Susumu Nohda

Setouchi high-Mg andesites revisited : geochemical evidence for melting of subducting sediments

Abstract In order to evaluate the mechanism of production of unusual high-Mg andesite (HMA) magmas, Pb–Nd–Sr isotopic compositions were determined for HMAs and basalts from the Miocene Setouchi volcanic belt in the SW Japan arc. The isotopic compositions of Setouchi rocks form mixing lines between local oceanic sediments and Japan Sea backarc basin basalts, suggesting a significant contribution of the subducting sediment component to the HMA magma generation. Mixing calculations using compositions of an inferred original mantle and local oceanic sediments suggest that a sediment-derived melt, neither an H 2 O-rich fluid nor an amphibolite/eclogite-derived melt, could have been produced first and served as a plausible metasomatic agent for the HMA magma source. The unusual tectonic setting, including subduction of a newly-borne hence hot plate, may be responsible for melting of subducting sediments.

Paleomagnetic evidence for the Miocene counter-clockwise rotation of Northeast Japan—rifting process of the Japan arc

Paleomagnetic results have been obtained from more than 700 oriented samples in igneous rocks of Cretaceous to Miocene age from Northeast Japan. The remanent magnetizations of welded tuffs with age between 32 Ma and 21 Ma old from 17 widely distributed sampling sites in Northeast Japan are fairly well grouped with a mean ofD = −41.2°, I = 56.5°, α95 = 7.2°. The remanent magnetization of welded tuffs of 14 Ma and 11 Ma from 4 sites shows a northerly direction (D = −11.7°, I = 60.5°, α95 = 15.3°). These data imply that Northeast Japan was apparently subjected to a counter-clockwise rotation through 47 ± 14° about a vertical axis on the representative point of Northeast Japan (141°E, 39°N) during the past 21 Ma. Comparing with the period of the clockwise rotation of Southwest Japan, the differential rotation between Northeast Japan and Southwest Japan is concluded to have occurred concurrently during the period between 21 Ma and 12 Ma. A rifting model of the Japan arc is proposed: Northeast Japan rotated counter-clockwise through 50° about a northern pivot at 146°E, 44°N, while Southwest Japan rotated clockwise through 54° about a southern pivot at 129°E, 34°N. The differential rotation about different rotation pivots gives rise to the opening of a diamond-shaped back arc basin, the Japan Sea.

Asthenospheric injection and back-arc opening: Isotopic evidence from northeast Japan☆

Nd and Sr isotopic compositions were determined for the Tertiary volcanics from the back-arc side of the NE Japan arc. Sr isotopes show a linear trend through time from an enriched signature (87Sr86Sr = 0.705437) to a depleted signature (87Sr86Sr = 0.70270). In a complementary fashion, Nd isotopes start at low value (ϵNd = −0.80) and show a gradual increase (ϵNd=8.3) with decreasing age. Isotopic change of Nd and Sr from the enriched signature to the depleted one is synchronous with the opening of the Japan Sea at ∼ 15 Ma. This synchronism indicates that the opening of the Japan Sea was initiated by the injection of the asthenosphere. During the pre-opening stage, the mantle wedge was composed of a two-layered structure: the sub-continental lithosphere and the underlying asthenosphere. The volcanics of this stage characterized by an enriched isotopic signature were derived from a source with a higher proportion of sub-continental lithosphere. The sub-continental lithosphere of the back-arc side was thinned by the injection of the depleted asthenosphere, which accelerated the growth of the MORB source within the mantle wedge of the back-arc side and resulted in magma generation with the depleted isotopic signature of Nd and Sr at the post-opening stage.

Opening of the Sea of Japan back-arc basin by asthenospheric injection

Abstract The position of a volcanic front on an arc is governed by dehydration reactions of amphibole + chlorite in down-dragged hydrated peridotite at the base of the mantle wedge. As these reactions are essentially pressure-dependent (ca. 3.5 GPa), the paleo-position of a volcanic front offers an estimate for the depth of the subducted oceanic lithosphere beneath it. This paper evaluates volcanism in the Northeast Japan arc since the Oligocene and discusses the mass transportation in the mantle wedge during the episode of back-arc extension in the Sea of Japan with reference to the migration of the volcanic front. The volcanic front during the Oligocene (ca. 30 Ma) was located along the western coast of the present Northeast Japan are and migrated about 200 km trenchwards at 23 Ma before the opening of the Sea of Japan back-arc basin. It follows that the angle of subduction became steeper during the period of 30-23 Ma, triggered by the injection of the asthenosphere into the mantle wedge. The back-arc extension of the Sea of Japan between 21 and 14 Ma is a manifestation of the dynamic process of the lithosphere due to the asthenospheric injection. The temperature of the injecting asthenospheric mantle was higher than that of the surrounding upper mantle or lithosphere. This caused the characteristic volcanism at 23-22 Ma: (1) High-magnesian andesite volcanism took place in the near-trench region; (2) across-arc compositional variation was not observed; (3) the volume of volcanic materials increases towards the back-arc side of volcanic arc. The volcanic front has migrated over a short distance inwards (less than 100 km) since 15 Ma, possible as a result of cooling in the mantle wedge, because both amphibole and chlorite break down at slightly higher pressures at lower temperatures. The upper mantle materials which injected into the mantle wedge and caused back-arc spreading are identical to those of the mid-oceanic ridge basalt source in Sr-Nd isotopic compositions, because volcanic rocks in the back-arc region of the Northeast Japan arc show secular variation in isotopic compositions from enriched to depleted isotopic characteristics during the last 30 Ma.

Counterclockwise rotation of Northeast Japan: paleomagnetic evidence for regional extent and timing of rotation

Early to Late Miocene welded tuffs have been sampled from northeast Honshu Island and the Oshima Peninsula of Hokkaido Island, Japan for paleomagnetic study. Characteristic directions with a high unblocking temperature component above 560°C are isolated from 24 sites. Westerly declinations (D = −1° ∼ −91°) are identified in welded tuffs older than 17 Ma from these areas. When combined with previous paleomagnetic data from welded tuffs, the Early Miocene paleomagnetic poles derived from northeast Honshu Island and the Oshima Peninsula are statistically identical and indicate that northeast Honshu Island and the Oshima Peninsula were subjected to counterclockwise rotation as a single tectonic block. These results establish that the rotating block of Northeast Japan extends north to the Oshima Peninsula to the latitude 43°N. The dimension of this rotating block is of the order of 500 km. The paleomagnetic data provide a best curve for the rotation process, with the counterclockwise rotation reaching a climax at 15.0 Ma and a net amount of rotation of 46.4°. The 500 km long Northeast Japan block was rotated counterclockwise through more than 45° and accompanied a clockwise rotation of the 600 km long Southwest Japan block at about 15 Ma.

Secular variation of magma source compositions beneath the Northeast Japan arc

Abstract Sr and Nd isotopic compositions were determined for Tertiary volcanic rocks with ages of 22, 16 and 8 Ma along the volcanic front in the NE Japan arc in order to examine the secular variation of magma source compositions in the mantle wedge. These rocks show no through-time variation in both Sr and Nd isotope ratios (0.70412–0.704759 and 2.09–4.12 ∈-units, respectively). It follows that the subduction components do not increasingly pollute the magma source region during the continuous subduction of oceanic lithosphere. The degree of pollution by the subduction components is controlled by the amount of clinopyroxene and aluminous phases in mantle wedge peridotites, because H 2 O which causes partial melting of the mantle wedge beneath a volcanic front is supplied by the breakdown of both amphibole and chlorite in the dragged hydrated peridotite formed by the addition of slab-derived fluid beneath the fore-arc region.

Formation of a third volcanic chain in Kamchatka: generation of unusual subduction-related magmas

The unusual development of three volcanic chains, all parallel to the trend of the subduction trench, is observed in Kamchatka at the northern edge of the Kurile arc. Elsewhere on the Earth volcanic arcs dominantly consist of only two such chains. In the Kurile arc, magmatism in the third volcanic chain, which is farthest from the trench, is also unusual in that lavas show concentrations of incompatible elements intermediate between those of the two trenchward chains. This observation can be explained by relatively shallow segregation of primary magmas and high degrees of partial melting of magmas in the third chain, compared to the conditions of magma separation expected from a simple application of the general acrossarc variation. Initial magmas in such an atypical third chain may be produced by melting of K-amphibolebearing peridotite in the down-dragged layer at the base of the mantle wedge under anomalously hightemperature conditions. Such an unusual melting event may be associated with the particular tectonic setting of the Kamchatka region, i.e. the presence of subductiontransform boundary. Such a mechanism is consistent with the across-arc variation in Rb/K ratios in the Kamchatka lavas: lowest in the third chain rocks and highest in the second chain rocks.

RbSr dating of acidic rocks from the middle part of the Inner Zone of southwest Japan: tectonic implications for the migration of the Cretaceous to Paleogene igneous activity☆

Abstract RbSr age determinations by mineral and whole-rock isochron methods have been performed on Cretaceous to Paleogene granitic and rhyolitic rocks from the middle part of the Inner Zone of southwest Japan in order to examine some chronological problems and tectonic models for the migration of igneous activity. Our chronological results include: (1) isotopic homogenization of granitic and rhyolitic rocks in the Miyazu-Izushi area was disturbed by mixing of acidic and basic magmas; (2) the Rokko granite was emplaced at 77.8 Ma and reheated at 72 Ma; (3) mineral isochron ages obtained for the acidic volcanic rocks range from 62.6 to 85.8 Ma and are considered to date the time of eruption. Igneous activity migrated eastward from ∼ 95 Ma to ∼ 70 Ma, which may be related to a similar eastward migration of the development of sedimentary basins in fore-arc regions. The observed eastward migration has been discussed with relevance to a ridge subduction model in which the heated position by subducted ridge moved from west to east. Although the cause for the migration of the igneous activity is poorly understood, several difficulties with the simple ridge subduction model are pointed out.

Geochemical stratification in the upper mantle beneath NE China

Compositions of primary magmas were estimated for Cenozoic Mg-rich basalts from the NE China region based on the olivine maximum fractionation model. These data together with available experimental data gave an estimate for magma segregation depth for each basalt magma, which is inversely correlated with 87Sr/86Sr isotopic ratio and positively correlated with 143Nd/144Nd isotopic composition. This observation may provide convincing evidence that the enriched upper mantle is underlain by more depleted asthenosphere mantle reservoir.

A distinctive characteristic within carbonates immediately above glacial deposits in the Neoprotrozoic Otavi Group of northern Namibia

アフリカ南部のナミビアには原生代後期の氷河堆積物とそれを覆う温暖な環境で堆積した炭酸塩岩が広く分布している. この堆積環境が極端に異なる岩相の組み合わせは, 地球史においても稀であり, 原生代後期の気候変動を考える上で大変重要な地質学的証拠と注目されている. 最近では氷床が当時の地球を広く覆っていたとするスノーボールアース(全球凍結)仮説が提唱され, その成因について活発な議論が行われている (例えば, Hoffman & Schrag, 2002). ここでは, 現地調査に基づきスノーボールアース仮説の根拠となった露頭の状況と氷河堆積物直上のドロマイト質炭酸塩岩に見られる特異な構造について紹介する.