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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   

2.
Three collisional cycles, the Tanzawa, Izu and Shichito, are known to have occurred in the South Fossa Magna, central Japan, since the late Miocene, based on geologic evidence. The cycles consist of six stages. At present the South Fossa Magna is in the later part of stage 5 of the Izu Cycle and stage 2 of the Shichito Cycle. Because the collisional processes are ongoing we can observe, measure and correlate them with the geologic records of the former cycles. The collisional processes are progressing intermittently because of the rupture and deformation of the collided and colliding island arc crusts. Rupture in the subducting crust can be explained by the geometry of the subducting plate along a boundary that is not straight. The delamination of the upper crust is detected from the geologic and crustal structure in the collided Tanzawa Block; it is an important factor in the deformation of the crust.  相似文献   

3.
WONN  SOH  KAZUO  NAKAYAMA & TAKU  KIMURA 《Island Arc》1998,7(3):330-341
The Pleistocene Ashigara Basin and adjacent Tanzawa Mountains, Izu collision zone, central Japan, are examined to better understand the development of an arc–arc orogeny, where the Izu–Bonin – Mariana (IBM) arc collides with the Honshu Arc. Three tectonic phases were identified based on the geohistory of the Ashigara Basin and the denudation history of the Tanzawa Mountains. In phase I, the IBM arc collided with the Honshu Arc along the Kannawa Fault. The Ashigara Basin formed as a trench basin, filled mainly by thin-bedded turbidites derived from the Tanzawa Mountains together with pyroclastics. The Ashigara Basin subsided at a rate of 1.7 mm/year, and the denudation rate of the Tanzawa Mountains was 1.1 mm/year. The onset of Ashigara Basin Formation is likely to be older than 2.2 Ma, interpreted as the onset of collision along the Kannawa Fault. Significant tectonic disruption due to the arc–arc collision took place in phase II, ranging from 1.1 to 0.7 Ma in age. The Ashigara Basin subsided abruptly (4.6 mm/year) and the accumulation rate increased to approximately 10 times that of phase I. Simultaneously, the Tanzawa Mountains were abruptly uplifted. A tremendous volume of coarse-grained detritus was provided from the Tanzawa Mountains and deposited in the Ashigara Basin as a slope-type fan delta. In phase III, 0.7–0.5 Ma, the entire Ashigara Basin was uplifted at a rate of 3.6 mm/year. This uplift was most likely caused by isostatic rebound resulting from stacking of IBM arc crust along the Kannawa Fault which is not active as the decollement fault by this time. The evolution of the Ashigara Basin and adjacent Tanzawa Mountains shows a series of the development of the arc–arc collision; from the subduction of the IBM arc beneath the Honshu Arc to the accretion of IBM arc crust onto Honshu. Arc–arc collision is not the collision between the hard crusts (massif) like a continent–continent collision, but crustal stacking of the subducting IBM arc beneath the Honshu Arc intercalated with very thick trench fill deposits.  相似文献   

4.
K–Ar and 40Ar/39Ar dates are presented for locations in the Izu–Bonin – Mariana (IBM) forearc (Ocean Drilling Program (ODP) sites 786 & 782, Chichijima, Deep Sea Drilling Program (DSDP) sites 458 & 459, Saipan), and Palau on the remnant arc of the Kyushu–Palau Ridge. For a number of these locations, the 40Ar/39Ar plateau and 36Ar/40Ar versus 39Ar/40Ar isochrons give older ages than the K–Ar results. The most important results are: (i) at site 786, initial construction of the proto-IBM (now forearc) basement occurred at least by ca 47–45 Ma, consistent with the age of the immediately overlying sediments (middle Eocene nannofossil Zone CP13c); the younger pulse of construction dated at ca 35 Ma by K–Ar could not be confirmed by 40Ar/39Ar analysis; (ii) 40Ar/39Ar ages for the initial construction of the Mariana portion of the IBM system are as old as those of the Izu–Bonin portion, for example at site 458, initial construction commenced at least by ca 49 Ma and at ca 47 Ma at Saipan (Sankakayuma Formation); and (iii) a combination of K–Ar and 40Ar/39Ar ages indicate continued boninite magmatism in the Izu–Bonin forearc (and remnant arc at Palau) until ca 35 Ma. Subduction inception including boninite series rocks along most of the exposed length of the IBM system, clearly preceded by some 5 million years the Middle Eocene (ca 43.5 Ma) change in Pacific plate motion. Boninitic series magmatism persisted at locations now exposed in the forearc for ～ 15 million years after arc inception concurrently with low-K tholeiitic series eruptions from a subaerial arc system, established at ≥ 40 Ma, on the Kyushu–Palau Ridge. For the Mariana portion of the IBM system, reconstruction of the proto-arc places this activity adjacent to the concurrent but orthogonally spreading Central Basin Ridge of the West Philippine Basin. It is possible that a combination of subduction of a young North New Guinea Plate beneath newly created back-arc basin crust may account for some of the features of the Mariana system. It is clear, however, that the understanding of the processes of subduction initiation and early IBM arc development is incomplete.  相似文献   

5.
Zircon U–Pb dating of the Tonaru metagabbro body in the Sanbagawa metamorphic belt, southwest Japan, suggests that igneous events at ca 200–180 Ma were involved in the protolith formation. The trace element compositions of the Tonaru zircons are enriched in U (a fluid‐mobile element) and Sc (an amphibole‐buffered element), and depleted in Nb (a fluid‐immobile element), suggesting that the parental magmas related to the Tonaru metagabbros formed in an arc setting. Integration of our results with previous studies of the metasedimentary rocks in the Tonaru body clearly indicates that the protoliths of the Tonaru body were produced by oceanic‐arc magmatism. With the previous geochronological and geological studies, the tectono‐magmatic–metamorphic history of the Tonaru and other mafic bodies in the Sanbagawa metamorphic belt may be summarized as follows: (i) the protolith formation by the oceanic‐arc magmatic event had occurred at 200–180 Ma; (ii) the protoliths were accreted in the trench at ca 130–120 Ma; and (iii) they were completely subducted into the depth of the eclogite‐facies condition after 120 Ma.  相似文献   

6.
De-Ru  Xu  Bin  Xia  Peng-Chun  Li  Guang-Hao  Chen  Ci  Ma  Yu-Quan  Zhang 《Island Arc》2007,16(4):575-597
Abstract Metabasites within the Paleozoic volcanic‐clastic sedimentary sequences in Hainan Island, South China, show large differences not only in the nature of protoliths, but also in zircon U‐Pb sensitive high mass‐resolution ion microprobe (SHRIMP) ages. The protoliths for the Tunchang area metabasites have intraoceanic arc geochemical affinities. In the east‐central island gabbroic to diabasic rocks and pillow lavas are also present, while the Bangxi area metabasites with back‐arc geochemical affinities in the northwest island consist of basaltic, gabbroic and/or picritic rocks. Three types of zircon domains/crystals in the Tunchang area metabasites are defined. Type 1 is comagmatic and yields concordant to approximately concordant 206 Pb/238 U ages ranging from 442.1 ± 13.7 Ma to 514.3 ± 30.2 Ma with a weighted U‐Pb mean age of 445 ± 10 Ma. Type 2 is inherited and yields a weighted 207 Pb/206 Pb mean age of 2488.1 ± 8.3 Ma. Type 3 is magmatic with a 207 Pb/206 Pb age of ca 1450 Ma. Magmatic zircons in the Bangxi area metabasites yield a weighted U‐Pb mean age of 269 ± 4 Ma. We suggest 450 Ma is the minimum age for crystallization of protoliths of the Tunchang area metabasites, because the age range of ca 440–514 Ma probably corresponds to both the time of igneous crystallization and the high‐temperature overprint. The presence of abundant inherited zircons strongly favors derivation of these rocks from a NMORB‐like mantle proximal to continental crust. A protolith age of ca 270 Ma for the Bangxi area metabasites probably records expansion of an epircontinental back‐arc basin and subsequent generation of a small oceanic basin. The presence of ophiolitic rocks with an age of ca 450 Ma, not only in Hainan Island, but also in the Yangtze block, highlights the fact that the South China Caledonian Orogeny was not intracontinental in nature, but characterized by an ocean‐related event.  相似文献   

7.
Understanding the petrologic and geochemical evolution of island arcs is important for interpreting the timing and impacts of subduction and processes leading to the formation of a continental crust. The Izu–Bonin–Mariana (IBM) Arc, western Pacific, is an outstanding location to study arc evolution. The IBM first arc (45–25 Ma) followed a period of forearc basalt and boninite formation associated with subduction initiation (52–45 Ma). In this study, we present new major and trace element data for the IBM first arc from detrital glass shards and clasts from DSDP Site 296, located on the northernmost Kyushu Palau Ridge (KPR). We synthesize these data with published literature for contemporaneous airfall ash and tephra from the Izu–Bonin forearc, dredge and piston core samples from the KPR, and plutonic rocks from the rifted eastern KPR escarpment, locations which lie within or correlate with KPR Segment 1 of Ishizuka, Taylor, Yuasa, and Ohara (2011). Our objective is to test ways in which petrologic and chemical data for diverse igneous materials can be used to construct a complete picture of this section of the Oligocene first arc and to draw conclusions about its evolution. Important findings reveal that widely varying primary magmas formed and differentiated at various depths at this location during this period. Changes in key trace element ratios such as La/Sm, Nb/Yb, and Ba/Th show that mantle sources varied in fertility and in the inputs of subducted sediment and fluids over time and space. Plutonic rocks appear to be related to early K‐poor dacitic liquids represented by glasses sampled both in the forearc and volcanic fronts. An interesting observation is that the variation in magma compositions in this relatively small segment encompasses that inferred for the IBM Arc as a whole, suggesting that sampling is a key factor in inferring temporal, across‐arc, and along‐strike geochemical trends.  相似文献   

8.
The Yongchun pluton is a Late Cretaceous adakitic intrusion in South Fujian Province, Southeast China, with associated metal mineralization. An understanding of the Yongchun pluton is helpful in tectono‐magmatic evolutionary processes, and is important in explaining the origin of related porphyry‐type deposits. Zircons from three samples of the pluton were analyzed by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS), yielding U–Pb ages of 99.50 ±0.87 Ma, 97.74 ±0.59 Ma, and 99.65 ±0.92 Ma. These ages are similar to those of the Sifang, Luoboling, and Sukeng plutons, all of which are related to Cu–Pb–Zn–Mo mineralization within the study area. The Yongchun pluton comprises high‐potassium, calc‐alkaline, metaluminous rocks, with average A/CNK values of 0.91, 87Sr/86Sr ratios of 0.705 51 to 0.706 83, εNd(t) values of ?4.63 to ?5.90, and two‐stage Nd model (T2DM) ages of 1.49–1.39 Ga, indicating the magmas were generated by partial melting of Mesoproterozoic continental crust mixed with mantle‐derived magmas. The pluton has geochemical characteristics typical of adakites, such as a high Sr content (average 553 ppm), and low Y (average 15.2 ppm) and Yb (average 1.61 ppm) contents, indicating that the parental magma was formed under high‐pressure conditions. The magmatism was associated with thickening of the lower crust during a change in subduction angle and convergence rate of the paleo‐Pacific Plate at 100 Ma. The compression was limited to South Fujian Province.  相似文献   

9.
Supracrustal rocks around the North Pole Dome area, Western Australia, provide valuable information regarding early records of the evolution of crustal processes, surface environments, and biosphere. Owing to the occurrence of the oldest known microfossils, the successions at the North Pole Dome area have attracted interest from many researchers. The Paleoarchean successions (Warrawoona Group) mainly comprise mafic‐ultramafic greenstones with intercalated cherts and felsic lavas. Age constraints on the sediments have been mainly based on zircon U–Pb geochronology. However, many zircon grains have suffered from metamictization and contain anomalously high contents of common Pb, which makes interpretation of the U–Pb data complicated. In order to provide more convincing chronological constraints, an U–Pb Concordia age is widely accepted as the best estimate. Most zircons separated from two adamellites also suffered from severe metamictization. In our analyses, less metamictized domains were selected using a pre‐ablation technique in conjunction with elemental mapping, and then their U–Pb isotopic compositions were determined with a laser ablation inductively coupled plasma mass spectrometry. Most analyzed domains contained certain amounts of common Pb (204Pb/206Pb > 0.000 1), whereas three and five U–Pb data points with less common Pb (204Pb/206Pb < 0.000 1) were obtained. These U–Pb datasets yielded U–Pb Concordia ages of ca 3 445 Ma and 3 454 Ma, respectively. These ages represent the timing of the adamellite intrusion, and constrain the minimum depositional age of the Warrawoona Group. In addition, a single xenocrystic zircon grain showed a 207Pb/206Pb age of ca 3 545 Ma, supporting the idea that the sialic basement of the Pilbara Craton existed prior to 3 500 Ma. The in situ U–Pb zircon dating combined with the pre‐ablation technique has the potentials to identify non‐metamictized parts and to yield precise and accurate geochronological data even from partially metamictized zircons.  相似文献   

10.
We present new Middle Miocene paleomagnetic data for the central Japan Arc, and discuss their implications for Miocene rotation. To obtain a refined paleodirection, we made magnetic measurements on basaltic to andesitic lavas and intrusive rocks from 12 sites in the Tsugu volcanic rocks (ca 15 Ma) in the northern part of the Shitara area, Japan. Significant secondary magnetizations in samples with strong magnetic intensities are interpreted as lightning‐induced components. Mean directions carried by magnetite and/or titanomagnetite were determined for all sites. An overall mean direction with a northerly declination was obtained from dual‐polarity site means for nine sites. This direction is indistinguishable from the mean direction for coeval parallel dikes in the northern part of the Shitara area, and also indistinguishable from the Miocene reference direction derived from the paleopole for the North China Block in the Asian continent. These comparisons suggest little or no rotation or latitudinal motion in the study area with respect to the North China Block since 15 Ma. We obtained a refined early Middle Miocene paleodirection (D = 9.7°, I = 52.5°, α95 = 4.8°; 30 sites) and paleopole (82.0°N, 230.8°E, A95 = 5.6°) for Shitara by combining data from the Tsugu volcanic rocks and a coeval dike swarm. An anomalous direction found at three sites could be a record of an extraordinary field during a geomagnetic polarity transition or excursion. Paleomagnetic data from Shitara suggest that: (i) the western wing of the Kanto Syntaxis, a prominent cuspate geologic structure in central Honshu, underwent a counterclockwise rotation with respect to the main part of the southwestern Japan Arc between ca 17.5 Ma and 15 Ma; (ii) collision between the Japan and Izu–Bonin (Ogasawara) Arcs began prior to 15 Ma; and (iii) clockwise rotation of the entire southwestern part of the Japan Arc had ceased by 15 Ma.  相似文献   

11.
The Indosinian Orogeny plays a significant role in tectonic background and magmatic evolution in Indochina and surrounding regions. Being a part product of the Indosinian magmatism in northwest Vietnam during late Permian–middle Triassic period, Muong Luan granitoid pluton dominantly consists of granodiorite, less diorite and granite. This pluton is located in the Song Ma suture and assigned to the Dien Bien complex. Geochemically, the Muong Luan granitoid rocks are characterized by a wide range of SiO2 contents (59.9–75.1 wt%) and high K2O contents. They display typical features of I‐type granites. The presence of hornblende and no muscovite and cordierite in the rocks further supports for I‐type character of granitoids. The emplacement age of the Muong Luan pluton obtained by LA–ICP–MS U–Pb zircon is at 242–235 Ma, corresponding to Indosinian time. Zircon εHf values of –5.6 to –10.4, in combination with moderate Mg values of 34–45 suggested that the Muong Luan granitoid was derived from partial melting of mafic crustal source rocks, which are probably Paleoproterozoic in age as revealed by Hf model ages (TDM2 = 1624–1923 Ma).  相似文献   

12.
The Kohistan–Ladakh Arc in the Himalaya–Karakoram region represents a complete section of an oceanic arc where the rocks from mantle to upper crustal levels are exposed. Generally this arc was regarded as of Jurassic–Cretaceous age and was welded to Asia and India by Northern and Southern Sutures respectively. Formation of this arc, timings of its collisions with Asia and India, and position of collision boundaries have always been controversial. Most authors consider that the arc collided with Asia first during 102–75 Ma and then with India during 55–50 Ma, whereas others suggest that the arc collided with India first at or before 61 Ma, and then the India–arc block collided with Asia ca 50 Ma. Recently published models of the later group leave several geological difficulties such as an extremely rapid drifting rate of the Indian Plate (30 ± 5 cm/year) northwards between 61–50 Ma, absence of a large ophiolite sequence and accretionary wedge along the Northern Suture, obduction of ophiolites and blueschists along the Southern Suture, and the occurrence of a marine depositional environment older than 52 Ma in the Indian Plate rocks south of the Southern Suture. We present a review based on geochemical, stratigraphic, structural, and paleomagnetic data to show that collision of the arc with Asia happened first and with India later.  相似文献   

13.
U–Pb geochronology and trace element chemistry of zircons in a microscale analysis were applied to the Ishizuchi caldera in the Outer Zone of Southwest Japan in order to estimate the timescale of the magma process, in particular, the magma differentiation. This caldera is composed mainly of ring fault complexes, major pyroclastic flow deposits, and felsic intrusion including central plutons. Using SHRIMP‐IIe, our new U–Pb zircon ages obtained from the major pyroclastic flow deposits (Tengudake pyroclastic flow deposits), granitic rocks from central plutons (Soushikei granodiorite and Teppoishigawa quartz monzonite), and rhyolite from the outer ring dike (Tenchuseki rhyolite) and the inner ring dike (Bansyodani rhyolite) are 14.80 ±0.11 Ma, 14.56 ±0.10 Ma, 14.53 ±0.12 Ma, 14.55 ±0.11 Ma and 14.21 ±0.19 Ma, respectively. Based on the U–Pb ages, the Hf contents and the REE patterns of the zircons, three stages are recognized in the evolutionary history of the magma chamber beneath the Ishizuchi caldera: (i) climactic Tengudake pyroclastic flow eruption; (ii) Tenchuseki rhyolite intrusion into the outer ring dike and central pluton intrusion; and (iii) Bansyodani rhyolite intrusion in the inner ring dike. These results indicate a magma evolution history of the Ishizuchi caldera system which took at least ca 600 kyr from the climatic caldera‐forming eruption to the post‐caldera intrusions. Our new geochronological data suggest that the Ishizuchi caldera formed as part of the voluminous and episodic magmatism that occurred in the wide zone along the Miocene forearc basin of Southwest Japan during the inception of the young Philippine Sea Plate subduction.  相似文献   

14.
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   

15.
Kazuo  Salto Koichi  Kato Shunji  Sugi 《Island Arc》1997,6(2):158-167
Abstract K-Ar age studies in the Ashigawa and the southern part of the Tokuwa granodioritic bodies, which consist of the southern part of the Kofu plutonic complex, revealed that they formed between 12 and 9 Ma. Quite a narrow range of ages obtained from the Ashigawa southernmost part of the Tokuwa pluton implies that they cooled rapidly. The southern part of the Tokuwa pluton, as a whole, shows a systematic age distribution with a decrease in age to the north. Compilation of currently available plutonic ages in the South Fossa Magna suggests that the plutonic activities occurred three times in this region. Episodic activity like this could be argued in relation to the tectonic development of this region.  相似文献   

16.
The Paleo‐Kuril Arc in the eastern Hokkaido region of Japan, the westernmost part of the Kuril Arc in the northwestern Pacific region, shows a tectonic bent structure. This has been interpreted, using paleomagnetic data, to be the result of block rotations in the Paleo‐Kuril Arc. To understand the timing and origin of this tectonic bent structure in the Paleo‐Kuril arc‐trench system, paleomagnetic surveys and U–Pb radiometric dating were conducted in the Paleogene Urahoro Group, which is distributed in the Shiranuka‐hill region, eastern Hokkaido. The U–Pb radiometric dating indicated that the Urahoro Group was deposited at approximately 39 Ma. Paleomagnetic analysis of the Urahoro Group suggested that the Shiranuka‐hill region experienced a 28° clockwise rotation with respect to East Asia. The degree of clockwise rotation implied from the Urahoro Group is smaller than that of the underlying Lower Eocene Nemuro Group (62°) but larger than that of the overlying Onbetsu Group (?9°). It is thus suggested that the Shiranuka‐hill region experienced a clockwise rotation of approximately 34° between the deposition of the Nemuro and Urahoro Groups (50–39 Ma), and a 38° clockwise rotation between the deposition of the Urahoro and Onbetsu Groups (39–34 Ma). The origin of the curved tectonic belt of the Paleo‐Kuril Arc was previously explained by the opening of the Kuril Basin after 34 Ma. The age constraint for the rotational motion of the Shiranuka‐hill region in this study contradicts this hypothesis. Consequently, it is suggested that the process of arc–arc collision induced the bent structure of the western Paleo‐Kuril Arc.  相似文献   

17.
Fission‐track (FT) and (U–Th–Sm)/He (He) analyses are used to constrain the denudation pattern and history of the Kiso Range, a Japanese fault‐block mountain range which has been uplifted since ca 0.8 Ma. We obtained nine zircon FT ages ranging 59.3–42.1 Ma, 18 apatite FT ages ranging 81.9–2.3 Ma, and 13 apatite He ages ranging 36.7–2.2 Ma. The apatite FT and He ages are divided into an older group comparable to the zircon FT age range and a younger group of <18 Ma. The younger ages are interpreted as a reflection of uplift of the Kiso Range because they were obtained only to the east of the Seinaiji‐touge Fault, and the event age estimated from apatite FT data is consistent with the timing of the onset of the Kiso Range uplift. On the basis of the distribution of the younger ages, we propose westward tilting uplift of the Kiso Range between the boundary fault of the Inadani Fault Zone and Seinaiji‐touge Fault, which implies a model of bedrock uplift that is intermediate between two existing models: a pop‐up model in which the Kiso Range is squeezed upward between the two faults and a tilted uplift model which assumes that the Kiso Range is uplifted and tilted to the west by the Inadani Fault Zone. The original land surface before the onset of uplift/denudation of the Kiso Range is estimated to have been uplifted to an elevation of 2700–4900 m. We estimated denudation rates at 1.3–4.0 mm/y and maximum bedrock uplift rates at 3.4–6.1 mm/y since ca 0.8 Ma. The Seinaiji‐touge fault is interpreted as a back thrust of the west‐dipping Inadani Fault Zone. The older group of apatite FT and He ages is interpreted to reflect long‐term peneplanation with a probable denudation rate of <0.1 mm/y.  相似文献   

18.
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   

19.
The 1 Myr tephra records of IODP (International Ocean Discovery Program) Holes U1436A and U1437B in the Izu‐Bonin fore‐ and reararc were investigated in order to assess provenance and eruptive volumes, respectively. In total, 304 tephra samples were examined and 260 primary tephra layers were identified. Tephra provenance was determined by means of major and trace element compositions of glass shards and distinguished between Japan and Izu‐Bonin arc origin of the tephra layers. A total of 33 marine tephra compositions were correlated to the Japan arc and 227 to the Izu arc. Twenty marine tephra layers were correlated between the two drilling sites. Additionally, we defined eleven correlations of marine tephra deposits to major widespread Japanese eruptions; from the 1.05 Ma Shishimuta‐Pink Tephra to the 30 ka Aira‐Tn Tephra, both from Kyushu Island. These eruptions provide independent time markers within the sediment record and six correlations were used to date tephra layers from Japan in Hole U1436A to establish an alternative age model for this hole. Furthermore, the minimum distal tephra volumes of all detected events were calculated, which enabled the comparison of the tephra volumes that derived from the Japan and the Izu‐Bonin arcs. For some of the major Japanese eruptions these are the first volume estimations that also include distal deposits. All of the Japanese tephras derived from events with eruption magnitude Mv ≥ 5.6 and three of the investigated eruptions reach magnitudes Mv ≥ 7. Volcanic events of the Izu‐Bonin arc have mostly eruption magnitudes Mv ≤ 5.  相似文献   

20.
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   

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