Research Article in Situ Geochemical Compositions

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GeoScienceWorld Lithosphere Volume 2020, Article ID 8878501, 24 pages https://doi.org/10.2113/2020/8878501

Research Article In Situ Geochemical Compositions of the Minerals in Basaltic Rocks from the West Philippine Basin: Constraints on Source Lithology and Magmatic Processes

1,2,3 1,2,3 1,2 1 1 Long Yuan, Quanshu Yan , Yanguang Liu, Shiying Wu, Ruirui Wang, 1,2 and Xuefa Shi

1Key Laboratory of Marine Geology and Metallogeny, First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China 2Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China 3College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China

Correspondence should be addressed to Quanshu Yan; [email protected]

Received 27 July 2020; Accepted 28 September 2020; Published 21 October 2020

Academic Editor: Songjian Ao

Since the early Cenozoic, the West Philippine Basin (WPB) and the whole Philippine Sea Plate (PSP) has undergone a complex geological evolution. In this study, we presented K-Ar ages, in situ trace element, and major element compositions of minerals of basalts collected from the Benham Rise and the Central Basin Fault (CBF) in the WPB, to constrain their magmatic process and regional geological evolution. Olivine phenocrysts and microlites in the alkali basalts (20.9 Ma) from the Benham Rise have forsterite (Fo) contents of 56.90%–76.10% and 53.13%-66.41%, respectively. The clinopyroxenes in the tholeiites (29.1 Ma) from : – : the CBF is predominantly diopside and augite, and it is depleted in light rare earth elements (LREEs) (LaN/YbN = 0 13 3 40) and large-ion lithophile elements (LILEs). The plagioclases in the basalts from both of the Benham Rise and the CBF are predominantly labradorite and andesine, with a minor amount of bytownite, and it is enriched in LREEs, Ba, Sr, and Pb and exhibits strong positive Eu anomalies. However, there exist obvious diﬀerences in plagioclase compositions between these two tectonic sites. The source lithology of the Benham Rise basaltic rocks could be garnet pyroxenite, and yet that of the CBF could be spinel-lherzolite. The calculated mantle potential temperature beneath the Benham Rise is 1439°C–1473°C, which is signiﬁcantly higher than that beneath the CBF (1345°C–1381°C), suggesting there existed thermal anomaly beneath the Rise during basaltic magmatism. This study also calculated the temperature and pressure of the clinopyroxenes and plagioclases, which have been used to indicate magmatic processes. Finally, we suggest that the Benham Rise basaltic rocks may be related to a mantle plume (e.g., the Oki-Daito mantle plume), and the CBF was once located in a back-arc spreading center behind an active subduction zone. The extinction of the Oki-Daito mantle plume activity might be at about 20.9 Ma, and cessation of the back-arc spreading of WPB was at about 29.1 Ma or younger.

1. Introduction trench to the north, the Mariana and Ogasawara Trench to the east, the Yap and Palau trenches to the south, and the The Philippine Sea is one of the largest marginal seas in the Philippine Trench to the west. The Philippine Sea Plate con- world, and it has been studied extensively. The Philippine sists of the West Philippine Basin (WPB), the Shikoku Basin, Sea Plate (PSP) lies at the intersection of the Eurasian Plate, the Parece Vela Basin, the Mariana Trough, and two remnant the Paciﬁc Plate, and the Indo-Australian Plate, and it is sur- arcs (the Kyushu-Palau Ridge and the West Mariana arc) [1– rounded by a series of subduction zones, with the Ryukyu 5]. Tectonic reconstructions of the Philippine Sea Plate

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suggest that the plate was located near the equator before rane in the north, including Gagua Ridge (123-105 Ma), the 50 Ma, and it migrated northward to its present position, Amami Plateau (~115 Ma), the Daito Ridge, and the Oki- with a nearly 90° clockwise rotation [1]. Daito Ridge (44.4–40.5 Ma); the Benham Rise (39.8– At present, there are three models for the origin of the 35.9 Ma) and the Urdaneta Plateau (41.6–37.2 Ma) in the West Philippine Basin. (1) The WPB is a segment of trapped west; and the CBF (49.0–35.0 Ma) in the middle (Figure 1) oceanic crust [3, 6]. In this model, the WPB is a marginal [3, 13, 16]. Since 50 Ma, there has been widespread magma- basin formed along the Kula-Pacific accretionary plate tism in the West Philippine Basin (e.g., [2, 16, 17]; Haraguchi boundary. (2) The WPB was developed by a back-arc spread- et al. 2011; [5];Yan and Shi 2013; [13]). The igneous rocks of ing system [7–11]. Hall [1] detailed the tectonic evolution of the Amami-Daito province are basalts, dacites, and tonalites. the western Pacific, including the PSP since 55 Ma. The PSP They have the geochemical characteristics of intraoceanic was originally located on the equator, and it has gradually island arc rocks, and they may have been affected by a mantle migrated northward since the Early Cenozoic. During its plume [4, 5, 16]. The igneous rocks in the Huatung Basin are northward migration, the West Philippine Basin formed basalts and gabbros, with E-MORB trace element signatures (50–30 Ma) and the proto-Izu-Bonin-Mariana (IBM) (35– and Indian MORB Sr, Nd, and Hf isotopic signatures [18]. 30 Ma) was rifted in turn. The rifting of the proto-IBM leads The igneous rocks of the Kyushu-Palau Ridge are tholeiitic, to the formation of the Shikoku Basin (25–17 Ma) and the and they have the geochemical characteristics of oceanic arc Parece Vela Basin (28–23 Ma), and the subduction region tholeiites (Haraguchi et al. 2011). The igneous rocks of the of the Pacific Plate is backward to the east of the Izu- Benham Rise and the Urdaneta Plateau are basalts, and they Bonin-Mariana arc [1–5, 7, 12]. In addition, a clockwise rota- have the geochemical characteristics of OIBs [2, 13, 16]. The tion of nearby 90° occurred as the PSP migrated northward igneous rocks of the CBF are basalts, and they have the geo- [1, 4]. (3) The development of the WPB is influenced by both chemical characteristics of MORBs [2, 17]. back-arc spreading and a mantle plume [1, 7, 13, 14]. In the The eleven basaltic rock samples in this study were col- West Philippine Basin, scientists have collected various rock lected from two dredge stations (100DS-Vinogradov Sea- types, including N-MORBs (Normal Mid-Ocean Ridge mount in the Benham Rise and 94DS in the CBF) during Basalts) from the Central Basin Fault (CBF); OIBs (Ocean China-Germany joint leg no. SO-57 cruise of R/V Sonne Island Basalts) from the Benham Rise, the Urdaneta Plateau, (Figure 1) [19]. Samples from 100DS are alkali basalts and the Amami Plateau, the Daito Ridge, and the Oki-Daito are of oceanic island basalt- (OIB-) like characteristics, and Ridge; and arc volcanic rocks from the Kyushu-Palau Ridge, those from 94DS are tholeiites and similar to back-arc basin and their geodynamics settings appear to be related to back- basalts (BABBs) [20]. The six samples from site 100DS arc spreading, a mantle plume, and volcanic arc processes, exhibit porphyritic textures and contain plagioclase and oliv- respectively [2, 4, 5, 13, 15–17]. The basalts erupted from ine phenocrysts, with some microlites (olivine and plagio- the spreading centers on the Philippine Sea Plate have the clase, <0.1 mm grain size) in the groundmass (Figures 2(a) isotopic characteristics of Indian Mid-Ocean Ridge basalts, and 2(b)). The phenocryst content of the samples from site rather than Pacific Mid-Ocean Ridge basalts [2]. Previous 100DS is 5%–10%, and the plagioclase content accounts for studies have shown that the Amami Plateau, the Daito Ridge, 70%–90% of the total phenocrysts. The grain size of the oliv- and the Oki-Daito Ridge are related to arc-plume interaction. ine phenocrysts in the basalts from site 100DS is mostly 0.2– The Benham Rise and the Urdaneta Plateau may have been 0.4 mm, and some are even as large as 1.5 mm (sample affected by a mantle plume (e.g., the Oki-Daito mantle 100DS7). The olivine phenocrysts are idiomorphic and hypi- plume) [5, 13, 16]. Until now, many studies have focused diomorphic, and some have been altered to iddingsite. The on the WPB, but few basement rock samples have been col- grain size of the plagioclase phenocrysts ranges from 0:04 × lected from the Benham Rise, and the CBF and the magmatic 0:20 to 0:2×2:00 mm2. The five samples from site 94DS processes of the WPB have not been studied in detail, which exhibit porphyritic textures and contain plagioclase and clin- hinders our understanding of the tectonic evolution of the opyroxene phenocrysts, with some microlites (clinopyroxene WPB and the PSP. and plagioclase) in the groundmass (Figures 2(c) and 2(d)). In this study, we obtained in situ trace element (using The phenocryst content of the samples from site 94DS is laser ablation inductively coupled plasma mass spectrometry 10%–20%, and the plagioclase content accounts for 50%– (LA-ICP-MS)) and major element (using EMPA) composi- 70% of the total phenocrysts. The grain size of the clinopyr- tions of the clinopyroxene, plagioclase, and olivine in the oxene in the basalts from site 94DS varies from 0.2 to basalts and investigated the source lithology, mantle poten- 0.3 mm. The clinopyroxene phenocrysts are idiomorphic tial temperature and primary melt compositions, and mag- and hypidiomorphic. The grain size of the plagioclase pheno- matic processes of the basalts from the Benham Rise and crysts ranges from 0:03 × 0:20 to 0:3×1:50 mm2, and most the CBF in the West Philippine Basin. are polysynthetic twins.

2. Geologic Setting and Sample Descriptions 3. Analytical Methods The West Philippine Basin is bounded by the Philippine 3.1. Dating Methods. Based on detailed petrographic observa- Trench to the west, the Ryukyu Trench to the north, and tions, we selected two samples (one for each dredge location) about 3000 km of the Kyushu-Palau Ridge to the east (Hara- with almost no containing olivine phenocrysts to be dated. guchi et al. 2011). The WPB consists of a Mesozoic arc ter- The analytical work was carried out in the State Key

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(a)

−10000 −8000 −6000 −4000 −20000 2000 4000

0 100 200 300 400km (b)

Figure 1: (a) Geologic map of the Philippine Sea region. (b) Geologic map of the West Philippine Basin showing the sample locations. The site numbers are Deep Sea Drilling Project and Ocean Drilling Program sites. HB: Huatung Basin. The white dashed line represents the CBF. The grey spots represent previous K-Ar/Ar-Ar dating site [2, 13].

Laboratory of Earthquake Dynamic (SKLED), Institute of using a G-3 ﬂame photometer. Parameters for age calcula- Geology, China Earthquake Administration. The detailed tion are as follows, 40Ar/36Ar = 295:5; λ =5:543 × 10−10 a−1; – −10 −l0 40 method of potassium argon dating is as follows, argon iso- λβ =4:962 × 10 /a; λe =0:581 × 10 /a; K/K = 1:167 × topes were measured in MM1200 mass spectrometer using 10−4 mmol/mol. the isotopic dilution method. Argon-38, whose purity is better than 99.9%, is used as the diluent, and the standard sample ZBH biotite (whose age is 132.6 Ma) was used during the 3.2. In Situ Compositional Analytical Methods for Minerals. calibration of diluent. Potassium content was measured The major elements were analyzed using the JXA-8230

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4. Results 4.1. Dating Results. The potassium–argon ages of basalts from the Benham Rise and the CBF are listed in Table 1. The apparent age of basalt (100DS16) from the Benham Rise is 20.9 Ma, which is obviously younger than the ages of basalts from IODP site 292 [2]. The apparent age of basalt (94DS1) from the CBF is 29.1 Ma, which is similar to the pre- (a) (b) vious K-Ar ages of basalts from CBF [12, 22].

4.2. In Situ Mineral Compositions 4.2.1. Olivine (1) Major Element Compositions. The analytical results of the olivine in the basalts from the West Philippines Basin are reported in Supplementary Table 1. For sample 100DS18, the Fo contents of the olivine phenocrysts and the microlites (c) (d) range from 57.80% to 76.10% and 56.90% to 60.00%, Figure 2: Representative photomicrographs of basalts. (a, b) Sample respectively. The Fo contents of the olivine microlites are 100DS7; (c) sample 94DS1; (d) sample 94DS0. Ol: olivine; Cpx: significantly lower than those of the phenocryst rims, while clinopyroxene; Pl: plagioclase. those of the other samples are similar to those of the phenocryst edges. In general, FeO, MnO, Al2O3,Ti2O3,and CaO increase and NiO decreases with decreasing Fo content. Electron Microprobe Analyzer (EMPA) at the Key Labora- The CaO contents of the olivine range from 0.144 to tory of Marine Geology and Metallogeny, First Institute of 0.51 wt%, which is within the range of basaltic phenocrysts Oceanography, Ministry of Natural Resources (MNR), (>0.1%) but is much higher than that of mantle peridotite China. The working conditions of the instrument were as fol- xenoliths (<0.1%) [23]. The grain size of the olivine is lows: the acceleration voltage was 15 kV, the electron beam relatively small, and no reaction rims or deformation − current was about 2×10 8 A, the electron beam spot was structures, which are common to olivine xenocrysts, were 1 μm, and the quantitative detection limit was about observed, indicating that the olivine phenocrysts in this 100 ppm. The standards used in these analyses were albite study are magmatic in origin [24]. In addition, NiO for Na and Si, orthoclase for K, diopside for Ca, olivine for decreases with decreasing Fo content, which differs from the Mg, hematite for Fe, garnet for Al, chromium oxide for Cr, mantle olivine array (Figure 3). rutile for Ti, rhodonite for Mn, and nickel silicide for Ni. The analytical results were corrected using the ZAF method For sample 100DS7, the Fo contents of big olivine (Z, A, and F represent the atomic number, absorption, and grain (>1.5 mm) range from 61.90% to 90.80%, and the fluorescence, respectively). The precision of the major ele- Fo contents of olivine microlites are about 58.60%, similar ment analyses (SiO2,Al2O3, and CaO) is better than 1%, to the rim of big olivine grain. The oxide contents (e.g., and that of the minor element analyses (Na2O, K2O, TiO2, TiO2, FeO, MnO, and CaO) of big olivine grain vary sig- fi P2O5, MnO, Cr2O3, and MgO) is better than 5%. During ni cantly from core to rim. The olivine has relative high the EMPA analysis, backscattering electron (BSE) images of Fo (%) (core: 90.30–90.80, rim: 61.90–62.80) and NiO typical minerals were obtained. The data were processed contents (core: 0.25–0.36, rim: 0.09–0.12) and low CaO using the Geokit geochemical software tool [21]. contents (core: 0.07–0.11, rim: 0.55–0.57) and MnO con- The in situ trace element analyses of the minerals in the tents (core: 0.12–0.17, rim: 0.45–0.51), indicating that the Late Cenozoic basaltic rocks from Thailand were conducted cores of olivine are xenocrysts from the mantle, and the at the Beijing Createch Testing International Co. Ltd. The rim of the olivine is formed in a magma chamber over laser sampling was performed using an ESI NWR 193 nm the course of a few years prior to eruption (Figure 3). excimer laser ablation system, and an Analytik Jena Plasma The presence of mantle-derived xenoliths indicates rapid Quant MS instrument was used to acquire the ion-signal magmatic ascent [25, 26]. intensities. The spot diameter of the laser beam was 35 μm, and the ablation frequency was 10 Hz. Helium was used as (2) Trace Element Compositions. The trace element concen- carrier gas. Each spot analysis incorporated approximately trations of the olivine from the West Philippines Basin are 20 s of background acquisition from the sample. The element listed in Supplementary Table 2. The total rare earth contents were calibrated against multiple reference materials element concentrations (ΣREE) of the olivine xenocrysts (NIST SRM612 and NIST SRM610). The data calibration was are very low, ranging from 0.05 to 0.43 ppm, with LaN/YbN – – – conducted using suitable internal standards and Glitter (Si of 1.78 22.29, CeN/YbN of 2.67 17.13, SmN/YbN of 0.29 – – was the internal standard for this experiment). The data for 4.76, LaN/NdN of 1.24 5.90, and LaN/SmN of 4.68 6.24. most of the elements have an accuracy of less than 5% and Based on their rare earth element concentrations, the olivine a precision of greater than 10%. microlites can be divided into two groups (Figures 4(a) and

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Table 1: Dating Results of potassium–argon geochronology of basalts from Benham Rise and Central Basin Fault.

Sample Tectonic Sample Water K 40Ar 40Ar Apparent age location Latitude rad rad Remark Reference region no. depth (m) (%) (grammol/gram) (%) (±1σ)/Ma Longitude Alkali This 100DS16 126.624 18.577 1850 0.37 9.2985E-11 66.05 20:99 ± 0:48 (K/Ar) Benham basalt study Rise Alkali — 124.651 15.819 —— — —35:6 − 36:2 (Ar/Ar) [2] basalts This 94DS1 128.743 17.513 3590 2.54 1.8850E-11 60.11 29:14 ± 1:08 (K/Ar) Tholeiite study 28:1±0:16, 26:1± CBF — 130.180 16.475 —— — — Basalts [22] 0:9 (K/Ar) Alkali — 132.500 15.000 —— — —27:4±1:6 (K/Ar) [12] basalt

4(b)). Group 1 has relatively high REE concentrations ranging than those from sample 94DS0 (Mg# = 80 – 88). The oxide from 100.82 to 156.01 ppm and exhibits light rare earth contents of the clinopyroxene from sample 94DS1 are similar element (LREE) enrichment, with relatively high LREE/HREE to those of the low Mg# clinopyroxene from the Okinawa – – (3.46 5.06) and LaN/YbN (9.39 19.30) ratios, and negative Ce Trough, while the oxide contents of the clinopyroxene from anomalies (Ce/Ce ∗ =0:45 – 0:86, with an average of 0.57) or sample 94DS0 are similar to those of the high Mg# ∗ ð ∗ Þ0:5 no obvious Ce anomalies (Ce/Ce =CeN/ LaN PrN ). clinopyroxene from the Okinawa Trough [32] (Figure 6). Group 2 has very low REE concentrations ranging from 1.85 to 3.28 ppm and exhibits LREE enrichment, with relatively Figure 6 shows the compositional spectrum of the clino- high LREE/HREE ratios (2.26–4.81) and obvious negative Ce pyroxene. The clinopyroxene is characterized by low Al2O3 (Ce/Ce ∗ =0:25)andEu(Eu/Eu ∗ =0:58)anomalies contents (less than 8.11%). Na2O and MnO do not systemat- ∗ ð ∗ Þ0:5 ically correlate with Mg#. The Cr O and SiO contents are (Eu/Eu =EuN/ SmN GdN ) (Supplementary Table 2). 2 3 2 positively correlated with Mg#. The FeO, TiO2,Al2O3, and On the primitive mantle-normalized trace element spider CaO contents are negatively correlated with Mg#. The diagrams (Figure 4(b)), most of the group 1 samples are gen- inverse FeO-Mg#Cpx trend, which is self-correlated because 2+ ð 2+ 2+Þ erally enriched in Nb, La, and Pb and depleted in Ta, Ce, Sr, the Mg# = 100 × Mg / Mg +Fe , is purposely plotted to ’ and Ti; while the group 2 samples are enriched in Ba, Nb, Ta, show the data s coherence [33]. The Cr2O3 contents of the Pb, Zr, Hf, and Ti and are depleted in Th, La, Ce, Pr, Sr, Sm, clinopyroxene decrease with decreasing Mg#Cpx. Cr is and Eu (Figure 4). highly compatible in clinopyroxene [33, 34]. The decrease in CaO is due to the fractionation of clinopyroxene since 4.2.2. Clinopyroxene clinopyroxene fractionation can reduce the CaO content of the residual melt [29]. (1) Major Element Compositions. The analytical results of the pyroxene are reported in Supplementary Table 3. The (2) Trace Element Composition. The trace element concentra- pyroxene from sample 94DS0 are augite, and those from tions of the clinopyroxene in the basalts from site 94DS are sample 94DS1 are diopside and augite [31] (Figure 5). The listed in Supplementary Table 4. SiO2 contents of the clinopyroxene from sample 94DS0 are relatively high and vary slightly, ranging from 50.86 The ΣREE values of the clinopyroxene from sample to 53.89 wt%, while the SiO2 contents of the clinopyroxene 94DS0 range from 15.98 to 17.39 ppm, with an average from sample 94DS1 are relatively low and vary consi- of 17.32 ppm, which are significantly lower than those of derably, ranging from 43.77 to 50.22 wt%. Several chemical the clinopyroxene from the Late Cenozoic basalts from differences between samples 94DS0 and 94DS1 were the South China Sea (average of 62 ppm) and Thailand fi identi ed in this study. For example, the TiO2, FeO, and (average of 84 ppm) [35, 36]. Compared to the clinopyrox- Na2O contents of the clinopyroxene from sample 94DS are ene from the Late Cenozoic basalts from the South China lower than those from sample 94DS1, whereas the Cr2O3 Sea and Thailand [35, 36], the clinopyroxene from sample contents of the former are higher than those of the latter, 94DS0 exhibits LREE depletion (Figure 7), with relatively indicating that during its evolution, the magma became low ratios of LREE/HREE (ranging from 0.57 to 0.66, with depleted in Fe, Na, and Ti. The Al2O3 contents of the an average of 0.60), LaN/YbN (0.13 to 0.17, average of clinopyroxene from sample 94DS (ranging from 1.75 to 4.87, 0.16), CeN/YbN (0.22 to 0.27, average of 0.25), SmN/YbN with an average of 3.46) are lower than those from sample (0.87 to 0.91, average of 0.90), LaN/NdN (0.27 to 0.33, 94DS1 (ranging from 3.73 to 8.11, with an average of 6.27). average of 0.29), and LaN/SmN (0.15 to 0.19, average of The Mg#s (Mg# = Mg/ðMg + FeÞ) of the clinopyroxene 0.17). The clinopyroxene from 94DS exhibits significant from sample 94DS1 (Mg# =56–75) are significantly lower negative Eu anomalies (Eu/Eu ∗ =0:77 – 0:79, with an

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0.9

0.8

0.7

0.6

0.5

0.4 Mno (%) Mno

0.3

0.2 Magmatic

0.1 Xenocrystic 0 50 60 70 80 90 100 Fo (%) (a) 0.7

0.6

0.5

0.4

Cao (%) 0.3

0.2 Magmatic 0.1 Xenocrystic 0 50 60 70 80 90 100

Fo (%) (b) 0.5 Hawaiian tholeiite olivines 0.4

0.3

Nio (wt%) Nio 0.2

0.1 Fractional crystallization Common olivines 0 50 60 70 80 90 100

Fo (%) (c)

Olivine xenocryst of sample 100DS7 Olivine phenocryst of sample 100DS18 Olivine microlite of sample 100DS7 Olivine microlite of sample 100DS18

Figure 3: Variations in the compositions of the olivine phenocrysts. The common olivine ﬁeld outlines the compositional ranges of the olivine from peridotite xenoliths, orogenic massifs and ophiolites, oceanic abyssal basalts, and MORBs [27]. In contrast, the Hawaiian tholeiite olivine ﬁeld denotes the range of olivine from Hawaiian tholeiite basalts [27]. The fractional crystallization trend and the mantle olivine trend are from Sato [28]. The dashed line that separates the magmatic and xenocrystic olivine on the basis of MnO and CaO is from Wang et al. [29] and Thompson and Gibson [23].

average of 0.78) but no obvious Ce anomalies, indicating The ΣREE values of clinopyroxene from sample 94DS1 the fractionation of plagioclase during the magmatic range from 35.33 to 112.07 ppm, with an average of processes. 59.67 ppm, which is slightly lower than those of

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1000

100

Olivine/chondrite 0.1

0.01

0.001 La Ce PrNd Sm Eu Gd Tb Dy Ho Er Yb Lu Lu (a)(a) 1000

100

1 Olivine/primitive mantle Olivine/primitive

0.1

0.01 Rb Ba T U Nb Ta La Ce Pb PrPr Nd Sr Zr Hf Sm Eu TiTi Gd TbDy Ho Y Er Yb Lu (b) 1100DS7-5-200DS7-5-2 1100DS7-5-700DS7-5-7 100DS7-6-100DS7-6-22 100DS7-5-3100DS7-5-3 1100DS7-5-800DS7-5-8 100D100DS7-6-3S7-6-3 1100DS7-5-400DS7-5-4 1100DS7-5-900DS7-5-9 1100DS7-6-400DS7-6-4 100DS7100DS7-5-6-5-6 100DS7-6-1100DS7-6-1

Figure 4: (a) Chondrite-normalized REE distribution patterns and (b) primitive mantle-normalized trace element spider diagram of the olivine in the basaltic rocks from the Benham Rise. Data for the chondrite and primitive mantle are from Sun and McDonough [30].

clinopyroxene from the Late Cenozoic basalts from the South On the primitive mantle-normalized trace element spider China Sea (average of 62 ppm) and Thailand (average of diagrams (Figure 7), most of the samples are generally 84 ppm) [35, 36]. Compared to the clinopyroxene from the depleted in Large-ion lithophile elements (LILEs) (such as Late Cenozoic basalts from the South China Sea and Thai- Ba and Sr), indicating the fractionation of plagioclase during land, the clinopyroxene from sample 94DS1 exhibits LREE the magmatic processes since Ba and Sr are mainly incor- depletion, with relatively low ratios of LREE/HREE (1.11– porated into K-rich and Ca-rich minerals in the form of – 3.40, average of 1.70), LaN/YbN (0.38 3.40, average of 1.26), isomorphism. The trace element and rare earth element – – CeN/YbN (0.55 1.70, average of 0.89), SmN/YbN (1.25 1.76, characteristics of the clinopyroxene from sample 94DS1 are – average of 1.54), and LaN/NdN (0.32 2.24, average of 0.72). similar to those of the low Mg# clinopyroxene from the Oki- In addition, the clinopyroxene from sample 94DS1 exhibits nawa Trough, while the trace element and rare earth element slight negative Ce anomalies (Ce/Ce ∗ =0:44 – 0:94, average characteristics of the clinopyroxene from sample 94DS0 are of 0.80) but no obvious Eu anomalies. similar to those of the high Mg# clinopyroxene from the

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10 90

20 80

30 70

40 60

50 50

Diopside Hedenbergite 60 40

70 Augite 30

80 20

90 Pigeonite 10

Clinoenstatite Clinoferrosilite 605040302010 70 80 90

En Fs 94DS0 94DS1 ⁎ ⁎ 94DS0 94DS1

Figure 5: Wo-En-Fs diagram of pyroxenes. ∗ refers to the clinopyroxene microlites.

Okinawa Trough, which is consistent with results of the plagioclase samples exhibit strong positive Eu anomalies; major element compositions [32] (Figures 6 and 7). LREE, Ba, U, Sr, Pb, and Eu enrichments; and Th, Nb, Ce, Zr, and Ti depletions. While the plagioclase phenocrysts 4.2.3. Plagioclase from sample 94DS0 exhibit negative Eu anomalies; Th, U, Pb, and Hf enrichments; and Ba, Nb, Sr, and Ti depletions (1) Major Element Compositions. The analytical results of the (Figure 9). The plagioclase phenocrysts from sample 94DS0 plagioclases are reported in Supplementary Table 5. The are similar to the MATA-1 plagioclase xenocrysts from the plagioclase in this study is predominantly labradorite and Late Cenozoic basalts from Thailand [36], indicating that bytownite, with minor andesine (Figure 8). The An values the plagioclase phenocrysts may be xenocrysts, rather than of the plagioclase from samples 100DS7 and 100DS18 range magmatic in origin, which were trapped by sample 94DS0 from 60.63 to 66.42 (average of 63.00) and 61.53 to 68.11 during its ascent to the surface. In addition, the trace (average of 63.67), respectively. While the An values of the element and rare earth element contents of the plagioclase plagioclase from samples 94DS0 and 94DS1 range from in the basalts from site 100DS are signiﬁcantly higher than 75.20 to 94.91 (average of 83.29) and 62.40 to 79.62 those from site 94DS, and the trace element and rare earth (average of 75.04), respectively. The chemical compositions element characteristics of the plagioclase from site 100DS of some of the plagioclase phenocrysts analyzed in this study are similar to those from Thailand [36]. show no obvious variations from core to rim, suggesting that the chemical compositions of the plagioclase phenocrysts 5. Discussion are relatively uniform. In addition, the An contents of the plagioclase microlites are similar to those of the 5.1. Timing for Benham Rise and CBF Volcanic Activity. The phenocryst rims. apparent age of basalt from the Benham Rise is 20.9 Ma, which is similar to the ages of basalts from Vinogradov (2) Trace Element Compositions. The trace element concen- Seamount (22.0 Ma), but is obviously younger than the trations of the plagioclases are reported in Supplementary ages of basalts from IODP site 292 (35.6-36.2 Ma) [2]. Table 6. On the chondrite-normalized REE patterns Thus, the age of 20.9 Ma could represent the extinction (Figure 9), all of the plagioclases are enriched in LREEs. time of the Oki-Daito mantle plume, and the younger Except for those from sample 94DS0, most of the age (relative to those for IODP site 292 basalts) could

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10 24

8 20

6 (wt%) 3 CaO (wt%) O 2 16

AL 4

2 12

0 1 (a) (e) 0.8

0.8 0.6

0.6 (wt%)

3 0.4 O 2

Cr 0.4 O (wt%) 0.2 2 Na

0.2

0 0 (b) (f) 4 56

3 52 (wt%)

2 (wt%) 2 48 2 TiO SiO

1 44

0 40 (c) (g) 0.5 14

0.4 12

0.3 10 FeO (wt%) MnO (wt%) 0.2 8

0.1 6

0 4 55 60 65 70 75 80 85 90 55 60 65 70 75 80 85 90

Mg# (cpx) Mg# (cpx)

(d) (h)

94DS0 ⁎ 94DS0 ought

⁎ High Mg# cpx of Okinawa Tr 94DS1 ught

94DS1 Low Mg# cpx of Okinawa Tro

Figure 6: Geochemical variations of the clinopyroxene phenocrysts. Mg# = 100 × Mg2+/ðMg2+ +Fe2+Þ. Data for the Okinawa Trough are from Li et al. [32].

represent late-stage volcanic activity that followed the of WPB was still active during this time, and this age could main stage of volcanism on the Benham Rise. represent the time of the cessation of the seafloor spreading The apparent age of basalt from the CBF is 29.1 Ma, which in WPB. The K-Ar ages of basalts from CBF show a decreasing is similar to the previous K-Ar ages of basalts from CBF trend from west-north to east-south, which indicates the time (27.4 Ma-28.1 Ma) [12, 22], suggesting the seafloor spreading of the cessation of the seafloor spreading in WPB is gradually

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100

10 Clinopyroxenes/chondrite

1 La Ce Pr Nd Sm Eu G Gdd Tb Dy Ho Er Tm Yb Lu ((a)a) 100

0.1 Clinopyroxenes/primitive mantle Clinopyroxenes/primitive

0.01 Rb BaTTaLaCePbPrSrNdHfSmEuDy Nb Y Yb Lu (b) 94DS1–1–1 94DS1–4 94DS1–1–2 94DS0–1 94DS1–1–3 94DS0–2 94DS1–1–4 94DS0–3 94DS1–1–5 High Mg# cpx of Okinawa Trough 94DS1–1–6 Low Mg# cpx of Okinawa Trough 94DS1–2 cpx of Tailand and South Chaina Sea 94DS1–3

Figure 7: (a) Chondrite-normalized REE patterns of the clinopyroxene from the basaltic rocks of the CBF, (b) primitive mantle-normalized trace element diagram for the clinopyroxene from the basaltic rocks from the CBF. Data for chondrite and primitive mantle are from Sun and McDonough [30]. Data for the Okinawa Trough are from Li et al. [32]. Data for Thailand and the South China Sea are from Yan et al. [35] and Yuan et al. [36].

decreasing from west-north to east-south. Based on the reli- 5.2. Nature of the Source Lithology and Partial Melting able K–Ar and Ar–Ar ages of basalts from WPB, the half- spreading rate of WPB can be calculated as 4.8 cm/y between 5.2.1. Constraints from Whole Rock Compositions. Basalts 29.1 and 44.4 Ma, and 5.2 cm/y between 29.1 and 36.2 Ma, have long been regarded to be partial melts of peridotite, respectively. The newly obtained values are slightly faster than while a great deal of experimental studies have shown that the estimate of 4.4 cm/y [7], but slightly lower than the esti- silica-deﬁcient alkali basalts can also be produced from mate of 5.5 cm/y [13]. silica-deﬁcient eclogite and garnet pyroxenite, hornblendrite,

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Or 100

10 90

20 80

30 70

40 60

50 50 Sanidine

60 40

70 30

80 20 Anorthoclase Anorthite 90 10

Andesine Labradorite Albite Bytownite 100 Oligoclase 0

0 10 20 30 40 50 60 70 80 90 100

Ab An 100D7 94DSO ⁎ ⁎ 100DS7 94DSO 100DS18 94DS1 ⁎ ⁎ 100DS18 94DS1

Figure 8: Classiﬁcation of the plagioclase in terms of its composition ∗ refers to plagioclase microlites.

and carbonated peridotite [35, 38–41]. In recent decades, from the Benham Rise is garnet pyroxenite, and the source researchers have successfully used the Fe/Mn ratio, FC3MS lithology of the basalts from the CBF is spinel-peridotite. − parameter (FeOT/CaO 3 × MgO/SiO2), CaO content, The estimated primary compositions of the basalts from Dy/Yb ratio, and Yb content to determine the nature of the the CBF and sites DSDP447 and ODP1201 plot within the source lithology [29, 35, 42–45]. The basalts of Benham Rise experimental field are defined by the partial melting of peri- have relatively low CaO contents (6.32%-9.36%) and high dotite, while the estimated primary compositions of the Ben- fi fi FC3MS parameter (0.72-1.14) and Fe/Mn ratio (51.23- ham Rise plot within the silica-de cient eclogite eld 70.92), which indicate that the source lithology of Benham (Figure 10) [38]. Furthermore, most of the basalts from the Rise basalts is pyroxenite. While basalts of CBF have rela- Benham Rise have high Fe/Mn (51.23-70.92) ratios and tively high CaO contents (9.66%-9.99%) and low FC3MS FC3MS values (>0.5), indicating that the mantle source of the basalts from the Benham Rise is pyroxenite rather than parameter (0.39-0.46) and Fe/Mn ratio (34.82-37.50), which peridotite. In addition, the basalts from the Benham Rise indicate that the source lithology of Benham Rise basalts is have low SiO2 (44.13-48.54) and high FeO (8.62-9.34) con- peridotite. Compared to spinel, garnet preferentially incorpo- tents, with no obvious depletions or enrichments in Zr and rates HREEs over LREEs, and basaltic rocks associated with Hf, suggesting that neither carbonated peridotite nor horn- garnet in the source have a high Dy/Yb ratio. The basalts of blendite is a suitable mantle source [46–48]. The effective Benham Rise have a relatively high Dy/Yb ratio (1.98-2.40), melting pressure and melting temperature based on the pri- indicating the mantle source of Benham Rise containing gar- mary melts are Pf =23:8 – 27:1 kbar (with the depth of about net. However, the basalts of CBF have relatively low Dy/Yb 70-80 km) and T = 1439°C – 1473°C (Table 2), which is in ratio (1.46–1.74), indicating the mantle source of Benham Rise agreement with a source lithology of garnet lherzolite/pyrox- containing spinel. Therefore, the source lithology of the basalts enite, suggesting that the basalts of the Benham Rise were

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1000

100

1 Plagioclases/chondrite

0.1

0.01 LaLa Ce Pr NdNd Sm Eu GdGd TbTb DyDy Ho Er Yb LuLu (a) 1000

100

1 Plagioclases/primitive mantle Plagioclases/primitive 0.1

0.01 Rb Ba T U Nb Ta La Ce Pb Pr Nd Sr Zr Hf Sm Eu Ti Gd Tb Dy Ho Y Er Yb Lu (b) 94DS1 100DS7⁎ 94DS1⁎ 94DS2 94DS1⁎ Plagioclases of Okinawa Trough basalt 100DS7

Figure 9: (a) Chondrite-normalized REE distribution patterns and (b) primitive mantle-normalized trace element spider diagram of the plagioclase from the basaltic rocks from the West Philippine Basin. Data for chondrite and primitive mantle are from Sun and McDonough [30]. Data for the Okinawa Trough are from Guo et al. [37].

derived from a garnet-bearing mantle source. Thus, the bulk of melting of the samples from the Benham Rise ranged rock major and trace element compositions give a consistent from 5% to 7%, which is consistent with the result of result, i.e., that the source lithology of the basalts from the whole-rock composition. Benham Rise is garnet pyroxenite, and the source lithology In conclusion, the source lithology of the basalts from the of the basalts from the CBF and sites DSDP447 and Benham Rise is garnet pyroxenite, the source lithology of the ODP1201 is spinel-peridotite. basalts from the CBF and sites DSDP447 and ODP1201 is The LREE concentrations more likely reflect the extent of spinel-peridotite, and the degree of partial melting of the the degree of partial melting. Compared to the CBF, the basalts from the Benham Rise is lower than that of those from basalts from the Benham Rise have higher La/Sm ratios the CBF. (7.12-7.97), indicating that the extent of the degree of partial melting of the basalts from the Benham Rise is lower than 5.2.2. Constraints from Olivine Compositions. Olivine is the that of those from the CBF. Calculated by the primary first silicate mineral that crystallizes from all mantle- melt composition, the degree of melting of the samples derived magma as they ascent to the surface. Thus, the olivine from the CBF ranged from 10% to 11%, while the degree can be used to fingerprint the mantle source of basaltic

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Patial melts of carbonated 20 silica–defcient eclogite 4

Partial melts of silica–excess 16 – 3 MORB like eclogite (wt%) (wt%) 12 3 2

Partial melts of silica O 2 TiO

2 poor eclogite Al

1 Partial melts of carbonated fertile peridotite Peridotite partial melts 4

0 40 44 48 52 40 44 48 52 (a) (b) 20 25

16 20

12 15 FeO (wt%) FeO MgO (wt%)

8 10

4 5

40 44 48 52 40 44 48 52 (c) (d) 2 20

1.6 16

1.2

(wt%) 12 3 O 2

0.8 CaO (wt%)

CaO/Al 8

0.4

0 40 44 48 52 40 44 48 52

SiO2 (wt%) SiO2 (wt%) (e) (f)

Samples of 100DS Samples of DSDP447 Samples of 94DS Samples of ODP1201D

Figure 10: Comparison of the fractionation-corrected basaltic rocks from the West Philippine Basin (compositions in equilibrium with Fo90.1, corrected by olivine addition; Table 1) with experimental partial melting. The ﬁelds of the experimental partial melts are modiﬁed from Dasgupta et al. [38].

magma. Sobolev et al. [49] showed that the presence of Mn/Zn versus Ni, Zn versus Mn, and 10000 ∗ Zn/Fe versus pyroxenite in mantle source can be identiﬁed by the existence 100 ∗ Mn/Fe in olivines are reliable indicators of pyroxenite of olivines containing high Ni contents and Fe/Mn ratios but versus peridotite sources [50]. Howarth and Harris [50] low Ca and Mn contents. In addition, the correlations of showed that olivines crystallized from pyroxenite source

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Table 2: Estimated primary melt compositions, mantle potential temperatures, and melting conditions for representative samples of basaltic ff rocks from the West Philippine Basin. F (degree of melting) is estimated by Eq. (A2) of Putirka et al. [57]. P1 to P3 are e ective melting fi fi pressures in kbar. P1is de ned by Albared [51]; P2 is de ned by Lee et al. [56]; P3 is the average of P1 to P2. T1 to T4 are the melting ° temperatures ( C) of the melt segregation. T1 is according to Albared [51]; T2 is according to Putirka [62]; T3 is according to Lee et al. ° [56]; T4 is the average of T1 to T3. TP1 to TP3 are the mantle potential temperature in C. TP1 is estimated using the MgO contents in the primary melts following Herzberg et al. [54]; TP2 is estimated following Kelley et al. [55]; TP3 is the average of TP1 to TP2. SD: standard deviation.

Region Central Basin Fault rift Benham Rise DSDP 447site ODP 1201 site Sample 447- 447- 447- 48R- 94DS0 94DS1 94DS15 100DS7 100DS16 100DS18 100DS22 47R 49R 52R 55R number 025 030 022 1

SiO2 50.7 49.8 50.0 46.2 46.3 46.1 45.3 48.3 50.3 50.3 50.1 49.6 49.3 48.7 49.9

TiO2 1.2 1.1 1.2 2.7 2.7 2.7 2.7 0.8 0.8 1.0 0.9 0.9 0.9 0.8 0.8

Al2O3 13.6 13.3 13.7 14.1 13.6 13.7 13.2 16.6 14.4 12.9 15.4 15.0 15.2 14.7 14.6

Cr2O3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 FeO 7.8 8.4 8.1 9.5 9.8 9.6 10.1 7.2 7.4 8.1 7.2 7.0 7.0 7.7 7.1 MnO 0.2 0.2 0.2 0.2 0.2 0.1 0.2 0.0 0.0 0.2 0.1 0.2 0.2 0.2 0.2 MgO 11.7 12.7 12.2 14.3 14.8 14.5 15.2 10.9 11.1 12.3 10.8 10.5 10.5 11.5 10.7 CaO 9.3 9.1 8.9 6.4 6.3 6.7 7.4 12.0 12.1 11.1 8.7 12.6 13.0 12.0 12.6

Na2O 2.4 2.2 2.4 2.7 2.4 2.5 1.9 2.3 1.8 2.1 3.5 2.0 1.8 2.0 1.9

K2O 0.3 0.3 0.5 2.3 2.1 2.1 2.0 0.0 0.1 0.0 1.3 0.2 0.3 0.4 0.2

Fe2O3 1.6 1.6 1.6 1.7 1.8 1.8 1.8 1.5 1.5 1.7 1.5 1.5 1.5 1.5 1.5 F 1111105 7 6 5 4 9 117 6 567

T1 1344 1375 1360 1441 1452 1448 1473 1338 1332 1362 1323 1319 1319 1352 1322

T2 1302 1328 1314 1370 1383 1376 1395 1281 1287 1318 1278 1272 1270 1298 1276

T3 1294 1323 1308 1415 1427 1423 1452 1304 1300 1331 1289 1289 1288 1320 1292

T4 1313 1342 1328 1409 1421 1416 1440 1308 1306 1337 1297 1293 1292 1323 1297 SD 22 23 23 30 29 30 33 23 19 19 19 20 20 22 19

TP1 1362 1394 1378 1440 1453 1447 1466 1334 1342 1382 1329 1320 1317 1356 1326

TP2 1328 1368 1347 1438 1461 1448 1480 1296 1307 1345 1295 1288 1284 1324 1293

TP3 1345 1381 1362 1439 1457 1448 1473 1315 1325 1364 1312 1304 1300 1340 1310 SD 17 13 15 1 4 1 7 19 17 19 17 16 17 16 16

P1 10.3 12.3 11.6 23 23 23 27 13.6 10.5 11.3 10.4 11.0 11.4 13.4 10.8

P2 9.7 11.4 11.3 25 25 25 27 11.3 8.2 9.9 12.6 8.8 8.8 11.5 8.5

P3 10.0 11.9 11.4 23.8 23.9 24.0 27.1 12.4 9.3 10.6 11.5 9.9 10.1 12.4 9.6 SD 0.3 0.5 0.2 1.2 1.1 1.0 0.1 1.1 1.2 0.7 1.1 1.1 1.3 0.9 1.1

melts have low Mn/Zn ratios (<13) while those from perido- tion, which modify the composition of the primary melts. tite source melts have high Mn/Zn ratios (>15). As shown in Previous studies have proposed that mantle-derived rock Figure 11, except olivine xenocryst, all the olivines of sample samples with MgO > 7:5wt% are generally believed to have 100DS7 trend toward pyroxenite sources, which are consis- only experienced the addition or subtraction of olivine [59, tent with the results of whole rock. However, the olivine 60]. Therefore, only the CBF basalts with MgO contents of xenocryst of sample 100DS7 plotted in the field of peridotite >8 wt% are chosen as the starting composition. In addition, source, suggesting that the olivine xenocryst originated from the predominant phenocrysts in the Benham Rise basalts peridotite source. are plagioclase, with minor olivine. We used the reverse crys- In summary, we conclude that the source lithology of the tallization model in Petrolog to minimize the influence of basalts from the Benham Rise is garnet pyroxenite, and the plagioclase, and then, we used the reversed melt as the start- source lithology of the basalts from the CBF and sites ing composition. The primary melt compositions were esti- DSDP447 and ODP1201 is spinel-peridotite. mated by adding small (0.1%) increments of olivine (Fo = 90:1) to the samples, assuming an olivine-melt distri- ffi ð Þol/liq : 2+ ð 2+ 5.3. Primary Melt Compositions and Mantle Potential bution coe cient KD Fe/Mg =031 and Fe / Fe +F Temperatures. The primary melt composition refers to the e3+Þ =0:9 in the melt [61]. partial melts that equilibrate with the source rocks, which Because the Fo contents of olivine phenocrysts are rela- can be used to constrain the thermal state of the mantle tively low (<76.10%), we do not use olivine phenocrysts to source and the source lithology [42, 51–58]. The magmatic calculate the mantle potential temperatures of Benham Rise. rocks usually undergo variable crystallization and assimila- The melting conditions and mantle potential temperatures

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3000

2000 Pyroxenite Peridotite source source Ni (ppm) Ni

1000

0 015 0152025 Mn/Zn (a) 3

Peridotite 2.5 source trend Pyroxenite source trend 2 Mn/Fe ⁎

100 1.5

0.5 020406080 ⁎ 10000 Zn/Fe (b) 8000

6000 Peridotite source trend

4000 Mn (ppm) Mn Pyroxenite source trend 2000

Olivines of sample 100DS7 Olivines xenocryst of sample 100DS7

Figure 11: Mn/Zn vs. Ni (a), 10000 ∗ Zn/Fe vs. 100 ∗ Mn/Fe (b), and Zn vs. Mn (c) values for chemical compositions of olivines from sample 100DS7. The peridotite source trend line and the pyroxenite source trend line are from Howarth and Harris [50].

were estimated based on the primary melt compositions [29, sites 447 (1315°C–1364°C, average of 1334°C) and 1201 42, 51, 55, 56, 62]; (Herzberg 2007) (Table 2). The mantle (1301°C–1363°C, average of 1323°C) (Table 2). The mantle potential temperature beneath the CBF varies from 1345°C potential temperature beneath the Benham Rise varies to 1381°C (average of 1363°C), which is similar to those from from 1439°C to 1473°C (average of 1454°C), which is

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900 1000 1100 1200 1300 1400 1500 1600 0

PI Sp Iherzolite

1 Sp Gt pyroxenite

Sp Gt Iherzolite

T 2 P

Pressure (GPa) Pressure 3

5 Temperatur

Lherzolite solidus Samples of 94DS Pyroxenite solidus Samples of DSDP447 Samples of 100DS Samples of ODP1201D

Figure 12: P-T estimates for the basalts from the West Philippine Basin. The P and T data for the West Philippine Basin are from Lee et al. [56]. The lherzolite solidus was calculated using the parameterization of Katz et al. [64]. The pyroxenite solidus was calculated using the model of Lambart et al. [65]. The spinel to garnet pyroxenite transition is from Herzberg [63], and the other transitions are from O’Neill [66].

significantly higher than that of the basalts from the CBF 5.4. Physical Conditions of Crystallization (△T =58°C – 128°C) and sites 447 (△T =75°C – 158°C) and 1201 (△T =76°C – 172°C), indicating that the mantle 5.4.1. Physical Conditions of the Crystallization of the source of the basalts from the Benham Rise is related to Basalts from Site 94DS ff a mantle plume. Based on the primary melts, the e ective (1) Crystallization Pressure and Temperature of the Clinopyr- melting pressure (Pf) and melting temperature (T) of the P : – : oxene. In recent decades, many researchers have proposed CBF basalts are f =100 11 4 kbar (average of 11.1 kbar, various thermobarometers for calculating the crystallization with the depth about 30-35 km) and T = 1313°C – 1342°C ° pressures and temperatures of clinopyroxene [62, 67, 68]. (average of 1328 C), which are similar to those of the Putirka et al. [68] established a series of thermodynamic P : – : T ° – basalts from sites 447 ( f =93 12 4 kbar; = 1306 C equations based on experimental work that relates the tem- ° P : – : T ° – 1337 C) and 1201 ( f =94 14 0 kbar; = 1279 C 13 perature and pressure to the equilibrium constants and 50°C) but are lower than those of the Benham Rise basalts allows for the construction of clinopyroxene-liquid thermo- ° (Pf =23:8 – 27:1 kbar, average of 24.7 kbar; T = 1409 C – barometers. However, these thermobarometers are only 1440°C, average of 1421°C). The degree of melting of the applicable to basaltic magma temperature and pressure cal- samples from the CBF ranged from 10% to 11%, while culations, with an application range of 1110°C–1475°C and the degree of melting of the samples from the Benham 0.4–0.9 GPa. Putirka [62] experimentally recalibrated the Rise ranged from 5% to 7%. In conclusion, the mantle thermobarometers and established a new equilibrium tem- potential temperature beneath the CBF is lower than that perature and pressure formula that can be applied to mafic beneath the Benham Rise, suggesting there existed thermal magmas and intermediate-silicic magmas. In this study, we anomaly during basaltic magmatism in the Benham Rise, used the Putirka [62] equation to calculate the crystallization which may relate to a mantle plume (Table 2). pressures and temperatures of the clinopyroxene (Table 3). ff cpx-liq Figure 12 shows a plot of the e ective melting pressure The KD (Fe-Mg) value can be used to estimate whether vs. temperature. We conclude that the temperatures and the clinopyroxene is in equilibrium with the host rock (the cpx-liq pressures of the primary melt compositions of the CBF equilibrium KD (Fe-Mg) ranging from 0.20 to 0.36), basalts plot above the spinel-garnet lherzolite transition and the KD (Fe-Mg) of the clinopyroxene in this study varies (~50–60 km), but the temperatures and pressures of the from 0.25 to 0.29, indicating that all of the clinopyroxene are primary melt compositions of the Benham Rise basalts in equilibrium with the host rock. The calculated results plot below the spinel-garnet lherzolite/pyroxenite field show that the temperature and pressure of the basalts from [63–66]. sample 94DS0 are 1170°C–1225°C (average of 1196°C) and

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Table 3: Crystallization temperatures and pressures of the clinopyroxenes of basaltic rocks from CFB. The crystallization temperature and pressure of the clinopyroxenes are estimated following Putirka et al. [68] and Putirka [62].

[62] (base on clinopyroxene [68] [62] only) cpx- cpx- ° KD (Fe-Mg) ° KD (Fe-Mg) ° Sample T ( C) P (GPa) liq T ( C) P (GPa) liq T ( C) P (GPa)

1209-1249 0.3-0.9 1170-1225 0.5-1.0 1209-1233 0.3-0.9 94DS0 0.24-0.33 (0.30) 0.28-0.29 (0.29) (1230) (0.6) (1195.5) (0.8) (1217) (0.6) 1089-1125 0.0-0.5 0.1-0.5 1064-1129 0.2-0.6 94DS1 0.20-0.32 (0.26) 1112-1157 (1127) 0.25-0.26 (0.26) (1104) (0.2) (0.3) (1103) (0.4)

Table 4: Crystallization temperatures and pressures of the plagioclases in basaltic rocks from the West Philippine Basin. The crystallization temperature and pressure of the clinopyroxenes and plagioclases are estimated following Putirka [62].

T ° P Pl-liq Sample ( C) (GPa) KD (Ab-An) H2O (wt%) 100DS7 1133-1140 (1136) 1.40-1.50 (1.40) 0.25-0.33 (0.30) 2.43-2.80 (2.60) 100DS18 1141-1145 (1143) 1.30-1.40 (1.40) 0.27-0.35 (0.32) 2.47-2.83 (2.70) 94DS0 1174-1179 (1176) 0.10-0.20 (0.10) 0.17-0.22 (0.19) 1.40-2.43 (1.65) 94DS1 1165-1180 (1172) 0.20-0.40 (0.30) 0.20-0.28 (0.24) 1.25-1.51 (1.37)

0.5–1.0 GPa (average of 0.8 GPa), respectively, while the tem- 0.1–0.2 GPa, respectively (Table 4), which are slightly lower perature and pressure of the basalts from sample 94DS1 are than those of the South Okinawa Trough (1218°C–1239°C 1112°C–1157°C (average of 1127°C) and 0.1–0.5 GPa (aver- and 0.6–0.9 GPa) but are similar to those of the Mariana age of 0.3 GPa), respectively. This suggests that the crystalli- Trough (975°C–1212°C) [37, 70]. zation depth of the clinopyroxene from sample 94DS0 was deeper than that of sample 94DS1, which is consistent with Compared with the crystallization pressures and temper- the clinopyroxene having higher Mg#s (Table 3). In addition, atures of the plagioclase and clinopyroxene from basalt sam- the crystallization temperatures and pressures of the clino- ple 94DS0, the crystallization pressures and temperatures of pyroxene in the basalts from site 94DS are similar to those the clinopyroxene from sample 94DS1 are slightly lower than from the middle of the Okinawa Trough (1121°C–1212°C those of the plagioclase, while the crystallization pressures and 0.1–1.2 GPa) [32]. Based on the calculated pressures, and temperatures of the clinopyroxene from sample 94DS0 the depth of the magma chambers of samples 94DS0 and are slightly higher than those of the plagioclase. Thus, the 94DS1 was calculated from the equilibrium temperatures results indicate that the order of crystallization of sample and pressures of the clinopyroxene, and melt can be inferred 94DS1 is plagioclase and clinopyroxene, while the clinopyr- to be 8–25 and 6–17 km, respectively. oxene from sample 94DS1 has no obvious negative Eu anomalies (Figure 7), suggesting that few or no plagioclase (2) Crystallization Pressure and Temperature of the Plagio- crystallized before the formation of the clinopyroxene. In clase. Kudo and Weill [69] created the ﬁrst plagioclase- addition, the obvious negative Eu anomalies in the microlite liquid geothermometer. Subsequently, many workers have clinopyroxene in sample 94DS0 are attributed to plagioclase various thermobarometers for calculating the crystallization crystallization before the formation of the clinopyroxene. pressures and temperatures of plagioclase formation. In addition, since the geothermometer of Kudo and Weill [69] does 5.4.2. Physical Conditions of the Crystallization of the Basalts not consider whether the melt and plagioclase reached equi- from Site 100DS. The crystallization temperatures and pres- librium or the eﬀect of water on plagioclase crystallization, sures of the plagioclase from basalt sample 100DS18 are we used the pressure and temperature calculations of Putirka 1141°C–1145°C(average value = 1143°C) and 1.3–1.4 GPa pl-liq : [62]. The KD (Ab-An) of the plagioclase in the basalts (average value = 1 4 GPa), respectively, and the crystalliza- from site 94DS indicates that most of the plagioclases are in tion temperatures and pressures of the plagioclase from equilibrium with the basalts composition (the equilibrium basalt sample 100DS7 are 1133°C–1140°C (average of pl-liq ° – KD (Ab-An) ranging from 0.16 to 0.38), and the plagio- 1136 C) and 1.4 1.5 GPa (average of 1.4 GPa), respectively. clase, which is not in equilibrium with the basalts composi- This indicates that the plagioclase from samples 100DS7 tion, was not considered in this study. The H2O content of and 100DS18 crystallized under similar physical conditions. the basalts from samples 94DS1 and 94DS0, which were cal- The crystallization temperatures and pressures of the plagio- culated using the thermobarometer of Putirka [62], is 1.25– clase from the Benham Rise can be comparable to those from 1.51 wt% (average of 1.37 wt%) and 1.40–2.43 wt% (average Thailand (1145°C–1214°C and 0.4–0.9 GPa) and the South of 1.65 wt%). The crystallization temperatures and pressures China Sea basin (927°C–1179°C) [36, 71]. In addition, the of the plagioclase from samples 94DS1 and 94DS0 are temperatures of the plagioclase from the Benham Rise basalts 1165°C–1180°C and 0.2–0.4 GPa and 1174°C–1179°C and are similar to those from the CBF, while the pressures of the

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40 Subduction component

Basalt of South

Sr/Y Okinawa Trough OIB

– N MORB MORB E–MORB PM

0 0 0.3 0.6 0.9 1.2 1.5

La/Y (a) 40

20 Ba/Y Subduction componentBasalt of South Okinawa Trough

OIB

MORB N–MORB E–MORB PM 0 0 0.3 0.6 0.9 1.2 1.5

La/Y (b) Calculated basaltic melt of 94DS1 Basalt composition of 94DS0 Calculated basaltic melt of 94DS0 Basalt composition of 94DS1 ⁎ Calculated basaltic melt of 94DS1 Basalt composition of DSDP447 ⁎ Calculated basaltic melt of 94DS0 Basalt composition of ODP1201

Figure 13: (a) Sr/Y vs. La/Y and (b) Ba/Y vs. La/Y diagrams for the plagioclase parental magma of the CBF. The N-MORB, PM, E-MORB, and OIB data are from Sun and McDonough [30], and the data for the basalt from the South Okinawa Trough are from Guo et al. [37]. The black arrows represent the degrees of element enrichment, which are interpreted to have been caused by the addition of subduction components. ∗ refers to plagioclase microlites.

– plagioclase from the Benham Rise basalts (1.3 1.5 GPa) are higher pressure. The H2O content of the melt under the Ben- signiﬁcantly higher than those from the CBF (0.1–0.4 GPa), ham Rise, which was calculated using the data for the plagio- suggesting that the plagioclase from the WPB formed at a clase, is signiﬁcantly higher than that of the CBF, indicating

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Ti/100

WPB A ZrN

B MORB C LAB

⁎ 1 Zr Y 3 10100 1000

Zr (ppm)

100DS 446 94DS 1201D 292 447 294 (a) (b)

Figure 14: Tectonic discrimination diagrams, proposed by Pearce and Cann [74] and Pearce and Norry [75]. (a) Y∗3-Ti/100-Y tectonic discrimination for the whole rock data; (b) Zr/Y vs. Zr tectonic discrimination diagram for the whole rock data. A: low-potassium tholeiites; B: ocean-ﬂoor basalts, low-potassium tholeiites, calc-alkali basalts; C: calc-alkali basalts; D: within-plate basalts; WPB: within- plate basalt; MORB: mid-ocean ridge basalt; IAB: island arc basalt.

that the magma under the Benham Rise had a higher H2O the basalt composition of sample 94DS0 are similar to those content (Table 4). Though the mantle potential temperature of MORB, and the Ba/Y ratio of the calculated melt of sample beneath the Benham Rise is significantly higher than that of 94DS0 and the basalt composition of sample 94DS0 are the CBF, the temperatures of the plagioclase from the Ben- higher than those of MORB, indicating the contribution of ham Rise basalts are similar to those of the CBF, suggesting a subduction component to the magma source of sample that the temperature conditions of the mantle source had lit- 94DS0. In addition, the Sr/Y and Ba/Y ratios of the basalts tle or no influence on the temperature conditions of the from site DSDP447 plot within the MORB field, while the magma crystallization differentiation at shallow depths. Sr/Y ratios of the basalts from site ODP1201 are slightly higher than those of MORB, suggesting that the subduction 5.5. Influence of the Subduction Component. Previous studies component had little or no influence on the magma source have shown that the start of the subduction under the Izu- of the basalts from sites DSDP447 and ODP1201 [17]. Bonin-Mariana arc, the onset of the rifting and spreading In conclusion, the magma source of site 94DS was of the WPB, and the effect of the mantle plume occurred in affected by a subduction component, while the subduction almost the same period [13]. Thus, it is important to discuss component had little or no influence on the magma source whether the magmatism in the WPB is affected by the sub- of the basalts from sites DSDP447 and ODP1201. duction component. La is mobile in high-temperature hydrous melts [72], Sr and Ba are mobile in low- 5.6. Tectonic Setting and Geological Evolution. The West Phi- temperature aqueous fluids, and HREEs, including Y, are lippine Basin contains various rock types, including N- immobile during subduction. Using the method of Sun MORB, OIB, and arc volcanic rocks, and its geodynamics et al. [73] and the trace element compositions of the plagio- background is related to back-arc spreading, a mantle plume, clase of basalts from site 94DS, we calculated the Sr/Y, and a volcanic arc [2, 4, 5, 13, 16, 17]; (Haraguchi et al. 2011). Ba/Y, and La/Y ratios of the hypothetical parental melt. In Thus, the tectonic setting of the samples in this study needs Figure 13, the Sr/Y and Ba/Y ratios of the calculated melt of to be further constrained. In the plot of Y∗3-Ti/10-Zr sample 94DS1 and the basalt composition of sample 94DS1 (Figure 14(a)) [74], most of the samples from site 100DS (similar to the calculated melt based on the composition of and sites IODP 292, 294, and 446 plot in the within plate plagioclase from the South Okinawa Trough) are higher than basalt field (WPB), while all of the samples from site 94DS that of MORB, which indicates the effect of a subduction and sites IODP 447 and ODP1201 plot within the ocean- component on the mantle source of sample 94DS1 [30, 37]. floor basalts or cal-alkali to low-potassium tholeiite fields. The Sr/Y ratio of the calculated melt of sample 94DS0 and On the plot of Zr vs. Zr/Y (Figure 14(b)) [75], most of the

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51–45 Ma plume arrival and Before 52 Ma spreading of WPB

Mesozoic terrane (Daito Ridge Group) DR AP Mesozoic terrane (Daito Ridge Group) ODR DR AP

Pacifc Plate Pacifc Plate

Mantle plume

(a) (b) 45–35 Ma 35–Ma plume forms plateaus plume forms Seamount chains as spreading continues before the spreading of WPB

Benham Rise Urdaneta Plateau ODR DR AP Benham Rise Urdaneta Plateau ODR DR AP

Pacifc Plate Pacifc Plate

Mantle plume Mantle plume

Benham Rise TS SS Urdaneta Plateau ODR DR AP Benham Rise TS SS VS Urdaneta Plateau ODR DR AP Extincted spreading center Extincted spreading center

Pacifc Plate

Mantle plume Mantle plume Pacifc Plate

(e) (f)

Figure 15: Schematic tectonic history for West Philippine Basin (WPB) (revised by Ishiuzuka et al. [13]). AP: Amami; DR: Daito Ridge; ODR: Oki-Daito Ridge.

samples from site 100DS and sites IODP 292, 294, and 446 the thicker crust (about 15 km) calculated from the gravity plot in the WPB field, while all of the samples from sites anomalies beneath the Benham Rise supports the viewpoint 94DS, DSDP447, and ODP1201 plot within the overlap that the volcanism is attributed to the interaction between a between the MORB and IAB (island arc basalt) fields. Based hotspot (or mantle plume) and a spreading ridge. on the results described above, the basalts from sites 94DS, Based on the reliable K-Ar and Ar-Ar ages of basalts from DSDP447, and ODP1201 were formed in a similar tectonic WPB, we proposed a tectonic evolution model for WPB setting, which is related to a mid-ocean Ridge setting. The (Figure 15). basalts from site 100DS and sites IODP 292, 294, and 446 Before 52 Ma, there existed some late Mesozoic terranes have a similar geodynamic background, which was affected (including Gagua Ridge, Daito Ridge Group, Daito Ridge, by a mantle plume. and Amami Plateau), and there was no volcanic activity in Recent studies have shown that magma with a surface the WPB (Figure 15(a)) [13]. area of about 0.11 Mkm2 and a volume of 0.13 Mkm3 Between 51-45 Ma, the Oki-Daito plume arrived at the extruded within 5 Myr, and the geochemical characteristics spreading center of WPB about 45 Ma, resulting in wide- of the basalts from the Benham Rise are similar to OIB, indi- spread OIB-like volcanic edifices and the spreading of WPB cating that the Benham Rise is likely a large igneous province until 35 Ma [13]. The spreading of the WPB may lead to [76]. In addition, the morphology of the Benham Rise and the split of Daito Ridge Group, and push part of Daito Ridge

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Group northward to form Daito Ridge in the north-east of Conflicts of Interest CBF (Figure 15(b)). During this period, Benham Rise was ﬂ formed on the south-west side of the spreading center, and The authors declare that they have no con icts of interest. a chain of age-progressive oceanic Plateaus (Urdaneta Pla- teau) was formed on the north-east side of the spreading cen- Acknowledgments ter of WPB (Figure 15(c)) [13]. At about 35 Ma, the spreading center jumped to north- We thank Prof. Sanzhong Li and two anonymous reviewers east of Oki-Daito plume, forming a chain of Seamounts for their constructive comments and suggestions, and Song- (e.g., Toog Seamount, Santos Seamount, and Akle Seamount) jian Ao for editorial handling. This work was supported by in the south-west of the spreading center (Figure 15(d)) [76]. the National Key Research and Development Program of However, the ages of these Seamounts are unknown, which China (Grant No. 2017YFC0602305), the National Natural hinder us to understand the eﬀect of the Oki-Daito plume. Science Foundations of China (grants nos. 41776070, Based on the age of basalts from CBF, the cessation time U1606401, 41276003, and 41322036), AoShan Talents Pro- of the spreading of WPB was about 29 to 27 Ma gram Supported by the Qingdao National Laboratory for (Figure 15(e)). After the cessation of the spreading of WPB, Marine Science and Technology (No. 2015ASTP-ES16), The Vinogradov Seamount formed near the spreading center and Taishan Scholarship from Shandong Province. We thank about 22 to 20 Ma (Figure 15(f)), which represents the LetPub (http://www.letpub.com) for its linguistic assistance extinction time of the Oki-Daito mantle plume. during the preparation of this manuscript.

6. Conclusions Supplementary Materials Supplementary Table 1: electron probe analyses (wt%) of (1) This study presents compositional characteristics of olivines in basalt from Benham Rise. Supplementary Table minerals in basalts from the Benham Rise and the 2: LA-ICP-MS analysis (ppm) of olivine in basalt from Ben- CBF as follows, Olivines from the Rise have forsterite ham Rise. Supplementary Table 3: electron probe analyses contents of 56.90%–90.80% and have high ΣREE (wt%) of clinopyroxenes in basalt from CFB. Supplementary values (0.05–0.43 ppm). The clinopyroxenes from Table 4: LA-ICP-MS analysis (ppm) of clinopyroxenes in the CBF are diopside and augite, and they are basalt from CFB. Supplementary Table 5: electron probe depleted in LREEs (La /Yb =0:13 – 3:40) and N N analyses (wt%) of plagioclases in basalt from the West Philip- LILEs. The plagioclases are predominantly labrador- pine Basin. Supplementary Table 6: LA-ICP-MS analysis ite and andesine, with minor bytownite, and there (ppm) of plagioclases in basalt from the West Philippine exist obvious differences in plagioclase compositions Basin. Supplementary Table 7: the major and part of trace between these two tectonic sites elements of basalts from the West Philippine Basin. (2) The source lithology of the Benham Rise basaltic (Supplementary Materials) rocks could be garnet pyroxenite, and yet that of the CBF could be spinel-lherzolite. The degree of melting References of the samples from the CBF ranged from 10% to “ 11%, while the degree of melting of the samples from [1] R. Hall, Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, the Benham Rise ranged from 5% to 7%. The magma ” ff model and animations, Journal of Asian Earth Sciences, source of sample 94DS was a ected by a subduction vol. 20, no. 4, pp. 353–431, 2002. component [2] R. Hickey-Vargas, “Origin of the Indian ocean-type isotopic (3) The calculated mantle potential temperature beneath signature in basalts from Philippine Sea plate spreading cen- ” the Benham Rise is 1439°C–1473°C, which is signifi- ters: an assessment of local versus large-scale processes, Jour- – cantly higher than that of the CBF (1345°C– nal of Geophysical Research, vol. 103, no. B9, pp. 20963 20979, 1381°C), suggesting there existed thermal anomaly 1998. [3] T. W. C. Hilde and C.-S. Lee, “Origin and evolution of the beneath the Rise during basaltic magmatism, which ” may relate to a mantle plume West Philippine Basin: a new interpretation, Tectonophysics, vol. 102, no. 1-4, pp. 85–104, 1984. (4) This study developed a conceptual model for WPB [4] X. F. Shi and Q. S. Yan, “Magmatism of typical marginal basins geological evolution and suggested the extinction of (or back-arc basins) in the West Pacific,” Advances in Earth the Oki-Daito mantle plume activity might be at Science, vol. 28, no. 7, pp. 737–750, 2013. about 20.9 Ma, and cessation of the back-arc spread- [5] Q. Yan and X. Shi, “Geological comparative studies of Japan ing of WPB was at about 29.1 Ma or younger arc system and Kyushu-Palau arc,” Acta Oceanologica Sinica, vol. 30, no. 4, pp. 107–121, 2011. [6] S. Uyeda and Z. Ben-Avraham, “Origin and development of Data Availability the Philippine Sea,” Nature Physical Science, vol. 240, no. 104, pp. 176–178, 1972. The data supporting the results of our study can be found in [7] A. Deschamps and S. Lallemand, “The West Philippine Basin: manuscript and supplementary table. an Eocene to early Oligocene back arc basin opened between

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