Petrogenesis of high-Mg volcanics in the Mikabu Belt

Japanese Magazine of Mineralogical and …, 2005

Abstract;Recent developments in instrumentation for in situ trace element analysis of peridotite minerals give us valuable data set of petrological and geochemical insights of the upper mantle. Here we summarize petrographical, petrological and geochemical characteristics ...

The Journal of the Geological Society of Japan

The Journal of the Geological Society of Japan

The Journal of the Geological Society of Japan

A thick deposit of the muddy Shimajiri Group (Upper Miocene-Early Pleistocene) is overlain by the calcareous sandy Chinen Formation (Pleistocene) in southern Okinawa-Jima, Southwest Japan. The abrupt facies change has been explained by a significant unconformity resulted from subaerial erosion (the Shimajiri disturbance) or the consequence of rapid and regional uplift leading to an abrupt environmental change (the Chinen disturbance). We identified sandy mass transfer deposits (MTD) directly overlying the Shinzato Formation the uppermost part of the Shimajiri Group in the large Urizun outcrop. The Shinzato-Chinen transition zone yields numerous dolomite concretions. The dolomite is strongly depleted in C, with δ δ C values of. to. , suggesting that the concretions were derived from anaerobic oxidation of methane (AOM). Zonation in δ δ O values across the concretions, from. in the centers to. at the rims, suggest that the precipitation temperature increased from. . to. . °°C , corresponding to a shallowing of the water depth from to m. The estimate is consistent with the analysis of benthic foraminiferal assemblage. Given the wide distribution of the Bottom Simulating Reflector (BSR) offshore of Okinawa-Jima, methane hydrates are assumed to have persisted in the Shimajiri Group throughout the Pliocene-Pleistocene. Abrupt uplift and relative sea-level fall during the Chinen disturbance at Ma must have triggered abrupt changes in facies and a massive methane hydrate dissociation. This disturbance resulted in a high methane flux, increased AOM, and the formation of the dolomite concretions in the Shinzato-Chinen transition zone. Methane-derived dolomite concretions are thus a geochemical record of an abrupt tectonic disturbance and hydrate dissociation offshore of Okinawa-Jima.

Journal of Geography (Chigaku Zasshi), 2001

The Journal of the Geological Society of Japan, 2010

We report a new discovery of Lower Triassic Spathian chert and siliceous claystone at two localities within the Mino terrane in the Mt. Funabuseyama area, central Japan. The two localities were thought to be underlain by the Hashikadani Formation, which is characterized by upper Lower to uppermost Permian siliceous rocks, representing deepwater facies that formed on the lower slope of an oceanic seamount and the surrounding ocean floor in the Panthalassa Ocean. The Lower Triassic siliceous rocks occur close to upper Middle to upper Upper Permian chert and siliceous claystone. The neighboring Permian and Triassic siliceous rocks show identical facing directions. The close proximity of the exposures, lithologic affinities, and identical facing indicate that the Lower Triassic siliceous rocks rest upon the Upper Permian Hashikadani siliceous rocks and were primarily accumulated as part of a Permo-Triassic siliceous rock succession. The inferred Permo-Triassic siliceous rock succession comprises upper Middle to upper Upper Permian grayish chert with minor black chert in the upper part ca. m thick; Follicucullus scholasticus-F. ventricosus Zone to Neoalbaillella ornithoformis Zone ; uppermost Permian gray chert, including alternating black chert, greenish gray siliceous claystone, and black claystone ca.-m thick; Neoalbaillella optima Zone ; and upper Lower Triassic rocks characterized by an upward increase in the proportion of grayish chert along with gray siliceous claystone and black claystone thickness unknown; Spathian. These new findings indicate that the stratigraphy and age of the Hashikadani Formation should be revised. Our data suggest that the accumulation of Permian siliceous sediments around a Panthalassic mid-oceanic seamount persisted until at least the late Early Triassic, across the Permo-Triassic boundary. The revised Hashikadani Formation has potential as an archive of long-term environmental changes in a mid-oceanic realm of the Panthalassa Ocean.

Annual Meeting of the Geological Society of Japan The 122nd Annual Meeting(2015' Nagano), 2015

The Journal of the Geological Society of Japan, 2016

The Isomatsu Formation in northeastern Japan comprises Miocene terrestrial to shallow-marine deposits displaying a typical lithofacies of so-called green tuff. Zircon fission track ages of the formation (~-Ma) are significantly younger than those expected from stratigraphic inferences. Here, we present the results of U-Pb geochronological analysis using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), performed on the same suite of zircon grains that yielded the young fission track age. The LA-ICP-MS analysis yielded a mean U-Pb age of ~. Ma for a dacitic pyroclastic deposit in a relatively low stratigraphic horizon of the Isomatsu Formation, suggesting deposition at or later than this time. This result is used to revise the chronostratigraphy of the lower Miocene sequence in the study area, and it constrains the depositional age of the Isomatsu Formation to the period from ~. Ma to ~ Ma. These ages further constrain the timing of: () felsic magmatic activity in and adjacent to the study area, () the marine transgression that caused the deposition of the Isomatsu Formation, and () the possible formation of a half-graben that may also have influenced deposition.

Chishitsu chosa kenkyu hokoku, 2010

We obtained a continuous 45 m-long core from the Miocene sedimentary sequence and basement Cretaceous granite at Kanamaru, northeast Japan. The Miocene sequence intercalates with a uranium-rich seam (U = 25-100 ppm; Th = 23-42 ppm). We analyzed the kerogen and biomarkers in the core to characterize the organic matter. Visual kerogen analysis indicated that coaly and woody kerogen is abundant in relatively organic-rich samples, while amorphous kerogen is abundant in organic-lean samples. Pyrolysis gas chromatography-mass spectrometry demonstrated that the organic matter in pyrolysates comprises mainly alkyl-aromatic hydrocarbons (alkyl-benzenes, alkyl-indenes, alkyl-naphthalenes, etc.) and n-alkanes. Acyclic isoprenoid alkanes (mainly pristane), n-alkenes, and n-fatty acids were detected as minor components. Most of these compounds are characteristic of the type-III kerogen that derived from terrestrial higher plants. These results suggest that amorphous kerogen originated from terrestrial higher plants, and we speculate that they were heavily degraded by oxidation and lost their woody texture. Thermal alteration index (TAI) of Pinus pollen was about 2.6, and Tmax values of Rock-Eval pyrolysis range from 441°C to 444°C, which indicate that the thermal maturation of the coaly shale reached the stage of early catagenesis. This maturity was also suggested by high abundance of diagenetically generated isomers of hopanes in pyrolysates. In the sediment sequence, a peak of uranium and uranium/thorium ratio is ca. 2 m shallower than the peak of organic carbon content, but it corresponds to a lithological boundary. This inconsistency suggests that organic matter was not involved in concentrating uranium. Uranium was possibly concentrated from groundwater at the lithological boundaries between different redox levels during the period of high groundwater level.

Shigen-Chishitsu, 1994