查看更多>>摘要：Zircon U-Pb weighted mean ages of seven samples from the Higo plutonic complex (HPC) in Kyushu were determined as follows; Shiraishino granodiorite: 109.3 +/- 0.9 Ma and 109.0 +/- 1.1 Ma; Kugushima granodiorite: 109.1 +/- 1.1 Ma; Miyanohara tonalite: 107.7 +/- 1.3 Ma and 108.3 +/- 0.9 Ma; Manzaka tonalite: 108.0 +/- 0.9 Ma; Joyama tonalite: 112.7 +/- 1.2 Ma. The plutonism of batholithic granitoids in the HPC occurred in 109-108 Ma, and granitic intrusions occurred in 113 Ma. The previously reported zircon U-Pb ages range from 107 to 98 Ma in northern Kyushu and concentrate around about 106 Ma in the Chikuhi area in the northern part of central Kyushu. The previously published age data and newly obtained data in this study demonstrate that Cretaceous igneous activity in Kyushu migrated northward from 114 to 98 Ma, accompanied by widespread igneous activity throughout central and northern Kyushu at 106-105 Ma. The HPC situated in the southernmost part of central Kyushu was the origin of this northward migration.
查看更多>>摘要：The South Kitakami Belt in the northeast Japan is unique in presence of a thick Paleozoic-Mesozoic sedimentary rocks. The Permian sedimentary succession in the Maiya area of this belt is divided into the Nishikori, Tenjinnoki, and Toyoma formations, in ascending stratigraphic order. The Tenjinnoki Formation includes the Yamazaki Conglomerate Member containing granitic clasts. We performed U-Pb dating for detrital zircon of one sample of tuffaceous sandstone from the Nishikori Formation, six samples of sandstone from the Tenjinnoki and Toyoma formations, and five granitic clasts from the Yamazaki Conglomerate using laser ablation-inductively coupled plasma-mass spectrometry. Our dating results show that the tuffaceous sandstone sample has two age peaks at 287 and 301 Ma for the Nishikori Formation, three age peaks at 320-300, 290, and 270 Ma for the Tenjinnoki and Toyoma Formation, and ages of 311, 300, and 270 Ma from granitic clasts of the Yamazaki Conglomerate. In addition, older ages of 452-435 and 380 Ma were obtained from some zircon grains of the sandstone and granitic clasts. Our results suggest igneous activity in these periods. The South Kitakami Belt's origin with respect to continental blocks has been discussed in regard of the margin of North China Block or South China Block. Based on the stratigraphic ages and timing of igneous activity, we conclude that during the Permian the South Kitakami Belt was located at the margin of the South Central Asian Orogenic Belt, near the Solonker-Xra Moron-Changchun suture and the North China Block in East Asia.
查看更多>>摘要：The present study deals with the biostratigraphic framework of Late Cretaceous sediments of the subsurface Well (DNG) "A" (2085-1840 m depth) from the Raghavapuram/Chintalapalli Shale of the Krishna-Godavari Basin and analyzed the palynological assemblage. The recorded palynological assemblage is dominated by marker dinoflagellate cyst viz. Areoligera coronata, Areoligera senonensis, Heterosphaeridium spinaconjunctum, Nelsoniella aceras, Nelsoniella semireticulata, Odontochitina porifera, Xenascus ceratioides, and Xenascus gochtii species. The last occurrence (LO) of significant species of dinoflagellate cysts is considered for the biostratigraphic establishment. On the basis of dinoflagellate cyst biostratigraphy, the examined sequences correspond to the Santonian-Campanian age. The dominance of gonyaulacoid dinoflagellate cysts is interpreted in terms of sea-level fluctuation and other paleoenvironmental signals through this biostratigraphic establishment. The recorded dinoflagellate cysts indicate the outer neritic, cool depositional environment. The sea-level rise, recorded in our study is linked with the transgressive phase during the Late Cretaceous (Santonian-Campanian) period.
查看更多>>摘要：The fossil record of the cephalopod genus Nautilus has been obscured because a few influential taxonomists during the 20th Century decided that fossils similar to Nautilus were instead other genera. We now recognize fossils once classified as species of other genera as species of Nautilus. This includes fossils from Miocene rocks of Taiwan that were previously described as Kummelonautilus taiwanum but herein recognized instead as being the northernmost Neogene record of Nautilus in the Indo-Pacific region. The name is corrected to Nautilus taiwanus, and now known to occur in two formations in central Taiwan, the early Miocene Shihmentsun and early to middle Miocene Houdongkeng formations. Miocene fossils from Indonesia that were placed in other genera are now considered to represent Nautilus as they were originally assigned, in addition to several Miocene species from Australia, which provide the southernmost Neogene fossil record for the genus. Some of these Indo-Pacific fossils may represent the same species, but more specimens are needed to determine the amount of variability within these Neogene taxa.
查看更多>>摘要：Pumice rafts that arrived at the Nansei Islands, Japan, provided a unique opportunity to investigate the Fukutoku-Oka-no-Ba (FOB) eruption of August 2021. Despite drifting for 2 months for similar to 1300 km, the drift pumice raft had a large volume and contained a variety of pumice clasts, some of which were deposited during a high tide in a typhoon, while others were washed up on a sandy beach. Most of the drift pumice clasts are gray in color, vesicular, and have a groundmass containing black enclaves. Rare black pumice and the main gray pumice components have similar trachytic compositions, with SiO2 = 61-62 mass% and total alkalis = 8.6-10 mass% (on an anhydrous basis). Both pumice types contain clinopyroxene, plagioclase, and rare olivine phenocrysts. Thin-section observations show that the gray pumice has more elongated vesicles as compared with the black pumice that has spherical vesicles, even where the two types of pumice are in the same clast. The glass in the black pumice is transparent and brown in color, while that in the gray pumice is colorless. No micro or nanocrystals were observed during electron and optical microscopy. Raman spectra of the brown-colored glass exhibit a clear magnetite peak, suggesting magnetite nanolites cause the brown color. High-Mg olivine in the black pumice has an equilibrium temperature of c. 1200 degrees C and a rim diffusion profile indicative of re-equilibration with the surrounding melt over a period of hours to days. The textural relationships between the gray and black pumice suggest that the black pumice had become black and viscous before the two types of pumice mixed. Therefore, crystallization of magnetite nanolites and a corresponding increase in melt viscosity were important in the eruption preparation process, which then resulted in a large-scale Plinian eruption.
查看更多>>摘要：One of the key areas and periods for dinosaur evolution is considered to be Early Cretaceous East Asia, of which paleoclimate, that affected dinosaur diversity, needs to be accurately estimated. This study reconstructed paleoenvironmental information from two paleosol sections of the Lower Cretaceous Tetori Group in Hokuriku province, Japan. Elemental and isotopic compositions indicate that the two sections (Sections K5 and T1) formed under similar mean annual temperature (MAT) and mean annual precipitation (MAP) but exhibit significantly different paleosol colors (red in the Section K5, and gray in the Section T1) and mineralogy of carbonate nodules (dolomite-siderite in the Section K5, and dolomite-calcite in the Section T1). In addition, the oxygen isotopic ratio of carbonate minerals in the Section K5 was significantly low. The differences in sedimentary features and isotopic composition were considered to reflect the differences of seasonal climatic factors (e.g., monsoon) and local settings (e.g., drainage condition) despite the two sections exhibiting similar MAT and MAP. The results of this study suggest that the northeastern margin of East Asia, where the Tetori Group was deposited, had temperate and humid climate with monsoonal contrast in humidity during Aptian to Albian.
查看更多>>摘要：Late Mesozoic lamprophyres are widespread in the Chaohu area of the Lower Yangtze region, eastern China. These mantle-derived rocks are of great significance for understanding the characteristics of mantle source and tectonic environment of eastern China during the Late Mesozoic. The lamprophyres from the Chaohu area can be divided into two types. The type-I lamprophyres have low SiO2 (43.8-44.5 wt.%) and TiO2 (1.06-1.14 wt.%), high MgO (13.5-17.5 wt.%) and high contents of compatible elements, and are enriched in LREE and LILE but depleted in HFSE. They have high initial Sr-87/Sr-86 (0.70767-0.70904) and negative epsilon Nd (t) (-4.8 to -4.4). The type-II lamprophyres are characterized by low contents of SiO2 (43.0-45.4 wt.%), high TiO2 (1.74-2.55 wt.%), MgO (9.17-15.4 wt.%), and compatible elements. They are also enriched in LREE and LILE but depleted in HFSE. They show enriched Sr-Nd isotopic compositions with initial Sr-87/Sr-86 values (0.70773-0.70886) and epsilon Nd (t) values (-8.6 to -7.8). The type-I lamprophyres show higher Mg-# and MgO contents but significant lower TiO2 than the type-II lamprophyres. In addition, the type-I lamprophyres have relatively similar initial Sr-87/Sr-86 ratios but higher epsilon Nd(t) values than the type-II lamprophyres. The difference of geochemical characteristics between the type-I and type-II lamprophyres may be caused by different degrees of partial melting of an amphibole-bearing garnet lherzolite mantle. The lithospheric mantle was metasomatised by the fluids caused by the rollback of the Paleo-Pacific plate subduction during the Late Cretaceous.
查看更多>>摘要：Application of Raman spectroscopy of carbonaceous material thermometry to samples of metasedimentary rock from the low-grade Sanbagawa belt in the central Kii peninsula reveals a progressive decrease in temperature from similar to 390 degrees C close to the northern boundary, a major continental shear zone-the median tectonic line (MTL)-to similar to 270 degrees C and similar to 7 km to the south and roughly constant temperature distribution thereafter. Within the Sanbagawa belt, the thermal structure is not significantly modified by slip-on fault boundaries between different geological units or folding. Meso- and microstructural observations combined with strain analysis using detrital grains in meta-mudstone indicate a similar deformation history throughout the area and no correlation between ductile strain and temperature gradients. These observations suggest the observed thermal structure was developed after the main stages of ductile deformation of the Sanbagawa belt were complete and are not due to localized preferential exhumation along with the MTL. The observations also require a heat source along with the MTL. Order of magnitude estimates suggest the influx of warm fluid along the MTL are viable causes of the observed thermal anomaly. Although shear heating would be another possible explanation, thermal calculations require anomalous fast slip rates along the MTL and much greater frictional strength than generally considered reasonable. For these reasons, fluid infiltration is our preferred model.
NSTL
Wiley