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1.
We have estimated the timescale of material circulation in the Sanbagawa subduction zone based on U–Pb zircon and K–Ar phengite dating in the Ikeda district, central Shikoku. The Minawa and Koboke units are major constituents of the high‐P Sanbagawa metamorphic complex in Shikoku, southwest Japan. For the Minawa unit, ages of 92–81 Ma for the trench‐fill sediments, are indicated, whereas the age of ductile deformation and metamorphism of garnet and chlorite zones are 74–72 Ma and 65 Ma, respectively. Our results and occurrence of c. 150 Ma Besshi‐type deposits formed at mid‐ocean ridge suggest that the 60‐Myr‐old Izanagi Plate was subducted beneath the Eurasian Plate at c. 90 Ma, and this observation is consistent with recent plate reconstructions. For the Koboke unit, the depositional ages of the trench‐fill sediments and the dates for the termination of ductile deformation and metamorphism are estimated at c. 76–74 and 64–62 Ma, respectively. In the Ikeda district, the depositional ages generally become younger towards lower structural levels in the Sanbagawa metamorphic complex. Our results of U–Pb and K–Ar dating show that the circulation of material from the deposition of the Minawa and Koboke units at the trench through an active high‐P metamorphic domain to the final exhumation from the domain occurred continuously throughout c. 30 Myr (from c. 90 to 60 Ma).  相似文献   

2.
Accurate pressure–temperature–time (P–T–t) paths of rocks from sedimentation through maximum burial to exhumation are needed to determine the processes and mechanisms that form high‐pressure and low‐temperature type metamorphic rocks. Here, we present a new method combining laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb with fission track (FT) dates for detrital zircons from two psammitic rock samples collected from the Harushinai unit of the Kamuikotan metamorphic rocks. The concordant zircon U–Pb ages for these samples vary markedly, from 1980 to 95 Ma, with the youngest age clusters in both samples yielding Albian‐Cenomanian weighted mean ages of 100.8 ± 1.1 and 99.3 ± 1.0 Ma (2σ uncertainties). The zircon U–Pb ages were not reset by high‐P/T type metamorphism, because there is no indication of overgrowth within the zircons with igneous oscillatory zoning. Therefore, these weighted mean ages are indicative of the maximum age of deposition of protolithic material. By comparison, the zircon FT data yield a pooled age of ca. 90 Ma, which is almost the same as the weighted mean age of the youngest U–Pb age cluster. This indicates that the zircon FT ages were reset at ca. 90 Ma while still at their source, but have not been reset since. This conclusion is supported by recorded temperature conditions of less than about 300 °C (the closure temperature of zircon FTs), as estimated from microstructures in the deformed detrital quartz grains in psammitic rocks, and no shortening of fission track lengths in the zircon. Combining these new data with previously reported white mica K–Ar ages indicates that the Harushinai unit was deposited after ca. 100 Ma, and underwent burial to its maximum depth before being subjected to a localized thermal overprint during exhumation at ca. 58 Ma.  相似文献   

3.
Abstract The chronological characteristics of Alpine metamorphic rocks are described and Alpine metamorphic events are reinterpreted on the basis of chronological data for the western and central Alps from 1960 to 1992. Metamorphic rocks of the Lepontine, Gran San Bernardo, Piemonte, Internal Crystalline Massifs and Sesia-Lanzo mostly date Alpine metamorphic events, but some (along with granitoids and gneisses from the Helvetic and Southern Alps) result from the Variscan, Caledonian or older events and thus predate the Alpine events. Radiometric age data from the Lepontine area show systematic age relations: U-Pb monazite (23-29 Ma), Rb-Sr muscovite (15–40 Ma) and biotite (15–30 Ma), K-Ar biotite (10-30 Ma), muscovite (15–25 Ma) and hornblende (25-35 Ma), and FT zircon (10-20 Ma) and apatite (5-15 Ma), which can be explained by the different closure temperatures of the isotopic systems. A 121 Ma U-Pb zircon age for a coesite-bearing whiteschist (metaquartzite) from the Dora-Maira represents the peak of ultra-high pressure metamorphism. Coesite-free eclogites and blueschists related to ultra-high pressure rocks in the Penninic crystalline massifs yield an 40Ar-39Ar plateau age of about 100 Ma for phengites, interpreted as the cooling age. From about 50 Ma, eclogites and glaucophane schists have also been reported from the Piemonte ophiolites and calcschists, suggesting the existence of a second high P/T metamorphic event. Alpine rocks therefore record three major metamorphic events: (i) ultra-high and related high P/T metamorphism in the early Cretaceous, which is well preserved in continental material such as the Sesia-Lanzo and the Penninic Internal Crystalline Massifs; (ii) a second high P/T metamorphic event in the Eocene, which is recognized in the ophiolites and calcschists of the Mesozoic Tethys; and (iii) medium P/T metamorphism, in which both types of high P/T metamorphic rocks were variably reset by Oligocene thermal events. Due to the mixture of minerals formed in the three metamorphic events, there is a possibility that almost all geochronological data reported from the Alpine metamorphic belt show mixed ages. Early Cretaceous subduction of a Tethyan mid-ocean ridge and Eocene continental collision triggered off the exhumation of the high pressure rocks.  相似文献   

4.
Zircon U-Pb thermal ionization mass-spectrometer (TIMS) and secondary ion mass-spectrometer (SIMS) dating, mica and amphibole 40Ar-39Ar dating and mineral Sm-Nd isotopic compositions of Huangzhen Iow temperature eclogite and country granitic gneiss are carried out. The zircon U-Pb weighted average SIMS age is (231.6±9.7) Ma for one eclogite.The mica 40Ar-39Ar isochron age is (232.6±2.1) Ma and the lowest plateau age is (221.7±2.4)Ma from same sample. U-Pb TIMS concordant ages from other eclogite zircons are from (221.3± 1.4) Ma to (222.5±2.3) Ma. U-Pb SIMS low intercept age from country granitic gneiss is (221±35) Ma. The retrograde amphibole 40Ar-39Ar isochron age is (205.9± 1.0) Ma. Except for mica,which may contain excess 40Ar, all the ages represent peak and retrograde metamorphism of low temperature eclogites. It is indicated that the Huangzhen low temperature eclogites differ from Xiongdian low temperature eclogites of north of the Northern Dabie Terrain in metamorphic ages.Huangzhen low temperature eclogites share one coherent HP-UHP terrain with high temperature eclogites from Southern Dabie Terrain and they may have differences in subduction depth and cooling rates during exhumation.  相似文献   

5.
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   

6.
Tetsumaru  Itaya  Hironobu  Hyodo  Tatsuki  Tsujimori  Simon  Wallis  Mutsuki  Aoya  Tetsuo  Kawakami  Chitaro  Gouzu 《Island Arc》2009,18(2):293-305
Laser step heating 40Ar/39Ar analysis of biotite and muscovite single crystals from a Barrovian type metamorphic belt in the eastern Tibetan plateau yielded consistent cooling ages of ca. 40 Ma in the sillimanite zone with peak metamorphic temperatures higher than 600°C and discordant ages from 46 to 197 Ma in the zones with lower peak temperatures. Chemical Th‐U‐Total Pb Isochron Method (CHIME) monazite (65 Ma) and sensitive high mass‐resolution ion microprobe (SHRIMP) apatite (67 Ma) dating give the age of peak metamorphism in the sillimanite zone. Moderate amounts of excess Ar shown by biotite grains with ages of 46 to 94 Ma at metamorphic grades up to the high‐grade part of the kyanite zone probably represent incomplete degassing during metamorphism. In contrast, the high‐grade part of the kyanite zone yields biotite ages of 130 to 197 Ma. The spatial distribution of these older ages in the kyanite zone along the sillimanite zone boundary suggests they reflect trapped excess argon that migrated from higher‐grade regions. The most likely source is muscovite that decomposed to form sillimanite. The zone with extreme amounts of excess argon preserves trapped remnants of an ‘excess argon wave’. We suggest this corresponds to the area where biotite cooled below its closure temperature in the presence of an elevated Ar wave. Extreme excess Ar is not recognized in muscovite suggesting that the entrapment of the argon wave by biotite took place when the rocks had cooled down to temperatures lower than the closure temperature of muscovite. The breakdown of phengite during ultrahigh‐pressure (UHP) metamorphism may be a key factor in accounting for the very old apparent ages seen in many UHP metamorphic regions. This is the first documentation of a regional Ar‐wave spatially associated with regional metamorphism. This study also implies that resetting of the Ar isotopic systems in micas can require temperatures up to 600°C; much higher than generally thought.  相似文献   

7.
A comparison between conventional KAr (biotite) ages and fission track (zircon and apatite) and UPb (zircon) ages obtained from stratigraphically well-constrained Priabonian (Late Eocene) volcano-sedimentary deposits of northern Italy is presented. Two sections at Priabona (one level) and Possagno (two levels) were dated. The application of fission track dating appears fruitful for obtaining reasonably precise (±4 to 5% 2σ errors) ages useful for time-scale calibration. The concordancy of apatite and zircon fission track ages, and the reproducibility of results provide the time of volcanic eruption and deposition. The UPb analysis of the zircons has not been unsuccessful, but discordancy does not permit accurate dating. Significant dates obtained from Possagno are: KAr method, 35.0 ± 0.5 Ma (duplicate analysis on K-rich biotite from the same level); fission track dating method, 35.8 ± 1.4 Ma (weighted mean age on 2 apatite and 3 zircon separates from the same level); UPb method, 36.7 ± 1.0 Ma (maximum age of discordant zircons from the same level). The comparison between the present results and recent multi-method and multi-laboratory results obtained from time equivalent Priabonian (Late Eocene) biotite-rich layers from the Apennines shows perfect agreement and supports the location of a Priabonian stage between about 37.5 Ma and about 33.7 (±0.5) Ma; the alternative ages preferred by the Decade of North American Geology convention should be abandoned and a large portion of this scale revised accordingly.  相似文献   

8.
For lack of reliable isotopic chronological data, the metamorphic rock series in the Faku region of northern Liaoning has long been regarded as the platform basement. Recent studies reveal that these deformed and metamorphosed rocks, with a variety of protoliths of plutonic intrusions and supracrustal volcanic and sedimentary rocks, were genetically related to later ductile shearing events, and they, together with the syntectonic intrusions, constituted the large-scale Faku tectonites. In this paper, we report new 40Ar/39Ar data on hornblende, biotite, and K-feldspar from typical granitic mylonites in this suite of tectonites. The plateau age 256 Ma of FK53 hornblende and the high-temperature plateau age 262 Ma of Fk51-1 biotite should represent the cooling ages when the granites, formed as a result of Paleozoic oceanic crustal subduction beneath the continental crust or collision of multiple micro-continental blocks, were exhumed into shallow crustal levels. The plateau age 231 Ma of FK51-1 boitite and the apparent age 227 Ma of Fk51-2 K-feldspar are interpreted to record the time of ductile deformation occurring under greenschist facies conditions, i.e. the formation age of the Faku tectonites, while the age gradient from 197 Ma to 220 Ma of Fk51-2 K-feldspar probably record the subsequent stable uplift-cooling process. The tectonic exhumation event indicated by the plateau age 180 Ma of Fk51-2 K-feldspar may be associated with the onset of paleo-Pacific subduction beneath the North China plate. In addition, the U-Pb dating of FK54 zircon from later-intruded granite yields the age of crystallization of this super-unit intrusion at 159 Ma, thus establishing an upper limit for the formation age of the Faku tectonites, while the plateau age 125 Ma of Fk54 K-feldspar most likely corresponds to the rapid cooling and tectonic denudation event associated with the final collision between the Siberian plate and the North China plate. These isotopic ages provide important geochronological constraints for re-evaluating the tectonic essence of the Faku Faulted Convex and ascertaining the suturing boundary between the North China Platform and the Xingmeng Fold System.  相似文献   

9.
Detrital zircon multi‐chronology combined with provenance and low‐grade metamorphism analyses enables the reinterpretation of the tectonic evolution of the Cretaceous Shimanto accretionary complex in Southwest Japan. Detrital zircon U–Pb ages and provenance analysis defines the depositional age of trench‐fill turbidites associated with igneous activity in provenance. Periods of low igneous activity are recorded by youngest single grain zircon U–Pb ages (YSG) that approximate or are older than the depositional ages obtained from radiolarian fossil‐bearing mudstone. Periods of intensive igneous activity recorded by youngest cluster U–Pb ages (YC1σ) that correspond to the younger limits of radiolarian ages. The YC1σ U–Pb ages obtained from sandstones within mélange units provide more accurate younger depositional ages than radiolarian ages derived from mudstone. Determining true depositional ages requires a combination of fossil data, detrital zircon ages, and provenance information. Fission‐track ages using zircons estimated YC1σ U–Pb ages are useful for assessing depositional and annealing ages for the low‐grade metamorphosed accretionary complex. These new dating presented here indicates the following tectonic history of the accretionary wedge. Evolution of the Shimanto accretionary complex from the Albian to the Turonian was caused by the subduction of the Izanagi plate, a process that supplied sediments via the erosion of Permian and Triassic to Early Jurassic granitic rocks and the eruption of minor amounts of Early Cretaceous intermediate volcanic rocks. The complex subsequently underwent intensive igneous activity from the Coniacian to the early Paleocene as a result of the subduction of a hot and young oceanic slab, such as the Kula–Pacific plate. Finally, the major out‐of‐sequence thrusts of the Fukase Fault and the Aki Tectonic Line formed after the middle Eocene, and this reactivation of the Shimanto accretionary complex as a result of the subduction of the Pacific plate.  相似文献   

10.
The protoliths of mafic-ultramafic plutons in the northern Dabie Mts. (NDM) (Hubei) include pyroxenite and gabbro. The zircon U-Pb dating for a gabbro suggests that emplacement of mafic magma took place in the post-collisional setting at the age of 122.9±0.6 Ma. It is difficult to obtain a reliable Sm-Nd isochron age, due to disequilibrium of the Sm-Nd isotopic system. Two hornblende40Ar/39Ar ages of 116.1±1.1 Ma and 106.6±0.8 Ma may record cooling of metamorphism in the mafic-ultramafic plutons in Hubei below 500°C. The hornblende40Ar/39Ar ages for the mafic-ultramafic rocks in Hubei are evidently 15–25 Ma younger than those for the same rocks in Anhui, indicating that there is a diversity of the cooling rates for the mafic-ultramafic rocks in Hubei and Anhui. The difference in their cooling rates may be controlled by the north-dipping normal faults in the NDM. The intense metamorphism occurring in the mafic-ultramafic rocks in Hubei may result from the Yanshanian magmatic reheating and thermal fluid action induced by the Cretaceous migmatization. The geochemical similarity of these mafic-ultramafic rocks wherever in Hubei and Anhui may be attributed to the same tectonic setting via an identical genetic mechanism.  相似文献   

11.
The protoliths of mafic-ultramafic plutons in the northern Dabie Mts. (NDM) (Hubei) include pyroxenite and gabbro. The zircon U-Pb dating for a gabbro suggests that emplacement of mafic magma took place in the post-collisional setting at the age of 122.9(0.6 Ma. It is difficult to obtain a reliable Sm-Nd isochron age, due to disequilibrium of the Sm-Nd isotopic system. Two hornblende 40Ar/39Ar ages of 116.1(1.1 Ma and 106.6(0.8 Ma may record cooling of metamorphism in the mafic-ultramafic plutons in Hubei below 500(C. The hornblende 40Ar/39Ar ages for the mafic-ultramafic rocks in Hubei are evidently 15-25 Ma younger than those for the same rocks in Anhui, indicating that there is a diversity of the cooling rates for the mafic-ultramafic rocks in Hubei and Anhui. The difference in their cooling rates may be controlled by the north-dipping normal faults in the NDM. The intense metamorphism occurring in the mafic-ultramafic rocks in Hubei may result from the Yanshanian magmatic reheating and thermal fluid action induced by the Cretaceous migmatization. The geochemical similarity of these mafic-ultramafic rocks wherever in Hubei and Anhui may be attributed to the same tectonic setting via an identical genetic mechanism.  相似文献   

12.
Within the Tethyan realm, data for the subduction history of the Permo–Triassic Tethys in the form of accretionary complexes are scarce, coming mainly from northwest Turkey and Tibet. Herein we present field geological, petrological and geochronological data on a Triassic accretionary complex, the A?vanis metamorphic rocks, from northeast Turkey. The A?vanis metamorphic rocks form a SSE–NNW trending lozenge‐shaped horst, ～20 km long and ～6 km across, bounded by the strands of the active North Anatolian Fault close to the collision zone between the Eastern Pontides and the Menderes–Taurus Block. The rocks consist mainly of greenschist‐ to epidote‐amphibolite‐facies metabasite, phyllite, marble and minor metachert and serpentinite, interpreted as a metamorphic accretionary complex based on the oceanic rock types and ocean island basaltic, mid‐ocean ridge basaltic and island‐arc tholeiitic affinities of the metabasites. This rock assemblage was intruded by stocks and dikes of Early Eocene quartz diorite, leucogranodiorite and dacite porphyry. Metamorphic conditions are estimated to be 470–540°C and ～0.60–0.90 GPa. Stepwise 40Ar/39Ar dating of phengite–muscovite separates sampled outside the contact metamorphic aureoles yielded steadily increasing age spectra with the highest incremental stage corresponding to age values ranging from ～180 to 209 Ma, suggesting that the metamorphism occurred at ≥ 209 Ma. Thus, the A?vanis metamorphic rocks represent the vestiges of the Late Triassic or slightly older subduction in northeast Turkey. Estimated P–T conditions indicate higher temperatures than those predicted by steady state thermal models for average subduction zones, and can best be accounted for by a hot subduction zone, similar to the present‐day Cascadia. Contact metamorphic mineral assemblages around an Early Eocene quartz diorite stock, on the other hand, suggest that the present‐day erosion level was at depths of ～14 km during the Early Eocene, indicative of reburial of the metamorphic rocks. Partial disturbance of white‐mica Ar–Ar age spectra was probably caused by the reburial coupled with heat input by igneous activity, which is probably related to thrusting due to the continental collision between Eastern Pontides and the Menderes–Taurus Block.  相似文献   

13.
As for the metamorphic rock series distributed on the “Faku Faulted Convex (FFC)”[1] of the northern Liaoning, the earliest geologic investigation divided it into the Langzishan, Dashiqiao and Gaixian forma-tions1) and considered it to be equivalent to the Liaohe Group in the eastern Liaoning, whereas Liaoning Bu-reau of Geology and Mineral Resources (LBGMR) included it into the Lower Proterozoic basement[1]. Recent 1︰50000 scale geological mapping2) reveals that this deformed and …  相似文献   

14.
Abstract The Ryoke metamorphic belt in south-west Japan consists mainly of I-type granitoids and associated low-pressure/high-temperature metamorphic rocks. In the Yanai district, it has been divided into three structural units: northern, central and southern units. In this study, we measured the Rb–Sr whole-rock–mineral isochron ages and fission-track ages of the gneissose granodiorite in the central structural unit. Four Rb–Sr ages fall in a range of ca 89–87 Ma. The fission-track ages of zircon and apatite are 68.9 ± 2.6 Ma and 57.4 ± 2.5 Ma (1σ error), respectively. Combining the newly obtained ages with previously reported (Th–)U–Pb ages from the same unit, thermochronologic study revealed two distinctive cooling stages; 1) a rapid cooling (> 40°C/Myr) for a period (~7 Myr) soon after the peak metamorphism (~ 95 Ma) and 2) the subsequent slow cooling stage (~ 5°C/Myr) after ca 88 Ma. The first rapid cooling stage corresponds to thermal relaxation of the intruded granodiorite magma and its associated metamorphic rocks, and to the uplift by a displacement along low-angle faults which initiated soon after the intrusion of the magma. Uplift by the later stage deformation having formed large-scale upright folds resulted in progress of the exhumation during the first stage. The average exhumation velocity of the stage is ≥ 2 mm/yr. During the second stage, the rocks were not accompanied by ductile deformation and were exhumed with the rate of 0.1–0.2 mm/yr. The difference in the exhumation velocity between the first and second cooling stages resulted from the difference in the thickness of the crust and in the activity of ductile deformation between the early and later stages of the orogenesis.  相似文献   

15.
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   

16.
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   

17.
New U–Pb ages of zircons from migmatitic pelitic gneisses in the Omuta district, northern Kyushu, southwest Japan are presented. Metamorphic zonation from the Suo metamorphic complex to the gneisses suggests that the protolith of the gneisses was the Suo metamorphic complex. The zircon ages reveal the following: (i) a transformation took place from the high‐P Suo metamorphic complex to a high‐T metamorphic complex that includes the migmatitic pelitic gneisses; (ii) the detrital zircon cores in the Suo pelitic rocks have two main age components (ca 1900–1800 Ma and 250 Ma), with some of the detrital zircon cores being supplied (being reworked) from a high‐grade metamorphic source; and (iii) one metamorphic zircon rim yields 105.1 ±5.3 Ma concordant age that represents the age of the high‐T metamorphism. The high‐P to high‐T transformation of metamorphic complexes implies the seaward shift of a volcanic arc or a landward shift of the metamorphic complex from a trench to the sides of a volcanic arc in an arc–trench system during the Early Cretaceous. The Omuta district is located on the same geographical trend as the Ryoke plutono‐metamorphic complex, and our estimated age of the high‐T metamorphism is similar to that of the Ryoke plutono‐metamorphism in the Yanai district of western Chugoku. Therefore, the high‐T metamorphic complex possibly represents the western extension of the Ryoke plutono‐metamorphic complex. The protolith of the metamorphic rocks of the Ryoke plutono‐metamorphic complex was the Jurassic accretionary complex of the inner zone of southwest Japan. The high‐P to high‐T transformation in the Omuta district also suggests that the geographic trend of the Jurassic accretionary complex was oblique to that of the mid‐Cretaceous high‐T metamorphic field.  相似文献   

18.
Yujiro  Nishimura  Philippa M.  Black  Tetsumaru  Itaya 《Island Arc》2004,13(3):416-431
Abstract A southwest dipping Mesozoic accretionary complex, which consists of tectonically imbricated turbiditic mudstone and sandstone, hemipelagic siliceous mudstone, and bedded cherts and basaltic rocks of pelagic origin, is exposed in northern North Island, New Zealand. Interpillow limestone is sometimes contained in the basaltic rocks. The grade of subduction‐related metamorphism increases from northeast to southwest, indicating an inverted metamorphic gradient dip. Three metamorphic facies are recognized largely on the basis of mineral parageneses in sedimentary and basaltic rocks: zeolite, prehnite‐pumpellyite and pumpellyite‐actinolite. From the apparent interplanar spacing d002 data for carbonaceous material, which range from 3.642 to 3.564 Å, the highest grade of metamorphism is considered to have attained only the lowermost grade of the pumpellyite‐actinolite facies for which the highest temperature may be approximately 300°C. Metamorphic white mica K–Ar ages are reported for magnetic separates and <2 µm hydraulic elutriation separates from 27 pelitic and semipelitic samples. The age data obtained from elutriation separates are approximately 8 m.y. younger, on average, than those from magnetic separates. The age difference is attributed to the possible admixture of nonequilibrated detrital white mica in the magnetic separates, and the age of the elutriation separates is considered to be the age of metamorphism. If the concept, based on fossil evidence, of the subdivision of the Northland accretionary complex into north and south units is accepted, then the peak age of metamorphism in the north unit is likely to be 180–130 Ma; that is, earliest Middle Jurassic to early Early Cretaceous, whereas that in the south unit is 150–130 Ma; that is, late Late Jurassic to early Early Cretaceous. The age cluster for the north unit correlates with that of the Chrystalls Beach–Taieri Mouth section (uncertain terrane), while the age cluster for the south unit is older than that of the Younger Torlesse Subterrane in the Wellington area, and may be comparable with that of the Nelson and Marlborough areas (Caples and Waipapa terranes).  相似文献   

19.
滇西临沧花岗岩基新生代剥蚀冷却的裂变径迹证据   总被引：13，自引：0，他引：13       下载免费PDF全文
为揭示临沧花岗岩基的剥蚀冷却历史，探讨印藏碰撞对滇西的影响，对6块临沧花岗岩基样品进行锆石和磷灰石裂变径迹测定，并利用模拟退火法对其中5块样品的磷灰石裂变径迹数据进行非线性热史反演，估算了不同时期的剥蚀量和抬升量. 结果表明，岩基自印藏陆陆碰撞以来经历了两期冷却事件，早期冷却速率仅5～10 ℃/Ma，晚期冷却速率明显增高，特别是近3 Ma以来的冷却速率达到16～20 ℃/Ma；两期总剥蚀厚度可达3300～3500 m. 分析表明冷却事件与印藏碰撞关系密切，早期冷却是在印藏碰撞影响下，临沧岩基卷入逆冲推覆运动而遭遇抬升、剥蚀的结果；晚期冷却则是上新世以来，特别是3Ma以来岩基经受整体的强烈抬升、剥蚀的结果，该期构造抬升量约为672～1263 m；裂变径迹资料还揭示印藏碰撞先影响南部岩体，随后才波及到岩基中北段.  相似文献   

20.
Nguyen D.  Nuong  Tetsumaru  Itaya    Hironobu  Hyodo  Kazumi  Yokoyama 《Island Arc》2009,18(2):282-292
Conglomerates of the Kuma Group, central Shikoku, southwest Japan contain Sanbagawa schist clasts with a variety of metamorphic grades and lithologies. K–Ar and 40Ar/39Ar dating of phengite show all the pelitic schist clasts from low- to high-grade zones have similar phengite ages (82–84 Ma) that are significantly older than those from the in situ Sanbagawa sequence of central Shikoku. This is because the Kuma–Sanbagawa sequence was exhumed earlier than the in situ Asemi sequence with an exhumation process intermediate between those for the Kanto Mountains and the in situ Asemi sequences. 40A/39Ar plateau ages (103 and 117 Ma) of phengite in amphibolites indicate the timing of the early stage of the exhumation of the metamorphic pile, probably close to the peak metamorphic age.  相似文献   

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