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Abstract Recent advanced chronological studies for the Tertiary volcanic rocks from the Northeast (NE) Japan arc revealed three volcanic fronts which differed in temporal and spatial distribution. These fronts were (i) the Matsumae-Shizukuishi-Shiogama line of 22–25 Ma which is obliquely across the Quaternary volcanic front (QVF); (ii) the Tomari-Shiogama line of 13–16 Ma which exists 30–50 km east of the QVF and (iii) a line of 0–8 Ma which is the same as the QVF. The first shifting of the 22–25 Ma line to the 13–16 Ma one was due to the counterclockwise rotation of the NE Japan arc during 20–12 Ma as proposed by Otofuji et al . (1985), and the second shifting of the 13–16 Ma line to the 0–8 Ma line could have contributed to a decrease in the dip of the slab of the Pacific plate which subducted beneath the NE Japan arc during 13–8 Ma. 相似文献
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Rasoul B Sorkhabi Arvind K Jain Susumu Nishimura Tetsumaru Itaya Nand Lal R M Manickavasagam Takahiro Tagami 《Journal of Earth System Science》1994,103(1):83-97
Thermotectonic history of the Trans-Himalayan Ladakh Batholith in the Kargil area, N. W. India, is inferred from new age data
obtained here in conjunction with previously published ages. Fission-track (FT) ages on apatite fall around 20±2 Ma recording
cooling through temperatures of ∼100°C and indicating an unroofing of 4 km of the Ladakh Range since the Early Miocene. Coexisting
apatite and zircon FT ages from two samples in Kargil show the rocks to have cooled at an average rate of 5–6°C/Ma in the
past 40 Ma. Zircon FT ages together with mica K−Ar cooling ages from the Ladakh Batholith cluster around 40–50 Ma, probably
indicating an Eocene phase of uplift and erosion that affected the bulk of the batholith after the continental collision of
India with the Ladakh arc at 55 Ma. Components of the granitoids in Upper Eocene-Lower Oligocene sediments of the Indus Molasse
in Ladakh supports this idea. Three hornblende K−Ar ages of 90 Ma, 55 Ma, and 35 Ma are also reported; these distinctly different
ages probably reflect cooling through 500–550°C of three phases of I-type plutonism in Ladakh also evidenced by other available
radiometric data: 102 Ma (mid-Cretaceous), 60 Ma (Palaeocene), and 40 Ma (Late Eocene); the last phase being localised sheet
injections. The geodynamic implications of the age data for the India-Asia collision are discussed. 相似文献
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In pelitic schists of the Sanbagawa metamorphic belt, sphene, rutile, and ilmenite occur as discrete grains, in composite aggregates, and as inclusions in garnets. Textural relationships, disposition of inclusions in garnet, and the compositions of ilmenites suggest that the titanium-bearing accessories stable at the peak metamorphic conditions were as follows: sphene in the chlorite zone, sphene plus rutile in the garnet zone, and ilmenite in the highest grade of the belt, the biotite zone. Rutile appears in the garnet zone as a consequence of Ca incorporation into garnet and a progressive increase in
.Retrograde reactions were responsible for the composite aggregates of rutile, sphene and ilmenite and these must be carefully evaluated before prograde relationships between titanium-bearing accessories can be properly understood. 相似文献
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Ar/Ar analyses of phengites and paragonites from the ultrahigh-pressure metamorphic rocks (zoisite–clinozoisite schist, garnet–phengite schist and piemontite schist) in the Lago di Cignana area, Western Alps were carried out with a laser probe step-heating method using single crystals and a spot dating method on thin sections. Eight phengite and two paragonite crystals give the plateau ages of 37–42 Ma with 96–100% of 39Ar released. Each rock type also contains mica crystals showing discordant age spectra with age fractions (20–35 Ma) significantly younger than the plateau ages. Phengite inclusions in garnet give ages of 43.2 ± 1.1 Ma and 44.4 ± 1.5 Ma, which are significantly older than the spot age (36.4 ± 1.4 Ma) from the matrix phengites, and the plateau ages from the step-heating analyses. Inclusion ages (43 and 44 Ma) are consistent with a zircon SHRIMP age (44 ± 1 Ma) in this area. These results suggest that the oceanic materials that underwent a simple subduction related UHPM, form excess 40Ar-free phengite and that the peak metamorphism is ca. 44 Ma or little older. We suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic 40Ar from the crystals. This has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age. 相似文献
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Abstract K-Ar ages have been obtained for mineral separates: plagioclases from two dolerites and biotites from one granite and four granodiorites in the Oban-Obudu massif of southeastern Nigeria. Dolerites in the Oban area give K-Ar plagioclase ages of 204.0 ± 9.9 Ma and 219.9 ± 4.7 Ma. The granite of the Obudu area yields a K-Ar biotite age of 507.6 ± 10.1 Ma whilst the granodiorites in the Oban area yield K-Ar biotite ages ranging from 474.6 ± 9.4 Ma to 511.8 ± 10.0 Ma. The dolerites are related to the tholeiitic basaltic magmatism at the early opening of the central Atlantic, and are compatible in age with the formation of the Ring Complexes and the rifting of the Benue Trough of Nigeria. The granites and granodiorites belong to the syntectonic Older Granite series in the Pan-African orogeny. The emplacement timing deduced from the reported Pb-Pb zircon age of 617 ± 2 Ma and the newly obtained biotite ages suggest that these older granites in southeastern Nigeria had a prolonged cooling history of ∼110 Ma. 相似文献
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Abstract Thick Middle (–Upper) Miocene turbiditic deposits filled very deep and narrow foredeep basins formed in the western margin of the Hidaka collision zone in central Hokkaido. Cobble- to boulder-sized clasts of eight monzogranites and a single granodiorite in the Kawabata Formation in the Yubari Mountains area yielded biotite K–Ar ages of 44.4 ± 1.0 to 45.4 ± 1.0 Ma and 42.8 ± 1.1 Ma, respectively. Major elemental compositions of the clasts all fall in the field of S-type granite on an NK/A (Na2 O + K2 O/Al2 O3 in molecule) versus A/CNK (Al2 O3 /CaO + Na2 O + K2 O in molecule) diagram, verifying their peraluminous granite character (aluminium saturation index (ASI): 1.12–1.19). These geochronological and petrographical features indicate that the granitoid clasts in the Kawabata Formation correlate with Eocene granitic plutons in the northeastern Hidaka Belt, specifically the Uttsudake (43 Ma) and Monbetsu (42 Ma) plutons. Foredeep basins are flexural depressions developed at the frontal side of thickened thrust wedges. The results presented here suggest that deposition of the Middle Miocene turbidites was coeval with rapid westward up-thrusting and exhumation of the Hidaka Belt. This early mountain building may have occurred in response to thrusting in the Tertiary fold-and-thrust system of central Hokkaido. 相似文献
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The Honvang serpentinite body in the Song Ma fault zone consists mainly of massive serpentinite, altered gabbro and rare chromitite. The serpentinite preserves relict chromian spinel with rare olivine inclusions. The compositional relationship between the Fo content of olivine (Fo90–92) and YCr [atomic ratio Cr / (Cr + Al) = 0.43–0.44] of chromian spinel suggests that the original peridotite was spinel-bearing lherzolitic harzburgite. Chromitite is typically a high-Al type, consisting of chromian spinel with YCr = 0.43–0.44. Saussuritized fine-grained gabbros display nearly flat rare earth element patterns, suggesting MORB-like affinity. Considering this petrotectonic information, we suggest that the serpentinite body of the Song Ma fault zone represents a remnant of paleo-oceanic lithosphere between the Indochina and South China blocks. The lherzolitic harzburgite may have formed in an environment with low degrees of melt depletion in a slow-spreading setting similar to some Tethyan paleo-oceanic lithospheres. 相似文献
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Satoshi Tonai Yusuke Suganuma Juichiro Ashi Tetsumaru Itaya Hisashi Oiwane Shoichi Kiyokawa 《Tectonophysics》2011,497(1-4):71-84
Over 300 samples for paleomagnetic analysis and K–Ar dating were collected from 27 sites at NW–SE and NE–SW trending dike swarms (herein, NW dikes and NE dikes, respectively) in the Koshikijima Islands, northern Ryukyu Arc. The NW dikes are Middle Miocene in age and have directions (D = ? 37.7°, I = 51.8°, α95 = 9.6°, and κ = 40.8) that are deflected westward relative to the stable eastern Asian continent. Conversely, the NE dikes, of Late Miocene age, have directions (D = 16.1°, I = 57.7°, α95 = 7.1°, and κ = 41.9) that show no such deflection. These differences are interpreted as indicating that the Koshikijima Islands underwent approximately 40° of counter-clockwise rotation during the Middle to Late Miocene. A synthesis of the paleomagnetic and structural data suggests a three-stage history of extensional deformation: (1) displacement upon normal faults (F1 faults) without vertical-axis block rotation, (2) strike-slip reactivation of F1 faults and oblique-normal displacement on NE–SW-trending faults (F2 faults) with vertical-axis block rotation, and (3) oblique-normal displacement on F2 faults without vertical-axis block rotation. Regional differences in the timing and amount of counter-clockwise vertical-axis block rotations indicate that the northern Ryukyu Arc rotated as several distinct rigid blocks. 相似文献