Alkali basalt from the Seifu seamount of the Japan Sea: post- spreading magmatism in the back-arc region

We report geochemical characteristics and Ar-Ar dating of a basalt dredged from the Seifu Seamount (SSM-basalt), located at northeast of the Tsushima Basin in the southwest Japan Sea, which is one of the western Pacific back-arc basin swarm. A plateau age of 8.33 ± 0.15 Ma (2σ) was obtained by the 40Ar-39Ar age spectrum of SSM-basalt. The SSM-basalt (8.3 Ma) was formed at an early stage after the termination of the Japan Sea back-arc opening. The SSM-basalt is high-K to shoshonitic alkaline basalt and is characterized by enrichment of light rare earth element (REE). The trace element pattern of 20 the SSM-basalt is similar to Ocean island-type basalt (OIB) whereas YbPM (=6) is distinctively higher than that of OIB, indicating of its formation by the low degree melting of the source mantle under spinel peridotite stability field. The Nd-Sr and Pb isotope compositions of the SSM-basalt are offset from the compositional trend of the Japan Sea back-arc basin basalts. The Sr-Nd isotope relationship of the SSM-basalt suggest its source can be formed by deplete MORB mantle source mixing with EM1-like component. The SSM basalt was formed as a post-back-arc spreading magmatism by low degree of partial 25 melting of a portion that is easily melted in the upwelling asthenosphere associated with the main back-arc magmatism.


Introduction
Many studies reported back-arc magmatisms during back-arc rifting/spreading (e.g., Martinez et al., 2001;Pearce and Stern, 2006). Sato et al. (2002) proposed that "post back-arc spreading magmatism" which is the formation and eruption of enriched basalts during the last stage of back-arc basin spreading and/or after cessation of spreading, as a common process in the late stage of back-arc development. Ishizuka et al. (2009) also examined "post back-arc spreading magmatism" in the Shikoku 5 Basin of the Izu-Bonin arc-back-arc system and suggested that it is characterized by two distinctive magma types.
The Sea of Japan (Japan Sea, hereafter) is located in the northern part of the western Pacific back-arc basin swarm and is regarded as one of the typical inactivated back-arc basins developed between island arc and continent (Tamaki and Honza, 1985;Uyeda and Kanamori, 1979). Numerous geophysical surveys were carried out to elucidate the architecture of the Japan Sea back-arc basin and its formation processes (e.g., Lee et al., 1999;Yoon et al., 2014;Sato et al., 2014). Geophysical 10 data coupled with direct sampling from the ocean floor including the Ocean Drilling Program (ODP) revealed the formation process of the northern Japan Sea   (Fig. 1). The breakup of the lithosphere and oceanic floor spreading were initiated from the eastern margin of continent at 32-28 Ma, and back-arc magmatisms were terminated at 18-15 Ma . The basins are mainly composed of extended continental crust and oceanic crust. Several seamounts and islands are distributed, and form chained array (Fig. 1). The radiometric age of volcanic rocks from the seamounts in the 15 Yamato basin is 6-13 Ma (Kaneoka et al., 1990;Kaneoka and Yuasa, 1988) whereas that of the Ulleung Island volcanism in the Tsushima basin (Ulleung Basin) is distinctively younger than 2 Ma (Kim et al., 1999a). Pouclet et al. (1995) summarized magmatic history of the circum-Japan Sea area including the southwest Japan arc, northeast China and the back-arc basin. The alkali basalt magmatism is frequent at the southwest Japan arc after termination of the Japan Sea opening (< 4 Ma). Kim and Yoon (2017) recently suggested that the seamount chain in the Tsushima Basin 20 was formed by the post-back arc spreading magmatism based on the geochemical data of alkali basalt samples studied by Lee et al. (2011).
A basaltic block containing peridotite xenoliths was dredged from the Seifu Seamount (Ninomiya et al., 2007), located at northeast of the Tsushima Basin in the southwest Japan Sea (Fig. 1). Radiometric dating, coupled with geochemical signatures, of volcanic rocks from the seamount would be the most direct method of addressing the tectonic and magmatic 25 history of the southwest Japan Sea. Here, we report the petrological, geochemical and geochronological data of the basalt sample from the Seifu Seamount, in comparing with data of volcanic rocks in the circum-Japan Sea area and discuss its origin in the context of post back-arc spreading magmatism.

Geological background and sample description
The Seifu Seamount is located at the northeastern margin of the Tsushima Basin, where is a junction of three basins between 30 continental crustal fragments (Fig. 1). The Tsushima Basin is now geomorphologically connected to southwestern margin of the Japan Basin by the Ulleung Interplain Gap (UIG) and is separated from the Yamato Basin by the Yamato Rise and the Oki  Tokyo, in 1985 (Dredge Station No. KT85-15 D-3: 38°12.20'-12.80'N, 132°34.70'E) (Shimamura et al., 1987). Lee et al. (1999) proposed that the basement of the Tsushima Basin is thick oceanic crust formed by incomplete spreading as well as extending of continental crust caused by westward propagation of spreading during the Japan Sea opening. The direction of the Tsushima Basin opening is N-S, evidenced from the NE-SW orientation of ridge-like feature 5 parallel to the Yamato Basin spreading . Kim et al. (2011) revealed that the chained seamounts buried by sediments are placed along the UIG. They expected that the volcanic ages of the UIG seamount chain are the same as that of the Yamato Seamount chain (Kim et al., 2011a). The Seifu seamount is seated on the Japan Basin of further ENE part of the UIG seamount chain.
The basalt sample contains peridotite xenoliths with variable size from a few millimetres to usually < 3cm (up to 10 10 cm) and their-derived xenocrysts (Ninomiya et al., 2007). The basalt sample shows porphyritic texture. The phenocrysts of the basalt sample are mainly olivine with small amounts of plagioclase, orthopyroxene, clinopyroxene and spinel. Plagioclase phenocryst shows anhedral with albite twins, oscillatory zoning and dusty zone, and is sometimes surrounded by tiny plagioclase crystals that is the same size as the groundmass. Orthopyroxene phenocrysts are rimmed by fine-grained mineral aggregate, which is similar to those found in orthopyroxene in peridotite xenoliths, indicating that all orthopyroxene 15 phenocrysts are of xenocryst origin. Spinel phenocryst, up to 0.5 mm in size, shows anhedral and rounded in shape. The groundmass of the sample is crystalline and shows intersertal textures mainly consisting of prismatic plagioclase with small amounts of olivine now partly serpentinized/altered, opaque mineral (ilmenite and titanomagnetite), apatite and clinopyroxene.
The prismatic plagioclase crystals in the groundmass sometimes show weak trachytic texture.

Analytical method
In preparation of the powdered sample, crustal and mantle xenolith fragments were removed as possible. Whole rock majorand trace-element compositions of basalt from the Seifu Seamount (SSM-basalt) are determined by EPMA and LA-ICP-MS at Kanazawa University. For EPMA and LA-ICP-MS measurements, fused whole-rock glass prepared by a direct fusion method was also used. Details are shown in Tamura et al. (2015). The major-element composition was also determined by 25 XRF in Senshu University (Sato, 2010). The analytical procedure used for chemical separation and mass spectrometry for Sr, Nd and Pb isotope determinations was outlined by Yoshikawa and Nakamura (1993), Shibata and Yoshikawa (2004) and Miyazaki et al. (2003). Mass spectrometry was performed on a Thermo-FinniganMAT262 equipped with nine Faraday cups, using a static multi-collection mode. Normalizing factors used to correct for isotopic fractionation in the Sr, Nd and Pb isotope analyses were 86  NIST 981. Total procedural blanks for Sr, Nd and Pb were less than 100, 10 and 10 pg, respectively. The geochemical data of the SSM-basalt is summarized in supplementary table S1.
We conducted the 40 Ar/ 39 Ar incremental heating analysis to obtain the radiometric age. The sample were crushed into a few mm grains, and separated fresh groundmass. Samples were wrapped in Al foil and contained in an Al capsule (70 mm in length, 10 mm in diameter) with flux monitors biotite of EB-1 (91.4 ± 0.5 Ma; Iwata, 1997), K2SO4 and CaF2. The samples 5 were irradiated for 24 hours in the Japan Material Testing Reactor (JMTR). During the irradiation, the samples were shielded by Cd-foil in order to reduce thermal neutron-induced 40 Ar from 40 K (Saito, 1994). The Ar extraction and Ar isotopic analyses were done at Radioisotope Center, University of Tokyo. During incremental heating, gases were extracted in 8 steps between 600 and 1300°C. The analytical methods are described by Ebisawa et al. (2004).

Results 10
The SiO2 and TiO2 contents of the SSM-basalt are 49 wt% and 1.9 wt%, respectively. Total alkali content (Na2O + K2O) is 5.8 wt% (Na2O and K2O are 4.0 wt% and 1.8 wt%, respectively). The SSM-basalt corresponds to alkaline basalt in terms of total alkali vs. SiO2 discrimination diagram (Miyashiro, 1978)  The 40 Ar-39 Ar age spectrum of SSM-basalt shows a plateau age of 8.33 ± 0.15 Ma (2σ) in 6 fractions at lower temperature ( Fig. 6), where the age should be accepted as the best estimate because the initial 40 Ar/ 36 Ar ratio corresponds to the atmospheric ratio (295.5) in the inverse isochrones (Fig. 6).

Origin of the SSM basalt and its tectonic setting
The trace-element patterns of the SSM alkali basalt is characterized by non-depletion of Nb and Ta relative to light-REE (e.g., 5 NbPM/LaPM = 1.5), and by high heavy-REE abundances (e.g., YbPM = 6) (Fig. 4). They are similar to OIB-type alkali basalts from oceanic islands (e.g., Ulleung Island) and the continental region of the east China whereas the high HREEs abundance distinguishes the SSM basalt from other alkali basalts in the circum-Japan Sea area (Fig 4). Alkali basalts with high Nb/La ratio and high-HREEs abundance have been also reported from the South Korean Plateau, located at the east of the SSM   (Fig. 4c). Relatively high HREEs in the SSM basalt suggest that garnet was not a residual phase in the source 10 mantle. The high-Nb SSM basalt with a high-HREEs can be formed by the low degree melting of the source mantle under pressure conditions shallower than garnet peridotite stability field. The age and geochemical constraints of the SSM basalt suggest that upwelling of the Japan sea back-arc asthenosphere 25 continued even after the main volcanism of back-arc spreading, so that a low degree of partial melting occurred in a mantle component, which is easily melted, of the upwelling asthenosphere.
The Nd, Sr and Pb isotopic compositions of the SSM-basalt are also clearly different from those of alkali basalts in the circum-Japan Sea area (Fig. 5). The SSM-basalt has depleted isotopic compositions: high-143 Nd/ 144 Nd and low-87 Sr/ 86 Sr ratios. In the Nd-Sr and Pb isotope diagrams, the SSM-basalt is plotted slightly offset from the compositional trend of the 30 Japan Sea BABB (Fig. 5). The alkali basalts from the South Korean Plateau also have distinctive Nd-Sr compositions similar to the SSM basalt (Fig. 5a) whereas their Pb isotopic compositions are comparable to the compositional trend of Japan sea BABB (Fig. 5bc). Remarkably, the isotopic compositions of the SSM basalt are comparable to those of 6-13 Ma trachyandesite from seamounts in the Yamato basin (Tatsumoto and Nakamura, 1991). Based on the geochemical data of the South Korean Plateau basalt (SKP basalt) and the tectonic history of the Tsushima basin, Kim and Yoon (2017) concluded that the SKP-basalt was a product of post-spreading magmatism. The age of SKP basalts has not been, however, determined yet. Many submarine volcanoes including seamounts buried by sediments were discovered in the Japan Sea Basin (e.g., Kim et al., 2011b;Kimura et al., 1987) (Fig, 1). These submarine volcanoes and seamounts were also likely formed by the post back-arc spreading. 5 Sato et al. (2002) reported post-spreading magmatism that formed the seamounts composed of the enriched basalt magma in the back-arc region of the Shikoku basin during and/or after the last stage of the back-arc spreading. They pointed out that the post-spreading magmatism is a common process at the back-arc basins in the western Pacific region. For example, in contrast to the spreading magmatism , enriched basalt volcanisms (Kinan Seamount Chain of Fig. 1: 7  were reported as the post-spreading magmatism in the Shikoku Basin in the Izu-Bonin back-arc system (Sato et al., 2002;10 Ishizuka et al., 2009). The age of the SSM basalt (8.3 Ma), which is about ~7 Mys after termination of the Japan Sea opening (~15 Ma), is comparable to the period of the post-spreading magmatism in the Shikoku Basin. Ishizuka et al., (2009) invoked that the post back-arc spreading magmatism in the Shikoku Basin is caused by heterogeneities of the asthenospheric mantle, which is upwelling and produces BABB melts beneath the back-arc spreading center. It is interesting to note that trace element compositions of the Kinan Seamount Chain alkali basalts are similar to those of the SSM basalt. 15 Because of the very minor contributions of slab component to the SSM basalt source, the SSM basalt is a window to explore mantle components beneath the Japan Sea back-arc region of the western Pacific region. Alkali basalts were also reported from the Philippine Sea plate (Kinan Seamount Chains: Sato et al., 2002;Ishizuka et al., 2009) and the Pacific plate (Petit spot magmas, see Fig. 1: Hirano et al., 2006;Machida et al., 2009) near the Japan Sea in the western Pacific region. The geochemical characteristics of the SSM basalt are similar to those of the Kinan Seamount Chains in the Philippine sea plate, 20 but different from those of the petit spot magmas in the Pacific plate (Figs. 3,4,5). The involvement of EM1 component in melt source is minor but is widely observed both in the Japan Sea and Shikoku basin in the western Pacific region.

Conclusion
A basaltic block containing peridotite xenoliths was dredged from the Seifu Seamount located at the northeastern margin of