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1.
Zircons separated from Cretaceous granitoids are dated from a south‐central transect of the Abukuma metamorphic and granitic terrane. The zircon ages do not follow ‘older’ and ‘younger’ granitoid ages that are used conventionally. In the western part of the study area (Zones I, II and III) where the Takanuki and Gosaisho metamorphic rocks are exposed, the Iritono quartz dioritic stock intruding the greenschist facies rocks in Zone III exhibits the oldest age of 121 Ma in the studied region. Quartz diorite located northward shows 112 Ma, but the other four granitoids intruding into the Takanuki and Gosaisho metamorphic rocks are younger and 103–99 Ma. Two‐mica and biotite granites belong to the youngest age group of 99 Ma. The granitic activities of both the Abukuma and Ryoke belts were initiated by intrusion of quartz dioritic magmas and were ended by two‐mica granite activity. The ages of the eastern two batholiths vary from 110 to 106 Ma (four samples), and show no age common to the Kitakami granitoids farther to the north. Throughout the Japanese Islands arc, Cretaceous granitic activities became younger toward the marginal sea side from the Kitakami Mountains, to the Abukuma Highland, and the Ryoke Belt, then to the Sanin belt of the Inner Zone of Southwest Japan.  相似文献   

2.
Plutonic rocks in the southern Abukuma Mountains include gabbro and diorite, fine‐grained diorite, hornblende–biotite granodiorite (Ishikawa, Samegawa, main part of Miyamoto and Tabito, Kamikimita and Irishiken Plutons), biotite granodiorite (the main part of Hanawa Pluton and the Torisone Pluton), medium‐ to coarse‐grained biotite granodiorite and leucogranite, based on the lithologies and geological relations. Zircon U–Pb ages of gabbroic rocks are 112.4 ±1.0 Ma (hornblende gabbro, Miyamoto Pluton), 109.0 ±1.1 Ma (hornblende gabbro, the Hanawa Pluton), 102.7 ±0.8 Ma (gabbronorite, Tabito Pluton) and 101.0 ±0.6 Ma (fine‐grained diorite). As for the hornblende–biotite granodiorite, zircon U–Pb ages are 104.2 ±0.7 Ma (Ishikawa Pluton), 112.6 ±1.0 Ma (Tabito Pluton), 105.2 ±0.8 Ma (Kamikimita Pluton) and 105.3±0.8 Ma (Irishiken Pluton). Also for the medium‐ to fine‐grained biotite granodiorite, zircon U–Pb ages are 106.5±0.9 Ma (Miyamoto Pluton), 105.1 ±1.0 Ma (Hanawa Pluton) and the medium‐ to coarse‐grained biotite granodiorite has zircon U–Pb age of 104.5 ±0.8 Ma. In the case of the leucogranite, U–Pb age of zircon is 100.6 ±0.9 Ma. These data indicate that the intrusion ages of gabbroic rocks and surrounding granitic rocks ranges from 113 to 101 Ma. Furthermore, K–Ar ages of biotite and or hornblende in the same rock samples were dated. Accordingly, it is clear that these rocks cooled down rapidly to 300 °C (Ar blocking temperature of biotite for K–Ar system) after their intrusion. These chronological data suggest that the Abukuma plutonic rocks in the southern Abukuma Mountains region uplifted rapidly around 107 to 100 Ma after their intrusion.  相似文献   

3.
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.  相似文献   

4.
The Ryoke Metamorphic complex has undergone low‐P/T metamorphism and was intruded by granitic magmas around 100 Ma. Subsequently, the belt was uplifted and exposed by the time deposition of the Izumi Group began. The tectonic history of uplift, such as the timing and processes, are poorly known despite being important for understanding the spatiotemporal evolution of the Ryoke Metamorphic Belt. U–Pb zircon ages from sedimentary rocks in the forearc and backarc basins are useful for constraining uplift and magmatism in the provenance. U–Pb dating of detrital zircons from 12 samples (four sandstones and eight granitic clasts) in the Yuasa–Aridagawa basin, a Cretaceous forearc basin in the Chichibu Belt of Southwest Japan, gave mostly ages of 60–110 Ma. Granitic clasts contained in conglomerate suggest that granitic intrusions predate the formation of Coniacian and Maastrichtian conglomerate. Emplacement ages of granitic bodies originated from granitic clasts in Coniacian conglomerate are (110.2 ±1.3) Ma, (106.1 ±1.8) Ma, (101.8+5.8–3.8) Ma, and (95.3 ±1.4) Ma; for granitic clasts in Maastrichtian conglomerate, (89.6 ±1.8) Ma, (87.3+2.4–1.8) Ma, (85.7 ±1.2) Ma, and (82.7 ±1.2) Ma. The results suggest that detrital zircons in the sandstones were mainly derived from volcanic eruptions contemporaneous with depositional age, and plutonic rocks of the Ryoke Metamorphic Belt. Zircon ages of the granitic clast samples also indicate that uplift in the provenance began after Albian and occurred at least during the Coniacian to Maastrichtian. Our results, together with the difference of provenance between backarc and forearc basins suggest that the southern marginal zone of the Ryoke Metamorphic Belt was uplifted and supplied a large amount of clastic materials to the forearc basins during the Late Cretaceous.  相似文献   

5.
A new U–Pb zircon geochronological study for the Hida metamorphic and plutonic rocks from the Tateyama area in the Hida Mountains of north central Japan is presented. The U–Pb ages of metamorphic zircon grains with inherited/detrital cores in paragneisses suggest that a metamorphic event took place at around 235–250 Ma; the cores yield ages around 275 Ma, 300 Ma, 330 Ma, 1 850 Ma, and 2 650 Ma. New age data, together with geochronological and geological context of the Hida Belt, indicate that a sedimentary protolith of the paragneisses is younger than 275 Ma and was crystallized at around 235–250 Ma. Detrital ages support a model that the Hida Belt was located in the eastern margin of the North China Craton, which provided zircon grains from Paleoproterozoic to Paleozoic rocks and also from Archean and rare Neoproterozoic rocks. Triassic regional metamorphism possibly reflects collision between the North and South China Cratons.  相似文献   

6.
Masahiro  Fujii  Yasutaka  Hayasaka  Kentaro  Terada 《Island Arc》2008,17(3):322-341
Abstract The Maizuru terrane, distributed in the Inner Zone of southwest Japan, is divided into three subzones (Northern, Central and Southern), each with distinct lithological associations. In clear contrast with the Southern zone consisting of the Yakuno ophiolite, the Northern zone is subdivided into the western and eastern bodies by a high-angle fault, recognized mainly by the presence of deformed granitic rocks and pelitic gneiss. This association suggests an affinity with a mature continental block; this is supported by the mode of occurrence, and petrological and isotopic data. Newly obtained sensitive high mass-resolution ion microprobe (SHRIMP) zircon U–Pb ages reveal the intrusion ages of 424 ± 16 and 405 ± 18 Ma (Siluro–Devonian) for the granites from the western body, and 249 ± 10 and 243 ± 19 Ma (Permo–Triassic) for the granodiorites from the eastern body. The granites in the western body also show inherited zircon ages of around 580 and 765 Ma. In addition, electron probe microanalysis (EPMA) monazite U–Th–total Pb dating gives around 475–460 Ma. The age of intrusion, inherited ages, mode of occurrence, and geological setting of the Siluro–Devonian granites of the Northern zone all show similarities with those of the Khanka Massif, southern Primoye, Russia, and the Hikami granitic rocks of the South Kitakami terrane, Northeast Japan. We propose that both the Siluro–Devonian and Permo–Triassic granitic rocks of the Northern zone are likely to have been juxtaposed through the Triassic–Late Jurassic dextral strike-slip movement, and to have originated from the Khanka Massif and the Hida terrane, respectively. This study strongly supports the importance of the strike-slip movement as a mechanism causing the structural rearrangement of the Paleozoic–Mesozoic terranes in the Japanese Islands, as well as in East Asia.  相似文献   

7.
The Permian–Triassic high pressure metamorphism and potassic magmatism in central Korea attest to the extension of the Dabie‐Sulu collision belt in central‐eastern China towards the Korean Peninsula and possibly the Japanese Islands. We present major and trace element and Sr–Nd isotope data for a ca. 230 Ma monzodiorite pluton emplaced in the Goesan area, central Okcheon belt, Korea. This pluton shows geochemical features comparable with those of the coeval monzonite–syenite–gabbro–mangerite suite documented recently in the Gyeonggi massif. The metaluminous and alkali–calcic signatures of the Goesan intrusives correspond to the Caledonian‐type post‐orogenic granitoids. The K2O/Na2O ratios of all analyzed samples are greater than 1, and are not correlative with their SiO2 contents. The enrichment of both large‐ion‐lithophile elements and highly compatible elements in the Goesan pluton is probably indicative of metasomatized mantle origin. The elemental fractionation in the source region must have occurred in the distant past, possibly the Paleoproterozoic, to generate significantly negative εNd(t) values (< –16). Chondrite‐normalized rare earth element patterns as well as Rb/Sr and Ba/Rb ranges suggest that the source consists of amphibole‐bearing rocks. Progressive decreases in negative Eu anomaly and Ba, Sr, Ni, Cr and V contents with increasing SiO2 contents reflect an important role of plagioclase, biotite and hornblende for the fractionation process. Zr is undersaturated in the potassic, metaluminous melt. The initial Sr–Nd isotopic compositions of the samples are correlated with their SiO2 contents, substantiating a role of crustal assimilation during the magmatic differentiation. The Sr–Nd elemental and isotopic modeling suggests that the Goesan pluton was initially slightly heterogeneous in its isotopic composition, and underwent concurrent assimilation and fractional crystallization. The occurrence of the Goesan pluton provides further evidence corroborating the amalgamation of allochthonous terranes within the Okcheon belt during the Permian–Triassic collisional orogeny.  相似文献   

8.
The tectonic setting of the late mesozoic of South China is in a debate between two schools of thought: an intra‐continental rift zone along a passive continental margin or active rifting associated with subduction of the paleo‐Pacific Plate. In this study, we present new sensitive high‐resolution ion microprobe (SHRIMP) U‐Pb zircon ages, along with geochemical data of three basic dikes that cross‐cut the Dexing porphyry copper deposit. The deposit is the largest of its kind in eastern China and part of large scale mineralization associated with Mesozoic magmatic activity in the area. Our results indicate that the dikes were emplaced in the Late Jurassic with an average U‐Pb age of 153.5 ± 2.4 Ma. The intrusions have bulk εNd(t) of ca +0.7 and zircon εHf(t) value of +1.54 to +6.92. Based on relatively enriched light rare earth elements (LREE) and depleted high‐field‐strength elements (HFSE) abundances with pronounced negative Ta–Nb, Hf–Zr and Ti anomalies in multi‐element diagrams, we propose that these dikes were derived from a subduction‐modified lithospheric mantle source. The variability in Hf isotopes identifies some degree of crustal contaminations. Our data support a scenario with a back‐arc extensional setting or an intra‐arc rift environment associated with the westward subduction of the paleo‐Pacific Plate at or prior to the late Jurassic as the most likely cause for these subduction signatures.  相似文献   

9.
We report Permian (ca. 272 Ma ±5.4 Ma) felsic dykes that intrude into the Neoproterozoic (ca. 750 Ma) magmatic suite of the Nagar Parkar Igneous Complex (NPIC), the western extension of the Malani Igneous Suite (MIS). The NPIC consists of Neoproterozoic basement amphibolites and granites (riebeckite–aegirine gray granites and the biotite–hornblende pink granites), all of which are intruded by several generations of mafic and felsic dykes. Granitic magmatism occurred in the Late Neoproterozoic (ca. 750 Ma) due to the subduction‐, followed by the rift‐related tectonic regime during the breakup of the Rodinia supercontinent. U–Th–Pb zircon and monazite CHIME age data of 700–800 Ma from the earlier generation porphyritic felsic dykes suggest the dyke intrusion was coeval or soon after the emplacement of the host granites. Our findings of Permian age orthophyric felsic dykes provide new insights for the prevalence of active tectonics in the MIS during late Paleozoic. Textural features and geochemistry also make the orthophyric dykes distinct from the early‐formed porphyritic dykes and the host granites. Our newly obtained age data combined with geochemistry, suggest the existence of magmatism along the western margin of India (peri‐Gondwana margin) during Permian. Like elsewhere in the region, the Permian magmatism in the NPIC could be associated with the rifting of the Cimmerian micro‐continents from the Gondwana.  相似文献   

10.
Granitoids in the Hida region of Japan encompass two main rock types: younger type‐1 granites and older type‐2 granites. Sensitive high mass‐resolution ion microprobe (SHRIMP) U–Pb zircon dating of older type‐2 granites collected from the Tateyama area show similar ages of 245 ± 2 Ma and 248 ± 5 Ma for two gneissose granites, while a significantly younger intrusion age of 197 ± 3 Ma was determined for the younger type‐1 granites collected from the Hayatsukigawa River which belongs to the Okumayama pluton. A felsic gneiss sample (07HI‐3) collected from the right bank of the Hayatsukigawa River yielded multiple complex ages at 330 ± 6 Ma, indicating the timing of the Hida regional tectono‐thermal events that formed the Hida gneisses; 243 ± 8 Ma, representing the timing of intrusion of the augen granite; and 220 Ma, indicating the timing of regional dextral ductile shearing that caused a repeated recrystallization of metamorphic rocks in the study area. Considering the geochronological data, the rock types and assemblages, basement, and Sr–Nd isotopic constraints, we propose that the Hida Belt separated from the Jiamushi massif, which is located in the eastern margin of the Central Asian Orogenic Belt.  相似文献   

11.
Geochronological and geochemical studies reveal the possible origin of the restricted body of mylonite rocks occurring at the eastern edge of Kyushu Island, Japan, just in contact with the Sashu Fault, a part of the Paleo‐Median Tectonic Line (Paleo‐MTL). The LA‐ICP‐MS zircon U–Pb dating of the quartz diorite mylonite in this mylonitic body indicates a crystallization age of 114.0 ±1.7 Ma. Moreover, the two tonalite samples appear as thin layers within the Permian fine‐grained mafic mylonite; a part of the same body yields the age of 113.7 ±2.3 Ma and 116.9 ±1.3 Ma, with extremely low Th/U ratio. These quartz diorite mylonite and tonalite are consistent with the late Early Cretaceous magmatism and coeval metamorphism similar to those in the Higo Plutono‐metamorphic Complex in western Kyushu, Japan. This newly characterized complex occurs just south of the Cretaceous Sambagawa metamorphic rocks. The newly characterized mylonitic rocks are lying structurally above the Sambagawa Metamorphic Complex and are distributed along the Paleo‐MTL. The extension of the Higo Plutonometamorphic Complex, as well as the structural relationship between this complex and the Sambagawa Metamorphic Complex, is still controversial but holds a key to reconstruct the tectonic evolution of Southwest Japan during the Late Mesozoic to Early Cenozoic period. Hence, this article provides new insight into the reconstruction of the evolution history of East Asia as an active convergent margin.  相似文献   

12.
The Chilas Complex is a major lower crustal component of the Cretaceous Kohistan island arc and one of the largest exposed slices of arc magma chamber in the world. Covering more than 8000 km2, it reaches a current tectonic width of around 40 km. It was emplaced at 85 Ma during rifting of the arc soon after the collision of the arc with the Karakoram plate. Over 85% of the Complex comprises homogeneous, olivine‐free gabbronorite and subordinate orthopyroxene–quartz diorite association (MGNA), which contains bodies of up to 30 km2 of ultramafic–mafic–anorthositic association (UMAA) rocks. Primary cumulate textures, igneous layering, and sedimentary structures are well preserved in layered parts of the UMAA in spite of pervasive granulite facies metamorphism. Mineral analyses show that the UMAA is characterized by more magnesian and more aluminous pyroxene and more calcic plagioclase than those in the MGNA. High modal abundances of orthopyroxene, magnetite and ilmenite (in MGNA), general Mg–Fe–Al spatial variations, and an MFA plot of whole‐rock analyses suggest a calc‐alkaline origin for the Complex. Projection of the pyroxene compositions on the Wo–En–Fs face is akin to those of pyroxenes from island arcs gabbros. The presence of highly calcic plagioclase and hornblende in UMAA is indicative of hydrous parental arc magma. The complex may be a product of two‐stage partial melting of a rising mantle diaper. The MGNA rocks represent the earlier phase melting, whereas the UMAA magma resulted from the melting of the same source depleted by the extraction of the earlier melt phase. Some of the massive peridotites in the UMAA may either be cumulates or represent metasomatized and remobilized upper mantle. The Chilas Complex shows similarities with many other (supra)subduction‐related mafic–ultramafic complexes worldwide.  相似文献   

13.
Basement rocks that occur along the northern margin of the South Kitakami Terrane in Japan consist of Ordovician ultramafic rocks (Hayachine ultramafic complex), gneissose amphibolite (Kuromoriyama amphibolite), and mafic rocks (Kagura igneous rocks, KIR). The KIR are composed of metagabbro, metadolerite, metabasalt, and minor felsic–intermediate dikes. Although the KIR contain green hornblende due to metamorphism of greenschist to epidote–amphibolite facies, they rarely retain primary brown hornblende. Approximately 30% of the metabasalt shows porphyritic textures, with phenocrysts of saussuritized plagioclase and/or altered mafic minerals. The geochemistry of the common metadolerite and metabasalt of the KIR shows a tholeiite trend, a low TiO2 content, and high Th/Nb and Ti/V ratios. The KIR are therefore indicative of a supra‐subduction zone tectonic setting, which implies a backarc origin (as also indicated by discrimination diagrams). Trace element patterns of the KIR resemble those of the backarc‐basin basalt of the Japan and Yamato basins in the Japan Sea. We propose that the KIR formed during backarc spreading from the Ordovician to Early Silurian. This view is supported by the geochemical data, the tectonic setting of the Hayachine ultramafic rocks, and the provenance of clastics within Silurian sedimentary rocks.  相似文献   

14.
Mesozoic volcanic rocks are widespread throughout the Great Xing'an Range of northeastern China. However, there has been limited investigation into the age and petrogenesis of the Mesozoic volcanics in the eastern Great Xing'an Range. According to our research, the volcanic rocks of the Dayangshu Basin, eastern Great Xing'an Range are composed mainly of trachybasalt, basaltic andesite, and basaltic trachyandesite, with minor intermediate–basic pyroclastic rocks. In this study, the geochemistry and geochronology of the Mesozoic volcanic rocks are presented in order to discuss the petrogenesis and tectonic setting of the Ganhe Formation in the Dayangshu Basin. Zircon U–Pb dating by laser ablation inductively coupled plasma–mass spectrometry indicates that the Mesozoic lavas formed during the late Early Cretaceous (114.3–108.8 Ma). This suite of rocks exhibits a range of geochemical signatures indicating subduction‐related genesis, including: (i) calc‐alkaline to high‐K calc‐alkaline major element compositions; (ii) enrichment of large ion lithophile elements (e.g. Rb, Ba, K) and light rare earth elements (LREEs/HREEs =7.33–9.85); and (iii) weak depletion in high field strength elements (e.g. Nb, Ta, Ti). Furthermore, Sr–Nd–Pb isotopic data yield initial 87Sr/86Sr values of 0.70450–0.70463, positive εNd(t) values of +1.8 to +3.3, and a mantle‐derived lead isotope composition. Combined with the regional tectonic evolution, the results of this study suggest that the Ganhe Group lavas are derived from decompression melting of a metasomatized (enriched) lithospheric mantle, related to asthenospheric upwelling, which resulted from lithospheric mantle delamination and produced extension of the continental margin following the subduction of the Paleo‐Pacific Plate.  相似文献   

15.
The Hakusan volcano, central Japan, is located in a region where two subducting plates (the Pacific Plate and the Philippine Sea Plate) overlap near the junction of four plates adjacent to the Japanese Islands (the Pacific Plate, the Philippine Sea Plate, the Eurasia Plate, and the North American Plate). The Hakusan volcano consists of products from four major volcanic episodes: Kagamuro, Ko‐hakusan, and Shin‐Hakusan I and II. To date the eruption events of the Hakusan volcano we applied thermoluminescence and fission track methods. 238U(234U)–230Th disequilibrium and 206Pb/238U methods were applied to date the zircon crystallization ages for estimating the magma residence time before the eruptions. The eruption ages we obtained are ca 250 ka for Kagamuro, ca 100 ka and ca 60 ka for Ko‐Hakusan, ca 50 ka for Shin‐Hakusan I, and <10 ka for Shin‐Hakusan II. They are concordant with previous reports based on K–Ar dating. Some of the pyroclastic rocks, possibly originating from Shin‐Hakusan II activities, are dated to be ca 36 ka or 50 ka, and belong to the Shin‐Hakusan I activity. The zircon crystallization ages show several clusters prior to eruption. The magma residence time was estimated for each volcanic activity by comparing the major crystallization events and eruption ages, and we found a gradual decrease from ca. 500 ky for the Kagamuro activity to ca. 5 ky for the Shin‐Hakusan II activity. This decrease in residence time may be responsible for the decrease in volume of erupted material estimated from the current topography of the region. The scale of volcanic activity, which was deduced from the number of crystallized zircons, is more or less constant throughout the Hakusan volcanic activity. Therefore, the decrease in magma residence time is most likely the result of stress field change.  相似文献   

16.
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   

17.
In order to provide references of the subduction process of the Paleo‐Pacific Plate beneath the Jiamusi Block, this paper studied the clastic rocks of the Nanshuangyashan Formation using modal analysis of sandstones, mudstone elements geochemistry, and detrital zircon U–Pb dating. These results suggest the maximum depositional age of the Nanshuangyashan Formation was between the Norian and Rhaetian (206.8 ±4.6 Ma, mean standard weighted deviation (MSWD) = 0.17). Whole‐rock geochemistry of mudstone indicates that source rocks of the Nanshuangyashan Formation were primarily felsic igneous rocks and quartzose sedimentary rocks, which were mainly derived from the stable continental block and a magmatic arc. Detrital zircon analysis showed the Nanshuangyashan Formation samples recorded four main age groups: 229–204 Ma, 284–254 Ma, 524–489 Ma and 930–885 Ma, and the provenances were attributed to the Jiamusi Block and a Late Triassic magmatic arc near the study area. Furthermore, the eastern Jiamusi Block was a backarc basin, affected by the subduction of the Paleo‐Pacific Plate in the Late Triassic, but the magmatic arc related to the subduction near the study area finally died out due to tectonic changes and stratigraphic erosion.  相似文献   

18.
The Japanese archipelago underwent two arc–arc collisions during the Neogene. Southwest Honshu arc collided with the Izu‐Bonin‐Mariana arc and the northeast Honshu arc collided with the Chishima arc. The complicated geological structure of the South Fossa Magna region has been attributed to the collision between the Izu‐Bonin‐Mariana arc and the southwest Honshu arc. Understanding the geotectonic evolution of this tectonically active region is crucial for delineating the Neogene tectonics of the Japanese archipelago. Many intrusive granitoids occur around the Kofu basin, in the South Fossa Magna region. Although the igneous ages of these granitoids have been mainly estimated through biotite and hornblende K–Ar dating, here, we perform U–Pb dating of zircon to determine the igneous ages more precisely. In most cases, the secondary post‐magmatic overprint on the zircon U–Pb system was minor. Based on our results, we identify four groups of U–Pb ages: ca 15.5 Ma, ca 13 Ma, ca 10.5 Ma, and ca 4 Ma. The Tsuburai pluton belongs to the first group, and its age suggests that the granite formation within the Izu‐Bonin‐Mariana arc dates back to at least 15.5 Ma. The granitoids of the second group intruded into the boundary between the Honshu arc and the ancient Izu‐Bonin‐Mariana arc, suggesting that the arc–arc collision started by ca 13 Ma. As in the case of the Kaikomagatake pluton, the Chino pluton likely corresponds to a granodiorite formed in a rear‐arc setting in parallel with the other granodiorites of the third group. The U–Pb age of the Kogarasu pluton, which belongs to the fourth group, is the same as those of the Tanzawa tonalitic plutons. This might support a syncollisional rapid granitic magma formation in the South Fossa Magna region.  相似文献   

19.
Precambrian basement rocks have been affected by Caledonian thermal metamorphism. Caledonian‐aged zircon grains from Precambrian basement rocks may have resulted from thermal metamorphism. However, Hercynian ages are rarely recorded. Zircon U–Pb Sensitive High Resolution Ion Microprobe (SHRIMP) dating reveals that zircon ages from the Huyan, Lingdou, and Pengkou granitic plutons can be divided into two groups: one group with ages of 398.9 ±5.3 Ma, 399 ±5 Ma, and 410.2 ±5.4 Ma; and a second group with ages of 354 ±11 Ma, 364.6 ±6.7 Ma, and 368 ±14 Ma. The group of zircon U–Pb ages dated at 410–400 Ma represent Caledonian magmatism, whereas the 368–354 Ma ages represent the age of deformation, which produced gneissosity. The three plutons share geochemical characteristics with S‐type granites and belong to the high‐K calc‐alkaline series of peraluminous rocks. They have (87Sr/86Sr)i ratios of 0.710 45–0.724 68 and εNd(t) values of ?7.33 to ?10.74, with two‐stage Nd model ages (TDM2) ranging from 1.84 Ga to 2.10 Ga. Magmatic zircon εHf(t) values range from ?3.79 to ?8.44, and have TDMC ages of 1.65–1.93 Ga. The data suggest that these granites formed by partial melting of Paleoproterozoic to Mesoproterozoic continental crust. A collision occurred between the Wuyi and Minyue microcontinents within the Cathaysia Block and formed S‐type granite in the southwest Fujian province. The ca 360 Ma zircon U–Pb ages can represent a newly recognized period of deformation which coincided with the formation of the unified Cathaysia Block.  相似文献   

20.
Abstract Zircon U–Pb sensitive high mass-resolution ion microprobe dating was carried out on three types of granitic rock (gneissose biotite granodiorite, biotite granite and two-mica granite) from the Cretaceous Ryoke belt of the Kinki district, Southwest Japan. The results give the ages of granitic magmatism in the Shigi-san area of between 87 and 78 Ma and suggest extensive melting of the Cretaceous Ryoke granitic crust to form the two-mica granite, probably at ca 80 Ma. Discrimination into older and younger granites based on development of gneissosity does not appear to represent the sequence of magma generation, although there is some scope in the interpretation of the zircon U–Pb data that would allow all three granites to form at 83 Ma. Compilation of chemical Th-U-total Pb isochron dating method ages, whole rock Rb–Sr isotope ages and U–Pb isotope ages indicates that most Ryoke plutonism occurred from ca 70 Ma to ca 100 Ma. Younger (85 Ma–70 Ma) plutonism with the formation of two-mica granite occurred only in the eastern sector of the Ryoke belt, including the Kinki District.  相似文献   

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