Geochemistry of Neogene Nansatsu volcanic rocks in South Kyushu, Japan

ABSTRACT We report the whole-rock major and trace element and Sr – Nd – Pb isotopic compositions of volcanic rocks from the Nansatsu area in South Kyushu. We analysed samples from the Oldest stage (middle Miocene), Older stage (late Miocene), and Middle stage (early Pliocene) Nansatsu volcanic rocks, which are andesite to dacite in composition, with an island-arc trace element signature. The compositional variabilities within the Older and the Middle stage rocks are relatively small compared to those in the Oldest stage and Quaternary volcanic rocks. Isotopic characteristics suggested that the Oldest stage volcanic rocks had magma sources similar to regional ilmenite-series granitoids of later ages (Outer Zone granitoids), which incorporated a substantial amount of graphite-bearing accretionary sediments (Shimanto Supergroup). The sources for the Older and Middle stage magmas were similar to those of primary Quaternary basalts in South Kyushu, indicating that Pliocene and Quaternary volcanic rocks in South Kyushu share a common mantle source, despite the change in the nature of volcanic activity from andesite-dominated to more variable volcanism of basalt to rhyolite.


Introduction
South Kyushu, located at the junction of the southwest Japan and Ryukyu arcs, is currently recognized as a locus of intense igneous activities associated with the subduction of the Philippine Sea Plate since Middle Miocene (Shinjo et al. 2000;Mahony et al. 2011).Previous studies have reported the petrological characteristics of these igneous activities in detail (e.g.Uto et al. 1997 and references therein), but the majority of reported chemical compositions are confined to Middle Miocene granites and Quaternary volcanic rocks; only sparse and dominantly major-element data are available on the chemical composition of pre-Quaternary volcanic rocks.
The southernmost region of the Satsuma Peninsula, in South Kyushu, known as the Nansatsu region (Figure 1) exhibits pre-Quaternary igneous activity.Nevertheless, previous publications on Nansatsu volcanic activities are mainly government reports which cannot be accessed online from search engines (e.g.Ministry of International Trade and Industry, hereafter MITI 1985), causing confusion in understanding of the regional stratigraphy.Therefore, this study first reviews the igneous activity in the Nansatsu region, and then reports new data on the whole-rock major and trace element and Sr-Nd-Pb isotopic compositions of the volcanic rocks.We utilize these data to investigate the possible origin of these volcanic rocks and to compare them with other igneous activities in South Kyushu.

Volcano-tectonic history
Southwest Japan is subdivided into two zones, the Inner and Outer zones, bound by the Median Tectonic Line (Figure 1(a)).The Shimanto belt, a Jurassic to Palaeogene accretionary prism composed of graphite-bearing sandstones and shales, is the southernmost and largest tectonic province in the Outer Zone (Figure 1(a); Wallis et al. 2020).
Granitoids of the middle Miocene age (17-12 Ma) are widely distributed in the Outer Zone of Southwest Japan, including the South Kyushu area (Figure 1(b); Kimura et al. 2005).These are referred to as the Outer Zone granitoids and are classified as ilmenite series rocks (Ishihara and Matsuhisa 1999).Most of these granitoids intrude into sedimentary rocks of the Shimanto belt (termed the Shimanto Supergroup).Tectonic reconstruction suggests that, in the middle Miocene, the Shikoku Basin, a relatively young part of the Philippine Sea Plate, was located further west and was subducting beneath Kyushu Island (Mahony et al. 2011).Formation of the Outer Zone granitoid is estimated to be related to the shallow subduction of the hot Shikoku Basin, with a relatively fast convergence rate (>10 cm/year; Kimura et al. 2005).
South Kyushu experienced a cessation of subductionrelated volcanism around 12 Ma (Kamata and Kodama 1994), the cause of which is a topic of ongoing debate.Volcanic activity resumed at approximately 7.5 Ma, along the western margin of South Kyushu (Mahony et al. 2011), and migrated eastward to the Kagoshima Graben, where the present volcanic front is located.Until the Pleistocene, volcanism was characterized by the formation of voluminous andesite lava plateaus, followed by basaltic to rhyolitic explosive and caldera-forming volcanism in the Quaternary period.These volcanic rocks lie unconformably on the Shimanto supergroup and Outer Zone granitoids.Tectonic reconstruction indicates that, during this period, the West Philippine Basin, an older part of the Philippine Sea Plate, was subducting beneath South Kyushu, with a moderate convergence rate (approximately 4 cm/year; Mahony et al. 2011).

Nansatsu volcanic rocks
For the middle Miocene -Pleistocene volcanic rocks in Nansatsu, different unit classifications have been used in different studies (as shown in Figure 1(d)), which have been confusing readers.In this study, we summarize the history of the unit classification of these volcanic rocks and describe their petrology.The most comprehensive study of igneous rocks in the Nansatsu area was conducted by MITI (1985), who classified the volcanic rocks into late Miocene Lower and Upper Nansatsu Groups, early Pliocene Nansatsu Middle Volcanics, and late Pliocene -Pleistocene Nansatsu Younger Volcanics.Kuroiwa et al. (1989) conducted an in-depth study of the Nansatsu Group and re-classified it into four units and indicated that three of the units (Hiramichiyama, Nishinooka, and Akime formations) were formed in the late Miocene; additionally, they suggested that one unit (Kasasakurose andesite) was formed during the middle Miocene period, owing to the fact that this unit was intruded by the outer zone granitoids (Yamamoto et al. 1970).Uto et al. (1997) first used the terms 'oldest/older/middle/younger stage Nansatsu Volcanic rocks', corresponding to the units found in the northern Satsuma Peninsula region (Hokusatsu area).These terms continue to be used, although the 'Oldest stage Nansatsu volcanic rock' has been omitted in recent compilations (Geological Survey of Japan GSJ 2022), without reasonable justification.In this study, we employ the terms 'oldest/ older/middle/younger stage Nansatsu Volcanic rocks', according to Uto et al. (1997) and Kawanabe et al. (2004).The correspondence relationships for each unit are explained in the following paragraphs and shown in (Figure 1(d)).
All stages of the Nansatsu volcanic rocks overlay the accretionary sediment basement rocks (Shimanto Supergroup) unconformably.The Shimanto Supergroup in this region consists mainly of sandstone, mudstone, shale, and a conglomerate and small amount of limestone, chert, mafic volcanic rock, and serpentinite (Uto et al. 1997).
The Oldest stage Nansatsu volcanic rocks correspond to the Kasasakurose andesite described by Kuroiwa et al. (1989).It consists of pyroxene andesitic lava with tuffaceous conglomerate, sandstone, and silt.The Oldest stage rocks generally underwent strong hydrothermal alteration, but fresh rocks contain plagioclase and clinopyroxene phenocrysts (Kuroiwa et al. 1989).Although no age data for the Oldest stage rocks are available, they are considered to have been formed in the middle Miocene, owing to the fact that the unit was intruded by the acidic rocks of the Outer Zone granitoids dated between 15 and 12 Ma (Yamamoto et al. 1980;Kuroiwa et al. 1989).To date, no associated mineralization has been reported for this volcanic rock unit.
The Older stage Nansatsu volcanic rocks correspond to the Hiramichiyama, Nishinooka, and Akime Formations (Kuroiwa et al. 1989) and the eastern part of the Lower and Upper Nansatsu Groups (MITI 1985).It consists mainly of pyroxene andesitic to dacitic lava and pyroclastic rock with tuffaceous mudstone, sandstone, and conglomerate, and overlies the Shimanto Supergroup (Morozuka Group) and the Outer Zone granitoids unconformably (Kawanabe et al. 2004).The lowermost pyroxene hornblende andesite has been dated at 7.6 ± 2.3 Ma (K-Ar dating; MITI 1985).The upper pyroxene hornblende andesite has been dated at 6.4 ± 0.3 Ma (K-Ar dating), and the pyroxene andesite has been dated at 5.9 ± 0.8 Ma (K-Ar dating) and 6.16 ± 0.45 Ma (fission-track dating) (MITI 1985), which agree with the results of palaeomagnetic dating conducted by Takeda et al. (2001).The Older stage Nansatsu volcanic rocks have generally undergone strong hydrothermal alterations and host several gold deposits distributed in the Nansatsu district (e.g.Kasuga and Iwato deposits).
The Middle stage Nansatsu volcanic rocks correspond to the Nansatsu Middle Volcanic Rocks analysed by MITI (1985).They consist of hornblende andesite lava and pyroclastic rocks with minor tuffaceous conglomerate and mudstone.The Middle stage Nansatsu volcanic rocks overlie the Cretaceous Shimanto Supergroup and the Older stage Nansatsu volcanic rocks unconformably and are overlain by the Younger stage Nansatsu volcanic rocks.The degree of alteration is weaker in the Middle stage rocks than the Older and the Oldest stage Nansatsu volcanic rocks.MITI (1985) deduced that the rocks had a fission-track age of 3.4-4.6Ma (with the exception of one sample with 2.3 Ma), which agrees with the results of palaeomagnetic dating conducted by Takeda et al. (2001).The Middle stage rocks host the Akeshi gold deposit (Metal Mining Agency of Japan 1993), which is mineralized in 4 ± 1 Ma (Takeda et al. 2001).The Middle stage rocks are regarded to be contemporaneous with gold mineralization in this area (Takeda et al. 2001;Tindell et al. 2018).
The Younger stage Nansatsu volcanic rocks (not analysed in this study) correspond to the Nansatsu Younger Volcanic Rocks analysed by MITI (1985).They consist of biotite hornblende andesite lava and pyroclastic rocks with minor pyroclastic rocks.The Younger stage Nansatsu volcanic rocks is not distributed in the studied area, but it overlies the Middle stage Nansatsu volcanic rocks near Kaimondake volcano (to the east of the studied area).This unit has been dated at 2.1 ± 0.4 Ma (K-Ar dating, MITI 1985).

Materials and methods
For this study, the volcanic rock samples were collected from three localities (A, B, and C in Figure 1(c)) in the Nansatsu area of South Kyushu, Japan, as shown in (Figure 1(c)).The samples were classified into different stages of Nansatsu volcanic rocks based on their location, mineral assemblage, and rock textures.Petrological information of the

Major and trace element composition of the Nansatsu volcanic rocks
The major-and trace-element compositions of the Nansatsu volcanic rocks analysed in this study are listed in Supplementary Table S1. Figure 2 shows variation diagrams for some major oxides (Al 2 O 3 , TiO 2 , FeO T , MnO, MgO, CaO, K 2 O, and P 2 O 5 ) for the targeted Nansatsu volcanic rocks in our study.The SiO 2 contents of the Nansatsu volcanic rocks in this study ranged from 57.2 to 68.5 wt.%, corresponding to andesite -dacite composition.For the Older and the Middle stage rocks, the major element compositions of the samples in this study were very similar to those reported by MITI (1985), although we sampled them in different locations from MITI (1985).This indicates that the compositional variability within the Older and Middle stage rocks is relatively small, and our data can give a first-order, general view of the geochemistry for these volcanic rocks, although further data can provide detailed information.The rather small compositional variation is also in line with the observation that pre-Quaternary volcanic activities were dominantly andesitic, in contrast to the Quaternary basaltic to rhyolitic activities (Watanabe 2005).Unlike the volcanic activities in Hohi volcanic zone in North Kyushu (Nakada and Kamata 1991), where they observed clear increase in K 2 O content in Quaternary, no systematic change of K 2 O content between pre-Quaternary and Quaternary rocks was observed.For the Oldest stage rocks, the samples in this study displayed much more felsic composition than those reported in Kuroiwa et al. (1989), suggesting certain compositional variability within the Oldest stage rocks or the presence of unknown sub-units in the Oldest stage rocks.Nevertheless, data from Kuroiwa et al. (1989) and this study share compositional features such as distinctly higher (>0.8%)TiO 2 concentrations than the Older and Middle stage rocks (Figure 2(b)).
A normal mid-ocean ridge basalt (N-MORB)-normalized multi-element concentration diagram of the samples analysed in this study is shown in Figure 3.The Oldest, Older and Middle stage Nansatsu volcanic rocks had elemental pattern characterized by the enrichment of large-ion lithophile elements (LILE; Rb, Ba, K, and Pb), along with Th and U, and the depletion of Nb, Ta, Ti, P, and Sr.These features are typical signatures of island-arc volcanic rocks (i.e.enriched in slab-derived materials), although the concentrations of Ti, Sr, and P may have been lowered due to fractional crystallization of magnetite, plagioclase, and apatite, respectively.The arc-like patterns support the view of previous studies that ocean-island basalt (OIB)like volcanic activities with weak Nb depletion in South Kyushu, indicative of mantle upwelling (Kita et al. 2012), are restricted in the back-arc side (western side) of Quaternary volcanic activities.

Source materials of magma for volcanic rocks in the Nansatsu area
The Sr -Nd-Pb isotopic compositions of the Nansatsu volcanic rocks analysed in this study are listed in Supplementary Table S2.In the Sr -Nd-Pb isotopic diagrams (Figure 4), the Oldest stage rocks and the  Older and Middle stage rocks show markedly different characteristics.The Oldest stage Nansatsu volcanic rocks in this study have isotopic compositions similar to those of the middle Miocene Outer Zone granitoids (Figure 4), suggesting that the origin and source materials of both were similar.Previous studies have suggested that the Shimanto Supergroup, the accretionary sediment basement rocks in the Kyushu area, contributed as a major source material (up to 70%) for Outer Zone granitoids with the most differentiated composition, based on S, O, and Sr -Nd-Pb isotopic evidence (Sasaki and Ishihara 1979;Shinjoe 1997;Ishihara and Matsuhisa 1999;Shin 2008).In (Figure 4), the Oldest stage volcanic rocks plotted near the region of the Shimanto Supergroup in Sr-Nd-Pb isotopic space, suggesting that the contribution of the Shimanto Supergroup is also likely for the Oldest stage rocks.At this moment, the nature of the parental magma before the contamination of the Shimanto Supergroup is unclear.Future studies on the Oldest stage rocks with more mafic composition, as reported in Kuroiwa et al. (1989), may be able to constrain the combined processes of assimilation and the fractional crystallization of these magmas.
In contrast, the Older and Middle stage Nansatsu volcanic rocks show Sr-Nd-Pb isotopic compositions similar to those of Quaternary basalts with most primary compositions (<52 wt.% SiO 2 ; Figure 4).Previous studies have shown that these basalts were formed by the melting of wedge mantle contaminated to variable degrees with fluids from a subducting slab (Shinjo et al. 2000).Therefore, these late Miocene to Pliocene volcanic rocks likely share a common mantle source with the Quaternary rocks, and it is probably the magma differentiation processes that brought about the difference in the nature of volcanic activity (dominantly andesitic in late Miocene to Pliocene vs more variable basaltic to rhyolitic in Quaternary; Figure 5).The differentiation process of magma is commonly influenced by the stress field of the crust (Zellmer and Annen 2008), and in south Kyushu, it has been pointed out that around 2 Ma, the subduction direction of the Philippine Sea Plate changed, leading to a shift from a compressional to an extensional tectonic setting (Watanabe 2005).It is therefore possible that such a change in the tectonic setting may have triggered more variable volcanic activity.

Figure 1 .
Figure 1.Geological maps of the studied area: (a) Shimanto belt, Outer Zone of Japan, modified according to Wallis et al. (2020); (b) distribution of igneous rocks in South Kyushu, modified according to GSJ (2022); (c) geological map of the Nansatsu area portraying sampling sites and gold deposit locations, modified according to Uto et al. (1997) and GSJ (2015) [gold deposit locations are from MITI (1985)].(d) the correspondence relationships for different units in the Nansatsu area.

Figure 2 .
Figure 2. Compositional variation diagrams of Nansatsu volcanic rocks for SiO 2 vs (a) Al 2 O 3 , (b) TiO 2 , (c) FeO T , (d) MnO, (e) MgO, (f) CaO, (g)K 2 O, and (h) P 2 O 5 .Classification of high-K, medium-K, and low-K was based onGill (1981).Gray dots show the composition of Quaternary volcanic rocks in South Kyushu from GEOROC database (detailed reference are given in Supplementary material).Data fromMITI (1985)  were reclassified into the Oldest, Older, and Middle stage Nansatsu volcanic rocks as well as volcanic rocks from other area, according to the sampling location.

Figure 3 .
Figure 3. Normal mid-oceanic ridge basalt (N-MORB)-normalized trace element patterns of Nansatsu volcanic rocks.Trace element concentrations of N-MORB are from Sun and McDonough (1989).

Figure 5 .
Figure 5. Summary of tectonic settings of igneous activity in South Kyushu since Middle Miocene (See also section 2.1).