Island Arc最新文献

A giant summit collapse in the Annapurna Himalaya was detected by Lavé et al. in 2023 and proposed to have happened at approximately 1190 AD. These authors concluded that the collapse transformed into a debris avalanche and subsequent debris flows, which reached the Pokhara Basin 60 km downstream to form the Pokhara Formation. Our geological investigations of the sediments in the Sabche Cirque and the valley-fill sediments in the Pokhara Basin demonstrate, however, that the Pokhara Formation is not equivalent to the mountain collapse deposit in Sabche Cirque. To the contrary, the Tallakot Formation—the oldest valley-fill formation, which is composed of a monomictic breccia of cataclastic texture, is equivalent to the sediments in the Sabche Cirque consisting entirely of the sediments derived from the Tethys Himalayan Sequence. The Ghachok Formation, which overlies the Tallakot Formation, is a wide-spread well-consolidated debris flow deposit also consisting of the debris derived from the Tethys belt. Several dating studies on the samples collected from the Ghachok Formation and overlying Phewa Formation, the dammed-up lacustrine deposits yielded¹⁴C IntCal20 ages between 15 and 10 ka, the oldest of which originates from a layer of humic soil at the base of the Ghachok Formation. These findings indicate that the series of events from the giant summit collapse to debris flows occurred at 15–14 ka. This timing coincides with the deglaciation period in the latest Pleistocene; it suggests that the melting of glaciers and permafrost weakened the rock strength and supported the mountain collapse. The main triggering agent of the collapse is attributed to an E-W extensional, normal fault-type earthquake that occurred in the Tethys belt. Unlike the Tallakot and Ghachok Formations, the Pokhara Formation is a polymictic heterometric almost nonconsolidated deposit that unconformably overlies the Ghachok Formation and is dated to be approximately 1250 AD.

To reassess the geotectonic identity of the Tsukuba Igneous Complex (TIC) granitoids, we compiled a comprehensive data set of the granitoids and associated microgranular enclaves (MGEs) using zircon U–Pb geochronology, magnetic susceptibility (MS) analysis, and whole-rock geochemistry. The TIC granitoids comprised high-K, calcic to calc-alkaline, and peraluminous I-type granite. SiO₂ values were relatively high, and enrichment in Pb was observed while Ti and Nb were both depleted. The trace element signatures are diagnostic indicators for rocks formed in subduction-related settings. The MGEs in the Kabasan granitoid body were almost coeval with the host granitoids at ca. 79–76 Ma. Taking into consideration the mineral and geochemical compositions between the MGEs and the host granitoids, it was concluded that the MGEs originated from diorite xenoliths. Moreover, our new zircon U–Pb dating of TIC granitoids and MGEs showed that the TIC emplacement ages are divided into two groups; that is, ca. 80–76 Ma and ca. 70–61 Ma. This fact clearly demonstrates that TIC magmatism occurred two times during the Late Cretaceous to Early Paleogene. On the other hand, the data of TIC MS is two or three orders of magnitude lower than that of San-in granitoids MS, and the TIC granitoids belong to the ilmenite-series. The MS differences between the TIC and San-in granitoids can be explained by the amount of involved sediment, indicating that the zircon U–Pb dating is the most appropriate proxy for the reassessment of the geological identity of TIC granitoids. Accordingly, we propose that the TIC granitoids have two separate origins based on the emplacement age: that is, the Late Cretaceous TIC granitoids belong to those of the Ryoke or San-yo belts, whereas the Early Paleogene TIC granitoids are considered as the eastern extension of the San-in belt.

There are limited examples of incremental emplacement models of pluton that can be demonstrated through field observations. Comprehensive field investigations identified the existence of magmatic structures of alternating layers of multiple magmatic enclave-rich and enclave-free granite sheets in the Kinpusan pluton in central Japan, which geologically constrains the geometry and duration of the intrusion time interval of the incremental magmatic unit. Most layer boundaries are inferred by the presence or absence of enclaves, and the microstructure of the granites comprising the different layers is hardly distinguishable. However, several outcrops that show clear subhorizontal structures due to grain size contrast in the host granite exist. The emplacement history consists of repetitive downward accretion of 20–200 m thick subhorizontal magma sheets with relatively high magma flux, and is a good candidate for comparison with thermal models used by many researchers in recent years. The observed grain size, aplite, and miarolitic cavities show a tendency to increase in the lower center of the body, indicating a longer period of high-temperature preservation than the pluton margin. Spatial patterns at this map scale were consistent with the predictions of the intrusion thermal model and the results of the zircon U–Pb dating together with a zircon saturation modeling, supporting the validity of the modeling.

The East Kunlun orogenic belt (EKOB), located in the northeastern part of the Qinghai-Tibet Plateau, is a significant Cu-Ni mineralization region associated with the geological evolution of the Paleo-Tethys and Proto-Tethys stages. Orogenic environments in this region exhibit considerable metallogenic potential, with two distinct metallogenic models: the subduction-related island arc model and the post-collisional extensional model. This study investigates the petrology, zircon U–Pb chronology, and geochemistry of ore-bearing intrusions in the Langmuri Cu-Ni deposit. The results reveal that the wehrlite yields a zircon U–Pb age of 420.0 ± 1.4 Ma, with an average ε_Hf(t) value of −5.5 and significant Nb-Ta negative anomalies. These features suggest that the parental magma was likely derived from the partial melting of a metasomatized continental lithospheric mantle. Additionally, crustal contamination during magma ascent triggered significant crustal sulfur assimilation. The Langmuri deposit represents a mineralization event formed during the final tectonic stage of the Proto-Tethys Ocean in a post-collisional extensional setting. Slab breakoff released hydrous fluids and melts that metasomatically enriched the lithospheric mantle, thereby providing the material basis for magmatic sulfide segregation.

The Yoshimi Metamorphic Rocks are composed mainly of mafic metamorphic rocks (e.g., garnet-amphibolite, garnet-clinopyroxenite, and orthopyroxene-clinopyroxene granulite) and fault-bounded pelitic gneiss. Peak metamorphic temperatures of mafic metamorphic rocks are estimated to have been approximately 800°C, based on garnet-clinopyroxene, garnet-hornblende, and orthopyroxene-clinopyroxene thermometers. Subsequently, the rocks were retrogressed and hydrated under the amphibolite facies at 0.6–0.8 GPa and around 600°C using a combination of several geothermobarometers. In mafic metamorphic rocks, exsolved rutile is found in garnet, orthopyroxene, and clinopyroxene. Such rutile probably formed during cooling from granulite to amphibolite facies. On the other hand, rutile exsolution is not observed in garnet from the pelitic gneisses, which only record a single stage of amphibolite facies metamorphism at 600°C–700°C and 1.1–1.5 GPa, as determined by garnet-biotite geothermometers and a garnet-biotite-muscovite-plagioclase-quartz geobarometer. Dating of zircon in garnet-amphibolite indicates that the protolith formed at ca. 120 Ma, with retrogressive amphibolite facies metamorphism at ca. 68 Ma. Zircons in the pelitic gneisses yield ages of ca. 66 Ma as an upper limit on the timing of original sedimentation and 63 Ma for metamorphism. The differences in the ages and metamorphism between the mafic metamorphic rocks and the pelitic gneisses clearly indicate they experienced different metamorphic histories, and that they were brought together during the exhumation process.

Two, outcrop-scale, basaltic, ring structures were discovered in Oligocene sandstone on an abrasion platform in northwestern Kyushu, Japan. This paper aims to report their occurrence and discuss their formation mechanisms. When viewed from above, they were shaped like a teardrop and an oval with diameters of ~16 and ~6 m, respectively; and were emplaced 5.5 m apart. There were neither radial nor concentric fractures in the country rock. The smaller structure had a ring dike and is underwater for most of the time, so its observational data are much less than those of the larger structure. The latter had a cone sheet with an intrusive contact with the host rock. The structures were probably feeder pipes of the Pliocene Higashi-Matsuura Basalts which cap hills around the structures at an altitude of ~140 m. The structures provide a rare opportunity with their small sizes to gain a panoramic view of volcanic conduits in relation to their host rocks, offering valuable insights into magma processes. Basaltic breccia and intrusions with lingulate shapes were surrounded by the cone sheet. Flow banding of this sheet indicated that the sheet was formed by repeated intrusion and destruction events, and that magma's ascent through a few-decimeter-thick annular spaces formed the cone sheet. The angular projections of the outer wall of this sheet and fractures in the country rock suggest that the structures expanded their diameters by stoping. Gusts of granular flow above the level of magma fragmentation likely smoothed the originally rugged wall of arcuate openings, in which parts of the sheet were formed. There were systematic joints around the structures, and the curvatures of some of the joints suggest that the ring structures were formed simultaneously with the jointing under weak, far-field, extensional stress with roughly north–south trending, minimum horizontal stress.

The dating of volcanic quartz from the same volcanic source, namely the Toya pyroclastic flow deposits and the ash-fall deposits, was conducted using the red thermoluminescence method (RTL). The results yielded the anticipated ages for the pyroclastic flow deposit (99–103 ka) and considerably older ages for the ash-fall deposits (51%–83% older, 150–188 ka). This discordance is attributed to changes in the annual dose rate due to elemental migration resulting from the weathering of the ash-fall deposit. X-ray diffraction (XRD) and solid ²⁹Si nuclear magnetic resonance (NMR) analysis of the ash-fall deposits indicate that allophane/imogolite was newly generated. Furthermore, solid ²⁷Al NMR measurements indicate that the fresh glass with tetrahedral Al has undergone a transformation to the octahedral Al of allophane/imogolite. A comparison of the Ti-normalized values of elements between the pyroclastic flow deposit and the ash-fall deposit, conducted using LA-ICP-MS measurements, revealed a significant reduction in alkali and rare earth elements (REEs) and an enlargement in aluminum in the ash-fall deposit. However, the Ti-normalized values of uranium (U) and thorium (Th) showed different migration trends depending on the sample. The following weathering factors are correlated with elemental migration: (1) The release of positive ions by the weathering of volcanic glass, (2) The adsorption and desorption of ions on the surface functional groups of clay (allophane/imogolite) and iron hydroxide, (3) The high hydrophilicity of the allophane/imogolite, and (4) Non-equilibration of the U and Th decay series due to Rn release. The annual dose rate of the ash-fall deposit has been subject to fluctuations as a consequence of the weathering process. Consequently, the adoption of the present annual dose rate for the dating may result in an unexpected age. It is therefore crucial to select sediments that can ensure a closed system of element transfer.