The Discovery and Geochemical Characteristics of an Eocene Peridotite Xenolith-Bearing Mafic Volcanic Neck in Coastal Southeast China

The mantle beneath coastal SE China evolved from enriched to depleted between the Cretaceous and the Neogene, although the precise timing of this change remains unclear. Here, we focus on a newly discovered Eocene mafic volcanic neck that contains peridotite xenoliths in the Fuding area of Fujian Province, and present new whole-rock Ar–Ar data which indicate that flood basalts formed during the Eocene (ages of 38.5 ± 1.2 and 42.3 ± 2 Ma). The basalt, gabbro, and diabase in the Fuding area are geochemically similar to ocean island basalt (OIB) and have SrI values that range from 0.703794 to 0.703911 (average of 0.703865) and εNd(t) values from 3.05 to 4.56 (average of 3.90). These samples yield two-stage Nd model (TDM2) ages of 0.61–0.73 Ga (average of 0.67 Ga). These data indicate that all of these units formed from magmas derived from an OIB-type mantle source, with both the gabbro and diabase units recording minor amounts of crustal contamination. Its OIB type geochemical characteristics may be inherited from crustal materials with similar characteristics. Peridotite xenoliths within the Fuding basalts provide evidence of the nature of the Eocene mantle in this area, especially the post late-Mesozoic evolution of the mantle beneath coastal SE China. The mantle beneath coastal SE China evolved from enriched in the Cretaceous to depleted in the Neogene, with this change occurring during the Eocene.


INTRODUCTION
Continental lithosphere is an important component of the Earth, and peridotite xenoliths captured by rapidly ascending basaltic magmas produced in some tectonic environments can provide direct samples of the upper mantle or lower crust of continental lithosphere. Such xenoliths also provide evidence of the nature and evolution of the mantle, the composition of the mantle, and the tectonic setting of the basaltic magmatism that captured the xenoliths. There are two main models for the petrogenesis of peridotite xenoliths: 1) they represent the refractory residue of the upper mantle after partial melting that generated basaltic magmas (Wang and Yang, 1987a;Wang and Yang., 1987b;Deng et al., 1988;Cong et al., 1996;Yan et al., 2007), or 2) they represent rock fragments of the upper mantle that were captured by ascending basaltic magma as accidental enclaves, meaning they played no direct role in the formation of the basaltic magmas (Tatsumoto et al., 1992;Liu et al., 1995;Li and Ma, 2002). Southeastern China has a complex geological history that includes late Mesozoic to Cenozoic lithospheric thinning and extensive basaltic magmatism. This magmatism peaked in coastal SE China after the late Mesozoic and has been the focus of many studies (Cui et al., 2011;Liu et al., 1995;Tang et al., 2010;G. ;Xie et al., 2005;Xing et al., 1999;Yan et al., 2005a;Zhou et al., 2006), including a large body of research devoted to the nature of the mantle during this magmatism (D. Chen et al., 1997;C. Chen et al., 2008;C. Dong C. W. et al., 2006;C. Dong Y. X. et al., 2006;He and Xu, 2011;Li, 2000;; X. J. Liu and Yan, 2007;Shen et al., 1999;D. Y. Wang et al., 2002;D.Z. Wang and Shen, 2003;Xiao et al., 2006;X. Xu et al., 2008;W. Zhang and Fang, 2014;Zhi and Qin, 2004;Zhi et al., 1994). This previous research focused on the Late Cretaceous and Miocene mantle in this area and was based on mantle xenoliths obtained mainly from late Neogene basaltic rocks. Peridotite xenoliths also occur within post-Neogene alkaline basalts that have been used to characterize the Cenozoic evolution of the mantle beneath SE China (Bodinier and Garrido, 2008;Dong et al., 1997;Dong et al., 1999;Wang et al., 2012).
However, the Paleogene (and especially the Eocene) evolution of the mantle in this area is poorly known, primarily due to the rarity of basic rocks hosting mantle xenoliths that formed at this time, meaning that the characteristics and the evolution of the Cretaceous to Neogene mantle beneath coastal SE China are uncertain.
A mafic volcanic neck located in the Fuding area of coastal SE China, within the southern section of the Cretaceous Fuding volcanic basin, is associated with a Mesozoic tectono-magmatic belt that is controlled by the NE-SW striking Fuding-Fuqing Fault. The Fuding basalts were erupted onto volcanic rocks of the lower Shimaoshan Formation (Figure 2), contain peridotite xenoliths, and were assigned by L. Li et al. (2011) to the late Oligocene to middle Pleistocene Fotan Group.
Here, we present new whole-rock Ar-Ar ages and geochemical data for mafic rocks from the Fuding mafic volcanic neck. These data constrain the formation age and tectonic setting of the Fuding mafic rocks, which contain peridotite xenoliths that provide clues to the characteristics of the Eocene mantle and to the post late-Mesozoic evolution of the mantle in coastal SE China.

GEOLOGICAL BACKGROUND AND PETROLOGICAL CHARACTERISTICS
The Fuding mafic volcanic neck is oval shaped and has a long axis that trends NW-SW, with a length of 790 m, a width of 420 m (Figure 2), and with clearly developed columnar joints. The neck is divided into three facies: an outermost eruption facies basaltic breccia volcano, an intermediate effusive facies basalt, and an innermost central neck facies volcano containing olivine-bearing gabbro-diabase rocks ( Figure 3A). The neck contains basaltic volcanic breccias, basalts, gabbros, and diabasic rocks. The lithofacies within the neck are heterogeneously distributed, with a columnar jointed cumulate gabbro section located above the diabase ( Figures 3B,C). The geological characteristics of the neck are as follows.

Volcanic Breccia
The volcanic breccias are gray-black, are brecciated and massive, and are dominated by breccias and crystals in a basaltic matrix. The rocks comprise angular-subangular breccia (60%-70% of the rock mass), and crystals (10%) of olivine, clinopyroxene, spinel, and others ( Figure 4A) in a basaltic matrix (20%) containing skeletal crystals.

Basalt
The basalt is gray-black, porphyritic, and has an almond-shaped structure. These rocks contain olivine (0.3-3 mm in size), clinopyroxene, and plagioclase phenocrysts that form 10%-20% of the rock. The amygdales in the basalt are filled with zeolite ( Figures 4B,C), and both amygdales and phenocrysts are hosted by an intergranular matrix that contains clinopyroxene (50%-55%), olivine (5%), feldspar (12%-15%), basaltic glass (15%), and minor ilmenite. The presence of basaltic glass indicates that the basaltic magma underwent rapid cooling to form a rock with minor amounts of iddingsite and chlorite alteration.

Gabbro
The gabbro is dark-gray, medium-grained, and has a cumulate gabbroic texture. The largest phenocrysts are~15 mm in diameter and the rock contains 60%-80% phenocrysts, mainly plagioclase, clinopyroxene, and olivine with minor and localized alkali feldspar. The clinopyroxenes are subhedral, columnar ( Figures 4D,E), zoned, and make up 15%-20% of the rock, whereas the plagioclase (25%-30%) is euhedral-subhedral and columnar, 0.2-6 mm in size, has clear polysynthetic twinning. The olivine gabbro is subhedral-subhedral, granular, and overprinted by the development of clinopyroxene megacrysts, with some of the olivine being visibly corroded. The olivines have diameters of 0.2-2 mm and make up 10%-20% of the rock. The alkali feldspars are platy and make up 5%-10% of the rock. These minerals occur in a fine-grained matrix of clinopyroxene, olivine, and plagioclase that forms 20%-35% of the rock. The gabbros also contain minor amounts of magnetite, ilmenite, zircon, apatite, and other accessory minerals, and the overall phenocryst abundance within the gabbro unit decreases from top to bottom.

Diabase
Diabase occurs beneath the gabbro and has a diabasic texture and a partly visible intersertal texture ( Figures 4F,G). Some of the clinopyroxenes within the diabase are twinned ( Figures 4H,I), and this unit has a similar mineralogy to the gabbros described above.

Whole-Rock Ar-Ar Analytical Procedures
The basalts dated during this study contain peridotite xenoliths and phenocrysts that were removed before analysis. Two basalt samples were dated by whole-rock Ar-Ar methods. The samples were crushed into small chips (≤5 mm) that were hand-picked under a binocular microscope to remove phenocrysts. The 40 Ar/ 39 Ar dating was carried out using a GV5400 mass spectrometer at the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, using the analytical procedures described by H. Qiu and Jiang (2007). All errors are reported at 2σ values and argon gas was extracted from the sample by step-heating using a MIR10 CO 2 laser. The released gases were purified using two Zr/Al getter pumps operated for 5-8 min at room temperature and~450°C, respectively. The background signal prior to analysis was <2 mV and the signal derived from the sample generally fluctuated between 40 and 200 mV. The dating results were calculated and plotted using ArArCALC software (Koppers, 2002), with a J-value of 0.00955 as determined using ZBH-2506 biotite (132 Ma) flux monitors. The results of these Ar-Ar analyses are given in Tables 1 and 2.

Whole-Rock Geochemical Analyses
Bulk-rock major, trace, and rare earth element (REE) concentrations were determined using X-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (IC-MS) at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences (CAGS), Beijing, China. Major element concentrations were determined by XRF and yielded analytical uncertainties better than 5%. Trace and REE concentrations were determined by ICP-MS, with the REE separated using cation exchange techniques. Analytical uncertainties during ICP-MS analysis were 10% for elements with concentrations of <10 ppm and~5% for those with concentrations of >10 ppm (Zeng et al., 2012). The results of these analyses are listed in Table 3.

Ar-Ar Geochronology
The Funding basalt samples (TMS2-2 and TMS2-1) from the Fotan volcanic succession dated by the 40 Ar/ 39 Ar method yielded apparent ages that were determined using age spectra and isotope correlation ( 36 Ar/ 40 Ar vs. 39 Ar/ 40 Ar) diagrams (Figures 5, 6; Tables 1,2). The latter provides a quantitative measure of the initial argon composition and allows the identification of excess argon that has 40 Ar/ 36 Ar ratios greater than the present day atmospheric value of 295.5. The data in Table 1 and Figure 2 indicate that little excess argon was present in the basalt samples.
The plateau age for sample TMS2-2 is 38.5 ± 1.2 Ma ( Figure 5A), a date that is within error of the isochron age for this sample (38.5 ± 2.3 Ma; Figure 5B). The second sample (TMS2-1) was collected from the basal section of the Fuding area and yielded an older plateau age (42.3 ± 2 Ma; Figure 6). As such, we interpret the plateau ages for these samples to represent the volcanism that formed the Fuding basalts, indicating that the final eruption of the Fotan Formation occurred at 42.3-38.5 Ma. Hence, the eruption of the Fuding basalt occurred at 42.3-38.5 Ma, during the Eocene.
Plotting data of the gabbro and diabase samples on Harker diagrams yields negative correlations between SiO 2 and CaO, FeOT, and MgO concentrations, and positive correlations between SiO 2 and Na 2 O, K 2 O, and Al 2 O 3 concentrations ( Figure 8).
All    Figure 9C). They have chondrite-normalized REE and primitive-mantle-normalized multi-element variation patterns that are similar to those of typical OIB compositions but differ from typical N-or E-type mid-ocean-ridge basalt (MORB) compositions.

Basalts
The basalts analyzed during this study contain higher concentrations of MgO (10.75-11.12 wt.%) than the gabbro and diabase samples, with both basalt samples classified as tephrites or basanites and both plotting above the alkaline curve ( Figure 7A), indicating that both of these basalts are alkaline basaltic rocks (Figures 7B,D).
Both of the basalt samples contain higher ΣREE concentrations (314.10-332.08 ppm; average of 323.09 ppm) than the gabbro and diabase samples, although the basalts are similar to these other samples in that they have OIB-type REE and multi-element variation diagram characteristics that contrast with typical N-or E-type MORB compositions ( Figures 9B,D).

Whole-Rock Sr-Nd Characteristics
The isotopic Rb-Sr and Sm-Nd ratios of the whole-rock samples are given in Table 4. The gabbro, diabase, and basalt samples have uniform initial εNd(t) (3.05-4.56, average of 3.90) and initial 87

Basalts
The basalts from the Fuding mafic volcanic neck are enriched in the REE and have highly differentiated HFSE compositions that are apparent in their OIB-type primitive-mantle-normalized multi-element diagrams (Figures 9B,D;Edwards et al., 1994; Frontiers in Earth Science | www.frontiersin.org June 2022 | Volume 10 | Article 950626 Sun and McDonough, 1989;Zhao and Zhou, 2007). Both of these samples are classified as within-plate alkali basalts in the Nb-Zr-Y diagram of Meschede ,1986; Figure 10A). These basalts have Ti/V ratios between 75 and 80 that are much higher than those of arc basalts, and they plot in the OIB field on a modified Ti versus V diagram ( Figure 10B; Shervais, 1982). Both samples plot in the OIB field on a Nb/Yb versus Th/Yb diagram ( Figure 10C; J. Pearce and Peate, 1995). They yield high Mg# values (= 100*MgO/(MgO + 0.85*Fe 2 O 3 T) and contain low concentrations of SiO 2 , suggesting that they originated from a  parental magma that was derived from an OIB-like mantle source in a continental rift environment, without any crustal input.

Gabbro and Diabase
The  Edwards et al., 1994;Sun and McDonough, 1989;Zhao and Zhou, 2007). However, the gabbro and diabase units have Mg# values of 63.96-66.58, suggesting that they record the minor differentiation of olivine and pyroxene. This fractionation is also evidenced by a negative Frontiers in Earth Science | www.frontiersin.org June 2022 | Volume 10 | Article 950626 11 correlation between SiO 2 and MgO concentrations (Figure 8). In addition, the positive correlation between Fe 2 O 3 T and MgO concentrations shown by the gabbro samples is indicative of the fractionation of olivine ( Figure 11A), whereas the positive correlation between CaO and MgO concentrations in these samples is evidence of the fractionation of clinopyroxene ( Figure 11B). This conclusion is also supported by the presence of positive correlations between Cr, Ni, and MgO concentrations ( Figures 11C,D).
The lack of any significant Eu anomalies (Eu/Eu* = 1.00-1.05; Figure 9A) indicates that the gabbro and diabase units do not record any plagioclase fractionation. However, crustal contamination can modify the composition of mantle-derived magmatic rocks. Crustal materials are enriched in the LILE, K 2 O, and Na 2 O, and are depleted in P 2 O 5 and TiO 2 . The gabbro and diabase units within the Fuding area contain low and invariant concentrations of K 2 O and Na 2 O, suggesting that they record minimal amounts of crustal contamination. Crustal contamination can also produce negative Nb and Ta, and positive Zr and Hf anomalies relative to the LILE and LREE (Sun and McDonough, 1989;Zhao and Zhou, 2007); however, these anomalies are absent in the primitive-mantle-normalized multi-element diagrams for the gabbro and diabase samples from the study area ( Figure 9C), again suggesting that these units record minimal crustal contamination. All of the samples show low initial 87 Sr/ 86 Sr values (average of 0.703865) and high εNd(t) values (3.05-4.56, average of 3.90). These data indicate that the basalt, gabbro, and diabase units in the Fuding area were derived from an OIB-type mantle source, with the gabbro and diabase units recording minor amounts of crustal contamination.
The trace and REE compositions of the Fuding mafic volcanic neck are different from those of the Neogene basalts of coastal SE China. The basalts from the former contain higher Rb (average of 87 ppm), Sr (average of 984 ppm), Ba (average of 807 ppm), and total REE (average of 243 ppm) concentrations and have higher light REE/heavy REE (LREE/HREE) ratios (average of 11.93) than the Neogene basalts of coastal SE China (average Rb, Sr, Ba, and total REE concentrations of 17.5, 339, 201, and 112 ppm, respectively, with an average LREE/HREE ratio of 4.22;Lu et al., 2000;J. Qiu et al., 1999;S. Wang, 1998;X. Wang et al., 2012;J. Zhang and Lu, 1997;Zouet al, 2000).
L. Li et al. (2011) reported that the Taimushan granite formed at 96.16 ± 1.6 Ma and suggested that the Fuding basalts formed part of the late Oligocene to middle Pleistocene Fotan Group. Our unpublished zircon U-Pb laser ablation (LA)-ICP-MS analyses yielded a Late Cretaceous age of 88.2 ± 1.0 Ma for these basalts, although these ages may be unreliable. The two basalt samples from the Fuding mafic volcanic neck yield similar Ar-Ar ages of 38.5 ± 1.2 and 42.3 ± 2 Ma, indicating that the basalts within both the neck and within the Fotan Group were emplaced during the Eocene.

Implications of the Presence of Peridotite Xenoliths
Cretaceous basalts in coastal SE China are usually associated with other contemporaneous bimodal volcanic rocks. The basalts in this area crop out over a much smaller area than associated acidic volcanic rocks and are thought to have been derived from either an enriched region of mantle, from a depleted region of the mantle and then either assimilated crustal material or underwent magma mixing, or were derived from the crust-mantle transition zone (C. Chen et al., 2008;Cui et al., 2011;He and Xu, 2011;H. Li et al., 1995;Tang et al., 2010;Xing et al., 1999;G. Xie et al., 2005;X. Xu et al., 2008;J. Yan et al., 2005b;Yu et al., 1993).
As mentioned above, both the petrogenesis of the basalts in coastal SE China and the nature of the mantle beneath this region remain uncertain. The main cause of this uncertainty is the lack of mantle rocks in this area; however, peridotite xenoliths within the Fuding basalts can be used to determine the characteristics of the Eocene mantle in this area, including the post late-Mesozoic evolution of the mantle beneath coastal SE China.
Both the late Mesozoic basalts and acidic volcanic rocks in coastal SE China have isotopic characteristics similar to those of enriched mantle (C. Dong C. W. et al., 2006;Y. Dong Y. X. et al., 2006;C. Dong et al., 2007;Xing et al., 1999;Xue et al., 1996;J. Zhou and Chen, 2001), suggesting that the Cretaceous upper mantle in this region was relatively stable but also weakly enriched.
The Paleocene basalts in coastal SE China generally crop out in the northern parts of the Jiangsu and Hefei basins, as well as in the Mingguang and Fushan areas (Qian and Li, 1996;Yang et al., 1999). These basalts contain abundant peridotite xenoliths, were emplaced at 64-56 Ma, have the characteristics of typical continental basalts, and were derived from weakly enriched mantle (D. Chen, 1992;Cong et al., 1996;X. Zhou, 1992).
Eocene basalts are sporadically exposed within coastal SE China and have been reported from the Sanshui Basin in Guangdong Province (Y. Dong Y. X. et al., 2006;Hu et al., 2013;Xiao et al., 2006;X. Zhang et al., 1993;W. Zhang and Fang, 2014;Zhu et al., 1991). X. Zhang et al. (1993) reported K-Ar ages of 45-55 Ma for these basalts, and Y. Dong Y. X. et al. (2006) obtained a zircon U-Pb age of 46-45 Ma for basaltic rocks in the Sanshui Basin. The latter basalts are thought to have been generated by partial melting of a depleted region of the mantle that generated continental rift-type magmatic rocks (Y. Dong Y. X. et al., 2006;W. Zhang and Fang, 2014), or partial melting of an enriched region of the mantle within a continental rift setting (Hu et al., 2013). The Sanshui basalts yield recalculated εNd(t) values of −0.77 to +1.23 (mean age 50 Ma) (Zhu et al., 1991) that reflect the weakly depleted nature of the Eocene mantle in coastal SE China. The basalt, gabbro, and diabase units within the Fuding area have OIB-type geochemical affinities with low initial 87 Sr/ 86 Sr ratios (average of 0.703865) and high εNd(t) values (average of 3.90) which similar with the depleted mantle. Its OIB type geochemical characteristics may be inherited from crustal materials with similar characteristics. Meanwhile, the widespread Neogene basalts in coastal SE China are exposed in the Xinchang area of Zhejiang Province, the Jiashan area of Anhui province, the Fangshan area of Jiangsu province, and the Sanshui Basin in Guangdong Province, and all of these Neogene basalts were derived from a depleted region of the mantle (R. Wang andYang, 1987a, Wang andYang, 1987b;Zhi et al., 1994). Thus, the mantle beneath coastal SE China evolved between the Cretaceous and the Neogene from enriched to depleted, and this change occurred during the Eocene.

CONCLUSION
Whole-rock Ar-Ar analyses of two basalt samples from the Fuding mafic volcanic neck yield ages of 38.5 ± 1.2 and 42.3 ± 2 Ma, indicating that the basalts were erupted at 42.3-38.5 Ma.
The basalt, gabbro, and diabase units within the Fuding area have OIB-type geochemical affinities with low initial 87 Sr/ 86 Sr ratios (average of 0.703865) and high εNd(t) values (average of 3.90). Both gabbro and diabase units record minor amounts of crustal contamination.
Peridotite xenoliths within the Fuding basalt provide evidence of the nature of the Eocene mantle in this area, especially the post late-Mesozoic evolution of the mantle beneath coastal SE China. The mantle beneath coastal SE China evolved from enriched to depleted between the Cretaceous and the Neogene, with this change occurring during the Eocene.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

AUTHOR CONTRIBUTIONS
XZ designed the study. CL wrote the first draft. YJ analyzed the geochemical data. GX drew some of the figures. MY and ZD completed the geochemical experimental analysis. All authors discussed the results and contributed to the preparation of the manuscript.

FUNDING
This work was supported by Geological Survey projects (DD20221633) of the China Geological Survey, National key research and development program (No 2016YFC0600205) and Geological Survey projects (Nos 121201008000150002, 1212011121098, 121201008000150002, and 12120113070800) of the China Geological Survey.