Geochronology and geochemistry of late Cenozoic volcanics on Naozhou Island, South China: insights into the interaction between the paleo-subduction slab and the Hainan mantle plume

Abstract

The mechanisms and processes of interaction between paleo-subducted slabs and mantle plumes are not well understood, primarily due to the challenges associated with direct observation. The Leiqiong Area (LQA), located in the northwestern South China Sea (SCS), may provide an ideal site to study the interaction between mantle plumes and paleo-subducted slabs. Extensive Late Cenozoic volcanic activities are present in the LQA, encompassing the Leizhou Peninsula (LP) and northern Hainan Island. This study conducted K-Ar dating, major and trace element analysis, and Sr-Nd-Pb-Hf isotopic analysis on volcanic rock samples from Naozhou Island, the largest volcanic island in the northeastern part of the LQA. The dating results show two periods of magmatic activities on Naozhou Island (3.6 Ma and 1 Ma). The geochemical results indicate that the columnar jointed basalts from Naozhou Island mainly show characters of oceanic island basalt (OIB). The isotopic data suggest origins of depleted MORB mantle (DMM) and Enriched Mantle (EM), with EM potentially originating from the Hainan mantle plume. In view of these findings, the study further integrates data (Geochronology, trace elements and isotopic composition) from other volcanic rocks in the LQA to explore the deep mechanisms of extensive volcanic activity and plume-slab interactions along the northwestern SCS margin. We discovered that the volcanic rocks from southern LP and northern Hainan Island are characterized by OIB, IAB and OIB-IAB transition like, however, the volcanic rocks from Naozhou Island (northern LP) and Weizhou Island (western LP) are characterized by OIB like merely. This can be explained by a branched Hainan mantle plume model and may indicate the interaction between Hainan mantle plume and paleo-subdected slab mainly focuse on the center location of the plume rather than distal margin. This conjecture is also in accord with the subduction direction of the Late Mesozoic subduction zone along the northern margin of the SCS.

1 Introduction

The interaction between mantle plumes and subducted slabs is a fundamental geodynamic process, exerting a significant influence on magmatic differentiation, volcanic edifice formation, and global geochemical fluxes (Foulger and Natland, 2003; Hofmann, 1997). The interaction may result in the flattening of subduction zones (Dalziel et al., 2000), the deflection of mantle plumes (Kincaid et al., 2013; Mériaux et al., 2015), extensive magmatic activity (Druken et al., 2014; Gazel et al., 2011; Yang et al., 2023), and compositional heterogeneity of mantle plumes (Xu et al., 2019; Xu et al., 2021; Yu et al., 2022), etc. Seismic tomography studies have revealed the presence of mantle plumes near certain subduction zones (Obrebski et al., 2010), suggesting potential interactions between mantle plumes and nearby subducting slabs (Mériaux et al., 2016; Toyokuni et al., 2022). Modern examples include the Tonga subduction zone and the Samoa plume (Price et al., 2014; Wendt et al., 1997; Chang et al., 2016), the Cascadia subduction zone and Yellowstone plume (Smith et al., 2009), as well as the Kamchatka subduction zone and the Kamchatka plume (Gorbatov et al., 2001). However, tomography has limitations in reconstructing slab-plume interactions from the pre-Cenozoic era. Evaluating pre-Cenozoic slab-plume interactions requires alternative proxies. Reconstructions of supercontinents and ancient large igneous provinces have demonstrated a spatiotemporal coupling between large igneous provinces and subduction abyssal systems (Wang et al., 2013), potentially indicating that slab-plume interactions were more widespread in ancient times. However, the processes by which subducted slabs are incorporated into mantle plumes and recycled back into the lithosphere remain unclear. The South China Sea (SCS), the largest marginal sea along the Western Pacific margin, has a complex tectonic history shaped by convergent and transform interactions among the Eurasian, Indo-Australian, and Pacific plates (Li et al., 2015; Li and Li, 2007; Briais et al., 2012; Taylor and Hayes, 1983) (Figure 1A). Its northern continental margin preserves a record of Late Mesozoic subduction history (Li et al., 2018; Cui et al., 2021), while the region is also characterized by the presence of the Hainan mantle plume (Xia et al., 2016) (Figure 1B). This makes the SCS a unique natural laboratory for studying interactions between mantle plumes and paleo-subducted slabs.

FIGURE 1

Briais et al., 2012; Taylor and Hayes, 1983); (b) Distribution of volcanoes ages in the LQA (Huang et al., 1993; Zhu and Wang, 1989; Zhang et al., 2020; Fan et al., 2006; Wang et al., 2021), Hainan Mantle Plume location (Xia et al., 2016); (c) Nayan Coast; (d) Turtle Coast.

Late Cenozoic basaltic rocks are extensively distributed across the northwestern margin of the SCS, particularly in the Leiqiong Area (LQA), which encompasses the Leizhou Peninsula (LP) and northern Hainan Island, covering approximately 7,000 km². Previous studies have proposed three models to explain the magma sources and volcanic activities in the LQA. ① Sub-continental lithospheric mantle (SCLM) model. Tu et al. suggested that the late Cenozoic volcanism on Hainan Island originated from the SCLM, with magma generation primarily influenced by the dynamics of the lithospheric mantle beneath the region (Tu et al., 1991). Similarly, Zhu and Wang, and Huang et al. investigated the Quaternary volcanoes in the LQA, in terms of geochronology, whole-rock major and trace elements, and Sr-Nd-Pb isotopes, proposed that volcanic activity was controlled by fault activities, with magma primarily derived from the lithospheric mantle (Huang et al., 1993; Zhu and Wang, 1989). ② Mantle plume model. Plenty of researchers have suggested that the typical OIB-type basalts in the LQA were derived from Hainan mantle plume, a deep-seated upwelling of hot material from the lower mantle (Ho et al., 2000; Lei et al., 2009; Zou and Fan, 2010; Liu et al., 2015). ③ Mantle plume interacts with subducted slab model. Recent studies suggest that the magma source is a mixture of depleted MORB mantle (DMM) and Enriched Mantle Ⅱ (EMⅡ) components (Zhang et al., 2020; Yung-Tan et al., 2022; An et al., 2017), indicating the interaction between the Hainan mantle plume and subduction slab. Wang et al. proposed that the LQA represents a rare example of a young mantle plume interacting with a deeply subducted slab. They suggested that the volcanic activity on Hainan Island was influenced by subduction-related processes (Wang et al., 2012). Zhao et al. proposed that, based on whole-rock major and trace elements and olivine geochemistry, the volcanic rocks in the LP range from typical OIB-type to IAB-type, likely linked to the subduction of the Paleo-Pacific Plate (Zhao et al., 2021). Futhermore, Chen et al. employed in situ Sr isotope disequilibrium in plagioclases of late Cenozoic basalts to show the influence of recycled subduction-related H₂O-enriched oceanic fluids/melts carried by the deep Hainan mantle plume across the entire LQA (Chen et al., 2023).

As mentioned above, the magma sources of the volcanic rocks in the LQA remain controversial and the spatial distribution of the interaction between Hainan mantle plume and subducted slab are still unclear. Additionally, previous studies mainly focused on the region of southern LP and Hainan Island, sparsely research on the northern LP. Considering these volcanics on the northern LP are located in the distal margin of the Hainan mantle plume, therefore, their geochemical features and ages may provide some valuable constraints about the evolution and spatial pattern of the plume, even the plume-slab interaction.

This study collected 20 volcanic rock samples from Naozhou Island, the largest volcanic island in South China Sea (Figures 1C,D). Through detailed geochemical analyses of whole-rock major and trace elements, coupled with high-precision Sr-Nd-Pb-Hf isotope geochemistry and K-Ar geochronology. We aim to delineate the magmatic sources and temporal evolution of the Naozhou Island basalts and discuss the spatial distribution of magma sources in the LQA combined with previously published data. This work fills the gap in petrological and geochemical records in the northern LP, where volcanic rock detail data have been lacking. By providing new insights into this underexplored region, our work makes a complementary contribution to the comprehensive understanding of mantle dynamics and magmatic processes across the entire LQA. This work not only refines our understanding of magmatic origins and mantle dynamics in the northwestern SCS margin but also provides a framework for future studies investigating plume-slab interactions in analogous tectonic settings globally.

2 Geological background and sampling

The SCS is bordered by three major tectonic plates: the Eurasian Plate, the Indo-Australian Plate, and the Pacific Plate (Taylor and Hayes, 1983). The LQA, located on the northwestern margin of the SCS, is a region characterized by extensive late Cenozoic volcanism (Figure 1B) (Huang et al., 1993; Zhu and Wang, 1989; Zhang et al., 2020; Fan et al., 2006; Wang et al., 2021). The LQA is intersected by numerous faults trending in SE-NW and NE-SW directions, as documented by (Zhang and Zhao, 1984). The volcanic activities in this region span from the late Miocene to the Holocene, with a notable peak in the Quaternary period (Huang et al., 1993; Zhu and Wang, 1989; Zhang, 1990).

Naozhou Island, located in the northeastern part of the LQA, stands as the largest volcanic island in South China. It spans an area of approximately 56 km² and extends in a northeast-southwest direction. The island features a gently sloping topography. The highest point, 81.6 m above sea level, is in the east, and the elevation gradually descends towards the west. Naozhou Island is mainly composed of late Cenozoic basaltic lava flows. These lava flows are interbedded with tuffaceous layers and overlaid by unconsolidated Quaternary sediments. Notably, on the seaward side of the volcanic crater in the eastern part of the island, there are two well-developed sets of columnar joints on display (Figure 1).

A total of 20 fresh volcanic rock samples were collected from Naozhou Island. The volcanic rocks exhibit a porphyritic texture, with phenocrysts of olivine, pyroxene, and plagioclase as revealed in the thin sections. For example, the sample NZ-12 (Figure 2A) shows such a porphyritic feature with phenocryst accounting for 15% and matrix for 85% (volume percentage). The matrix consists mainly of acicular microcrystalline plagioclase interspersed with finer pyroxene and magnetite. The phenocrysts are dominated by subhedral olivine, pyroxene, and euhedral plagioclase.

FIGURE 2

3 Methods

3.1 K-Ar chronology

K-Ar dating was performed at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology, China. Rock samples were crushed to 60 mesh, and porphyritic minerals were removed under a stereoscope. The samples were then cleaned using ultrasonic oscillation in alcohol, deionized water, and acetone, and subsequently divided into two portions.

One portion of the sample was precisely weighed (±0.001 mg) and wrapped in pure aluminum foil. The sample was then vacuum-baked for 48 h to remove adsorbed gases before being placed in a double-vacuum furnace for complete melting and gas extraction. The released gases were purified using a U-shaped liquid nitrogen cold trap and two zirconium-aluminum getter pumps operating at 450°C and room temperature, respectively. A known quantity of ³⁸Ar was introduced as a diluent, and the sample’s Ar isotopic composition was analyzed using an Argus VI rare gas mass spectrometer. The analysis results were corrected for background, atmospheric Ar, and mass discrimination. The radiogenic ⁴⁰Ar* content was calculated using the measured Ar isotope ratios and the known amount of ³⁸Ar diluent.

The second portion of the sample was weighed, and its K content was determined by atomic absorption spectrometry. The ⁴⁰K content was then derived from the measured K content, assuming a constant ⁴⁰K/K ratio within Earth’s lithosphere. The ⁴⁰K-⁴⁰Ar age was calculated using the standard isotopic decay equation:

Where is the total decay constant at ⁴⁰K (Potassium). The decay constant adopted the recommended value of 5.543 × 10⁻¹⁰ from Steiger and Jager (Steiger and Jäger, 1977). Standard substance adopts ZBH-25 black mica. Standard sample test results are included in the Supplementary Information.

3.2 Major and trace element

Major element analysis of whole-rock samples was performed using an Axios MAX XRF at Nanjing Hongchuang Geological Exploration Technology Service Co., Ltd. The procedure was as follows: (Foulger and Natland, 2003): Sample powders (200 mesh) were dried at 120°C for 8 h; (Hofmann, 1997); Approximately 0.5–1.0 g of dried sample was weighed in a constant-weight ceramic crucible and heated in a muffle furnace at 1,000°C for 200 min. The sample was then cooled to room temperature to calculate the loss on ignition (LOI); (Dalziel et al., 2000); A mixture of Canadian Claisse flux (6.0000 g ± 0.3 mg, 49.75% Li₂B₄O₇: 49.75% LiBO₂: 0.5% LiBr) and 0.6000 g ± 0.3 mg of dried sample was prepared and homogenized using a quartz rod. The mixture was transferred to a platinum crucible and melted at 1,100°C. After melting, the glass bead was cooled and prepared for XRF analysis.

Trace element analysis was conducted using an Elan DRC-e ICP-MS at Nanjing Hongchuang Geological Exploration Technology Service Co., Ltd. The sample digestion procedure was as follows: (Foulger and Natland, 2003): Sample powders (200 mesh) were dried at 105°C for 12 h; (Hofmann, 1997); 50 mg of dried sample was weighed and placed in a Teflon bomb; (Dalziel et al., 2000); 1.5 mL of HNO₃, 1.5 mL of HF, and 0.1 mL of HClO₄ (all ultrapure) were added to the Teflon bomb; (Kincaid et al., 2013); The Teflon bomb was sealed in a stainless steel pressure jacket and heated at 190°C for 48 h; (Mériaux et al., 2015); After cooling, the solution was evaporated to near dryness on a hotplate at 140°C, and then 3 mL of HNO₃ was added and evaporated again; (Druken et al., 2014); 3 mL of 50% HNO₃ was added, the bomb was resealed, and heated at 190°C for 12 h; (Gazel et al., 2011); The final solution was diluted to 100 g with Milli-Q water and spiked with 1 mL of a Rh + Re mixed standard solution (1 mg/L). Major elements were tested using GBW07104, GBW07105, GBW07310, GBW07312, GBW07314, and GBW07316 standards, while trace elements were tested using AGV-2 and BHVO-2 standards. Analytical precision was better than 3% for major elements and 1% for trace elements. Results of standard sample tests are included in the Supplementary Information.

3.3 Sr-Nd-Pb-Hf isotope

Geochemical reference materials (USGS BCR-2, BHVO-2, AVG-2, RGM-2) were used for quality control, with results agreeing with previous publications within analytical uncertainty (Weis et al., 2007; Weis et al., 2006). Detailed results of standard sample tests are provided in the Supplementary Information.

4 Results

4.1 K-Ar chronology

This work performed K-Ar dating results for three volcanic rock samples from Naozhou Island, which reveal two distinct periods of volcanic activity. The analytical data are listed in Table 1. On the Nayan Coast, to east of the volcanic crater, the age of upper part of the strata is 1.04 Ma (NZ-28), while the age of lower part is 1.11 Ma (NZ-22) (Figure 1C). On the Turtle Coast, to the southwest of the crater at a relatively lower elevation, sample NZ-12 was dated at 3.6 Ma (Figure 1D). These results suggest that volcanic activity on Naozhou Island occurred from the late Pliocene to the Pleistocene.

The columnar joints observed on Naozhou Island serve as clear indicator of magma solidification process. The ages obtained from these joints provide reliable constraints on the timing of volcanic eruption cycles. Our findings corroborate the stratigraphic contact relationships observed in the field (Wang et al., 2023). Based on these results, it can be inferred that magmatic activity on Naozhou Island occurred at least two periods: about 3.6 Ma and 1 Ma (1.11–1.04 Ma).

4.2 Major elements

The major element data are summarized in Table 2. SiO₂ content ranges from 50.2 to 54.11 wt%, TiO₂ from 1.46 to 1.8 wt%, Al₂O₃ from 15.32 to 18.13 wt%, total Fe₂O₃ (^TFe₂O₃) from 7.8 to 10.38 wt%, MgO from 3.61 to 6.75 wt%, Na₂O from 2.85 to 3.95 wt%, K₂O from 1.03 to 1.45 wt%, and Mg# from 47.44 to 59.8. The loss on ignition (LOI) values are generally low, less than 2 wt% (Table 2). In the total alkali-silica (TAS) diagram (Figure 3) (Le Maitre et al., 2002), these volcanic rocks classified as basalt and basaltic andesite. The 1 Ma volcanic rocks have higher alkalinity, with combined Na₂O + K₂O values around 5%, while the 3.6 Ma volcanic rocks have values closer to 4%.

FIGURE 3

Le Maitre et al. (2002).

The TAS diagram shows that most of the 3 Ma rocks fall within the basaltic andesite field, indicating that these magmas were relatively more evolved, with moderate SiO₂ content and lower alkalinity. The 3 Ma samples show similar SiO₂ contents with those of 1 Ma samples. Some of the 1 Ma samples fall into the basalt field and exhibit higher Na₂O+ K₂O values, indicating a trend toward higher alkalinity. Overall, the TAS diagram illustrates a clear distinction between the 3 Ma and 1 Ma volcanic rocks, with the latter showing evidence of higher alkalinity.

4.3 Trace elements

Despite two distinct periods of volcanic activity, the trace element patterns of these samples show no signifcant differences, indicating a stable mantle source over time (Table 3).

The total REE contents (ΣREEs) of the samples range from 92.71 to 134.66 ppm, with a mean of 111.13 ppm, which is lower than the typical OIB average of 198.9 ppm (Sun et al., 1989). In the chondrite-normalized REE diagram (Figure 4) (Sun et al., 1989; Niu and O'Hara, 2003; Plank and Langmuir, 1998), these volcanic rocks exhibit a clear enrichment in light REEs (LREEs), with (La/Yb)_N values ranging from 7.58 to 12.79, averaged at 10.98. The Eu anomalies (Eu/Eu*) range from 0.98 to 1.10, with a mean of 1.06, indicating minimal Eu anomalies. The slight enrichment in Eu relative to typical OIB values may reflect a weak influence of fractional crystallization, particularly involving plagioclase. The REE distribution patterns resemble the typical OIB.

FIGURE 4

Sun et al. (1989). Data for IABs are from Niu and O'Hara (2003); (b) Primitive mantle-normalized trace element diagrams. Data for primitive mantle, OIBs, E-MORBs are from Sun et al. (1989). Data for IABs are from Niu and O'Hara (2003). Data for GLOSS are from Plank and Langmuir (1998).

The trace elements, patterns are also consistent with OIB, are featured by enrichment in large-ion lithophile elements (LILEs, such as Ba) and LREEs. Moreover, a few samples show a positive Pb anomaly. No significant negative Nb anomaly and U anomaly display in our samples.

Overall, the trace element and REE patterns observed in the Naozhou Island volcanic rocks suggest a mantle source that has remained stable across different volcanic episodes. The geochemical signatures are consistent with those of typical OIBs, indicating a mantle source enriched in LREEs and showing typical trace element behaviors associated with OIBs.

4.4 Sr-Nd-Pb-Hf isotopes

The Sr-Nd-Pb-Hf isotope ratios for the Naozhou Island volcanic rock samples are as follows (Table 4): ⁸⁷Sr/⁸⁶Sr = 0.703733–0.704173, ¹⁴³Nd/¹⁴⁴ Nd = 0.512788–0.512896, ²⁰⁶Pb/²⁰⁴Pb = 18.43722–18.672847, ²⁰⁷Pb/²⁰⁴Pb = 15.62841–15.70108, ²⁰⁸Pb/²⁰⁴Pb = 38.66635–39.028549, and ¹⁷⁶Hf/¹⁷⁷Hf = 0.283054–0.283086. These isotope ratios are characteristic of OIB-type compositions (Figure 5).

FIGURE 5

Zhao et al. (2021)), southern Leizhou Peninsula (Huang et al., 1993; Zhu and Wang, 1989; Zhao et al., 2021), Weizhou Island (Zhang et al., 2020; Fan et al., 2008; Li et al., 2014), and northern Hainan Island (Huang et al., 1993; Zhu and Wang, 1989; Wang et al., 2012; Zhao et al., 2021). The Naozhou Island data are included in the Northern Leizhou Peninsula. DMM, EMⅠ, EMⅡ and HIMU end-member (Zindler and Hart, 1986), Shaded areas representing end-members are from the data set (www.earthchem.org). OIBs end-member (Staudigel et al., 1984). SCS data (Yan et al., 2008; Yan et al., 2014; Yan et al., 2015). Northern margin SCS data (Tu et al., 1991; Zou and Fan, 2010; Zou et al., 2000). Indochina block data (An et al., 2017; Hoàng et al., 2013; Hoang et al., 1996). Indian ocean-type MORB end-member (Mahoney et al., 2012).

In the ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb diagrams (Figure 6), the Naozhou Island samples plot above the Northern Hemisphere Reference Line (NHRL), resembling the Dupal anomaly observed in the Southern Hemisphere (Tu et al., 1991; Hart, 1984; Flower et al., 1992). The Dupal anomaly typically indicates the presence of EMII in the mantle source, and the position above the NHRL suggests that these rocks contain components that are more enriched than those from a DMM.

FIGURE 6

Dupré and Allègre (1983). The NHRL is North Hemisphere reference line (Hart, 1984).

5 Discussion

5.1 Geochemical insights into crustal contamination and magmatic sources

5.1.1 Crustal contamination and fractional crystallization on Naozhou Island

Evaluating whether the magma experienced crustal contamination during its ascent is crucial for accurately determining the magma source and understanding the magma evolution process (Xu et al., 2005; Dai et al., 2018; Zeng et al., 2013; Wang et al., 2019). Nb/U and Ce/Pb ratios are sensitive indicators of crustal contamination, as crustal material typically lowers these ratios. The geochemical data of volcanic rocks from Naozhou Island exhibit primitive Nb/U ratios (30.88–60.21, average: 39.87) and Ce/Pb ratios (4.2–14, average: 11.6). These ratios are significantly higher than those typical of continental crust (Nb/U ≈ 6.15, Ce/Pb ≈ 3.91), suggesting that the magma was not significantly contaminated by crust (Rudnick and Gao, 2003; Salters and Stracke, 2004). The absence of negative Nb anomalies in the trace element diagrams also suggests minimal crustal contamination (Figure 4). Furthermore, the ratios of sensitive elements (Nb/U, Ce/Pb) and isotopic data (⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb) are plotted against the contents of MgO and SiO₂ (Figure 7). The results demonstrate that there are no systematic trends of variation in these relationships, providing evidence that the influence of crustal contamination is limited (Zhang et al., 2020; Yan et al., 2018).

FIGURE 7

Fractional crystallization represents a crucial process in the evolution of magma. The Naozhou Island volcanic rocks exhibit Mg# contents ranging from 44.8 to 57.2 and Cr contents spanning from 192 to 357 ppm. These values are lower than those of primitive basalts (Mg# value >70, Cr contents >1,000 ppm) (An et al., 2017; Wilkinson and Maitre, 1987), which clearly indicates that these volcanic rocks have experienced fractional crystallization. Mg# displays a positive correlation with ^TFe₂O₃, SiO₂, and CaO/Al₂O₃, a negative correlation with K₂O and Al₂O₃, and no significant correlation with CaO (Figure 8). This pattern implies an evolutionary sequence of fractional crystallization, starting with the early crystallization of Olivine and pyroxenes and then transitioning to a medium stage dominated by plagioclase.

FIGURE 8

5.1.2 Magmatic source characteristics on Naozhou Island

Given that fractional crystallization does not alter the isotopic composition of magma (Staudigel et al., 1984; Hamelin and Allègre, 1985), the isotopic ratios thus can be used to indicate the characteristics of their magma source. The volcanic rocks’ isotopic ratios can be used to infer the characteristics of their magma source. The Sr-Nd-Pb-Hf isotope data of volcanic rocks from Naozhou Island, showing between DMM and EMⅡ reference values (Figure 5). Combined with those from adjacent regions (Zou and Fan, 2010; Zhang et al., 2020; Wang et al., 2012; Zhao et al., 2021; Yan et al., 2008; Yan et al., 2014), may suggest that basalt can be explained by a binary mixed model involving DMM and EMⅡ endmember components. The DMM endmember represents a depleted mantle source, likely originating from the Indian MORB mantle, which is prevalent in late Cenozoic intraplate volcanism across Southeast Asia (An et al., 2017; Yan et al., 2018; Hoàng et al., 2013; Hoang et al., 1996). The EMⅡ endmember is characterized by high ⁸⁷Sr/⁸⁶Sr (>0.705), low ¹⁴³Nd/¹⁴⁴Nd ratios (<0.5125) (Figure 5), and high Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 18.5–19.5) (Figure 6), they are usually associated with recycled continental or oceanic crust and sediments (von Huene et al., 2004; Wang et al., 2018). Previous studies have suggested that EMⅡ in this region could originate from the SCLM or a mantle plume (Zhang et al., 2020; An et al., 2017; Yan et al., 2018; Yan et al., 2014; Yan et al., 2015).

The geochemical data from Naozhou Island suggest that the EMⅡ corresponds to the mantle plume (Figure 5). The lithospheric mantle exhibits significant Nd-Hf isotopic decoupling due to fluid-driven metasomatism (Choi and Mukasa, 2012). The volcanic rocks on Naozhou Island exhibit the same linear array as those in the surrounding areas (Zhang et al., 2020; An et al., 2017; Yan et al., 2018; Yan et al., 2015) (Figure 5D). This indicates that EMⅡ is more likely to originate from a mantle plume rather than SCLM. Secondly, the Nd-Hf isotopic composition of the SCLM is typically different from that of oceanic island basalts. The samples from Naozhou Island all fall within the OIB range (Figure 5D), further ruling out the influence of the SCLM. Additionally, SCLM often shows more enriched Sr isotopic signatures, which are not observed in Naozhou Island. Based on seismic tomography studies, a low-velocity conduit extending from the lower mantle to the shallow lithosphere has been identified, indicating the presence of the Hainan mantle plume in the region (Figure 1B) (Toyokuni et al., 2022; Xia et al., 2016; Lei et al., 2009; Lebedev and Nolet, 2003; Chen et al., 2021; Hua et al., 2022). In summary, geochemical and geophysical evidence suggests that the EMⅡ endmember in Naozhou Island may originate from the Hainan mantle plume, consistent with findings from surrounding areas (Figures 5, 6, 9).

FIGURE 9

Zhang et al., 2020; Fan et al., 2008); Leizhou Peninsula and Northern Hainan Island data (Ho et al., 2000; Liu et al., 2015; Yung-Tan et al., 2022; Wang et al., 2012; Zhao et al., 2021). Lines integrated from regional data average and the shadows were fields ranges.

5.2 Temporal evolution of volcanic activities in the Leiqiong Area

This study conducted K-Ar dating revealing two major periods of volcanic activity on Naozhou Island: approximately 3.6 Ma (late Pliocene) and 1 Ma (Pleistocene). These K-Ar dating results provide a reliable framework for understanding the timing of volcanism Naozhou Island. The volcanic rock ages across the LQA (Table 5; Figure 10A) reveals that the volcanic activities period predominantly occurred during the Quaternary period (Huang et al., 1993; Zhu and Wang, 1989; Zhang et al., 2020; Fan et al., 2006; Wang et al., 2021). The volcanic rocks in northern Hainan Island display the widest age span, ranging from 34.78 Ma to 0.013 Ma. In contrast, the volcanic rocks in the southern LP range from 12.46 Ma to 0.48 Ma, while those in the northern LP range from 11.51 Ma to 0.1 Ma. The shortest age span is observed on Weizhou Island, where volcanic rocks date from 1.42 Ma to 0.036 Ma. This distribution suggests that volcanic activity of LQA has persisted into modern times, with the earliest activity beginning in the southern region and gradually progressing toward the northwest.

FIGURE 10

Huang et al., 1993; Zhu and Wang, 1989; Zhang et al., 2020; Fan et al., 2006; Wang et al., 2021). The boxes indicate the range between the first (25%) and the third (75%) quantiles. The solid lines in the boxes indicate the median. The two caps indicate the age data of 10%–90% range; (b) DMM-EMⅡ mixing end-member proportions derived from Sr-Nd isotope calculations versus K-Ar ages (Huang et al., 1993; Zhu and Wang, 1989; Zhang et al., 2020; Wang et al., 2012; Zhao et al., 2021; Fan et al., 2008; Li et al., 2014); (c) Schematic diagram of slab and Hainan Mantle Plume mixing (Li et al., 2018; Xia et al., 2016).

The earlier onset and longer duration of volcanic activity in northern Hainan Island are likely due to the mantle plume. In contrast, the southern LP, northern LP, and Weizhou Island, being relatively further from the plume’s core, experienced volcanic activity later as magma migrated through distal branches of the plume (Figure 1). This spatial and temporal pattern may indicate that the initial phase of volcanism was driven by lithospheric thinning due to tectonic extension, while later phases were predominantly influenced by upwelling of the Hainan mantle plume. The progressive northward expand of volcanic activity concurs with the hypothesis that magma generation and eruption were increasingly controlled by the mantle plume as tectonic influences related to the SCS extension diminished (Xie et al., 2023). This transition from tectonically-driven to plume-dominated volcanism provides crucial insights into the geological evolution of the LQA and the underlying mechanisms driving volcanic activity across the region.

5.3 Interaction between Hainan mantle plume and paleo-subducted slab

Previous studies have shown that the volcanic rocks in the LQA are predominantly of the OIB type with certain IAB type, mainly originating from the Hainan mantle plume or as a result of mixing between mantle plume materials and ancient subducted slab components (Ho et al., 2000; Zou and Fan, 2010; Zhang et al., 2020; Zhao et al., 2021). Geochemical analyses indicate that the volcanic rocks in the LQA predominantly exhibit OIB characteristics, reflecting the pivotal role of the mantle plume in controlling the chemical composition and eruption patterns of the magma. Sr-Nd-Pb-Hf isotope data further reveal that the mantle source of these volcanic rocks reflects the geochemical heterogeneity of a mantle plume, with isotopic characteristics commonly associated with DMM and EMⅡ signatures observed across nearly entire study areas (Ho et al., 2000; Zou and Fan, 2010; Zhang et al., 2020; Zhao et al., 2021). Based on the linear mixing model of DMM and EMⅡ end-members calculated from the ⁸⁷Sr/⁸⁶Sr-¹⁴³Nd/¹⁴⁴Nd isotopic diagram, combined with K-Ar geochronological data (Figure 10B), it is shown that since 6 Ma, the contribution of the EMⅡ to the source of Cenozoic volcanic rocks in the LQA has increased. A possible mechanism is the progressive upwelling of the mantle plume, which leads to an enrichment of the EMⅡ component. Another possibility is the incorporation of residual Mesozoic subducted slabs during the upwelling process, allowing recycled materials to enter the mantle and occupy a larger proportion in the magma source region.

The temporal and spatial distribution characteristics of volcanic rocks in the region provide new insights into the possible mechanisms of ancient subduction zones' involvement in magmatic activities. Temporally, volcanic activity in the region initiated earlier in the south and progressively became active toward the north. The volcanic rocks on Weizhou Island, located west of the same latitude as northern LP, are generally younger, while volcanic activity in the southern LP occurred earlier than the northern LP, with northern Hainan Island showing the earliest volcanic activity. This temporal difference is closely related to the spatial distribution pattern. The Hainan mantle plume is generally believed to be located in northeast of Hainan Island, which suggests that volcanic activity in northeastern Hainan Island, being closer to the core of the plume, occurred earlier, while Weizhou Island and northern LP, being relatively farther from the plume, experienced later volcanic activity. This trend is consistent with the position of the Hainan mantle plume (Toyokuni et al., 2022; Xia et al., 2016; Lei et al., 2009; Lebedev and Nolet, 2003; Chen et al., 2021; Hua et al., 2022).

The volcanic rocks in the southern LP, as well as northern Hainan Island, display characteristics of OIB, IAB and OIB-IAB transition types (Zhang et al., 2020; Yung-Tan et al., 2022; An et al., 2017; Wang et al., 2012; Zhao et al., 2021; Chen et al., 2023), which differ from the typical OIB rocks associated with mantle plumes. Previous studies suggested that the mantle source regions in these areas were not only influenced by the mantle plume but also possibly mixed with recycled materials from ancient subducted slabs (Zhang et al., 2020; Yung-Tan et al., 2022; An et al., 2017; Wang et al., 2012; Zhao et al., 2021; Chen et al., 2023). This hypothesis is further supported by our combined isotopic data, particularly in Sr-Nd and Pb isotope diagrams, where samples from southern LP and northern Hainan Island show greater dispersion (Figures 5, 6), indicating that the mantle source regions in these areas may have undergone complex mixing processes. However, the Naozhou Island located in northern LP (this study) and the Weizhou Island located in western LP (Zhang et al., 2020) merely show OIB type basalts which may indicate deriving from the Hainan mantle plume rather than paleo-subducted slab. This distribution can be explained by the branched Hainan mantle plume model suggested by Xia et al. (Xia et al., 2016) and concurs with the subduction direction of the Late Mesozoic subduction zone along the northern SCS margin, which trends approximately from southeast to northwest (Li et al., 2018), implying that the influence of ancient subducted slab materials focused on the center location of the Hainan mantle plume merely (Figure 10C).

6 Conclusion

This study conducted K-Ar dating on basalt samples from Naozhou Island, revealing two periods of volcanic activities: approximately 3.6 Ma (late Pliocene) and 1 Ma (Pleistocene), providing a refined temporal framework for volcanic activity on the island. The basalts on Naozhou Island have undergone limited crustal contamination but intermediate fractional crystallization. Geochemical and isotopic analyses indicate that the volcanic rocks on Naozhou Island are of the OIB-type, primarily derive from a mixture of DMM and EMⅡ, may originate from the Hainan mantle plume. By integrating data from other volcanic rocks in the LQA, this study further elucidates the temporal and spatial evolution of volcanic activity across the entire LQA region. The results demonstrate that the Hainan mantle plume is the primary driver of magmatic activity in the LQA. Moreover, isotopic analysis across different regions reveals dispersion, particularly in southern LP and northern Hainan Island, coupled with the presence of IAB and OIB-IAB transitional characteristics, suggests a significant role of recycled subducted slab material may participate in the magma source. However, the Naozhou Island located in northern LP and the Weizhou Island located in western LP merely show OIB type basalts which may indicate deriving from the branched Hainan mantle plume rather than paleo-subducted slab. The spatial distribution feature of the interaction between branched Hainan Mantle Plume and paleo-subducted slab is in accord with the subduction direction of the Late Mesozoic subduction zone along the northern margin of the SCS.

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