Based on Sr isotopic data for Tertiary and Quaternary basaltic rocks from the NE Japan arc, relationships are discussed between the temporal variation of magma source characteristics and the opening of the Japan Sea. The basaltic rocks from the trench side and from the transitional zone show initial87Sr/86Sr ratios (Sri ratios) in the range of 0.70411–0.70546 but no temporal variation in Sri ratios. The back-arc side basaltic rocks with ages of 29.8 to ≈ 15 Ma have Sri ratios similar to those of the trench side and the transitional zone, and these values also show no temporal change. In contrast, the basaltic rocks from the back-arc side, with ages younger than ≈ 15 Ma, show significantly lower Sri ratios (0.70396 to 0.70290), which are slightly higher than those of N-type MORB. These Sr isotopic features may imply that at least before ≈ 15 Ma the magma source regions (the sub-continental mantle) beneath the NE Japan arc had an enriched chemical character and that after ≈ 15 Ma, the magma sources for volcanic rocks from the back-arc side show a drastic change in Sr isotopic character, from an enriched nature to a depleted one. The depleted magmas may have been formed as a result of injection of depleted asthenosphere (or of a depleted mantle diapir) into the subcontinental mantle under the back-arc side of the NE Japan arc, during the spreading of the Japan Sea back-arc basin. The middle Miocene basaltic rocks from the back-arc side are characterized by lower contents of LIL elements such as K2O and Rb compared with those from the trench side, suggesting that during the middle Miocene (syn-opening stage of the Japan Sea) the degree of partial melting may have been higher in the back-arc side mantle than in the trench side mantle. High degree of partial melting in the back-arc side mantle can be attributed to an increasing geothermal gradient in the mantle due to the injection of hot asthenosphere. This injection might also have caused the melting of the lower crust from which the voluminous middle Miocene acidic volcanics in the back-arc side and transitional zone may have been produced.

The relationships between drastic changes in Sr isotope ratios of magma sources beneath the NE Japan arc and the spreading of the Japan sea back-arc basin

On the basis of Sr and Nd isotopic data for Tertiary to Quaternary volcanic rocks, the isotopic characteristics of the upper mantle beneath the NE Japan arc and genesis of Middle Miocene bimodal volcanism are discussed. Basaltic rocks from the back-arc side, erupted from 35 Ma to present, can be divided into two different groups based on initial Sr isotopic ratios (Sri), initial eNd values, and ages. One group has higher Sri ratios (∼0.7040–∼0.7058) and lower eNd values (∼0–∼+3) than the other (∼0.7029–∼0.7040 and ∼+3–∼+8). Basaltic activity of the high Sri group took place continually from 30 to 10 Ma, whereas activity of the other group occurred mainly in two periods; i.e., at around 34–35 Ma and after –15 Ma. A similar pattern is also observed in basaltic rocks from the transitional zone and the trench side, where basaltic activity characterized by higher Sri ratios took place continuously during ca. 15–0 Ma, whereas eruption of lower Sri basalts occurred at around 15 Ma and after about 3 Ma. One interpretation of these features is that the sub-continental mantle beneath the NE Japan arc may be layered; comprising upper isotopically undepleted (Sri = ∼0.7040–∼0.7058, eNd = ∼0–∼+3) and lower depleted (Sri = ∼0.7029–∼0.7040, eNd = ∼+3–∼+8) parts. Basaltic magmas with an undepleted isotopic signature are considered to have formed from the upper layer, while those with a depleted isotopic signature are from the lower layer. The sharp increase in basaltic activity with a depleted isotopic character after –15 Ma in the back-arc side may be due to the upwelling of hot depleted layer from a deeper region in the mantle (asthenosphere) to the upper undepleted layer beneath the back-arc side of the NE Japan arc during the opening of the Japan Sea. This asthenospheric upwelling might have also caused melting of the lower crust to produce the Middle Miocene acidic volcanic rocks in the back-arc side and the transitional zone. Rhyolitic rocks from Tobishima Island and the Tsugawa-Tadami area in the back-arc side show significantly higher Sri ratios (0.7055–0.7072) and lower eNd values (∼0–∼-5) than the associated basaltic rocks, indicating a lower crustal origin for these rhyolitic rocks. On the other hand, the similarity in eNd values (∼+4) of dolerites and rhyolites from the Kakudate-Rokugo area of the transitional zone suggests derivation from a common mantle source with an isotopic signature between the upper undepleted and lower depleted layers.

Middle Miocene bimodal volcanism by asthenospheric upwelling: Sr and Nd isotopic evidence from the back-arc region of the Northeast Japan arc.

Late Oligocene to Middle Miocene adakitic andesites are found in the southern part of Okushiri Island, the northern Noto Peninsula and in theToyama region in the present-day back-arc margin of the SWand NE Japan arcs. On Okushiri Island, adakitic andesite is accompanied by moderately alkaline basalt, whereas on the Noto Peninsula, adakitic andesite has been erupted along with high magnesian andesite (HMA), bronzite andesite and tholeiitic basalt. Adakitic andesites from all three locations are characterized by high Sr/Yand lowY, and have higher MgO contents than adakitic melts generated by experimental melting of metabasalt and amphibolite. They also have higher Ni and Cr contents than either Archaean tonalite^trondhjemite^granodiorite (TTG) suites or Early Cretaceous adakitic granites, which have been attributed to partial melting of subducted oceanic crust. The Noto Peninsula adakitic andesite has Sr and Nd isotopic compositions identical to normal mid-ocean ridge basalt (N-MORB), whereas the Okushiri Island and Toyama adakitic andesites are more isotopically primitive than N-MORB.The Noto Peninsula primary adakitic melt was derived from subducted oceanic N-MORB crust, whereas the Okushiri Island andToyama primary adakites are interpreted as melts of subducted N-MORB and sediment that have subsequently interacted with the overlying mantle wedge peridotite.To explain the comagmatism of adakite, HMA and basalt, the following model is proposed. A hydrated adakitic diapir ascends from the subducting slab and is heated because it enters the overlying hot mantle wedge.The subsequent establishment of thermal and H2O gradients in the adakitic diapir and surrounding mantle wedge peridotite results in concurrent generation of adakitic andesite magma in the inner adakitic diapir region (low temperature and high H2O content), HMAand bronzite andesite magmas in the intermediate peridotite region (intermediate temperature and H2O content), and tholeiitic basalt magma in the outer peridotite region (high temperature and lower H2O content).

Origin of Late Oligocene to Middle Miocene Adakitic Andesites, High Magnesian Andesites and Basalts from the Back-arc Margin of the SW and NE Japan Arcs

Orthopyroxene-rich and orthopyroxene-poor peridotite xenoliths were sampled from quaternary basaltic to andesitic lava flows of the Shiribeshi seamount, Japan Sea. These xenoliths were affected by reactions with the host magma during transportation to the surface, which caused partial orthopyroxene dissolution and intergrowth with vermicular spinel. Chromian spinel and clinopyroxene major element compositions in the Shiribeshi peridotite are similar to those in abyssal peridotites. REE modeling indicates that the Opx-rich peridotite experienced decompression partial melting from the garnet to the spinel peridotite stability field. Rare earth element (REE) patterns of clinopyroxene in the Opx-rich peridotite show various degrees of enrichment in light REE, which resulted from melt percolation through the reaction with host magma. Comparison with peridotite xenoliths from two other localities (Seifu and Oshima-Ōshima) in the Japan Sea suggests that the Oshima-Ōshima peridotite record higher degree of partial melting than the Shiribeshi and Seifu peridotites. Oxygen fugacities calculated from chromian spinel in the Japan Sea peridotites are comparable to those of arc peridotites. The high degree of partial melting of the Oshima-Ōshima peridotite was possibly caused by the infiltration of a H2O-bearing flux released from the subducted slab. The Shiribeshi peridotite is interpreted as the residue formed after the extraction of depleted back-arc basalts during a later stage of the Japan Sea opening in the Middle Miocene, whereas the Oshima-Ōshima peridotite is residual after the extraction of enriched basalts during an earlier stage of the opening of the Japan Sea.

Peridotite xenoliths from the Shiribeshi Seamount, Japan Sea: insights into mantle processes in a back-arc basin

Petrogenesis of Middle Miocene Primitive Basalt, Andesite and Garnet-bearing Adakitic Rhyodacite from the Ryozen Formation: Implications for the Tectono-magmatic Evolution of the NE Japan Arc

Shiribeshi Seamount is located to the east of the Okushiri Ridge, in the northeast Japan Sea Whole rock K-Ar age of olivine-augite andesite dredged from the Seamount was determined to be 09 ± 02 Ma (Tsuchiya et al, 1989), indicating that Shiribeshi Seamount is a Quaternary volcano in the back-arc region off the junction of the Northeast Japan and Kurile arcs Shiribeshi volcano is composed of basalt to rhyolite, which show a typical island arc calc-alkaline nature on the basis of petrographical characteristics of 95 samples dredged from four sites Abundances of incompatible elements including K, Rb, Sr, Nb, P, Ti, Y and Zr in 16 representative rocks are discussed, together with those in the Quaternary volcanic rocks from the NE Japan and Kurile arcs in terms of compositional variation across the arcs The estimated composition of the primary magma of Shiribeshi volcano is characterized by higher incompatible element contents and a higher Zr/Y ratio than primary magmas in the volcanic front side Based on HFS element concentrations the degree of partial melting for three primary magmas of Oshima-Oshima, Shiribeshi and Rishiri volcanoes in the northeast Japan Sea may decrease gradually with increasing distance from the volcanic front However, LIL element contents, especially K and Rb are lower in the primary magma of Rishiri volcano located far from the volcanic front than in the remaining two primary magmas, which would imply that LIL/HFS ratios (or degree of contribution of LIL elements originating from the subducted oceanic crust) become minimal at Rishiri volcano One basalt and three andesites from Shiribeshi volcano have the restricted range of low87Sr/86Sr ratios of 070297–070300, which indicates that the magma source for Shiribeshi volcano may be slightly more enriched in Sr isotopic compositions than theN-type MORB source

S. Tamura

Papers

Geochemistry of newly discovered quaternary Shiribeshi Volcano, Northeast Japan Sea