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1.
A new U–Pb dating and oxygen isotope analysis of zircons collected from a granitic mylonite and an undeformed granite in the Kamioka area, in the Hida Belt of southwest Japan, was conducted using a sensitive high‐resolution ion microprobe (SHRIMP) to restrict the timing of the mylonitization in the Funatsu Shear Zone, which is situated on the eastern and southeastern margins of the Hida Belt. Here, undeformed granite intrudes into the granitic mylonite deformed by mylonitization in the Funatsu Shear Zone. The granitic mylonite and the undeformed granite yielded U–Pb zircon ages of 242.6 ±1.9 Ma and 199.1 ±1.9 Ma, respectively. The granitic mylonite and the undeformed granite also yielded zircon oxygen isotope ratios (δ18OVSMOW) of 7.74 ±0.37 ‰ and 5.74 ±0.17 ‰, which suggests that these rocks are derived from different magmas. Therefore, the timing of the mylonitization in the Funatsu Shear Zone is constrained to be at least 242.6–199.1 Ma, which is consistent with other data from the Tateyama area. The U–Pb zircon ages of the banded gneiss in the Kamioka area also reveals that the protolith is a sedimentary rock deposited at approximately 256 Ma, and regional metamorphism occurred at 245.0 ±6.6 Ma, which indicates that the mylonitization in the Funatsu Shear Zone occurred after the metamorphism in the Hida Belt. These geochronological and geochemical data give new insight into the relationship between the Hida Belt and the eastern margin of the Asian continent: the geochronological and geochemical data in this study support the possibility that the Funatsu Shear Zone is comparable with the Cheongsan Shear Zone located at the center of the Ogcheon Belt on the Korean Peninsula.  相似文献   

2.
U–Pb ages of detrital zircons and white mica K–Ar ages are obtained from two psammitic schists from the western and eastern units of the Sanbagawa Metamorphic Belt located in the Sakuma–Tenryu area. The detrital zircons in the sample from the western unit (T1) show an age cluster around 95 Ma, and the youngest age in the detrital zircons is 94.0 ± 0.6 Ma. The detrital zircons in the sample from the eastern unit (T5) show a main age cluster in the Late Cretaceous with some older ages, and the youngest age in the detrital zircons is 72.8 ± 0.9 Ma. The youngest zircon ages restrict the older limit of the depositional ages of each sample. White mica K–Ar ages of T1 and T5 are 69.8 ± 1.5 Ma and 56.1 ± 1.2 Ma, respectively, which indicate the age of exhumation and restrict the younger limit on the depositional age of each sample. The results show that the western and eastern units were different in their depositional and exhumation ages, suggesting the episodic subduction and exhumation of the Sanbagawa Belt in the Sakuma–Tenryu area. These results also suggest simultaneous existence of subduction and exhumation paths of metamorphic rocks in the high‐P/T Sanbagawa Metamorphic Belt.  相似文献   

3.
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   

4.
The new result of SHRIMP U–Pb zircon dating of the Kinshozan Quartz Diorite from the Kanto Mountains, Japan, provides 281.5 ± 1.8 Ma. The age is 30 m.y. older than the available age of the Kinshozan Quartz Diorite obtained by hornblende K–Ar method. The new U–Pb zircon age represents the time of crystallization of the Kinshozan Quartz Diorite. The hornblende K–Ar age indicates the time that the Kinshozan Quartz Diorite cooled down to 500 °C which is the closure temperature of the systematics. Permian granites are found in small exposures in Japan, and frequently referred to as 250 Ma granites. The Kinshozan Quartz Diorite is considered as a type of the 250 Ma granites, and the age was influential in establishing a model of Paleozoic tectonic evolution for the Japanese Islands. The new age of the Kinshozan Quartz Diorite provides the opportunity to re‐examine the model. The Kinshozan Quartz Diorite and other Permian granites in the south of the Median Tectonic Line of Japan were constituents of the Paleo‐Ryoke Belt. The geochemical characteristics of the granitic rocks in the Paleo‐Ryoke Belt indicated that the granitic rocks were formed in a primitive island arc environment, and the new trace element data also support this interpretation. Examination of the available data and results of the present study suggests the late Paleozoic granitic activity in Japan as follows. At about 310–290 Ma, arc magmatism generated adakitic granites and other granites in the South Kitakami Belt. Quartz diorite and tonalites of primitive characteristic, such as the Kinshozan Quartz Diorite and granites in the Maizuru Belt appear to have been formed at the immature island arc, and accreted to the Japanese Islands at the end of Paleozoic or early Mesozoic era. During 260–240 Ma, granitic activity took place in the Hida and Maizuru Belts as a part of the Asian continent.  相似文献   

5.
Abstract The tectonic history of the Okcheon Metamorphic Belt (OMB) is a key to understanding the tectonic relationship between South Korea, China and Japan. The petrochemistry of 150 psammitic rocks in the OMB indicates that the depositional environment progressively deepened towards the northwest. These data, combined with the distribution pattern of oxide minerals and the abundance of carbonaceous material, support a half‐graben basin model for the OMB. Biotite and muscovite K–Ar dates from metasediments in the central OMB range from 102 to 277 Ma. K–Ar ages of 142–194 Ma are widespread throughout the area, whereas the older ages of 216–277 Ma are restricted to the metasediments of the middle part of the central OMB. The younger (Cretaceous) ages are only found in metasediments that are situated near the Cretaceous granite intrusions. The 216–277 Ma dates from weakly deformed areas represent cooling ages of M1 intermediate pressure/temperature (P/T) metamorphism. The relationship between age distribution and deformation pattern indicates that the Jurassic muscovite and biotite dates can be interpreted as complete resetting ages, caused by thermal and deformational activities associated with Jurassic granite plutonism. Well‐defined 40Ar/39Ar plateau ages of 155–169 Ma for micas from both metasediments and granitic rocks can be correlated with the main Jurassic K–Ar mica ages (149–194 Ma). U–Pb zircon dates for biotite granite from the southwest OMB are 167–169 Ma. On the basis of the predominantly Jurassic igneous and metamorphic ages and the uniformity of d002 values for carbonaceous materials in the study area, it is suggested that the OMB has undergone amphibolite facies M2 metamorphism after M1 metamorphism. This low P/T M2 regional thermal metamorphism may have been caused by the regional intrusion of Jurassic granites. The OMB may have undergone tectono‐metamorphic evolution as follows: (i) the OMB was initiated as an intraplate rift in the Neoproterozoic during break‐up of Rodinia, and may represent the extension of Huanan aulacogen within the South China block; (ii) sedimentation continued from the Neoproterozoic to the Ordovician, perhaps with several unconformities; (iii) M1 intermediate P/T metamorphism occurred during the Late Paleozoic due to compression caused by collision between the North and South China blocks in an area peripheral to the collision zone; and (iv) during the Early to Middle Jurassic, north‐westward subduction of the Farallon‐Izanagi Plate under the Asian Plate resulted in widespread intrusion of granites, which triggered M2 low P/T regional thermal metamorphism in the OMB. This event also formed the dextral Honam shear zone at the boundary between the OMB and Precambrian Yeongnam massif.  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.
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   

8.
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   

9.
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   

10.
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   

11.
Abstract Granitoids are widely distributed in the Ryoke belt and have been divided into four main igneous stages based on their field setting. In this paper, we present Rb–Sr isochron ages for the younger Ryoke granitoids (second stage to fourth stage) in the Kinki district. The Yagyu granite (second stage) gave a Rb–Sr whole‐rock isochron age of 74.6 ± 10.9 Ma with an initial 87Sr/86Sr ratio of 0.70938 ± 0.00016, and a Rb–Sr mineral isochron age of 71.8 ± 0.1 Ma. The Narukawa granite (second stage) yielded a Rb–Sr mineral isochron age of 79.5 ± 0.4 Ma. A Rb–Sr whole‐rock isochron age of 78.3 ± 3.0 Ma with an initial 87Sr/86Sr ratio of 0.70764 ± 0.00014 was obtained for the Takijiri adamellite (third stage). The Katsuragi quartzdiorite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 85.1 ± 18.3 Ma (initial 87Sr/86Sr ratio of 0.70728 ± 0.00006), and mineral isochron ages of 76.9 ± 0.5 Ma and 74.8 ± 0.5 Ma. The Minamikawachi granite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 72.8 ± 2.0 Ma with an initial 87Sr/86Sr ratio of 0.70891 ± 0.00021. These age data indicate that the igneous activity in younger Ryoke granitoids of Kinki district occurred between 80 and 70 Ma, except for the Katsuragi quartz diorite. The isotopic data on the various igneous stages in Kinki district correspond with the relative timing from field observations. Based on the initial 87Sr/86Sr ratios, the granitoids of the Ryoke belt in Kinki district are spatially divided into two groups. One is granitoids with initial 87Sr/86Sr ratio of 0.707–0.708, distributed in the southern part of the Ryoke belt. The other is granitoids with initial 87Sr/86Sr ratio of 0.708–0.710 distributed in the northern part of the Ryoke belt. The initial 87Sr/86Sr ratios of granitoids increase with decreasing (becoming younger) Rb–Sr whole‐rock isochron ages.  相似文献   

12.
Baogutu copper deposit in Western Junggar area is a mesoscopic porphyry deposit found in recent years. Study on its geochronology will help further understand ore genesis and regional ore-forming pattern. A series of small quartz-diorite and granodiorite stocks outcrop at Baogutu area, numbered I―X according to their size. A detailed exploration on Number V stock confirmed it as a mesoscopic scale copper deposit, and various exploration work has been carried out on other stocks with ore-forming evidence. Th...  相似文献   

13.
Being a part of the Paleo‐Tethys Ocean, closing of the Buqingshan‐Anyemaqen oceanic basin left a rich geologic record in the East Kunlun Orogenic Belt. The genesis and tectonic setting of the granites including quartz monzodiorite, granodiorite and mozogranite is discussed in light of the geochemical and U–Pb chronological data obtained. U–Pb dating studies on zircon from the quartz monzodiorite and monzogranite of the research area yielded ages of 220.11 ± 0.49 Ma ((Mean Square Weighted Deviates) MSWD = 0.046) and 223.33 ± 0.54 Ma (MSWD = 0.14), respectively, by Laser Ablation Multiple Collector Inductively Coupled Plasma Mass Spectrometry (LA–MC–ICP–MS) method. According to sedimentological and structural investigations, the Paleo‐Tethys Ocean in the Qimantag region began to close at about 235 Ma, and completely disapperared at about 220 Ma. The three types of granites in this study are considered to intrude the syn‐ to post‐collisional stages. The quartz monzodiorite and granodiorite belong to the I‐type granite whereas the monzogranite is of the S‐type granite. These two types of granites were formed by different ways of partial melting: first, partial melting of the lower crust took place as a result of asthenosphere upwelling triggered by break‐up of the leading edge or tearing of the descending oceanic slab. Subsequently partial melting of the middle–lower crust was caused by the underplating of basaltic magma formed by partial melting of the mantle wedge fluxed by fluids liberated by the oceanic slab dehydration. The magma responsible for the formation of S‐type granites appears to have originated from partial melting of the upper crustal material at a shallower level with a clear signature of continental crust.  相似文献   

14.
Once a mafic intrusive rock has become altered, it is generally difficult to obtain a reliable intrusion age using conventional isotopic dating methods. To overcome this problem, this study used zircon fission track (ZFT) thermochronometry to determine the timing of crystallization of altered mafic intrusions. ZFT dating was carried out on samples of baked granite country rock adjacent to dolerite dikes (5–10 m thick) in the Takato area of central Japan. Three granite samples collected within 8 mm of a dike contact yielded consistent ZFT ages of 17–16 Ma, with confined track lengths indicative of the complete annealing of pre‐existing tracks by reheating due to dike intrusion. An older ZFT age was obtained for one granite sample collected within 20 mm of the contact, but confined track length measurements indicate that this is an incompletely reset age that lies between the ZFT age of the unbaked granitic country rocks (ca. 55 Ma) and the emplacement age of the dike. Petrographic examinations suggest that post‐intrusion hydrothermal activity did not influence the ZFT ages. We conclude that the 17–16 Ma ZFT age represents the emplacement age of the dikes. Our results show that ZFT dating of baked country rock is an effective tool for dating altered mafic intrusions, for which other dating techniques are not applicable. In the eastern part of Southwest Japan, dispersed volcanic activity occurred in the late Early to early Middle Miocene (18–15 Ma), and the volcanic belt extended into the forearc. This pulse of activity was possibly related to the injection of asthenospheric material into the trench‐side mantle wedge beneath the Japan arc. We also present young apatite fission track ages (ca. 4 Ma) that may reflect a Middle Miocene or later thermal event associated with local magmatic activity near the Takato area.  相似文献   

15.
The Mawat ophiolite is part of the Mesozoic Neo‐Tethyan ophiolite belt of the Middle East and is located in the Zagros Imbricate Zone of Iraq. It represents fossil fragments of the Neo‐Tethyan oceanic lithosphere within the Alpine collisional system between the Arabian and Eurasia Plates. The first U–Pb zircon dating of the Daraban leucogranite from the Mawat ophiolite provides a 207Pb–206Pb age of 96.8 ± 6.0 Ma. The age is 59.0 ± 6.0 m.y. older than the previously published age of the Daraban leucogranite obtained by 40Ar–39Ar muscovite dating method. The U–Pb dating of magmatic zircons collected from the Daraban leucogranite, which intrudes into the Mawat ophiolite, reveals that melting of the pelagic sediment beneath the hot Zagros proto‐ophiolite in an intra‐oceanic arc environment led to anatexis at the subduction front and the generation of granitic melts at 96.8 ± 6.0 Ma, which were emplaced in the overlaying mantle wedge. This process was a response to the initial formation of the Neo‐Tethys ophiolite above a northeast‐dipping intra‐oceanic subduction zone at 96.8 ± 6.0 Ma. Published 40Ar–39Ar muscovite dating from the same leucogranite dike yields plateau ages of 37.7 ± 0.3 Ma, reflecting that the age was reset during the Arabia–Eurasia continental collision. Therefore, the bimodal age populations from the granitic intrusion in the Mawat ophiolite preserve a record of the subduction to the collision cycle of the Zagros Orogenic Belt. The 59.0 ± 6.0 m.y. age difference from the Daraban leucogranite represents the duration of the subduction‐collision cycle of the Zagros Orogenic Belt in the Kurdistan region of Iraq and the time span for the closure of the Neo‐Tethys Ocean along the northern margin of the Arabian plate.  相似文献   

16.
Late Triassic A‐type granites are identified in this study in Sarudik, SW Sumatra. We present new data on zircon U–Pb geochronology, whole‐rock major and trace elements and Sr‐Nd‐Hf isotope geochemistry, aiming to study their petrogenesis and tectonic implications. LA‐ICP‐MS U–Pb dating of zircon separated from one biotite monzogranite sample yields a concordia age of 222.6 ±1.0 Ma, indicating a Late Triassic magmatic event. The studied granites are classified as weakly peralumious, high‐K calc‐alkaline granites. They exhibit high SiO2, K2O + Na2O, FeO/(FeO + MgO) and Ga/Al ratios and low Al2O3, CaO, MgO, P2O5 and TiO2 contents, with enrichment of Rb, Th and U and depletion of Ba, Sr, P and Eu, showing the features of A‐type granites. The granites have zircon εHf(t) values from ?4.6 to ?0.4 and whole‐rock εNd(t) values from ?5.51 to ?4.98, with Mesoproterozoic TDM2 ages (1278–1544 Ma) for both Hf and Nd isotopes. Geochemical and isotopic data suggest that the source of these A‐type granites is the Mesoproterozoic continental crust, without significant incorporation of mantle‐derived component, and their formation is controlled by subsequent fractional crystallization. The Sarudik A‐type granites are further assigned to A2‐type formed in post‐collisional environment. Combined with previous knowledge on the western SE Asia tectonic evolution, we conclude that the formation of the Late Triassic A‐type granites is related to the post‐collisional extension induced by the crustal thickening, gravitational collapse, and asthenosphere upwelling following the collision between the Sibumasu and the East Malaya Block.  相似文献   

17.
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   

18.
Zircons from two samples of the Sukeng pluton in the southwest Fujian Province, China, were analyzed by LA–ICP–MS with the aim of determining the timing of formation. The zircons from the two samples yield similar U–Pb ages of 100.47 ± 0.42 and 102.46 ± 0.69 Ma, indicating that the Sufeng pluton is contemporaneous with the Sifang and Luoboling plutons, all of which are also related to Cu–Au–Pb–Zn–Mo mineralization within the study area. All three plutons have geochemical features of I‐type granites, are high‐ to mid‐K calc‐alkaline metaluminous rocks, and have average molar Al2O3/ (CaO+Na2O+K2O) values of 0.95, initial 87Sr/86Sr ratios of 0.70465–0.70841, εNd(t) values at 101 Ma from –1.72 to –7.26, and two‐stage Nd model ages (T2DM) from 1.16 to 1.60 Ga. Zircons within these plutons have εHf(t) values at 101 Ma from –3.5 to 6.25 and T2DM ages from 0.74 to 1.46 Ga, suggesting these I‐type granites formed from magmas generated by partial melting of Mesoproterozoic to Neoproterozoic continental crust that mixed with mantle‐derived magmas. The magmatism was associated with thickening of the lower crust caused by collisions between microcontinents in the Cathaysian Block, which were driven by Early Cretaceous subduction of the Pacific Plate.  相似文献   

19.
Zircon U–Pb ages of two acidic tuff and two turbidite sandstone samples from the Nakanogawa Group, Hidaka Belt, were measured to estimate its depositional age and the development of the Hokkaido Central Belt, northeast Japan. In the northern unit, homogeneous zircons from pelagic acidic tuff from a basal horizon dated to 58–57 Ma, zircons from sandstone from the upper part of the unit dated to 56–54 Ma, and zircons from acidic tuff from the uppermost part dated to 60–56 Ma and 69–63 Ma. Both of the tuff U–Pb ages are significantly older than the youngest radiolarian fossil age (66–48 Ma). Therefore, the maximum depositional age of the turbidite facies in the northern unit is 58 Ma and the younger age limit, estimated from the fossil age, is 48 Ma. In the southern unit, homogeneous zircons from turbidite sandstone dated to 58–57 Ma. Thus the depositional age of this turbidite facies was interpreted to be 66–56 Ma from the fossil age, probably close to 57 Ma. Most of the zircon U–Pb ages from the Nakanogawa Group are younger than 80 Ma, with a major peak at 60 Ma. This result implies that around Hokkaido volcanic activity occurred mainly after 80 Ma. Older zircon ages (120–80 Ma, 180–140 Ma, 340–220 Ma, 1.9 Ga, 2.2 Ga, and 2.7 Ga) give information about the provenance of other rocks in the Hidaka Belt. It is inferred that the Nakanogawa Group comprises protoliths of the upper sequence of the Hidaka Metamorphic Zone, which therefore has the same depositional age as the Nakanogawa Group (66–48 Ma). The depositional ages of the lower sequence of the Hidaka Metamorphic Zone and the Nakanogawa Group are probably the same.  相似文献   

20.
Diagnostic mineral assemblages, mineral compositions and zircon SHRIMP U–Pb ages are reported from an ultrahigh‐temperature (UHT) spinel–orthopyroxene–garnet granulite (UHT rock) from the South Altay orogenic belt of northwestern China. This Altay orogenic belt defines an accretionary belt between the Siberian and Kazakhstan–Junggar Plates that formed during the Paleozoic. The UHT rock examined in this study preserves both peak and retrograde metamorphic assemblages and microstructures including equilibrium spinel + quartz, and intergrowth of orthopyroxene, spinel, sillimanite, and cordierite formed during decompression. Mineral chemistry shows that the spinel coexisting with quartz has low ZnO contents, and the orthopyroxene is of high alumina type with Al2O3 contents up to 9.3 wt%. The peak temperatures of metamorphism were >950°C, consistent with UHT conditions, and the rocks were exhumed along a clockwise P–T path. The zircons in this UHT rock display a zonal structure with a relict core and metamorphic rim. The cores yield bimodal ages of 499 ± 8 Ma (7 spots), and 855 Ma (2 spots), with the rounded clastic zircons having ages with 490–500 Ma. Since the granulite was metamorphosed at temperatures >900°C, exceeding the closure temperature of U–Pb system in zircon, a possible interpretation is that the 499 ± 8 Ma age obtained from the largest population of zircons in the rock marks the timing of formation of the protolith of the rock, with the zircons sourced from a ～500 Ma magmatic provenance, in a continental margin setting. We correlate the UHT metamorphism with the northward subduction of the Paleo‐Asian Ocean and associated accretion‐collision tectonics of the Siberian and Kazakhstan–Junggar Plates followed by rapid exhumation leading to decompression.  相似文献   

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