Geochemical constraints on the spatial distribution of ...

Lithos 296–299 (2018) 382–395

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Geochemical constraints on the spatial distribution of recycled oceanic crust in the mantle source of late Cenozoic basalts, Vietnam Thi Hong Anh Hoang a, Sung Hi Choi a,b,⁎, Yongjae Yu a,b, Trung Hieu Pham c, Kim Hoang Nguyen c, Jong-Sik Ryu d a

Department of Astronomy, Space Science and Geology, Chungnam National University, Daejeon 34134, South Korea Department of Geology and Earth Environmental Sciences, Chunganm National University, Daejeon 34134, South Korea Faculty of Geology, University of Science VNU-HCM, Vietnam d Division of Earth and Environmental Sciences, Korea Basic Science Institute, Ochang, Chungbuk 28119, South Korea b c

a r t i c l e

i n f o

Article history: Received 23 June 2017 Accepted 11 November 2017 Available online 21 November 2017 Keywords: Sr-Nd-Pb-Mg isotopes Basalt Recycled oceanic crust Eclogite Vietnam

a b s t r a c t This study presents a comprehensive analysis of the major and trace element, mineral, and Sr, Nd, Pb and Mg isotopic compositions of late Cenozoic intraplate basaltic rocks from central and southern Vietnam. The Sr, Nd, and Pb isotopic compositions of these basalts define a tight linear array between Indian mid-ocean-ridge basalt (MORB)-like mantle and enriched mantle type 2 (EM2) components. These basaltic rocks contain low concentrations of CaO (6.4–9.7 wt%) and have high Fe/Mn ratios (N60) and FeO/CaO–3MgO/SiO2 values (N 0.54), similar to partial melts derived from pyroxenite/eclogite sources. This similarity is also supported by the composition of olivine within these samples, which contains low concentration of Ca and high concentrations of Ni, and shows high Fe/Mn ratios. The basaltic rocks have elevated Dy/Yb ratios that fall within the range of melts derived from garnet lherzolite material, although their Yb contents are much higher than those of modeled melts derived from only garnet lherzolite material and instead plot near the modeled composition of eclogite-derived melts. The Vietnamese basaltic rocks have lighter δ26Mg values (−0.38 ± 0.06‰) than is expected for the normal mantle (−0.25 ± 0.07‰), and these values decrease with decreasing Hf/Hf* and Ti/Ti* ratios, indicating that these basalts were derived from a source containing carbonate material. On primitive mantle-normalized multielement variation diagrams, the central Vietnamese basalts are characterized by positive Sr, Eu, and Ba anomalies. These basalts also plot within the pelagic sediment field in Pb\\Pb isotopic space. This suggests that the mantle source of the basalts contained both garnet peridotite and recycled oceanic crust. A systematic analysis of variations in geochemical composition in basalts from southern to central Vietnam indicates that the recycled oceanic crust (possibly the paleo-Pacific slab) source material contains varying proportions of gabbro, basalt, and sediment. The basalts from south-central Vietnam (12°N–14°N) may be dominated by the lowest portion of the residual slab that contains rutile-bearing plagioclase-rich gabbroic eclogite, whereas the uppermost portion of the recycled slab, including sediment and basaltic material with small amounts of gabbro, may be a major constituent of the source for the basalts within the central region of Vietnam (14°N–16°N). Finally, the southern region (10°N–12°N) contains basalts sourced mainly from recycled upper oceanic crust that is basalt-rich and contains little or no sediment. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Intraplate continental basaltic magma are generally considered to be generated by the partial melting of sub-continental lithospheric mantle, asthenospheric mantle, or both types of material within extensional tectonic regimes. The geochemical diversity of these magmas most likely reflects different degrees of partial melting of the lithospheric mantle metasomatized to various extents during its long-term attachment to the continental crust (e.g., Arndt and Christensen, 1992; McKenzie, ⁎ Corresponding author at: Department of Geology and Earth Environmental Sciences, Chunganm National University, Daejeon 34134, South Korea. E-mail address: [email protected] (S.H. Choi).

https://doi.org/10.1016/j.lithos.2017.11.020 0024-4937/© 2017 Elsevier B.V. All rights reserved.

1989). More recent research has suggested that the presence of recycled dehydrated oceanic slab (pyroxenite/eclogite) within the mantle source regions plays a significant role in the generation and chemical heterogeneity of intraplate magma (Herzberg, 2011; Hirschmann et al., 2003; Kogiso et al., 2003; Sobolev et al., 2005, 2007). Late Cenozoic intraplate basaltic volcanism is widespread in east and southeastern Asia and the western Pacific in a region that has been described as a diffuse igneous province (e.g., Hoang et al., 1996, 2013 and references therein). The basaltic plateaus in Indochina, including Vietnam, Thailand, Laos and Cambodia, crop out over an area of ~ 70,000 km2 (Hoang and Flower, 1998; Hoang et al., 1996, 2013; Zhou and Mukasa, 1997; Fig. 1). The eruptive units in Vietnam and eastern Cambodia represent the most voluminous sections of this

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rF ve

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2. General geology

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as well as the nature, scale, and implications of chemical heterogeneity within this mantle source region.

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Fig. 1. Map of Indochina (after Hoang et al., 1996) showing fault systems (thin lines), Cenozoic volcanic centers (shaded), national boundaries (thin dashed lines), and lithospheric sectors (thick dashed lines): I. northern accretionary belt (Precambrian to Paleozoic), II. central-Kontum massif (Archean, Proterozoic, Paleozoic), III. southwestern-Khorat plateau (Precambrian) and surrounding Paleozoic and Mesozoic belts, IV. southeastern-undifferentiated Precambrian overlain by Mesozoic. Literature data for basaltic rocks from central and southern Vietnam are from Hoang et al. (2013) and An et al. (2017), respectively.

province and individual unites often exceed 100 km in diameter and reach up to several hundred meters in thickness (e.g., Hoang and Flower, 1998). Research over the past twenty years has attempted to determine the geochemical characteristics and petrogenetic histories of these basaltic rocks (e.g., An et al., 2017; Hoang et al., 1996, 2013; Hoang and Flower, 1998; Liu et al., 2015; Wang et al., 2012; Zhou and Mukasa, 1997). However, the ultimate origin of these magmas remains controversial, with proposed models including (1) interaction of a midocean-ridge basalt (MORB) source with subcontinental lithosphere (Flower et al., 1992; Hoang et al., 1996; Zhou and Mukasa, 1997), (2) a petrogenetic association with the Hainan mantle plume (An et al., 2017; Wang et al., 2012), and (3) derivation from the melting of a mixed peridotite/pyroxenite source (An et al., 2017; Huang et al., 2015; Liu et al., 2015). As such, the basalts in Vietnam provide an ideal opportunity to examine the petrogenesis of part of the most voluminous late Cenozoic intraplate volcanism in southeastern Asia. Identifying the sources of basaltic magmas is a key part of determining the origin of basaltic volcanism. Radiogenic isotopic (Sr, Nd, Pb) and trace element data can provide useful insights into the nature of the sources of basaltic rocks (e.g., Hart, 1984; Hofmann et al., 1986; Zindler and Hart, 1986). More recent research has highlighted the usefulness of magnesium isotopic data as an independent tool for understanding the nature of mantle heterogeneity (e.g., Huang and Xiao, 2016; Wang et al., 2014a, 2014b; Yang et al., 2012; Zhong et al., 2017). The late Cenozoic intraplate basaltic rocks in Vietnam are generally located within the central and southern parts of the country, and samples were collected throughout the entirety of this region during the present study (Fig. 1). The nature of the source of the basalts in this region and their petrogenetic history were investigated using whole-rock major and trace elements, and Sr, Nd, Pb, and Mg isotopic data, as well as the compositions of minerals within late Cenozoic basaltic rocks from central and southern Vietnam. The data provide insights into the role of recycled oceanic crustal material in the source region of these basalts,

Widespread late Cenozoic volcanic activity occurred in Vietnam generated a basaltic lava flow that crops out over an area of ~23,000 km2. This lava is located primarily in the Tay Nguyen Highland area of Vietnam, including within the Phuoc Long (15–4 Ma), Pleiku (6.4–0.2 Ma), Buon Ma Thuot (8.7–0.3 Ma), and Kon Plong (7.0–16.5 Ma) plateaus (Fig. 1). Basaltic lava flows are also found on near-shore islands such as Ly Son (Quang Ngai Province, 11.0–0.4 Ma) and Con Co (1.3–0.3 Ma) (Bat et al., 2002; Fedorov and Koloskov, 2005; Hoang, 2005a, 2005b; Hoang and Flower, 1998; Hoang et al., 1996; Tri and Khuc, 2009; Fig. 1). In the present study, basaltic rock samples were obtained from five mainland lava flows at Kong Plong, Pleiku, Song Cau, Dalat and Xuan Loc (Fig. 1). The Kong Plong flow crops out over an area of 2000 km2 in Vietnam and has a thickness of ~150 m (Tri and Khuc, 2009), but remains understudied. The flow records two phases of activity at 10.5–16.5 Ma and 7.0–8.2 Ma, and these basalts are interbedded with sandstone, siltstone, and claystone containing flora remains. The samples for this study were collected from the area near the Ba To part of the northernmost body of the flow (Fig. 1). The Pleiku flow crop out over an area of ~7000 km2 and has a central area where the flow reaches a thickness of up to ~500 m (Quoc and Giao, 1980; Tri and Khuc, 2009), forming a broad shield-like edifice capped by thinner late-stage flows. The early phase (6.4–3.4 Ma) consists of extensive quartz tholeiite and olivine tholeiite units that were erupted from extensional fissures that formed a plateau of ~150 m in height. The late phase (2.4–0.2 Ma) of activity involved central eruptions of olivine tholeiite, alkali basalt, and basanite units that reach a maximum thickness of ~ 130 m (Hoang et al., 1996, 2013). The Dalat flows consist of four major phases (16.7–6.3 Ma, 4.0–2.1 Ma, 1.8–0.9 Ma, and 0.7–0.4 Ma), crop out over an area of ~ 3500 km2, and are up to 300 m thick (Hoang et al., 1996; Tri and Khuc, 2009). The Xuan Loc flows contain three major phases (11.6–11.4 Ma, 2.6–2.2 Ma, and 1.1–0.2 Ma), crop out over an area of ~ 2000 km2, and are up to ~ 300 m thick (Hoang et al., 1996; Tri and Khuc, 2009). Many well-preserved volcanic craters have been reported in this area (Hoang et al., 1996; Tri and Khuc, 2009), as have several pyroclastic deposits that contain ultramafic xenoliths (Hoang et al., 1996; Tri and Khuc, 2009). The Song Cau flows are divided into three major phases (10.5–7.0 Ma, 5.0 Ma, and 1.6–0.7 Ma), crop out over an area of ~500 km2, and have a maximum thickness of ~200 m (Hoang et al., 1996; Tri and Khuc, 2009). Indochina can be divided into four lithospheric sectors based on tectonic suture zones, namely: (I) the Precambrian to Paleozoic northern accretionary belt; (II) the Archean to Paleozoic central Kontum massif, which is a quasi-cratonic block with a 2.8-Ga core surrounded by concentric sub-sectors of Proterozoic to Permo-Triassic sutures; (III) the Precambrian southwestern Khorat plateau and associated Paleozoic to Mesozoic accretionary belts; and (IV) the Precambrian to Mesozoic southeastern accretionary sector (Tung and Tri, 1992; Fig. 1). Tri and Khuc (2009) suggested that the cratonic core of the Kontum massif records significant metamorphism that began in the Early to Middle Ordovician and culminated in the Late Permian to Early Triassic. The Kontum massif in Vietnam is therefore unlikely to represent uplifted Precambrian crystalline basement material (Tri and Khuc, 2009). The basaltic rocks sampled during this study are located within sectors II (Pleiku, Song Cau) and IV (Dalat, Xuan Loc). Two major tectonic events are recorded in northwestern Vietnam: (1) the Permo-Triassic Indosinian orogeny, which involved the oblique collision of Indochina with South China along the NW-SE oriented Song Ma Suture; and (2) India–Asia collision and subsequent lateral extrusion of Indochina along the left-lateral Ailao Shan-Red River Fault between ~ 27 and 22 Ma (e.g., Chung et al., 1997; Lan et al., 2000;

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Lepvrier et al., 2008; Tapponnier et al., 1986; Fig. 1). The basaltic magmatism in Vietnam occurred shortly after the cessation of movement along the Ailao Shan–Red River Fault and was nearly coeval with the termination of seafloor spreading in the South China Sea (Hoang et al., 2013). 3. Petrography Eight basalt samples were collected from the Pleiku area of the Kontum massif of central Vietnam (Fig. 1). In addition, one sample from Song Cau in central Vietnam, as well as two samples from Dalat, and three samples from Xuan Loc in southern Vietnam, all of which were collected by An et al. (2017), are included in this analysis (Fig. 1). Petrographic details of the samples from Song Cau, Dalat and Xuan Loc are described by An et al. (2017). The majority of the Pleiku samples are fresh (Fig. S1a–j), but sample VN16171 is relatively altered (Fig. S1 k, l). The Pleiku samples are porphyritic and contain up to 10% phenocrysts of olivine and minor plagioclase. The olivine phenocrysts are euhedral to subhedral, up to 0.1 cm in size (Fig. S1a–d), and are altered to iddingsite along grain margins or fractures (Fig. S1e, f). Plagioclase phenocrysts are generally present as elongate laths up to 1 mm long (Fig. S1a, b), and some show growth zoning with sieve textures (Fig. S1 g, h). These phenocrysts are hosted by a groundmass of plagioclase laths, granular olivine and/or clinopyroxene, and opaque minerals such as titanomagnetite or magnetite (Fig. S1a-l). Some samples contain sub-spherical vesicles and secondary calcite (Fig. S1c, d, g, h). The Pleiku alkaline basalts often contain mantle xenoliths that range in composition from garnet lherzolite to spinel lherzolite and harzburgite (Fig. S2; Hoang et al., 1996, 2013). 4. Analytical procedures Mineral compositions were determined using a JEOL JXA-8100 wavelength-dispersive electron microprobe at Gyeongsang National University, Jinju, South Korea. The instrument was operated with an accelerating voltage of 15 kV, beam current of 10 nA, beam diameter of 1 μm, and count time of 10 s; standard ZAF correction procedures were employed. Natural minerals were used as standards for Na, Si, Fe, K, Al, Mn, Ca, Mg, and Ti, and synthetic oxides were used for Cr and Ni. The results of this analysis are given in Tables S1 and S2. Prior to whole-rock analysis, samples were crushed into b 0.5-cm pieces in a tungsten carbide mortar before being ultrasonically cleaned with Milli-Q water. The clean crushed samples were then pulverized in an agate ball mill prior to geochemical analysis. Whole-rock major elemental analysis was conducted using X-ray fluorescence spectrometry (XRF) at Actlabs, Ontario, Canada. The resulting data were reduced with a weighted regression line based on analysis of the BIR-1, DNC-1, and W-2 standards, yielding a combined sample preparation and analytical precision of better than 5%. The results of these analyses are given in Table S3. Whole-rock trace element concentrations were obtained using inductively coupled plasma mass spectrometry (ICP-MS) at Actlabs. This analysis has a precision of ±10% based on replicate analyses of BIR-1, JR-1, and DNC-1 international standards. The results are given in Table S3. Chemical separation of Sr, Nd, and Pb for whole-rock isotope analysis was performed at Chungnam National University, Daejeon, South Korea, following the methods outlined by (An et al., 2017). Sr and Nd were separated using standard ion exchange techniques employing a Bio\\Rad AG50W × 8 resin for Sr and the rare earth elements (REE), and a HDEHP-coated Teflon resin for Nd. Lead was separated using Bio\\Rad AG1 × 8 resin chromatography. Analyses of Sr, Nd, and Pb isotopes were undertaken using thermal ionization mass spectrometers (TIMS) employing a VG Sector instrument at the Korea Basic Science Institute (KBSI), Ochang, South Korea. The resulting 87Sr/86Sr and 143 Nd/144Nd ratios were corrected for instrumental mass fractionation by normalizing to 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219,

respectively. Replicate analyses of the NBS-987 and JNdi-1 standards yielded values of 87Sr/86Sr = 0.710252 ± 0.000004 (N = 10, 2σ) and 143 Nd/144Nd = 0.512110 ± 0.000006 (N = 10, 2σ). Measured Pb isotope ratios were corrected for instrumental mass fractionation of 0.1% amu− 1 by reference to replicate analyses of the NBS-981 standard. Blank values averaged 30 pg for Sr and Nd and 50 pg for Pb. The results of these analyses are given in Table S4. The chemical separation and analysis of Mg isotopes were undertaken at KBSI. Powdered samples were dissolved in Teflon beakers in a concentrated mixture of HF–HCl–HNO3 before Mg was separated from matrix elements by cation exchange chromatography with Bio\\Rad AG50W-X8 resin (200–400 mesh) in 1 N HNO3 media. Loading of the sample was followed by the elution of matrix elements with 5 mL of 0.15 M HF, followed by 10 mL of a mixture of 0.5 M HCl and 95% acetone, and 8 mL of 1 M HNO3 before Mg was collected using 12 mL of 1 M HNO3. Pure Mg solutions were then dried and re-dissolved in 5% HNO3. The concentrations of cations (including Al, Ca, Fe, K, Mn, Na, and Ti) were measured by ICP-MS, with the results indicating a cation/Mg (mass/mass) ratio of b0.01. All procedural blank values were negligible (b3 ng). Magnesium isotopic compositions were determined using a sample-standard bracketing method employing a Neptune multicollector (MC)-ICP-MS instrument at KBSI that has been upgraded with a large dry interface pump. Sample intensities were matched to within 10% of the intensities of the standards. The approach using both Jet sample and X-skimmer cones yielded a sensitivity of ~ 100 V/ppm at mass 24 at a typical uptake rate of 100 μL/min. All 26 Mg/24Mg and 25Mg/24Mg ratios are reported in delta notation relative to DSM-3, where δxMg = [(xMg/24 Mg)sample/(xMg/24Mg)DSM-3–1] × 1000 and x = 25 or 26. Samples were analyzed in triplicate or quadruplicate (n = 3 to 4), and uncertainties are reported as two standard deviations (2σ). Three reference materials (DSM-3, CAM-1, and SRM980) were repeatedly measured during the analysis period. DSM-3 yielded a δ26Mg value of 0.01 ± 0.03‰ (2σ, n = 51), CAM-1 yielded a δ26Mg value of −2.70 ± 0.09‰ (2σ, n = 44), and SRM980 yielded a δ26Mg value of −4.36 ± 0.09‰ (2σ, n = 52) (Table S5), all of which are in good agreement with reported values (Brenot et al., 2008; Galy et al., 2003; Huang et al., 2009; Tipper et al., 2008). The chemical purification method, and the precision and accuracy of the Mg isotope measurements were validated using the USGS powder standards and International Association for the Physical Sciences of the Oceans (IAPSO) seawater standards, with BCR-2 yielding a δ26Mg value of − 0.32 ± 0.09 (2σ, n = 9), BHVO-2 yielding a δ26Mg value of −0.25 ± 0.1 (2σ, n = 6), BIR-1 yielding a δ26Mg value of − 0.23 ± 0.08 (2σ, n = 7), and IAPSO seawater yielding a δ26Mg value of − 0.88 ± 0.06 (2σ, n = 46) (Table S5). These values are comparable to reported values (Huang et al., 2009; Teng et al., 2007; Tipper et al., 2008) and are summarized in Table S5. 5. Results 5.1. Mineralogy The chemical compositions of the minerals analyzed during this study are given in Tables S1 and S2. Olivines have a wide range of core Fo (68.1–91.2), MnO (0.10–0.34 wt%), CaO (0.05–0.30 wt%), and NiO (0.11–0.42 wt%) compositions, with CaO and NiO being negatively and positively correlated with Fo values, respectively (Tables S1 and S2). There are no discernible chemical differences between olivines within tholeiitic and alkali basalts in this study area (Fig. 2). These olivines are also slightly normally zoned, with Fo values and NiO concentrations that decrease from the core to the rim, and the CaO and MnO contents that increase from core to rim (Tables S1 and S2). The low Ca-olivines (CaO b 0.1 wt%) within these samples may represent xenocrysts derived from disaggregated peridotite xenoliths (Thompson and Gibson, 2000). However, these low-Ca olivines are present within the tholeiitic basalts rather than the mantle xenolith-bearing alkali basalts. Furthermore, these olivines do not have the typical characteristics of xenocrystic

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5.2. Whole-rock major and trace elements

(a) Olivines: Derivative Magmas (8-20 wt% MgO)

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Fo (mol%) Fig. 2. Composition of olivine phenocrysts in basaltic rocks from central Vietnam compared to calculated olivines from partial melts of a fertile peridotite and a stage 2 pyroxenite, after Herzberg (2011). Also shown are Koolau olivines (Sobolev et al., 2007).

The whole-rock major and trace element compositions of the samples are given in Table S3. The loss on ignition (LOI) values are generally b1 wt%, although some samples have much higher alteration-related LOI values (up to 6.9 wt%). On the total alkali versus silica (TAS) diagram (Fig. S4), the samples from central Vietnam are tholeiitic basalts, basaltic andesites, or alkaline basalts and trachybasalts, whereas the samples from southern Vietnam are tholeiitic basalts, alkaline basanites, and basaltic trachyandesites. These samples have MgO contents of 4.2 to 10.5 wt% and Mg# (= 100 Mg/(Mg + Fe2 +)) values of 44.2 to 66.0 (Table S3). These Mg# values are lower than those expected for primitive basalts (Mg# N 70; Frey et al., 1978), indicating that these basalts record the fractionation of ferromagnesian minerals (An et al., 2017). Fig. S5 shows the major element concentrations plotted against MgO as a differentiation index along with previously published data for Vietnamese basaltic rocks (An et al., 2017; Hoang et al., 2013). A negative correlation between MgO and Al2O3, when combined with rarity of plagioclase phenocrysts (Fig. S1), indicates that the magmas that formed these basalts underwent negligible plagioclase fractionation. The large variations in SiO2, CaO, Na2O, K2O, MnO, TiO2, and total Fe recalculated as FeO (FeOT) cannot be explained by the simple fractional crystallization of a single parent magma. Overall, the tholeiitic rocks contain higher concentrations of SiO2 and lower concentrations of FeOT, K2O, Na2O, and TiO2 than the alkaline rocks at the same concentration of MgO. The samples contain 50–270 ppm Ni, 26–52 ppm Co, 30–300 ppm Cr, and 14–22 ppm Sc (Table S3). The variations in the concentrations of these element are shown in Fig. S6 as a function of MgO. The Ni and Co concentrations show a rough positive correlation with MgO and were most likely controlled by olivine fractionation (Fig. S6a, b). In comparison, Sc, Cr, CaO, and CaO/Al2O3 show no correlation with MgO until a value of ~6 wt% of MgO, above which they show negative correlations (Figs. S5c, i, and S6c, d), indicating that clinopyroxene was not a major fractionating phase in samples with MgO b 6 wt%. The majority of moderately to highly incompatible elements do not correlate with MgO (Fig. S6e-i), suggesting these samples record variations in degrees of partial melting of possibly multiple sources. The chondritenormalized REE patterns of these basalts are shown in Fig. 3a and b. These basaltic rocks have oceanic island basalt (OIB)-like light rare earth element (LREE)-enriched chondrite-normalized variation patterns [(La/Yb)N = 5.5 to 20.6]. A primitive mantle-normalized multielement variation diagram for the Vietnamese basalts (Fig. 3c and d) shows enrichment in the large-ion lithophile elements (LILE) without significant depletion of high-field strength elements (HFSE), again resembling typical OIB, although the alkaline samples are more enriched in the LILE and the HFSE than the tholeiitic basalts. The basalts from central Vietnam show positive Ba, Eu, and Sr anomalies (average Eu/Eu* = 1.06, and Sr/Sr* = 1.11, where Eu* = (Eu)N/[(Sm)N × (Gd)N]0.5 and Sr* = (Sr)N/[(Ce)N × (Nd)N]0.5), that are not present in the basalts from southern Vietnam. The K anomalies in these samples may reflect alteration. 5.3. Sr-Nd-Pb isotopes

olivines, such as kink bands, reaction rims, and resorption boundaries. The CaO contents of olivines from magmatic cumulates and low-CaO melts can be b0.1 wt% (Kamenetsky et al., 2006; Li et al., 2012), suggesting that these low-Ca Vietnamese olivines are considered to be phenocrystic rather than xenocrystic. Clinopyroxenes within the basalts have diopside to augite compositions (Wo41.8–49.5 En36.4–46.1Fs12.3–14.1) with Mg# (= 100 Mg/[Mg + Fe]) values of 72.4 to 79.4 (Table S1; Fig. S3a). Their Al2O3 and TiO2 contents range from 2.9 to 7.0 wt%, and from 0.9 to 3.2 wt%, respectively. The plagioclases in these basalts has a labradorite composition (An56.1– 62.2; Table S1; Fig. S3b) and shows weak normal zoning (Table S1).

The Sr-Nd-Pb isotopic compositions of the Vietnamese basalts are given in Table S4 and are shown in Sr\\Nd and Pb\\Pb isotopic correlation diagrams (Figs. 4 and 5). Reference points representing the hypothetical mantle end-members of depleted MORB mantle (DMM) and enriched mantle types 1 and 2 (EM1 and EM2, respectively; Zindler and Hart, 1986) are shown on these diagrams for comparison. The MORB field and representative EM1-type OIBs from the Pitcairn Islands and EM2-type OIBs from Samoa (http://georoc.mpchmainz.gwdg.de/georoc/) are also included on the plots for reference. The samples from this study are not age-corrected, as they are relatively young (ca. 0.2–16.5 Ma; An et al., 2017; Hoang et al., 1996, 2013).

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(c) Rock/Primitive mantle

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Fig. 3. (a, b) Rare earth element patterns normalized to chondrite values (Sun and McDonough, 1989) and (c, d) extended trace element abundances normalized to the composition of the primitive mantle (Sun and McDonough, 1989) for Vietnamese basaltic rocks. Data for a typical oceanic island basalt (OIB; Sun and McDonough, 1989), average continental crust (Rudnick and Gao, 2003), and the average of global subducting sediment (GLOSS; Plank and Langmuir, 1998) are shown for comparison.

The Vietnamese basalts yield 87Sr/86Sr = 0.70381–0.70466, 143 Nd/144Nd = 0.512746–0.512930 (εNd = + 2.1 to + 5.7), 206 Pb/204Pb = 18.14–18.95, 207Pb/204Pb = 15.54–15.69, and

0.5132

Central Vietnam

Pacific/Atlantic MORB

Tholeiite (This study) Alkaline(This study) Tholeiite (Literature) Alkaline (Literature)

0.5131

208

Pb/204Pb = 38.25–39.16. These samples plot outside of the MORB field but within the EM2-type OIB field on a Sr\\Nd isotopic diagram (Fig. 4), and above the Northern Hemisphere reference line (NHRL; Hart, 1984) along an Indian MORB-EM2 mixing array in Pb\\Pb isotope space. There are no discernible differences in Sr-Nd-Pb isotopic compositions between the tholeiite and alkaline samples, indicating they were derived from a similar source, although the central Vietnamese basalts have elevated 207Pb/204Pb ratios at given 206Pb/204Pb ratios compared with the southern Vietnamese basalts (Fig. 5a).

143Nd/144Nd

Southern Vietnam Tholeiite (This study) Alkaline (This study) Tholeiite (Literature) Alkaline (Literature)

0.5130 Indian MORB

0.5129

EM

0.5128

2

0.5127 1 EM

0.5126

0.703

0.704

0.705

87Sr/86Sr

Fig. 4. 87Sr/86Sr vs. 143Nd/144Nd isotopic ratios for Vietnamese basaltic rocks. Data sources: mid-ocean ridge basalts (MORB; An et al., 2017 and references therein), EM1-type oceanic island basalts from the Pitcairn Islands and EM2-type oceanic island basalts from the Samoa Islands (http://georoc.mpch-mainz.gwdg.de/georoc/), and Vietnamese basaltic rocks (An et al., 2017; Hoang et al., 2013). Error values (2σ) are smaller than the size of symbols.

5.4. Mg isotopes The Mg isotopic compositions of the Vietnamese basaltic rocks are given in Table S4. These basalts have Δ25Mg' (Δ 25Mg' = δ25Mg' – 0.521 × δ26Mg', where δ25,26Mg' = 1000 × ln[(δ25,26Mg + 1000)/ 1000]; Young and Galy, 2004) values of − 0.02 ± 0.01‰ (Table S4). All of these samples plot along the terrestrial equilibrium mass fractionation curve in a δ25Mg vs. δ26Mg diagram (Fig. 6a), with a slope of 0.521. These samples have variable but generally light Mg isotopic compositions with δ25Mg values of − 0.34‰ to − 0.15‰ (− 0.22 ± 0.04‰, 2SD, n = 14) and δ26Mg values of − 0.62‰ to − 0.28‰ (− 0.38 ± 0.06‰, 2SD, n = 14). The tholeiitic basalts yield a limited range of δ26Mg values (− 0.40‰ to − 0.28‰), that slightly overlap with the lower end of normal mantle compositions (δ26Mg = −0.25 ± 0.07‰; Teng et al., 2007, 2010; Fig. 6b). In comparison, the alkaline samples show heterogeneous δ26Mg values (− 0.62‰ to − 0.30‰) that are much lighter than values expected for the normal mantle (Fig. 6b).

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L

39

15.5

RB

O nM

EM1

ia

O PA L DU

38

Pacific pelagic sediment

EM1

In d

Ind

Pacific/Atlantic MORB

RB

15.6

M O

R NH

EM2

(b)

IB

40

ia n

15.7

Pacific pelagic sediment

208Pb/204Pb

(a) 207Pb/204Pb

EM2

GEO CHRO N

15.8

387

37

DMM

Pacific/Atlantic MORB

NH

DMM

RL

15.4

15.3

17

18

19

17

20

18

206Pb/204Pb

19

20

206Pb/204Pb

Fig. 5. (a) 207Pb/204Pb vs. 206Pb/204Pb and (b) 208Pb/204Pb vs. 206Pb/204Pb isotopic ratios for Vietnamese basaltic rocks. Northern Hemisphere reference line (NHRL) from Hart (1984), and mantle components are from Zindler and Hart (1986). Data for Pacific pelagic sediments (Godfrey et al., 1997; Othman et al., 1989; Yu, 2005), MORBs (An et al., 2017 and references therein) and Vietnamese basaltic rocks (An et al., 2017; Hoang et al., 2013) were sourced from the literature. The dashed line in (b) is the DUPAL isotopic anomaly (An et al., 2017 and references therein). Symbols are as in Fig. 4. Error values (2σ) are smaller than the size of symbols. Abbreviation: MORB = mid-ocean ridge basalt, DMM = depleted MORB mantle, EM1 and EM2 = enriched mantle types 1 and 2, respectively.

6. Discussion 6.1. Crustal contamination Prior to eruption, the magmas that formed the Vietnamese basaltic rocks ascended through the continental crust. As such, it is important to assess the possible role of crustal contamination in their petrogenesis. Continental crustal material has high concentrations of SiO2 and the LILEs (i.e., Rb, Ba, U, K, and Pb), is depleted in the HFSEs (i.e., Nb, Ta, and Ti), and has enriched Sr and Nd isotopic compositions. This means that magmas assimilated crustal material show increase in SiO2 and LILE concentrations and the 87Sr/87Sr ratios, but have lower HFSE concentrations and 143Nd/144Nd ratios compared with uncontaminated basalts with similar petrogenetic histories. There are no meaningful correlations between SiO2 or MgO and the 143Nd/144Nd or 87Sr/87Sr ratios within the Vietnamese basaltic rocks (Fig. S7a–d), indicating that these basalts record negligible or no crustal contamination. Both Ce/Pb and Nb/U are sensitive indicators of crustal contamination, and the majority of the Vietnamese alkaline samples have Ce/Pb and Nb/U ratios that fall within the range of oceanic basalts (Ce/Pb = 25 ± 5, Nb/U = 47 ± 10; Hofmann et al., 1986) and are distinctly higher than the ratios expected for typical continental crustal material (Fig. S8a, b). In comparison, some of the tholeiitic basalts from the study area have relatively low Ce/Pb and Nb/U ratios that indicate a degree of crustal

contamination. However, a lack of meaningful correlations between Ce/Pb or Nb/U ratios and εNd values (Fig. S8c, d) or HFSE depletions (Fig. 3c, d) rules out the possibility of crustal contamination in the genesis of the magma that formed these basalts. The Vietnamese basalts also plot within the oceanic basalt field of Sr-Nd-Pb isotopic correlation diagrams (Figs. 4, 5 and S9), again suggesting they do not record significant crustal contamination. The lower continental crust is likely to either have a mantle-like Mg isotopic composition (δ26 = −0.25 ± 0.07‰; Li et al., 2010; Teng et al., 2010; Yang et al., 2016) or a composition that is slightly heavier than that of normal mantle (δ26Mg = ~ −0.18‰; Teng et al., 2013). In comparison, the upper continental crust has a heterogeneous Mg isotopic composition (δ26Mg = − 0.52 to + 0.92‰), but with a mantle-like weighted average δ26Mg value of − 0.22‰ (Li et al., 2010). The Vietnamese basalts have light Mg isotopic compositions (δ26Mg = −0.38 ± 0.06‰, 2SD, n = 14; Table S4) relative to the normal mantle, which is the opposite trend to that expected for samples that record crustal contamination. In addition, the presence of abundant mantlederived xenoliths within Vietnamese alkali basalts (Fig. S2; Hoang et al., 1996, 2013) suggests that the magmas that formed these basalts ascended rapidly to the surface, therefore avoiding substantial crustal contamination. All of this means that these basaltic rocks record very little crustal contamination and as such can be used to determine the nature of the mantle source for the magmas that formed these units. 0.0

(a)

(b)

-0.1

-0.2

δ26Mg

δ25Mg

-0.2

-0.3

-0.4 -0.8

-0.25±0.07

-0.4

-0.6

-0.6

-0.4

δ26Mg

-0.2

0.0

-0.8

4

6

8

10

MgO (wt%)

Fig. 6. (a) Magnesium isotope plot of the samples used in this study. The solid line represents the terrestrial equilibrium mass fractionation line with a slope of 0.521 (Young and Galy, 2004). (b) δ26Mg vs. MgO (wt%) for Vietnamese basaltic rocks. The grey bar represents the widely accepted δ26Mg of the normal mantle (−0.25 ± 0.07, Teng et al., 2010). Error bars represent 2σ uncertainties. Symbols are as in Fig. 4.

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5

6.2. Nature of the source lithology of the Vietnamese basaltic magmas

0%

4 Eclogite (Cpx:Gnt=75:25) 0% 5%

Dy/Yb

The Vietnamese basalts define a mixing array between the Indian MORB-like DMM and EM2 components in Sr-Nd-Pb isotope spaces (Figs. 4, 5, and S9). The mixing of these components is characteristically recorded by late Cenozoic basaltic rocks in Southeast Asia (An et al., 2017; Choi et al., 2006; Zhou and Mukasa, 1997). Some of samples analyzed during this study have elevated DUPAL-like Pb isotopic compositions (Fig. 5b) that have previously recognized in volcanic rocks within the western Pacific (Hoang et al., 1996; Hoang and Flower, 1998; Mukasa et al., 1987). Several origins have been proposed for the enriched EM2 component of the mantle, including an enriched region of the asthenospheric mantle (Flower et al., 1992), subducted marine sediments (Mukasa et al., 1996), subcontinental lithosphere (Flower et al., 1992; Hoang et al., 1996; Zhou and Mukasa, 1997), mantle plumes (Wang et al., 2012), and recycled oceanic crust-derived eclogite/pyroxenite (An et al., 2017; Liu et al., 2015). The next section discussed the potential source lithologies of the enriched component within the Vietnamese basalts.

Eclogite

5%

10% (Cpx:Gnt=82:18) 0% 5% 10% 25%

3 10%

0% 25% 50%

15%

2

Garnet Lherzolite 20%

5%

50%

Garnet-Spinel Lherzolite

Spinel Lherzolite

10% 15%

10%

5%

1 0

1

2

3

4

Yb (ppm) 6.2.1. Constraints from whole-rock compositions The aluminous phase in the upper mantle shifts from plagioclase to spinel to garnet with increasing depth (Klemme and O'Neill, 2000). The transition from spinel to garnet lherzolite occurs at ~ 19 kbar at 1200 °C and at ~26 kbar at 1500 °C (Klemme and O'Neill, 2000). Garnet is also an essential phase of eclogite/pyroxenite, as is clinopyroxene. As such, we thus first assess the presence of garnet in the mantle source of Vietnamese basalts. Garnet preferentially incorporates the heavy rare earth elements (HREE) over the LREE (e.g., McKenzie and O'Nions, 1991), indicating that basalts derived from a garnet-bearing mantle source will have high Dy/Yb ratio and relatively low Yb contents. The Dy/Yb ratios and Y contents of Vietnamese basaltic rocks range from 2.4 to 3.4 and 17 to 33 ppm, respectively, as shown in Fig. 7, along with melting curves for various source lithologies (spinel/garnet lherzolite and eclogite) modeled using the non-modal batch melting equation (Shaw, 1970). A spinel lherzolite source for the Vietnamese basaltic rocks can be excluded, as partial melts derived from this type of mantle materials have low Dy/Yb ratios (less than ca. 1.5; Fig. 7). In comparison, the majority of the Vietnamese samples have elevated Dy/Yb ratios that plot within the range of melts derived from garnet lherzolite (Fig. 7). However, their Yb contents are higher than the modeled garnet lherzolite melts and instead plot near eclogite-derived melts, suggesting the magmas that formed the Vietnamese basaltic rocks were derived from a hybrid garnet lherzolite-eclogite source, as suggested by An et al. (2017). The contribution of mafic eclogite/pyroxenite lithologies to the magma that formed the Vietnamese basalts can be examined using major element data. Calcium is highly incompatible in olivine (DOl Ca = 0.02, Leeman and Scheidegger, 1977), but is compatible in clinopyroxene (DCpx Ca = 1.82–1.95, Pertermann and Hirschmann, 2002). This means that the low-degree partial melting of a pyroxenite source would generate melts with lower CaO concentration than melts derived from peridotite. The Vietnamese basaltic rocks have low CaO contents and plot below the peridotite-derived partial melt field (Fig. 8a) in a diagram limited to basaltic rocks (MgO N 6 wt%) to minimize the effect of clinopyroxene fractionation (see above). The Fe/Mn ratios can also be used as an indicator of source lithology (Wang et al., 2012). Garnet, orthopyroxene, and clinopyroxene generally have DFe/Mn b 1, where DFe/Mn = (Femineral/Femelt)/(Mnmineral/Mnmelt), whereas olivine has DFe/Mn N 1 (Liu et al., 2008 and references therein). Pyroxenite-derived melts are therefore likely to have higher Fe/Mn ratios (N60; Liu et al., 2008 and references therein) than dry peridotitederived melts (b60; Liu et al., 2008 and references therein). The Vietnamese basaltic rocks have high Fe/Mn ratios that fall within the pyroxenite-derived melt field (Fig. 9b).

Fig. 7. Dy/Yb ratio versus Yb concentration (ppm) for Vietnamese basaltic rocks. Also shown are the melt curves for non-modal batch melting of spinel lherzolite, garnet lherzolite and spinel-garnet lherzolite, and for modal batch melting of eclogite (Cpx:Gnt = 75:25 and 82:18, respectively) using the partition coefficients from McKenzie and O'Nions (1991). The “enriched”-depleted mid-ocean ridge basalt (MORB) mantle (E-DMM; Workman and Hart, 2005) and normal MORB (Sun and McDonough, 1989) compositions were used for lherzolite and eclogite modeling, respectively. The phase proportions (by weight) in the solid mode are Ol55Opx25Cpx18Sp2 for spinel lherzolite, Ol55Opx25Cpx10Gnt10 for garnet lherzolite, and Ol50Opx25Cpx19Gnt3Sp3 for spinel-garnet lherzolite. The phase proportions (by weight) in the melt mode are Ol10Opx20Cpx68Sp2 for spinel lherzolite, Ol5Opx5Cpx45Gnt45 for garnet lherzolite, and Ol7Opx10Cpx50Gnt25Sp8 for spinel-garnet lherzolite. Abbreviations: Ol = olivine; Opx = orthopyroxene; Cpx = clinopyroxene; Sp = spinel; Gnt = garnet. Symbols are as in Fig. 4.

Yang and Zhou (2013) identified a FC3MS parameter (FeO/CaO3*MgO/SiO2, all in wt%) that can be used to identify basalt source lithologies. They determined that experiment peridotite melts have average FC3MS values of − 0.07 ± 0.51 whereas pyroxenite melts tend to have higher values of 0.46 ± 0.96. In addition, the upper limit of the FC3MS value for peridotite-derived melts is ~0.5. This means that basaltic rocks with FC3MS values of N0.5 are unlikely to have been derived from melts generated by the partial melting of peridotite alone. The majority of the Vietnamese basalts have FC3MS values of N 0.5 and plot within the pyroxenite-derived melt field (Fig. 8c). This is similar to Hawaiian basalts derived from a hybrid peridotite-pyroxenite source (Fig. 8a–c; Sobolev et al., 2005). This suggests that the magma that formed the Vietnamese basalts were derived from an eclogite/ pyroxenite source that also contained garnet peridotite. Porter and White (2009) reported that residual oceanic slabs subducted into the deep mantle tend to have Ce/Pb and Nb/U ratios outside of the range of oceanic basalts (Ce/Pb = 10.3, Nb/U = 25.8 compared with Ce/Pb = 25 ± 5, Nb/U = 47 ± 10, respectively; Hofmann et al., 1986; Porter and White, 2009; Fig. S8a, b). Some of the tholeiitic Vietnamese basalts have relatively low Ce/Pb and Nb/U ratios that plot near the residual slab compositions (Fig. S8a, b), suggesting they were derived from a source containing recycled slab material. In contrast to the southern Vietnamese basalts, the central Vietnamese basalts show significant positive Sr and Eu anomalies and are enriched in Ba relative to Th (Fig. 3a, c). Europium is compatible in plagioclase relative to the other REEs (Bédard, 2006), and Sr is more compatible in plagioclase than are the neighboring REEs (Pr and Nd; Bédard, 2006). Barium is weakly compatible but Th is incompatible in plagioclase (Bédard, 2006), and the samples from the study area rarely contain plagioclase. This suggests that these positive Sr and Eu anomalies are unlikely to have been caused by the accumulation of plagioclase

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(a)

14

Hawaiian basalt L - Ol

CaO (wt%)

12

L - Ol - Cpx - Plag

Peridotite partial melts

10

8

Pyroxenite partial melts 6

CaO = 13.81 - 0.274MgO 0

6

12

18

24

30

MgO (wt%) 120

(b)

Fe/Mn

100

Pyroxenite partial melts

80

60

40 Hydrous peridotite

partial melts Dry peridotite partial melts 20 0.05

0.1

0.15

0.2

0.25

0.3

MnO (wt%)

(c)

FeOT/CaO - 3*MgO/SiO2

1.2

0.8

Pyroxenite partial melts Average = 0.46±0.96 0.4

0

Peridotite partial melts Average = -0.07±0.51

-0.4 4

8

12

16

20

24

MgO (wt%) Fig. 8. MgO vs. CaO (a), MnO vs. Fe/Mn (b), and MgO vs. FeOT/CaO – 3*MgO/SiO2 (FC3MS) (c) values for Vietnamese basaltic rocks (MgO N 6 wt%). The thick purple line in (a) separates peridotite- and pyroxenite-sourced primary melts (Herzberg and Asimow, 2008). The broken green arrow in (a) indicates the typical liquid line of descent for primary magmas that crystallize gabbro in the crust (Herzberg and Asimow, 2008). Black and green broken lines in (b) represent average FC3MS values of pyroxenite and peridotite partial melts, respectively (Yang and Zhou, 2013). Data for Hawaiian basalts are from Gaffney et al. (2005), Hauri (1996), and Humayun et al. (2004). The fields showing experimental melts in (c) were modified from Liu et al. (2008). Abbreviations: L = liquid; Ol = olivine; Cpx = clinopyroxene; Plag = plagioclase. Symbols are as in Fig. 4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

in the basalts. Instead, the anomalies are likely to have resulted from the presence of recycled plagioclase-rich cumulate gabbro material in the basalts source. The deep subduction of mafic rocks such as basalts or gabbros into the mantle causes these rocks to be converted to eclogite. If this metamorphic transformation occurs in a closed system, then this eclogite can retain Sr, Eu, and Ba enrichments that originally

389

resulted from the presence of cumulus plagioclase (Frey et al., 2016). This means that the incorporation of recycled gabbroic eclogite material into the mantle source of basaltic melts can generate melts with plagioclase-type trace-element signatures. The central Vietnamese basalts show more radiogenic Pb isotopic compositions than the southern Vietnamese samples (Fig. 5). This means that the presence of recycled mafic material alone cannot explain all of the observed geochemical properties of these basalts. The majority of the central Vietnamese basalts plot within the Pacific Ocean pelagic sediment field in Pb\\Pb isotope space (Fig. 5a, b). Continental sedimentary material is generally enriched in highly incompatible elements and has significant negative Nb and Ta anomalies relative to La and Th (Fig. 3d). This means that Nb/Nb* values [where Nb* = NbN / (ThN × LaN)0.5, with N indicating normalization to the primitive mantle compositions of Sun and McDonough, 1989], can be used to identify the presence of sediments within basaltic source regions. Marine sediments have higher 232Th/238U and 87Sr/86Sr ratios, and lower 143Nd/144Nd ratios than the primitive mantle (Othman et al., 1989). The 87Sr/86Sr and 207 Pb/204Pb ratios of the Vietnamese basalts show a rough negative correlation with Nb/Nb*, whereas 143Nd/144Nd values show a positive correlation with Nb/Nb* values (Fig. S10a–c). The presence of a significant negative correlation between 208Pb*/206Pb* and Nb/Nb* values (Fig. S10d) also indicates the source region has a high long-term Th/U ratio in the source. These observations indicate the presence of various recycled sedimentary components in the mantle source of the magmas that formed the Vietnamese basalts. 6.2.2. Constraints from olivine compositions Olivine is the first silicate mineral that crystallizes from almost all mantle-derived magmas as they ascend to the surface. This means that olivine can be used to fingerprint the mantle source of basaltic magmas. Sobolev et al. (2005, 2007) suggested that the presence of pyroxenite/eclogite in a mantle source can be identified by the existence of olivine phenocrysts containing low concentrations of Ca, high concentrations of Ni, and with high Fe/Mn ratios. In addition, olivine that crystallizes from magmas with low CaO tends to inherit this low-CaO signature (Herzberg and Asimow, 2008). The Vietnamese basaltic rocks have low CaO contents that resemble those of pyroxenitederived partial melts (Fig. 8a). Olivine phenocrysts from the central Vietnamese basalts also have low Ca contents, lower than those of olivines from peridotite-derived melts for a given Fo content (Fig. 2a). The Ni contents of the olivines from the central Vietnamese basalts are lower than those of olivine with the same Fo content derived from pyroxenite melts (Fig. 2b). This may reflect the mixing of pyroxeniteand peridotite-derived melts (Herzberg and Asimow, 2008). The high Fe/Mn ratios of the central Vietnamese basaltic rocks (Fig. 8b) are also reflected in the Fe/Mn ratios of their olivine phenocrysts, which are higher than those of olivines derived from peridotite melts (Fig. 2c). These high Fe/Mn ratios are primarily associated with the retention of Mn in a garnet-rich source region (e.g., eclogite or garnet pyroxenite) during melt generation (Herzberg and Asimow, 2008). These olivine compositions are also similar to those of olivines from the Hawaiian Koolau basalts, which are thought to have been derived from a mantle source containing significant amounts of recycled oceanic crustal material (garnet pyroxenite; Sobolev et al., 2007; Fig. 2a–c). In summary, we conclude that the mantle source of the magmas that formed the Vietnamese basalts contains two components: garnet peridotite and recycled oceanic crustal material (i.e., basalt, gabbro, and sediment). These recycled materials are heterogeneously distributed throughout central and southern Vietnam, with gabbroic and sedimentary material being concentrated in the central region. 6.3. Origin of the light Mg isotopic compositions The Vietnamese basaltic rocks have much lighter δ26Mg values (− 0.62 to − 0.28‰) than are expected for normal mantle material

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0.0

20

(a)

18

-0.2

-0.25±0.07

Nb/Ta

δ26Mg

(b)

16

-0.4

14

-0.6

12

Accumulation of ilmenite Accumulation of ilmenite 10

-0.8 1.5

2.0

2.5

3.0

1

1.5

TiO2 (wt.%) 0.0

0.0

(c)

2.5

3.0

-0.4

(d)

-0.2

-0.25±0.07

δ26Mg

δ26Mg

-0.2

2.0

TiO2 (wt.%)

-0.25±0.07

-0.4

-0.6

-0.6

-1.6 -2.0

-1.6 -2.0

Carbonatite 0

0.05

0.7

0.8

0.9

1.0

1.1

Hf/Hf*

Carbonatite 0

0.01 0.6 0.7 0.8 0.9 1.0

1.1 1.2

Ti/Ti*

Fig. 9. (a) δ26Mg vs. TiO2, (b) Nb/Ta vs. TiO2, (c) δ26Mg vs. Hf/Hf*, and (d) δ26Mg vs. Ti/Ti* for Vietnamese basaltic rocks. Black stars represent the average ratios of Hf/Hf* (0.01), Ti/Ti* (0.004) for magnesio- and calico carbonatites (Bizimis et al., 2003; Hoernle et al., 2002). The δ26Mg value of carbonatite is from Wang et al. (2014b). Hf/Hf∗ = HfN/(SmN × NdN)0.5, Ti/ Ti∗ = TiN/(SmN × DyN)0.5 where the subscript N indicates normalization to chondrites (Sun and McDonough, 1989). The yellow arrows represent the ilmenite accumulation trend. Orange arrows indicate involvement of recycled carbonate in the mantle source. Grey bar represents average mantle (δ26Mg = −0.25 ± 0.07, Teng et al., 2010). Error bars represent 2σ uncertainties. Symbols are as in Fig. 4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(−0.25 ± 0.07‰, Teng et al., 2010; Table S4). These δ26Mg values do not correlate with MgO concentrations (Fig. 6b), indicating that the basalts underwent insignificant Mg isotope fractionation during basalt differentiation. This inference is consistent with the results of previous studies, which found limited Mg isotope fractionation during magmatic differentiation (e.g., Li et al., 2010; Telus et al., 2012; Teng et al., 2007, 2010). The fact that crustal contamination is unlikely to have been a major factor in the petrogenetic history of the Vietnamese basalts (see above) means that these light Mg isotopic compositions are likely to have been inherited from a mantle source region. There are two mantle-source components that can cause these light Mg isotopic compositions, namely the presence of ilmenite or carbonate (Huang et al., 2015; Sedaghatpour et al., 2013; Tian et al., 2016; Yang et al., 2012). Sedaghatpour et al. (2013) reported the Mg isotopic compositions of low-Ti and high-Ti lunar basalts. The low-Ti basalts have normal mantle-like δ26Mg values (average δ26Mg of −0.25 ± 0.10‰) whereas the high-Ti basalts have very light Mg isotopic compositions (average δ26Mg value of − 0.49 ± 0.14‰), indicating that ilmenite has lighter Mg isotopic compositions than coexisting olivine and pyroxene (Sedaghatpour et al., 2013). Ilmenite preferentially incorporates Ta over Nb, with a DTa/Nb value of ~ 1.3 (Dygert et al., 2013). This means that magma derived from a mantle source region containing significant amounts of ilmenite yield a negative correlation between Nb/Ta values and TiO2 concentrations. The Vietnamese basaltic rocks have δ26Mg values that decrease with increasing TiO2 content (Fig. 9a), but they also contain much lower concentrations of TiO2 (b3.5 wt%) than the lunar high-Ti basalts (6.0–9.0 wt% TiO2; Sedaghatpour et al., 2013). In addition, the lack of a meaningful correlation between Nb/Ta ratios and TiO2 contents in the Vietnamese samples (Fig. 9b) indicates that

ilmenite accumulation in the mantle source cannot account for the light Mg isotopic compositions of the Vietnamese basaltic rocks. Light Mg isotopic compositions have been identified within b110 Ma basalts from eastern China that are thought to have been generated from a mantle source region that interacted with isotopically light carbonatite melts derived from subducted Pacific slab material (Huang et al., 2015; Tian et al., 2016; Yang et al., 2012). Carbonates have notably light Mg isotopic compositions, with δ26Mg values of − 3.65 ± 2.17‰ for calcite/aragonite, − 1.89 ± 1.08‰ for dolomite, and −1.11 ± 0.75‰ for magnesite (Huang and Xiao, 2016). The Mg isotopic compositions of oceanic crustal material is insignificantly modified during subduction-related prograde metamorphism (Li et al., 2014; Teng et al., 2013; Wang et al., 2014a). This means that deeply recycled carbonated eclogite material might preserve the light Mg isotopic compositions of carbonate-bearing oceanic crustal protoliths (Hu et al., 2015). Experimental studies indicate that the near-solidus partial melting of carbonated eclogite can produce carbonatite melts (Kiseeva et al., 2013; Yaxley and Brey, 2004). Song et al. (2016) reported that carbonatites in the Qinling orogeny in China that were generated by the melting of subducted carbonate-bearing slab material have low δ26Mg values (−1.89 to −1.07‰). Carbonatites are also characterized by significant LILE enrichment and HFSE (e.g., Hf, Zr and Ti) depletion (Bizimis et al., 2003). The δ26Mg values of Vietnamese basaltic rocks also decrease with decreasing Hf/Hf* and Ti/Ti* values, and plot near the values expected for carbonatite melts (Fig. 9c, d). This indicates that recycled carbonated eclogite material may be present in the source region of the magmas that formed the Vietnamese basalts. The alkaline Vietnamese basalts tend to have slightly lighter Mg isotopic compositions and lower Hf/Hf* and Ti/Ti* values than tholeiite

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3.2

(a)

2.4

391

(b)

2.8

Ta/Ta*

Nb/Nb*

2.0 1.6

2.0

1.2

1.6

0.8

1.2 4

2.0

(c)

1.8

(d)

1.6

3

1.4

Nb/La

Ti/Ti*

2.4

1.2

2

1.0 0.8 0.6

1

(e)

60

(f)

3

Sr/Sr*

Sr/Nd

50 40

2

30 20

1

(g)

(h)

200

1.4

Ba/Th

Eu/Eu*

160 1.2

120

80

1.0

40 10

11

12

13

14

15

16

o

Latitude ( N)

10

11

12

13

14

15

16

o

Latitude ( N)

Fig. 10. Latitude (° N) vs. Nb/Nb* (a), Ta/Ta* (b), Ti/Ti* (c), Nb/La (d), Sr/Nd (e), Sr/Sr* (f), Eu/Eu*, and Ba/Th (h) values for Vietnamese basaltic rocks. Nb/Nb∗ = NbN/(ThN × LaN)0.5, Ta/Ta ∗ = TaN/(ThN × LaN)0.5, Ti/Ti* = TiN/(SmN × DyN)0.5, Eu/Eu* = EuN/(SmN × GdN)0.5 and Sr/Sr* = SrN/(CeN × NdN)0.5, where the subscript N indicates normalization to chondrites (Sun and McDonough, 1989). Symbols are as in Fig. 4.

basalts in the study area (Fig. 9c, d). This suggests that the alkaline basalts were derived from a source region containing more carbonate material than the source region of the tholeiitic basalts. However, these Mg isotopic compositions do not correlate with the Sr, Nd or Pb isotopic compositions of the basalts (Fig. S11a–d), suggesting that either the carbonate in the source region contained very low abundances of Sr, Nd, and Pb, or the source region contained relatively young recycled carbonated eclogites. The δ26Mg values of the basalts show a negative correlation with the melting-sensitive Dy/Yb ratio (Fig. S12), which is consistent with the quantitative evaluation of Zhong et al. (2017) that

changes in degrees of partial melting of a garnet-bearing source region can affect the Mg isotopic composition of the resulting parental melts. Ytterbium is more compatible than Dy in garnet, indicating that the Dy/Yb ratios of partial melts from a garnet-bearing source decrease with increasing degree of partial melting (Fig. 7). The magmas that formed the alkaline basalt samples were generated during lowerdegree partial melting than the magmas that formed the tholeiite samples (Fig. 7). In addition, the recycled oceanic crust solidus is 50–150 °C lower than typical peridotite solidus temperatures for a given pressure (Hirschmann and Stolper, 1996; Kogiso et al., 2003). Carbonation also

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significantly lowers the solidus of MORB-like pyroxenite relative to carbonate-free pyroxenite (Gerbode and Dasgupta, 2010). Consequently, carbonated eclogite may melt first during the partial melting of a hybrid peridotite-pyroxenite source, meaning that this material may dominate the contributions to melts generated by low-degree partial melting (Gerbode and Dasgupta, 2010; Huang et al., 2015). Experimental studies and field observations indicate that carbonatitic melts evolve into carbonated silicate melts, and finally to alkali basaltic melts with increasing degrees of partial melting (Dasgupta et al., 2007; Kiseeva et al., 2013; Zhang et al., 2017). Therefore, the Vietnamese alkaline basalts that were generated by low-degree partial melting are likely to contain more isotopically light incipient carbonatitic melts than their tholeiitic counterparts. 6.4. Implications of the chemical heterogeneity of the mantle source of late Cenozoic basalts from southern to central Vietnam and its implications The origin and spatial distribution of chemical heterogeneity in the mantle source of Vietnamese basaltic volcanism was investigated by plotting representative trace element ratios and Sr-Nd-Pb-Mg isotopic 0.7055

compositions of basaltic rocks (SiO2 b 55 wt%) as a function of latitude or longitude (Figs. 10, 11, S13, and S14). There is no meaningful longitudinal (i.e., eastward or westward) variation in the geochemical compositions (Figs. S13 and S14). This observation, combined with the fact that there is no relevant west-to-east age propagation recorded within the Vietnamese basalts, suggests that the eastward extrusion of asthenospheric material possibly related to the Indo-Eurasian collision event (Hoang et al., 1996; Hoang and Flower, 1998) may not have been a factor in the generation of theses basalts. In comparison, the basaltic magmatism in southern and central Vietnam can be divided into three latitudinal geochemical regions, namely southern Vietnam (10°N–12°N), south-central Vietnam (12°N–14°N), and central Vietnam (14°N–16°N). The south-central basalts have the highest Nb/Nb*, Ta/Ta*, Ti/Ti*, Nb/La, Sr/Nd, Sr/Sr*, Eu/Eu*, and Ba/Th ratios (Fig. 10a–h), whereas basalts from the central region tend to have slightly higher Ta/Ta*, Ti/Ti*, Sr/Sr*, and Ba/Th ratios than those from the southern region (Fig. 10b, c, f, h). Rutile is a common accessory phase in eclogites, and a major host mineral for the HFSE (e.g., Nb, Ta and Ti; Schmidt et al., 2009). Accessory phase rutile melts out after about N5–10% of partial melting (Pertermann and Hirschmann, 2003),

(b)

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Fig. 11. Latitude (° N) vs. 87Sr/86Sr (a), 143Nd/144Nd (b), 207Pb/204Pb (c), 208Pb/204Pb (d), [(208Pb/204Pb)sample – 29.532]/[(206Pb/204Pb)sample – 9.306]. Symbols are as in Fig. 4.

10

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Pb/206⁎Pb (e), and δ26Mg (f) values of Vietnamese basaltic rocks.

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indicating that the melting of rutile-bearing eclogites can thus explain the elevated Nb/Nb*, Ta/Ta*, Ti/Ti*, and Nb/La ratios of the basalts in the region. In addition, the high Sr/Sr*, Eu/Eu*, Ba/Th, and Sr/Nd ratios recorded in the melts may be caused by the involvement of source regions containing recycled plagioclase-rich cumulate gabbroic material, as discussed above. These observed differences in the Sr/Sr*, Eu/Eu*, Ba/Th, Sr/Nd, Nb/La, Nb/Nb*, Ta/Ta*, and Ti/Ti* ratios between the three regions suggests that the mantle source for these basalts contains variable amounts of recycled rutile-bearing gabbroic eclogite. Basalt 87Sr/86Sr, 207Pb/204Pb, 208Pb/204Pb, and 208Pb*/206Pb* isotopic ratios increase somewhat from the southern to the central regions, whereas 143Nd/144Nd values decreases (Fig. 11a–e). These trends indicate varying amounts of recycled sediment in the mantle source for the basalts, as discussed above. This suggests that the mantle source of the basalts within the central region contains the largest proportion of recycled sedimentary material. However, the Mg isotopic compositions of these basalts do not vary with latitude, unlike their Sr-Nd-Pb isotopic compositions (Fig. 11f). Basalt δ26Mg values are relatively constant across all of these regions (Fig. 11f), indicating that the carbonates within the altered recycled oceanic crust are uniformly distributed throughout the mantle source of the magmas that formed the Vietnamese basalts. Subduction of the (paleo-)Pacific Plate beneath the Eurasian Plate has played a significant role in a number of magmatic events since the late Mesozoic, as evidenced by the widespread Late Cretaceous calcalkaline magmatism and Sn\\W mineralization along the continental margin of East Asia including eastern Vietnam (Cheng et al., 2016; Nguyen et al., 2004a, 2004b; Shellnutt et al., 2013). Seismic tomographic observations beneath northeast Asia indicate that the subducted Pacific slab flattens and stagnates in the mantle transition zone (ca. 410–660 km depth), with its leading edge reaching a longitude of 120° E (e.g., Huang and Zhao, 2006; Zhao et al., 2009). However, slab stagnation and buckling in the transition zone are only temporary, meaning that the slab is likely to eventually descend into the deeper lower mantle (Fukao et al., 2001). Machetel and Humler (2003) suggest that the Cretaceous mantle thermal high may be a consequence of a catastrophic mantle avalanche that began at the 670 km discontinuity. In addition, Fukao et al. (2001) suggested that Rayleigh–Taylor-type gravity instability occurred extensively throughout the transition zone during the Eocene, resulting in a major reorganization of global plate motion. This means that the majority of the avalanche slabs in the western Pacific may have reached the core–mantle boundary, whereas the present-day subducting slab still resides in the transition zone (Fukao et al., 2009). Late Cenozoic intraplate volcanism in Vietnam may be related to the nearby Hainan plume, which has been imaged as a subvertical zone extending to a depth of ~1000 km (e.g., Huang and Zhao, 2006; Xia et al., 2016). An et al. (2017) estimated that the effective melting pressure (Pf = 30–33 kbar) and temperature (1470–1480 °C) of the southern Vietnamese basalts are within the ranges of the primary Hainan basalts (Pf = 18–32 kbar; T = 1420–1530 °C), thereby supporting a sub-lithospheric Hainan plume origin. In addition, the ubiquitous presence of recycled oceanic crustal materials in the source of the Vietnamese basalts (as outlined above) rules out any significant role of the lithospheric mantle. Instead, it is likely that the paleoPacific slab that resides in the lower mantle was entrained into the rising Hainan plume, causing the basaltic volcanism recorded in Vietnam. In summary, we suggest that the enriched components within the mantle source of the magmas that formed the Vietnamese basaltic rocks were derived from recycled paleo-Pacific oceanic crustal material and consist of varying proportions of sediment, basalt, and gabbro. The south-central region of basaltic volcanism appears to be dominated by the lowermost portion of the residual slab, which consists mainly of rutile-bearing plagioclase-rich gabbroic eclogite. In comparison, the uppermost portion of the recycled slab, including sediments and basalts with minor amounts of gabbro, may be a major constituent of the source of the basalts located within the central region. The southern region is

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Fig. 12. Distribution of recycled oceanic crust (sediment, basalt, and gabbro) in the mantle source of late Cenozoic basaltic rocks from southern to central Vietnam.

dominated by recycled upper oceanic crustal material, which is basaltrich and contains little or no sediment (Fig. 12). 7. Conclusions (1) The mantle source of late Cenozoic Vietnamese basaltic rocks is dominated by garnet peridotite and recycled oceanic crustal material (sediment, basalt, and gabbro). (2) The light Mg isotopic compositions of the Vietnamese basaltic rocks are most likely inherited from recycled carbonates in the mantle source. The alkaline basalts tend to have slightly lighter Mg isotopic compositions than the tholeiite basalts, suggesting that the lower-degree partial melting that generated the alkaline basalts involved more isotopically light incipient carbonatitic components than is the case for the tholeiite basalts. (3) The basaltic magmatism in the region between southern and central Vietnam can be geochemically divided into southern (10°N–12°N), south-central (12°N–14°N), and central (14°N– 16°N) regions. The basalts in the southern region most likely have recycled basalt as their primary enriched component, whereas the central region is dominated by the uppermost portion of the residual slab (sediment + basalt ± gabbro) and the south-central region basalts obtain their enriched signature from rutile-bearing recycled gabbroic eclogite. (4) The Vietnamese basaltic magmatism may have been generated as a result of entrainment of the accumulated paleo-Pacific slab into the rising Hainan plume at lower-mantle depth.

Supplementary data to this article can be found online at https://doi. org/10.1016/j.lithos.2017.11.020. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF2016R1A2B4007113). Insightful reviews by Greg Shellnutt and Martin Flower greatly improved the manuscript. We thank Andrew Kerr for the editorial handling.

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