Journal of Earth Science  2018, Vol. 29 Issue (2): 265-279   PDF    
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Carboniferous Arc Setting in Central Hainan: Geochronological and Geochemical Evidences on the Andesitic and Dacitic Rocks
Shubo Li1, Huiying He1, Xin Qian1, Yuejun Wang1, Aimei Zhang2    
1. Guangdong Provincial Key Lab of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-Sen University, Guangzhou 510275, China;
2. Third Institute of Oceanography, State Oceanic Administration, Xiamen 361005, China
Abstract: Volcanic rocks in the Bangxi-Chenxing tectonic zone provide important carries for better understanding the Late Paleozoic tectonic evolution in Hainan and its temporal-spatial pattern of the eastern Paleotethyan evolution. This paper presents a set of new geochronological and geochemical data on the andesitic and dacitic rocks along the Bangxi-Chenxing tectonic zone in central Hainan. The representative andesitic and dacitic samples yield similar zircon U-Pb ages of 353±3 and 351±7 Ma, respectively, being of Early Carboniferous origin. These volcanic rocks are characterized by low TiO2 and high Al2O3 contents and are enriched in LILEs and LREEs but depletion in HFSEs, along with negative εNd(t) values of -1.4– -4.7 and high 87Sr/86Sr(i) ratios of 0.707 2–0.710 1. Geochemical signatures suggest that the andesitic and dacitic samples might originate from a metasomatized wedge modified by the slab-derived component in a continental arc setting. In combination with the available data, it is proposed for the development of a Carboniferous continental arc in response to the eastern Paleotethyan evolution. The Bangxi-Chenxing tectonic zone might westerly link with the Jinshajiang-Ailaoshan-Song Ma suture zone, constituting an assemblage boundary between the South China and Indochina blocks.
Keywords: Early Carboniferous    volcanic rocks    continental arc setting    Bangxi-Chenxing zone    central Hainan    Paleotethyan evolution    
0 INTRODUCTION

Southeast Asia is a complex assemblage preserved abundant geological relicts related to the Paleotethyan evolution (e.g., Wang et al., 2017, 2016; Qian et al., 2016, 2015; Metcalfe, 2013, 1996; Sone and Metcalfe, 2008; Yin and Harrison, 2000). The line of evidence shows that Changning-Menglian tectonic zone represents the Paleotethyan main suture zone and links northerly with the Longmucuo-Shuanghu zone and southerly the Inthanon-Bentong-Raub zone (Wang et al., 2017, 2010; Metcalfe, 2013, 2011, 2002, 1998, 1996; Zi et al., 2012; Hennig et al., 2009; Sone and Metcalfe, 2008). In Southeast Asia, several back-arc basins/ branches also have been identified including the Jinshajiang, Ailaoshan, Song Ma, Luang Prabang, Nan and Loei suture zones, which separated into numerous blocks/fragments (e.g., South China, Indochina, Sibumasu and East Malaya) that derived from Gondwana but subsequently assembled during the Late Paleozoic to Early Mesozoic (Fig. 1; e.g., Wang et al., 2017, 2016; Qian et al., 2016, 2015; Metcalfe, 2013, 2011; Feng et al., 2008, 2004; Feng, 2002). Abundant geological signatures are preserved along the Jinshajiang-Ailaoshan suture zone in Southwest China and the Song Ma suture zone in northern Vietnam separating the South China and Indochina blocks (e.g., Qian et al., 2016; Yang et al., 2016; Zhang et al., 2016; Fan et al., 2015, 2010; Wang et al., 2010; Hennig et al., 2009; Metcalfe, 2002; Sengör, 1976). However, due to the Cenozoic shearing dislocation along the Red River shear zone, the eastward extension of the Jinshajiang-Ailaoshan-Song Ma suture zone still remain controversial (e.g., Guangdong BGMR, 1988). Hainan Island is tectonically located among the India-Australia, Euro-Asian and Pacific plates (Fig. 1a) and is geographically considered to correspond to the extension of the Jinshajiang-Ailaoshan-Song Ma suture zone. Thus, more attention has been paid to the area for revealing the possible effect of Paleotethyan evolution in Hainan Island.

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Figure 1. (a) Tectonic outline of Southeast Asia (revised after Wang et al., 2010), (b) geological map of Hainan Island showing sampling locations and (c)–(d) simplified geological maps for the Bangxi and Chenxing areas, respectively (revised after Guangdong BGMR, 1988).

In spite that numerous works have been done to reveal the Paleozoic tectonic pattern in Hainan Island, the Late Paleozoic tectonic nature is still disputed. Zhang et al. (1997) and Shui (1987) supposed that Hainan Island had an affinity to the Cathaysia of the South China Block (SCB), but Chen et al. (1994) and Hsü et al. (1990) considered it as a part of the Indochina Block (e.g., Guangdong BGMR, 1988). More and more researchers believed that Hainan might be divided into two tectonic parts of North Hainan and South Hainan by the Jiusuo-Lingshui fault or the Changjiang-Qionghai fault (e.g., He et al., 2017; Zhang et al., 2011; Li et al., 2002; Xia et al., 1991a, b), or NW Hainan and SE Hainan by NE-trending Baisha fault (Metcalfe, 1996), tectonically equivalent to the South China and Indochina blocks, respectively. In addition, in Hainan Island, the Permian I-type granitoids and the Early Triassic WNW-trending structural deformation, along with the Late Paleozoic volcanic rocks (Fig. 1b; e.g., Chen et al., 2011; Zhang et al., 2011; Li et al., 2006; Metcalfe, 2002, 1996) suggested that the Hainan might be a key area for revealing the eastern Paleotethyan tectonic evolution (e.g., He et al., 2017; Chen et al., 2014; Zhang et al., 2011; Xu et al., 2008; Li et al., 2002; Ma et al., 1998; Metcalfe, 1996). In this paper, we presented a set of new zircon U-Pb geochronological data and whole-rock geochemical, elemental and Sr-Nd isotopic data of representative volcanic rocks at the Chenxing and Bangxi areas in central Hainan, with the aim for better constraining the formation age and petrogenesis of the volcanic rocks, and further understanding the temporal-spatial relationship with the Jinshajiang-Ailaoshan-Song Ma zone in response to the Paleotethyan evolution in Southeast Asia.

1 GEOLOGICAL SETTINGS

Hainan Island, which is separated from China mainland by the Qiongzhou Strait, is an important element in tectonic reconstructions between the South China and Indochina blocks (Guangdong BGMR, 1988). It is characterized by four EW-trending Wangwu-Wenjian, Changjiang-Qionghai, Jianfeng-Diaoluo and Jiusuo-Lingshui faults and two NE-trending Gezhen-Lingao and Baisha faults, respectively (Fig. 1b; e.g., Xie et al., 2009; Wang et al., 1992, 1991; Xia et al., 1991a, b, 1990; Guangdong BGMR, 1988). The stratigraphical sequences are mainly characterized by Precambrian Baoban and Shilu groups, along with Paleozoic marine and Mesozoic terrestrial packages (e.g., Long et al., 2002; Ma et al., 1998). Mesoproterozoic Baoban Group, mainly preserved in western Hainan, has been traditionally considered to be the crystallized basement in Hainan. However, the relationship between the Baoban and Shilu groups is poorly observed (e.g., Li et al., 2002; Wang et al., 1992, 1991; Guangdong BGMR, 1988). Available data indicated that the previously-defined Baoban and Shilu groups might be the Mesoproterozoic (~1 420 Ma) Complex constituted by granitic gneiss, migmatite, metamorphic volcanics, paragneiss, quartz-mica schist, quartzite and a small amount of amphibolite fragments/ pods (e.g., Li et al., 2002; Ma et al., 1998; Wang et al., 1991). Detrital zircons from the Baoban and Shilu supracrustal sedimentary rocks had tectonothermal records of the Columbia breakup and Grenvillian orogenic events (Li et al., 2002). In Hainan, the Neoproterozoic rocks are poorly identified. The Lower Paleozoic sequences mainly outcropped in central Hainan and are dominated by Cambrian and Ordovician siltstone, sandstone and slate as well as Lower Silurian sandstone (Long et al., 2007; Hu et al., 2001; Tang and Feng, 1998; Wang et al., 1992, 1991; Xia et al., 1990). The Upper Paleozoic sequences have commonly undergone the greenschist-facies metamorphism and consist of Devonian sandstone, Carboniferous slate and volcanics, Lower Permian limestone and Middle Permian sandstone to north of the Jiusuo-Lingshui fault (Long et al., 2007; Hu et al., 2001; Tang and Feng, 1998; Wang et al., 1992, 1991; Xia et al., 1990). The Mesozoic strata in Hainan mainly involve the Upper Triassic siliciclastic rocks and Cretaceous sandstones (Wang et al., 1991). Middle Triassic sandstone is unconformably underlain by pre-Triassic package and overlain by Jurassic or Lower Cretaceous terrestrial siliciclastics.

The foliated and gneissic granites in Hainan are dominated by the Mesoproterozoic Baoban granitic gneiss (e.g., Gongai and Ledong) and the Permian Wuzhishan and Wanning gneissic granites (e.g., Chen et al., 2011; Li et al., 2002; Wang et al., 1991). The granitic rocks with the ~60% acreage of Hainan Island composed of Indosinian (e.g., Qiongzhong and Jianfengling) and Yanshanian monogranites, biotite granites (e.g., Danxian, Tunchang, Qianjia and Baochen) monogranites, biotite granites and granodiorites (Chen et al., 2013; Wang et al., 1991). Small amount of mafic rocks has been observed in Hainan Island, mainly including the Mesoproterozoic Baoban metabasites, Late Paleozoic–Early Mesozoic basaltic, gabbroic and doleritic rocks. The Cenozoic rift-related basalt is widespread. The Carboniferous volcanic rocks are recently identified along the Bangxi-Chenxing area and are characterized by MORB-like metabasite (He et al., 2017; Li et al., 2002).

In the Chenxing (Tunchang) area of central Hainan, our field investigation identified small amount of andesitic and dacitic rocks, which occurred in Permian strata (Guangdong BGMR, 1988). The volcanic samples experienced the shear deformation. In Bangxi (Changjiang) area, the dacitic samples exposed in the previously-mapped Paleozoic volcanic-clastic sedimentary package. All samples have a similar mineral association of plagioclase, hornblende and biotite with a porphyrotopic texture. Their phenocrysts include hornblende and biotite, which usually occurs as a euhedral-subhedral columnar crystal in the range of 0.2–0.8 centimeters, whereas the matrix mainly consists of fine-grained plagioclase and volcanic tuffaceous material. Apatite, sphene and zircon are dominating accessory minerals.

2 ANALYTICAL METHODS 2.1 Zircon U-Pb Dating

Zircon grains from representative samples were separated using conventional heavy liquid and magnetic techniques in the mineral separation laboratory of the Bureau of Geology and Mineral Resources of Hebei Province. The zircon grains were mounted in epoxy and were documented with cathodoluminescence (CL) images to reveal their internal structures via a JXA-8100 scanning electron microprobe at the Sun Yat-Sen University, Guangzhou. Representative CL images of zircon grains are inserted in Fig. 2. The Neptune Plus MC-ICP-MS coupled with an ArF-193 nm laser ablation system (Resonetics Resolution M-50-HR) at the Institute of Geochemistry (GIG), Chinese Academy of Sciences (CAS) was used as laser ablation ICP-MS (LA-ICP-MS) zircon U-Pb analyses. The zircon standards 91500 and Plešovice were used to calibrate the U-Th-Pb ratios. In Tables, individual analyses and plots are presented with 1σ errors, and uncertainties in ages are quoted at the 95% confidence level. The detailed analytical procedure follows Xia et al. (2011) and the age calculations and plots and data collection and dealing with questions were made using ICPMSDataCal and ISOPLOT of Ludwig (2003), respectively. The analytical results are shown in Table 1 with 1σ level of uncertainties and 95% confidence level of mean ages for pooled 206Pb/238U results.

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Figure 2. Concordia diagrams of zircon U-Pb data for the Chenxing sample (11HN-01A) (a) and Bangxi sample (11HN-21A) (b), central Hainan. Insets show the representative cathodoluminescence (CL) images for the zircon grains.
Table 1 LA–ICP–MS zircon U–Pb dating results for the Chenxing and Bangxi samples (11HN-01A and 11HN-21A) in the Bangxi-Chenxing zone
2.2 Whole-Rock Geochemical Analyses

The representative samples were collected from the Chenxing and Bangxi areas in central Hainan with removed the altered surfaces before crushed to millimeter-scale chips using an agate mill. Chips from all samples were crushed to 200-mesh in an agate mill for major oxide, trace element, and Sr-Nd isotopic analyses. The analytical results are given in Table 2.

Table 2 Major oxides (wt.%), trace element (ppm) and Sr-Nd isotopic results for the andesite and dacite in central Hainan

Major and trace elements were performed by the X-ray fluorescence (XRF) techniques on the Rigaku RIX 2000 spectrometer and Perkin Elmer Sciex ELAN 6000 inductively coupled plasma mass spectrometry (ICP-MS), respectively at the GIG, CAS. The analysis precision generally ranges from 1% to 5%. Detailed sample preparation and analytical procedure followed Liu et al. (1996). Analyses of Sr and Nd isotopic ratios were performed on a Neptune Plus multi-collection mass spectrometry equipped with nine Faraday cup collectors and eight ion counters at the GIG, CAS. The analytical procedures are similar to which were reported by Yang et al. (2006). Cation columns and HDEHP coated Kef columns were used to separate Sr, rare earth elements and Nd, respectively. The total procedural blank is in the range of 200–500 pg for Sr and less than 50 pg for Nd. The mass normalization for 87Sr/86Sr ratio and 143Nd/144Nd ratios are based on 86Sr/88Sr=0.119 4 and 146Nd/144Nd=0.721 9, respectively. The reported 86Sr/88Sr were adjusted to (NIST) SRM987 standard 86Sr/88Sr=0.710 265±12 (2σ) and 146Nd/144Nd were adjusted to the La Jolla standard 146Nd/144Nd=0.511 862± 10 (2σ) on this MAT-261 mass spectrometer during the present study, respectively. The Rb, Sr, Sm and Nd abundances measured by ICP-MS are used to calculate the 87Rb/86Sr and 146Nd/144Nd ratios.

3 RESULTS 3.1 Geochronological Results

The representative andesitic (11HN-01A) and dacitic (11HN-21A) samples were selected for zircon U-Pb dating. The majority of the analyzed grains is euhedral and light brown or colorless with oscillatory zoning (Fig. 2).

Andesitic sample (11HN-01A): This sample was taken from the site (19º25′30″N, 109º57′55″E) at the Chenxing area. Twenty-one analytical spots were performed on 21 zircons. A weighted mean 206Pb/238U age of 353±3 Ma (MSWD=0.5) are defined by seven spots. The other eight analyses give a weighted mean 206Pb/238U age of 432±4 Ma (MSWD=0.2; Fig. 2a), which are interpreted as xenocrysts, suggestive of the Caledonian thermo-tectonic event. The similar data have been reported by Xu et al. (2007). The remaining eight analyses show older apparent ages ranging from 900 to 2 450 Ma, also interpreted as xenocrysts. Taking into account their weak oscillatory zoning in the CL image (inset in Fig. 2a), it is proposed that the mean age of ~350 Ma represent its formation age of the sample.

Dacitic sample (11HN-21A): This sample was collected from the site (19º24′47″N, 109º07′56″E) nearby the Bangxi Town. Five spots from 15 grains give the apparent 206Pb/238U ages of 415–442 Ma with a weighted mean age of 433±5 Ma (MSWD=3.2) and seven analyses show the apparent ages from 944 to 2 532 Ma, representing the xenocryst grains. The remaining three analyses yield a coherent group with weighted mean age of 351±7 Ma with MSWD=3.3 (Fig. 2b), reflective of the crystallization age of the dacitic sample. Such an age is also consistent with the eruption age of Chenxing andesitic sample.

3.2 Geochemical Characteristics

Whole-rock major oxides, trace elements and Sr-Nd isotopic data for the representative samples are listed in Table 2. The Chenxing samples have SiO2=55.34 wt.%–65.41 wt.%, MgO=2.93 wt.%–4.12 wt.%, TiO2=0.62 wt.%–0.90 wt.% and K2O/Na2O=1.10–2.78. In comparison with those of typical andesites, the Chenxing samples have similar Al2O3 (14.51 wt.%–21.07 wt.%), higher K2O (2.46 wt.%–4.18 wt.%), but lower CaO (2.65 wt.%–4.47 wt.%) contents. The Bangxi samples show weak variation with SiO2 ranging from 60.69 wt.% to 65.50 wt.%, Al2O3 from 14.85 wt.% to 16.53 wt.%, MgO from 1.84 wt.% to 2.45 wt.% and TiO2 from 0.60 wt.% to 1.11 wt.%. Their K2O/Na2O ratios are in the range of 0.82–2.44. The Chenxing and Bangxi samples can be classified as calc-alkalic series rocks in the Zr/TiO2-Nb/Y diagram and fall in the fields of andesite and dacite in the TAS diagram (Figs. 3a3b, Winchester and Floyd, 1977). In the Harker diagram (Fig. 4), MgO, Al2O3, FeOt, TiO2 and P2O5 show a sharply decreasing but a poor variety for CaO with increasing SiO2.

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Figure 3. (a) TAS (after Le Bas et al., 1986) and Zr/TiO2 vs Nb/Y (after Winchester and Floyd, 1977) classification diagrams for the andesite-dacite association from the Bangxi-Chenxing zone.
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Figure 4. Plots of SiO2 vs. MgO (a), FeOt (b), Al2O3 (c), CaO (d), TiO2 (e) and P2O5 (f) for the andesite-dacite association from the Bangxi-Chenxing zone.

Our Chenxing and Bangxi samples show similar right-sloping chondrite-normalized REE patterns (Fig. 5a) with Eu/Eu*=0.61–0.90. Their (La/Yb)N and (Gd/Yb)N ratios range from 5.81 to 8.07 and 1.18 to 1.60, respectively. On the primitive mantle-normalized multi-element spidergram (Fig. 5b), these samples are characterized by enrichment in LILEs (e.g., Rb and Ba) and depletion in HFSEs (e.g., Nb, Ta and Ti) with Sr negative anomalies. They have high Zr/Nb (13.79–20.75) and Th/La (0.29–0.36) ratios, resembling to arc volcanic rocks (e.g., Peng et al., 2008). The Sr-Nd isotopic analysis for seven representative samples from the Chenxing and Bangxi areas were shown in Table 2 and Fig. 6. The Chenxing samples have initial 87Sr/86Sr(i) ratios ranging from 0.708 5 to 0.710 1 and εNd(t) values from -1.4 to -2.0. The initial 87Sr/86Sr(i) ratios for the Bangxi samples range from 0.707 2 to 0.708 2 and have εNd(t) values from -3.4 to -4.7. Their high initial 87Sr/86Sr(i) ratios are interpreted herein to be reflective of the seawater alteration.

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Figure 5. (a) Chondrite-normalized REE pattern and (b) primitive mantle-normalized trace element spidergram for the andesite-dacite association from the Bangxi-Chenxing zone. Data of Lancangjiang Early Triassic arc volcanic rocks are from Peng et al. (2008). The patterns of E-MORB, OIB and normalized values for chondrite and primitive mantle are from Sun and McDonough (1989).
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Figure 6. Plot of initial 87Sr/86Sr(i) vs. εNd(t) for the andesite-dacite association from the Bangxi-Chenxing zone.
4 DISCUSSION 4.1 Petrogenesis

The Chenxing and Bangxi samples display slightly high loss of ignition (LOI) contents (1.24 wt.%–3.57 wt.%). Together with the Sr isotopic deviation from the mantle-evolved array in Fig. 7, these characteristics suggest that they may have been undergone some degree of alteration. However, our samples display insignificant correlations between the LOI values and HFSEs or LILEs and isotopic ratios (not shown), indicative of the immobility during the alteration, and thus are selected herein for the petrogenetic discussion.

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Figure 7. Plots of (a) SiO2 vs. Nb/La, (b) SiO2 vs. εNd(t), (c) Yb vs. Tb/Yb and (d) Yb vs. La/Yb for the andesite-dacite association from the Bangxi-Chenxing zone.

The Chenxing and Bangxi samples have relatively high silica contents, even up to 65.50 wt.%, and low Ni (20.9 ppm–74.9 ppm) and Cr (36.2 ppm–60.7 ppm) contents, along with the xenocrystal zircons, possibly suggesting the preservation of crustal contamination. However, the following signatures might indicate the insignificance of the crustal contamination during the magma ascending: (1) lower Nb/La ratios (0.29–0.37) than those of average continental crust (~0.7) (Fig. 7a), (2) relatively constant Nb/La ratios and εNd(t) values with increasing SiO2 contents (Fig. 7b; e.g., Sun and McDonough, 1989; Hoffman and Ranalli, 1988), and (3) lower Ce/Pb (1.93–4.99) and Nb/U (3.08–4.70) ratios in comparison with those of average continental crust. However, the fractional crystallization and source heterogeneity might be involved during magma evolution on the basis of the correlations between Yb and Tb/Yb and La/Yb ratios in Figs. 7c7d. Our data suggest that the incompatible elements ratios and isotopic compositions are more likely to the inheriting of the magma source.

The Mg# for the Chenxing samples ranges from 44.5 to 46.3, Cr contents from 36.2 ppm to 66.6 ppm and Ni contents from 20.9 ppm to 34.5 ppm, respectively, suggesting that their magma has been experienced certain degree of fractional crystallization of clinopyroxene. The Bangxi samples have relatively lower MgO contents (1.84 wt.%–2.45 wt.%) and higher Ni contents (25.9 ppm–74.9 ppm) than those of Chenxing samples, indicative of olivine fractionation crystallization. The Eu negative anomalies and Sr positive anomalies of all samples argue against strong plagioclase fractionation during the magma process (Figs. 5a5b).The negative correlations between SiO2 and TiO2 and FeOt (Figs. 4b and 4e) are related to the fractionation crystallization of Ti-Fe oxides. The andesitic and dacitic samples from the Bangxi and Chenxing areas might be explained as the generation of contiguous fractional crystallization according to the similar trends in Harker, SiO2-Nb/La, La-La/Yb and Yb-La/Yb diagrams (Figs. 4 and 7).

Three models have been proposed for the formation of the andesitic samples involving (1) hypomigmatization of lower crust, (2) the mixing of basic magma and acid magma, and (3) the partial melting of the mantle wedge metasomatized by subduction-related components (Dungan and Davidson, 2004). The slightly high Mg# of the Chenxing and Bangxi samples ranges from 37.1 to 46.8 (commonly > 40), distinct from the product from partial melting of the granulite or eclogite crust, which commonly has low Mg# (< 40; Rapp et al., 1999, 1991). In combination with the enrichment in LILEs and LREEs and relatively low Nb/La ratios (0.29–0.37), it is suggested for these samples being unlikely derived from the lower crust of the South China Block (Gao et al., 1999; Taylor and McLennan, 1995). In addition, the relatively constant εNd(t) values for our samples indicate that the mixing model of basic with acid magma might be an appropriate candidate in the petrogenesis. Our Bangxi-Chenxing samples have high SiO2 (55.34 wt.%–65.50 wt.%), Al2O3 (14.51 wt.%– 19.41 wt.%) and low TiO2 (0.60 wt.%–1.11 wt.%) contents, as well as enrichment in LILEs and LREEs and depletion in Nb, Ta and Ti. Such signatures, along with high Sc (13.5 ppm–27.1 ppm) and V (102 ppm–160 ppm) contents and Nb/La, Nb/U, Nd/Pb, Ce/Pb ratios, as well as negative εNd(t) values, suggest the petrogenetic possibility of origination of a recycled component-modified enriched source (e.g., Stolz et al., 1990).

The Th/Yb ratios for these samples range from 2.33 to 3.82 and Ba contents from 235 ppm to 575 ppm. In Fig. 8a, the plot in the Kitakami andesite range interpreted as derivation of the subducted metasomatism source. These samples have high Th/Zr and Ba/Y ratios but low Nb/Zr and Nb/Y ratios in Figs. 8b8c, pointing to an involvement of the fluid-related component in Stolz et al. (1990) and Sun and McDonough (1989). The synthesis of these data indicates that the Bangxi-Chenxing volcanic rocks might be the product of significant crystallization fractionation of the derivation from an enriched mantle modified by the fluid-and sediment-related recycled components. Available data show the presence of the Carboniferous MORB-like metabasites in the Bangxi-Chenxing area, which formed in a back-arc basin setting (He et al., 2017). Thus, it is herein inferred that Carboniferous volcanics formed at a continental arc setting.

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Figure 8. Plots of (a) Ba vs. Nb/Y, (b) Th/Zr vs. Nb/Zr and (b) Nb/Y vs. Ba/Y (after Wang et al., 2004) for the andesite-dacite association from the Bangxi-Chenxing zone.
4.2 Tectonic Implications

A key issue remains to whether or not the Carboniferous arc or/and back-arc basin in Bangxi-Chenxing tectonic zone geodynamically links the Paleopacific or Paleotethyan domain. Li and Li (2007) and Li et al. (2006) considered that the westward flat-slab subduction of the Paleopacific Plate beneath the South China Block resulted into the formation mechanism of the Wuzhishan I-type granites (267–262 Ma) since Permian. However, numerous researches indicate the westward subduction of the Pacific Plate being not initiated until the Middle Jurassic (e.g., Wang et al., 2013, 2007; Shu et al., 2008; Zhou and Li, 2000 and references therein). In addition, more and more researchers identified the development of contemporaneously Late Paleozoic to Early Mesozoic igneous rocks along the Jinshajiang-Ailaoshan-Song Ma tectonic zone. For example, Jian et al.(2009a, b, 1998) and Wang et al. (2000) obtained zircon U-Pb ages of 383–334 Ma from the Ailaoshan suture zone, and Zhang et al. (2014) and Vượng et al. (2013) reported the 387–313 Ma mafic-ultramafic rocks in the Song Ma suture zone. These data indicate that the Bangxi-Chenxing zone has similar magmatic activity with the Jinshajiang-Ailaoshan-Song Ma tectonic zone (e.g., He et al., 2017; Chen at al., 2013; Xu et al., 2007; Li et al., 2006, 2002). In addition, the Devonian fish fossils and Late Permian Dicynodon in the Indochina Block are same with those in the South China Block (e.g., Thanh et al., 2007; Janvier et al., 1994), arguing for continental linkage between the South China and Indochina blocks. To the south of Bangxi-Chenxing tectonic zone, it is reported for abundant Permian–Triassic granitoids interpreted as the 272–233 Ma products of the Bangxi-Chenxing back-arc basin (e.g., Chen et al., 2011, 2006; Zhang et al., 2011; Li et al., 2006), also geochemcially identical with Permian–Triassic igneous rocks along the Truong Son tectonic zone in central Vietnam (e.g., Lai et al., 2014; Maluski et al., 2005) and Jinshajiang-Ailaoshan zone in Southwest China (e.g., Liu et al., 2015; Zi et al., 2012). These data suggest the close relationship of the Bangxi-Chenxing and Jinshajiang-Ailaoshan-Song Ma tectonic zones. As a result, our data support a continental arc setting along the Bangxi-Chenxing tectonic zone between the Indochina and South China blocks in response to the Paleotethyan evolution.

5 CONCLUSIONS

A set of new zircon U-Pb geochronological and whole-rock elemental and Sr-Nd isotopic data for the Bangxi-Chenxing volcanic rocks in central Hainan gave the following conclusion. (1) These volcanic rocks along the Bangxi-Chenxing zone were formed at ~350 Ma, indicative of the Early Carboniferous origin. (2) These volcanic rocks originated from a recycled components-modified source. (3) The Carboniferous andesitic-dacitic rocks formed in a continental arc setting, tectonically linking with the Jinshajiang-Ailaoshan-Song Ma zone.

ACKNOWLEDGMENTS


We would like to thank Drs. Yuzhi Zhang, Feifei Zhang, Xinyue Chen and Huichuan Liu for their help during fieldwork, geochronological and geochemical analyses. This study was jointly supported by the Projects from China (Nos. U1701641, 41506050, 2016ZT06N331 and 2017M612794). The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0936-0.


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