The evolution of the lithospheric mantle beneath Ia Bang, Pleiku plateau, Central Vietnam

Accepted Manuscript Full length article The evolution of the lithospheric mantle beneath Ia Bang, Pleiku plateau, Central Vietnam The Cong Nguyen, Youngwoo Kil PII: DOI: Reference:

S1367-9120(18)30511-X https://doi.org/10.1016/j.jseaes.2018.12.011 JAES 3727

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Journal of Asian Earth Sciences

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26 July 2018 10 December 2018 11 December 2018

Please cite this article as: Cong Nguyen, T., Kil, Y., The evolution of the lithospheric mantle beneath Ia Bang, Pleiku plateau, Central Vietnam, Journal of Asian Earth Sciences (2018), doi: https://doi.org/10.1016/j.jseaes.2018.12.011

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The evolution of the lithospheric mantle beneath Ia Bang, Pleiku plateau, Central Vietnam

The Cong Nguyen, Youngwoo Kil* Department of Energy and Resources Engineering, Chonnam National University * Corresponding author: E–mail: [email protected] Telephone: +82–62–530–1731 Fax: +82–62–530–1729

ABSTRACT: Mantle xenoliths are scattered in alkali basalt in central and southern Vietnam. The xenoliths from Ia Bang, Pleiku plateau, consist of fertile spinel lherzolites and refractory spinel harzburgites. The spinel peridotites display varying textures from protogranular through porphyroclastic to equigranular. Primitive mantle-normalized rare earth elements (REE) of clinopyroxene in the peridotites reveal depleted, spoon-shaped, and enriched patterns, suggesting a depletion event followed by various enrichment processes. The correlation between (Yb)n and (Y)n of clinopyroxene in spinel peridotites indicates that the subcontinental lithospheric mantle (SCLM) beneath Ia Bang has been affected by 1%~20% fractional melting. The presence of glass and secondary clinopyroxene, coupled with the enrichment of light REEs (LREEs), high field strength elements (HFSEs), and large ion lithophile elements (LILEs) without introductions of new minerals suggests that the xenoliths underwent cryptic metasomatism. Metasomatic agents were identified as Na-alkali silicate, carbonate-rich melts, and H2O-CO2 fluids. The peridotites were at equilibrium temperatures of 841oC ~1131oC before being brought up by alkali

basalt from depths of 36 to 50 km. The SCLM beneath Ia Bang experienced a thinning process in which the refractory lithosphere was thinned and replaced by asthenosphere during the Phanerozoic, following the asthenosphere-lithospheric interaction. Keywords: Ia Bang, lithospheric mantle, spinel peridotite, metasomatism

1. Introduction

Mantle xenoliths are of key importance for directly investigating the heterogeneities in composition, mantle processes, and evolution of the SCLM (Chen, 2017; Griffin et al., 1998; Nixon, 1987; O’Reilly and Griffin, 2013; Pearson and Nowell, 2002; Pearson et al., 2003). However, the SCLM beneath the Indochina Block, Southeast Asia, has not yet received sufficient attention from petrologists. Previous studies only mentioned or generally described the characteristics of mantle xenoliths within very limited areas in Thailand (Promprated et al., 1999; Sutthirat et al., 2018) and Vietnam (Hoang and Flower, 1998; Hoang, 2005; Xuan, 2006). The Ia Bang area is situated on the Pleiku plateau, about 24 km southeast of Pleiku city, Gia Lai province (Fig. 1). The Ia Bang xenoliths, which are enclosed in alkali basalt, were rarely mentioned in the early studies by Hoang (2005) and Xuan (2006). The samples in these studies were all spinel lherzolites with high contents of clinopyroxene (10%~15%) and displayed protogranular texture. The enrichment in REE and some incompatible trace elements indicated that the lherzolites experienced silicate metasomatism. Isotope compositions of the mantle xenoliths were different from those of the host alkali basalt, suggesting that they did not share the same source of primary magma. However, information on harzburgite was absent in the previous studies. Moreover, the detailed characteristics of the mantle xenoliths remain unclear,

and there persists a lack of petrological data, geothermometry, and knowledge of extraction depth and metasomatism. Here, we aim to provide a systematic study on the petrography and chemistry of the mantle xenoliths from Ia Bang. The lherzolites and harzburgites were identified as residual mantle products after various degrees of partial melting. In addition, they were altered by different episodes of metasomatism. The temperature conditions were estimated to clarify the depths of the equilibrium state for the mantle xenoliths prior to their transport to the surface. Eventually, through evidence of the thinning and replacement of the lithospheric mantle, we were able to interpret the evolution of the SCLM beneath Ia Bang. This work provides a premise for elucidating the characteristics of the lithospheric mantle under Vietnam in order to understand the evolution of the SCLM beneath the Indochina Block.

2. Geological background

The Late Cenozoic basalts in the Indochina Block erupted following the India-Asia collision and have been described as a “Diffuse Igneous Province” (Hoang and Flower, 1998). In the Vietnam region, the intraplate basalts are spread over a large area, mostly in central and southern localities. The Pleiku basalts cover about 4,000 km2 in the central and they exhibit two eruption episodes. The early eruption of quartz and olivine tholeiite formed a shield that is overlain by thinner, later-stage eruptions of olivine tholeiite, alkali basalt, and basanite (Hoang et al., 1996, 2013). The uppermost alkali basalt (0.4–0.2 Ma) was generally extruded from northwestsoutheast aligned volcanoes (Hoang et al., 1996). Xenolith-bearing alkali basalt is scattered in the continent and continental shelf in central and southern Vietnam (Fig. 1). Of these, the Pleiku

plateau is considered as the largest area to host numerous xenoliths from the SCLM, e.g., spinel lherzolite, spinel harzburgite, dunite, websterite, pyroxenite, and megacrysts of pyroxene and olivine. Alkali basalt in the Ia Bang area, Pleiku plateau erupted during the Quaternary period (2.4~0.2 Ma; Hoang et al., 2013) and contained various types of such mantle xenoliths.

3. Petrography

Twenty-three peridotite xenoliths were carefully selected for modal analysis. These peridotites were composed of olivine, orthopyroxene, clinopyroxene, and spinel without a hydrous phase, e.g., amphibole and phlogopite; therefore, they are classified as spinel peridotites. In a ternary diagram using modal composition (Table 1), 13 samples fall into the lherzolite field and 10 samples fall into harzburgites (Fig. 2). The former group is composed of olivine (47%~75%), orthopyroxene (16%~30%), clinopyroxene (5%~22%), and spinel (1.3%~2.7%), whereas the latter are composed of olivine (60%~78%), orthopyroxene (19%~36%), clinopyroxene (2%~4%), and spinel (0.3%~1.6%). The peridotites’ textures are primarily protogranular and porphyroclastic (Figs. 3a, 3b) with a small amount of equigranular (Mercier and Nicholas, 1975). Some xenoliths show transitions between protogranular and porphyroclastic textures or between porphyroclastic and equigranular textures. They reflect an increasing intensity of deformation from coarse grain through medium to fine grain types, implying that the lithosphere beneath Ia Bang is deformed SCLM. Olivine grains are relatively large (up to 3mm) and display rounded to irregular shapes. Some grains have polygonal shapes, creating triple junctions at grain boundaries (Fig. 3a). Kinked and undulose extinction features were commonly observed. In some samples, olivine

occurs as mineral inclusions in orthopyroxene and clinopyroxene (Figs. 3c, 3d). The size of neoblastic olivine in the equigranular samples is about 0.3 mm, and the neoblastic olivine has a typically round or polygonal shape with curved grain boundaries. Less commonly, olivine crystals are elongated, indicating a weak preferential orientation (Fig. 3e). Somewhere at margins of the mantle xenoliths, large olivine grains have been crosscut by the basaltic melt into fine grains. Orthopyroxene grains exhibit equant or tabular morphologies and show irregular, curved, or trip junction grain boundaries. The size of orthopyroxene is the largest (up to 4 mm) among the mineral phases in the xenolith samples. The orthopyroxene also shows undulatory phenomena as olivine; especially, some orthopyroxene grains contain parallel exsolution lamellae of clinopyroxene. Occasionally, orthopyroxene hosts olivine and clinopyroxene as mineral inclusions. Clinopyroxene grains with a protogranular texture are less abundant and smaller in size (~2mm) compared to olivine and orthopyroxene. With the porphyroclastic and equigranular textures, they occur in void spaces as neoblast grains (~0.2 mm). They are mostly subhedral or have an irregular shape. Some clinopyroxene grains show a sieve texture (Fig. 3f) in which their surfaces are coated by small patches of melts. Secondary clinopyroxene grains are small (~0.1mm), have a rounded to irregular shape, and present as clusters or interstices between primary minerals or along their margins. They were formed by the interaction between basaltic magmas and the peridotites. Spinel occur as discrete, dispersed grains (0.1~0.5 mm) with irregular, vermicular, or weakly elongated morphologies. They are dispersed within the neoblast or present as intergrowths in olivine and pyroxene hosts.

Glasses are dark brown in color and occur in some spinel peridotites as veins or patches, implying infiltrations of magma fluids. They infrequently present as skeletal forms or quenched crystals as plagioclases. Glasses are often observed together with secondary clinopyroxene grains in the peridotites (Figs. 3g, 3h).

4. Analytical results

4.1. Major elements Table 2 shows representative major elements of the minerals in the Ia Bang spinel peridotites, and Table 3 shows representative average major elements of whole-rock peridotite, mineral inclusion, secondary clinopyroxene, glass, and host rock (alkali basalt) from Ia Bang. The Fo (100*Mg/[Mg+Fe2+]) of olivine in Ia Bang varies from 89.0 to 91.3, and it lies within the composition range of the subcontinental lithosphere (Arai, 1994). In addition, olivine grains in refractory harzburgites have slightly higher Fo content (89.0–91.3) than those in fertile lherzolites (89.0~90.9). Olivine exhibits very low concentrations of Al2O3 and CaO (<0.1 wt%). The NiO and MnO contents of olivine range from 0.20 to 0.31 wt% and 0.13 to 0.21 wt%, respectively. Orthopyroxene with compositional ranges of Wo 0.7-1.6En87.7-90.8Fs7.8-10.8 is categorized as enstatite. Mg# (Mg/[Mg + Fe2+]) of orthopyroxene varies from 0.89 to 0.92, similar to the Fo of olivine. Orthopyroxene in the lherzolites contains ~0.61 wt% CaO, which is similar to that of most harzburgites. In addition, orthopyroxene in lherzolites has lower Mg# (~0.90 vs. ~0.92) and Cr2O3 (~0.36 vs. 0.53 wt%), but higher Al2O3 (~3.98 vs. 2.52 wt%) than those in harzburgites. Clinopyroxene with compositional ranges of Wo45.3-50.7En44.3-51.0Fs3.0-6.6 is classified as Cr–

diopside. Clinopyroxene in the lherzolites has lower Mg# (~0.909 vs. ~0.924) and Cr# (Cr/[Cr+Al]) (~0.13 vs. ~0.24) contents than harzburgites. Clinopyroxene exhibits relatively high CaO with identical values in most peridotites (~20.44 wt%). Lherzolites have a higher content of Al2O3 (2.83~7.00 wt%) than do harzburgites (2.22~5.52 wt%). Secondary clinopyroxene has high contents of Na2O (1.99~2.68 wt%) and Cr2O3 (3.44~4.49 wt%), but it is relatively low in Al2O3 (0.41~1.46 wt%). Spinel is categorized as Cr–spinel and has Cr2O3 and Al2 O3 contents ranging from 6.34 to 50.00 wt% and 24.77 to 63.24 wt%, respectively. These values cover a large field of continental peridotite (Arai, 1994). Spinel in lherzolites has lower content of Cr# (0.06~0.36 vs. 0.16~0.58) but higher Mg# (0.73~0.81 vs. 0.62~0.68) than do harzburgites. Glass has low MgO (~2.25 wt%) content and high Al2O3 (~19.63 wt%) and CaO (~2.72 wt%). It exhibits high alkali compositions, i.e., NaO (~3.83 wt%) and K2O (~2.23 wt%). Given that the N2O/K2O ratios range from 1.18 to 2.94, it is classified as Na-alkali silicate glass (Cortolti et al., 2000). Whole-rock peridotite compositions were calculated by the mass balance of mineral chemistries and modal abundances. Compared to fertile lherzolites, the harzburgites overall have slightly lower whole-rock Al2O3 and CaO, but higher MgO, i.e., 0.57~1.50 wt% vs. 1.63~3.37 wt%, 0.55~0.95 wt% vs. 1.25~3.27 wt%, and 42.72~45.47 wt% vs. 34.68~43.05 wt%, respectively. 4.2. Trace elements in clinopyroxene and glass Clinopyroxene in spinel peridotites is the primary host of trace elements (Nagasawa et al., 1980; Stosch, 1982); therefore, it was used to investigate behaviors of trace compositions in the Ia Bang mantle xenoliths. Representative trace element compositions of clinopyroxene from the

peridotite xenoliths are shown in Table 4. (REE)n patterns of clinopyroxene display depleted, spoon-shaped, and enriched patterns (Figs. 4a, 4b, 4c). The depleted patterns are observed in the fertile lherzolite samples (Fig. 4a), and this subset exhibits a depletion of LREEs compared with the middle and heavy REEs (MREEs and HREEs). Values for (La/Yb)n and (Sm/Yb) range from 0.05 to 0.61 and from 0.12 to 1.09, respectively. The spoon-shaped patterns are also obtained in the lherzolites (Fig. 4b), and they reveal a depletion in the HREEs to MREEs with (Sm/Yb)n ratios of 0.23~1.54, followed by an enrichment in the LREEs with (La/Yb)n of 0.39~12.11. Enriched patterns are observed in the refractory harzburgites (Fig. 4c), and these patterns are featured by the enrichment of LREEs and MREEs; i.e., the (La/Yb)n values are 7.06~83.74 and the (Sm/Yb)n values are 1.54~13.92. Clinopyroxene displays negative anomalies in some HFSEs, e.g., Ta, Zr, and Ti; conversely, they reveal weak positive anomalies in Sr. In addition, they exhibit an enrichment of highly incompatible elements, i.e., Th and U. (REEs)n of secondary clinopyroxene display an enriched pattern. All REE contents of the secondary clinopyroxene are higher than those of primary clinopyroxene (Fig. 4d). Their (La/Yb)n and (Sm/Yb)n values are 4.99~12.25 and 7.07~9.00, respectively. The secondary clinopyroxene reveals negative anomalies in HFSEs, i.e., Ti and Hf. Glass exhibits values of (La/Yb)n from 8.12 to 20.13 and values of (Sm/Yb)n from 2.44 to 6.50. Their (REE)n display similar depleted patterns to the host rock alkali basalt (Fig. 4d), indicating that they originated from the host.

5. Discussion

5.1. Partial melting

The varieties in modal and major compositions of the Ia Bang mantle xenoliths are representative of the variable degrees of partial melting from a mantle source. Clinopyroxene in spinel peridotite is the principal mineral that melted during partial melting. It is manifested by a positive correlation between clinopyroxene/orthopyroxene and clinopyroxene abundances (Fig. 5a).

The inverse correlation between Al2O3 and MgO in

clinopyroxene indicates a depletion in the “basaltic” composition followed by an enrichment in the “refractory” composition through the melting process (Fig. 5b). Furthermore, the positive correlations between whole rock MgO and olivine modal and between whole rock CaO and clinopyroxene modal also reflect the nature of partial melting and melt extraction (Figs. 5c, 5d). The degree of partial melting of peridotite xenoliths from Ia Bang was calculated by using the less incompatible Y and Yb in clinopyroxene with the assumed initial bulk composition values (C0) for the primitive mantle (Hofmann, 1988; Norman, 1998). The obtained values were then compared with those from the primitive mantle after batch and fractional melting. In comparison with the Y and Yb values from the primitive mantle after batch melting, the Ia Bang peridotites yielded values from 1% to 90% (Fig. 6a). These high values have been regarded as unrealistic for degrees of partial melting (mostly less than 40%) for peridotites (Arai, 1994). The correlations of (Y)n and (Yb)n in the clinopyroxene in the fractional melting model (Fig. 6b) indicate that the samples experienced very low to moderate degrees of fractional melting (less than 1% to 20%). For this, the highest degree of depletion (~20%) was obtained in the harzburgite sample and the lowest degree of depletion (<1%) was obtained in the lherzolite sample. 5.2. Criptic metasomatism Numerous studies have described that metasomatism is one of the mantle processes and it

frequently occurs on spinel peridotite xenoliths in the SCLM (Goldschmidt, 1922; Harte, 1983; Liang and Elthon, 1990; Menzies et al., 1985; Roden and Murthy, 1985; Wiechert et al., 1997; Xu et al., 2000). The spinel peridotites from Ia Bang are absent of hydrous minerals, e.g., mica and amphibole, which suggests a lack of modal metasomatism. The spoon-shaped patterns of the lherzolites and enriched patterns of the harzburgites display the various enrichments of LREEs. In addition, the increases in incompatible trace elements, such as Th, U, La, Hf, Eu, Zr, Sr, Rb, and Pb, indicate that they have undergone cryptic metasomatism (Dawson, 1984). Three metasomatic agents are supposed to be likely to cause cryptic metasomatism in the SCLM, including silicate melts, carbonate melts, and H2O-CO2 fluid (Dawson, 1984; Roden and Murthy, 1985). The sample plots in the (La/Yb)n vs. Ti/Eu diagram demonstrate that the mantle xenoliths beneath Ia Bang experienced both silicate and carbonatite metasomatisms (Fig. 7). It has been alleged that glass in peridotite xenoliths is the reaction product of primary mantle assemblage and migrating melt (Coltorti et al., 2000, Shaw and Edgar, 1997; Shaw et al., 2006). Therefore, the chemical compositions of glass can be used to estimate reactions and to identify original metasomatic agents. The Na2O/K2O ratios of glass in the Ia Bang peridotites range from 1.18 to 1.94, corresponding to Na-alkali silicate melt (Coltorti et al., 2000). In comparison, the (LREE)n patterns of glasses are similar to those in the alkali basalt host rock, but the former compositions have lower LREE contents than the latter. This unconformity indicates that the original melts experienced reduced trace elements during the reaction and the subsequent formation of the glass was greatly controlled by the reaction. The presence of a sieve texture on the clinopyroxene grains also provides evidence of the reaction between melts and the mantle xenoliths (Shaw et al., 2006). In addition, the conjunctions between the thin veins of glasses and the host rock indicate that the glasses could originate from their host magma. Secondary Cr-rich

clinopyroxene is commonly observed in situ with glass in the Ia Bang peridotites. They are also generally considered to be the reaction products of peridotite and migrating silicate melts (Kelemen et al., 1992, 1993; Shaw and Edgar, 1997; Shaw et al., 2006). In addition to the glass and secondary clinopyroxene, the presence of composite xenoliths in the Ia Bang peridotites strongly supports silicate metasomatism. Through the reaction with alkali-silicate melts, both olivine and orthopyroxene in the peridotites were dissolved and clinopyroxene was precipitated. The melts first reacted with the peridotites and formed glass, then crystallized clinopyroxene and orthopyroxene, or they mixed with the original melts and continued to crystallize clinopyroxene, producing pyroxene-rich lithologies (Coltorti et al., 2000; Kelemen et al., 1992). The Ia Bang harzburgites display very high contents of LREEs, enriched HFSEs (e.g., Th, U, La, Hf, Eu, and Zr) and LILEs (e.g., Sr, Rb, and Pb), implying that they had undergone carbonatite metasomatism. In addition, the remarkable Ti and Zr negative anomalies in the chondrites-normalized trace element spidergrams (Fig. 4c), and the low Ti/Eu relative to high (La/Yb)n, indicate that the refractory harzburgites were affected by carbonate-rich melt (Bizimis et al., 2003; Coltorti et al., 1999; Nelson et al., 1988). Noticeably, these expressions were absent in the fertile lherzolites. Therefore, we suggest that two episodes of metasomatism occurred in the SCLM beneath Ia Bang: first, the harzburgites were modified by carbonate metasomatism, and subsequently, both lherzolites and harzburgites were affected by silicate metasomatism during transportation to the surface by the alkali basalt host rock. The carbonate-rich agent is supposed to upwell from the asthenosphere, which was caused by the subduction of the Pacific Plate under the Eurasia Plate (Nguyen et al., 2004; Taylor and Hayes, 1983). This interaction resulted in the thinning and replacement of the lithospheric mantle beneath Ia Bang (discussed in a later section). H2O-CO2 fluids released from the subducting slab are possibly the additional

metasomatic agent. Infiltrations of H2O-CO2 fluids can affect the lithospheric mantle, producing new phases such as amphibole or phlogopite, and/or enrich incompatible trace elements (O’Reilly and Griffin, 1988; Roden and Murthy, 1985). The absence of the new minerals in the spinel peridotites with enrichment in Th, U, and LREE suggests that the H2O-CO2 fluids contributed to the metasomatism to a lesser extent. 5.3. Geothermometry and extraction depths The minerals of the Ia Bang mantle xenoliths display a triple junction in grain boundaries. In the chemical composition, a homogeneous major composition between cores and rims as well as an absence of reaction rims indicate that the spinel peridotites have reached an equilibrium state before being captured by the alkali basalt host rock (Kil, 2002). In addition, the similarities in the major compositions of the inclusions and primary phases also offer evidence of equilibrium (Li et al., 2014). Coexisting minerals within the peridotites, therefore, can be used as parameters for estimating equilibrium temperatures and pressures. Nevertheless, current geobarometers are unrealistic for accurately determining the equilibrium pressure for spinel peridotite (Kil, 2002; Medaris et al., 2014). Equilibrium temperatures have been calculated at an assumed pressure of 15kb, which is the mid-range of pressures in the stability field of spinel lherzolite (Green and Ringwood, 1970). The temperature results were obtained using single minerals, mineral pairs, or mineral assemblages, i.e., geothermometers of two–pyroxene (Bertrand and Mercier, 1985; Brey and Köhler, 1990; Wood and Banno, 1973), single–orthopyroxene (Brey and Köhler, 1990), and olivine–orthopyroxene–spinel (Witt‐Eickschen and Seck, 1991). The calculated data are shown in Table 5. The geothermometers in Brey and Köhler (1990), based on a calcium exchange reaction between orthopyroxene and clinopyroxene, were considered the most accurate geothermometers due to their independence of pressure. The equilibrium temperatures estimated

for spinel peridotites from the Ia Bang range from 841oC to 1131oC. Mineral geothermometry is currently an indirect means for estimating the extraction depths of mantle xenoliths (Medaris et al., 2014). The extraction depths of spinel peridotites from Ia Bang can be deduced from the equilibrium temperatures, heat flow data combined with the geothermal gradient, and Moho depth using the method of Sclater et al. (1980) and Medaris et al. (2014). No heat flow data have been reported in the continental area of Vietnam. Comparatively, heat flow on the surface of Ia Bang was inferred from the values of the closest areas, i.e., at the boundary between Laos and Thailand and the boundary between South China and Vietnam (The international heat flow commission, 2011; Toshiyasu and Uyeda, 1995). From this, the heat flow values in Ia Bang were suggested to be around 70 to 80 mW/m2, with an assumed average heat flow of 75mW/m2 used to calculate the geothermal gradient. The Moho is located at a depth of 34km under the continental crust and the base of the lithospheric mantle is about 85km (Starostenko et al., 2009). The derived depths of the Ia Bang spinel peridotites were determined to be from 36 to 50km, in the uppermost part of the lithospheric mantle (Fig. 8). 5.4. Mantle evolution beneath Ia Bang Interaction of the asthenosphere-lithosphere resulting in thinning of the lithosphere has been commonly observed in the SLCM worldwide (Boyd, 1989; Griffin et al., 1998; Kusky et al., 2007; Menzies et al., 1985, 1987; O’Reilly et al., 2001). In the Mg# of olivine vs. modal olivine diagram, most lherzolites with low Mg# of olivine (≤90) are in the Phanerozoic field and most harzburgites with higher Mg# of olivine (>91) are in the Proterozoic field (Fig. 9). This distinction suggests that the SCLM beneath Ia Bang has experienced an interaction event. The refractory harzburgites, which were considered to be the old lithospheric mantle, were thinned and partly replaced by younger fertile lherzolites that uprose during the Phanerozoic. This

mechanism generally occurred beneath the North and South China Blocks (Li et al., 2014; Liu et al., 2012, 2013; Lu et al., 2013; Sun et al., 2012; Wang et al., 2010; Xu et al., 2000; Zheng et al., 1998, 2004). It was generally recognized that the mantle beneath the eastern Eurasia continent was disturbed by the westward subduction of the Pacific oceanic crust through the late MesozoicCenozoic time (Arai et al., 2007; Cheng et al., 2016; Isozaki, 1996; Isozaki et al., 2010; Lapierre et al., 1997; Maruyama and Seno, 1986; Nguyen et al., 2004; Taylor and Hayes, 1983). In central and southern Vietnam, the voluminous plutonic and coeval volcanic rocks are the products of the subduction event related to the western Pacific Plate (Nguyen et al., 2004; Taylor and Hayes, 1983). The subducting oceanic slab has disturbed and decompressed the asthenospheric mantle beneath Ia Bang, resulted in an uprising of hot magmas and fluids. Subsequently, the refractory lithospheric mantle was thinned and replaced by new fertile materials. Carbonatitic melts derived from the upwelling asthenosphere possibly modified the refractory harzburgites. The influence of the subducting Pacific plate leading to the thinning processes is also widely recognized in the lithospheric mantle beneath southeastern China (Liu et al., 2012; Lu et al., 2013; Snyder et al., 1997; Zheng et al., 1998). Therefore, we suggest that the SCLM beneath Ia Bang, Indochina Block has shared an analogous lithospheric evolution with the South China Block during the Phanerozoic time.

6. Conclusions

(1) The spinel peridotites from Ia Bang have variations in modal compositions and chemistries, indicating the differentiations between fertile lherzolites and refractory harzburgites.

The peridotites exhibit various textures, from protogranular through porphyroclastic to equigranular, suggesting a deformation in the SCLM beneath Ia Bang. The (Y)n and (Yb)n correlations in clinopyroxene indicate that the mantle xenoliths underwent 1%~20% fractional melting. (2) The primitive mantle-normalized REEs of clinopyroxene display depleted, spoonshaped, and enriched patterns. In addition, trace elements exhibit the increase of HFSEs and LILEs, implying that the peridotites are affected by multiple metasomatism. From this, the early episode of carbonatite metasomatism coupled with H2O and CO2 fluids modified the harzburgites and the later episode of silicate metasomatism affected both lherzolites and harzburgites during their transport to the surface. The equilibrium temperatures of the Ia Bang spinel peridotites range from 841oC to 1131oC, as deduced from Brey and Köhler (1990) geothermometers. Combined with heat flow data, Moho depth, and lithospheric base, the peridotites were detectably derived from 36~50km in the uppermost mantle. (3) The Mg# in olivine and olivine modal correlations unveiled that the refractory harzburgites were present in the Proterozoic and that fertile lherzolites were produced in the Phanerozoic. This observation represents the thinning and replacement of the old lithospheric mantle by the younger magmas rising from the asthenosphere as a result of westward subduction of the Pacific Plate.

Acknowledgments We are grateful to Mr. Dinh Quang Sang of PetroVietnam University for supporting to collect the samples from Ia Bang. This work was supported by 1) the Energy Efficiency and Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant

funded by the Korea Government Ministry of Trade, Industry and Energy (No. 20152510101980)

and

2)

the

National

Research

Foundation

of

Korea

(NRF:

2018R1D1A3B07048228).

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Appendix Methodology Mantle xenoliths were selected from fresh samples without any secondary alteration or contaminated materials. Modal compositions were estimated on thin sections (ca.70 µm thickness) using ImageJ software (Abramoff, 2004). Each mineral phase of spinel peridotites was separated under a binocular microscope for major compositional analysis. Subsequently, quantities of major element compositions for olivine, orthopyroxene, clinopyroxene, spinel, and silicate glass were determined using an Electron Micro Probe Analyzer (EPMA, Shimadzu 1600) at Chonnam National University. Both the core and rim of the minerals were analyzed for comparison. The operating conditions were established at 15 kV acceleration voltage, 1 µm

beam size, and 20 s peak counting time. The counting precision for EPMA measurements was ±0.5%. Ferric concentrations for the total iron of minerals were estimated using the stoichiometric criteria (Droop, 1987). Selected geothermometers were used to calculate equilibrium temperatures using the PTMafic (v.2.00) software package (Soto and Soto, 1995). Trace compositions of primary clinopyroxene, secondary clinopyroxene, and glass were determined

using

a

laser

ablation

inductively

coupled

plasma

mass

spectrometer

(193nm Excimer LA–ICP–MS) at the Korea Basic Science Institute (KBSI). The analysis process was conducted at 10 Hz frequency, 5 J/cm2 pulse energy, 110 µm beam size for primary clinopyroxene, and 65 µm beam size for secondary clinopyroxene and glass. The calcium contents from EPMA data and NIST 612 were used as internal and external standards, respectively. The Glitter software package was adopted for data reduction. The relative standard deviation (RSD) was within 5%~10%.

Figure 1. Localities of the study area and other major xenolith-bearing alkali basalt in central and southern Vietnam. IB = Ia Bang, EL = EaHleo, DT = Dakton, PR = Prenn, DD = Don Duong, PH = Phu Hiep, NN= Nui Nua, XT = Xuan Tan, BR = Ba Ria, PQ = Phu Quy. Map modified from Hoang et al. (1996, 2013), Sone and Metcalfe (2008), and Xuan et al. (2004).

Figure 2. The modal composition of spinel peridotites from Ia Bang. Ol: olivine, Opx: orthopyroxene, Cpx = clinopyroxene.

Figure 3. Thin section photomicrographs of spinel peridotites from Ia Bang. a) Protogranular texture, kinked olivine displays trip junction boundary (IB13); b) Porphyroclastic texture, large orthopyroxene surrounded by olivine grains (IB14); c-d) Olivine inclusion in clinopyroxene (IB64);e) Oriented olivine and orthopyroxene grains (IB1).f) Sievetexture on clinopyroxene (IB75). g) Small secondary clinopyroxene aggregates together with Na-alkali brown glasses (IB69). h) Secondary clinopyroxene and glass in BSE image. Ol: olivine, Opx: clinopyroxene, Cpx: clinopyroxene, CpxII: secondary clinopyroxene, Gl: glass.

Figure 4. Primitive-mantle REE patterns and chondrite-normalized trace-element spidergrams of the peridotites from Ia Bang. a) Depleted patterns for lherzolites. b) Spoon-shaped patterns for lherzolites. c) Enriched-patterns for harzburgites. d) Patterns for average values of glass, host rock, and secondary clinopyroxene. Values of host rock are from Hoang et al. (2013).The primitive mantle and chondrite data are from Hofmann (1988) and Sun and McDonough (1989), respectively.Cpx: clinopyroxene, Avg: average value.

Figure 5. Diagrams of various characteristics of spinel peridotites from Ia Bang. a) Cpx modal vs. Cpx/Opx modal ratio. b) MgO vs. Al2O3 in Cpx. c) Olivine modal vs. whole rock MgO. d) Cpx modal vs. whole rock CaO. Ol: olivine, Opx: orthopyroxene, Cpx: clinopyroxene, Spl: spinel. (See Fig. 2 for explanations of symbols.)

Figure 6. Comparison of melting model for (Y)n and (Yb)n contents of clinopyroxene in spinel peridotites from Ia Bang. a) Batch melting (from less than 1% to 90%). b) Fractional melting (from less than 1% to 20%). (See Fig. 2 for explanations of symbols)

Figure 7. Ti/Eu vs. (La/Yb)n ratios of clinopyroxene in spinel peridotites from Ia Bang. Trends of silicate and carbonatite metasomatism are from Coltorti et al. (1999). (See Fig. 2 for explanations of symbols.)

Figure 8. Extraction depths of spinel peridotites from Ia Bang deduced from the temperature condition. Temperatures were estimated using the Brey and Köhler (1990) geothermometer. The crust thickness is 34km and the base of the lithospheric mantle is 85km (Trieu et al., 2004). The heat flow is 75mW/m2 (The International Heat Flow Commission, 2011; Toshiyasu, 1995). The geothermal gradient curve is deduced from the heat flow by the method of Sclater et al. (1980). The boundary of Pl-p (plagioclase peridotite) and Sp-p (spinel peridotite) is from O’Neil (1981) and Gasparik (1987), and the boundary of Sp-p and Gt-p (garnet peridotite) is from Köhler and Brey (1990).

Figure 9. Modal olivine contents vs. Mg# in olivine from the Ia Bang spinel peridotites. The

oceanic-peridotites depleted trend is from Boyd (1989). The Archean, Proterozoic, and Phanerozoic fields are from Boyd (1997) and Griffin et al. (1998).

Table 1. Modal composition of spinel peridotites (%) from Ia Bang. Ol: olivine, Opx: orthopyroxene, Cpx: clinopyroxene, Spl: spinel, Spl–ha: spinel harzburgite, Spl-lh: spinel lherzolite.

Table 2. Representative major element compositions (wt%) of olivine, orthopyroxene, clinopyroxene, and spinel in spinel peridotitesfrom Ia Bang. All values were performed by EPMA. Ol: olivine, Opx: orthopyroxene, Cpx: clinopyroxene, Spl: spinel.

Table 3. Representative average major element compositions (wt%) of whole-rock peridotite, mineral inclusion, secondary clinopyroxene, glass, and host rock alkali basalt from Ia Bang. Values of inclusion, secondary clinopyroxene, and glass were performed by EPMA. Whole-rock peridotite compositions were calculated from modals and mineral compositions. Host rock values are from Nguyen Hoang et al. (2013).Ol: olivine, Opx: orthopyroxene, Cpx: clinopyroxene, Spl: spinel, CpxII: secondary clinopyroxene; n: sample numbers.

Table. 4. Representative trace element compositions (ppm) of primary clinopyroxene and average values of secondary clinopyroxene, glass, and host rock from Ia Bang.

Values of primary clinopyroxene, secondary clinopyroxene, and glass were performed by LAICP-MS. Values of host rock from Nguyen Hoang et al. (2013). Spl-lh: spinel lherzolite,Spl-ha: spinel harzburgite, CpxII: secondary clinopyroxene.

Table 5. Temperature data for Ia Bang spinel peridotite xenoliths. Temperatures were calculated at a pressure of 15Kb. T: geothermometer, T BK: Brey and Köhler (1990), TBM: Bertrand and Mercier (1985), TWB: Wood and Banno (1973), TWS: Witt-Eickschen and Seck (1991).

a)

b) Ol

Ol

Opx

Ol Ol

Ol

Opx c)

d) Cpx

Cpx

Ol

Ol

e)

f)

Opx

Ol

Ol

Opx

Cpx Ol

g)

Ol

h)

CpxII

Gl

Gl

Ol

Gl CpxII

Ol

Ol CpxII

CpxII

Spl Ol

Gl CpxII

Ol

Gl

1000 IB2 IB6 IB18 IB19 IB20

100

10

1

100 Clinopyroxen/chondrite

Clinopyroxene/Primitive mantle

a) 1000

10 1

0.01 0.001

0.1

Rb

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

100

10

IB3 IB7 IB14 IB15 IB24 IB27 IB28

1

c) 1000

La

Ce

Pr

Sr

Zr

Nd

Sm

Hf

Eu

Ti

Gd

Tb

Dy

Ho

Yb

Er

Lu

10 1 IB3 IB7 IB14 IB15 IB24 IB27 IB28

0.1 0.01

Rb

Ba

Th

U

Ta

La

Ce

Pr

Sr

Nd

Zr

Hf

Sm

Eu

Ti

Gd

Tb

Ho Yb Dy Er Lu

1000 IB1 IB5 IB9 IB10 IB11 IB12 IB13 IB21

1

100 Clinopyroxen/chondrite

Clinopyroxene/Primitive mantle

Ta

100

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

10

U

0.001

0.1

100

Th

Ba

1000

1000

Clinopyroxen/chondrite

Clinopyroxene/Primitive mantle

b)

IB2 IB6 IB18 IB19 IB20

0.1

10 1

IB1 IB5 IB9 IB10 IB11 IB12 IB13 IB21

0.1 0.01

0.001

0.1

Rb

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

Ba

Th

U

Ta

La

Ce

Pr

Sr

Nd

Zr

Hf

Sm

Eu

Ti

Gd

Tb

Dy

Ho

Er

Yb

Lu

d) 1000 Avg glass Avg host rock

100

Avg CpxII

10

1

Clinopyroxen/chondrite

Clinopyroxene/Primitive mantle

1000

100

10

Avg glass

1

Avg host rock Avg CpxII

0.1

0.1

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

Rb

Ba

Th

U

Ta

La

Ce

Pr

Sr

Nd

Zr

Hf

Sm

Eu

Ti

Gd

Tb

Dy

Ho

Er

Yb

Lu

b)

1.0

Al2O3 in Cpx (wt%)

a)

Cpx/Opx

0.8 0.6 0.4 0.2 0.0

6

4 2 0

0

4

8 12 16 Cpx modal %

20

c)

13

14 15 16 MgO in Cpx (wt%)

17

0

4

16

d) 4 Whole rock CaO (%)

50

Whole rock MgO (%)

8

45 40 35 30

3 2

1 0

40

50

60 70 Ol modal (%)

80

a)

8 12 Cpx modal (%)

b)

100

100 Batch melting

Fractional melting Primitive mantle

Primitive mantle

10

5

10

1 90

50

30

(Yb)n

(Yb)n

10

1

3

5 1

20

1

3

10 15 20 25

0.1

0.1

0

1

2

3

(Y)n

4

5

6

0

1

2

3

(Y)n

4

5

6

100

(La/Yb)n

80 Carbonatitic metasomatism

60 40 20

Silicate metasomatism

0 0

4000

8000

Ti/Eu

12000

Sample IB2 IB3 IB6 IB7 IB14 IB15 IB18 IB19 IB20 IB23 IB24 IB27 IB28 IB1 IB5 IB9 IB10 IB11 IB12 IB13 IB16 IB21 IB25

Rock type

Spl-lh

Spl-ha

Ol 65.54 63.12 59.57 69.33 74.56 66.89 69.18 46.95 69.31 64.02 68.85 65.55 56.42 59.94 70.57 77.81 76.11 74.66 76.61 76.63 77.85 72.00 76.72

Opx 22.19 30.10 26.68 23.73 15.56 24.96 23.25 27.94 21.05 22.40 19.64 24.16 26.61 35.58 26.69 19.81 20.25 20.19 19.50 19.14 18.50 22.86 19.05

Cpx 9.62 5.34 11.54 5.25 8.55 6.52 5.95 22.40 7.37 12.42 10.04 9.02 15.36 2.85 1.77 2.01 3.20 3.77 3.58 3.95 2.60 3.52 3.54

Spl 2.65 1.45 2.22 1.69 1.33 1.63 1.62 2.71 2.27 1.16 1.47 1.26 1.61 1.64 0.97 0.37 0.44 1.38 0.30 0.28 1.05 1.61 0.69

Cpx/Opx 0.43 0.18 0.43 0.22 0.55 0.26 0.26 0.80 0.35 0.55 0.51 0.37 0.58 0.08 0.07 0.10 0.16 0.19 0.18 0.21 0.14 0.15 0.19

Sample Mineral

IB2–Lherzolite

IB3–Lherzolite

IB6–Lherzolite

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

SiO2

40.08

53.73

54.47

0.02

39.98

55.08

55.89

0.02

40.81

55.13

54.60

0.05

TiO2 Al2O3

0.02 0.01

0.13 4.66

0.41 6.77

0.05 58.46

0.02

0.02 2.95

0.05 2.83

0.06 40.38

0.02 0.03

0.16 4.58

0.64 7.00

0.13 59.42

Cr2O3 FeO NiO MnO MgO CaO Na2O K2O Total

0.02 9.71 0.25 0.16 49.29 0.04 0.01 99.58

0.36 6.03 0.04 0.12 33.87 0.49 0.06 0.03 99.52

1.22 1.97 0.03 0.12 13.40 19.97 1.71 100.09

12.82 7.98 0.15 0.12 19.46 0.01 99.07

0.05 8.89 0.26 0.16 49.75 0.06 99.18

0.50 5.63 0.09 0.10 34.98 0.71 0.02 0.01 100.08

0.84 1.85 0.02 0.09 15.83 21.23 0.47 99.09

33.67 9.66 0.09 0.17 16.82 0.01 0.01 100.86

10.28 0.26 0.20 47.91 0.05 99.55

0.33 6.38 0.07 0.14 31.79 0.67 0.11 99.34

1.05 2.61 0.03 0.10 13.67 18.95 1.91 100.57

11.54 9.60 0.21 0.12 19.13 0.01 0.01 100.21

Sample Mineral SiO2 TiO2 Al2O3 Cr2O3 FeO NiO MnO MgO CaO Na2O K2O Total

IB7–Lherzolite

SiO2 TiO2 Al2O3 Cr2O3 FeO NiO MnO MgO CaO

IB15–Lherzolite

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

40.62 0.02 0.03 9.01 0.22 0.17 49.13 0.05 0.01 99.26

56.46 0.04 3.14 0.40 5.95 0.08 0.15 33.39 0.60 0.05 0.02 100.27

55.94 0.15 3.66 1.14 1.71 0.01 0.10 14.72 20.51 1.14 99.08

0.05 0.07 45.21 27.75 9.91 0.13 0.16 17.66 100.93

40.02 0.01 0.02 10.20 0.28 0.17 48.46 0.05 99.21

56.79 0.02 3.72 0.38 6.17 0.07 0.14 32.77 0.63 0.12 0.01 100.79

54.34 0.08 4.31 1.32 2.82 0.04 0.07 14.39 20.76 1.76 99.90

0.05 0.02 52.57 17.73 11.95 0.21 0.18 18.12 100.84

40.77 0.01 0.05 9.62 0.20 0.16 48.70 0.02 99.53

56.75 0.07 3.80 0.44 6.04 0.06 0.13 32.92 0.48 0.05 100.74

54.46 0.22 5.16 1.70 2.55 0.03 0.07 13.12 20.91 1.72 99.95

0.02 0.05 54.53 16.07 10.69 0.14 0.13 18.28 0.01 0.01 99.94

Sample Mineral

IB14–Lherzolite

IB18–Lherzolite

IB19–Lherzolite

IB20–Lherzolite

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

41.15 9.12 0.27 0.13 48.68 0.02

56.85 0.06 3.45 0.43 6.05 0.02 0.16 32.98 0.67

53.68 0.13 4.21 0.88 2.46 0.01 0.07 15.36 21.34

0.02 0.03 53.95 16.53 11.63 0.16 0.17 18.02 -

40.83 0.02 0.02 10.80 0.24 0.18 47.15 0.06

55.73 0.20 4.87 0.23 6.64 0.04 0.16 32.05 0.66

52.10 0.73 6.94 0.86 3.52 0.03 0.11 13.64 19.59

0.05 0.15 63.24 6.34 10.84 0.26 0.13 19.91 -

40.79 0.01 9.99 0.30 0.17 47.98 0.02

56.46 0.07 4.08 0.35 6.34 0.07 0.15 32.62 0.57

54.13 0.26 5.75 1.36 2.77 0.05 0.11 13.55 21.20

0.04 0.03 58.79 12.04 10.57 0.17 0.14 18.80 0.01

Na2O K2O Total

99.36

0.02 0.01 100.70

1.17 99.31

0.01 100.50

99.30

0.12 0.01 100.71

2.13 99.65

100.93

99.25

0.06 100.77

1.77 0.01 100.95

100.57

(Continues)

(continued) Sample Mineral

IB23–Lherzolite

IB24–Lherzolite

IB27–Lherzolite

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

SiO2 TiO2 Al2O3 Cr2O3 FeO

40.71 0.01 10.72

55.08 0.15 4.50 0.23 6.85

51.81 0.69 6.79 0.87 3.06

0.07 0.15 62.22 7.08 11.46

40.89 0.03 9.69

55.94 0.07 3.88 0.34 6.05

52.86 0.38 6.08 1.39 2.59

0.04 0.08 57.63 12.52 11.28

41.21 0.01 0.01 9.43

56.14 0.06 3.83 0.37 6.39

52.93 0.22 6.01 1.57 2.25

0.04 0.05 57.72 12.54 10.10

NiO MnO MgO CaO Na2O K2O

0.22 0.20 47.52 0.06 -

0.04 0.15 31.33 0.66 0.15 -

0.10 14.39 19.81 1.93 0.01

0.23 0.10 19.36 0.01

0.28 0.15 48.41 0.06 -

0.05 0.17 32.00 0.71 0.11 -

0.06 0.13 14.71 20.31 1.80 -

0.27 0.12 18.87 -

0.27 0.18 48.16 0.04 -

0.04 0.17 32.87 0.47 0.03 -

0.07 14.32 21.37 1.66 -

0.24 0.14 18.72 -

Total

99.44

99.13

99.45

100.67

99.51

99.30

100.31

100.83

99.31

100.37

100.40

99.55

Sample Mineral

IB28–Lherzolite

IB1–Harzburgite

IB5–Harzburgite

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

SiO2 TiO2

41.27 -

55.96 0.11

53.01 0.58

0.06 0.14

40.38 0.01

55.26 0.08

55.20 0.24

0.02 0.11

40.13 -

57.43 0.02

57.27 0.05

0.04 0.07

Al2O3 Cr2O3 FeO NiO MnO MgO

0.01 0.05 10.41 0.23 0.19 47.08

4.30 0.28 6.48 0.05 0.15 32.08

6.50 1.06 2.80 0.02 0.10 14.68

60.02 9.51 10.79 0.23 0.12 19.34

8.56 0.25 0.14 50.14

2.43 0.51 5.87 0.06 0.13 35.25

3.52 1.80 2.15 0.04 0.09 14.48

32.85 38.27 15.13 0.09 0.19 13.82

0.03 8.86 0.25 0.13 49.87

2.14 0.61 5.52 0.05 0.16 33.90

2.63 1.82 2.05 0.04 0.08 15.36

28.14 43.80 13.60 0.10 0.24 14.38

CaO Na2O K2O Total

0.06 0.01 99.30

0.68 0.12 0.01 100.20

19.89 1.88 0.01 100.53

100.21

0.03 99.52

0.71 0.03 100.31

21.61 0.96 0.01 100.10

100.47

0.05 0.01 99.32

0.69 0.11 100.63

20.02 1.47 100.81

100.35

Sample Mineral SiO2 TiO2 Al2O3 Cr2O3 FeO

IB9–Harzburgite

IB10–Harzburgite

IB11–Harzburgite

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

40.71 0.01 0.03 8.63

56.36 0.05 2.83 0.58 5.51

55.70 0.14 4.06 2.22 1.93

0.02 0.10 30.67 41.23 11.09

40.81 0.01 0.03 8.76

54.88 0.04 2.59 0.50 5.56

56.05 0.05 2.83 1.06 2.06

0.04 0.06 31.69 42.03 11.00

41.30 0.01 0.02 8.90

57.05 0.03 2.00 0.52 5.47

57.16 0.03 2.22 1.06 1.93

0.03 0.02 24.77 50.00 11.50

NiO MnO MgO

0.25 0.16 49.45

0.06 0.14 33.52

0.06 0.08 14.69

0.07 0.17 16.17

0.28 0.13 49.37

0.08 0.13 34.75

0.06 0.07 16.02

0.09 0.17 15.47

0.31 0.13 48.61

0.03 0.12 34.01

0.04 0.08 15.72

0.08 0.20 14.05

CaO Na2O K2O Total

0.04 99.28

0.64 0.06 99.74

19.51 1.57 99.96

99.52

0.06 99.44

0.73 0.05 99.31

20.78 0.70 99.68

100.55

0.05 99.33

0.69 0.13 100.05

19.45 1.36 99.04

100.67

(Continues)

(continued) Sample Mineral

IB12–Harzburgite

IB13–Harzburgite

IB16–Harzburgite

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

Spl

SiO2 TiO2 Al2O3 Cr2O3 FeO NiO

40.18 0.01 0.03 9.12 0.27

56.86 0.10 2.80 0.53 5.63 0.06

55.63 0.03 2.54 1.30 2.59 0.05

0.05 0.24 41.78 27.49 14.07 0.15

40.93 0.01 0.01 0.03 8.87 0.26

57.98 0.02 2.04 0.47 5.43 0.03

55.74 0.04 2.48 1.19 2.88 0.02

0.03 0.06 28.96 41.09 14.49 0.08

40.75 0.02 0.02 0.03 8.81 0.27

57.72 0.15 2.10 0.48 5.45 0.06

54.19 0.32 2.83 1.19 2.43 0.03

0.04 0.57 27.75 40.23 15.90 0.14

MnO MgO CaO Na2O K2O Total

0.21 49.27 0.02 0.01 99.12

0.13 33.55 0.36 100.02

0.12 15.79 20.31 1.59 99.95

0.22 15.93 0.01 99.93

0.17 49.47 0.04 99.77

0.13 33.91 0.65 0.11 100.77

0.09 15.75 20.12 1.67 0.01 99.98

0.24 14.89 99.84

0.16 49.07 0.06 99.18

0.13 34.03 0.68 0.03 100.82

0.09 15.99 21.05 1.05 0.03 99.19

0.30 14.62 0.02 0.01 99.57

Sample Mineral SiO2 TiO2 Al2O3 Cr2O3 FeO NiO MnO MgO CaO Na2O K2O Total

IB21–Harzburgite

IB25–Harzburgite

Ol

Opx

Cpx

Spl

Ol

Opx

Cpx

40.38 0.02 10.78 0.25 0.19 47.46 0.07 0.01 0.01 99.16

56.79 0.04 2.89 0.62 5.70 0.07 0.13 33.78 0.72 0.11 0.01 100.85

53.96 0.09 3.39 1.48 2.38 0.04 0.11 16.34 20.65 1.55 99.99

0.05 0.11 36.15 33.75 13.87 0.13 0.25 15.56 99.86

41.51 0.02 0.02 0.02 8.54 0.24 0.15 49.23 0.07 99.79

56.03 0.37 3.40 0.49 5.66 0.05 0.13 33.00 0.70 0.08 99.91

51.44 1.46 5.52 1.64 2.55 0.02 0.10 15.32 20.85 1.24 0.01 100.14

Spl 0.05 1.13 44.51 22.55 13.31 0.20 0.18 17.69 0.01 99.63

Whole-rock peridotites Mean values

SiO2 TiO2 Al2O3 Cr2O3 FeO NiO MnO MgO CaO Na2O K2O P2O5 Total

Sample Elements Li Sc Ti V Cr Co Ni Zn Ga Sr Rb Y Zr Nb Cd Sb Cs Ba La Ce Pr Nd Sm

IB2 Spllh 3.94 59.4 2804 267 6457 21.8 302 8.85 3.57 17.7 7.33 17.72 22.56 0.05 0.06 0.01 0.02 0.39 2.15 0.35 2.60 1.32

IB3 Spllh 2.10 53.7 334 188 6630 21.8 376 9.58 1.26 7.5 3.12 4.70 4.37 0.68 0.10 0.31 0.01 0.06 1.14 2.70 0.35 1.37 0.18

Spl-Lh (n=13)

Spl-Ha (n=10)

Ol (n=7)

Opx (n=4)

Cpx (n=4)

Spl (n=4)

44.83 0.07 2.55 0.47 8.26 0.18 0.15 40.74 2.19 0.19 99.63

44.32 0.03 0.94 0.52 8.07 0.21 0.15 44.49 0.81 0.06 99.60

41.17 0.01 9.04 0.19 0.13 49.05 0.04 0.01 99.65

54.58 0.06 3.62 0.36 6.42 0.04 0.19 33.65 0.39 0.03 0.01 99.34

52.94 0.06 5.20 0.84 1.98 0.02 0.11 16.58 21.17 0.43 0.01 99.33

0.02 0.04 40.71 33.39 9.57 0.09 0.15 15.85 0.01 99.84

IB6 Spllh 1.75 56.9 4088 262 4912 21.0 322 8.31 3.97 31.2 12.97 17.44 32.15 0.12 0.08 0.02 0.04 1.67 4.41 0.67 4.47 1.56

IB7 Spllh 7.72 68.6 826 226 6206 19.8 347 8.36 1.88 10.5 4.37 6.68 3.86 1.03 0.19 0.08 0.02 0.01 2.10 3.91 0.33 0.88 0.37

Host rock (Alkali basalt) (n=5) 49.04 2.56 13.99 11.64 0.15 8.74 8.14 3.26 1.91 0.69 100.12

Mineral inclusions

IB14 Spllh 1.62 67.5 639 234 7184 20.1 363 16.62 2.11 18.5 7.68 7.70 1.15 0.17 0.27 0.02 0.01 0.03 7.69 15.15 1.30 3.38 0.74

IB1 Splha 8.69 73.1 1442 233 10100 17.5 296 8.54 1.92 75.9 31.52 6.96 22.69 0.21 0.14 0.66 0.12 0.14 4.90 12.51 1.63 7.64 1.84

IB5 Splha 0.99 80.7 244 206 10320 21.5 351 9.91 2.00 104.4 43.36 7.31 88.31 0.90 0.13 0.05 0.03 0.20 18.14 41.59 4.83 19.11 2.73

IB9 Splha 1.53 78.8 815 234 12971 20.9 344 8.06 1.85 73.4 30.47 6.54 16.49 0.81 0.03 0.01 0.11 13.68 37.82 3.86 11.24 1.35

CpxII (n=6)

Glass (n=12)

54.53 0.11 0.78 3.94 1.69 0.01 0.11 17.06 18.80 2.28 0.03 99.32

IB10 Splha 9.20 56.9 299 174 5637 21.0 364 9.82 1.25 11.9 4.93 1.66 3.70 0.60 0.26 0.09 0.04 5.12 8.33 0.78 2.65 0.41

56.69 1.61 19.63 0.03 3.89 0.13 2.25 2.72 3.83 2.23 93.00

IB11 Splha 1.72 48.9 145 123 6165 19.9 352 10.70 1.09 98.3 40.83 1.40 14.53 0.45 0.25 0.30 0.01 0.36 12.81 19.59 1.66 4.18 0.46

Average values CpxII

Glass

7.23 81.5 479 245 21714 17.8 376 24.94 1.57 57.1 22.78 20.40 34.26 4.27 0.93 0.05 0.05 14.67 14.47 47.02 7.39 35.62 9.03

4.67 11.3 2818 59 4587 29.0 551 80.56 6.92 117.8 49.54 7.89 91.85 36.16 0.18 0.06 0.58 147.38 14.12 29.24 3.39 13.51 2.85

Host rock 169 238 209 767 62.9 25.8 246 65.8 0.82 675. 44.4 88.2 9.82 40.4 8.40

Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

0.59 1.92 0.47 3.01 0.66 1.89 0.27 1.57 0.27 0.86 0.75 0.02 -

Themometers Mineral used Method IB2 IB3 IB6 IB7 IB14 IB15 IB18 IB19 IB20 IB23 IB24 IB27 IB28 IB1 IB5 IB9 IB10 IB11 IB12 IB13 IB16 IB21 IB25

0.09 0.31 0.06 0.44 0.13 0.40 0.06 0.35 0.09 0.16 0.07 0.36 0.10 0.03

Rock type

Spllh

Splha

0.74 2.37 0.46 3.55 0.62 1.98 0.24 1.84 0.28 0.96 0.02 2.19 0.62 0.19

0.17 0.56 0.12 0.95 0.20 0.75 0.11 0.78 0.11 0.23 0.03 0.63 0.58 0.20

0.27 0.62 0.13 1.20 0.27 0.98 0.13 1.12 0.13 0.02 0.01 0.25 0.06

0.59 1.99 0.29 1.56 0.28 0.70 0.08 0.47 0.09 0.96 0.03 0.33 0.07 0.03

0.94 1.90 0.32 1.65 0.22 0.61 0.07 0.50 0.08 0.40 0.20 10.57 1.75 0.42

0.48 1.47 0.21 1.20 0.31 0.61 0.09 0.51 0.08 0.59 0.09 1.23 1.06 0.31

0.12 0.34 0.04 0.26 0.06 0.23 0.05 0.29 0.04 0.11 0.09 0.56 0.94 0.24

0.13 0.39 0.06 0.24 0.05 0.13 0.03 0.22 0.03 0.34 0.05 1.83 1.91 0.36

2.79 8.07 1.11 5.82 0.88 1.81 0.20 1.20 0.18 0.45 0.14 0.75 0.15 0.03

0.87 2.47 0.34 1.88 0.31 0.72 0.09 0.60 0.08 1.85 1.45 1.74 2.06 0.63

TBK (oC) Cpx-Opx

TBM (oC) Cpx-Opx

TWB (oC) Cpx-Opx

TBK (oC) Opx

TWS (oC) Ol-Opx-Sp

Ca partitioning

Ca partitioning

Miscibility gap

Ca content in Opx

Ca, Al partitioning

1032 1131 1101 1047 927 856 882 991 871 979 983 841 994 979 1078 1079 1130 1110 991 1021 995 982 1004

980 1163 1163 1052 958 739 1057 997 1037 1126 1155 1011 1175 961 1075 1128 1156 1187 1055 1086 1120 1152 1132

992 1083 1026 965 973 908 989 999 871 1021 1035 969 1045 981 1000 1012 1074 1045 1035 1054 1023 1047 1080

912 1002 912 912 912 912 949 1002 912 912 1002 912 912 1002 1002 912 1002 1002 785 912 912 1002 1002

907 871 917 891 933 901 872 872 879 878 889 863 885 986 1008 1062 956 1009 914 999 997 1045 947

2.74 7.33 1.07 5.34 0.91 2.15 0.27 1.56 0.21 5.78 4.67 3.56 5.94 1.32

Highlights

- Spinel peridotite xenoliths from Ia Bang reveal variations in modal and chemical compositions, indicative of the variation from fertile lherzolites to refractory harzburgites. - The mantle xenoliths have undergone 1%~20% fractional melting and cryptic metasomatism. - The equilibrium temperatures of the Ia Bang spinel peridotites range from 841 oC to 1131oC, corresponding to 36~50km in the uppermost mantle. - The spinel peridotites are present as the Proterozoic harzburgites and the Phanerozoic lherzolites.

Graphical abstract 100

100 Fractional melting 80

5

1

(La/Yb)n

(Yb)n

10 1

3

Carbonatitic metasomatism

60

40

10

15

20

20 25 0.1

Silicate metasomatism

0 0

1

2

3

4

5

6

0

(Y)n

4000

8000

Ti/Eu Spinel peridotites from Ia Bang

Lherzolite

Harzburgite

12000

Declaration of interests

The authors declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Authors: The Cong Nguyen Young-woo Kil