Search

Ordovician Japan formed a mature arc-trench system developed along the palaeo-Pacific (Panthalassa) margin of the Greater South China (GSC) continental block. GSC consists of South China, East China Sea, SW–NE Japan and the Khanka–Jiamusi–Bureya megablock in the Far East; Paleozoic GSC was thus, in total, twice as large as the South China components by themselves (Yangtze and Cathaysia). The Ordovician crust of Proto-Japan comprised coeval arc-related rocks, such as granitoids, supra-subduction zone ophiolites and fore-arc basin strata, although most of them were considerably fragmented. The Ordovician and middle–late Paleozoic fossils from Japan are highly limited but suggest that Proto-Japan was positioned in the low-latitude domains probably of the palaeo-Pacific Ocean in connection to Paleo-Tethys. GSC became separated from Rodinia in the Neoproterozoic, and its Proto-Japan segment evolved as a collision-free subduction margin for nearly 500 myr since the mid-Cambrian. The GSC framework provides critical constraints to the palaeogeographical reconstruction of circum-Pacific continental blocks. First, the Cambro-Ordovician GSC should have been isolated from Australia/India/East Antarctica that formed East Gondwana by a relatively wide ocean domain for keeping ‘subduction potential’. Second, the Cathaysian margin of GSC should have faced to an extensive ocean without major continents since the Cambrian. The palaeo-Pacific is the only possible candidate for this.

With a particular focus on the earliest Cambrian diversification of small shelly fossils (SSFs), stratigraphic analysis was conducted on the lower Cambrian Zhongyicun Member at the Hongjiachong section in the Chengjiang area, Yunnan, China. From ca. 3-m-thick bedded phosphorites (Unit A) in the lower part of the member, we recovered unique SSFs. This interval with 4 SSF-bearing horizons is characterized by the dominance in blade-shaped SSFs, including Halkieria spp., Brushenodus prionodes, Sinosachites delicatus, and Pteronus sp., and the absence of typical tube/cap-shape SSFs common in the Fortunian. This interval tentatively named “Halkieria-dominant interval” is stratigraphically positioned ca. 3 m below the previously known level of the oldest mollusks in the continuous bedded phosphorite sequence between the Anabarites trisulcatus-Protohertzina anabarica Assemblage Zone and the overlying Paragloborilus subglobosa-Purella squamulosa Assemblage Zone of the Fortunian (the earliest Cambrian). The “Halkieria-dominant interval” yields some SSFs likely affiliated with ostracods in view of size and morphology, suggesting that arthropod body fossils appeared in the Fortunian, considerably earlier than previously believed.

Although the Phanerozoic geology of the Central Asian Orogenic Belt is extensively studied, there is still much to reveal about the initiation and early stages of tectonism. The Gargan block of the Tuva-Mongolia terranes at the Mongolian-Russian border is one of the Archean-Proterozoic continental crustal fragments that record the earliest evolutionary history of the belt. To better constrain the age of its basement, extent, and provenance in regional tectonic framework, we present new zircon U Pb geochronologic data from previously undated basement gneisses and the overlying Neoproterozoic meta-sedimentary rocks of southern part of the Gargan block. Comparison of available basement ages and Precambrian detrital zircon spectra from nearby Archean-Proterozoic continental crustal fragments show complex relationship amongst them. We propose that the Gargan block travelled by itself in most of the Proterozoic or was derived from an unknown continental block. Obtained age confirms the existence of Neoarchean-Neoproterozoic basement in the region; however, for the first time, the 2749–814 Ma Salig Complex reveals multiple stages of Neoproterozoic metamorphic events within the Central Asian Orogenic Belt. Older of which, at ∼1 Ga, may relate to an assembly of several of the pre-existing continental crustal fragments in Mongolia.

To constrain the redox conditions and related nitrogen cycles during the Middle Permian (Guadalupian) to latest Late Permian (Lopingian) deep mid-Panthalassa, we determined the abundances of major, trace, and rare earth elements along with the carbon and nitrogen isotope ratios in shales interbedded with deep-sea cherts that are well-exposed at the Gujo-Hachiman section in the Mino-Tanba belt, SW Japan. The positive Ce anomalies together with low Mo and U enrichment factors (MoEF

The Mikabu greenstones form accreted fragments of an ancient oceanic plateau, which are currently incorporated within the Cretaceous high P/T-type Sanbagawa metamorphic belt in SW Japan. Previous geological and petrological studies indicate that protoliths of the Mikabu greenstones were mafic/ultramafic volcano-plutonic rocks of oceanic plateau and/or islands. Despite their rock assemblage and metamorphic nature, their protolith age has been poorly constrained. By measuring U-Pb age of igneous zircon grains using LA-ICPMS, this study identified for the first time a Late Jurassic age for the protoliths of the Mikabu greenstones in the eastern Kii Peninsula (the Shurei complex). With an overlap within 2-sigma error, measured U-Pb ages of zircon grains have a weighted mean age of 154.6 ± 1.6 Ma (the Kimmeridgian, Late Jurassic), which is slightly younger than the radiolarian ages (Middle Jurassic) of deep-sea pelagic cherts associated with the Mikabu greenstones. This age difference indicates that mafic magmas intruded pelagic deep-sea sediments to form a volcanic edifice onto the pre-existing oceanic lithosphere. New measurements of trace element composition of the dated zircon grains further confirmed that the dated zircon grains are more similar to those in oceanic crusts rather than in continental crusts. In addition, the trace element composition suggests that the host mafic magma of the zircon grains was derived from depleted mantle similar to the mid-oceanic ridge-type source, rather than fertile mantle source like Hawaii and Iceland. On the basis of the common occurrence of similar mafic rocks from the Shatsky Rise in the modern NW Pacific Ocean, we conclude that the protoliths of the Mikabu greenstones had formed a part of an oceanic plateau/island, which was generated by the Late Jurassic intra-plate mafic magmatism in ancient mid-Pacific, probably together with the Shatzky Rise.

The end-Permian mass extinction (EPME) was the most severe mass extinction event of the Phanerozoic, and was associated with the development of global oceanic anoxia. The intensification of ocean anoxia preceded the EPME, but the degree of intensity and timing of oceanic redox changes in the mid-Panthalassa Ocean remain debated. Here we present the results of geochemical and multivariate statistical analyses of a late Guadalupian to Lopingian (middle–late Permian) bedded chert succession from the Iwaidani section, Japan, which preserves pelagic deep-sea facies from the ocean floor to the lower flank of a mid-Panthalassan seamount. The entire section yields a low manganese-enrichment factor (MnEF 2S-rich euxinic conditions developed during the latest Lopingian and around the time of the EPME. The cause of the shift toward more reducing conditions in the early–middle Lopingian is unknown, but PCA results suggest that the euxinic conditions occurred in association with intensified continental weathering in response to a temperature rise during the ca. 200 kyr before the EPME.

Despite the recent development in radiometric dating of numerous zircons by LA-ICPMS, mineral separation still remains a major obstacle, particularly in the search for the oldest material on Earth. To improve the efficiency in zircon separation by an order of magnitude, we have designed/developed a new machine – an automatic zircon separator (AZS). This is designed particularly for automatic pick-up of 100 μm-sized zircon grains out of a heavy mineral fraction after conventional separation procedures. The AZS operates in three modes: (1) image processing to choose targeted individual zircon grains out of all heavy minerals spread on a tray, (2) automatic capturing of the individual zircon grains with micro-tweezers, and (3) placing them one-by-one in a coordinated alignment on a receiving tray. The automatic capturing was designed/created for continuous mineral selecting without human presence for many hours. This software also enables the registration of each separated zircon grain for dating, by recording digital photo-image, optical (color) indices, and coordinates on a receiving tray. We developed two new approaches for the dating; i.e. (1) direct dating of zircons selected by LA-ICPMS without conventional resin-mounting/polishing, (2) high speed U-Pb dating, combined with conventional sample preparation procedures using the new equipment with multiple-ion counting detectors (LA-MIC-ICPMS). With the first approach, Pb-Pb ages obtained from the surface of a mineral were crosschecked with the interior of the same grain after resin-mounting/polishing. With the second approach, the amount of time required for dating one zircon grain is ca. 20 s, and a sample throughput of >150 grains per hour can be achieved with sufficient precision (ca. 0.5%).We tested the practical efficiency of the AZS, by analyzing an Archean Jack Hills conglomerate in Western Australia with the known oldest (>4.3 Ga) zircon on Earth. Preliminary results are positive; we were able to obtain more than 194 zircons that are over 4.0 Ga out of ca. 3800 checked grains, and 9 grains were over 4300 Ma with the oldest at 4371 ± 7 Ma. This separation system by AZS, combined with the new approaches, guarantees much higher yield in the hunt for old zircons. Keywords: Hadean, Zircon, U-Pb dating, LA-ICPMS, Automatic mineral separator, Primordial crust

U-Pb ages of detrital zircons were newly dated for 4 Archean sandstones from the Pilbara craton in Australia, Wyoming craton in North America, and Kaapvaal craton in Africa. By using the present results with previously published data, we compiled the age spectra of detrital zircons for 2.9, 2.6, 2.3, 1.0, and 0.6 Ga sandstones and modern river sands in order to document the secular change in age structure of continental crusts through time. The results demonstrated the following episodes in the history of continental crust: (1) low growth rate of the continents due to the short cycle in production/destruction of granitic crust during the Neoarchean to Paleoproterozoic (2.9–2.3 Ga), (2) net increase in volume of the continents during Paleo- to Mesoproterozoic (2.3–1.0 Ga), and (3) net decrease in volume of the continents during the Neoproterozoic and Phanerozoic (after 1.0 Ga). In the Archean and Paleoproterozoic, the embryonic continents were smaller than the modern continents, probably owing to the relatively rapid production and destruction of continental crust. This is indeed reflected in the heterogeneous crustal age structure of modern continents that usually have relatively small amount of Archean crusts with respect to the post-Archean ones. During the Mesoproterozoic, plural continents amalgamated into larger ones comparable to modern continental blocks in size. Relatively older crusts were preserved in continental interiors, whereas younger crusts were accreted along continental peripheries. In addition to continental arc magmatism, the direct accretion of intra-oceanic island arc around continental peripheries also became important for net continental growth. Since 1.0 Ga, total volume of continents has decreased, and this appears consistent with on-going phenomena along modern active arc-trench system with dominant tectonic erosion and/or arc subduction. Subduction of a huge amount of granitic crusts into the mantle through time is suggested, and this requires re-consideration of the mantle composition and heterogeneity. Keywords: Detrital zircon, U-Pb age, Continental growth, Subduction erosion, Preservation bias

To investigate the secular changes in Permian seawater chemistry and to constrain the global environmental changes during the end-Guadalupian (Middle Permian) extinction, we analyzed 87Sr/86Sr ratios in Capitanian (Upper Guadalupian) shallow-marine limestones in NE Japan and in southern Primorye, Far East Russia. These limestones were deposited on the continental shelf/platform of the northern part of Greater South China, which faced the northern connecting seaway between the Tethys and Panthalassa. The measured limestone samples were collected from the Capitanian fusuline (Lepidolina)-bearing intervals and overlying beds at Iwaizaki in the South Kitakami belt, NE Japan, and at Senkina Shapka in the Sergeevka belt, southern Primorye. The present analysis of bulk 87Sr/86Sr ratios clarified extremely low 87Sr/86Sr ratios of 0.7068–0.7070 from all measured samples. These Sr isotopic values support a Capitanian age for these strata. The topmost 30 m-thick interval of the Iwaizaki Limestone, in particular, witnessed a stepwise biodiversity decline in the shallow marine warm-water biota and the collapse of the carbonate factory after the reef development during the Capitanian in the northern part of Greater South China. The disappearance of the tropical biota and the collapse of the reef during the late Capitanian suggest that the biotic responses to a significant environmental change appeared relatively earlier at mid-latitudes than in the tropical regions.

Understanding the pre-Cenozoic geotectonic framework along the Japan Sea coast in southwest Japan is critical in clarifying the relationship between oceanic subduction-related and collision-related geotectonic belts along the continental margin of East Asia. Although a thick Neogene-Quaternary volcanic-sedimentary cover often conceals the pre-Cenozoic units, the Western Honshu is one of the pivotal regions especially the Mizoguchi gneiss (high-grade gneisses up to amphibolite facies with Rb-Sr isochron age of 185 Ma) exposed near Mt. Daisen in western Tottori prefecture. This unit, together with a neighboring 199 Ma granitoid, was previously correlated with the Hida gneiss in Central Japan, located nearly 250 km to the east. The present study examines this interpretation using zircon U-Pb geochronology. Ages of three gneissose rocks of the Mizoguchi gneiss reveals that the protolith is composed of and represented by granitoid, accretionary complex, and/or high pressure/low temperature (HP/LT) type metamorphosed units. In terms of protolith rock type and zircon age spectra, the Mizoguchi gneiss is different from the Hida gneiss that is interpreted to have been derived from sedimentary rocks/volcanics with continental shelf, and also from the Oki gneiss characterized by its preservation of abundant Precambrian detrital zircon grains. The high-grade metamorphism of the Mizoguchi gneiss is interpreted to have occurred after middle Jurassic as constrained by the youngest detrital grain. The protolith of the Mizoguchi gneiss belongs to the Suo belt (Jurassic HP/LT type metamorphosed accretionary complex) in southwest Japan; however, the unit is interpreted to have been metamorphosed by a Jurassic thermal event, which overlaps that of the Jurassic felsic magmatism in the Hida belt (the Funatsu episode). The Mizoguchi gneiss in western Honshu is unique, because it marks the northernmost or the most proximal occurrence of Phanerozoic subduction-accretion complex in Southwest Japan, clearly on the south of non-accretionary Oki and Hida belts.

In order to examine paleogeographic link between Primorye and Japan with respect to other continental blocks in East Asia, middle-upper Paleozoic shallow marine sandstones of the Khanka block and Sergeevka belt were analyzed by LA-ICPMS U-Pb dating of detrital zircons. The age spectra of detrital zircons of the Devonian Lyutorgian Formation, Lower Carboniferous Shevelevka Formation, and Permian Lyudyanza Formation in the Vladivostok-Nakhodka area share more or less the same age range; the majority of detrital zircons are dated 500–400 Ma (Cambrian to Silurian). These ages are common in previously dated granitoids with arc signature in Primorye and also in Japan, thus suggest a probable derivation from a single subduction-related Paleozoic batholith belt along the Pacific rim. The zircon age spectra of the Devonian and Carboniferous sandstones show similarities to those of coeval sandstones in NE and SW Japan. The middle-late Paleozoic sedimentary basins in Primorye and Japan commonly had a provenance featuring early Paleozoic arc rocks. In addition, all of these Paleozoic terrigenous clastics in Primorye and Japan consistently contain Proterozoic zircons. Occurrence of Meso- to Neoproterozoic (ca. 1500 and 600 Ma) grains positively suggests a link to the South China block, as other blocks in East Asia lack crusts of this age of pan-African affinity. The common age spectra of detrital zircons of the Paleozoic sandstones in southern Primorye and NE-SW Japan support the concept of “Greater South China” that is comprised of conterminous South China to extend to Japan/Primorye, which is consistent with the mutual similarities recognized in Paleozoic marine fauna. Paleozoic South China was much larger than previously imagined, and this greater size, almost double the present conterminous South China, requires much larger area in the paleogeographical reconstruction of Rodinia.

A recent study on quadruple sulfur isotopes (32S, 33S, 34S, and 36S) of sedimentary pyrite suggested that the end-Guadalupian extinction was caused by shoaling of the sulfidic deep-water. This scenario is based on the assumption that sulfur isotopic compositions of pyrite from hosting sediments were controlled by benthos activities, thus by the redox conditions of the sedimentary environments. Nonetheless, the relationship between the sulfur isotope records and redox conditions, reconstructed from litho- and bio-facies, are poorly known. In order to examine the effect of bioturbation in sediments, quadruple sulfur isotopic compositions of sedimentary pyrite from the end-Guadalupian succession in Chaotian, South China, were analyzed. Black mudstones of deep-water facies immediately below the extinction horizon have consistently high Δ33S values of ca. +0.079‰, clearly suggesting a sulfate reduction in the anoxic water column. Our new data are consistent with the emergence of a sulfidic deep-water mass prior to the end-Guadalupian extinction; the upwelling of the toxic deep-water may have contributed to the extinction. In contrast, shallow-marine bioclastic limestones with burrows deposited under oxic conditions have negative Δ33S values. This anomalous isotopic signal indicates the mixing of two distinct types of pyrite; one generated during the sulfate reduction in an open system and the other in a closed system. We interpret that bioturbation supplied sulfate in the sediments and promoted sulfate reduction and in-situ sulfide precipitation within the sediments. The negative Δ33S values of oxic sediments in Chaotian are inconsistent with the previous model and demonstrate that the sedimentary sulfur cycle associated with bioturbation was more complicated than previously thought. Our study also implies that, more generally, the role of bioturbation in increasing seawater sulfate concentration in the Phanerozoic may have been overestimated in the previous studies, because bioturbation may have enhanced sulfide burial or sulfur output from the oceans.

In this article we make a review of recent findings on seismic imaging of the Asian orogens and subduction zones. High-resolution images of seismic tomography and receiver functions of the Asian region are obtained, revealing significant lateral heterogeneities in the crust and upper mantle, which are caused by active plate subductions and continental orogeny. A significant advance in the seismic imaging is tomographic inversions for three-dimensional distribution of seismic anisotropy in the crust and mantle, which provides important new information on the lithospheric deformation and mantle convection associated with the continental orogeny and plate subductions. The intraplate volcanism in Northeast Asia is caused by hot and wet upwelling flows in the big mantle wedge above the stagnant Pacific slab in the mantle transition zone (MTZ). The age distribution of the subducting Pacific slab beneath East Asia is estimated, shedding new light on the evolution of the Pacific slab, as well as the East Asian tectonics during the Late Mesozoic to the Cenozoic. The nucleation of great earthquakes, such as the 2008 Wenchuan earthquake (M 8.0), the 2011 Tohoku-oki earthquake (M 9.0) and the 2015 Nepal earthquake (M 7.9), is controlled by structural heterogeneities in and around the seismogenic fault zones. It is considered that fluids are involved in the nucleation and rupture processes of all types of earthquakes. The cause of deep earthquakes is still not very clear, though transformational faulting triggered by metastable olivine transforming to spinel in the cold, stressed core of the subducting slab is a viable mechanism, and a metastable olivine wedge is revealed within the western Pacific subducting slab at the MTZ depths. The 2015 Bonin deep earthquake (M 7.9, ∼670 km depth) occurred at the MTZ bottom within the vertical Pacific slab which is penetrating into the lower mantle. This very unusual deep event was caused by joint effects of several factors, including the slab’s fast deep subduction, slab tearing and thermal variation, stress changes and phase transformations in the slab, and complex interactions between the slab and the ambient mantle.

CAOB occupies a vast area that extends from the Urals to the Far East Asia and from the Siberian craton to the North China and Tarim cratons (Fig. 1, A). In order to better constrain Precambrian tectonic evolution of the CAOB, it is important to revisit Precambrian terranes of Mongolia as outlined in Badarch et al. [2002] that contain Archean to Proterozoic metamorphic basement and Neoproterozoic metasedimentary and volcanic rocks.