Island Arc最新文献

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.

The Cenozoic Himalayan orogeny resulted from the continental collision between the Tibetan block and the northern Indian Precambrian shield. The latter, replete with evidence of Columbian supercontinent assembly, likely comprised the north Indian continental margin that was reworked mechanically and thermally during the Himalayan orogeny, and still survives as Precambrian vestiges in the Himalaya. Parts of this Paleoproterozoic crust, which now occur as nappes and klippen, were tectonically transported by the Main Central Thrust southwards over the Lesser Himalayan sedimentaries during the orogeny. The Absence of Columbian metamorphic signatures in these thrust sheets has intrigued geologists for long. We present evidence for a Middle Orosirian metamorphic event from the pelitic gneisses of the Almora Group in the Baijnath Klippe from NW Himalaya. The physical conditions of metamorphism have been inferred using mineral chemistry, bulk-rock chemistry, and phase section modeling using Perple_X software in the MnNKCFMASHT model system. Zircon U–Pb geochronology for the Dangoli pelitic gneisses yielded a robust upper intercept at 1891 ± 12.82 Ma. The P–T phase diagram indicates that the peak mineral assemblage stabilized in the P–T range of 0.41–0.46 GPa and 675°C–700°C suggesting upper amphibolite facies metamorphism. Integrated metamorphic and geochronological results indicate that the Dangoli pelitic gneisses were derived by muscovite dehydration melting of metasediments during the peak metamorphism related to syn-collisional setting broadly coeval with the Paleoproterozoic magmatism during the Columbia supercontinent assembly. The evidence for definite involvement of Paleoproterozoic high-grade metamorphic rocks of the northern Indian shield in the Himalayan orogeny is being documented.

The Middle Miocene was a period of tectonic transition in the southwest Japan arc. The cessation of backarc spreading at 15 Ma has been thought to have caused a pronounced change in the overall stress state from tension to compression in the arc-perpendicular direction. However, the spatial variation in stress throughout the arc, especially in the forearc, which reflects the characteristics of plate subduction, has yet to be delineated. This study investigates the stress field of the forearc region and its temporal change by applying a stress tensor inversion technique to outcrop-scale dilatant fractures. We measured the attitudes of clastic dikes and mineral veins in the Lower–Middle Miocene Tanabe Group, the paleo-forearc basin deposits in the southwestern Kii Peninsula. Results of paleostress analyses show that the Tanabe Group was influenced by (i) stress with an E–W minimum compressional axis before 15 Ma and (ii) stress with a NW–SE maximum horizontal compressional axis after 15 Ma. In contrast to conventional interpretations, we find that the former stress differed from arc-perpendicular tension in the backarc region before 16 Ma. The latter stress differed from trench-parallel tension on the southernmost Kii Peninsula. Such spatial non-uniformity of the forearc stress can be explained by the occurrence of a dynamic backstop, such as a landward area of the out-of-sequence thrust, beneath the Tanabe Group.

Zircon U–Pb dates for detrital grains in the Shakubetsu Formation of the uppermost Eocene Urahoro Group and the Upper Miocene Atsunai Formation distributed in the eastern part of Hokkaido have been determined for the first time. Both weighted mean 48.8 ± 1.4 Ma (n = 80) for < 70 Ma zircons and youngest grain 36.5 + 1.8, −2.5 Ma (Middle to Late Eocene) ages of the Urahoro Group are consistent with the sedimentary ages and Paleogene magmatic arc origin of the detritus clarified in the previous studies. Although the Zircon U–Pb ages (n = 24) of the Atsunai Formation range between 5 and ~1700 Ma, a weighted mean age of 7.05 ± 0.57 Ma (n = 10) for < 10 Ma zircons (youngest cluster) could constrain the maximum sedimentary age, which is consistent with the previous ages from diatom biostratigraphy and K-Ar method. However, while the K-Ar age was obtained from an Ol-Cpx-Opx basalt in tuff breccia, the dated zircons were probably derived from felsic tuff including many pumice grains. Therefore, the bimodal volcanism could have occurred at c. 7 Ma in the present forearc region, indicating that the volcanic front at this age migrated south significantly, correlated to the one which also occurred coevally in the seaward side of the present volcanic front in the NE Japan arc. Further, it has been found that the eastern boundary of the Shiranuka Hills is characterized by an ESE verging, large-scale monocline based on the present field study and previous geological maps. Since the Atsunai Formation was involved in the formation of the large-scale monocline, the structure started to form after the sedimentation at c. 7 Ma. The fact implies that the ongoing collision of the Kuril forearc sliver with the NE Japan arc has propagated to this area since the latest Late Miocene.

The southeastern edge of the South China Block (SCB) experienced the subduction of the Paleo-Pacific Block, resulting in varying tectonic settings across different regions of the SCB during the subduction. To explore the tectonic settings during sediment deposition in the southeastern edge of the SCB in the early Late Cretaceous, we analyzed the Jiao Yangjing-Yetang section of the Xingning Basin, Guangdong province. This section contains a sedimentary sequence of the Upper Cretaceous Yetang Formation, facilitating lithofacies analysis, U–Pb zircon chronology, trace elements analysis, crustal thickness estimation, Hf isotopes analysis, and age distribution comparison of detrital zircons. Our findings indicate that the Late Cretaceous Yetang Formation was deposited during the early Late Cretaceous Cenomanian stage (99.6 ± 1.9 Ma), and identifies five groups of detrital zircon U–Pb ages: 2320–1810, 1100–670, 500–400, 230–160, and 130–96 Ma. The early Late Cretaceous ℇ_Hf (t) values ranged from −7.4 to 0.6, suggesting that the debris originated from the southwestern Fujian and northern Guangdong regions. Trace elements analysis of detrital zircons indicates increasing crustal thickness during the early Late Cretaceous. The Xingning Basin was in a compressive environment during the late Early Cretaceous, and the compressive condition continued until the early Late Cretaceous period (99.6 ± 1.9 Ma).

Sri Lanka occupies a pivotal geological position within East Gondwana, featuring four prominent high-grade Precambrian lithotectonic units: the Highland Complex (HC), the Wanni Complex, the Vijayan Complex (VC) and the Kadugannawa Complex. Despite reasonable geochemical and geochronological investigations into rocks in the VC, a comprehensive understanding of their petrological characteristics, particularly their pressure–temperature–time evolution, remains limited, impeding accurate terrane correlations. In this study, a detailed analysis of mineral assemblages, reaction textures, and U–Pb zircon ages in samples collected from the eastern margin of the HC to the eastern coast, in the VC were conducted. Results indicate that the VC preserved textural evidence for prograde dehydration reactions indicating upper amphibolite to granulite facies transition. The peak metamorphism is followed by a stage of near isobaric cooling stage. Conventional geothermobarometric analysis coupled with phase equilibria modeling of VC gneisses reveal peak metamorphic P–T conditions of 790°C–830°C and 7–8 kbar. Retrograde metamorphism has occurred until the temperature decreased to 740°C. The U–Pb zircon dates at ca. 1010–960 Ma suggest protolith ages of the VC coinciding with Rodinia amalgamation and dispersal, aligning metamorphic thermal peaks at ca. 575–545 Ma with the Kuunga orogeny during Gondwana amalgamation. Our data signifies connections between the VC and both the Lurio foreland in Mozambique and the Innhovde Suite and Yamato-Belgica Complex in East Antarctica, however, the differences in metamorphic conditions and geochronological signatures necessitate further investigation into its tectonic history and paleogeographic evolution.