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Mitsuhiro Nagata Kazuhiro Miyazaki Hideki Iwano Tohru Danhara Hideyuki Obayashi Takafumi Hirata Koshi Yagi Yoshikazu Kouchi Koshi Yamamoto Shigeru Otoh 《Island Arc》2019,28(4)
We have estimated the timescale of material circulation in the Sanbagawa subduction zone based on U–Pb zircon and K–Ar phengite dating in the Ikeda district, central Shikoku. The Minawa and Koboke units are major constituents of the high‐P Sanbagawa metamorphic complex in Shikoku, southwest Japan. For the Minawa unit, ages of 92–81 Ma for the trench‐fill sediments, are indicated, whereas the age of ductile deformation and metamorphism of garnet and chlorite zones are 74–72 Ma and 65 Ma, respectively. Our results and occurrence of c. 150 Ma Besshi‐type deposits formed at mid‐ocean ridge suggest that the 60‐Myr‐old Izanagi Plate was subducted beneath the Eurasian Plate at c. 90 Ma, and this observation is consistent with recent plate reconstructions. For the Koboke unit, the depositional ages of the trench‐fill sediments and the dates for the termination of ductile deformation and metamorphism are estimated at c. 76–74 and 64–62 Ma, respectively. In the Ikeda district, the depositional ages generally become younger towards lower structural levels in the Sanbagawa metamorphic complex. Our results of U–Pb and K–Ar dating show that the circulation of material from the deposition of the Minawa and Koboke units at the trench through an active high‐P metamorphic domain to the final exhumation from the domain occurred continuously throughout c. 30 Myr (from c. 90 to 60 Ma). 相似文献
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Summary A statistical investigation of the world-wide electric current-systems corresponding to s.c's has been carried out. The storm-time variationDst and the disturbance diurnal variationDs of s.c's are derived separately from the results of the average local-time variations in the magnetic field at different latitudes.A conspicuousDs current-system is found in the inner polar regions, and a concentrated current-flow is also confirmed along the geomagnetic equator resembling the current-system of an enhancedSq-field. The inner polar region currents flow towards the meridian at about 9h local-time and are supposed to complete their path along the outer auroral zone in opposite directions in the forenoon and afternoon hemispheres. It is of interest that the pattern of this atmospheric electric current-system is very similar to that caused by an electric doublet centred on the highiy conducting region near the pole.Some observed facts concerning the statistical nature of s.c's can be explained by these current-systems. 相似文献
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Age gap between the intrusion of gneissose granitoids and regional high‐temperature metamorphism in the Ryoke belt (Mikawa area), central Japan 下载免费PDF全文
Kota Takatsuka Tetsuo Kawakami Etienne Skrzypek Shuhei Sakata Hideyuki Obayashi Takafumi Hirata 《Island Arc》2018,27(1)
The relationships between the intrusion of gneissose granitoids and the attainment of regional high‐T conditions recorded in metamorphic rocks from the Ryoke belt of the Mikawa area, central Japan, are explored. Seven gneissose granitoid samples (tonalite, granodiorite, granite) were collected from three distinct plutonic bodies that are mapped as the so‐called “Older Ryoke granitoids.” Based on bulk‐rock compositions and U–Pb zircon ages obtained by laser ablation inductively coupled plasma mass spectrometry, the analyzed granitoids can be separated into two groups. Gneissose granitoids from the northern part of the area give weighted mean 206Pb/238U ages of 99 ±1 Ma (two samples) and 95 ±1 Ma (one sample), whereas those from the southern part yield 81 ±1 Ma (two samples) and 78–77 ±1 Ma (two samples). Regional comparisons allow correlation of the northern granitoids (99–95 Ma) with the Kiyosaki granodiorite, and mostly with the Kamihara tonalite found to the east. The southern granitoids are tentatively renamed as “78–75 Ma (Hbl)?Bt granite” and “81–75 Ma Hbl?Bt tonalite” (Hbl, hornblende; Bt, biotite). and seem to be broadly coeval members of the same magmatic suite. With respect to available age data, no gneissose granitoid from the Mikawa area shows a U–Pb zircon age which matches that of high‐T metamorphism (ca 87 Ma). The southern gneissose granitoids (81–75 Ma), although they occur in the highest‐grade metamorphic zone, do not seem to represent the heat source which produced the metamorphic field gradient with a low dP/dT slope. 相似文献
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