Tadashi Usuki

Late Triassic–Late Cretaceous accretion/subduction in the Taiwan region along the eastern margin of South China – evidence from zircon SHRIMP dating

To examine the tectonic history of the Taiwan segment of the eastern margin of South China, six rock samples from the Tailuko belt, the metamorphic basement of Taiwan, were selected for zircon SHRIMP dating. The aim was to identify evidence shedding light on the timing of the change from passive to active tectonics for this part of the continental margin since South China separated from the supercontinent of Rodinia. The results lead to two age groups, 190–200 and 88–90 Ma. These age groups, augmented by the previously published age data, suggest that they could have resulted from two Mesozoic accretion/subduction events. In addition, this mid‐late Mesozoic Tailuko belt might have also been reactivated and structurally complicated by the late Cenozoic collision/accretion of the Luzon arc with the Eurasian continent. Records of older tectonic events, such as those derived from the Japanese Islands, are absent in this metamorphic basement. An important finding of this study is the existence of the 191±10 Ma Talun metagranite, the oldest granitic intrusion ever reported in the Taiwan region and along the eastern coast area of South China. In spite of a large age uncertainty, the occurrence of this metagranite is not consistent with the apparent younging trend of Jurassic‐Cretaceous igneous activity toward the coastline in South China, and should be taken into consideration by future studies.

Generation of Cenozoic granitoids in Hokkaido (Japan): Constraints from zircon geochronology, Sr-Nd-Hf isotopic and geochemical analyses, and implications for crustal growth

The island of Hokkaido is a young accretionary terrane, basically built with a Jurassic accretionary complex and Cretaceous arc in the west (= NE Japan arc terrane), a Cretaceous-Paleogene forearc basin and accretionary complex with the Hidaka metamorphic belt in the center, and a Cenozoic island arc with Cretaceous basement in the east (= Chishima or Kuril arc terrane). Though volumetrically small, Paleogene and Neogene granitoids are widespread in central Hokkaido (Hidaka Belt). Granitoids are the most representative component of the continental crust, so in this work we aimed to study the mode of generation and source characteristics of these granitoids in order to assess the crustal composition of Hokkaido and examine the general problem of continental growth. New zircon geochronology on nine granitic and one gabbroic rocks from the Hidaka Belt reveals three distinct magmatic episodes, two in the Eocene at 45–46 Ma (3 granites), and 37.0 ± 0.5 Ma (1 granite), and one in the Miocene at 18 to 19 Ma (5 granites and 1 gabbro). The Miocene episode represents the most important granitic emplacement in Hokkaido. The early Eocene zircon ages of 45 to 46 Ma are identified for the first time for granitoids that occur in the northern part of the Hidaka Belt. The zircon age of 37 Ma for a granite from Shirataki is rather rare in Hokkaido, but similar ages had been reported for a tonalite and a granite from the Hidaka metamorphic belt. Geochemically, all granites are slightly peraluminous but not S-type, and they possess volcanic arc granitoid characteristics. Their REE distribution patterns are typically “granitic,” showing fractionated patterns with LREE enrichment and distinct negative Eu anomaly. The whole-rock isotopic signatures [ISr = 0.7044 to 0.7061; εNd(t) = +1.0 to +4.7; TDM-1 = 400-1000 Ma] reveal their largely juvenile characteristics. This is corroborated by the zircon Hf isotopic compositions [εHf(t) = +8 to +19]. The Eocene granites were most probably generated by melting of subducted accretionary complex in a prolonged period from 46 to 37 Ma in supra-subduction zone; whereas the Miocene granites were also generated by melting of accretionary complex in a back-arc rifting setting. In both cases, the involved accretionary complex was probably dominated by the mantle-derived lithological component with little Paleozoic or older crustal material. Hokkaido provides an excellent example of juvenile crust addition to the continental crust.

Hf isotope and REE compositions of zircon from jadeitite (Tone, Japan and north of the Motagua fault, Guatemala): implications on jadeitite genesis and possible protoliths

Zircon separates from one jadeitite sample (JJ) from Tone, Japan and one from Guatemala (GJ) were studied for mineral inclusions, age dating, trace-element determination and Hf isotope analysis. These zircons can be categorized into two types. Type I (igneous) zircons are characterized by the presence of mineral inclusions, among others K-feldspar, which is not present in jadeitite matrix. They also show higher Th/U ratios, larger Ce anomalies and higher 176 Lu/ 177 Hf ratios. Type II (metasomatic/solution-precipitate) zircons contain omphacite/jadeite inclusions and exhibit lower Th/U ratios, smaller Ce anomalies and lower 176 Lu/ 177 Hf ratios. Both types of zircons display high eHf( t ) values, slightly lower than the depleted mantle evolution line. The JJ sample contains both type I and II zircons. SHRIMP and geochemical data indicate that this jadeitite sample was formed through the mechanism of whole-sale metasomatic replacement at ~80 Ma from an igneous protolith of juvenile origin with an age of 136 ± 2 Ma. The GJ sample contains only type II zircons and may have formed through a mechanism of, or close to, vein precipitation at 98 ± 2 Ma. The two samples therefore testify that both mechanisms may have been in operation during jadeitite formation. Based on Hf isotope composition of type I zircons and the back-calculated REE pattern of the presumed protolith, the geochemical characteristics of the protolith of the Tone jadeitite were shown to be similar to those of oceanic plagiogranites derived from partial melting of cumulate gabbros or subduction-zone adakitic granites originated from partial melting of subducted oceanic crust. The latter, however, is a more probable candidate because the former is known to be poor in K 2 O, which, in contrast, is a notable chemical component in Tone jadeitite. On the basis of the available data, it is also suggested that the protolith, the physicochemical conditions and the extent of jadeitization may all play a role in dictating the chemical variations of jadeitites.

Intraoceanic unroofing of eclogite facies rocks in the Omachi Seamount, Izu-Bonin frontal arc

Electron-microscope and electron-microprobe observations of amphibolites from the Omachi Seamount in the Izu-Bonin frontal arc show that they contain micrometer-sized inclusions of omphacite ± kyanite, indicating eclogite facies metamorphism. The garnet-zoisite amphibolite was subjected to metamorphic conditions of 600–700 °C and ∼2 GPa in the early stage of metamorphism. These metabasites were subducted to a depth of ∼70 km beneath the Izu-Bonin arc and were exhumed together with serpentinite schist. Their intraoceanic unroofing was probably induced by backarc extension of the upper plate, whose axes were intersected and cooled by the newly formed Izu-Bonin subduction system. This intraoceanic exhumation associated with suprasubduction extension can be a modern analogue for ancient on-land eclogite terranes accompanied by ophiolites.

Anisotropic Fe-Mg diffusion in biotite

Abstract Marked variations of biotite Fe-Mg compositional profiles on approach to interfaces with garnet as a function of biotite crystallographic orientation are found in a natural high-temperature metamorphic rock. For biotite with (001) planes nearly parallel to the interface, Mg increases and Fe decreases steeply toward the interface. If oblique, the profile is less steep, and if nearly perpendicular, the profile is flat. These variations are evidences for anisotropic Fe-Mg intradiffusion within biotite during cooling. For precise application of geothermobarometry and geospeedometry, the crystallographic orientation and compositional gradient in biotite must be considered

Preliminary study for the Middle Miocene Kawabata Formation, Hobetsu district, central Hokkaido, Japan; special reference to the sedimentary system and the provenance.

北海道中央部, 穂別地域に分布する中部中新統川端層について堆積相解析を行った.その結果, 川端層はタービダイトと関連粗粒重力流堆積物からなり, 構成岩相の累重様式・水平分布は, 堆積盆東縁に発達した小規模あるいは短命のチャネルやスロープエプロンなどによる斜面基部～堆積盆底環境での形成を示唆する.また, その堆積の後期には, 側方の陸域からの供給が大きく寄与する側方流卓越型のシステムが成立していた可能性があり, 川端層の堆積に際する軸流と側方流の寄与は地域あるいは層準によって大きく異なっていたと推定される.また川端層砂岩には蛇紋岩片等の不安定な岩片や, 片麻岩起源のざくろ石などが含まれる.そのため, 川端層の供給源としては東に隣接する枝幸-日高帯南部地域があげられ, この時期にはすでに枝幸-日高帯南部が陸化していたことが示唆される.このことは従来の古地理の復元や日高帯の上昇時期について再考を促すものである.

Melt Pockets and Spongy Clinopyroxenes in Mantle Xenoliths from the Plio-Quaternary Al Ghab Volcanic Field, NW Syria: Implications for the Metasomatic Evolution of the Lithosphere

Spongy minerals, especially clinopyroxenes, and fine-grained, often glass-bearing mineral assemblages , commonly referred to as melt pockets, occur in many mantle xenolith suites worldwide, but their origins remain far from being clearly understood. We describe a suite of spongy clinopyroxene- and melt pockets-bearing peridotite xenoliths from the Plio-Quaternary volcanic field in the Al Ghab Depression, a pull-apart basin of the Dead Sea Fault System in northwestern Syria. The melt pockets comprise fine-grained olivines, clinopyroxenes, spinels and feldspars ± amphiboles ± glasses. Petrography and major and trace element mineral chemistry suggest that the xenoliths have experienced at least two stages of metasomatism with the formation of the melt pockets being associated with the latest event. The first metasomatic episode featuring LREE, Na, Th, U enrichment and relative Ti and Zr depletion in the cores of primary clinopyroxenes involved metasomatism by a low-silica, CO2-rich agent. The second metasomatic episode was associated with the development of melt pockets which evolved from decompressional and perhaps metasomatism-induced incongruent melting of clinopyroxene ± spinel. The spongy clinopyroxenes that occur as coronas around clear, primary clinopyroxenes represent a transitional stage of the partially melted crystals. The incongruent melting produced a liquid that evolved within the melt pockets and eventually migrated out to form amphiboles and micas elsewhere in the lithosphere. Albeit with some uncertainty, geothermobarometric estimations reveal significant, systematic differences in the equilibration pressures between the primary minerals (0.8–1.4 GPa), and spongy and melt-pocket minerals (0.7–0.9 GPa), lending good support for the decompressional origins of the spongy clinopyroxenes and melt pockets. It is interpreted that decompression resulted from local transtension associated with the development of the Al Ghab pull-apart basin, a step-over zone of the Dead Sea Fault System, in Plio-Quaternary time.

Dating thin zircon rims by NanoSIMS: the Fengtien nephrite (Taiwan) is the youngest jade on Earth

Nephrite in the Fengtien area of the eastern part of the Central Mountain Range, Taiwan, is associated with antigorite-serpentinite within the Yuli belt, a late Cenozoic subduction–accretionary complex related to the eastward subduction of the South China Sea plate forming the Luzon arc. Diopsidite and clinozoisite rock are two other metasomatic components accompanying nephrite between serpentinites and the greenschist-facies country rock (carbonaceous material-)quartz-mica schist. Detrital zircons were separated from one clinozoisite rock sample, formed through metasomatic replacement after mica-quartz schist at temperatures of 320–420°C or slightly lower, which is lower than the metamorphic temperature conditions of the Yuli belt. Most of the detrital zircons have thin zircon rims less than 15–20 μm wide. These zircon rims, considered as newly formed during metasomatism leading to nephrite/diopsidite/clinozoisite rock formation, were dated by a high lateral resolution secondary ion mass spectrometer (CAMECA NanoSIMS NS50). The resulting 238U/206Pb-204Pb/206Pb inverse isochron gave an age of 3.3 ± 1.7 Ma. The collision of the Eurasian continental margin with the Luzon arc has been suggested to have begun at ca. 6.5 Ma in the Taiwan area. The nephrite formation processes therefore clearly post-dated South China Sea plate subduction. The present date, substantiated by the metamorphic and metasomatic temperature information, demonstrates that the fluid–rock interaction forming Fengtien nephrite would have taken place during a Barrovian-type metamorphic overprint resulting from arc-continent collision, leading to the exhumation of the Yuli belt. This conclusion on nephrite formation with regard to regional tectonics can serve as a working model for future studies on other nephrite deposits with similar occurrences, mostly embedded within Mesozoic or older subduction-accretionary complexes. The Fengtien nephrite deposit is therefore the youngest one of its kind exposed on Earth’s surface.

SHRIMP age of detrital zircon from the rock of Taiwan

The macroscale morphology of Archean stromatolites has been used as evidence of early microbial ecosystems. But as Archean stromatolites only rarely contain fossil microbes, their biogenicity is tacitly assumed on the basis of macroscopic morphological comparisons with modern structures. Biogenetic definitions, however, require microscopic examination of suspected stromatolites. We obtained an unique collection of pristine samples from a diamond drillhole that intersected the 2.7Ga Tumbiana Formation, Australia (Pilbara Drilling Project, Van Kranendonk et al., 2006). We report the occurrence of micronsized globules of organic carbon intimately associated with the host micritic carbonate. Scanning Transmission X-ray Microscopy (STXM) analysis revealed that these organic globules are composed of organic carbon with aromatic, aliphatic and carboxyl functional groups. High Resolution Transmission Electron Microscopy (HRTEM) analysis revealed that the organic material occurs in intimate association with clustered, 50-200nm rounded bodies of aragonite. These nano-aragonite aggregates show striking similarities with nano-carbonate spheroids associated with microbial cells and polymers in modern microbialites. Our results indicate that Tumbiana stromatolites were likely formed via in-situ microbial lithification. They also extend the geologic record of aragonite back more than 2,300 million years, with profound implications on the environmental conditions prevailing on Early Earth.