Satoshi Okamura

Multiple magma sources involved in marginal-sea formation: Pb, Sr, and Nd isotopic evidence from the Japan Sea region

The Cenozoic basaltic rocks in Sikhote-Alin and Sakhalin exhibit a change over 55 m.y. from subduction-zone-related to continental-rift-related volcanism as the Japan Sea opened between eastern Sikhote-Alin and the Japan arc. The temporal geochemical trends in volcanic rocks erupted before and during the opening of the Japan Sea, suggesting that there was a change in magma source from the lithosphere to the depleted asthenosphere as the Japan Sea opening progressed. Two geochemical groups can be identified within the basalts that postdate the Japan Sea opening: tholeiitic basalts and alkalic basalts. As the tholeiitic basalts have EMI-type signatures, we propose that the tholeiites had a significant contribution from an EMI-type Precambrian subcontinental lithospheric mantle. The alkalic basalts, yielding geochemical characteristics similar to enriched oceanic island basalts, may have been derived from the subcontinental asthenospheric mantle. The opening of the Japan Sea was triggered by lateral migration of asthenospheric mantle from beneath northeast China toward the Japan arc.

Geochemistry of Mesozoic intracontinental basalts from Yunnan, southern China: Implications for geochemical evolution of the subcontinental lithosphere

SummarySouthwestern Yunnan, comprising the Yangtze and Shan-Thai microcontinents and the Simao block, has successively undergone subduction of an oceanic plate, followed by a collision of the microcontinents and intracontinental rifting associated with basaltic volcanism during Late Paleozoic to Mesozoic.The Triassic Nanjian basalts, erupted on the Yangtze microcontinent, have more enriched isotopic ratios and higher LREE/HFSE and LREE/HREE ratios. This suggests the existence of an enriched subcontinental lithosphere under the Yangtze microcontinent which stabilized over long periods of the earths history (> 2Ga).The Middle Jurassic Simao basalts have more depleted geochemical features and also have element enrichments characteristic of a subduction zone environment, although the basalts were erupted in an intracontinental graben. It may be inferred that the lithospheric mantle of the Simao block was modified by subduction processes during Latest Carboniferous to Late Triassic prior to the onset of the Middle Jurassic continental rifting. The lack of correlation between depletion of HFSE, Y and HREE, and relative enriched Nd isotopic ratios suggests that the source depletion of the Simao basalts is not an old feature and has been contemporaneous with the subduction-related enrichment through mantle metasomatism shortly before the basalts were produced.The Middle Jurassic Baoshan basalts which erupted during the continental rifting on the Shan-Thai microcontinent have an Sr-Nd isotopic composition similar to the bulk earth and higher concentrations of incompatible trace elements. These features suggest that the subcontinental lithosphere under the Shan-Thai microcontinent underwent mantle metasomatism just prior to eruption of the Baoshan basalt.ZusammenfassungSüdwestjünnan umfaßt den Jangtse und den Shan-Thai Mikrokontinent und den Simao Block. Das Gebiet wurde von aufeinander folgenden Subduktionsphasen einer ozeanischen Platte betroffen, auf die Kollision der Mikrokontinente und interkontinentales Rifting folgte. Dieses war mit basaltischem Vulkanismus während des späten Paläozoikums bis ins Mesozoikum assoziiert.Die triassischen Nanjian-Basalte, die auf dem Jangtse Mikrokontinent eruptierten, haben mehr angereicherte Isotopenverhältnisse und höhere LREE/HFSE und LREE/HREE Verhältnisse. Dieses weist auf eine angereicherte subkontinentale Lithosphäre unter dem Jangtse Mikrokontinent hin, die sich während langer Perioden der Erdgeschichte stabilisierte (>2Ga).Die mittel jurassischen Simao-Basalte haben eine mehr verarmte geochemische Signatur aber auch Elementanreicherungen, die für ein Subduktionszonen-Milieu charakteristisch sind, obwohl die Basalte in einem interkontinentalen Graben ausgetreten sind. Man kann daraus schließen, daß der lithosphärische Mantel des Simao-Blockes durch Subduktionsprozesse während des jüngsten Karbons bis in die späte Trias vor dem Beginn des mittel-jurassischen kontinentalen Riftings modifiziert worden war. Das Fehlen einer Korrelation zwischen der Anreicherung von HFSE, Y und HREE und relativ angereicherter Nd-Isotopenverhältnisse weist darauf hin, daß die Verarmung der Quelle der Simaobasalte nicht weit zurückreicht. Sie dürfte viel eher gleichaltrig mit der subduktions-bedingten Anreicherung durch Mantel-Metasomatose kurz vor der Entstehung der Basalte sein.Die mittel-jurassischen Baoshan-Basalte, die während des kontinentalen Riftings auf den Shan-Thai Mikrokontintent eruptierten, haben eine Sr-Nd-Isotopensignatur, die ähnlich der Gesamterde ist, jedoch höhere Konzentrationen inkompatibler Spurenelemente zeigt. All dies legt nahe, daß die subkontinentale Lithosphäre unter dem Shan-Thai-Mikrokontinent kurz vor der Eruption der Baoshan-Basalte von Mantel-Metasomatose betroffen worden ist.

Breakdown of residual zircon in the Izu arc subducting slab during backarc rifting

High field strength elements (HFSEs) are depleted in subduction-related melts because of the residual minerals in the slab. It has been suggested that high slab-surface temperatures cause the breakdown of residual zircon in the slab. In this study, we found that two geochemical types can be identified within basalts from the active backarc rift zone of the Izu arc (northwest Pacific Ocean). Low-Zr/Y basalts (LZBs) were collected from across the entire area of the active rift zone, whereas the high-Zr/Y basalts (HZBs) were identified only at the Sumisu Rift. Although enrichment of large-ion lithophile elements (LILEs) and Nb-Ta depletion are recognized in both types, a minimal negative Hf concentration anomaly (Hf/Hf* = 0.98) characterizes the HZB. In Nd-Hf isotope space, the HZB lies close to the Indian/Pacific mantle boundary, displaced from the LZB. Variations in Nb/Yb and Zr/Yb indicate differences in the fertility and/or degree of melting of source mantle that can explain the HFSE systematics in the LZB, but not in the HZB. Geochemical modeling demonstrates that mixtures of mantle wedge and ~2% slab-derived melt, not in equilibrium with residual zircon, produces the observed trace-element and isotopic characteristics in the HZB. Moreover, the range of occurrence of the HZB corresponds with the range in seismic low-velocity anomalies in the upper slab. We suggest that asthenospheric material injected into the mantle wedge beneath the Sumisu Rift is a possible heat source for the breakdown of residual zircon in the slab.