ISSN 0001-4370, Oceanology, 2016, Vol. 56, No. 2, pp. 267–274. © Pleiades Publishing, Inc., 2016. Original Russian Text © E.A. Chernysheva, D.V. Eroshenko, 2016, published in Okeanologiya, 2016, Vol. 56, No. 2, pp. 287–294.

MARINE GEOLOGY

Signs of Continental Rifting in the Southwestern Japanese Island Arc E. A. Chernysheva and D. V. Eroshenko Atlantic Division, Shirshov Institute of Oceanology, Russian Academy of Sciences, Kaliningrad, Russia e-mail: [email protected], [email protected] Received September 16, 2014

Abstract⎯The southwestern margin of the Japan Arc evolved in the geodynamic regime of continental rifting during the Miocene–Pleistocene. This has been verified by broad manifestations of metasomatosis of mantle peridotites that underlie the lithosphere of the Japan Islands and by episodes of deep magmatism (kimberlites and melilitites) in the region. The high enrichment of deep melts in incompatible rare and rare earth elements is partially preserved in melts of regional basalts from smaller depths. In contrast, spreading basalts of the Sea of Japan and subduction basalts from the Nankai trench at the boundary with the Philippine Plate are extremely depleted in rare elements. DOI: 10.1134/S0001437016020041

INTRODUCTION Marginal seas of the Western Pacific formed as a result of long-term interaction between the eastern Eurasian Plate and adjacent moving oceanic plates (see [1, 4, 15, etc.]). At the continent–ocean boundary, the most important is separation of geochemical types of igneous rocks typical of different geodynamical settings. The influence of oceanic processes on the composition of igneous rock series has been studied in the most detail: oceanic basalts from juvenile seas, products of oceanic plate subduction [4, 6, 16], and other signs have been distinguished. To a lesser degree, the influence of the continent on the character of magmatism in marginal seas is not well studied. However, the East Asian continental plate is one of the most ancient and one of the largest cratons; it includes the Sino-Korean diamond-bearing province encompassing North and Northeast China and the northern Korean Peninsula. During the Paleozoic and Cenozoic, the all of Eastern Asia underwent intensive rifting that supposedly led to the appearance of the marginal seas of the Western Pacific [4, 5]. Simultaneously, west of the coast, the continental East China rift system formed; it controlled the activation zones of kimberlite fields. The aim of this communication is to analyze the literature data on manifestations of continental magmatism in southwestern Japan. The Sea of Japan region had completely formed within the limits of the continental Eurasian (East Asian) Plate [6]. Tectonic processes of tension, shear, thrust, and rifting, which had been being developed for a long time, were accompanied by thinning of the continental plate until its breaking, which led to the

appearance of an oceanic basin in the northern part of the region ca. 20 Ma BP [13]. Southward and southeastward expansion of the basin took place in the same regime of tension and breaking of the Earth’s crust. Complete opening of the Sea of Japan ended 14–16 Ma BP. Volcanic activity within the limits of the Japan Arc is mostly less than 12 Ma in age. Relic blocks of the continental crust can be found in the southern part of the Sea of Japan, represented by rises and ridges divided by grabenlike depressions [4]. The character of interaction between the Japan island arc and plates within the limits of the Pacific makes it possible to distinguish three segments along the arc (Fig. 1, [1]): (1) from the Kuril Islands to the northeastern Japan arc, the region rests on the North American Plate, which is subducted by the Pacific Plate; (2) the Izu-Bonin arc rests on the Philippine Plate and is also subducted by the Pacific Plate; (3) from southeastern Japan to Ryukyu Island, the region rests on the Eurasian Plate, which is subducted by the Philippine Plate. However, during the opening of basin of the Sea of Japan, the Philippine Sea Plate was located far to the south and approached southwestern Japan as late as 2 Ma ago, so that this part of the arc barely suffered subduction and instead has preserved traces of the earlier geological events directly related to the East Asian Plate. The earliest such event is intrusion of lamprophyre dikes in central Shikoku Island, Shingu area (Fig. 2), ca. 21 Ma BP [20]. The dikes contain abundant xenoliths of mantle and crustal rocks, but most important is the fact that mantle xenoliths contain diamond microinclusions [14]. Xenoliths are represented by peridotites, pyroxenites, and pyroxene xenocrystals. Peridotites are classified as transitional between plagioclase and spinel lherzolites. Pyroxenes pertain to two

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yu

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Fig. 1. Tectonic scheme of the Japan Islands region, after [1]. Dashed lines denote plate boundaries. Arrows indicate motion of oceanic plates.

varieties, “green” and “black”; isolated diamond microinclusions are revealed only in the “green” variety of pyroxene. As part of the inclusion, diamond can come directly in contact with carbonate and aqueous minerals; cavities always contain gaseous CO2. The chemical composition of xenolith-containing dike rocks indirectly indicates the large depth of their formation: they are characteristic of low SiO2, high TiO2, and very high CO2 (6–8 wt %) contents, with a low H2O content, in contrast to other alkali rocks of this region (table). Petrological analysis [14] suggests that diamonds and host minerals formed at depths of 150– 170 km, at pressure of about 5.5 GPa and temperature of about 1500°С, in the carbon-saturated mantle substrate. In terms of age, lamprophyre dikes of Shikoku Island are older than other Miocene volcanic rocks of the island and their age is close to that of spreading basalts of the Sea of Japan. Manifestations of diamondcontaining dikes and diatremes are known within the

limits of fault zones in the western coast of the sea (in North Korea, Primorye, and Sikhote-Alin). Perhaps the combined system of deep faults had existed that reached the mantle, and this system joined the Japan arc and the Eurasian Plate [4, 5]. The second manifestation of the “characteristic” continental rocks of the Japan arc is located in southwestern Honshu Island, in the area of Hamada [19]. This is a small lava flow atypical of the Japan Islands (melilite-olivine nephelinites), occupying an area of about 2 km2 with a maximum height of 186 m above sea level (Fig. 2). The flow superimposes Jurassic shales in the East and Early Miocene rhyolites in the West. Based on absolute K–Ar dating, Hamada melilite-olivine nephelinites formed 5.75 ± 0.20 and 6.10 ± 0.19 Ma BP. The mantle xenoliths of lherzolite and wehrlite of 1–3 to 6 cm in size are reported in lavas. Nephelinites possess extremely fine-grained texture with rare olivOCEANOLOGY

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Fig. 2. Volcanism manifestations in southwestern Japan Islands: (1) lamprophyre dikes with xenoliths of diamond-bearing peridotites in Shingu area, after [14]; (2) lavas of melilite–olivine nephelinites in Hamada area, after [19]; (3) volcanoes composed of subalkali basalts from Honshu Island, after [12]; (4) Nanzaki volcano composed of basanites. Izu Peninsula, after [16]; (5) Philippine Sea Plate boundary (Nankai trough), after [17].

ine and titanomagnetite nephelinites (up to 2 and 0.5 mm in size, respectively), and the finer and rarer clinopyroxene and melilite phenocrysts. The groundmass of rock is composed by clinopyroxene, olivine, melilite, titanomagnetite, nepheline, and apatite. Chemical composition of Hamada melilite-olivine nephelinites differs from other volcanic rocks of Honshu Island in an extremely low SiO2 content of about 37 wt %, lower Al2O3 and higher TiO2, CaO, MgO, alkali, and phosphorus contents, and, importantly, anomalously high contents of incompatible rare elements Sr, Ba, Nb, Zr, and light REE (table, Fig. 3). These features characterize volcanic rocks of Hamada as close-to-primitive deep melts of very low degree of melting. Isotope compositions of Sr and Nd in meliliteolivine nephelinites also indicate their deep mantle source: 87Sr/86Sr = 0.703794–0.703909; 143Nd/144Nd = 0.512824–0.512932 [19]. The researchers believe that such an unusual enrichment in nephelinites with rare OCEANOLOGY

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elements was caused by intense metasomatosis of mantle lherzolites and wehrlites at the lower boundary of the lithospheric plate under the effect of carbonatite or kimberlite melts. As noted above, mantle xenoliths with signs of metasomatosis are widespread in southwestern Japan [1, 10]. Based on the data above, we can state that Hamada volcanic rocks refer to the olivine melilite–nephelinite–phonolite series (see [8, 21, 22] and others), which is well known in many continents, in regions that manifested riftogenic alkali magmatism; in these regions, rocks of this kind often associate with kimberlites or are represented as parts of complex massifs of alkali rocks with carbonatites. A classic example of how this type of volcanism manifestation is Central and Western Europe, where the collision between the African and European continental plates in the Cenozoic led to large-scale movements in the upper mantle [22]. Uplift of the basement resulted in tension and

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Volcanic rocks of Sea of Japan region Sample nos.

1

Sample SNG-N-01 SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 CO2 LOI Total Sc V Cr Co Ni Cu Zn Rb Ba Sr Y Nb Ta Zr Hf La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Pb Th U

41.69 2.05 14.70 1.19 8.83 0.16 7.31 8.99 2.85 1.65 0.46 8.14 1.76 99.82 – – 300 – 100 – – – – – – – – – – – – – – – – – – – – – – – – – – –

2 A-8 43.76 2.07 14.82 10.53 – 0.16 7.68 8.95 2.47 1.88 0.52 – – 92.84 22 – 210 37.8 130 – – 46 1370 670 21.3 – 3 200 13.8 32.1 69.5 – 30.39 6.01 2 – 0.8 – – – – 1.62 – – 5.78 –

3

4

6

7

8

9

10

HM 18-1 HM 11-1 HM 21L

AR14

TKY

B201

T802

797

35.63 2.59 11.42 14.78 – 0.34 8.63 14.46 4.62 2.31 2.28 0.07 1.85 99.98 – – – – 72 – – 61 1891 2937 52 158 – 360 – 195 326 – 152 21.6 4.5 – – 12.2 – – – 4.8 – 9 23 –

44.19 1.93 13.15 – 10.08 0.17 12.05 11.33 1.74 1.53 0.49 – – 96.62 – – 470 – 310 – – 57 730 620 26 48 – 180 – 36 71 – 33 6.3 2.0 – – – – – – 2.1 0.34 – 5.3 –

46.90 1.80 14.24 – 9.68 0.16 10.45 10.59 2.97 0.89 0.51 – – 98.18 – – 360 – 230 – – 40 720 670 25 45 – 180 – 40 76 – 45 6.8 2.0 – – – – – – 2.0 0.36 – 6.5 –

42.44 1.198 14.88 11.31 – 0.204 12.44 12.57 1.80 0.81 0.41 – 2.13 100.20 35 304 730 55 310 100 30 22 598 945 20.4 30.6 2.06 61 1.6 28.7 51.1 6.22 25.3 5.6 1.74 4.94 0.79 4.3 0.82 2.1 0.294 1.77 0.249 5 3.81 1.22

42.91 1.441 14.96 11.55 – 0.20 10.34 12.57 2.18 0.35 0.52 – 2.51 99.53 36 336 570 49 240 80 90 10 450 616 22.3 37.8 2.08 104 2.4 28.6 54.5 6.34 27.6 5.87 1.83 5.31 0.81 4.42 0.82 2.17 0.301 1.89 0.299 5 5.14 –

49.27 1.79 17.33 11.05 – 0.45 11.65 3.13 3.58 0.38 0.21 – 7.36 100.07 44.1 480 294 40.9 55 70 90 2 70 153 35 7 0.333 121 3.11 8.21 21.6 – 13.6 4.12 1.52 – 0.86 – – – – 3.65 0.522 3 0.37 0.22

36.23 2.58 11.29 14.96 – 0.34 8.80 14.04 4.71 2.53 2.34 0.07 1.67 99.66 – – – – 73 – – 63 1424 2787 51 149 – 348 – 215 361 – 155 23.6 5.0 – – 13.4 – – – 5.0 – 10 23 –

5

36.89 2.56 11.58 14.55 – 0.33 9.09 14.70 2.92 1.89 2.22 0.05 4.04 100.85 – – – – 76 – – 76 1708 2622 51 153 – 357 – 210 350 – 155 23.1 4.9 – – 13.0 – – – 5.2 – 10 22 –

Columns 1 and 2 correspond to lamprophyre dikes [15, 20]; 3–5, to Hamada melilite–olivine nephelinites [19]; 6 and 7, to subalkali basalts of the Kibi volcano area [12]; 8 and 9, to basanites of the Nanzaki area (scoria and massive lava) [16]; 10, spreading basalt from the Sea of Japan (borehole 797 ODP) [7]. Dash means no data. OCEANOLOGY

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Fig. 3. Geochemical characteristics of Hamada melilite–olivine nephelinites (upper line). Kibi volcano basalts and Nanzaki volcano basanites (Honshu Island). Distributions of rare earth (a) and incompatible admixture elements (b) are normalized on chondrite and primitive mantle, respectively, after [18].

associated thinning of the lithospheric plate, as well as in cracking and riftogenesis. In the known rift zones and grabens of Germany, France, and Czech Republic, lavas and volcanic edifices are composed by alkali basalts, basanites, olivine nephelinites, and melilitites, whose source was probably the mantle peridotites metasomatically enriched as a result of influx of rare and volatile (СО2) components from the sublithospheric mantle. Note that volcanic rocks of the melilite-nephelinite-phonolite series in these rift zones and in similar structures of Africa, America, and other regions possess very similar characteristics of Sr and Nd isotope composition (see [21, 22] and others). The sequence of events in southwestern Japan also follows this scenario—from metasomatosis of mantle peridotites to sequential formation of rocks pertaining to the kimberlite and melilitite series—and was undoubtedly related to tectonic activation of the East Asian Plate, whose thick subcontinental mantle yields flows of thermal, volatile, and rare elements into the tension and riftogeneous zone along the southwestern margin of the Japan Islands. DISCUSSION Study of the carbon behavior in the Earth’s interior based on many facts shows that magmatic processes in the upper and lower mantle are achieved for high activity of alkali carbonatite melts with a small degree of melting, which are in equilibrium with rocks of basic and ultrabasic paragenesis [3]. Carbonatite metasomatosis can lead to considerable enrichment of mantle peridotites in incompatible rare elements. The high dissolubility of CO2 in carbonatite–silicate mantle melts is an important factor for the transport of diaOCEANOLOGY

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mond-bearing kimberlites from depths of 150–250 km [2]. The analytical research data on inclusions from mantle minerals and the experimental research data indicate that origination and growth of diamond crystals in peridotites saturated with alkali-carbonate melt are possible under lithospheric mantle conditions [3]. It was shown experimentally [2, 11] that in the presence of a high amount of СО2, in equilibrium with mantle peridotites, all types of melts from peridotites to kimberlites can form. The transition from a kimberlite to melililite melt is achieved by a reduction in the CO2 pressure from 8–6 to 4–3 GPa, and with a further decrease in pressure, the melt may become a basalt or picrite composition (see [2, 3, 11, etc.]). Hence, СО2 contents of 6–8 wt % in the lamprophyre dikes of Shikoku [10] probably reflect the actual high CO2 pressure conditions in the formation of xenoliths of diamond-bearing peridotites. As noted above, the tectonic events that took place ca. 20 Ma BP in the northern Sea of Japan led to rupturing of the lithospheric plate and spreading of the newly formed oceanic crust [13]. Then, the Japan Islands were shifted to the southeastern Sea of Japan; meanwhile, tectonic processes were activated in the East Asian Plate. Crustal tension, compression, and shearing during southward motion of the island arc were not accompanied by rupturing of the continental crust [4], but by thermal and fluid flows directed from the craton towards the rifting zone [6]. The main result from the effect of these flows was metasomatic alteration of peridotites underlying the continental plate of the island arc. Perhaps, the shattered base of the continental plate in the rifting zone was a screen or trap for carbonatite melts enriched in volatile and incompati-

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100

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20

La Ce Pr NdSm Eu Gd Tb Dy Ho Er Tm Yb Lu

10

3

RbBaThNb U La Ce PbNd Sr SmZr Eu Y YbLu

Fig. 4. Geochemical characteristics of spreading basalts from Sea of Japan based on data from borehole ODP 797 [13]. Distributions of rare earth (a) and incompatible admixture elements (b) in basalts are normalized on chondrite and primitive mantle, respectively, after [18].

ble rare elements and transported from the subcratonic mantle. The appearance of lavas anomalously enriched in rare and rare earth elements of melilite– olivine nephelinites in Honshu Island was the result of melting of the enriched “base” of the continental plate ca. 6 Ma BP. The tension regime of the lithospheric plate (rifting) is the most favorable for the appearance of melts with a small degree of melting (kimberlites and melilitites). The change in the deep melts of Shikoku kimberlite series (150–170 km depth) to those of Hamada melilitite series (about 90 km depth) should be accompanied, as noted above, by a decrease in CO2 pressure from 6–5 to 4–3 GPa [11]. Further uplift of magma led to its degassing at depths of 30–60 km and to an increase in the degree of melting of the initial substrate. It is this type of volcanic rocks that is represented in the same region of southwestern Honshu (Chugoku area) in the form of multiple volcanoes composed of subalkali and alkali basalts and their differentiation products of 12 to 0 Ma in age (Fig. 2) [12]. The isotopic composition of volcanic rocks varies toward lower 143Nd/144Nd and higher 87Sr/86Sr values, compared to Hamada nephelinites [15]. In contrast to volcanic rocks of deeper origin, basalts possess a different mineral composition and are considerably more depleted in REE and other incompatible elements, but they are surprisingly similar in the character of distribution of these elements (Figs. 3a and 3b). For comparison, we show data on compositions of the least differentiated basalts from Kibi volcano (9 Ma in age) and basanites of the young Nanzaki volcano (0.43 Ma) on the Izu Peninsula (Fig. 2); note that the Izu Peninsula is the eastern terminus of Honshu Island and is believed (however, with some doubts) to be part of the Izu–Bonin island arc [16]. In terms of isotope compo-

sition, Nanzaki basanites considerably differ from volcanic rocks of other parts of this island arc, but are close to basalts of Honshu Island: 87Sr/86Sr = 0.703143– 0.703272; 143Nd/144Nd = 0.512992–0.513023 [16]. The difference between the ages of Kibi and Nanzaki volcanic rocks does not affect the composition of basaltoids, indicating that the enriched source of basalts in Honshu Island was the same. Most likely, this source was metasomatically enriched (carbonated peridotites from the basal part of the lithospheric plate of the Japan Islands: at the early stage and under different conditions, Hamada melilite nephelinites of deeper origin melted from these rocks. Importantly, basalts of Honshu Island differ from both the spreading basalts of the northern Sea of Japan (borehole 797 ODP [7]) (Figs. 4a and 4b) and subduction basalts of the Nankai trough (borehole 808 ODP [17]) in terms of geochemical characteristics. Spreading basalts are the most depleted rocks from the smallest depths and are close to oceanic basalts (MORB) with continental crust relics, as indicated by the clearly pronounced Pb anomaly in the spider plot. Comparison between the compositions of volcanic rocks of the Japan island arc (Fig. 5a) and those of rock series from grabens and rifts of Central and Western Europe (Fig. 5b) suggests certain similarities. Figures 5a and 5b show the dependence of the degree of enrichment of melts (La/Yb) on melting depth (Ce/Y) [9]. In both plots, melilitites and olivine nephelinites of the deepest origin are the most enriched, in contrast to basanites and basaltoids. Basalts and basanites of Honshu Island partially inherited the higher enrichment of the deep riftogenic substrate. Volcanic rocks with similar geochemical OCEANOLOGY

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Fig. 5. Variations in degree of enrichment of volcanic rocks with rare elements (La/Yb) depending on depth of melting (Ce/Y), after [9]. (a) Japan island arc: 797 spreading basalts from Sea of Japan; crosses mean subalkali basalts from southwestern Honshu Island, after [12] (differentiated units are not shown); (1) Kibi volcano basalts. after [12]; (2) Nanzaki volcano basanites, after [16]; Hamada means melilite–olivine nephelinites of Honshu Island, after [19]. (b) Rifts and grabens of Central and Western Europe: melilitites and olivine nephelinites are indicated by closed circles; basanites, open circles; alkali and subalkali basalts; crosses— all after [21, 22].

characteristics are rarely found among rocks within the Pacific (for example, borehole 801 ODP). They were probably formed under similar conditions and may be a reference to the geodynamic setting of continental rifting. CONCLUSIONS In the southwestern Japan island arc, some signs of continental rifting in the margin of the East Asian Plate have been preserved. Evidence for this is (a) the widely manifested metasomatic alteration of mantle peridotites in the region and (b) sequential formation of rocks pertaining to the Shingu kimberlite series (ca. 21 Ma BP) and lavas pertaining to the Hamada melilitite series (5–7 Ma BP). These events are related to activation of deep tectonic processes in the subcratonic mantle beneath the East Asian Plate and to the inflow of alkali carbonatite melts to the riftogeneous zone within the destructing continental plate of the Japan island arc. The tension regime within this plate ensured the formation of the deepest melts with a small degree of melting, which were the most enriched in incompatible rare and rare earth elements. Higher enrichment is partially inherited by volcanic rocks from smaller depths and can be a reference when reconstructing ancient geodynamical settings. REFERENCES 1. S. Arai, “Petrologic features of peridotites in upper mantle under island arches of Japan. Petrogenesis of spinel peridotites,” Geol. Geofiz., No. 1, 14–31 (1991). OCEANOLOGY

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Translated by N. Astafiev

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