Journal of GEOLOGY, Series B, No

I. INTRODUCTION

The eastern margin of Eurasia incorporates numerous Cenozoic alkaline basalt fields that temporally replaced suprasubduction Cretaceous magmatism and marked continental margin extension zones. The geodynamic environment of alkaline volcanites is represented by grabens and pull-apart structures formed as a distant result of the Indo-Eurasian collision [15, 17, 27, 28; etc.]. However, their magmatic sources remain a debatable subject.

There are two general competing opinions connecting the magma formation process either to the contribution of enriched continental lithospheric sources or to plumes originating in the upper or lower mantle. Based on the combined geochemical and geophysical data, the latter model (lower mantle upwelling) is often thought to be more reliable.

It is remarkable that in any petrologic genetically model of Cenozoic alkaline basalts from the marginal extension zones (MEZB) of eastern Asia, data on the compo-sition of OIB from the central Pacific ocean as chemical indicators of within-plate basalt (WPB) are used. This portion of the ocean was previously distinguished as the Southern Pacific Isotopic and Thermal Anomaly (SOPITA) belonging to DUPAL mantle [6, 26, 48; etc.). The problem of magma source correlation of MEZB and OIB was usually not discussed, because of the considerable difference in the geodynamic settings of these volcanic complexes: continental lithosphere in contrast to intraoceanic area. The problem of the correlation of components of MEZB and OIB sources has become urgent in connection with the distribution of the DUPAL anomalous mantle not only in the Indian and Pacific oceans [20, etc.], but also in a large region of eastern and southeastern Asia [17, 46; etc.]. It was also shown that both the territory of Pacific OIB and the eastern margin of Eurasia display low-velocity P-wave seismic anomalies [10,18; etc.) differing, however, in morphology and depth of occurrence.

II. COMPARATIVE CHARACTERISTICS OF ALKALINE BASALTS FROM EURASIAN CONTINENTAL MARGIN AND THE PACIFIC OCEAN

The extension structures that originated in the Cenozoic at the eastern margin of Eurasia (Sikhote Alin, Korea-Japan region, eastern China, and Indochina) in response to the India-Eurasia collision were accompanied mainly by alkaline basalts (MEZB) (Fig. 1). They form numerous dispersed small areas of Cenozoic (Eocene-Holocene) volcanism of the WPB type. The age of rocks varies from site to site, which is related to short-term and intermittent volcanic activity in eastern Asia. The formation of MEZB areas was related to fissure eruptions and the activity of small monogenetic or large shield volcanoes. The typical MEZB area morphology is a flat-summit lava plateau formed by numerous near horizontal flows.

Areas of MEZB distribution can be exemplified by the Korean Peninsula (Fig. 1) whose southern and northern parts and Jejudo Island have been extensively studied [12-15; etc.]. In this territory and adjacent regions of northeastern China, islands of the Sea of Japan, and southwestern and central parts of Honshu Island, Japan (Fig. 1), volcanic occurrences related to continental margin destruction were dated from the Eocene up to present time [11, 19, 29, 31, 47, 57; etc.]. Despite their general affinity to WPB-type alkaline series, the Cenozoic MEZB of this region are characterized by rather wide variations of geochemical and isotopic parameters, which reveal sometimes the presence of a subduction-related component. The situation is complicated by the local occurrence of Early Miocene tholeiitic basalts, which correspond to the spreading stage of the Sea of Japan [1,7; etc.] and are included into the thick column of Cenozoic alkaline volcanics.

All rocks belong to WPB type and range from strongly alkaline, transitional, to tholeiitic basalts. Fractional silicic volcanic form minor volume. Their trace element profiles are characterized by incompatible element enrichment similar to DUPAL ocean island basalts. The isotopic composition clearly depicting the involvement of DUPAL components. The MEZB of the alkaline and moderate-alkaline series are characterized by high Nb, Ta, and Zr concentrations, elevated La_N/Yb_N values, and the P-OIB signature [1, 4, 13, 24, 40, 41; etc.). The tholeiitic basalts form a lenticular body of primarily E-MORB rocks in the thick Cenozoic (alkaline MEZB) sequence.

On the whole, the Cenozoic MEZB of eastern Asia is not geochemically different from the Pacific OIB [23, 59]. They show greatly variable contents of particular elements including total alkalis. It is remarkable that considerable variations of MEZB compositions are revealed even from a single area, which is exemplified by the Tkhongchong and Paennyondo regions of central Korea and islands in the Tsushima Strait and the Sea of Japan.

The isotopic compositions of MEZB also show resemblances to OIB compositions plotting above the NHRL, within the enriched I-MORB (or the DUPAL anomaly) fields. Although MEZB do not include all the extreme isotopic end-members that are found in OIB [59], their concentrations of radiogenic isotopes are considerably scattered. The relationships ¹⁴³Nd/¹⁴⁴Nd–⁸⁷Sr/⁸⁶Sr of the Miocene-Holocene alkaline basalts (with occasional flows of enriched tholeiitic basalts) from Korea, Jejudo, and Honshu (Chugoku area) form a linear trend extending from E-MORB-like compositions to EMI. The MEZB of different areas of the Korea-Japan region occupy different segments of this trend, which emphasized the lateral heterogeneity of the volcanic.

All the Cenozoic MEZB of the Korea-Japan region and the tholeiitic basalts of the Sea of Japan show high ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb (at given ²⁰⁶Pb/²⁰⁴Pb), which classifies them as volcanic related to the DUPAL anomaly. In the ⁸⁷Sr/⁸⁶Sr - ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb - ²⁰⁶Pb/²⁰⁴Pb diagrams, the Pliocene-Quaternary alkaline basalts of Ulreung-do and Dog islands are close to EMI, and the volcanic of Oki-Dogo Island combines the characteristics of two components, EMI and EMII. The Early Miocene tholeiitic basalts of the E-MORB type form linear trends in these diagrams between DMM and EMII, although their ⁸⁷Sr/⁸⁶Sr - ²⁰⁶Pb/²⁰⁴Pb relationships suggest the influence of the EMI component. Points of the most depleted tholeiitic basalts of Hole 797 form a separate group in these diagrams approaching DMM.

Thus, the Sr, Nd, and Pb isotopic compositions of the MEZB are located between three end-members: enriched MORB, EMI, and EMII, and the former two components are predominant. The lateral isotopic heterogeneity of the alkaline volcanic suggests their relationships to different sources rather than mixing of magmas with contrasting isotopic characteristics.

It is important to point out that WPB-type rocks accompanying Cenozoic extensional structures of other regions of the eastern Eurasian margin exhibit relationships to the same extreme components, MORB, EMI, and EMII (i.e., characteristics of DUPAL anomaly). The proportions of these three constituents vary over the area. For instance, the Cenozoic alkaline basalts of northeastern China form a trend between DMM and EMI on the ¹⁴³Nd/¹⁴⁴Nd - ⁸⁷Sr/⁸⁶Sr, ⁸⁷Sr/⁸⁶Sr - ²⁰⁶Pb/²⁰⁴Pb, and ²⁰⁷Pb/²⁰⁴Pb - ²⁰⁶Pb/²⁰⁴Pb diagrams with a minor role of EMII [4, 53; etc.]. In contrast, the basalts of Indochina are dominated by MORB and EMII components at a minor contribution from EMI [42, 46; etc.].

The OIB of the Pacific Ocean occur within the DUPAL isotopic mantle anomaly, which forms a global belt at about 30°S [20]. It was found [10] that this territory corresponds to a low-velocity mantle zone. The criteria for distinguishing the DUPAL-type mantle included some basalt properties: high values of ⁸⁷Sr/⁸⁶Sr (>0.7035), unusually high ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb (at a given ²⁰⁶Pb/²⁰⁴Pb), and, in general, the presence of the isotopic components EMI, EMII, and HIMU [9, 20, 21, 59]. The origin of HIMU and EMI components of the Pacific OIB is related to the recycling of subducted oceanic crust of varying age and the lower mantle upwelling [21-23; etc.]. Sometimes continental crust is proposed as a source of EM components, especially EMII [2; etc.]. However, EMII-type alkaline basalts from the Samoa Islands, which are considered a superplume analogue, have a high (lower mantle type) He isotope content (R/Ra > 8-22, where R is the ³He/⁴He value in rock and Ra is the He component in atmosphere) [54].

The comparison of the isotopic compositions of the Cenozoic MEZB of the continental margin and Cretaceous-Cenozoic Pacific OIB demonstrated their substantial similarity and certain differences. The available data on the Korea-Japan region suggest that MEZB contain isotopic groups similar to those distinguished among the Pacific OIB. Group 1 of OIB (dominated by EMII) are similar to some basalts from Ulreungdo and Oki-Dozen islands. These rocks are close to the Cretaceous OIB of Hemler (Magellan chain) and Wilde (Wake Archipelago) seamounts and the Cenozoic OIB of the Samoa, Societies, and some of the Marquesas Islands. OIB of isotopic group 4 (dominated by EMI) are much more widespread in the Korea-Japan region including the MEZB of Honshu, central Korea (Chugaryong area), and part of Ulreungdo, Oki-Dogo, and Oki-Dozen islands. These rocks are analogues of the Cretaceous volcanic of Limalok and Wodejebato seamounts (Marshall Islands) and the Cenozoic islands of Rarotonga and Pitcairn in Polynesia.

No complete analogues of OIB group 3 with a significant contribution from the N-MORB component were found among the alkaline rocks of the Korea-Japan region. Nevertheless, the basalts from the Phohang graben, some rocks of the Chugaryong area in Korea, and some Jejudo volcanics tend toward this isotopic end-member. The alkaline basalts of these regions can be distinguished into group 3a with transitional isotopic signatures formed by mixing of the MORB and EM components. The Korean rocks of isotopic group 3a are similar to some rocks from the Cretaceous seamounts of the Magellan chain and some Cenozoic volcanic of the Societies and Samoa.

Thus, the principal similarity between the isotopic compositions of the Pacific OIB and MEZB of the continental margin is related to the presence of alkaline basalts with EMI and EMII characteristics. On other hand, in comparison with the OIB, the MEZB are isotopically less variable and show no evidence for the influence of HIMU and depleted mantle compositions (DMM). The MORB-like component in the alkaline basalts of the Korea-Japan region was found only in combination with EM components. Tholeiitic basalts similar to MORB were formed only in extension zones during the opening of the Japan basin. However, even in that zones the majority of rocks belong to enriched varieties. There is another essential difference between MEZB and OIB: a subduction component is sporadically manifested in the former (Ta-Nb minimum, low Ti content, etc.), whose significance decreases in general from the early to later stages of continental margin destruction and MEZB accumulation.

III. DEEP STRUCTURE OF THE PACIFIC OCEAN AND ITS WESTERN CONTINENTAL MARGIN

Later studies [17, 46; etc.] demonstrated that similar DUPAL-like isotopic anomalies occupied a considerable area in southeastern and eastern Asia (together with adjacent marginal basins and the Korea-Japan region, which is discussed in this paper) up to the latitude of Lake Baikal. The northerly DUPAL mantle "penetration" is often explained [17-27; etc.] by the India-Eurasia collision. However, such a mechanism of mantle mass movement from the Southern Hemisphere seems to be hardly possible. It cannot account for the appearance of similar basalts as far as in northeastern Asia.

The western and central segments of the Pacific Ocean (SOPITA region) show a swell on the geoid surface, elevated heat flow, and low velosity anomalies (LVa) of seismic P-waves in the underlying mantle [10, 18, 36, 38, 48, 50, 55; etc.]. All these data (together with magnetometer results) formed a basis for understanding the mechanism of mantle upwelling and reconstruction of the Middle Cretaceous Pacific lower mantle superplume. During the Cretaceous-Holocene, this superplume has supplied to the surface heat energy and relatively undepleted material [18, 32, 33, 37, 49; etc.], which was the main source of OIB. The origin of Pacific superplume was a result of subduction of the coldest oceanic slab corresponding to high velocity anomalies (HVa) of P-wave to the hottest mantle [38]. The HVa immediately above the core-mantle boundary (CMB) within the Pacific Ocean, suggest the presence of slab graveyards formed at geological period much older than the breakup of Rodinia. Mechanism of superplume formation is explained [38] by transformation of post-perovskite of slab graveyard to perovskite at the top of the D" layer at ca. 2550-2900 km depth, and at a T = 2000-3500 K. This polymorphic transformation is exothermic, releasing heat injection to convert the low-T slab graveyard to a region of hot superplume with time. The exothermic polymorphic transformation raises the temperature of CMB to 4000° K that is resulted to melting the recycled MORB to form the Pacific superplume corresponding to the LVa. Besides, the major discharge of heat and light elements from the core is into the superplume.

On the global scale, seismic tomography studies have showed the complex structure of vertical ascending plumes and the possibility of lateral movement of hot mantle material with branching out of daughter plumes at various levels often separating from the main body. The daughtert plumes can also lose contact with the main feeding plume [5, 55; etc.]. The high lateral mobility of low-density lower mantle intercalating vertically with high-velocity anomalies (HVa) of subducted oceanic slabs results in a layered distribution of materials with different seismic properties in the mantle [37; etc.].

In the periphery of the Pacific Ocean, a half-ring LVa occurs at depths of 35-200 km [55]. It is composed of two main segments, western and eastern. The western arcuate segment extends almost continuously at the depth of 35-200 km from the Chukchi Peninsula to New Zeland and is composed of several cells with minima in P-wave velocity. One of the strongest minima comprises the territory of the East China Sea, Korea, and southwestern Honshu. The southernmost LVa minimum is situated east of Australia in the area of the Fiji Sea and the Fiji Archipelago. It is main Pacific negative P-wave superanomaly with a complex variable morphology and structure which traced continuously to the lower mantle and the D" layer. The remaining portion of the western arcuate negative anomaly attenuates gradually by already at a depth of 350 km. At a depts of 400 km it is changed by a continuous arcuate HVa, which embraces the territory of the Pacific Ocean and near-continental part from the Chukchi Peninsula through the Sikhote Alin, northeastern China, Korea, South China Sea region, and New Hebrides islands bonded on the south by the Pacific negative superanomaly. This arcuate HVa is interpreted [5, 18; etc.] as a subducted slab. Seismic profiles suggest that the slab changes from an inclined position in the shallow mantle levels to almost horizontal in the 410-660 km transitional zone.

It is worth to note that the arcuate LVa in the western periphery of the Pacific Ocean reappears below this high-velocity slab already at a depth of 660 km, initially in fragments and at a depth interval of 1270-1470 km as an essentially continuous western periphery band, which again contacts on the south the main LVa of the Pacific superplume. This western arcuate LVa in the middle mantle (1270-1470 km) is wider than the upper mantle LVa anomaly and comprises the entire area of the Bering Sea (including the Aleutian arc), western Northwestern Basin of the Pacific Ocean, the region of the Shatsky Rise and Emperor Seamounts, and, on the south, part of the Philippines Sea, eastern Marianna basin, and Darvin Rise and connects further on the south with the meridional branch of the main Pacific low-velosity superanomaly.

In contrast to the arcuate LVa in the western periphery, the Pacific low-velocity superanomaly is traced continuously through the upper and lower mantle to a depth of 2700 km showing a varying morphology. At middle and lower mantle depths (1270 to 2670 km), the superanomaly is composed of two channels of lower mantle upwelling: western Pacific and southern Pacific. The Pacific superanomaly is rootless on the whole: the corresponding area at the CMB is occupied by an extended high-velosity P-waves anomaly [50, 55] corresponding to the ancient slab graveyard.

The tomographic data revealed expansion of the superplume head at depths of 1470-1870 km and even more extensive lateral enlargment of lower mantle material at middle (1270-1470 km) and upper mantle (35-200 km) levels. This was accompanied by the formation of arcuate lateral plumes in the western Pacific margin. Thus, the synthesis of distribution of low-velocity seismic anomalies in the mantle revealed the Pacific superplume of complex morphology and spatially related circum-Pacific arcuate "layer" bodies at several depth levels. This model allows us to explain the appearance of deep-derived lower mantle material in the shallow upper mantle of the periphery and continental framing of the Pacific Ocean and, thus, approach the solution of the problem of the source of WPB volcanism in the Eurasian margin.

IV. MECHANISM OF INFLUENCE OF PACIFIC SUPERPLUME ON THE EURASIAN CONTINENTAL MARGIN

A variety of geological, geochemical, isotopic, and geophysical data suggests that the unusually extensive Middle-Cretaceous lower mantle upwelling resulted in formation of Cretaceous-Holocene WPB-type basalts in the inner parts of the Pacific Ocean and Cenozoic basalts in its margins. The dynamics of the superplume that has originated at the CMB at 130-120 Ma was controlled by the combination of the vertical ascent of mantle material and lateral movements along the boundaries of rheological (temperature and composition) instabilities into circum-Pacific regions. The area of the maximum and longest (Middle Cretaceous - Holocene) of the Pacific superplume action was situated directly above the head of the vertical stem of the superplume, whereas its lateral limbs attained the oceanic periphery with a certan time delay, which was responsible for the respective later WPB volcanism in the Cenozoic on the continental margin. In this context, the analysis of relationships of OIB with lower mantle upwelling provides a basis for understanding the mechanism and sources of MEZB formation.

The beginning of the superplume activity in the Pacific Ocean was marked by prolonged cessation of magnetic inversions [32, 33] and changes in the geochemical and isotopic parameters of Darwin Rise MORB at 130 Ma [25, 26]. The basalts of spreading zones with ages of 151-130 Ma are chemically similar to N-MORB and related to a strongly depleted sourse of the DMM type containing 2-10 % of HIMU admixture (indicating a contribution from ancient recycled crust in asthenospheric sources) and complete absence of EM components.The younger MORB (125-100 Ma) of the Darwin Rise bear evidence for the participation of EMI in addition to DMM and HIMU. Thus, in addition to the formation of OIB and the SOPITA region in general, the begining of the Middle Cretaceous superplume event was marked by the enrichment of spreading zone basalts in lower mantle EMI component.

The similarity of isotopic and geochemical characteristics of OIB from the Darwin Rise and Polynesia suggests that at least during the past 130 Ma. magma sources did not change in the SOPITA region [8, 48; etc.]. The isotopic data indicate their connection with lower mantle material of the Pacific superplume. The latter was heterogeneous owing to entrainment of the ancient recycled oceanic crust during upwelling; interaction with upper mantle and lithosphere material; and contamination by fragments of younger subducted oceanic crust with overlying sediments and, probably, WPB volcanics of previous geologic epochs. The OIB are usually dominated by basalts of the HIMU-EMII isotopic series at a minor role of EMI [48, etc.]. However, the data on the Darwin Rise OIB demonstrated wide occurrence of HIMU and EMI components in the Cretaceous basalts supplementing the N-MORB component in basalts of Wace, Mid-Pacific, and Line islands. The EMII component occurs in rare instances (Wide and Hemler Guyots). The Cenozoic OIB of Polynesia show even more pronounced HIMU properties (Rururtu, Tubuai, and Mangaia islands of the Cook-Austral chain) and the dominating role of EMII (Samoa, and some islands of the Marquesas and Cook-Austral chains). In the OIB of this region, EMI is manifested loccally in Macdonald and Pitcairn islands.

Thus, the earliest activity of the Pacific superplume produced OIB with EMI signatures. This component enriched the asthenospheric sources of P-MORB in the Middle Cretaceous. These data support the opinion [21, etc.] on the lower mantle nature of the EMI component. Basalts of EMII compositions were predominant in at the Cenozoic stages of superplume activity. It is conceivable that EMII has also a lower mantle nature, at least in part. In contrast to the EM components, the HIMU component of OIB is not genetically linked with the lower mantle matter of superplume. According to [21, 23, 59; etc.], HIMU basalts are distributed also outside the DUPAL anomaly. For instance, the presence of the HIMU component in the Jurassic-Early Cretaceous MORB before the beginning of the Middle Cretaceous superplume activity. The DMM component of OIB reflects involvement of depleted asthenospheric sources into magma formation.

In order to elucidate dynamics of intraplate magmatism within the Pacific region, it is necessary to account for the general shift of its occurrence with time from northwest to southeast (in accord with oceanic plate motion), repeated renewal of volcanism within individual sites, and mosaic distribution of OIB with contrasting isotopic characteristics. For instance, along the Marshall chain, the basalts of Ratak Guyot, Bicar Guyot, Erikub Atoll, and Majuro Atoll with pronounced HIMU signatures are abruptly changed by the basalts of Limaloc Island with the EMI component. Such unusually wide variations in ²⁰⁶Pb/²⁰⁴Pb (18.6 - 21.1) with a single volcanic chain cannot be explained by the activity of a single isotopically extremely heterogeneous source. More probable is the involvement in the superplume magmatism of numerous episodically activated sources with different geochemical and isotopic properties (Fig. 2).

It was shown previously [37; etc.] that plumes of various orders could separate from the Pacific superplume at boundary mantle levels. According to the model [26], the entire compositionally heterogeneous SOPITA volcanism was related to the activity of high-order idividual plumes (plumelets, daughter plumes, hot fingers), which ascended from the head of the superplume and often separated spatially from it transforming into rootless bodies (Fig. 2). Such plumelets differ usually in age (Fig. 2) but could also function simultaneously in various portions of the large SOPITA area. An abrupt change of a particular OIB isotopic composition within an individual volcanic chain and appearance of volcanics with contrasting isotopic signatures suggest the extinction of an individual plumelet and origination of another, if they were connected with different areas of the heterogeneous feeding superplume (Fig. 2). There are several possible reasons for the separation of plumelets from the main plume including instability at the boundary of two substances with different viscosities, shear strain at the boundary of these two media, and upper mantle convection.

The model of action of numerous plumelets connecting with the main superplume can be aplied to explain the Cenozoic WPB-type volcanism of the western continental framing of the Pacific Ocean (Fig. 2). These volcanics are related to a near horizontal lower mantle plume branching out of the vertical superplume. The penetration of the lateral lower mantle plume into shallow upper mantle levels of the western Pacific periphery resulted in that even the earliest Eocene-Oligocene structures of initial marginal extension were accompanied by eruptions of alkaline basaltoids of the WPB type with predominant lower mantle EMI composition.

Symptomatically, the tholeiitic volcanics of the Early Miocene stage of the Japan basin extension, which intensified loccally were affected by lower mantle material, which resulted in the appearance of DUPAL characteristics. The decrease of extension resulted in the extinction of the Japan spreading zones and the toleiitic marginal basin magmatism was changed by the accumulation of alkaline basalts, which compose modern islands in this basin. In its framing, in the eastern China, Korea, and Honshu, MEZB eruptions did not interrupt during the entire stage of the Japan basin formation, which suggests the continuous entering of shallow lower mantle material.

Previously, the shallow position of the sources of Cenozoic alkaline magmattism in the continental margin was used as a basis for the conclusion on their genetic relation (as well as EMI and EMII) to upper mantle levels of the cratonic lithosphere [4, 43, 46, 53, 56; etc.]. This contradicted the opinion [3, 40, 41; etc.] on a link between MEZB and a lower mantle plume. The mechanism of the lateral extending of lower mantle material into various levels of the upper mantle (Fig. 2) eliminates this contradiction.

The penetration of lower mantle material (enriched in EMI and EMII) into the upper mantle of the perioceanic and adjacent continental regions, which was supported by seismic tomography and isotopic geochemistry, was responsible for the appearance in this territory of anomalous mantle with DUPAL characteristics [7, 17, 43; etc.], i.e., similar to the DUPAL mantle of the Indian and Pacific Oceans. In addition, the existence of a system of perioceanic lateral plumes (often transformed into rootless bodies) explains the mechanism of formation of the "marble cake" structure of the upper mantle, i.e. an alternation of depleted and enriched mantle areas.

A specific feature of the Cenozoic volcanism in the Eurasian margin is the influence of a subduction component on the early melt portions erupted in MEZB grabens. This influence strongly decreased with time and is hardly distinguishable or absent in the Miocene-Quaternary alkaline basalts. The presence of this component in the alkaline basalts of the continental margin (completely missing in the SOPITA OIB) is understandable. At least during the entire Late Mesozoic, the lithospheric mantle of the Eurasian margin occured in the suprasubduction environment of the Katasian magmatic belt and was affected by intense fluid metasomatism. In this respect, amphibole-phlogopite domains of the cratonic mantle, wich are old suprasubduction magma chambers imparted IAB signatures to early WPB-type melt portions of the Cenozoic extension zones. Thus, this Mesozoic suprasubduction component had no genetic relation to the superplume, neither to the EMI and EMII components nor ancient modified recycled oceanic crust entrained by the superplume at various levels.

Isotopic studies revealed a mosaic character of the lateral distribution of Cenozoic basalts dominated by a particular component, E-MORB, EMI, or EMII, in the extensional zones of the continental margin. This suggest that spatially separated magma sources of the WPB type with different isotopic characteristics were manifested nearly contemporaneously during the Cenozoic. Thus, WPB from the continental margin zone also supports the mechanism of plumelet (hot finger) activity connected with various parts of a heterogeneous lateral plume that branching out from the Pacific superplume. Such plumelets could occur in different spatial and temporal combinations (Fig.2), which resulted in the spatial dispersion of separated MEZB areas.

V. CONCLUSION

The comparision of chemical and isotopic properties of the Cenozoic basalts of extensional zones (MEZB) of eastern Asia and Cretaceous-Cenozoic OIB of the Pacific isotopic and thermal anomaly (SOPITA) was aimed to exhibit their considerable similarity, primarily, affiliation to WPB-type volcanics and enrichment in EM components. Certain differences were also revealed. The Cretaceous OIB of the Darwin Rise and the Cenozoic OIB of Polynesia show the maximum variability in Sr, Nd, and Pb isotope compositions, which involved four end-members: N-MORB, HIMU, EMI, and EMII. The composition of MEZB from the Korea-Japan region and adjacent territories of the Eurasian margin is dominated by EMI and (or) EMII. There is no evidence for the influence of HIMU, and the most depleted component is reprsented by compositions similar to E-MORB. In addition, occasionally, these alkaline basalts show evidence for the influence of the metasomatized suprasubduction mantle of the continental lithosphere (moderately low Ti content and Ta-Nb minimum), which does not occur in the Pacific OIB.

The origin of SOPITA OIB is related to lower mantle superplume ca. 130-120 m.y. ago, which corresponds to the low-velocity P-waves seismic anomaly (LVa) of complex morphology. Global seismic tomographic images of various levels [55] demonstrated that this main superanomaly gives rise to a system of lateral circum-Pacific low-velocity anomalies forming a half-ring perioceanic structure. In the western periphery of the Pacific Ocean, lateral LVa branching out of the main superanomaly are detected in the middle mantle (1270-1470 km) and in the upper mantle (35-200-350 km). In seismic tomographic profiles [5], the continuation of the perioceanic upper mantle LVa (cut by a high velocity subducted slab) is traced to middle and lower mantle depths. During the lower mantle upwelling, layer expansion of hot low-density lower mantle material from the main stem of the Pacific superplume occurred at several levels and resulted in the formation of lateral lower mantle plumes.This model based on seismic tomography allows us to explain the penetration of deep-derived lower mantle material into the shallow upper mantle as the source of MEZB. Since the superplume evolved in time from CMB toward the shallow mantle levels, the daughter lateral plumes in the western Pacific periphery are relatively young, which explains the later (Cenozoic) occurrence of alkaline volcanism in the continental Eurasian margin in comparison with the intraoceanic area, where the head of the superplume was projected in the Middle Cretaceous.

Thus, it is reasonable to suggest that the system of the Pacific superplume and branching lateral plumes was a general cause of WPB magmatism in the islands of the Darwin Rise and Polynesia in the Cretaceous-Holocene and in the extension zones of continental Eurasian margins in the Cenozoic. The shallow (0-200 km) distribution of the lower mantle lateral plume in the western margin of the Pacific Ocean allows us to explain a seemingly paradoxical phenomenon, that is the accumulation of WPB volcanics showing evidence for a connection with deepest lower mantle material at early stages of extension of the Eurasian continental margin, whereas depleted astheno-spheric sources were active during later and more intense extension in the Japan basin.

Melt evolution at the formation of OIB is evolved from EMI-dominated compositions in the Cretaceous (Darwin Rise islands) to EMII-dominated ones in the Cenozoic (Polynesian islands). The initial stage of the Pacific superplume was accompained not only by OIB formation with EMI signatures but also by the enrichment MORB in this component, which suggest on the lower mantle nature of EMI. ³He/⁴He data suggest lower mantle origin of EMII component of OIB. The HIMU component related to ancient modified oceanic lithosphere. In addition, the geterogeneity of the lower mantle superplume resulted from the involvement of the DMM component of the oceanic lithosphere. The prolonged formation of the latter in the Pacific Ocean excluded the possibility of significant contribution from the continental litosphere in OIB sources.

The EMI-dominated isotopic compositions of the China-Korea-Japan MEZB (with local occurrence of EMII signatures) are indicative of a connection between the WPB-type Cenozoic volcanics of the continental margin and the lower mantle material of a shallow later plume. The data of seismic tomography allow understanding another paradox, the shallow localization of sources of deepest-derived (with lower mantle isotopic signatures) material feeding the MEZB. Furthermore, the upwelling of lower mantle from these shallow sources accompanied by the entering of EMI and EMII components explaines in general the origination of the DUPAL signatures of the upper mantle of the Eurasian continental margin and adjacent perioceanic (including the Sea of Japan) regions.

However, in contrast to OIB, the composition of MEZB of the Eurasian margin was influenced (to a degree strongly decreasing with time) by the continental lithospheric mantle altered metasomatically in a ancient suprasubduction zones.

Another common property of the Pacific OIB and MEZB of eastern Asia is the lateral mosaic in the distribution of basalts with different isotopic signatures and significant area dispersion of volcanic occurrences. It was explained within the model [26]. According to this model, the volcanics were formed under the action of numerous individual plumelets, which branched out at the process of lower mantle upwelling either from various parts of the head of the heterogeneous vertical superplume (Pacific OIB) or from the lateral lower mantle (also heterogeneous) plumes (MEZB of the Eurasian margin).

This work was supported by the Russian Academy of Sciences (Program No. 14), the RFBR (Project No. 08-05-00748), and Project No. Nsh-651.2008.5.

REFERENCES

1. Allan J.F., Gorton М.Р, 1992. Geochemistry of igneous rocks from legs 127 and 128, Sea of Japan. Proc. ODP Sci. Res., 127-128/2: 905-929.

2. Anderson D.L, 1982. Isotopic evolution of the mantle: A model. Earth Planet. Sci. Lett., 57: 13-24.

3. Baker P.E., Castillo P.R., and Condliffe E, 1995. Petrology and geochemistry of igneous rocks from Allison and Resolution Guyots, sites 865 and 866. Proc. ODP Sci. Results, 143: 245-261. College station.

4. Basu A.R., Wang J.W., Huang W.G. et al, 1991. Major element, REE, Pb, Nd, and Sr isotopic geochemistry of Cenozoic volcanic rocks of Eastern China: Implication for origin from subvolcanic-type mantle reservoirs. Earth Planet. Sci. Lett. 105/79: 149-169.

5. Bijwaard H., Spakman W., Engdahl E.R, 1998. Closing the gap between regional and travel time tomography. J. Geophys. Res., 103/12: 30055-30078.

6. Castillo P., 1988. The DUPAL anomaly as a trace of the upwelling lower mantle. Nature, 336: 667-670.

7. Cousens B.L., Allan J.F, 1992. A Pb, Sr, and Nd isotopic study of basaltic rocks from the Japan Sea, Legs 127/128. Proc. ODP Sci. Res., 127/128/2: 805-818.

8. Davis A.S., Pringle L.G., Pickthorn D.A. et al, 1989. Petrology and age of alcalic lava from the Ratak Chain of the Marshall Islands. J. Geophys. Res.vol.94. 5757-5774.

9. Dupre B., Allegre C.J., 1983. Pb-Sr isotope variation in India Ocean basalts and mixing phenomena. Nature, 303: 142-146.

10. Dziewonski A., 1984. Mapping the lower mantle: Determination of lateral heterogenity in P-velocity up to degree and order 6. J. Geophys. Res., 89: 5929-5952.

11. Fan Q., Hooper P.R., 1991. The Cenozoic basaltic rocks of Eastern China: Petrology and chemical composition. J. Petrology, 32/4: 765-810.

12. Fedorchuk A.V., Filatova N.I., 1993. Cenozoic magmatism of Northern Korea and geodynamic conditions of its accumulation. J. Petrologiya, 1/6: 645-656.

13. Fedorov P.I., Filatova N.I., 2002. Cenozoic volcanism of the Korea and adjacent areas. Geokhimiya, 1: 3-29.

14. Filatova N.I., Chang K.Kh., 1999. Late Mesozoic lateral series of conditions of sediment accumulation in the Korea-Japan Region. Dokl. Russ. Akad. Nauk, 369/1: 100-104.

15. Filatova N.I., Fedorov P.I., 2001. Cenozoic magmatism in the extension zones of continental margins: An example of the Korea-Japan area. J. Petrologiya, 9/5: 519-546.

16. Filatova N.I., Fedorov P.I., 2003. Cenozoic magmatism in the Korean-Japanese region and its geodynamic setting. Geotectonics, 1: 49-70.

17. Flower M., Tamaki K., Hoang N., 1998. Mantle extrusion: A model for dispersed volcanism and DUPAL-like asthenosphere in East Asia and the Western Pacific. Mantle dynamics and plate interactions in East Asia. AGU, 67-88. Washington.

18. Fukao Y., Maruyama S., Obayashi M., Inone H., 1994. Geological implication of the whole mantle P-wave tomography. J. Geol. Soc. Japan, 100: 4-23.

19. Geologiya Korei, 1993. The Geology of Korea. Pjungjang: Books in foreign languages (in Russian).

20. Hart S.R., 1984. A large-scale isotope anomaly in the southern hemisphere mantle. Nature, 309: 753-757.

21. Hart S.R., 1988. Heterogeneous mantle domains: Signatures, genesis and mixing chronologies. Earth Planet. Sci. Lett., 90: 273-296.

22. Hofman A.W., White W.M., 1982. Mantle plumes from ancient oceanic crust. Earth Planet. Sci. Lett., 57/4: 421-436.

23. Hofman A.W., 1997. Mantle geochemistry: The message from oceanic volcanism. Nature, 385/6613: 219-229. London.

24. Iwamori H., 1992. Degree of melting and source composition of Cenozoic basalts in southwest Japan: Evidence for mantle upwelling by flux melting. J. Volcanol. Geother. Res., 97: 10983-10995.

25. Janney Ph.E., Castillo P.R., 1997. Geochemistry on Mesozoic Pacific mid-ocean ridge basalt: Constraints on melt generation and the evolution of the Pacific upper mantle. J. Geophys. Res., 102/B3: 5207-5229.

26. Janney Ph.E., Castillo P.R., 1999. Isotope geochemistry of the Darwin Rise seamounts and the nature of long-term mantle dynamics beneath the south central Pacific. J. Geoph. Res., 104/B5: 10571-10589.

27. Jolivet L., Tamaki K., 1992. Neogene kinematics of the Japan Sea region and volcanic activity of the Northeast Japan arc. Proc. ODP Sci. Res., 127/128-2:1311-1332.

28. Jolivet L., Shubuya H., Fournier M., 1995. Paleomagnetic rotations and Japan Sea opening: Active marginal basins of the western Pacific. Amer. Geol. Union, pp. 355-369. Washington.

29. Kaneoka I., Takigami Yu., Takaoka N. et al., 1992. ⁴⁰Ar-³⁹Ar analysis of volcanic rocks recovered from the Japan Sea floor: Constraints on the age of formation of the Japan Sea. Proc. ODP Sci. Res., 127/128-2: 819-836.

30. Kang P.Ch., Cho J.D., Choi J.H, et al., 1996. Bouguer gravity anomaly map, the southern part of Korea. Scale 1:1 000 000. KIGAM.

31. Kim K.H., Tanaka T., Nagao K., Jang S.K., 1999. Nd and Sr isotopes and K-Ar ages of the Ulreungdo alkaline volcanic rocks in the East Sea, South Korea. Geochem. J., 33/5: 317-341.

31. Kurasawa H., 1968. Isotopic composition of lead and concentration of uranium, thorium and lead in volcanic rocks from Dogo of the Oki islands, Japan. Chem. Geol., 2:11-28.

32. Larson R.L., Olson P., 1991. Mantle plumes control magnetic reversal frequency. Earth Planet. Sci. Lett., 107: 437-447.

33. Larson R.L., 1991. Latest pulse of Earth: Evidence for a Mid-Cretaceous superplume. Geology, 19: 547-550.

34. Lee J.S., Pouclet A., 1988. Le volcanisme Néogène de Pohang (SE Corée), nouvelles constraintes géochronologiques pour l'ouverture de la Mer du Japon. C. R. Acad. Sci. Paris, 307/II: 1405-1411.

35. Liu J., 1987. Study on geochronology of the Cenozoic volcanic rocks in Northeast China. Acta Petr. Sinica, 4: 21-31.

36. Loper D.E., Lay T., 1995. The core-mantle boundary region. J. Geophys. Res., 100/B4: 6397-6420.

37. Maruyama Sh., 1994. Plume tectonics. J. Geol. Soc. Japan, 100/1: 24-49.

38. Maruyama Sh., Santosh M., Zhao D., 2007. Superplume, supercontinent, and post-perovskite: Mantle dynamics and anti-plate tectonics on the Core-Mantle boundary. Gondwana Res., 11: 7-37.

39. Morris P.A., Kagami H., 1989. Nd and Sr isotope systematics of Miocene to Holocene volcanic rocks from Southwest Japan: Volcanism since the opening of the Sea of Japan. Earth Planet. Sci. Lett., 92/3-4: 335-346.

40. Nakamura E., Campbell J.H., McCullouch M.T., and Sun Sh.-S., 1989. Chemical geodynamics in a back arc region around the Sea of Japan: Implications for the genesis of alkaline basalts in Japan, Korea and China. J. Geoph. Res., 94/84: 4634-4654.

41. Nakamura E., McCullouch M.T., Campbell J.H., 1990. Chemical geodynamics in the back arc region of Japan based on the trace elements and Sr-Nd isotopic composition. Tectonophysics, 174/3-4: 207-233.

42. Nguyen H., Flower M.F.J., Corlson R.W., 1996. Major trace elements and isotopic composition of Vietnamese basalts: Interaction of hydrous EMI-rich asthenosphere with thinned Eurasian lithosphere. Geochim. Cosmochim. Acta, 60/22: 4329-4351.

43. Okamura S., Arculus R.J., Martynov Yu.A. et al., 1998. Multiple magma sources involved in marginal-sea formation: Pb, Sr, and Nd isotopic evidences from the Japan Sea region. Geology, 26/7: 619-622.

44. Park J.-B., Kwon S.-T., 1993. Geochemical evolution of the Cheju volcanic island (II): Trace element chemistry of volcanic rocks from the northern part of Cheju island. J. Geol. Soc. Korea, 29/5: 477-492.

45. Park J.-B., Park K.-H., 1996. Petrology and petrogenesis of the Cenozoic alkaline volcanic rocks in the middle part of Korean peninsula: Petrography, mineral chemistry and whole-rock major element chemistry. J. Geol. Soc. Korea, 32/3: 223-249.

46. Smith A.D., 1998. The geodynamic significance of the DUPAL anomaly in Asia. Mantle dynamics and plate interactions in East Asia, AGU: 89-105. Washington.

47. Song S., Lee H.K., Yun H., 1997. Petrogenesis of the Tertiary volcanic rocks from the southeastern part of Korea. J. Volcanоl. Geotherm. Res., 75: 345-368.

48. Staudigel H., Park K.H., Pringle M. et al., 1991. The longevity of the South Pacific isotopic and thermal anomaly. Earth Planet. Sci. Lett., 102: 24-44.

49. Stein M., Hofmann A.W., 1994. Mantle plumes and episodic crustal growth. Nature, 372/3: 63-68. London.

50. Su W.-J., Woodward R.L., Dziewonski A.M., 1994. Degree 12 model of shear velocity heterogeneity in the mantle. J. Geophys. Res., 99/B4: 6945-6980.

51. Tatsumoto M., Nakamura Y., 1991. DUPAL anomaly in the Sea of Japan: Pb, Nd and Sr isotopic variations at the eastern Eurasian continental margin. Geochim. Cosmochim. Acta, 55/65: 3697-3708.

52. Tamaki K., Suyehiro K., Allan S., et al., 1992. Tectonic synthesis and implications of the Japan Sea ODR drilling. Proc. ODR Sci. Res., 127/128-2: 1333-1350.

53. Tatsumoto M., Basu A.R., Huang W. et al., 1992. Sr, Nd and Pb isotopes of ultramafic xenoliths in volcanic rocks of eastern China: Enriched components EMI and EMII in subcontinental lithosphere. Earth Planet. Sci. Lett., 113/1:107-128.

54. Turner S., Hawkesworth Ch., 1998. Using geochemistry to map mantle flow beneath the Lau Basin. Geology, 26: 1019-1022.

55. Vasco D.W., Johnson L.R., 1998. Whole Earth structure estimated from seismic arrival times. J. Geoph. Res., 103/132: 2633-2671.

56. Wang J., Xie G., Tatsumoto M., Basu A.R., 1989. Sr, Nd, Pb isotope geochemistry and magma evolution of the potassic volcanic rocks, Wudalianchi, Northeast China. Chinese J. Geochem., 8/4: 322-330.

57. Yun S.H., Won Ch.K., Lee M.W., 1993. Cenozoic volcanic activity and petrochemistry of volcanic rocks in the Mt. Paektu Area. J. Geol. Soc. Korea, 29/3: 291-307.

58. Zhang M., Zhou X.-M., Zhang J.-B., 1998. Nature of the lithosphere mantle beneath NE China: Evidence from potassic volcanic rocks and mantle xenoliths. Mantle dynamics and plate interactions in East Asia. AGU: 197-219. Washington.

59. Zindler A., Hart S., 1986. Chemical geodynamics. Ann. Rev. Earth Planet. Sci., 14/2: 493-541