Journal of Earth Science  2019, Vol. 30 Issue (4): 754-762   PDF    
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Geochemical and Mineralogical Investigation on Different Types of Cenozoic Basalts in the Sanshui Basin: Implications for Magma Mixing Processes
Wei Zhang 1, Nianqiao Fang 2, Xiaobo Yuan 2, Lukai Cui 3     
1. Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;
2. School of Ocean Sciences, China University of Geosciences, Beijing 100083, China;
3. College of Earth Sciences, East China Institute of Technology, Nanchang 330013, China
ABSTRACT: Two kinds of basalts with different chemical compositions were obtained in the Sanshui Basin, they share the same eruption period. Although they have significant differences in major and trace element concentrations, their isotopic ratios suggest a similar magma source. Based on mineral geochemical analysis, orthopyroxene with reaction rim and zoned clinopyroxene are observed in Zidong (ZD) basalts, but not found in Wangjiegang (WJG) basalts. Linear scanning of these minerals suggests compositional variation between within-plate tholeiite and within-plate alkali-basalt. Presence of a double-layer magma chamber under the Sanshui Basin, and occurrence of magma mixing between upper and lower chambers is proposed. This magma mixing leads to different chemical composition and mineral constitution of two kinds of basalts, as well as reaction rim and compositional variation of minerals in ZD basalts.
KEY WORDS: Sanshui Basin    alkali basalt    tholeiite    magma mixing    magma chamber    geochemistry    
0 INTRODUCTION

Tectonic extension occurred in the East Asian continental margin since the Early Cenozoic (Ru and Piggot, 1986; Taylor and Hayes, 1983). As a part of the East Asian continental margin, South China has been dominated by this tectonic extension, which led to formation of numerous volcanic basins (Huang et al., 2018; Zou, 2001, 1995). The Sanshui Basin is one of such volcanic basins, whose volcanism was widely believed to have occurred from the Late Cretaceous to Late Paleocene (Li et al., 1989; Zhu et al., 1989).

Alkali volcanic rocks are the most important eruption in the Sanshui Basin. Although trachyte dominates most of the eruption, basalt is also a very common formation. Based on petrological and geochemical analysis, most basalts in the Sanshui Basin are tholeiitic (Chung et al., 1997; Zhu et al., 1989), while some alkali basalts are also obtained. On basis of previous studies (Dong et al., 2006; Xiao et al., 2006), the relation between trachyte and basalt was well-researched. Trachytic and basaltic samples from Sanshui Basin were erupted from the similar magma source with different crystallization processes. However, the relationship between tholeiite and alkali basalts remains unclear.

In this study, two kinds of basalts with different alkalinities were obtained. Analysis for whole-rock and mineral compositions was carried out for the difference and relation between them. The discussions on analytical results will help us to understand different formation processes of the two kinds of basalts, as well as the magmatism under the Sanshui Basin.

1 GEOLOGICAL SETTINGS

The Sanshui Basin is located in South China, and its volcanism was widely believed to have occurred from 65 to 43 Ma (Hou et al., 2007; Dong et al., 2006; Zhu et al., 1989). On basis of previous studies, more than 1 500 m of volcanic rocks erupted during the Early Paleocene to Middle Eocene (Bureau of Geology and Mineralogy of Guangdong Province, 1988), and most of them are trachyte (more than 1 000 m) and tholeiite (Dong et al., 2006; Chung et al., 1997; Tang, 1994, 1984; Zhu et al., 1989). Based on major and trace element analysis, volcanism in the Sanshui Basin was widely considered to be the consequence of regional asthenospheric mantle upwelling in the continental rift environment (Dong et al., 2006; Xiao et al., 2006). This asthenospheric mantle upwelling commenced at the Lianping and Heyuan basins and migrated southward, and may have finally led to the opening of the South China Sea in Late Paleocene (Zhu et al., 2004, 1989; Chung et al., 1997).

Basalt samples from Wangjiegang (WJG) and Zidong (ZD) were both collected. WJG locates in the south of the Sanshui Basin, while ZD is only 2 km away from WJG in its west (Fig. 1). On the basis of the K-Ar geochronology, WJG basalts were erupted in the last eruption cycle with an age of 43.1±1.3 Ma (Zhu et al., 1989). Meanwhile, the ZD basalts were discovered overlaying Late Paleogene sediment, and were erupted during the same period as the WJG basalts (Dong et al., 2006). Generally speaking, it is highly probable that the ZD and WJG basalts formed in the same eruption period from a unique magma chamber.

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Figure 1. Sketch map showing the locations for the ZD and WJG basalts in the Sanshui Basin (after Hou et al., 2010).
2 SAMPLES AND ANALYTICAL METHODS

Only fresh samples without significant alterations were carefully selected for analysis. These samples were ultrasonically cleaned in distilled water for 60 min, with the water changed every 15 min, and were left to dry in an oven at 60 ℃. They were then crushed into < 200 mesh particles, prior to analysis.

Eight samples (3 from the WJG basalts, 5 from the ZD basalts) were selected for major elemental analysis; and X-ray fluorescence spectrometry (XRF) on fused glass disks was conducted using a Shimadzu XRF-1500 instrument, at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. Relative precision for elements present in concentrations > 1.0 wt.% was ±1.3 wt.%, and was about ±10 wt.% for elements present in concentrations < 1.0 wt.%. However, only total Fe oxide contents were obtained by the XRF analysis. FeO concentrations were obtained using the method by dissolution with HCl titration and potassium dichromate solution. This analysis was also finished at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing.

These samples were also selected for trace elemental analysis (including rare earth elements), using inductively coupled plasma mass spectrometry (ICP-MS), at the Institute of Oceanology, Chinese Academy of Sciences, Qingdao. Sample powders were weighed to be precisely 40 mg, and were transferred into teflon beakers. 1.5 mL HF and 0.5 mL HNO3 were added to the sample powders, and heated at 150 ℃ for 24 h on an electro-thermal plate. Then, at temperature of 120 ℃, 0.2 mL HClO4 was added, and after the white smoke had dissipated, 1 mL HNO3 and 1 mL H2O were added to the residue prior to sealing and heating at less than 120 ℃ for 12 h. The dissolved samples were then diluted a 1 000-fold using 2% distilled super pure HNO3. The solutions were analyzed using Re as the internal standard, and all elements have RSD < 4%.

Seven samples (four from the ZD basalts, three from the WJG basalts) were selected for Sr, Nd isotopic analysis. Sample powders were left to dry in the oven at 80 ℃ for 3 h, and diluted with relevant diluents (85Rb+84Sr or 145Nd+149Sm). Then, HF and HClO4 were added for the dissolution. Then, they were separated and purified with Dowex 50×8 cation exchange resin before the analysis on the thermal ionization mass spectrometry (Triton) at the Wuhan Center of Geological Survey, Ministry of Land and Resources of the People's Republic of China. During the whole analysis, relevant reference materials (NBS987, NBS607 and GBW04411 for Rb-Sr isotopic analysis; GBW04419 and JNdi-1 for Sm-Nd isotopic analysis) were analyzed for comparison (Table 1).

Table 1 Sr and Nd isotopic ratios and chronology results for basalts from the Sanshui Basin

Beside geochemical analysis of whole-rock, electron probe micro analysis (EPMA) on clinopyroxene phenocrysts was also conducted using a JXA-8230(JEOL), at the EPMA Laboratory in Ocean University of China, Qingdao. Si, Na, Mg, Al, K, Ca, Ti, Fe, Mn and Cr were determined under 15 kV acceleration voltages, 20 ncA current, and a beam size of 2 μm. Peak counting times were 60 s for Fe and Mg, and 30 s for remaining elements. Abundance uncertainties were mostly < ±3%.

3 RESULTS 3.1 Major and Trace Element Concentrations

The major element results of the WJG and ZD basalts are listed in Table 2. Two kinds of basalts have higher Na2O+K2O (WJG: 5.8 wt.%–6.0 wt.%; ZD: 4.8 wt.%–5.5 wt.%) and relatively higher TiO2, which is similar to typical alkali series volcanic rocks. However, they have obviously different major element concentrations. Compared with the ZD basalts, the WJG basalts have obviously lower SiO2, and higher concentrations of TiO2, Na2O+K2O, Al2O3 and CaO. The different SiO2 and total alkali between them lead to the different alkalinities. The WJG basalts have extremely high Rittmann indices of 11.5–16.2, while those of the ZD basalts are only 3.1–3.9. In this regard, the WJG basalts can be attributed to peralkaline series (Rittmann: > 9), while the ZD basalts to calc-alkaline (Rittmann: < 3.3) to alkaline series (Rittmann: 3.3–9.0). Compared with the studied basaltic samples by previous works (Dong et al., 2006; Xiao et al., 2006; Chung et al., 1997), most previously studied samples are similar to the ZD basalts with relatively low total alkali and variable SiO2 contents, while a few samples (sample Si-31 and Si-32 from Dong et al., 2006) have high total alkali contents which are similar to the WJG basalts. Such difference between the WJG and ZD samples can be observed in TAS diagram (Fig. 2), while the ZD basalts locate on the boundary line between alkaline and sub-alkaline series, the WJG basalts locate in alkaline series area.

Table 2 Major (in wt.%) and trace elements (in ppm) concentrations of the WJG and ZD basalts from the Sanshui Basin
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Figure 2. SiO2 vs. Na2O+K2O diagram for the ZD and WJG basalts in the Sanshui Basin (the classification is after le Maitre et al., 2002; data of previous study are from Chung et al., 1997; dashed line representing the boundary between alkaline series and subalkaline series is from Rollinson, 1993). Ol. olivine; q. quartz.

The trace element results of the WJG and ZD basalts are also listed in Table 1. Compared with the ZD basalts, the WJG samples have higher concentrations of some large ion lithophile elements (LILE) such as Cs, Rb, Ba, Sr, etc., and some light rare earth elements (LREE) such as La, Ce, Pr and Nd (Fig. 3). Both of them are similar to alkali-basalt (Sun and McDonough, 1989) in incompatible elements concentrations. Compared with the basaltic samples from previous studies (Zhu et al., 2004; Chung et al., 1997), our samples have higher concentrations on most incompatible elements, especial in some highly incompatible elements (Th, U, Nb, K, etc.).

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Figure 3. Primitive mantle-normalized multi-elemental patterns for basalts from the Sanshui Basin (data of previous study are from Chung et al., 1997; normalizing values are from Sun and McDonough, 1989).
3.2 Sr-Nd Isotopic Compositions

The Sr and Nd isotope compositions of the WJG and ZD basalts are listed in Table 2. Both the WJG and ZD basalts have high 143Nd/144Nd ratios with a limited variation ranging from 0.512 881 to 0.512 894, they also have very similar 87Sr/86Sr ratios with a relatively wider variation ranging of 0.703 9–0.704 3. Compared with the basaltic samples of previous studies (Xiao et al., 2006; Zhu et al., 2004, 1989; Chung et al., 1997), our samples have obviously limited variation in Sr-Nd isotope composition and are comparable with some samples (88-13 and 81K-40) of the last eruption period (47.57–37.98 Ma, Zhu et al., 2004) with relatively higher 143Nd/144Nd but lower 87Sr/86Sr ratios (Fig. 4). In general, the similarity of Sr and Nd isotope ratios of the WJG and ZD basalts suggest a common source for them.

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Figure 4. 143Nd/144Nd vs. 87Sr/86Sr for basalts from the Sanshui Basin (data of previous studies are from Zhu et al., 2004; Chung et al., 1997).
4 DISCUSSION 4.1 Compositional Differences and Source Characteristics of WJG and ZD Basalts

Based on previous studies (Dong et al., 2006; Xiao et al., 2006; Chung et al., 1997; Zhu et al., 1989), large quantities of volcanic rocks (basalt, trachyte, rhyolite, etc.) were found in the Sanshui Basin, and most basaltic samples are sub-alkaline.

In this study, two kinds of basalts with obviously different alkalinities are obtained with systematic differences in major and trace element concentrations. The WJG basalts have higher Na2O+K2O (~6.00 wt.%), TiO2 (> 2.50 wt.%) and significant lower SiO2 contents (45.07 wt.%–46.03 wt.%), which are similar with typical alkali basalt. On the other hand, ZD basalts have relatively lower Na2O+K2O (4.78 wt.%–5.51 wt.%), TiO2 (< 2.00 wt.%) and higher SiO2 contents (50.20 wt.%–51.20 wt.%). Nevertheless, the two kinds of basalts have higher Na2O+K2O contents than most tholeiitic basalts from the Sanshui basin (3.11 wt.%–4.70 wt.%). The different alkalinities (Rittmann indice) of the WJG and ZD basalts are mainly the consequence of different SiO2 contents, other than different total alkali contents (Na2O+K2O). On the other hand, the two kinds of basalts have differences in their trace element concentrations. The WJG basalts have higher concentrations of most highly incompatible elements (Rb, Ba, Th, Nb, Sr and LREE), but lower Zr and Hf contents than those of the ZD basalts. In addition, they also have different ratios of some higher incompatible to less incompatible elements such as Nb/Y, Ti/Zr and La/Yb (Table 1). Although there are some systematic compositional differences, the WJG and ZD basalts have very similar 143Nd/144Nd and 87Sr/86Sr ratios (Fig. 4 and Table 1), which indicates a similar source. Their 143Nd/144Nd and 87Sr/86Sr ratios are different from those of Early Cenozoic basalts from the adjacent basin (e.g., Heyuan and Maoming), but comparable with those of the alkali basalts from Leiqiong Basin formed after the expanding of the South China Sea (Zhu et al., 2004). For these basalts, their compositions are very different from basalts derived from the normal asthenospheric mantle. According to the previous studies on the Sanshui Basin, the all volcanic rocks from the Sanshui Basin exhibit a very wide variation in 87Sr/86Sr ratios, but relatively limited 143Nd/144Nd ratios (Fig. 4). As Sr is more incompatible than Nd, the concentration of Sr is obviously higher than Nd in the crust. A small amount of crustal contamination may lead to the significant shifting on Sr isotope ratios without significant changing in Nd ratios (Chung et al., 1997).

The compositional difference between the two kinds of basalts may not result from different mantle sources, but may relate to the different partial melting degrees. Elements which are more incompatible tend to enter into melt prior to less incompatible elements, and this process lead to the enrichment of highly incompatible elements in samples with low partial melting degree. In La/Sm versus La diagram (Fig. 5). La is more incompatible than Sm, and tends to enter into the melt prior to Sm. The samples with low partial melting degree (i.e., the WJG basalts) are apt to have higher La/Sm ratios and La contents than those samples with high partial melting degree (i.e., the ZD basalts and other samples).

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Figure 5. La/Sm vs. La for basalts from the Sanshui Basin (data of previous study are from Chung et al., 1997).

In general, we propose that the ZD and WJG basalts were erupted from similar magma source with small amount of crustal contamination, and the compositional difference between the ZD and WJG basalts is the result of different partial melting degrees.

4.2 Mineral Evidences for the Magma Mixing Processes

Most previous studies on the Sanshui Basin were based on geochemical analysis of whole-rock. The whole-rock composition is the combination of mineral phenocryst and matrix compositions, which might be affected by several different crystallization and mixing processes. However, minerals are more sensitive to sudden modifications in the melt, and may record information in their textural and compositional zoning patterns (Chen et al., 2016; Li et al., 2016; Viccaro et al., 2010; Salisbury et al., 2008; Davidson et al., 2007; Ginibre and Wӧrner, 2007; Ginibre et al., 2002a, b). On basis of the whole-rock chemical analysis above, the WJG and ZD basalts show differences in major and trace elements concentrations. These compositional differences may be generated by the different mineral constitutions. For better understanding the differences between the WJG and ZD basalts, electron probe X-ray micro-analysis (EPMA) on phenocryst compositions are conducted, which will be a powerful tool for distinguishing different processes during the crystallization.

4.2.1 Different mineral compositions

On basis of EPMA analysis, the WJG and ZD basalts have different mineral compositions. All analyzed pyroxene phenocrysts in the WJG basalts are calcium-rich clinopyroxene (Cpx) (CaO: 18 wt.%–21 wt.%), and are categorized as diopside. However, two types of pyroxenes with different calcium contents are found in the ZD basalts. One is similar to the WJG basalt's Cpx, and the other is orthopyroxene (Opx) with extremely low calcium contents (CaO less than 2 wt.%).

Beside compositions, the two kinds of pyroxene phenocrysts show different mineral shapes. Using backscattered electron (BSE) image observations, it can be seen that most Opx phenocrysts in the ZD basalt develop 100 μm wide spongy textured rims (Fig. 6a), which are not observed around Cpx phenocrysts. Based on chemical analysis, these rims show high Ca and low Mg, and are comparable to the groundmass around the Opx phenocrysts. These kinds of rims were commonly found in mantle xenoliths, which were formed by magma mixing during mantle uplifting (Shaw et al., 2006). When minerals are deprived, the compositional imbalance between minerals and melt may lead to chemical reaction on the mineral surface, resulting in spongy textured rims. Thus we believe that the ZD basalts might suffered magma mixing process during crystallization, and those Opx phenocrysts might be derived from the mixed magma.

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Figure 6. Image of spongy-texture rims of Opx phenocrysts by BSE (a) and POM (b).
4.2.2 Zoned clinopyroxene phenocryst: implications for the magma mixing processes

Beside the different mineral constitution, the magma mixing process has also been recorded by some Cpx phenocrysts in the ZD basalts. On basis of observation by BSE and polarized-light microscope, some Cpx phenocrysts with obvious zones are observed in the ZD basalts (Fig. 7), while this phenomenon is not observed in the WJG basalt.

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Figure 7. BSE image (a) and EPMA traceline (b) of Cpx from ZD basalt (numbers beside the traceline are point numbers).

For these zoned Cpx phenocrysts in the ZD basalts, liner scan of EPMA is conducted to obtain 57 compositions, and four zones (rim, Zone 1, Zone 2 and core) are divided by these compositions. Most of the core and rim's points have similar compositions; however, Zone 1 and Zone 2 have wide variation in compositions (Fig. 8). Compared with other zones, Zone 2 has highest MgO (13.92 wt.%–16.28 wt.%) and CaO (18.80 wt.%–20.69 wt.%) (Table S1).

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Figure 8. Mg# and Ti/Al variation (a) and multi-elemental contents (b) of Cpx from the ZD basalt.

On basis of comparison between these four zones, three obvious compositional variations can be distinguished. The first one (Step 1) is between the core and Zone 2. From point 17, the significant increase in MgO accompanies the decrease in CaO, FeO, and Al2O3, and their Mg# (100×MgO/(MgO+TFeO)) and Ti/Al display a rapid rise and decline, respectively. The second one (Step 2) is between Zone 2 and Zone 1. From point 26, the compositions of points exhibit rapid increase in Al2O3, FeO and CaO, but decrease in MgO. This process is also accompanied with relevant variation in their Mg# and Ti/Al. The third one is (Step 3) between Zone 1 and the Rim. The compositions of the points display slowly shifting in Zone 1, but dramatic fluctuations in the last points (37–41) with rapid decrease in FeO and TiO2. This process may be related to the crystallization of Fe-Ti oxides, which is confirmed by the observation by BSE and polarized-light microscope.

It is widely believed that clinopyroxene's composition varies with the chemistry of its host lava (Leterrier et al., 1982), and the Ti versus Ca+Na discrimination diagram (Fig. 9) is conducted for distinguishing the characteristics of their host lava. Almost all Zone 2 points locate in the tholeiite area with low Ti and low Ca+Na, while points from the other three zones locate in alkaline area. In this regards, the ZD magma commenced crystallization in alkali-basaltic magma, and shifted composition to tholeiite in step 1. Then it turned back to alkaline from Zone 2 to the Rim, in steps 2 and 3. The similar variation is also observed in SiO2 versus Al2O3 diagram (Fig. 10) for distinguishing the alkalinity changing of their host lavas (Nisbet and Pearce, 1977). From the core to Zone 2, the alkalinity shows a sustained decline from alkaline to sub-alkaline. However, it returns to per-alkaline from point 27, and go back to the beginning in Step 3.

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Figure 9. Discrimination diagram of Ti vs. Ca+Na for Cpx phenocryst from the ZD basalt (Leterrier et al., 1982; apfu represent atoms per formula unit).
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Figure 10. Discrimination diagram of SiO2 vs. Al2O3 diagram for Cpx phenocryst from the ZD basalt (Nisbet and Pearce, 1977).

Generally speaking, all of these evidences above suggest a similar chemical variation for the formation of the ZD basalts. It started at alkali basaltic core with low MgO and high Ti/Al. Then it influenced by a less alkali magma with higher MgO and lower Ti/Al in Step 1, which lead to the compositional and alkalinity shifting. However, it turned back in steps 2 and 3, which explained the compositional similarity of the core and the Rim.

4.3 Magma Chamber under the Sanshui Basin

On basis of the previous studies, plenty of basaltic samples with different ages were collected in a large area in the Sanshui Basin. In this study, the ZD and WJG basalts were collected in two places only 2 km away from each other. On basis of chronology research, they were both erupted in a similar period (57.1–59.7 Ma). In other words, they were highly probably erupted from a unique magma chamber at the similar eruption period, and that explain the similarity in their isotopic ratios (143Nd/144Nd and 87Sr/86Sr). However, these two kinds of basalts have different chemical compositions and mineral constitutions. Furthermore, the zoned Cpx and rimy Opx in the ZD basalt reveal that it experienced the mixing between magmas with different alkalinities, while this mixing process did not occur for the WJG basalt. How could these happen in one magma chamber at the similar time?

Chung et al. (1997) assumed a "double diffusive" magma chamber model, which explains the generation of the Sanshui bimodal volcanism. We believed that this model can also explain the ZD and WJG basalts' contemporary eruption, as well as the zoned Cpx and Opx with spongy-textured rim (Fig. 11).

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Figure 11. Magma chamber model for the Sanshui Basin (after Chung et al., 1997).

Firstly, an interface formed in the magma chamber, which generated relatively independent differentiations in the upper and lower chambers, respectively. Two types of magma with different density accumulated in the upper and lower chambers, forming alkaline series and calc-alkaline series rocks (Chung et al., 1997). It is proposed that not only trachyte and rhyolite, but also a small amount of alkali basalt, crystallized in the upper chamber. On the other hand, a huge amount of tholeiitic magma crystallized in the lower chamber. This explains why most of basalts in the Sanshui Basin are tholeiitic, other than alkali.

Following this, small amount of magma mixing occurred between the upper and lower chambers though the interface, which lead to compositional shifting and different mineral constitution in the ZD basalt. This mixing process was recorded by zoned Cpx, and some Opx xenoliths were also captured. These Opx formed the spongy textural rims due to reaction with surrounding alkali-basaltic melt. However, the WJG basalt is irrelevant to this mixing process, whose composition represents typical products in the upper chamber.

Generally speaking, although the ZD and WJG basalts were formed by similar source, and crystallized in the same magma chamber, they show different chemical composition and mineral constitution, which is mainly the consequence of different magmatism processes.

5 CONCLUSIONS

(1) Although the Sanshui Basin's ZD and WJG basalts show systematic difference in their chemical compositions, the similarity of their isotopic ratios suggests that they were derived from a similar magma source.

(2) The ZD and WJG basalts have different mineral constitutions. Pyroxene phenocrysts in the WJG basalt are all calcium-rich type, while those in the ZD basalt can be divided into two types with different calcium concentrations.

(3) Opx with spongy textured rim and zoned Cpx are observed in the ZD basalt, which indicates mixing between magmas with different alkalinities.

(4) A magma chamber under the Sanshui Basin is proposed with alkaline and calc-alkaline magma accumulating in upper and lower chambers, and formed alkali-basalt and tholeiite, respectively. Furthermore, small amount of magma mixing occurred between these two magmas, which lead to the different chemical compositions and mineral constitutions of the ZD and WJG basalts.

ACKNOWLEDGMENTS

We thank Prof. Qi Li, Drs. Shu-Ying Yang, Ye Jin, and Jun-Jie Hu for helping us to collect samples. We also appreciate many suggestions from anonymous reviewers. This study was financial supported by the National Natural Science Foundation of China (Nos. 41276047, 41030853, 41572207, 41440041) and the Special and Frontier Foundation for the Twelve Five Plan of the China Ocean Mineral Resources Research and Development Association (No. DY125-22-QY-21). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1208-y.

Electronic Supplementary Material: Supplementary material (Table S1) is available in the online version of this article at https://doi.org/10.1007/s12583-019-1208-y.


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