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1.
The Central Tianshan Tectonic Zone (CTTZ) is anarrow domain between an early Paleozoic southernTianshan passive continental margin and a late Paleo-zoic northern Tianshan arc zone, which is character-ized by the presence of numerous Precambrian meta-morphic basement blocks. Proterozoic granitoidgneisses and metamorphic sedimentary rocks,namely Xingxingxia and Kawabulag and Tianhugroups, are the most important lithological assem-blages in these metamorphic basement blocks, and alittle of …  相似文献   

2.
The geochronologic history of the northern portion of the Western Gneiss Region, south-central Norway has been investigated by UPb analyses of zircon and titanite from various basement units and one supracrustal gneiss. A six-point (four zircon and two titanite) discordia line from a migmatite melt-pod and its host-gneiss (Åstfjord migmatite gneiss) defines upper- and lower-intercept ages of 1659.1 ± 1.8 Ma and 393.6 ± 3.6 Ma, respectively. The upper-intercept age is interpreted as a time of tonalite emplacement and migmatite formation in the region.A seven-point (three zircon and four titanite) discordia line from the Ingdal granite gneiss has similar upper- and lower-intercept ages (1652.9 ± 1.7 Ma and 396.1 ± 4.9 Ma, respectively) and the upper-intercept age is interpreted as the time of granite crystallization. The lower-intercept age from both theÅstfjord migmatite gneiss and the Ingdal granite gneiss is interpreted as the time of regional metamorphic resetting and is evidence for Caledonian influence in the region.Titanite from these two gneiss units, as well as from other tonalite, granite, and supracrustal gneisses throughout the Western Gneiss Region displays a remarkably uniform pattern of discordance. Titanite discordance is variable from 6% to 100% and, in general, the degree of discordance is proportional to the Caledonian metamorphic grade of nearby supracrustal schists. Episodic, diffusional lead loss from titanite during Caledonian metamorphism probably caused the discordance pattern, but a combined mechanism of mixing and diffusional lead loss cannot be ruled out. In any case, the seventeen-point titanite and zircon discordia line clearly indicates that: (1) the bulk of the granitoid terrane in this portion of the Western Gneiss Region was emplaced, migmatized, and cooled in a short time interval about 1657 Ma ago; (2) a second, short-lived thermal event that exceeded the blocking temperature of titanite occurred about 395 Ma ago; and (3) titanite and zircon in the region were not isotopically disturbed by comparable geologic events in the period from 1657 to 395 Ma, or at any time after 395 Ma.  相似文献   

3.
Abstract CHIME (chemical Th-U-total Pb isochron method) ages were determined for monazite from gneisses and granitoids of the Ryoke belt in the Iwakuni area. The CHIME monazite ages are 99.6 ± 2.4, 98.9 ± 2.1 and 98.2 ± 5.7 Ma for the Ryoke gneiss, 90.7 ± 2.2, 89.7 ± 2.0 and 89.3 ± 2.2 Ma for the Tajiri Granite, 91.0 ± 3.2, 90.6 ± 3.2 and 89.9 ± 3.2 Ma for the Namera Granite, 89.3 ±3.3 and 88.6 ± 5.6 Ma for a small stock at Shimizu, and 87.3 ± 1.6 and 86.6 ± 2.1 Ma for the post-tectonic Shimokuhara Granite. The CHIME monazite ages, interpreted as the time of the first attainment at the amphibolite facies conditions for the gneisses and as the time of emplacement for the granites, respectively, agree with the field intrusive sequence. The present dating documented that the Ryoke metamorphism in the Iwakuni area reached the amphibolite facies conditions at ∼98 Ma, was complete at -87 Ma, the time of emplacement of the post-tectonic Shimokuhara Granite.  相似文献   

4.
Abstract We report new field, petrological and isotopic data and interpretations from one of New Zealand’s major basement geological boundaries, the contact between the east side of the Median Batholith (formerly Median Tectonic Zone) and the allochthonous Mesozoic terranes of the Eastern Province. In the Nelson and Hollyford–Eglinton areas this contact is a Cenozoic fault, the Median Tectonic Line of earlier workers. However, in the Longwood Range, unfaulted pre-Cenozoic geological relations are preserved intact. Our new Ar–Ar, U–Pb and isotopic data show that the Median Batholith in the Longwood Range consists of two suites. (i) Eastern, isotopically primitive (87Sr/86Sri = 0.702 to 0.703; ?NdT = + 7 to + 8) trondhjemite and gabbroic rocks of Permian age that we believe are part of the intraoceanic Brook Street arc of the Eastern Province. (ii) Western, isotopically more evolved (87Sr/86Sri = 0.703 to 0.704; ?NdT = + 3 to + 5) quartz diorites, quartz monzodiorites and rare granites of Middle Triassic to Early Jurassic age that we correlate with a pulse of magmatism elsewhere in the Median Batholith. Field observations in the Longwood Range indicate intrusive, not faulted, contacts between these units and constrain accretion of the Brook Street Terrane to Gondwana to have occurred 230–245 Ma. Intra-batholith shear zones (T ~ 600°C and P ~ 0.2–0.3 GPa) were active at approximately 220 Ma. Modelling of K-feldspar Ar incremental heating ages indicate that most of the Longwood Range had cooled below 175°C by the Middle Jurassic and experienced no subsequent reheating. Significant additional post-accretionary Early Cretaceous and Cenozoic thermotectonic activity in Median Batholith in the Hollyford-Eglinton area is indicated by a new 140 ± 2 Ma U-Pb zircon date on a Largs ignimbrite, as well as by Cenozoic K-feldspar Ar–Ar ages in the Middle Triassic Mistake Diorite.  相似文献   

5.
As the core block of the East Gondwana Land, the East Antarctic Shield was traditionally thought, before 1992, as an amalgamation of a number of Archaean-Paleoproterozoic nuclei, be-ing welded by Grenville aged mobile belts during 1400—900 Ma, while the …  相似文献   

6.
Geology of the Grove Mountains in East Antarctica   总被引：2，自引：0，他引：2  
Grove Mountains consists mainly of a series of high-grade (upper amphibolite to granulite facies) metamorphic rocks, including felsic granulite, granitic gneiss, mafic granulite lenses and charnockite, intruded by late tectonic gneissic granite and post-tectonic granodioritic veins. Geochemical analysis demonstrates that the charnockite, granitic gneiss and granite belonged to aluminous A type plutonic rocks, whereas the felsic and mafic granulite were from supracrustal materials as island-arc, oceanic island and middle oceanic ridge basalt. A few high-strained shear zones disperse in regional stable sub-horizontal foliated metamorphic rocks. Three generations of ductile deformation were identified, in which D1 is related to the event before Pan-African age, D2 corresponds to the regional granulite peak metamorphism, whereas D3 reflects ductile extension in late Pan-African orogenic period. The metamorphic reactions from granitic gneiss indicate a single granulite facies event, but 3 steps from mafic granulite, with P-T condition of M1 800°C, 9.3×105 Pa; M2 800–810°C, 6.4 × 105 Pa; and M3 650°C have been recognized. The U-Pb age data from representative granitic gneiss indicate (529±14) Ma of peak metamorphism, (534±5) Ma of granite emplacement, and (501±7) Ma of post-tectonic granodioritic veins. All these evidences suggest that a huge Pan-African aged mobile belt exists in the East Antarctic Shield extending from Prydz Bay via Grove Mountains to the southern Prince Charles Mountains. This orogenic belt could be the final suture during the Gondwana Land assemblage.  相似文献   

7.
Gettysburg Bank forms the western end of the Gorringe Seamount which is situated in the North Atlantic 110 km west of the tip of the Iberian Peninsula, on the eastern end of the Azores/Gibraltar fracture zone.Gabbros dredged from the Gettysburg Bank record a complex history of events. K-Ar ages of separated mineral phases fall into three concordant groups (plus some discordant ages). The oldest ages are from three brown kaersutitic hornblendes and their mean age of135 ± 3Ma is taken to be that of their formation. Six plagioclase feldspars yielded concordant ages of105 ± 3Ma which is possibly a consequence of a thermal event occurring at that time. Ages from three deformed plagioclases are concordant with a mean of82 ± 3Ma which is believed to relate to a phase of shearing, perhaps occurring during transform motion at the plate boundary.  相似文献   

8.
Suturing of the supercontinent Rodinia in the Grenville event (˜ 1000 Ma) was followed by rifting in the late Proterozoic (˜ 800-700 Ma), reorganization to Gondwana in the Pan-African (˜ 700-500 Ma) and further accretion to develop Pangea at the end of the Paleozoic. One of the Rodinian rifts followed part of the Grenville suture, it produced the margin of eastern North America and southern Baltica and the contrasting margin of west Gondwana in present South America. The Paleozoic accretionary wedge against the Grenville-age margin of North America and Baltica contains Avalonian/Cadomian terranes that exhibit Pan-African erogenic events ± sediments apparently developed while the terranes were in or near Gondwana. These terranes carry lower-Paleozoic fauna (Acado-Baltic) that are not indigenous to North America and Baltica.U---Pb zircon ages range from 1500-1000 Ma in Grenville terranes and from 800–500 Ma with minor inheritance in Avalonian terranes; they are generally much older in Cadomian terranes, implying very little resetting during Pan-African events. TDM ages are generally 2000–1200 Ma in Grenville terranes, 1300–600 Ma in Avalonian terranes and 2000–1200 in Cadomian terranes. These summary data show that: (1) the Grenville orogenic event produced almost no juvenile crust; (2) the Avalonian terranes of North America contain crust that evolved primarily in the late Proterozoic, possibly as a mixture of juvenile Pan-African material and Grenville or slightly older material; (3) the Cadomian terranes of Europe consist of old (middle-Proterozoic to Archean) crust with minor juvenile Pan-African material. The Avalonian terranes apparently evolved near, and partly on, the Grenville-age crust now in South America during the intense orogeny associated with rotation of Gondwana away from North America. The Cadomian terranes of Europe, however, appear to be fragments of other parts of Gondwana, probably West Africa.  相似文献   

9.
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   

10.
UPb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the Ancient Gneiss Complex. The oldest episode at3644 ± 4Ma(2σ) produced the dominant type of zircon, characteristically purplish-brown and massive in texture; this probably constitutes unaltered zircon precipitated from the original magma. Recrystallization accompanied (and obscured) by early Pb loss took place within the oldest grains at3504 ± 6Ma and3433 ± 8Ma. Whole new grains grew at these times also. We interpret the post-3644 Ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss. In addition, many grains are visibly overgrown by two layers of younger zircon of different colour and texture, dated at2986 ± 20Ma and2867 ± 30Ma. Euhedral, finely-zoned whole grains having the 2986 Ma age are present also, evidently contributed by very thin felsic veins associated with the nearby Lochiel granite. The age of3644 ± 4Ma combined with precise zircon UPb dating of volcanics from the Onverwacht Group reported elsewhere demonstrates that at least part of the Ancient Gneiss Complex is older than the Barberton Greenstone Belt.  相似文献   

11.
We report precise 207Pb/206Pb single zircon evaporation ages for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, and from granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group yield ages between 3445 +/- 3 and 3416 +/- 5 Ma and contain older crustal components represented by a 3504 +/- 4 Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between 3259 +/- 5 and 3225 +/- 3 Ma in age which we interpret to reflect the time of crystallization. A surprisingly complex xenocryst population in one sample documents ages from 3323 +/- 4 to 3522 +/- 4 Ma. We suspect that these xenocrysts were inherited, during the passage of the felsic melts to the surface, from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite-trondhjemite plutons along the southern and western margins of the BGB, and units predating any of the exposed greenstone or intrusive rocks. Several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 Ma. coeval with or slightly older than Onverwacht felsic volcanism, while the Kaap Valley pluton along the northwestern margin of the belt is coeval with Fig Tree dacitic volcanism. These results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites. Some of the volcanic plutonic units contain zircon xenocrysts older than any exposed rocks. These indicate the existence of still older units, possibly stratigraphically lower and older portions of the greenstone sequence itself, older granitoid intrusive rocks, or bodies of older, unrelated crustal material. Our data show that the Onverwacht and Fig Tree felsic units have distinctly different ages and therefore do not represent a single, tectonically repeated unit as proposed by others. Unlike the late Archaean Abitibi greenstone belt in Canada, which formed over about 30 Ma. exposed rocks in the BGB formed over a period of at least 220 Ma. The complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones. A surprising similarity in rock types, tectonic evolution, and ages of the BGB in the Kaapvaal craton of southern Africa and greenstones in the Pilbara Block of Western Australia suggests that these two terrains may have been part of a larger crustal unit in early Archaean times.  相似文献   

12.
The intrusive bodies studied include Mafan diorites ((462.7±1.5) Ma,40Ar/39Ar amphibole plateau age), Duhudian granites ((293±12) Ma, U-Ph zircon age) and Suxianshi granites ((146.2±0.9) Ma) in Beihuaiyang area at the northern foot of Dabie Mountains, central China. Petrological studies indicate that all of them belong to I-type granitoid rocks. Among them, the Mafan and Duhudian stocks were formed by arc magmatism, while the Suxianshi pluton is a post-collisional granitic body. Three intrusive bodies have distinctive characteristics of structural deformation. The Mafan stock has a rather complicated structure pattern resulting from polyphase deformation during the Caledonian and Mesozoic, the Duhudian stock has been pronouncedly deformed during the Hercynian-Yanshanian events, while regional foliation is not pronounced within the Yanshanian Suxianshi stock. Combination of regional stratigraphic, regional structural and geochronological data shows that the Yangtze plate has experienced two episodes of subduction northward beneath the North China plate during the Paleozoic and following collisional events. The first phase of collision at about 400 Ma resulted in the formation of the Beihuaiyang crystalline basement and the Caledonian high-pressure metamorphism in Dabie orogenic zone, and a late phase of continent-continent collision (~230 Ma) is responsible for the Triassic ultrahigh- and high-pressure metamorphism in Dabie Mountains and for orogenic uplift of the Dabie Mountains. It is suggested that the Beihuaiyang tectonic belt at the northern foot of the Dabie Mountains is a multicyclic suture.  相似文献   

13.
The Linglong granitoid complex (LGC) is composed of four major plutonic units that intruded and cooled in the Middle Jurassic (170-155 Ma). Gravity-anomaly modeling indicates that the LGC is a sheet-like laccolith, less than 10 km thick, that dips shallowly below the surface toward the Tancheng-Lujiang (Tan-Lu) fault, a major lithospheric structure in Eastern China. Measurements of foliation in the field and measurements of planar and linear magnetic fabrics from the study of anisotropy of magnetic susceptibility in the LGC indicate that foliation is dominantly shallowly dipping and magnetic lineation is mainly parallel to the dip direction of the laccolith toward the Tan-Lu fault zone. The trend of lineations is consistent with flow of magma up the thrust to reach shallower levels. The magma of the LGC probably originated by crustal melting within the Tan-Lu fault zone and the emplacement of magma occurred along a shallowly-dipping thrust that drained the Tan-Lu fault zone, the mechanism of which is mainly dike-fed model.  相似文献   

14.
Two contacts between Sudbury norite and northwest-trending diabase dikes and two contacts between the overlying micropegmatite and northwest dikes were investigated in order to estimate the depth of burial of the present erosion surface at the time of dike emplacement. A zone of hybrid paleomagnetic direction representing the vectorial sum of an older host component and an intrusion component of decreasing highest blocking temperature and intensity with distance from the intrusion was sought. Subtracting the calculated thermal effect of the intrusion from this highest blocking temperature yields the temperature of the host at the time of magma emplacement. Dividing this host temperature by an estimated paleogeothermal gradient yields the burial depth of the present erosion (or sampling) surface at the time of magma emplacement. Remanence direction in one of the dikes and norite contact zones is not typical for the Sudbury dike swarm of 1250 Ma age, and this contact is not further considered. An earlier published result for a norite-dike contact was reconsidered because of complicated dike geometry and included in this study. In one of the four usable contacts the hybrid zone is represented by three samples, in another by one sample, and in the remaining two only the contact zone width could be used. The final host temperature results are based on 4 individual calculations and show fair consistency with mean values of 287°C (s.d. 13°) and 267°C (s.d. 11°) calculated without and with a correction for viscosity of the host remanence respectively. Using a gradient of 26°C/km for 1250 Ma ago indicates a burial depth of9.5 ± 2km at that time. The fair consistency encourages the use of the method to deduce quantitatively the history of vertical motions of Precambrian terranes, the detail obtained being dependent on the presence of hybrid zones and of intrusions of various ages.  相似文献   

15.
Ion microprobe measurements of Pb isotope ratios in monazites have been obtained, in situ, from thin sections using the Cambridge ISOLAB 120. Molecular interferences are sufficiently resolved at an RP of 6500 to allow 207Pb/206Pb dating of monazite with precisions as low as 4–5 Ma (2σ). The results presented here provide important information on the chronological history of the Late Archean metamorphism of the Wind River Range, Wyoming (USA).

Matrix monazites and monazite inclusions in garnets from a metapelite from the northern Wind River Range have been analysed by SIMS. In a previous study peak metamorphic conditions (T = 800°C; P = 8 ± 1 kb*) were estimated using inclusion assemblages in garnets from this same sample. Isolated monazite inclusions in garnet yield 207Pb/206Pb age estimates of 2781 ± 6 to 2809 ± 10 Ma. Those along fractures yield lower ages (2603–2687 Ma) which are similar to TIMS and SIMS ages of matrix monazites. A single large (500 μm) monazite grain locally preserves growth zoning, but has a recrystallised core and a resorbed (recrystallised?) rim. Age estimates for these three regions are 2788 ± 9 Ma, 2663 ± 4 and 2523 ± 6 Ma, respectively. Thus the inclusion assemblages of Sharp and Essene* may record peak metamorphic conditions at ca. 2.8 Ga, and indicate a phase of metamorphism that predates by over 100 Ma the emplacement of the Bridger Batholith, the major lithologic component of the northern Wind River Range.

The analysed monazite grains appear to preserve ca. 300 Ma history, even within a single grain. Monazite inclusions in garnet that are fully armoured may provide estimates for the time of garnet growth, even in high grade terranes where most chronometers are reset. The age pattern preserved by the large monazite grain cannot be simply related to diffusion controlled closure. Instead, a chronology is preserved which can be related to the petrographic setting of indicidual grains through in situ analysis.  相似文献   


16.
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   

17.
During late Mesozoic subduction of paleo‐Pacific lithospheric plates, numerous gold vein deposits formed in the Dabie–Sulu Belt of east‐central China plus its east‐Asian extensions, and in the Klamath Mountains plus Sierran Foothills of northern California. In eastern Asia, earlier transpression and continental collision at about 305–210 Ma generated a high pressure–ultrahigh pressure orogen, but failed to produce widespread intermediate to felsic magmatism or abundant hydrothermal gold deposits. Similarly in northern California, strike‐slip ± minor transtension–transpression over the interval of about 380–160 Ma resulted in the episodic stranding of oceanic terranes, but generated few granitoid magmas or Au ore bodies. However, for both continental margin realms, nearly head‐on Cretaceous destruction of oceanic lithosphere involved sustained underflow; reaching magmagenic depths of about 100 km, the descending mafic‐ultramafic plates dewatered, producing voluminous calc‐alkaline arc magmas. Ascent of these plutons into the middle and upper crust released CO2 ± S‐bearing aqueous fluids and/or devolatilized the contact‐metamorphosed wall rocks. Such hydrothermal fluids transported gold along fractures and fault zones, precipitating it locally in response to cooling, fluid mixing, and/or reactions with wall rocks of contrasting compositions (e.g. serpentinite, marble). In contrast, where sialic crust was subducted to depths of about 100 km, only minor production of granitoid melts occurred, and few major coeval Au vein deposits formed. The mobilization of precious metal‐bearing fluids in continental margin and island arc environments apparently requires long‐continued, nearly orthogonal descent of oceanic, not continental, lithosphere.  相似文献   

18.
We have measured concentrations of heat producing elements (Th, U, and K) in 58 samples representative of the main lithologies in a 100 km transect of the Superior Province of the Canadian Shield, from the Michipicoten (Wawa) greenstone belt, near Wawa, Ontario, through a domal gneiss terrane of amphibolite grade, to the granulite belt of the Kapuskasing Structural Zone, near Foleyet. This transect has been interpreted as an oblique cross section through some 25 km of crust, uplifted along a major thrust fault, and thus provides an opportunity to examine in detail a continuous profile into deep continental crust of Archean age. Mean heat production values for these terranes, based on aereal distribution of major rock types and calculated from their Th, U, and K concentrations are: Michipicoten greenstone belt = 0.72 μW m−3; Wawa domal gneiss terrane (amphibolite grade) = 1.37 μW m−3; Kapuskasing granulites = 0.44 μW m−3. Among the silicic plutonic rocks (tonalites, granites, and their derivative gneisses), the relatively large variation in heat production correlates with modal abundances of accessory minerals including allanite, sphene, zircon, and apatite. We interpret these variations as primary (pre-metamorphic). The relatively high weighted mean heat production of the domal gneiss terrane can be accounted for by the larger proportion there of late-stage Th-, U-, and K-rich granitoid plutons. These may have been derived from the underlying Kapuskasing granulite terrane, leaving it slightly depleted in heat producing elements. Transport of Th, U, and K, therefore, could have taken place in silicate melts rather than in aqueous or carbonic metamorphic fluids. This conclusion is supported by the lack of a statistically significant difference in heat production between tonalites, tonalite gneisses and mafic rocks of amphibolite versus granulite grade.The pre-metamorphic radioactivity profile for this crustal section is likely to have been uniformly low, with a mean heat production value less than 1 μW m−3. This result is distinctly different from measured profiles in more silicic terranes, which show decreasing heat production with depth. This implies fundamental differences in crustal radioactivity distributions between granitic and more mafic terranes, and may be an important factor in selective reactivation of lithologically different terranes, possibly resulting in preferential stabilization of basic terranes in the geological record. Our results indicate that a previously determined apparently linear heat flow-heat production relationship for the Kapuskasing area does not relate to the distribution of heat production with depth. Low, but significant heat production, 0.4–0.5 μW m−3, continues to lower crustal depths with no correlation to the depth parameter from the linear relationship. This low heat production may be a minimum average granulite heat production and suggests that, in general, heat flow through the Moho is 8–10 mW m−2 lower than the reduced heat flow calculated from the heat flow-heat production regression.  相似文献   

19.
The Higo metamorphic terrane situated in west-central Kyushu island, southwest Japan, is composed of greenschist- to granulite-facies metamorphic rocks. The southern part of the metamorphic terrane consists mainly of garnet–biotite gneiss and garnet–cordierite–biotite gneiss, and orthopyroxene or cordierite-bearing S-type tonalite with subordinate amounts of hornblende gabbro. Rb–Sr, Sm–Nd and K–Ar isotopic ages for these rocks have been determined here. The garnet–biotite gneiss gives an Sm–Nd age of 227.1 ± 4.9 Ma and a Rb–Sr age of 101.0 ± 1.0 Ma. The hornblende gabbro has an Sm–Nd age of 257.9 ± 2.5 Ma and a K–Ar age of 103.4 ± 1.1 Ma. These age differences of the same samples are due to the difference in the closure temperature for each system and minerals. The garnet-cordierite–biotite gneiss contains coarse-grained garnet with a zonal structure conspicuously distinguished in color difference (core: dark red; rim: pink). Sm–Nd internal isochrons of the garnet core and the rim give ages of 278.8 ± 4.9 Ma (initial 143Nd/144Nd ratio = 0.512311 ± 0.000005) and 226.1 ± 28.4 Ma (0.512277 ± 0.000038), respectively. These ages are close to formation of the garnet core and the rim. It has been previously suggested that the Higo metamorphic terrane belongs to the Ryoke metamorphic belt. But Sr and Nd isotopic features of the rocks from the former are different from those of the Ryoke metamorphic rocks, and are similar to those of the granulite xenoliths contained in the Ryoke younger granite.  相似文献   

20.
This study is concerned with the tectono‐thermal history of the Kathmandu nappe and the underlying Lesser Himalayan sediments (LHS) that are distributed in eastern Nepal. We carried out zircon fission‐track(ZFT) dating and obtained 16 ZFT ages from the eastern extension of the Kathmandu nappe, the Higher Himalayan Crystalline, Kuncha nappe, and the Main Central Thrust (MCT) zone. The ZFT ages of the frontal part of the Kathmandu nappe range from 13.0 ±0.8 Ma to 10.7 ±0.7 Ma and exhibit a northward‐younging tendency. These Middle Miocene ZFT ages indicate that the frontal part of the Kathmandu nappe remained at a temperature above 240 °C until the termination of its southward emplacement at 12–11 Ma. The ZFT ages of the LHS range from 11.1 ±0.9 Ma in the southern part of the Okhaldhunga Window to 2.4 ±0.3 Ma of the augen gneiss in the northern margin and also exhibit a northward‐younging age distribution. The ZFT ages show the northward‐younging linear distribution pattern (?0.16 Ma/km) along the across‐strikesection from the frontal part of the Kathmandu nappe to the root zone, without a significant age gap. This distribution pattern indicates that the Kathmandu nappe, the underlying MCT zone, and the Kuncha nappe cooled from the frontal zone to the root zone as a thermally united geologic body at a temperature below 240 °C. An older ZFT age (456.3 ±24.3 Ma), which was partially reset at the axial part of the Midland anticlinorium in the central part of the Okhaldhunga Window, was explained by downward heating from the “hot” Kathmandu nappe. The above evidence supported a model that southward emplacement of the hot Kathmandu nappe resulted in a thermal imprint on the upper part of the LHS; however, the lower part did not reach 240 °C.  相似文献   

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