral separation techniques and hand-picked under a binocular microscope. Cathodoluminescence (CL) images were taken by Gantan MiniCL imaging system attached to a JSM-IT100 tungsten filament scanning electron microscope. Zircon U–Pb geochronology was carried out by Laser Ablation-Inductively Coupled Plasma-mass spectrometer (LA-ICP-MS) at the Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Land and Resources, Jilin University, Changchun, China. The GeoLasPro 193 nm ArF excimer laser by COMPExPro in Germany was used for laser ablation. The laser beam diameter was 32 μm, the laser fluence of 10 J/cm² and repetition rate of 8 Hz. The ICP-MS instrument used was an Agilent 7900. High-purity helium was used as carrier gas to provide efficient aerosol transport to the ICP and minimize aerosol deposition around the ablation site and within the transport tube with a gas flow of 600 mL/min. Argon was used as make-up gas with a gas flow of 1.15 L/min. The standard used was reference material NIST 610 (Liu et al. 2010), an artificial synthetic silicate glass developed by the National Institute of Standards and Technology (NIST) in the United States. The collection time for ⁴⁹Ti, ²⁰⁶Pb and ²⁰⁸Pb was 20 ms, for ²⁰⁷Pb 30 ms, for ²³²Th and ²³⁸U 15 ms, and for other elements, it was 6 ms. Isotope ratio correction used the standard zircon 91500 (1062 Ma) as the primary reference material and 91500 was ablated every five unknowns (Wiedenbeck et al. 1995). Uranium, Th and Pb concentrations were calibrated using ²⁹Si as an internal standard. Glitter (ver4.0, Macquarie University) was used to calculate isotope ratio and age values of ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U. Uncertainties of individual analyses are reported with 2s error. Concordance is calculated as: age >1000Ma (²⁰⁷Pb/²⁰⁶U age)/ (²⁰⁶Pb/²³⁸U age) × 100 (%); age<1000Ma, (²⁰⁷Pb/²³⁵U age)/ (²⁰⁶Pb/²³⁸U age) × 100 (%). Only analyses > 90 % and < 110% concordant were used to generate the age histograms, Concordia diagrams and discuss in the data interpretation (Supplementary Fig. S2). The calculation of the weighted average age and the drawing of Concordia diagrams was done with the Isoplot/Ex (3.0) programme (Ludwig, 2003). Weighted mean ages were calculated at 2s confidence level. The zircon Plešovice was dated as the secondary reference material, yielding a weighted mean ²⁰⁶Pb/²³⁸U age of 338.18 ± 5.2 Ma (n = 14, 2 s), which is in excellent agreement with the recommended age of 337.13 ± 0.37 Ma (Sláma et al. 2008). The data and zircon U-Pb concordia diagram are at 2 s error. The specific analytical methods and procedures, as well as the common Pb correction, can be found in Andersen (2002, 2005) and Yuan et al. (2004).

We intentionally selected detrital zircon grains with different geometric shapes, internal structures, little cracks, and inclusions for testing experiments. A total of 380 zircons are selected for U-Pb dating from the four groups of samples. After discarding the age data with concordance lower 90% and greater 110%, 377 effective concordant age analysis points were obtained (Table S1). For zircons with ages younger than 1000 Ma, ²⁰⁶Pb/²³⁸U was used as the best age, while the ages older than 1000 Ma, ²⁰⁷Pb/²⁰⁶Pb was used as the best age. The kernel density estimation (KDE) method was used to visualize all the concordant ages (Vermeesch, 2012).

Uranium-Pb age probability density plots (PDP) of the samples were drawn and analysed visually (e.g. Zhang et al. 2020 a). However, in order to eliminate the influence of subjective judgement on visual analysis and provide a more detailed and quantitative characterization of the similarity among samples, we conducted a quantitative analysis of the similarity of detrital zircon U-Pb age data including cross-correlation coefficient (Saylor et al. 2012, 2013), likeness (Satkoski et al. 2013), similarity (Gehrels, 2011), a Kolmogorov-Smirnov test (Saylor et al. 2012) and multidimensional scaling analysis (MDS) (Vermeesch, 2013; Wissink et al. 2018).

4. Results

The zircon crystals in the samples of this study are 75–120 μm in diameter, with subangular–subrounded shapes, and idiomorphic–hypidiomorhic, transparent, prismatic or granular textures, and have length–width ratios of 1.2:1 to 2.5:1 (Supplementary Fig. S2). Most zircon grains show obvious oscillatory growth zoning in CL images. Th/U ratios of zircon can be used to infer a magmatic or metamorphic origin (Wu & Zheng, 2004). All Th/U ratios of samples from the Zusailin Formation with ages younger than 500 Ma are greater than 0.1, while only a few grains older than 500 Ma are less than 0.1 (Fig. 4). The average Th/U ratios of BT-2, BT-3, BT-5 and BT-7 samples are 0.48, 0.53, 0.67 and 0.54, respectively. Among them, seven zircons in BT-2, three zircons in BT-3, 6 zircons in BT-5 and 3 zircons in BT-7 have Th/U values less than 0.1. Combining the typical oscillatory zoning structure and Th/U ratios of most zircons in the CL images, it can be inferred that most of the detrital zircons from the lower Silurian Zusailing Formation are of magmatic origin, with only a small percentage of metamorphic zircons.

Figure 4. Th/U ratio–ages diagrams of detrital zircon analyses.

Zircon ages of the Zusailing Formation samples in Nanhao village, Baoting County show distribution from the Mesoarchaean to the early Palaeozoic (434-3174 Ma), with most of them being early Palaeozoic age. The five major peaks are as follows: 2800–2200 Ma, 2100–1350 Ma, 1250–950 Ma, 600–480 Ma and 480–430 Ma and mainly distributed in 480–430 Ma (Supplementary Fig. S1). The major age peak of sample BT-2 is ca. 451 Ma (MSWD = 1.9, n = 18), sample BT-3 is ca. 449 Ma (MSWD = 2.4, n = 12), sample BT-5 is ca. 448 Ma (MSWD = 3.4, n = 12), sample BT-7 is ca.446 Ma (MSWD = 0.62, n = 13).

5. Discussion

5.a. Depositional age

In this paper, the age range of the four sample groups is narrow, so the YC2s (weighted mean age corresponding to 2 s) is used to determine the maximum depositional age of the strata (Dickinson & Gehrels, 2009). The YC2s ages of BT-2, BT-3, BT-5 and BT-7 samples are respectively 436.6 ± 9.4 Ma (MSWD = 0.002), 435.5 ± 7.8 Ma (MSWD = 0.01), 435.9 ± 6.1 Ma (MSWD = 0.09) and 438.3 ± 9.1 Ma (MSWD = 0.02) (Fig. 5a). Combined with the abundant palaeontological fossils from the Zusailing Formation, it can be inferred that the maximum depositional age of the Zusailing Formation is Telychian (upper Llandovery).

Figure 5. (a) The youngest detrital zircon weighted average age of Zusailing Formation; Similarity analysis among samples; (b) Cross-correlation; (c) Likeness; (d) Similarity; (e) K–S Test D statistic.

The results of the similarity analysis of the Zusailing Formation samples (Fig. 5b-e) show that the results of cross-correlation R², likeness and similarity are close to 1, while the results of K-S test D statistic are close to 0. Combined with the probability density plot of detrital zircon U-Pb ages (Supplementary Fig. S2), the weighted mean age of the youngest detrital zircon (Fig. 5a), and the chi-square statistics and p-values of the Zusailing Formation (Table S4), it can be concluded that the detrital zircon age composition of the Zusailing Formation in the study area is very similar, indicating a common provenance. The four groups of samples from Zusailing Formation are combined and discussed later.

5.b. Sediment sources

The detrital zircon age spectra have 9.2% Archaean ages, 32.9% Palaeoproterozoic ages, 20.5% Mesoproterozoic ages, 13.1% Neoproterozoic ages and 24.3% early Palaeozoic ages. The age peaks are ca. 1769 Ma, ca 1612 Ma, ca. 1430 Ma, ca 1119 Ma, ca. 574 Ma and ca. 449 Ma. The cumulative proportion diagram (Cawood et al. 2012) shows that the cumulative curves of the four sample are similar, although a small number of zircons (about 20%) have crystallization ages that are close to the sedimentation age, indicating that the sediment was deposited in a collisional basin (Fig. 6b).

Figure 6. (a) Kernel density estimate (KDE) diagrams of detrital zircon U–Pb ages from Zusailing Formation and Crustal thickness between 435–1800 Ma modelled by zircon anomalies Eu/Eu* (chondrite-normalized ${\rm{Eu}}/\sqrt {{\rm{Sm}} \times {\rm{Gd}}} $ ). The black and green curves represent global compilation (Tang et al. 2021) and the margin of East Gondwana continents (after Wu et al. 2023); (b) Cumulative ratio curve of the difference between measured crystallization age and sedimentary age of detrital zircons, convergent (A: red field), collisional (B: blue field), extensional settings (C: green field) (after Cawood et al. 2012).

The 2800–2200 Ma and 2100–1350 Ma detrital zircons show euhedral-subhedral morphologies, but they have complex internal structures, characterized by brighter CL images, weak and planar zoning, and metamorphic recrystallization (Supplementary Fig. S1). Therefore, we speculate that the detrital zircons in this age group are likely from nearby areas and/or reflect inputs from multi-cycle sediment sources. Detrital zircons in the Zusailing Formation have main age peaks at ca. 1769 Ma, ca. 1612 Ma and ca. 1430 Ma, as well as multiple small peaks, which are almost identical with the ca. 1.78 Ga, ca. 1.60 Ga and ca. 1.43 Ga age zircons from the Baoban Complex in the basement of Hainan Island (Fig. 7c-f). Previous studies have shown that the complex magmatic and metamorphic events in the Baoban Complex are related to the Nuna (Columbia) supercontinent cycle, and it is highly possible that Hainan Island was connected to western Laurentia and eastern Australia during the Mesoproterozoic to Palaeoproterozoic (Zhou et al. 2015 a; Yao et al. 2017; Zhang et al. 2018 a, 2019; Xu et al. 2019; Zhao et al. 2020, 2021; Suzuki et al. 2023). The detrital zircon ages of the Zusailing Formation have similar Mesoproterozoic–Palaeoproterozoic peaks as those of the Belt–Purcell Supergroup in western Laurentia and the lower to middle Rocky Cape Group in Tasmania in eastern Australia. In summary, we argue that the detrital zircon age groups in the samples from 2800–2200 Ma and 2100–1350 Ma record the tectonic events of the Nuna supercontinent, and the ultimate source regions were likely western Laurentia and eastern Australia.

Figure 7. The speculated palaeoposition of Hainan Island and summary of detrital zircon age of distribution of sedimentary rocks of this study and previous work. (a) Showing the connection of Hainan with Laurentia and Australia at –1.43 Ga during the break-up of Nuna (modified after Yao et al. 2017; Zhang et al. 2018 a, 2019; Xu et al. 2019); (b) showing the proposed configuration of Hainan, Yangtze, Cathaysia, India, Australia, Antarctica and Laurentia at 1.3–0.9 Ga assembly of Rodinia supercontinent (modified after Li et al. 2018; Qi et al. 2020; Zhang et al. 2020 b; Wu et al. 2023); Abbreviations: BPB, Belt-Purcell Basin; CB, Cathaysia Block; PB, Perth Basin; VB, Vindhyan Basin; YB, Yangtze Block; YC, Yilgarn Craton. Green arrows show hypothetical transport pathways of sediments from sources. (c) South Hainan Island; (d) Baoban Complex from Hainan Island ; (e) Belt-Purcell Supergroup from western Laurentia; (f) lower to middle part of the Rocky Cape Group from Tasmania; (g) lower-middle Rocky Cape Group from Tasmania; (h) Truong Son Belt in Central Vietnam and Eastern Laos; (i) Perth Basin in west Australia; (j) Sibumasu. Data sources for (c) – this study; (d) – Li et al. 2002, 2008; Yao et al. 2017; Zhang et al. 2018 a; (e) – Ross et al. 1991, 1992; Rämö et al. 2003; Ross & Villeneuve, 2003; Link et al. 2007; Stewart et al. 2010; Doe et al. 2012; Malone et al. 2017; Mulder et al. 2017; (f) – Black et al. 2004; Halpin et al. 2014; Mulder et al. 2016; (g) – Malone et al. 2008; Mckenzie et al. 2011, 2013; Turner et al. 2014; (h) – Wang et al. 2016, 2021 a; (i) – Cawood & Nemchin, 2000; Veevers et al. 2005; Condie et al. 2009; (j) – Cai et al. 2017; Wu et al. 2023. All data are based on analyses within 90–110% of concordance. n=number of concordant analyses.

During the Mesoproterozoic–Neoproterozoic Grenvillian Orogeny (1.3–0.9 Ga), the residual fragments of the Columbia supercontinent that were not completely disintegrated converged to form the Rodinia supercontinent (Fig. 7b) (Cawood, 2020; Zhang et al. 2020 b; Wu et al. 2023). The Grenvillian Orogeny (1.3–0.9 Ga) is extensively recorded in both Laurentia and Gondwana (Fig. 7c-j). The upper Vindhyan sequence of the Vindhyan Basin in northwestern India contains a large amount of 1200–1000 Ma detrital zircons (McKenzie et al. 2013; Turner et al. 2014). There is an age peak of 964 Ma and a detrital zircon age group of 1160–1076 Ma in the Cambrian–Devonian sandstone of the Truong Son Belt in the northern Indochina Block (Wang et al. 2016, 2021 a). The Silurian-Ordovician strata in the Perth Basin in Western Australia record 1200–1050 Ma detrital zircons from the Albany-Fraser Belt on the southern margin of the Archaean Yilgarn Craton (Fitzsimons, 2000; Meert, 2003; Wang et al. 2016; Olieook et al. 2019). In northern East Gondwana, Sibumasu also contains a significant percentage of Greenlandian detrital zircons (Cai et al. 2017; Wu et al. 2023). But at this time, Sibumasu had not fully formed, with the eastern and western Sibumasu blocks receiving sediment from northern India and western Australia, respectively (Zhang et al. 2018 b). Through the comparison of detrital zircon age groups, we found that both the Vindhyan Basin in northwest India and the Perth Basin in Western Australia contain 1300–900 Ma detrital zircons reflecting the Grenville Orogeny. However, considering the distance between the study area and the Indian Plate when these basins were formed, the possibility of Vindhyan Basin providing detritus to the study area was excluded. On the other hand, plates such as the Truong Son Belt and Sibumasu formed relatively later, and their Neoproterozoic detrital zircons are of a recycled origin. Therefore, the provenance of the 1250–950 Ma detrital zircons in the sample may be the Albany–Fraser Belt in Western Australia.

During the Pan-African Orogeny (ca. 550 Ma), subduction of the proto-Tethys Ocean caused a series of microcontinents to coalesce to the northern margin of East Gondwana (Wang et al. 2016; Zhou, 2018; Qi et al. 2020; Zhou et al. 2021 a; Du, 2022; Wu et al. 2023). Palaeomagnetic results show that during the Pan-African Orogeny, India, East Antarctica, and Australia converged to form East Gondwana (Fitzsimons, 2000, 2003; Powell et al., 2001; Pisarevsky et al., 2003). From 570 to 510 Ma, the collision of India–Antarctica and Australia–Antarctica resulted in the formation of the Kuunga Orogeny (Meert, 2003; Meert & Lieberman, 2008; Boger, 2011). The area between India and Western Australia, which is called the Pinjarra Orogen (Fitzsimons, 2003; Harley et al. 2013), records significant regional deformation and high-grade metamorphism because of convergence (Halpin et al. 2008; Pankhurst et al. 2008; Boger, 2011). However, there are no magmatic events in South China (Yangtze and Cathaysia Blocks) during this period (Wang et al. 2010). In summary, we argue that the ca. 574 Ma age peak and 600–480 Ma detrital zircon in the samples record the tectonic cycles of the Gondwana and the source area was the Pinjarra Orogen between India and Western Australia.

The 480–430 Ma detrital zircons show a significant ca. 449 Ma age peak, and the zircons exhibit typical magmatic zoning with euhedral to subhedral crystals (Supplementary Fig. S2), indicating that most of the sediments were deposited proximally. In the north of Baoting County, rocks of this age are widely reported. A 453–448 Ma andesite rock related to subduction events have been identified in Bangxi and Tunchang areas in northern Hainan Island (Zhou et al. 2021 b; Du, 2022). Under the influence of Kwangsian Orogeny, the Wuyi-Yunkai Orogen in the Cathaysia Block preserves many early Palaeozoic magmatic and metamorphic records, most of which are S-type granites. These granites are mainly derived from Precambrian basement with limited mantle-derived components (Li et al. 2010; Liu et al. 2010; Xia et al. 2014). Provenance studies of lower Palaeozoic strata in South China indicate that the detritus in the Yangtze block originated from the rocks associated with the Kwangsian orogeny in the Cathaysia Block (Xu et al. 2012, 2013; Wang et al. 2010). The main Silurian strata exposed north of the Jianfeng-Diaoluo fault in the Hainan Island is the Tuolie Formation, which comprises a regressive sedimentary sequence of deep to bathyal to shallow sea origin (Yao et al. 1999; Zeng et al. 2003). However, the Tuolie Formation is not exposed in the study area on the south side of the Jianfeng-Diaoluo fault. Here, the Silurian strata show regression through a succession of shallow marine, neritic shelf, and foreshore or nearshore origin consisting of S₁k, S₁d, S₁kq and S₂z (Zeng et al. 2003, Li et al. 2007). In conclusion, the Silurian deposits on Hainan Island suggest regression, evident in both the deeper water sequences to the north and the shallower water sequences to the south. Furthermore, the Zusailing Formation was neither sourced from the ca. 450 Ma rocks in the north of Baoting County nor from the ca. 440-Ma-old detritus widespread in early Palaeozoic sedimentary rocks in South China. We propose that the ca. 449 Ma age peak in the detrital zircon ages of the Zusailing Formation likely signifies a hitherto unidentified tectonic event in southern Hainan Island. The provenance of the Zusailing Formation provides sedimentary evidence for a more detailed and accurate reconstruction of the early Palaeozoic tectonic evolution of Hainan Island and its position within East Gondwana.

5.c. Early Silurian tectonic environment in the Hainan Island

Trace elements indicate that the magmatic zircons are mostly derived from granitoids (Fig. 8a-d). Relative to the primitive mantle, U is usually significantly enriched and Yb is slightly depleted in island arc magma, resulting in a higher U/Yb ratio than mid-ocean-ridge basalt (MORB) (Grimes et al. 2015). In the diagram of U/Yb-Hf and U/Yb-Y zircon genesis, most of the 600–434 Ma detrital zircons in the Zusailing Formation, indicate that the zircons are predominantly of continental crust origin (Fig. 8e, f). The relative contents of Th, U, Hf and Nb in zircon can reflect the tectonic background during crystallization, and the differences in these elements can be used to distinguish the tectonic background of host magmas. Niobium depletion of arc magma is more obvious than that of intraplate magma, so the zircons formed by arc magma crystallization have lower Nb/Hf ratio and higher Th/Nb ratio (Yang et al. 2012 a, b). In the Nb/Hf-Th/U and Hf/Th-Th/Nb diagrams, the 434–600 Ma detrital zircons from the Zusailing Formation samples almost all plot in the island arc/orogenic belt field (Fig. 8g, h). Considering that the detrital zircons are mainly formed in the continental crust, it can be inferred that the 434–600Ma detrital zircons in the Zusailing Formation samples record orogenic events on the southern side of Hainan Island.

Figure 8. Trace element diagrams of Palaeozoic zircon grains from dated samples in the Zusailing Formation. (a) Y–U; (b) Nb–Ta; (c) Y–Tb/Sm; (d) Y–Nb/Ta (after Belousova et al. 2002); (e) U/Yb–Hf; (f) U/Yb–Y (after Grimes et al. 2007); (e) Nb/Hf–Th/U; (f) Hf/Th–Th/Nb (after Yang et al. 2012 a, b).

Recently, the application of Eu anomaly (Eu/Eu*) of detrital zircon as a chemical mohometer to estimate crustal thickness has gained widespread use (Tang et al. 2020, 2021; Brudner et al. 2022; Carrapa et al. 2022; Gao et al. 2022a; Wu et al. 2023). In this paper, we used zircon europium anomaly [chondrite-normalized (Sun & McDonough, 1989) Eu/√(Sm×Gd)] based empirical equation of detrital zircons from Zusailing Formation to estimate the crustal thickness during Mesoproterozoic to Early Palaeozoic in the provenance region (Fig. 6a). The empirical equation calculates crustal thickness (Tang et al. 2020):

$$\rm z = (84.2 \pm 9.2) \times Eu/Eu*zircon + (24.5 \pm 3.3) $$

where z is the crustal thickness (in km). To eliminate the influence of metamorphic zircons and those affected by inclusions, data with Th/U<0.1 and/or La>1 parts per million (ppm) are removed before calculating the crustal thickness (Hoskin & Schaltegger, 2003). The highest 10% and the lowest 10% Eu/Eu* data within 5-year intervals are removed when the average crustal thickness is calculated. This selection method will only reduce the scatter of data and will not change the calculated crustal thickness pattern. Our estimate of the crustal thickness of the Hainan Island region before ca. 600 Ma is in good agreement with the continental thickness of the margin of East Gondwana (Wu et al. 2023). However, after ca. 600 Ma, the crustal thickness of the Hainan Island region rapidly increased and even exceeded the average thickness of crust, based on global compilations (Tang et al. 2021). Thick continental crust usually occurs in continental arcs (Ducea et al. 2015; Tang et al. 2020), and in Bangxi and Tunchang areas in the northern Hainan Island, there are 453Ma-448Ma andesites that are associated with subduction (Zhou et al. 2021b; Du, 2022). Thickened crust reflects the uplift of the sediment source area, which is consistent with the transformation of the lower Silurian strata, recording a transition from marine to terrestrial in Hainan Island. Therefore, the sedimentary environment of the Zusailing Formation in the Hainan Island is back-arc basin in a continental arc, recording the event of crustal thickening between 600 to 434 Ma on the southern side of Hainan Island.

5.d. Sedimentary response to the closure of the Proto-Tethys Ocean

The age spectrum of detrital zircons from the lower Silurian Zusailing Formation in the Hainan Island is compared with those of the Silurian–Devonian strata in northern East Gondwana, including Truong Son, Kontum, Baoshan, Sibumasu, Western Australia, Lhasa, South and North Qiangtang, as well as the Cathaysia and Yangtze Blocks (Fig. 9a-l). We found that the Simao Block, the Truong Son Block of the northern Indochina Block, Kontum of the southern Indochina Block, Baoshan, Sibumasu, Western Australia, Lhasa, and southern Qiangtang have similar ca. 574 Ma age peaks and 600–480 Ma age groups to the samples in this study, indicating that these microcontinents might have recorded Pan-African tectonic events, received sediment from the Kuunga Orogeny and were relatively close to the orogenic belt (Fig. 10a). The majority of the ca. 600–434 Ma detrital zircons from the lower Silurian Zusailing Formation in the Hainan Island were crystallized in granitic melts in continental crust. The tectonic setting of the granites is island arc/orogenic belt, recording the subduction of the Proto-Tethys Ocean during Neoproterozoic to early Palaeozoic. However, the evolution of the Proto-Tethys Ocean along the northern margin of the entire Gondwana is different. A large number of rock formations related to the subduction of the Proto-Tethys Ocean have been found along the northern margin of the West Gondwana, such as 576–536 Ma granitic rocks and 579–536 Ma metabasic rocks in Kuh-e-Sarhangi area, central Iran (Rossetti et al. 2015; Hosseini et al. 2015), 512–467 Ma granitic rocks and included ca. 477 Ma crustal xenoliths from central-western Nepal (Cawood et al. 2007), Andean-type granites of 480-466 Ma in the Utla and Mansehra regions of northwestern Pakistan (Sajid et al. 2018). There is also a lot of evidence for the subduction of the Proto-Tethys Ocean in the northern margin of India-Australia in East Gondwana. The Lhasa Terrane reported 522–496 Ma ultrapotassic rhyolites, 536–492 Ma meta-rhyolite, and 492 Ma meta-basalt (Zhu et al. 2012; Ding et al. 2015). The 505–468 Ma granitic gneisses and a small amount of 463±5 Ma meta-basalts in the South Qiangtang Terrane (Guynn et al. 2012; Peng et al. 2014), 502–471Ma magmatism in Gemuri area (Wang et al. 2020) and the Duguer region has also reported Late Silurian high-pressure eclogites (Zhang et al. 2014). The Tengchong Terrane has widely distributed 488–484 Ma leucogranites, 495–487 Ma granite gneisses and 492-485 Ma light-coloured granites (Eroğlu et al. 2013; Wang et al. 2013 a; Zhao et al. 2016). The Proto-Tethys Ocean subducted towards the northern margin of the Gondwana, with the western part earlier than the eastern part, and initiation of the subduction towards the northern margin of the India-Australia in East Gondwana should not be later than the Early Cambrian (Fig. 10). The ca. 574 Ma peak age in the lower Silurian Zusailing Formation of the Hainan Island and the detrital zircons of the 600–480 Ma age group reflect this subduction event. The provenance of Hainan Island at this time should be the Pinjarra Orogen between India and Western Australia.

Figure 9. Comparison of the provenance of each microcontinent along the northern margin of East Gondwana. (a) Hainan Island; (b) Simao Block; (c) North Indochina Truong Son; (d) South Indochina Kontum;(e) Baoshan Terrane; (f) Sibumasu Block; (g) West Australia; (h) Lhasa Terrane; (i) North Qiangtang Terrane; (j) South Qiangtang Terrane; (k) Cathaysian Block; (l) Yangtze Block. Data sources for (a) – this study; (b) – Xia et al. 2016; Zhao et al. 2017; (c) – Wang et al. 2016, 2021 a; (d) – Wang et al. 2021 a; (e) – Nie, 2016; (f) – Cai et al. 2017; Wu et al. 2023;(g) – Cawood & Nemchin, 2000; Veevers et al. 2005; Condie et al. 2009; (h) – Leier et al. 2007; Zhu et al. 2011; Wang et al. 2021c ; (i) – Peng et al. 2019; Fu et al. 2022; (j) – Pullen et al. 2008; Dong et al. 2011; Zhu et al. 2011; Liu et al. 2022 b; (k) – Wang et al. 2010; Yao et al. 2011; (l) – Wang et al. 2010; Duan et al. 2011; Xu et al. 2012; Yang et al. 2012 a, b; Xia et al. 2016; Ma et al. 2018; Ren et al. 2023. All data are based on analyses within 90–110% of concordance. n=number of concordant analyses. (m) Showing the proposed configuration of Northeastern Gondwana at ca. 0.45 Ga during assembly of the Gondwana supercontinent. (The MDS plot is based on the Kolmogorov-Smirnov (K-S) statistical approach, which is used to separate zircon grains that came from various sources. Different coloured dots correspond to the colours of the microcontinents and KDE diagrams of detrital zircons from them. There is no weight significance for the X and Y axes and do not reflect the real numerical difference. The samples with the highest similarity values are plotted closest to each other. Solid and dashed lines connect the samples with their closest and second-closest neighbours in this plot, respectively.) (modified after Xu et al. 2014; Zhang et al. 2018 b; Qi et al. 2020; Zhou et al. 2021 a; Zhang et al. 2022, 2023; Dodd et al. 2023; Wu et al. 2023).

Figure 10. Reconstruction of the Palaeogeography and Tectonic Evolution of the Eastern Proto-Tethys Ocean during Neoproterozoic-Palaeozoic. (modified after Xu et al. 2014; Zhang et al. 2018 b; Qi et al. 2020; Zhou et al. 2021 a; Zhang et al. 2022, 2023; Dodd et al. 2023; Wu et al. 2023).

The Simao Block, Truong Son in the northern Indochina Block, Kontum in southern Indochina Block, Baoshan Terrane and Sibumasu Block all have 480–430 Ma detrital zircon age groups. Multidimensional scaling analysis (MDS) is often used for statistical comparison of detrital zircon age compositions and potential provenance or adjacent topography (Vermeesch, 2013). The MDS plot based on the Kolmogorov-Smirnov (K-S) statistical method (Fig. 9m), shows that the Silurian Zusailing Formation in the Hainan Island has a strong affinity with the Simao Block, Truong Son in the northern Indochina Block, Kontum in southern Indochina Block, Baoshan Terrane and Sibumasu Block. This indicates that during 480-430 Ma, these microcontinents were in close proximity to each other in the region, and may be in the same source-to-sink system, recording similar tectonic events (Fig. 10). The South China Block shows a younger age peak of ca. 420 Ma, and does not have the ca. 574 Ma age peak discussed above, which is significantly different from other microcontinents (Fig. 9k, l). Our interpretation is that during formation of East Gondwana in the early Palaeozoic, the far-field response of subduction and collision of the Proto-Tethys Ocean towards the South China Block triggered the intracontinental orogeny (Kwangsian intracratonic orogenic), forming Wuyi-Yunkai Orogen (Wang et al. 2010; Xu et al. 2012, 2013; Du, 2022). This indicates that the South China Block was further away from the main subduction zone of the Proto-Tethys Ocean and other microcontinents (Fig. 10a, b), which resulted in it being affected by the Proto-Tethys Ocean subduction later than the microcontinents. The amphibolite facies metamorphism of 462 Ma in the Hainan Island is similar to the granulite age of 468 Ma in the Indochina (Faure et al. 2018; Zhang et al. 2022). In addition, the ε _Hf (t) values of detrital zircons from the Ordovician–Silurian (460–430 Ma) in the Hainan Island and Indochina are also very similar (Zhang et al. 2023). These phenomena are all related to the collision closure of the Proto-Tethys branch ocean (Tran et al. 2014; Wang et al. 2021 a, b; Nguyen et al. 2019). Hainan Island was closer to Indochina block in the early Palaeozoic. The ocean basin between Hainan Island and East Gondwana in the early Palaeozoic was an extension of the Tam Ky-Phuoc Son Ocean, which was a larger branch ocean basin of the Proto-Tethys Ocean adjacent to the northern margin of India-Australia.

This study shows that there was a transition from a marine to a continental environment in the Hainan Island region during the early Silurian. The 480–430 Ma detrital zircons in the Zusailing Formation were supplied by the granites of a continental arc. The crustal thickness estimated by the Eu anomaly of the detrital zircons reflects the subduction of the Proto-Tethys Ocean in southern Hainan Island at that time, which thickened the crust under the Andean-type orogenic zone. Meanwhile, the subduction of the Proto-Tethys Ocean underneath northern East Gondwana during the Ordovician to Silurian is also widely recorded. In southwestern Yunnan, there are ophiolitic melanges in the Longmucuo-Shuanghu and the Changning-Menglian suture zones (Liu et al. 2017, 2021), abundant high-pressure metamorphic rocks in the Lancang Group (Peng et al. 2022), 459–456 Ma Huimin metamorphic volcanic rocks (Nie, 2016; Xing, 2016), 445–439Ma Nantinghe gabbro (Wang et al. 2013 b; Nie, 2016). The Truong Son, Tam Ky-Phuoc Son and Kontum areas in the Indochina block have abundant evidence of magmatic activities related to the bidirectional subduction of the Early Palaeozoic Tam Ky-Phuoc Son Ocean (Shi et al. 2015; Nakano et al. 2021; Wang et al. 2021 a, b). The Sukhothai Terrane belonged to the Indochina Block in the Early Palaeozoic, and the 450-420 Ma feldspar/quartz schist in this area was formed in the plate collision environment (Nie, 2016). The Simao Block is considered to be the extension of the Indochina Block, and there is a great abundance of 446–420 Ma magmatic rocks related to the subduction of the Proto-Tethys Ocean in this area (Liu et al. 2018, 2019; Zhao, 2019). The Baoshan Block, which is generally recognized as the northward extension of the Sibumasu, has 499 ± 2 Ma meta-basalt and 502-448 Ma granite rocks related to the retreat or break-off of the Proto-Tethys Oceanic (Yang et al. 2012c ; Dong et al. 2013; Metcalfe, 2013; Wang et al. 2013 b; Li et al. 2016; Zhang et al. 2018 a; Zhou et al. 2020). However, Sibumasu did not separate from Western Australia in the Early Palaeozoic and thus records the characteristics of Gondwana (India-Australia) (Metcalfe, 2013; Liu, 2020; Gao et al. 2023), whereas the Hainan Island and the Indochina block have the characteristics of North Vietnam-South China (Xing, 2016; Zhao, 2019; Dan et al. 2022). The subduction of the Proto-Tethys Ocean along the northern margin of the East Gondwana in the late Palaeozoic was bidirectional. In the late Neoproterozoic to early Cambrian, the Proto-Tethys Ocean only subducted towards the northern edges of India and Australia. By the end of the early Palaeozoic, the Proto-Tethys Ocean kept subducting towards the northern edges of India-Australia while increasing its subduction to the North Vietnam-South China (from unidirectional to bidirectional subduction) (Fig. 10c, d). The ca. 449 Ma age peak of the Early Siluria Zusailing Formation and the 480–430 Ma detrital zircons age group in the Hainan Island well record the subduction events of the Proto-Tethys Ocean, with the provenance of the continental arc formed by the southward subduction of the Proto-Tethys Ocean towards Hainan Island (Fig. 10b, d). Additionally, due to the bidirectional subduction of the Proto-Tethys Ocean, Hainan Island collided with the Sibumasu on the margin of Western Australia in the early Silurian, resulting in a transformation of the sedimentary environment of Hainan Island from marine to continental and an influx of material from the Pinjarra Orogen. After the collision, the southern part of Hainan Island uplifted and remained in a state of weathering and erosion for a long time (Fig. 10e). In the late Palaeozoic, the break-up of the Gondwana caused Hainan Island to separate from Western Australia, so that the magmatic rocks with subduction-collision related information were eroded or collapsed in the basin, with the result that no early Palaeozoic subduction-collision magmatic rocks were discovered in the southern part of Hainan Island.

In summary, the age spectrum of detrital zircons from the Zusailing Formation in the Hainan Island shows peaks at ca. 574 Ma and ca. 449 Ma, which reflect two tectonic events. The ca. 574 Ma age peak records the subduction of the Proto-Tethys Ocean beneath the northern margin of India-Australia in East Gondwana, and the ca. 449 Ma age peak records the subduction of the Proto-Tethys Ocean beneath the North Vietnam-South China. The microcontinents in northern East Gondwana documenting the closure of the Proto-Tethys Ocean can be roughly divided into two stages. The first stage, 600–480 Ma, records subduction of the Proto-Tethys Ocean towards the northern margins of India-Australia, and the subduction began no later than the Early Cambrian. In the second stage, 480–430 Ma (youngest records in the South China Block), there was subduction of the Proto-Tethys Ocean towards the North Vietnam-South China.

6. Conclusion

1. (1) The detrital zircon U-Pb ages of the Zusailing Formation in the early Silurian exhibit five age groups of 2800–2200 Ma, 2100–1350 Ma, 1250–950 Ma, 600–480 Ma and 480–430 Ma and are mainly distributed in 500–430 Ma, with a ca. 449 Ma age peak. The 2800–2200 Ma and 2100–1350 Ma detrital zircons are derived from the ancient basement of Hainan Island, with inferred provenance from western Laurentian craton and eastern Australia. The 1250-950Ma detrital zircons are sourced from the Albany-Fraser belt in Western Australia. The provenance of the 600–480Ma detrital zircons is the Pinjarra Orogen located between India and Western Australia. The 480–430 Ma detrital zircon source area is a continental arc formed by the subduction of the Proto-Tethys Ocean to the south of Hainan Island.

2. (2) The subduction of the Proto-Tethys Ocean beneath the northern margin of India–Australia in East Gondwana had been unidirectional from 600 to 480 Ma. Moreover, from 480 to 430 Ma, the Proto-Tethys Ocean evolved to subduct beneath both East Gondwana and North Vietnam-South China, which includes Hainan Island (bidirectional subduction). The two age peaks of ca. 574 Ma and ca. 449 Ma of the Zusailing Formation in the Hainan Island record the tectonic evolution from unidirectional to bidirectional subduction during the closure of the Proto-Tethys Ocean.