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1.
We present Re–Os, Sm–Nd and Pb–Pb isotope and trace element data for the Konchozero sill, a layered mafic–ultramafic intrusion in the Early Proterozoic Onega plateau, one of the oldest continental flood basalt provinces on Earth. The Sm–Nd and Pb–Pb combined mineral and whole-rock isochron ages of 1988±34 and 1985±57 Ma for the sill coincide with the age of ferropicrites from Pechenga (the Kola Peninsula). The lithostratigraphic, chemical and isotope evidence suggest the derivation of Pechenga lavas and the Onega plateau volcanics from a single mantle plume. Peridotite and gabbro whole-rock samples, and primary ulvospinel and ilmenite mineral separates from the sill yield a Re–Os isochron with a slope corresponding to an age of 1969±18 Ma, γOs(T) =−0.61±5.9. This age is consistent with the other isotope data, and indicates the closed-system behavior of Re and Os in the rocks. The peridotites and ulvospinel have high Os concentrations (2.5–14 ppb) and low 187Re/188Os ratios (0.35–1.1), thus allowing a more accurate determination of the weighted average initial 187Os/188Os of 0.1144±0.0019 (2σpop), γOs(T) =+0.77±1.7. This value is lower than that determined by Walker et al. (Geochim. Cosmochim. Acta 61 (1997) 3145–3160) for the Pechenga lavas (γOs(T) =+6.1±0.7), and implies a substantial Os-isotope heterogeneity in this ancient plume. Compared to the Onega plateau primary basalt magma, Pechenga ferropicrites are relatively enriched in iron and Ni, have lower (Nb/Th)N ratios (2.1 vs 1.1) and less radiogenic Nd-isotope compositions (Nd(T) = +3.1 and +1.4, respectively), but share similar low-radiogenic Pb-isotope characteristics (μ1=8.57 and 8.60). Incorporation of small amounts (1.5%) of outer core material into the hotter central part of the plume and subsequent contamination of the Pechenga ferropicritic magmas with the 2.9 Ga Belomorian gneisses can explain the observed chemical and isotope variations in the two provinces provided that the core had <0.25 ppm of Pb.  相似文献   

2.
In order to understand the evolution of the crust-mantle system, it is important to recognize the role played by the recycling of continental crust. Crustal recycling can be considered as two fundamentally distinct processes: 1) intracrustal recycling and 2) crust-mantle recycling. Intracrustal recycling is the turnover of crustal material by processes taking place wholly within the crust and includes most sedimentary recycling, isotopic resetting (metamorphism), intracrustal melting and assimilation. Crust-mantle recycling is the transfer of crustal material to the mantle with possible subsequent return to the crust. Intracrustal recycling is important in interpreting secular changes in sediment composition through time. It also explains differences found in crustal area-age patterns measured by different isotopic systems and may also play a role in modeling crustal growth curves based on Nd-model ages. Crustal-mantle recycling, for the most part, is a subduction process and may be considered on three levels. The first is recycling with only short periods of time in the mantle (<10 m.y.). This may be important in explaining the origin of island-arc and related igneous rocks; there is growing agreement that 1–3% recycled sediment is involved in their origin. Components of recycled crustal material, with long-term storage (up to 2.5 b.y.) in the mantle as distinct entities, has been suggested for the origin of ocean island and ultrapotassic volcanics but there is considerably less agreement on this interpretation. A third proposal calls for the return of crustal material to the mantle with efficient remixing in order to swamp the geochemical and isotopic signature of the recycled component by the mantle. This type of recycling is required for steady-state models of crustal evolution where the mass of the continents remains constant over geological time. It is unlikely if crust-mantle recycling has exceeded 0.75 km3/yr over the past 1–2 Ga.Good evidence exists that selective recycling is an important process. Sedimentary rocks preserved in different tectonic settings are apparently recycled at different rates, resulting in a bias in the sediment types preserved in the geologic record. Selective recycling has important implications for the interpretation of Nd model ages of old sedimentary rocks and in the analysis of accreted terranes. Although there is evidence that continental crust was formed prior to 3.8 Ga, the oldest preserved rocks do not exceed this age. It is likely that the intense meteorite bombardment, which affected the earth during the period 4.56–3.8 Ga, coupled with rapid mantle convection, which resulted from greater heat production, caused the destruction and probable recycling into the mantle of any early formed crust.Although crust-mantle recycling is seen as a viable process, it is concluded that crustal growth has exceeded crust-mantle recycling since at least 3.8 Ga. Intracrustal recycling has not been given adequate consideration in models of crustal growth based on isotopic data (particularly Nd model ages). It is concluded that crustal growth curves based on Nd model ages, while vastly superior to those based on K/Ar or Rb/Sr, tend to underestimate the volume of old crust, due to crust-mantle and/or intracrustal recycling.  相似文献   

3.
Abstract Rb–Sr and Sm–Nd isochron ages were determined for whole rocks and mineral separates of hornblende‐gabbros and related metadiabases and quartz‐diorite from Shodoshima, Awashima and Kajishima islands in the Ryoke plutono‐metamorphic belt of the Setouchi area, Southwest Japan. The Rb–Sr and Sm–Nd whole‐rock‐mineral isochron ages for six samples range from 75 to 110 Ma and 200–220 Ma, respectively. The former ages are comparable with the Rb–Sr whole‐rock isochron ages reported from neighboring Ryoke granitic rocks and are thus due to thermal metamorphism caused by the granitic intrusions. On the contrary, the older ages suggest the time of formation of the gabbroic and related rocks. The initial 87Sr/86Sr and 143Nd/144Nd ratios of the gabbroic rocks (0.7070–0.7078 and 0.51217–0.51231 at 210 Ma, respectively) are comparable with those of neighboring late Cretaceous granites and lower crustal granulite xenoliths from Cenozoic andesites in this region. Because the gabbroic rocks are considered to be fragments of the lower crustal materials interlayered in the granulitic lower crust, their isotopic signature has been inherited from an enriched mantle source or, less likely, acquired through interaction with the lower crustal materials. The Sr and Nd isotopic and petrologic evidence leads to a plausible conclusion that the gabbroic rocks have formed as cumulates from hydrous mafic magmas of light rare earth element‐rich (Sm/Nd < 0.233) and enriched isotopic (?Sr > 0 and ?Nd < 0) signature, which possibly generated around 220–200 Ma by partial melting of an upper mantle. We further conclude that they are fragments of refractory material from the lower crust caught up as xenoblocks by granitic magmas, the latter having been generated by partial melting of granulitic lower crustal material around 100 Ma.  相似文献   

4.
Abstract Rb–Sr and K–Ar chronological studies were carried out on granitic and metamorphic rocks in the Ina, Awaji Island and eastern Sanuki districts, Southwest Japan to investigate the timing of intrusion of the granitoids in the Ryoke belt. Intrusions of 'younger' Ryoke granitic magmas took place in the Ina district between 120 Ma and 70 Ma, and cooling began immediately after the emplacement of the youngest granitic bodies. Igneous activity in Awaji Island was initiated at 100 Ma and continued to 75 Ma. Along-arc variations of Rb–Sr whole-rock isochron ages suggest that magmatism began everywhere in the Ryoke and San-yo belts at almost the same time ( ca 120 Ma). The last magmatism took place in the eastern part of both belts. Rb–Sr and K–Ar mineral ages for the granitoids young eastwards. The age data suggest that the Ryoke belt was uplifted just after the termination of igneous activity. Initial Sr and Nd isotopic ratios for the Ryoke granitoids indicate that most were derived from magmas produced in the lower crust and/or upper mantle with uniform Sr and Nd isotopic compositions. Several granitoids, however, exhibit evidence of assimilation of Ryoke metamorphic rocks or older Precambrian crustal rocks beneath the Ryoke belt.  相似文献   

5.
Ion microprobe zircon ages, a Nd model age and RbSr whole-rock dates are reported from the high-grade gneiss terrain at Sabaloka on the River Nile north of Khartoum, formally considered to be part of the Archaean/early Proterozoic Nile craton. The granulites, which are of both sedimentary and igneous derivation, occur as remnants in migmatites. Detrital zircon ages range from ≈ 1000 to ≈ 2650 Ma and prove the existence of Archaean to late Proterozoic continental crust in the sedimentary source region. The Nd model age for one sedimentary granulite is between 1.26 (TCHUR) and 1.70 (TDM) Ga and provides a mean crustal residence age for the sedimentary precursor. Igneous zircons in enderbitic gneiss crystallized at 719 ± 81 Ma ago, an age that also corresponds to severe Pb loss in the detrital zircons and which probably reflects the granulite event at Sabaloka. The RbSr data indicate isotopic homogenization at about 700 Ma ago in the granulites and severe post-granulite disturbance at ≈ 570 Ma in the migmatites. We associate this disturbance with hydration, retrograde metamorphism and anatexis that produced undeformed granites ≈ 540 Ma ago. The ≈ 700 Ma granulite event at Sabaloka suggests that this part of the Sudan belongs to the Pan-African Mozambique belt while the ancient Nile craton lay farther west. The gneisses studied here may represent the infrastructure of the ancient African continental margin onto which the juvenile arc assemblage of the Arabian-Nubian shield was accreted during intense horizontal shortening and crustal interstacking of a major collision event.  相似文献   

6.
Up to now the age of granulite gneisses intruded by the Zabargad mantle diapir has been an unsolved problem. These gneisses may represent either a part of the adjacent continental crust primarily differentiated during the Pan African orogeny, or new crust composed of Miocene clastic sediments deposited in a developing rift, crosscut by a diabase dike swarm and gabbroic intrusions, and finally metamorphosed and deformed by the mantle diapir. Previous geochronological results obtained on Zabargad island and Al Lith and Tihama-Asir complexes (Saudi Arabia) suggest an Early Miocene age of emplacement for the Zabargad mantle diapir during the early opening of the Red Sea rift. In contrast, SmNd and RbSr internal isochrons yield Pan African dates for felsic and basic granulites collected 500–600 m from the contact zone with the peridotites. Devoid of evidence for retrograde metamorphic, minerals from a felsic granulite provide well-defined RbSr and SmNd dates of 655 ± 8 and 699 ± 34 Ma for the HP-HT metamorphic event (10 kbar, 850°C). The thermal event related to the diapir emplacement is not recorded in the SmNd and RbSr systems of the studied gneisses; in contrast, the development of a retrograde amphibolite metamorphic paragenesis strongly disturbed the RbSr isotopic system of the mafic granulite. The initial143Nd/144Nd ratio of the felsic granulite is higher than the contemporaneous value for CHUR and is in agreement with other Nd isotopic data for samples of upper crust from the Arabian shield. This result suggests that source rocks of the felsic granulite were derived at 1.0 to 1.2 Ga from either an average MORB-type mantle or a local 2.2 Ga LREE-depleted mantle. Zabargad gneisses represent a part of the disrupted lower continental crust of the Pan African Afro-Arabian shield. During early stages of the Red Sea rifting in the Miocene, these Precambrian granulites were intruded and dragged upwards by a rising peridotite diapir.  相似文献   

7.
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   

8.
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9.
云南地壳和上地幔的岩石学结构   总被引：4，自引：0，他引：4       下载免费PDF全文
通过对地表出露变质岩、深部地震测深资料和高温高压岩石波速测试资料的综合分析 ,研究了云南地壳和上地幔岩石组成。结果表明 ,云南上、中、下地壳分别由绿片岩相 (顶部为沉积层 )、角闪岩相和麻粒岩相变质岩组成或分别由与之相当的花岗岩类、闪长岩类、辉长岩类组成 ,部分地区地壳底部有镁铁质榴辉岩存在。上地幔由橄榄岩组成 ,部分地区 (兰坪思茅坳陷和滇中坳陷 )壳幔过渡带可能由镁铁质榴辉岩和橄榄岩组成  相似文献   

10.
Liu  Yican  Xu  Shutong  Li  Shuguang  Jiang  Laili  Wu  Weiping  Chen  Guanbao  Su  Wen 《中国科学:地球科学(英文版)》2000,43(1):178-188

The petrologic geochemical and Sr−Nd isotopic compositions of the eclogites from the mafic-ultramafic rock belt (MUMRB) in the Northern Dabie Mountains indicate that: (1) the protoliths of most of eclogites are tholeitic basalt and a few may be gabbro, and most of them produced from the Yangtze subducted continental crust (lower crust and formed during the deep subduction) and a part may be from paleo-oceanic relics between the Yangtze and North China continental plates; (2) their positive Nb anomalies and related trace element characteristics show that they did not form in the island-arc setting; (3) the metamorphosed MUMRB with eclogite and meta-peridotite blocks along the southern part of the Mozitan-Xiaotian fault zone may represent the suture zone produced during the collision between the Yangtze and North China continental plates, which included the Yangtze subducted continental crust and paleo-oceanic relics.

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11.
Granitoids in the Hida region of Japan encompass two main rock types: younger type‐1 granites and older type‐2 granites. Sensitive high mass‐resolution ion microprobe (SHRIMP) U–Pb zircon dating of older type‐2 granites collected from the Tateyama area show similar ages of 245 ± 2 Ma and 248 ± 5 Ma for two gneissose granites, while a significantly younger intrusion age of 197 ± 3 Ma was determined for the younger type‐1 granites collected from the Hayatsukigawa River which belongs to the Okumayama pluton. A felsic gneiss sample (07HI‐3) collected from the right bank of the Hayatsukigawa River yielded multiple complex ages at 330 ± 6 Ma, indicating the timing of the Hida regional tectono‐thermal events that formed the Hida gneisses; 243 ± 8 Ma, representing the timing of intrusion of the augen granite; and 220 Ma, indicating the timing of regional dextral ductile shearing that caused a repeated recrystallization of metamorphic rocks in the study area. Considering the geochronological data, the rock types and assemblages, basement, and Sr–Nd isotopic constraints, we propose that the Hida Belt separated from the Jiamushi massif, which is located in the eastern margin of the Central Asian Orogenic Belt.  相似文献   

12.
The petrologic geochemical and Sr?Nd isotopic compositions of the eclogites from the mafic-ultramafic rock belt (MUMRB) in the Northern Dabie Mountains indicate that: (1) the protoliths of most of eclogites are tholeitic basalt and a few may be gabbro, and most of them produced from the Yangtze subducted continental crust (lower crust and formed during the deep subduction) and a part may be from paleo-oceanic relics between the Yangtze and North China continental plates; (2) their positive Nb anomalies and related trace element characteristics show that they did not form in the island-arc setting; (3) the metamorphosed MUMRB with eclogite and meta-peridotite blocks along the southern part of the Mozitan-Xiaotian fault zone may represent the suture zone produced during the collision between the Yangtze and North China continental plates, which included the Yangtze subducted continental crust and paleo-oceanic relics.  相似文献   

13.
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   

14.
The isotopic compositions of Sr, Nd and Pb in leucogranites which are intercorrelated (Bernard-Griffiths et al., 1985 [1]) may be explained by the mixing of ancient basement (1800 Ma) with juvenile crust (late Precambrian or early Palaeozoic). This hypothesis does not involve the existence of Mid-Proterozoic crust, as apparently indicated by theTDM model ages of the leucogranites (ranging between 1600 and 1100 Ma). The Nd isotopes reveal the crustal reworking while Sr isotopes mainly record juvenile crust formation. This paradox is explained by the geochemical heterogeneity of the sources involved.  相似文献   

15.
The granulite facies assemblages of the anorthositic rocks of the Bergen Arcs (stable at 800–900°C and 10 kbar) have been transformed to eclogite facies assemblages (stable at 700–750°C and 16–19 kbar) in the vicinity of Caledonian shear zones. This section of the root zone of the Caledonian mountain chain reveals a deep polymetamorphic crust where Precambrian granulites (mean density 3.02 g/cm3) and Caledonian eclogites (mean density 3.19 g/cm3) alternate on a scale of meters over a minimum area of 3 × 12 km. Detailed mapping of three localities shows that eclogites account for up to 30–45% of the rock volume. The stabilitization of the eclogite mineralogy is controlled by fluids penetrating these deep crustal shear zones. The eclogitization is independent of preexisting compositional variation in this anorthosite-norite complex. The Bergen Arcs example suggests that the amount of eclogite versus granulites in the lowermost crust is a function of deformation and fluid access, rather than being controlled byT, P and rock composition alone. These relationships may explain the gradual increase in seismic velocity observed in some deep crustal sections and also the complex reflection pattern obtained from the lowermost crust in many areas.  相似文献   

16.
The effect that recent relief changes may have on the distribution of rock ages with elevation is investigated for a range of thermochronometers. From the solution of the heat transport equation in a crustal block undergoing uplift and surface erosion, the temperature history of rock particles that are exhumed at the Earth’s surface today is computed. These T-t paths are then used to calculate apparent isotopic ages for the (U–Th)/He system in apatite, characterized by a low (≈70°C) closure temperature. The results show that recent relief changes strongly affect the distribution of ages with elevation (notably the slope of the age–elevation relationship). The calculations presented here predict that, in most situations, regions that have undergone a steady decrease in surface relief in the recent past should be characterized by an inverted age–elevation relationship, that is older ages should be found near valley bottoms and younger ages near summit tops. It is also shown how the wavelength of the topography, the geothermal gradient, the exhumation rate and the duration of the relief reduction event affect this result.  相似文献   

17.
The high-K calc-alkaline granitoids in the northern part of the Mandara Hills are part of the wellexposed post-collisional plutons in northeastern Nigeria.The calc-alkaline rock association consists of quartz monzodiorite,hornblende biotite granite,biotite granites and aplite which intruded the older basement consisting mainly of low-lying migmatitic gneisses and amphibolites during the Neoproterozoic Pan-African Orogeny.Petrological and geochemical studies have revealed the presence of hornblende,iron oxide,and metaluminous to slightly peraluminous characteristics in the granitoids which is typical of I-type granite.The granitoids are also depleted in some high field strength elements(e.g.Nb and Ta) as well as Ti.Plots of Mg#versus SiO_2 indicate that the granite was derived from partial melting of crustal sources.Lithospheric delamination at the waning stage of the PanAfrican Orogeny possibly triggered upwelling of hot mafic magma from the mantle which underplated the lower crust.This,in turn,caused partial melting and magma generation at the lower to middle-crustal level.However,the peculiar geochemical characteristics of the quartz monzodiorite especially the enrichment in compatible elements such as MgO,Cr,and Ni,as well as LILE element(e.g.K,Ce,Cs,Ba,and Sr),signify that the rock formed from an enriched upper mantle source.The emplacement of high-K granites in the Madara Hill,therefore,marked an important episode of crustal reworking during the Neoproterozoic.However,further isotopic work is needed to confirm this model.  相似文献   

18.
The Nd isotopic systematics of the sources of crustal granitic rocks are used to estimate the Sm/Nd ratio of the continental crust as a function of its age. It is found that the Sm/Nd value of granite magma sources in continental crust increases from about 0.47 to 0.64 times the chondritic value with decreasing age from the Early Archean to the Late Proterozoic. This trend is opposite to that inferred for the crust from rare earth element patterns in sedimentary rocks. The observed trend may apply strictly only to the felsic portions of the crust, but unless older crust contains a much higher percentage of mafic material than young crust (50% versus 0%), the direction of the trend also applies to the bulk crust. Because some portion of the earth's oldest crust has probably been destroyed by subsequent processes, the trend could conceivably be the result of preservational bias rather than a real shift in crustal composition with time. The isotopic data, combined with the crustal age distribution, indicate that the Sm/Nd value of the bulk continental crust is not lower than 0.60 times the chondritic value. This limit and estimates of the Nd concentration of the crust are consistent with the mass balance that equates the Nd in the continents to that missing from the upper mantle down to a depth of about 700 km.  相似文献   

19.
Poly-deformed and poly-metamorphosed glaucophane-eclogite mega-boudins beneath the Samail Ophiolite, Oman record an early subduction-related high-P metamorphism as well as subsequent overprinting deformation and metamorphism related to exhumation. Previously published Rb/Sr ages of 78 Ma and 40Ar/39Ar ages of 82-79 Ma record the major NE-directed shearing event that partially exhumed the eclogites to a shallower crustal level. New Sm/Nd garnet-garnet leachate-whole rock isochron data from garnet-bearing eclogite assemblages in the As Sifah subwindow in NE Oman are 110±9 Ma (DG02-87D); 5-point isochron) and 109±13 Ma (DG02-86E; 3-point isochron). On the basis of microfabric and field structural relationships these ages are interpreted to reflect the timing of prograde, peak high-P metamorphism in the rocks structurally beneath the Samail Ophiolite. This metamorphism clearly predates the age of formation of the obducted Samail oceanic lithosphere (97-94 Ma) as well as the subsequent obduction onto the margin (80-70 Ma). A U-Pb SHRIMP zircon age from small (<200 μm in length) zircons with herring-bone textured zoning from DG02-87D indicate that rapid zircon growth associated with high-Si phengites occurred at 82±1 Ma. Zircon growth is possibly related to liberation of Zr on garnet breakdown during decompression metamorphism under high-P conditions with exhumation. These data require that crustal stacking models attendant with ophiolite obduction are inappropriate to explain the Oman high-P metamorphism.  相似文献   

20.
Metasedimentary and metavolcanic rocks from the Archaean of West Greenland have been examined for evidence of crustal components greater than 3.8 Ga in age and for their compatibility with the presently adopted bulk Earth Sm-Nd parameters. Sm-Nd isotopic data have been obtained for the garbenschiefer metagabbro unit, metasediments from the Isua supracrustal belt, gneisses interior to the Isua belt and metasediments from the Malene supracrustal belt.Using estimates of emplacement age (T) of between 3.77 and 3.67 Ga for the parental volcanics to the garbenschiefer unit, initial143Nd/144Nd ratios yield positiveεNdT values between +1.0 and +3.1 (relative to the CHUR parameters) for seven out of eight samples. Model Sm-Nd ages for the Isua gneisses and metasediments are only compatible with their estimated stratigraphic ages if their sources were ca.+2εNd relative to CHUR at those times. Similarly, model Sm-Nd ages for the Malene samples are only compatible with stratigraphic age constraints when based on a source evolution with positiveεNdT. Implications of these results for the early development of the Earth's mantle are discussed.  相似文献   

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