Abstract

A recently discovered basanite dike in the Zaolin area of Jingdezhen, South China, contains mantle xenocrysts such as kink-banded olivines, olivines + orthopyroxenes assemblage, and chromites. In addition, polymorphic carbonates of the MgCO3–FeCO3 series occur as augens, either independently or interspersed with diopside and spinel in the matrix. The rock is characterized by high Cr and Ni contents, high whole-rock Mg# values (0.66–0.72), and high Ca/Al (0.72–1.03) and TFeO/MgO (1.1–1.3) ratios and is alkali-rich with Na2O > K2O. The trace-element partition patterns are similar to those of other basanites in eastern China as well as ocean island basalts. Whole-rock geochemical analyses show depleted Sr and Nd isotopic compositions (86Sr/87Sr=0.703580.703853, εNd=2.526.73). These data indicate that the rock has experienced negligible crustal contamination, should be derived from asthenospheric mantle, or mixed by the MORB with EMI/EMII mantle and have been carbonated. The calculated T–P conditions of the melt in equilibrium with xeno-olivine are 1160–1320°C at the mantle depth. The high Cr# values of the spinel xenocrysts indicate that the lithospheric mantle under the Jingdezhen area was probably relict Proterozoic mantle. The Ar–Ar plateau age and the isochron and inverse isochron ages for the matrix of the basanite are all 44 Ma. The basanite, as well as other alkaline basalt or lamprophyre dikes in southeastern China, formed in a rifting regime during the Eocene.

1. Introduction

Cenozoic alkaline basalt provinces occur in every continent and are commonly associated with active lithospheric extension in localized continental rifts or over broader continental regions ([1]). Large volumes of Cenozoic alkaline basalts are also widespread in the eastern Asian continental margin (Figure 1(a)). These basalts outcrop along coastal areas and adjacent offshore shelf regions from north of Heilongjiang province to south of Hainan Island, as well as in the South China Sea along the eastern margin of China, together constituting the eastern China volcanic belt ([2]; Figure 1(b)). They were erupted mainly during the Miocene and Pleistocene (20.7-0.38 Ma, [3]). The petrogenesis and tectonic implications of these rocks have been described previously in studies that the NNE-trending belt of Cenozoic mantle-xenolith-bearing alkaline basalts represents a rift setting that evolved from an island-arc setting during the late Mesozoic ([1, 316]). Compared with the Miocene to Pleistocene alkaline basalts, the Paleogene basalts (65–23.5 Ma) are less voluminous and have received less attention [3]. Despite the smaller volumes of volcanic products, the Paleogene is still considered to be an important stage of active volcanism based on basin drilling data and vein properties ([8, 9, 17, 18]). Cong et al. [8] proposed that during the Oligocene, eastern China entered a stage of continental-margin rifting based on the occurrence of Paleocene tholeiitic basalts (58.7–59.7 Ma) and Oligocene alkaline basalts (22.7–28.8 Ma) in the Subei Basin.

Paleogene (especially Eocene) alkaline basalts have rarely been reported in Jiangxi Province and include 41–40 Ma basalt dikes that crop out in Guangfeng City, southeast of Jingdezhen ([19]; Figure 1(a)), and a 44 Ma lamprophyre dike in the Anyuan area south of Jingdezhen ([20]; Figure 1(a)). Recently, a basanite dike was discovered in Permian carbonate rocks in the Zaolin area of Jingdezhen City, Jiangxi Province (Figure 1(a)), at the eastern margin of the Yangtze Block (Figure 2). However, the geochronology, source, petrogenesis, and geodynamic setting of this dike are unknown. This paper reports Ar–Ar ages and mineralogical, petrologic, geochemical, and Sr and Nd isotope data for this dike to ascertain its origin and genesis and provide insights into the composition and physical behavior of the subcontinental lithospheric mantle of eastern China.

2. Geological Setting

The eastern China volcanic belt is composed of widespread Cenozoic basaltic rocks that are distributed along coastal areas and adjacent offshore shelf regions and extends over 2000 km from north to south along the eastern edge of the Asian continent (Figure 1(a)). Tectonically, this belt occupies (from north to south) the Xing’anmeng Block and the eastern parts of the North China, Yangtze, and Cathaysia Block (Figure 1(b)). These blocks and plates have generally been considered an assembly of exotic continental terranes that broke away from Gondwanaland, with their amalgamation having been largely completed during the early Mesozoic [2, 7, 21, 22]. Continental extension and upper mantle convection in eastern China, induced by subduction of the Pacific plate underneath the Asian continent, have been widely accepted as a possible mechanism for the genesis of the Cenozoic intraplate basaltic volcanism ([23] and references therein). Besides, alternative mechanisms including upwelling of a mantle plume [2428] or eastward asthenospheric flow from beneath western China to eastern China [29, 30] also have been proposed.

South China, as a member of this volcanic belt, is composed of the Yangtze Block in the west and the Cathaysia Block (including Taiwan) in the east. Since the Indosinian, the eastern part of South China has become an active continental margin as a result of the collision between the paleo-Pacific and Eurasian plates after the Neoproterozoic amalgamation of the North China and Yangtze Blocks. Subsequently, long-term crustal deformation and lithospheric thinning started, a series of NE-trending faults and graben basins developed from north to south in the eastern part of South China ([9, 17, 3134]), and sediments were deposited during Paleozoic to early Mesozoic period. Red-bed and fluvial–lacustrine facies are well developed in the graben basins ([35]; Figure 1). During the early Cenozoic, the eastern part of South China entered a stage of continental rifting, with frequent volcanic eruptions and magmatic intrusions that are thought to have been related to paleo-Pacific plate subduction and are distributed in basins or along NE- and NNE-trending faults ([3, 9, 17, 36]). Cenozoic alkaline basaltic rocks are exposed in Jiangsu, Zhejiang, and Fujian provinces as well as near and southeast of the Dapu–Zhenghe Fault (e.g., Ming Xi, Longyou, Xilong, and Niutoushan). The basaltic magma was derived from the deep mantle, and its ascent was controlled by the Mesozoic structural framework [3]. The recently discovered basanite dike studied here is located in a quarry in Zaolin (29°539.67N, 117°1023.26E), about 30 km south of Jingdezhen City in South China (Figure 2).

3. Lithology and Petrography

Neoproterozoic and minor Paleozoic and Mesozoic strata strike NE–SW, corresponding to the NE-oriented faults in the Zaolin area (Figure 2). The Zaolin basanite dike outcrops about 25–30 m long and 0.8–4.5 m wide and intrudes Permian carbon-bearing limestones (Figure 3(a)). A shear fault cut off the basanite dike body into two sections with a displacing distance of about 2 meters (Figure 3(a)). The Permian limestones are also intruded by Late Jurassic–Early Cretaceous granitoid porphyries that host the giant Zhuxi W–(Cu) copper ore deposit [37].

The basanite rock is very fresh except for minor fade alteration (indicated by a grayish-white color) and crack deformation within a narrow contact zone (1–5 cm) with the Permian strata (Figures 3(b)–3(e)). The basanite is grayish dark, massive, and porphyritic (Figures 3(c)–3(e)). Phenocrysts (0.05–0.20 mm) occupy 5–10 vol% of the rock mass and are mainly olivine with minor clinopyroxene, carbonate, and spinel (Figure 4). The matrix consists predominantly of microliths or microcrystals of clinopyroxene, with lesser amounts of spinel, olivine, and apatite, as well as glass-like nepheline and sanidine (Figure 5). In contrast to basanites found elsewhere [38, 39], the Zaolin basanite does not contain plagioclase.

Magma-crystallized olivine in the basanite is predominantly euhedral or subhedral, with most grains being fresh, although a few phenocrysts are serpentinized or carbonized (Figures 3(c)–3(d) and 4(a)–4(c)). Some olivine grains have well-developed cleavages and are fractured and tabular in shape (Figure 4(b)). Some of the olivines contain inclusions of spinel + clinopyroxene + sanidine + nepheline, identical to the assemblage of the groundmass (Figure 5(a)). Olivines in the matrix are anhedral, show minor alteration, and locally occur in aggregates (Figures 5(a) and 5(b)). Clinopyroxene phenocrysts are prismatic and euhedral with well-developed zoning (Figure 5(b)), with some occurring as aggregates. In contrast to olivine, which dominantly occurs as phenocrysts, clinopyroxene occurs mainly as tiny laths in the groundmass (Figures 4(a), 4(d), and 5). Spinel is opaque, euhedral, or anhedral and occurs as fine grains scattered in the groundmass (Figures 4 and 5). Carbonate, in fact being polycrystalline, occurs mainly as augen-like phenocrysts (Figures 4(a) and 5(c)–5(f)) or anhedral matrix grains among silicate minerals (Figures 5(d), 5(e), and 5(g)). Augen carbonates contain inclusions of spinel (Figures 4(a) and 5(c)) and are locally intergrown with clinopyroxene (Figure 5(c)), and the silicate matrix lacks quenching rim and fissures where in contact with the carbonates. We therefore attribute the carbonates to silicate–carbonate-liquid immiscibility products (augen phenocrysts) or coprecipitated products (in the matrix) within the magma, but not to capture foreign inclusions or veins. Apatite occurs in the groundmass as very tiny needles (Figures 5(a) and 5(g)).

Mantle-derived xenocrysts occur in the basanite as 0.1–2 mm in size and consist of olivine, olivine + spinel, or olivine + orthopyroxene assemblages (Figures 4(b)–4(d)). This type of olivine shows clear cores and rims, is anhedral, and is the largest mineral in the rock. Kink bands are observed in the olivine xenocrysts (Figure 4(b)). The spinel xenocrysts (0.05–0.50 mm) are euhedral or anhedral, mostly brown in color, and dark along their rims and fractures (Figures 4(d), 5(f), and 5(g)).

Felsic enclaves are locally observed and are inferred to have been captured from wall rocks along the volcanic conduit through which the dike-forming magma traveled.

4. Analytical Methods

49 thin-sections were prepared for petrographic analyses and mineral identification and imaging of the components, texture, and structure of the basanite samples using a Leica 4500P microscope and an energy dispersive spectroscope (EDS) (JEOL JSM–5610LV, focusing distance of 18 mm and accelerating voltage of 20 kV) at the Institute of Geology, Chinese Academy of Geological Sciences (IG–CAGS), Beijing, China. Representative minerals were selected and analyzed using an electron probe microanalyzer (JEOL JXA-8100) at IG–CAGS, with a focused beam diameter of 5 μm, an accelerating voltage of 15 kV, a beam current of 20 nA, and collection time of 10 s.

Two batches of five and seven samples were analyzed for whole-rock major- and trace-element compositions at the National Research Center for Geo-analysis of Beijing, CAGS, Beijing, China. Major elements were analyzed by X-ray fluorescence spectrometry (XRF) based on certified standards (GSR-1 and GSR-3) and duplicate analyses, with the analytical uncertainties being <5%. Ferric and ferrous iron measurements were determined by wet chemical analyses (titration). Trace elements were determined using solution inductively coupled–plasma mass spectrometry (ICP–MS).

Rb–Sr and Sm–Nd isotopes were analyzed by thermal ionization mass spectrometry (TIMS, Finnigan MAT-262) at the Laboratory for Radiogenic Isotope Geochemistry, University of Science and Technology of China, Hefei, China. 87Sr/86Sr ratios were normalized to 86Sr/88Sr=0.1194, and 143Nd/144Nd ratios were normalized to 146Nd/144Nd=0.7219, to correct for isotopic fractionation. 87Sr/86Sr ratios were adjusted to NBS-987 Sr standard=0.710250, and 143Nd/144Nd ratios were adjusted to Shin Eston Jndi-1 Nd standard=0.512115. The uncertainties for concentration analyses by isotopic dilution are ±2% for Rb, ±0.5%–1.0% for Sr, and 0.5% for Sm and Nd, depending on concentration levels. The overall uncertainty for Rb/Sr is ±2% and for Sm/Nd is ±0.2%–0.5%. The procedural blanks were 100 pg for Rb and Sr and 50 pg for Sm and Nd.

Matrix minerals in the samples were obtained after carefully removing the olivine and pyroxene phenocrysts and xenocrysts, as well as the felsic xenoliths to increase the K concentration. The matrix minerals were then irradiated in a quartz vial in a nuclear reactor at the Chinese Institute of Atomic Energy, Beijing, China. 40Ar/39Ar age determinations were performed at the Ar–Ar laboratory at CAGS. The decay constant for 40K used in the calculations was 5.543×1010 yr−1 [40]. 40Ar/39Ar spectra were constructed using Isoplot [41].

5. Results

5.1. Mineral Chemistry

Representative microprobe data for minerals in the Zaolin basanite are listed in Supplementary Data Tables 1–6.

Olivine xenocrysts have uniform compositions and the highest MgO contents of the studied minerals, with Fo values of 89.5–90.7 in cores and 79.6–85.2 in rims (Figure 4(b); Supplementary Data Table 1). In the cores, NiO and CaO contents are >0.3 wt% and <0.06 wt%, respectively. Orthopyroxene xenocrysts are interspersed between olivine mineral xenocrysts (Fo=90) (Figure 4(c)) and have a composition of En89Wo1Fs10 and an Al2O3 content of 3.74 wt% (Supplementary Data Table 2). Spinel xenocrysts are anhedral or euhedral, with clearly defined cores and rims (Figures 4(d) and 5(f)–5(h)). Of the studied minerals, these spinels have the lowest contents of TiO2 (<1 wt%), the highest contents of MgO (18 wt%) and Cr2O3 (43 wt%), and yield Cr# values of 25–65 (Cr#=atomCr/Cr+Al) and Mg# values of 44–69 ((Mg#=atomMg/Mg+Fe2++Fe3+) (Supplementary Data Table 3). The very low TiO2 (mostly <0.3 wt%) contents suggest that the spinel xenocrysts are residual chromite [42]. The rims are rich in TiO2 and FeO, similar to the spinels in the matrix, and show Cr# values of 38–57 and Mg# values of 2–17, suggesting the rims are a product of reaction between melt and spinel xenocrysts. Strong erosion by the magma converted the high-Cr spinel to Ti-rich chromite and Ti-magnetite (Figures 5(f) and 5(g)). The relationship between spinel Cr# and olivine Fo indicates the xenocrysts were derived from a mantle source [38, 42, 43]. In addition, on the basis of the high Fo, En, and Mg# values of olivine and pyroxene, the xenocrysts are analogous to spinel lherzolite and harzburgite inclusions in Cenozoic alkaline basalts of eastern China ([5, 9, 17, 20, 44]) and are from the lithospheric mantle.

Magma-crystallized minerals including olivine, clinopyroxene, spinel, carbonate, sanidine, nepheline, and apatite have been measured. The magma-crystallize dolivine with Fo end-member composition has Fo values of 79–85 (mostly 83–85), differing from olivine xenocryst cores but similar to olivine xenocryst rims (Supplementary Data Table 1). These olivines have higher CaO (0.2–0.5 wt%) and lower NiO (0.1–0.2 wt%) contents than the olivine xenocrysts. Near felsic inclusions, the Fo content of the magmatic olivine drops markedly to 45–53 (Supplementary Data Table 1, analysis points 93 and 94), suggesting that Fe–Mg partitioning between the melt and minerals (olivine and clinopyroxene) was affected by temperature and pressure when cold foreign enclaves were introduced.

The clinopyroxenes plot in the diopside domain in a pyroxene classification diagram (Figure 6). These clinopyroxenes commonly contain TiO2 and show compositional zoning (Supplementary Data Table 2). Clinopyroxenes in the matrix are homogenous and rich in Ti and Al, having the highest TiO2 (7 wt%) and Al2O3 (13 wt%) contents of the studied clinopyroxenes. Diopside is mainly CaTiAl2O6, CaAl2SiO6, NaAlSi2O6, CaFe2SiO6, and NaFeSi2O6. If only the Wo, En, and Fs end-members are considered, the diopside has compositions of En38–32Wo53–48Fs16–9, similar to those of Cenozoic basanites in other areas such as Nushan, Tashan and Fangshan (Subei; [5, 14, 45]), Mt. Vulture volcanic complex (Italy; [46]), and Izu–Bonin volcanic arc (Japan; [38]).

The magmatic spinel shows clear compositional heterogeneity even in tiny matrix grains. Spinel centers are rich in Cr2O3 and MgO and poor in TiO2 and FeO relative to the edges, with Cr# values of 38–60 and Mg# values of 1–40. The high TiO2 and FeO contents of the edges are similar to those measured for the rims and fractured parts of spinel xenocrysts (Supplementary Data Table 3). The compositions of the magmatic spinels are similar to those of spinels found in kimberlite [47] and leucite basanite (7.28 wt% TiO2, 6.40 wt% Al2O3, 28.13 wt% Cr2O3, 50.46 wt% FeO, and 5.27 wt% MgO; [48]) in that Cr2O3, MgO, TiO2, Al2O3, and FeO occur together in single crystals, in contrast to basanite in the Izu–Bonin arc [38], which contains Ti-free chromite or evolved basanite with Ti-magnetite [46]. We infer that the spinels in the matrix of the studied Zaolin basanite were inherited from lithospheric spinels that were trapped and altered by Ti-rich alkaline silicate melts, given that spinel in the matrix contains Cr, Ti, Mg, Fe, and Al together in single crystals and that the basanite contains mantle-derived xenoliths.

The carbonates are polyphasic, occur mainly as solid solutions of FeCO3 and MgCO3 with minor CaCO3 end-members (Figures 5(c)–5(f)), and show clear compositional heterogeneities (Supplementary Data Table 4). The darker parts of these carbonates in BSE images (Figures 5(d) and 5(e)) correspond to magnesite with compositions of Cc1–7Sid18–41Mag80–52, and the lighter parts correspond to siderite with compositions of Cc1–19Sid52–82Mag14–47. Carbonate in the groundmass coexists with alkaline feldspars and diopside and has a sideritic composition (Figures 5(d) and 5(g)). Locally, carbonate with MgCO3 and FeCO3 contents of ~50% occurs adjacent to carbonate augens or is distributed along the margins of, and fractures within, olivine. Ionov et al. [49] considered that magnesite (MgCO3–FeCO3) from carbonate-bearing spinel lherzolite inclusions in the Spitsbergen basalt was a secondary, posteruption product. The MgCO3–FeCO3 system at 600–800°C and 15 kbar is completely solid solution, and CaCO3 has limited solubility in this system even at 800°C [50]. When the temperature drops below 300°C, MgCO3–FeCO3 solid solution might separate. These characteristics of the MgCO3–FeCO3 series carbonate within the basalt differ markedly from those of olivine basalt in the CaCO3 [46, 51], CaCO3–CaMg(CO3)2 [52], CaCO3–FeCO3 [53], and FeCO3–CaMg(CO3)2 series [54], which might be associated with a melt rich in CaO to which a carbonate component was subsequently added.

The sanidine and nepheline in the basanite are closely intergrown and very fine (~20 μm in size), as confirmed by electric microprobe analyses (Figures 5(a), 5(b), and 5(g)). These two minerals appear only as intergranular phases in the matrix. In BSE images, the relatively light-colored minerals correspond to sanidine, with compositions of Or84–55Ab41–11An7–0 (Supplementary Data Tables 5 and 6). The dark-colored regions correspond to nepheline that is darker at higher SiO2 contents, which range from 46.4 to 54.8 wt%.

Compared with nepheline in basalts [38, 46, 5559], the nepheline-like crystals in the Zaolin basanite are even more enriched in SiO2 and Al2O3 and are deficient in alkalis (Na2O+K2O<14wt%), which suggests that the basanitic magma was poorly differentiated and crystallized at the time of crystal growth. Calculations show that the Si contents of the nepheline-like crystals are lower than those of normative feldspar but higher than those of normative nepheline. Relative to synthesized silicon-rich nepheline [55], based on 8 oxygens, the nepheline has very low Na + K, high Si + Al + Fe3+ (>4.0), and minor Mg and Ca. However, considering that the Ca contents are very low and those of Mg a little higher, we speculate that the nepheline-like crystals contain minor pyroxene or occurred as solid solution with minor pyroxene, as inferred from the ternary diopside–nepheline–sanidine system at 2 GPa [60]. Except for a few analysis points (Supplementary Data Table 5), the compositions are restricted to the field defined by the Barth join, and the quartz end-member exceeds 40 (Table 1), which might be relevant to the nepheline in the basanite appearing in a quenched glass or metastable phase [61] and containing minor diopside [60].

Apatite, confirmed by EDS analysis (Figures 5(a), 5(b), and 5(g)), occurs as very fine needles in the matrix. This apatite might contribute to the whole-rock phosphorus content (P2O5>1wt%).

5.2. Whole-Rock Chemistry

The analytical major- and trace-element data for 12 samples of the basanite are listed in Table 1. The oxide compositions have a narrow range: SiO2=40.0944.30wt%, TiO2=2.222.43wt%, Al2O3=10.9111.61wt%, Fe2O3=3.394.65wt%, FeO=7.0210.01wt%, MnO=0.170.19wt%, MgO=9.0211.49wt%, CaO=9.1012.35wt%, Na2O=3.334.75wt%, K2O=1.102.47wt%, P2O5=1.121.23wt%, CO2=0.173.80wt%, and H2O=1.943.12wt%, with Na2O > K2O. The loss on ignition (LOI) is 1.98–5.46 wt%, which is slightly lower than the total content of CO2 + H2O. The LOI should be a function of the content of carbonate and serpentine because the samples are fresh and show negligible alteration. Other basanites have Al2O3 and CaO contents that are commonly >12.0 wt% and>10.0 wt%, respectively [51, 6265], or Al2O3 contents of >12.0 wt% [66]. In comparison, the Zaolin basanite has lower Al2O3 and CaO (except one sample, ZX14-38), which likely explains the lack of plagioclase that crystallized from the magma. In a total alkali–silica (TAS) classification diagram (Figure 7), the data fall in the basanite field. Based on the various aforementioned features, including SiO2 contents of <50 wt% and normative nepheline and olivine contents of >10%, the rock samples can be classified as basanite. The high Mg# (0.75–0.66) (Mg#=atomicMg/Mg+Fe2+) values imply that the basanitic magma is close to primary magma [67]. The TFeO/MgO ratios lie in the range of 1.1–1.3, showing that the magma experienced minor differentiation.

The Zaolin basanite is characterized by high contents of Ni (244–299 ppm), Cr (307–389 ppm), Ba (352–618 ppm), and Sr (1183–1466 ppm), reflecting the characteristics of primary magma [67]. In primitive-mantle-normalized spidergrams (Figure 8(a)), all samples show overall enrichment in highly incompatible elements relative to less incompatible elements, with positive Nb and Ta anomalies and negative K and Pb anomalies, but no depletion of high-field-strength elements (HFSEs), similar to the patterns of oceanic island basalt (OIB; [68]) and of Cenozoic basalts in southeastern China (CBSEC) (Figure 8). The chondrite-normalized rare-earth element (REE) patterns of the basanite samples are characterized by enrichment of light REEs (LREEs) (La/Yb=46.153.8) and a lack of Eu or Ce anomalies (Figure 8), implying the absence of low-temperature alteration and plagioclase fractional crystallization. High LREE contents (La–Eu: 256–318 ppm) and Gd/Yb ratios (6.0–6.7) and low heavy REE (HREE) contents (Gd–Lu: 18.7–21.6 ppm) indicate residual garnet in the source. The samples also show strong negative Ti anomalies and weak negative Zr and Hf anomalies, with high Zr/Hf ratios of 38.0–46.0, also similar to those of OIB [69] and CBSEC (Figure 8).

5.3. Sr–Nd Isotope Compositions

The Sr and Nd isotope ratios of 12 basanite samples are listed in Table 2. The 86Sr/87Sr ratios lie in a narrow range of 0.70358–0.70385 and the 143Nd/144Nd ratios in a narrow range of 0.512715–0.512913, similar to basanites in the Hefei basin [8], Nüshan [70], Shandong [71], and Jiangsu [72] in eastern China (Figure 9). The calculated εNd(t) and εNd(0) values are 2.52–6.73 and 1.50–5.36, respectively. These isotope values are also similar to those of the OIB array intermediate between DMM and EMI (Figure 9). Accordingly, crustal contamination is inferred to have been negligible.

5.4. Geochronology of the Basanite

To determine the age of crystallization of the basanite, large and high-density phenocrysts and xenocrysts were removed by handpicking and gravity separation to maximize the potassium concentration. The Ar–Ar isotope data for the matrix of the basanite are listed in Table 3. The analysis yielded a plateau age of 44.05±0.52Ma, an isochron age of 43.5±0.9Ma with an initial 40Ar/36Ar ratio of 298.2±4.6, and an inverse isochron age 43.5±2.5Ma with an initial 40Ar/36Ar ratio of 298±14 (Figures 10(a)–10(c), respectively). Therefore, the basanitic magma cooling age is interpreted as 44 Ma, confirming that the magmatism was unrelated to the large-scale Mesozoic W–Cu metal mineralization (J3–K1) in the area. This age is close to the 44 Ma age of lamprophyre [20] and the 41 Ma age of alkaline basalt [19] in Jiangxi Province, reflecting an Eocene magmatic event in this part of eastern China. Subsequent alkaline basaltic magmatism migrated eastward and southward and was widely distributed across eastern China during the Miocene ([3, 9, 17]).

6. Discussion

6.1. Temperature and Pressure of Formation of the Basanitic Magma

To estimate the temperature and pressure conditions of the magma, olivine–melt equilibrium and clinopyroxene–melt equilibrium geothermobarometers were applied [73]. In Figure 11, only xenocrystalline olivine is in equilibrium with the whole-rock chemical composition as melt with KDol–liq(Fe–Mg) close to 0.30±0.03 [63]. CO2 would facilitate fluid exsolution from such magma, meaning that the H2O content is low in some CO2-rich basanite and phonolite magmas, generally <1 wt% [74]. Therefore, we present results for two types of magmatic system: hydrous and CO2-bearing [65, 73, 75], using the geothermobarometer of Putirka [73] for the hydrous system and that of Dasgupta et al. [75] for the CO2-bearing system. As the water and CO2 contents in the Zaolin magma are unknown, we assumed a melt pressure of 8–20 kbar and water contents of 0.5, 1.0, and 5.0 wt% [65]. According to equation 22 of Putirka [73], the calculated temperature of the melt for these three water contents is 1254–1320°C, 1243–1307°C, and 1160–1216°C, respectively. In contrast, the calculated melt temperature is 1100–1202°C with a pressure of 8–20 kbar based on the equation of Dasgupta et al. [75] for a CO2-bearing system. Combining these results for the two systems (hydrous and CO2-bearing), the conditions for melt in equilibrium with xenocrystalline olivine are estimated to be 1100–1320°C with CO2 and 0.5–5.0 wt% H2O in the upper mantle [76].

When the clinopyroxene–melt equilibrium geothermobarometer [73] was applied, melt–clinopyroxene was found to be in disequilibrium on account of the large differential of KDcpx–liq(Fe–Mg) from 0.27±0.03 (Figure 11). If a barometer based on clinopyroxene only with equations 32c and 32d of Putirka [73] is used with KDFeMg=0.2820.299, then the calculated temperature–pressure (T–P) range is 932–1244°C and 10–26 kbar. These temperature estimates are lower than those for the melt in equilibrium with xenocrystalline olivine (1100–1320°C) as well as those for Zhejiang Cenozoic tholeiitic and weakly alkaline basalts [14]. These lower estimates are probably attributable to the magma system containing H2O and CO2, which causes a decrease in the solvus line of the melt system.

6.2. Magma Source of the Basanite

Most of the Zaolin basanite samples have relatively low LOI values of 2.2 to 3.4 wt.% (Table 2), as well as Ba/Rb values (10.8–14.1 with a mean of 12.0) similar to those of intraplate basalts (~12; [68]), suggesting that postmagmatic alteration of the samples was insignificant. This inference is consistent with the petrographic observations, which show that the samples are generally fresh, and confirms that the whole-rock geochemical and mineral chemical data are reliable.

Some elements and isotopes can be used to determine the source of basalt, such as HFSEs being depleted relative to LREEs in the lithospheric mantle, with the Nb/La ratio being lower in OIBs originating from the lithospheric mantle (≤0.5) and higher in OIBs originating from the asthenospheric mantle (≥1.0; [77]). De Paolo and Daley [78] also considered that La/Nb<1 indicates an asthenospheric source and La/Nb>1 a lithospheric source. Melts originated from asthenospheric mantle have La/Ta ratios close to 10 [79]. The Zaolin basanite yields Nb/La ratios of 1.2–1.6 and La/Ta ratios of 11–13.4, suggesting that the melt was derived from an asthenospheric source but not lithospheric mantle [80]. Zr/Ta and Nb/Hf ratios, which are independent of metasomatism, crustal contamination, and differentiation, can also indicate source characteristics. High Zr/Ta (>200) and low Nb/Hf (<4) ratios reflect a depleted source, whereas low Zr/Ta (<200) and high Nb/Hf (>4) ratios indicate an enriched asthenospheric mantle source [81, 82]. The Zaolin basanite samples have low Zr/Ta ratios (50–55) and high Nb/Hf ratios (12.0–16.3; Table 1). Furthermore, the high 143Nd/144Nd and low 86Sr/87Sr ratios, as well as the trace- and rare-element patterns, are in accordance with an OIB-type asthenospheric source. These characteristics are consistent with those of the Cenozoic alkaline basalts in eastern China, which are generally thought to have been derived from low-degree partial melting of asthenospheric mantle ([3, 9, 14, 17, 3034, 8386]).

Two likely sources have been identified for the Cenozoic alkaline basalts in eastern China: reconstructed silicon-deficient and silicon-enriched pyroxenite [85, 87] and carbonated mantle peridotite containing minor eclogite and pyroxenite [39, 88]. The inclusions of ephritic–trachyandesitic and tholeiitic glasses (SiO2=46.456.2wt%) in olivine phenocrysts of the alkaline basalts of eastern China are thought to have been captured from early melts of the source, equivalent to pyroxenite [87]. However, much earlier stage melts have been observed in anhydrous spinel and garnet peridotite xenoliths in alkaline basalts of northeastern, northern, central, and southern China [89], in which melt inclusions in olivine and pyroxene comprise SiO2- and alkaline-rich glasses (SiO2=6068wt% and K2O > Na2O), with Al2O3–CaO–TiO2-rich clinopyroxene, spinel, and feldspar daughter crystals [89]. These melt inclusions are interpreted to be captive mantle fluid materials and are multistage inclusions [87, 8992]. Thus, we infer that the captive early melts within and between grains would have been affected by mixing, crystallization, and differentiation during magma ascent, with the components captured by olivine and pyroxene in the magma being evolved products, meaning that the glasses entrained in olivines are not necessarily representative of the primary composition of the source. In contrast, the high Mg# and positive εNd(t) values of the Zaolin basanite show that the source underwent a low degree of partial melting. The inclusion assemblage in olivine phenocrysts is Ti-diopside + spinel + nepheline + sanidine, similar to the Zaolin basanite itself (Table 1; Figure 5(a)), suggesting that the early melt resembled the late melt compositionally and that the melt experienced weak differentiation. In the basanite, the olivine phenocrysts that crystallized from magma are rarely zoned, and they have narrow rims. The clinopyroxene phenocrysts are also rarely zoned, and most of the clinopyroxene occurs in the matrix. The melt might have resided within the upper mantle, considering the above-calculated T–P conditions of olivine and clinopyroxene crystallization.

Zeng et al. [39] proposed that the source of alkaline basalts in eastern China was mainly carbonated peridotite with minor eclogite, on the basis that carbonatite is highly enriched in incompatible elements but not in K, Zr, Hf, or Ti and has extremely high Zr/Hf and Ca/Al ratios. K, Zr, Hf, and Ti anomalies are expected on account of their much higher bulk partition coefficients compared with REEs under mantle conditions for carbonatite melt [93]. The addition of a carbonate component or CO2 flux to peridotite can lead to enrichment in REEs but not in K, Zr, Hf, or Ti [64]. Trace-element modeling using depleted upper mantle plus 0.3 and 1.0 wt% carbonatite yielded similar trace-element patterns to those of natural alkaline basalts, as well as high Zr/Hf ratios. Silica-deficient garnet pyroxenite or eclogite cannot produce negative Ti anomalies unless there is residual rutile in the mantle source, which would result in Ti, Nb, and Ta depletion in melts [94]. However, zircon would not significantly fractionate Zr from Hf [95]. Therefore, garnet pyroxenite (garnet < pyroxene) cannot produce melts with both high Zr/Hf ratios and negative Zr and Hf anomalies. Eclogite (garnet > pyroxene) can produce melts with negative Zr and Hf anomalies but with low Zr/Hf ratios [96]. Therefore, pyroxenite and eclogite cannot explain the fractionation of Zr and Hf and are not the main contributors to alkaline basalts [66].

Lithospheric mantle metasomatized by CO2 fluid yields a silica-deficient melt that shows high CaO/Al2O3 and La/Zr ratios and low Ti/Eu ratios [9799]. The Zaolin basanite has LaN/ZrN (3.5–4.4) and TiN/EuN (0.6–0.7) ratios close to those of the calculated parental magma (LaN/ZrN=3.811.3; TiN/EuN=0.180.54) in equilibrium with carbonatite liquid [100]. The HfN/YN ratios (4.1–4.4) of the basanite are also similar to those of an alkaline basalt complex in Brazil (3.9–9.4) [100]. In addition, olivine with high Ca/Al ratios overlapping those of HIMU basalt is interpreted to be related to carbonated metasomatism of the sublithospheric mantle [101]. Some of the olivine (analysis points 20, 39, and 72 in Supplementary Data Table 1) in the Zaolin basanite have high Ca/Al ratios (36.5, 29.8, and 31.7, respectively; others<16.2). Together, the above characteristics indicate the carbonated nature of the source for the Zaolin basanite, but not a metasomatic origin.

Petrologic experiments have confirmed that low-degree partial melting of carbonated peridotite can yield nepheline-bearing silicate magma [75, 102]. Moreover, CO2-bearing melt and fluid inclusions [70, 103106] and carbonate-bearing pyrolite xenoliths [49] in basalt are commonly observed, showing that carbonate exists in the mantle and participates in mantle reactions. Such CO2-bearing fluid inclusions record very high pressures, corresponding to near-mantle depths [104]. The presence of carbonate minerals; carbonate-like trace-element patterns of K, Pb, Ti, and Zr; negative Hf anomalies; high Zr/Hf ratios (38–46); and high Ca/Al ratios of olivine in the Zaolin basanite (Figures 4(a), 5(c)–5(g), and 8(b); Tables 1 and 2) suggests that the source of the alkaline basalts of southeastern China was carbonated peridotite [39, 45, 107].

6.3. Evolution of the Basanite

Various studies have been conducted on alkaline basalts and their relationship to carbonate [46, 49, 52, 53]. Experiments have shown that partial melting of carbonated peridotite can generate carbonate melting, or CO2-rich silicate melting and carbonate liquidation, or carbonated silicate melt after crystal fractionation and differentiation [108, 109]. Carbonate ocelli in silicate (basalt, kimberlite, and eclogite), which were initially explained in terms of liquidation between silicate and carbonate liquids, have subsequently been recognized as the products of magmatic differentiation or evolved products [109]. The early crystallized carbonate from a carbonate–silicate melt comprises dolomite and magnesium calcite, and the late-crystallized carbonate is siderite [49]. Carbonate-bearing silicate magma dissociates along the solidus line and forms coprecipitated carbonate and silicate crystals. As the surface energy of carbonate is higher than that of silicate melt, carbonate melt exists between silicate crystals that are unable to independently migrate from the silicate melt and are removed only after crystallization of the silicate or liquidation [109, 110]. Therefore, carbonate crystallizes later than silicate melt.

Though carbonate-bearing basalts are rarely reported in Cenozoic in China, they are much more common in Mesozoic alkaline basalts [54, 111, 112]. These rocks are all found in sedimentary carbonate strata, and the wall rock with high fCO2 probably prevents CO2-rich fluids of alkaline basaltic magma from diffusing during solidus depression and magma upwelling [54, 111, 112]. The studied Zaolin basanite also outcrops within the Permian carbonate strata. Carbonate minerals in this basanite occur with two types of texture: polycrystalline (which might have formed during late, rapid crystallization) and augen-like. The MgO + FeO content of the carbonate in the basanite is much higher than 50 wt% (Supplementary Data Table 4), which means that this carbonate was miscible with basanitic silicate magma at high temperature and pressure and separated later during solidus depression, according to the experimental results of Lee and Wyllie [109]. The carbonates have heterogeneous chemical compositions (Figures 5(c)–5(e), coarser grains), indicating a change from high-temperature miscible to low-temperature immiscible phases. Furthermore, the Zaolin basanite carbonates differ markedly from the carbonate of the host rock (Permian strata), which is mainly calcite. Few fractures are observed around the carbonate augens and siderite, and some of the carbonate minerals are interspersed with diopside (Figure 5(c)) and Ti-magnesite (Figure 4(a)), suggesting that the carbonate melt produced minerals similar in size to that of diopside in the matrix and crystallized around the silicates and oxides. The finer siderite observed in the silicate minerals of diopside, sanidine, and nepheline-like minerals probably coprecipitated with these minerals (Figures 5(c), 5(d), and 5(g), fine-grained areas). The other type of carbonate, distributed along the margins of, and fractures in, olivine phenocrysts, was formed by late-stage hydrothermal fluid from the melt when P–T conditions had dropped substantially. The hydrothermal fluids likely separated from the basanitic magma as a result of the hydrous nature of the melt and CO2 escaping from the magma during ascent at shallow levels in the crust.

The lack of zoning in olivine and clinopyroxene and the high Mg# of the melt indicate that the residence time of the melt within the crust was short and that polybaric crystallization did not occur [63]. The low Fo values (79–85) of olivine, in contrast to the high Mg# values and the existence of mantle xenoliths, show that olivine and clinopyroxene accumulated in the melt but that differentiation was negligible. In addition, the high Ni and Cr contents of the magma also suggest insignificant fractional crystallization of olivine and negligible fractional crystallization of clinopyroxene in the Zaolin basanite [113]. Zr/Y and Zr/Nb ratios also can indicate the degree of partial melting, with high Zr/Y and low Zr/Nb ratios indicating low-degree partial melting and vice versa [114]. The Zaolin basanite is inferred to have originated from low-degree partial melting on account of its high Zr/Y (11.1–12.8) and low Zr/Nb (2.8–3.4) ratios, consistent with the values of Mg#, Cr, and Ni [80].

During the partial melting of garnet lherzolite and spinel lherzolite, garnet preferentially incorporates HREEs relative to spinel [115, 116], resulting in high La/Yb and Gd/Yb ratios in the melt. The chondrite-normalized REE patterns of the Zaolin basanite indicate a garnet residue in the source and that the melt had high La/Yb (46–54) and Gd/Yb ratios (6.1–6.7) and low HREE contents (18.7–21.6 ppm; Table 2). In addition, all of the olivines in the basanite plot in the common olivine field for peridotite xenoliths, orogenic massifs and ophiolites, abyssal peridotite, and MORB [117], without an abnormally high NiO content, suggesting that the source lacked a pyroxenite component (Figure 12; [118]). The following factors indicate that apatite existed in the source: the Zaolin basanite contains more P2O5 (>1 wt%) than other basanites (<1 wt%) [51, 6265], the melt inclusions of olivine in the Baekdusan basanite [119] and the Ross Island alkaline magmas [74], and the observed positive P anomaly in primitive-mantle-normalized REE patterns. When partial melting occurred, apatite would have readily formed in the melt, thereby increasing the positive P anomaly. In addition, experiments have shown that when the melt temperature drops below 1100°C and SiO2 is ≥48 wt%, apatite starts to crystallize from the melt [74], which might explain the presence of apatite in the matrix of the Zaolin basanite.

In accordance with the above findings, the evolution of the Zaolin basanite can be inferred from its mineral assemblages and chemistry, geochemistry, and source. During the evolution of the basanitic magma, melt converged and crystallized at mantle depths, and olivine and clinopyroxene phenocrysts grew. Owing to the rapid ascent of the magma under high-temperature (~1250°C) and low-viscosity (low-SiO2) conditions, olivine and clinopyroxene did not readily crystallize and differentiate, meaning that crystal sizes were small (<0.5 mm), and the melt had high Mg# values and high Cr and Ni contents and was in disequilibrium with olivine and pyroxene. Microliths of clinopyroxene and olivine, followed by spinel, sanidine + Si-rich nepheline, and carbonate, crystallized while the melt ascended through the crust. Ca was incorporated largely into diopside and less readily into MgCO3–FeCO3 [50], leading to the formation of carbonate (and feldspar) and late-crystallized minerals poor in Ca. During the late stage, Al and Si entered mainly the sanidine and nepheline-like melt. K was mainly consumed and incorporated into sanidine. Na and the remaining K, as well as Si, were incorporated in the nepheline, which was the last mineral to crystallize, with variable SiO2 contents ranging from 45 to 52 wt% (Table 1). No plagioclase was formed in the magma owing to the insufficient Si, Al, and Ca in the melt. Carbonate melt crystallized late and was controlled mainly by early crystallized silicates; these carbonates formed as polycrystalline and ocellar carbonates, with sizes similar to those of the silicate minerals [110], as either phenocrysts or in the matrix.

Previous studies on mantle xenoliths captured by Cenozoic alkaline basalts show that the lithospheric mantle of eastern China is fertile, reflecting the upwelling of asthenospheric mantle that replaced ancient lithospheric mantle [20, 44, 86, 120]. South China is also considered to have fertile juvenile lithospheric mantle and locally preserved relict Proterozoic mantle [20, 44]. The Cr# of spinel and clinopyroxene is a sensitive indicator of the extent to which mantle peridotites have lost their basaltic components [20, 67, 121]. The Zaolin basanite entrained mantle xenocrysts of olivine and spinel, in which the olivine has a very narrow range of Fo (89.6–90.7), similar to the values for peridotite xenoliths in eastern China. However, the Cr# (0.28–0.64) and Mg# (0.52–0.68) values of the spinels are refractory rather than fertile, reflecting that the lithospheric mantle is relict. Three spinel xenocrysts contain much higher Cr2O3 (>40 wt%) and lower Al2O3 (<12.0 wt%) contents (Supplementary Data Table 3) than those of spinel lherzolite and harzburgite xenoliths in alkaline basalts in eastern China ([5, 9, 20, 44]) and resemble those from peridotite inclusions in kimberlite [122]. These features suggest that the lithospheric mantle beneath Jingdezhen underwent high-degree partial melting and partly formed at very high pressure, with the residual lithospheric mantle fragments being captured by rapidly ascending basanitic melt and brought to the surface.

Our calculations show that the Zaolin basanite melt was in equilibrium with olivine at 1160–1320°C and pressures of 8–20 kbar. Considering the existence of carbonate melt as well as garnet (2.5–3.0 GPa) in the source, the Zaolin basanitic magma is inferred to have been derived from a depth of >70 km [62, 109, 123, 124]. Spinels in the matrix of the Zaolin basanite are clearly inherited from spinel of the relict lithospheric mantle, as shown by the high Cr2O3 contents of these spinels, their compositional inhomogeneity, and the presence of both Cr2O3 and TiO2 in single spinel grains (Table 1). As the basanitic magma formed at high temperature and given that spinel has a lower melting point than silicates (olivine and pyroxene), the trapped spinel from the relict lithospheric mantle was easily melted and reacted with alkaline silicate melt, then was dispersed and subsequently crystallized late when the temperature dropped during magma ascent. Owing to the slow rate of Cr3+ diffusion, the characteristics of high Cr2O3 content and compositional inhomogeneity of the dispersed spinel were retained in the matrix. Therefore, the occurrence of the Zaolin basanite illustrates that the lithospheric mantle under Jingdezhen, Jiangxi Province, in the Yangtze Block has not been completely replaced by upwelling asthenosphere mantle and was captive by the basanitic magma generated by low-degree partial melting of deep carbonated peridotite.

Major element and REE concentrations and U–Th–Pb, Sm–Nd, and Rb–Sr isotope systematics reported for Cenozoic volcanic rocks in northeastern and eastern China have confirmed that these volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system that developed as a result of subduction of the western Pacific plate beneath the eastern Asiatic continental margin. This subduction and rifting caused extension-induced passive asthenospheric upwelling and decompression melting ([35] and references therein), thereby explaining the petrogenesis of Cenozoic basalts in eastern China. Our Ar–Ar dating shows that the Zaolin basanite was formed at 44 Ma, similar to the ages of the Guangfeng alkaline basalt dike and the Anyuan lamprophyre dike. The dynamic setting inferred for the Cenozoic basalts in eastern China can also reasonably explain the dynamics for the Zaolin basanite, although more evidence is needed.

7. Conclusion

A systematic study of the petrography, mineral chemistry, major and trace elements, Sr–Nd isotopes, and Ar–Ar ages of the Zaolin basanite from southeastern China allows us to constrain its petrogenesis including forming conditions, source, and evolution processes and thereby the origin of continental basalts. The major conclusions of the study are as follows.

  • (1)

    The basanitic magma was in equilibrium with xenocryst olivine at 1100–1320°C and mantle depths. Olivine and clinopyroxene phenocrysts might have been retained in the mantle and later rapidly ascended to the surface in the melt with only a short residence time in the crust

  • (2)

    The Zaolin basanite contains mantle xenocrysts, i.e., kink-banded olivines, olivines + orthopyroxenes, and chromites, and shows high Cr and Ni contents and depleted Sr and Nd isotopes. These features indicate an origin from low-degree partial melting of asthenosphere mantle and enriched mantle, which was subsequently metasomatized by carbonate melt

  • (3)

    The evolution of the Zaolin basanite was established from the mineral assemblages and chemistry, geochemistry, and source. During rapid ascent, the basanitic melt captured fragments of lithospheric mantle. Small amount of olivine and pyroxene phenocrysts began to crystallize in the spinel stability field at high T and P. CO2 largely dispersed, with only a small amount being retained in the silicate melt with the pressure decrease during ascent. Olivine and pyroxene were first crystallized and then carbonate and spinel, followed by sanidine and then nepheline. Owing to the lack of Si, Al, and Ca, no plagioclase formed in the magma

  • (4)

    The lithospheric mantle beneath the Jingdezhen area in Jiangxi Province was probably relict Proterozoic mantle that underwent a high degree of partial melting or melt extraction. In contrast to the fertile lithospheric mantle of eastern China, the lithospheric mantle in Jiangxi Province has not been completely replaced by asthenospheric mantle. The occurrence of the Zaolin basanite, as well as other alkaline basalt or lamprophyre dikes, shows that Eocene (ca. 44 Ma) magmatism ever took place in southeastern China. This magmatism was related to the rift tectonic setting of the eastern China continent

Data Availability

Representative microprobe data for minerals in the Zaolin basanite are listed in Supplementary Data Tables 1–6. The analytical major- and trace-element data for 12 samples of the basanite are listed in Table 1. The Sr and Nd isotope ratios of 12 basanite samples are listed in Table 2. The Ar–Ar isotope data for the matrix of the basanite are listed in Table 3. Figure captions Figure 1: simplified Cenozoic volcanic geology and tectonic framework of eastern China (modified after E and [9, 39]). Figure 2: geological map of the Zaolin area, Jingdezhen, Jiangxi Province (modified after [37, 45]). Figure 3: field photographs and photomicrographs of the Zaolin basanite: (a, b) photographs showing the basanite (dotted white line) within Permian carbonate strata and displaced by a fault (solid white line); (c) photograph of a hand specimen (ZL-36); (d) photomicrograph of the Zaolin basanite (cross-polarized light) showing porphyritic texture with olivine phenocrysts; (e) clinopyroxene phenocrysts (plane-polarized light). ol: olivine; Cpx: clinopyroxene. Figure 4: photomicrographs of basanite under an optical microscope. (a) Augen carbonates (transmitted light). The cpx occurs as columnar crystals in the matrix. Spinel occurs as inclusions in carbonate and between carbonate grains. (b) Kink-banded olivine xenocryst with a clear margin (cross-polarized light). Other phenocrysts are olivine that crystallized from magma. (c) Ol + opx xenocrysts (reflected light). (d) Brown spinel xenocryst and tiny dark spinels (transmitted light). ol: olivine; opx: orthopyroxene; cpx: clinopyroxene; sp: spinel; Car: carbonate; rim: rim of olivine or spinel. Figure 5: back-scattered electron images of the Zaolin basanite. (a) Inclusions of cpx, sp, and ne in olivine, similar to the mineral assemblage in the matrix. Needle-like apatites occur in the matrix. (b) Zoned clinopyroxene phenocryst. In the matrix, sanidine and nepheline appear as light and dark, respectively, distributed between columnar clinopyroxene grains. (c) Augen of siderite interspersed with cpx and sp. (d) Augen of carbonate showing distinct phase separation, with the dark parts of the carbonate being Mg-rich magnesite and light parts being Fe-rich siderite. (e) Heterogeneous carbonate with MgO and FeO contents of ~50%. (f) Spinel xenocryst eroded by magma, showing a Ti- and Fe-rich rim and fractures, whereas its inner part is primarily chromite, rich in Mg and Cr. (g) Eroded spinel with several compositional zones. Near the spinel is siderite coexisting with ne and cpx. (h) Eroded spinel, rich in Ti and Fe, or Ti-magnesite. The tiny (light-colored) minerals scattered in the matrix of images (a–h) are Cr–Ti–Mg–Fe spinel. ol: olivine; cpx: clinopyroxene; sp: spinel; Car: carbonate; ap: apatite; ne: nepheline; sa: sanidine. Figure 6: classification of pyroxene [125] into diopside, augite, pigeonite, enstatite, and ferrosilite. The data point in the enstatite field is of opx interspersed with olivine xenocrysts. The circle near the center, on the diopside–augite boundary, represents the compositions of the cpx near felsic enclaves. Figure 7: (Na2O + K2O) versus SiO2 diagram of the Zaolin basanite (classification from [126]). The plotted data were adjusted for LOI, and the oxide contents were recalculated to 100%. Figure 8: trace-element variation diagrams for the Zaolin basanite. (a) Chondrite-normalized REE diagrams (chondrite REE data from [127]). (b) Primitive-mantle-normalized spider diagrams of incompatible elements for the basanite (primitive-mantle data from [127]). For comparison, the average compositions of present-day OIB [68] and Cenozoic basalts in southeastern China (CBSEC; [128]) are also plotted. Figure 9: 86Sr/87Sr and 143Nd/144Nd isotope data for the Zaolin basanite and comparison with data of Cenozoic basalts from southeastern China (from [2]) and other localities (from Stille et al., 1983; Storey et al., 1988). Small circles: this study. Figure 10: Ar–Ar isotope analytical spectrum of the Zaolin basanite: (a) plateau age; (b) isochron age; (c) inverse isochron age. Figure 11: assessment of equilibrium between olivine and melt based on the Fe–Mg exchange reaction (Roeder and Emslie, 1970). Figure 12: NiO–Fo compositional variations of olivine phenocrysts and xenocrysts (after [118]). The fractional crystallization trend is from Sato (1977). The field “common olivines” depicts the compositional range of olivines from peridotite xenoliths, orogenic massifs and ophiolites, oceanic abyssal basalts, and MORB [117], whereas “Hawaiian tholeiite olivines” denotes the range of olivines from Hawaiian tholeiite basalts [117] and the “Hainan basalt olivines” field is from Wang et al. [24].

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the Program of Department of Science and Technology (No. 2016YFC0600203), the National Natural Science Foundation of China (No. 41873059), and the China Geological Survey Project (No. DD20190001). We thank Dr. Zhiming Yang for discussions on petrologic genesis. We express gratitude to Dr. Xiaohong Mao for assistance with microprobe analyses at the Key Laboratory of Continental Dynamics, Department of Natural Resources, China. We also thank geological engineers including Limin Shu, Luchuan Luo, and Tao Xie from the 912 Geological Team, Bureau of Geology and Mineral Resources of Jiangxi Province, for their help with field sampling. Comments and suggestions from Dr. Leonid Danyushevsky greatly improved an earlier version of the manuscript.

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