Island Arc最新文献

Sediment samples tend to dry out during storage and are, therefore, stored refrigerated at about 4°C after wrapping in plastic foil. During XRF core scanning however, the samples must be taken out of their cover, increasing the risk of drying and formation of desiccation cracks on the surface. Because scan times can often amount to several hours and at highest resolution may take over a day to complete, the core will progressively dry out during scanning. With this study we aim to increase our understanding of how this slow drying of the samples during scanning and storage influences the XRF signal because of changes in water content, sediment surface topography, and the development of small, but slowly expanding cracks in the sediment core. Results show that the desiccation of samples during scanning and storage influence the XRF measurements in several ways. Most importantly, slow desiccation of the cores results in both a general lowering of the sample surface, and a shortening of the core due to shrinkage. Larger distance between sediment surface and detector leads to increased noise levels and poor reproducibility for many elements, while the shrinking of cores may shift individual data points between runs, resulting in poor reproducibility and offsets between datasets obtained at different times. Moreover, the loss of light elements, such as hydrogen and oxygen, can influence the matrix effect, especially for organic-rich sediment. Because the XRF signals of individual elements are affected to different degrees, these changes may induce artificial shifts and biases in many elemental ratios commonly used for paleoenvironmental reconstruction.

The XV mafic-ultramafic intrusion is located in the western part of the Posht-e-Badam Block (PBB) within the Central Iranian Micro-Continent (CIMC). Petrographically, the intrusion is composed of gabbro and pyroxenite. Apatite U–Pb dating has established the crystallization age of this intrusion to be 363 ± 67 Ma. The XV intrusive rocks are tholeiitic to slightly calc-alkaline in nature and are characterized by an enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and Heavy Rare Earth Elements (HREE), respectively. The major oxide elements display continuous trends relative to SiO₂. The ⁸⁷Sr/⁸⁶Sr(i) ratios range from 0.7045 to 0.7056, and the εNd(i) values range from 2.63 to 3.30. In addition, the ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios exhibit a narrow range, varying from 18.68 to 18.70, 15.67 to 15.71, and 38.84 to 38.99, respectively. The geochemical and isotopic characteristics suggest that the parental magma was derived from a Sub- Continental Lithospheric Mantle (SCLM) that was modified by oceanic slab-derived components. The locations of the XV intrusive rocks in εNd(i) versus TDM (Ga) and Nb/La versus discrimination diagrams further support this conclusion. Fractional crystallization is identified as the dominant process influencing the formation of distinct lithological units within the XV intrusive rocks. Our newly presented isotopic and geochronological data, when considered in the regional context, suggest that the XV intrusive rocks were formed in an extensional tectonic setting. In this scenario, upwelling from the asthenospheric mantle induced heating, leading to the melting of previously subduction-modified SCLM. Comparative analysis with previously published ages indicates that extensional magmatism in the PBB continued into the Middle Paleozoic.

The regionally prominent main boundary thrust (MBT) of the Himalayan fold-thrust belt in northwest India is typically defined by the presence of Proterozoic rocks in the hanging wall and Cenozoic rocks in the footwall. The present study focuses on identifying the MBT contact across Gambar River section in Himachal Pradesh, India, using alternative methodologies, such as the meter-scale litho-structural mapping, followed by detrital zircon U–Pb geochronology to precisely identify the thrust contact and provide insights on the deformation history of the MBT zone. We have identified a sharp change in the age (from ~600 to ~61 Ma) of the sedimentary units along a narrow zone in the study area by detrital zircon U–Pb geochronology using LA-ICP-MS. The sharp change in the detrital zircon U–Pb age data thus delineate the MBT occurring in the area along a < ~1 m thickness. The lithological assemblage and the age data indicate the unified maximum depositional age from ~700 to ~600 Ma for the hanging wall rocks, which have been equated with the Krol Group of the Lesser Himalayan Sequence (LHS). In comparison, the footwall rocks exhibit the maximum depositional age of ~61 Ma and have been equated with the Cenozoic Subathu Formation of the Sub-Himalayan Sequence (SHS).

This study investigated geological evidence for near-surface crustal deformation in a high-strain shear zone that has been geodetically identified but which is not associated with obvious tectonic landforms. Fieldwork was conducted in the east–west-trending southern Kyushu high-strain shear zone (SKHZ), Japan, focusing mainly on occurrences of fracture zones, which are defined by a visible fracture density of >1 per 10 cm² and are commonly associated with cataclasite, fault breccia, and gouge. The area in which east–west-trending fracture zones are dominant is restricted to the east–west-trending, ~2-km-wide aftershock area of the 1997 Northwestern Kagoshima Earthquakes. Analysis of slip data from minor faults using the multiple inverse method, irrespective of whether the faults are in fracture zones, reveals that the area where the calculated main stress field is consistent with the current stress field estimated from focal-mechanism solutions of microearthquakes is restricted to the east–west-trending aftershock area. This finding for the SKHZ contrasts with the case of the Niigata–Kobe Tectonic Zone, which is a major strain-concentration zone with many exposed active faults in central Japan and for which the stress field estimated using fault-slip data is considered to be uniform and coincides with the current stress field. The cumulative amount of displacement estimated from the areal density of fracture zones in the SKHZ study area is smaller than that estimated from geodetically measured strain rates. Investigations based on slip data from minor faults and fracture-zone occurrence could help to identify concealed faults that are too small to generate tectonic landforms but which are sufficiently large to trigger major earthquakes.

Early Miocene sediments of the Morozaki Group in central Japan contain deep-sea fossils that have been dated using biostratigraphic and radiometric data. In this study, we utilize magnetostratigraphy to provide a more precise age for mudstones from just below the layer containing the fossils. Rock magnetic experiments suggest that both magnetic iron sulfide and Ti-poor titanomagnetite carry the remanent magnetization of the mudstones. Two different stratigraphic sites have normal polarity directions with a northeastern declination, which can be correlated with Chronozone C5Dn. Given their magnetostratigraphic position near the C5Dn/C5Dr chronozone boundary (17.466 Ma) and a high sedimentation rate, the estimated age for both the sites and the deep-sea fossils is ~17.4 Ma. The northeasterly-directed site-mean directions suggest clockwise tectonic rotation, most likely due to the Early Miocene clockwise rotation of Southwest Japan associated with the back-arc opening of the Japan Sea. The deep-sea fossils, dated at ~17.4 Ma, represent organisms deposited within a submarine structural depression formed by crustal extension during the back-arc opening stage.

The Xiangshuyuan Formation (middle Rhuddanian to middle Aeronian stages of the Llandovery Series, lower Silurian) records a shelly fauna representing recovery after the end-Ordovician mass extinction in a well-oxygenated shallow carbonate platform of the Upper Yangtze region, South China Block. Carbon isotope stratigraphy is documented from limestone sequences of the formation at the Qiankou section, northeast Guizhou. The early Aeronian carbon isotope excursion (EACIE, with an amplitude of about 2 ‰ and peak value of 2.44 ‰) is identified in the middle and upper parts of the formation (Ozarkodina obesa conodont Biozone). The EACIE recorded herein correlates well with those in Baltica, Canada, and the United States; together with its records from organic material (δ¹³C_org) the data verify that the EACIE is a global event. The beginning of the EACIE can be used as a chemostratigraphic marker defining the Rhuddanian/Aeronian boundary in strata that lack high-resolution biostratigraphic constraints.

Granulite facies metamorphism and crustal anatexis exist in the East Cathaysia Block, the exact timing of granulite facies partial melting and its link with orogenesis have not been well constrained. In this study, we carried out petrography, whole rock geochemistry, and zircon U–Pb dating, trace elements and Hf isotopes analyses on Dazhe gneissic granite and banded migmatite from the Badu Group in southwest Zhejiang province in the East Cathaysia Block. The melts were produced through the dehydration of biotite, such as biotite + quartz + plagioclase = orthopyroxene + K-feldspar + melt and biotite + quartz + plagioclase + sillimanite = garnet + K-feldspar + melt. Zircons from these rocks show clear core-rim structure and yield rim and core concordant ages at 233 Ma and 1.83 Ga, respectively. The zircon rims suggesting the melts and the cores are suggesting the protolith of Dazhe gneissic granite and banded migmatite were crystallized from an evolving magma. The zircon cores and rims have negative ε_Hf(t) = −2.2 ~ −6.3 and ε_Hf(t) = −22.8 ~ −32.4, and they give suggestion of the presence of Neoarchean components. Although the major-element compositions of the gneissic granite and banded migmatite are slightly different, the trace-element spider diagram and REE pattern show they are similar, and then we find that the protoliths are A-type granodiorite/diorite. Combined with the published data, we suggested that the Dazhe gneissic granite and banded migmatite were formed through granulite facies partial melting at 233 Ma, which was promoted by crustal shortening and thickening of the collision orogeny between East Cathaysia Block and an unknown terrane with a NNE trend structure line. The protoliths (granite or granodiorite) of Dazhe gneissic granite and banded migmatite crystallized at 1.83 Ga by reworking of the Neoarchean components of East Cathaysia Block. The Paleoproterozoic (1912–1819 Ma) collisional orogeny and the later intraplate rifting stage are corresponding to the aggregation and breakup of the Columbia supercontinent.

Paleogene surface tectonics in Japan is not well understood because of the paucity of onshore Paleogene stratigraphic records except for those from accretionary complexes. Paralic Paleogene formations remaining in SW Japan are usually so thin that it is difficult to decipher the tectonics from them. However, the Eocene paralic sedimentary package with a thickness of kilometers indicates syn-depositional tectonic subsidence by a few kilometers in the Amakusa archipelago, west of Kyushu Island. Thus, we made a detailed geological map of the Eocene formations in an area of ~50 square kilometers in the northwestern part of the archipelago. We identified NE-SW and NW-SE trending normal faults, most of which were recognized by previous researchers, and also discovered low-angle faults. NW-SE trending ones are known to be of the Miocene. NE-SW trending and low-angle normal faults are the oldest map-scale structures in the Eocene ones. It is not obvious within the above-mentioned area whether those normal faults are accompanied by growth strata. However, the significant southeastward thickening of the Eocene formations across the Amakusa archipelago suggests that they filled a large half graben with the basin margin fault along the eastern side of the archipelago. This basin model is consistent with the N-S to NW-SE transport directions of the low-angle and NE-SW trending normal faults. Since many NE-SW to EW trending Eocene grabens were formed in the offshore regions west of Kyushu Island and in the East China Sea, the Amakusa region was probably a northeastern branch of the rift system. The geologic structures and depositional ages of the Eocene formations indicate that the Eocene extensional tectonics removed the overlying strata to some extent for the high-P/T Takahama Metamorphic Rocks which crops out to the south of our study area.

Internal microtextures of ternary alkali feldspars in sanidine trachyte from Oki-Dogo Island were examined using an electron microprobe analyzer, a scanning electron microscope, a transmission electron microscope and cathodoluminescence instruments, to develop the understanding of volcanic processes of alkaline magmas related to feldspar crystallization. The examined trachyte is an evolved rock of the Oki-Dogo Pliocene trachyte group. Its phenocryst feldspars are commonly associated with lamellar-wavy-domain textures with scales approximately from 100 nm up to several hundreds of μm that show complex and gradual variations in composition: however, anti-rapakivi zoning textures common in other Oki-Dogo alkaline rocks are almost completely absent in the trachyte. These textures are produced by extensive magmatic ion-exchange replacement reactions progressively advanced in the evolved magma. Characteristic braided fluorite alignments are developed consistently with lamellar-wavy-domain textures in phenocryst feldspars, and similar braided alignments are also present in groundmass feldspars with complicated microtextures. Most of fluorite grains are <100 nm in diameter, and the patterns of braided fluorite alignments vary greatly in individual feldspars. The whole occurrence of the feldspar microtextures represents an extreme example of diffusion-controlled replacement reactions, progressively advanced in the dry (relatively anhydrous) trachyte magma. The genetic processes forming fluorite alignments in feldspars are related to magma compositions, especially F and P contents, and the crystallization of F-bearing minerals, especially of fluorapatite.

The crystallization of nanolites within magma chambers has recently raised a strong interest due to their impact on increasing melt viscosity and triggering magmatic eruptions. In 2021, the Fukutoku-Oka-no-Ba (FOB) underwater eruption produced large quantities of pumices that eventually formed rafts drifting at the surface of the ocean to the East coasts of Japan. Pumices collected along the shore shortly after grounding show various colors, microscopic and Raman analyses made by Yoshida et al. (Island Arc, 31, 1, 2022) revealed the presence of magnetite nanolites in some of them. In this study, we explore the magnetic properties of a batch of pumices of different colors from the FOB eruption, aiming to refine characterization of iron oxide nanolites. We used various analytical techniques such as SEM and FEG-SEM observations, EDS-X analyses, and rock magnetic experiments, including thermomagnetic analyses, hysteresis curves, coercivity analyses and FORC measurements. Our findings reveal that the iron oxides present in the FOB samples are Ti-magnetite, with minor amounts of Mg and Al. The magnetic crystals show a wide range of sizes, from extra small iron oxide nanolites (ESION) in the pumices with the lighter colors, to more bulky grains reaching the micrometer size in some of the dark color samples, significant diffusion is inferred in that case. Consequently, the magnetic characterization of iron oxide crystals within the Fukutoku-Oka-no-Ba pumices reveals varying stages of nucleation, dissolution, growth, and diffusion processes, providing evidences for the heterogeneous state of the magma during the eruption.