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1.
M.E. Wilks 《Earth and Planetary Science Letters》1988,87(1-2)
Present crustal evolution models fail to account for the generation of the large volumes of continental crust in the required time intervals. If the oceanic crust was appreciably thicker in the Archaean, as geothermal models would indicate, then oceanic crustal collision on the scale of the present-day Himalayan continental collision zone may have been a frequent occurrence in the Archaean, resulting in extensive partial melting of the hydrous underthrust oceanic crust to produce voluminous tonalite melts leaving a depleted stabilised basic residuum. 相似文献
2.
There is a linear relationship between the spacing of Pliocene-Pleistocene volcanoes and the thickness of the lithosphere and attenuated crust in the East African rift valley. Assuming that the physical-chemical properties of the Archaean and Cenozoic lithosphere and crust were broadly similar, we use the spacing of volcanic centres in the Abitibi greenstone belt of southern Canada to determine lithospheric and crustal thickness in the Archaean. The abitibi volcanoes have been deformed and so have elliptical cross-sections. In order to arrive at their original form we have removed the effects of tectonic strain by two alternative mechanisms of pure and simple shear which give comparable results. A mean original volcano spacing of 84–88 km suggests that the lithsophere was 80–90 km thick and that the crust was probably 35–45 km thick in this greenstone belt about 2700 m.y. ago. The crustal values are comparable with those determined by geochemical parameters and are consistent with the suggestion that greenstone belts formed in extensional marginal basins between crustal-thickened continental masses, deep sections of which are now seen in Archaean high-grade regions. 相似文献
3.
New chemical and isotopic data permit the recognition of a cryptic suture zone between two Archaean continental masses within the Nagssugtoqidian mobile belt of West Greenland. This discovery has important implications for Precambrian crustal evolution: suture zones may not always be identifiable from geological field observations, with the consequence that mobile belts in which undetected sutures exist may be mis-identified as ensialic, and thought to require special non-plate tectonic models to account for their development.The Nagssugtoqidian belt consists mainly of Archaean gneisses reworked during the Proterozoic, with metamorphic grade and degree of isotopic disturbance increasing towards the centre of the belt. At the centre of the belt the Nagssugtoqidian includes metasediments and calc-alkaline volcanic and plutonic rocks of Proterozoic age, almost always strongly deformed and metamorphosed. From isotopic evidence (Sri ca. 0.703; model μ1 values ca. 8.0; initial εNd ca. 0) it is clear that the Proterozoic igneous rocks do not include any significant contributions derived from the Archaean crust, and the chemistry of the rocks, together with the isotope data, suggests that they were formed at a destructive plate margin. The Proterozoic rocks are found in a narrow zone (up to 30 km wide) between the Archaean gneisses to the north and south of Nordre Strømfjord, and are interpreted as reflecting the existence of a suture between two Archaean continental blocks. Zircon UPb data and other isotope evidence show that subduction started before ca. 1920 Ma ago, and lasted until ca. 1850 Ma when collision occurred, with consequent crustal thickening, high-grade metamorphism and local anatexis. Given the time-span for the operation of subduction, the existence of a wide Nagssugtoqidian ocean can be inferred, even for slow rates of plate motion.The Proterozoic and Archaean gneisses in the Nagssugtoqidian belt are very similar lithologically and chemically, and it has only been possible to distinguish between them using isotopic criteria. Suture zones of this kind are very difficult to detect, and may be present elsewhere within the reworked Archaean terrains of northern Greenland and Canada. 相似文献
4.
N.B.W. Harris C.J. Hawkesworth P. Van Calsteren F. McDermott 《Earth and Planetary Science Letters》1987,83(1-4)
Nd isotopic data from the Zimbabwe and Kaapvaal cratons and the Limpopo, Kalahari, Namaqualand and Damara mobile belts imply that over 50% of present-day continental crust in this region had separated from the mantle by the end of the Archaean and that< 10% of continental crust of southern Africa has formed in the last 1.0 Ga. Such a growth rate implies that average erosion rates through geological time were high and that evolution of continental crust has been dominated by crustal growth prior to 1.4 Ga, and crustal reworking since that time. The evolution of average crust is not represented directly by clastic sediment samples but may be determined from sediment analyses if both the time of orogeneses and the average erosion rate are known. Both trace element data from southern Africa granitoids and the high erosion rates implied by the isotopic study suggest that growth of continental crust in the Archaean was by underplating rather than lateral accretion, but arc accretion was the dominant mechanism after 2.0 Ga. 相似文献
5.
The geochemistry of Lewisian amphibolite-facies gneisses from northwest Scotland is described with particular reference to the rare earth elements (REE) and compared with the geochemistry of Lewisian granulite-facies gneisses. The results show that there are no significant differences between “Laxfordian” amphibolite-facies and “Scourian” granulite-facies gneisses in terms of REE and other immobile trace elements (at equivalent silica levels), although the mobile radioactive heat-producing elements, K, Rb, Th, U, are significantly lower in the granulites. In both types the basic gneisses have moderately fractionated REE patterns while the intermediate and acid gneisses have strongly fractionated REE patterns with low heavy REE abundances and decreasing levels of total REE with increasing SiO2. The most silicic gneisses develop large positive europium anomalies.These gross chemical similarities between gneisses from intermediate (amphibolite-facies) and lower (granulite-facies) crustal levels constrain models for the evolution of the Archaean crust. The depletion of K, Rb, Cs, Th and U in granulites, but not other incompatible trace elements cannot be explained by magmatic processes. The positive Eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation. Fractionation of radioactive heat-producing elements from other trace elements is a result of granulite-facies metamorphism with these elements being removed by an active fluid phase. The apparent lack of partial melting in lower crustal granulites suggests a model for Archaean crustal growth largely through underplating by primary tonalitic magmas. 相似文献
6.
Estimates of the chemical composition of the Archaean mantle, derived from elemental abundance ratios in komatiites combined with ultramafic xenolith data, support a model of a multistage heterogeneous accretion history of the Earth and synchronous core formation, 4.6 Ga ago.Most refractory lithophile element abundance ratios in komatiites and xenoliths are close to chondritic except for V/Ti and Ca/Al. Depletion of vanadium is likely due to its partial incorporation into the iron core; whereas fractionation of Ca/Al observed in Archaean Al-undepleted komatiites (1.20 times chondrites) and in some modern fertile spinel lherzolite xenoliths (1.15 times chondrites) could be due to small amounts of garnet (rich in Al but poor in Ca) segregation into the lower mantle during partial or complete melting of the upper mantle in the very early history of the Earth. However, this process may have had only a small effect on the overall chemical composition of the upper mantle.Simultaneous occurrence of early Archaean Al-undepleted (Al/Ti chondrites) and Al-depleted (Al/Ti 0.5 chondrites, and depletion of Sc and heavy REE) peridotitic komatiites in the Barberton area, S. Africa, and late Archaean Newton Township, Canada, argue against derivation of peridotitic komatiites from a circum-global magma ocean. Garnet separation from a mantle diapir which intersects the solidus at great depth ( 200 km) in a hotter early Archaean mantle could explain the chemical characteristics of Al-depleted komatiites. Alternatively, these two types of komatiites could have been derived from different layers in a fractionated mantle. A limited amount of Hf isotope data for Archaean komatiites seems to suggest that both mechanisms are important. This chemically and minerallogically layered mantle, if it existed, was homogenized by mantle convection after early Archaean times.Constant P2O5/TiO2, Ni/Mg, Co/Mg, Fe/Mg ratios (siderophile/lithophile) and PGE abundances, estimated for the mantle sources of komatiites from Archaean to modern times, strongly argue against continuous growth of the Earth's core since the early Archaean.Extensive crustal contamination might have been involved in the generation of Archaean-early Proterozoic siliceous high magnesian basalts with “boninite affinity”. However, involvement of chemically modified ancient continental lithosphere may also be important in the generation of these basalts. 相似文献
7.
The pre-3100 m.y. old Ameralik dykes from West Greenland show a range in primary composition from primitive low-K, low-Ti tholeiites virtually identical in composition to ridge basalts of modern ocean crust, to more differentiated basaltic rocks similar to some present-day continental tholeiites. Primary variations are distinguished from secondary metasomatism using REE patterns, Ni, Sr, Ti and Zr contents and Mg number as a guide to the stage of differentiation reached by a particular sample and comparing this to the amount of alkalis present. The chemistry of the dykes is compared to that of metabasalts from Archaean greenstone belts and the use of chemistry alone to distinguish the crustal environment under which Archaean basic rocks were formed is questioned. 相似文献
8.
Ion probe UPb age determinations on zircons from two samples of metasediment belonging to the Malene supracrustals of southern West Greenland closely constrain the age of sedimentation, between the youngest age obtained from detrital material and the age of metamorphic overgrowth. For both samples, older and younger limits of ca. 2900 Ma and ca. 2650 Ma, respectively, are indicated. Some of the detrital zircons are best interpreted as derived from their source rock after the regional high-grade metamorphism at ca. 2800 Ma: if so, the older limit of the age of sedimentation is younger than 2800 Ma. The hypothesis that all Malene supracrustal rocks pre-date the middle to late Archaean Nuˆk gneisses is no longer valid. This has major implications for interpretations of the late Archaean crustal evolution of western Greenland: the period between 2800 and 2500 Ma was characterised by major tectonic activity and metamorphism. 相似文献
9.
Geologic discontinuities across the Cheyenne Belt of southeastern Wyoming have led to interpretations that this boundary is a major crustal suture separating the Archaean Wyoming Province to the north from accreted Proterozoic island arc terrains to the south. Gravity profiles across the Cheyenne Belt in three Precambrian-cored Laramide uplifts show a north to south decrease in gravity values of 50–100 mgal. These data indicate that the Proterozoic crust is more felsic (less dense) and/or thicker than Archaean crust. Seismic refraction data show thicker crust (48–54 km) in Colorado than in Wyoming (37–41 km). We model the gravity profiles in two ways: 1) thicker crust to the south and a south-dipping ramp in the Moho beneath and just south of the Cheyenne Belt; 2) thicker crust to the south combined with a mid-crustal density decrease of about 0.05 g/cm3. Differences in crustal thickness may have originated 1700 Ma ago because: 1) the gravity gradient is spatially related to the Cheyenne Belt which has been immobile since about 1650 Ma ago; 2) the N-S gradient is perpendicular to the trend of gravity gradients associated with local Laramide uplifs and sub-perpendicular to regional long-wavelength Laramide gradients and is therefore probably not a Laramide feature. Thus, gravity data support the interpretation that the Cheyenne Belt is a Proterozoic suture zone separating terrains of different crustal structure. The gravity “signature” of the Cheyenne Belt is different from “S”-shaped gravity anomalies associated with Proterozoic sutures of the Canadian Shield which suggests fundamental differences between continent-continent and island arc-continent collisional processes. 相似文献
10.
A. Krner R.J. Stern A.S. Dawoud W. Compston T. Reischmann 《Earth and Planetary Science Letters》1987,85(1-3)
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
11.
P Bormann P Bankwitz E Bankwitz V Damm E Hurtig H Kmpf M Menning H.-J Paech U Schfer W Stackebrandt 《Journal of Geodynamics》1986,6(1-4)
On the basis of geological and geophysical data, a contribution is made to understanding the structure and evolution of the Earth's crust across the Princess Astrid Coast, East Antarctica, especially in the area of the Schirmacher Oasis and the Wohlthat Massif. The crustal material is mostly composed of medium- to high-grade metamorphics which underwent three important tectogenic events (close of the Archaean, within the Proterozoic and at the close of the Precambrian). The intrusive age of the Eliseev Anorthosite Massif could be determined (2,400 Ma). The dominant structural features in the Schirmacher Oasis show a strike of some 40° to 60° and indicate a NNE-SSW oriented compressional stress regime. It is probably connected with the last major thermotectonic event some 400 to 600 Ma ago. The Phanerozoic history is controlled by the younger Gondwana break-up and N-S oriented tension.The continental crust as a whole is of relatively acid composition. This is also reflected in the negative regional magnetic anomaly. Probably, its origin is linked to late Precambrian thermotectonic activation. A typical feature is the crustal thinning to the north (from some 50 to 20 km) in connection with deep-reaching E-W crustal faults indicating northward down-faulted blocks. 相似文献
12.
New rare earth element (REE) data for Archaean basalts and spinifex-textured peridotites (STP) show a range of La/Sm ratios (chondrite-normalized) from 0.36 to 3.5, with the bulk of the data in the range 0.7–1.3. This supports the hypothesis, based on Sr isotope initial ratios, that the Archaean mantle was chemically heterogeneous. We suggest that the bulk mantle source for Archaean basaltic magmas was close to an undepleted earth material. An average chemical composition of the Archaean mantle is estimated using chemical regularities observed in Archaean STP and high-magnesian basalts. TiO2 and MgO data show an inverse correlation which intersects the MgO axis at about 50% MgO (Fo92). TiO2 abundance in the mantle source is measured on this plot by assigning anMgO= 38% for the mantle. Concentrations of other elements are also estimated and these data are then used to obtain a composition for the bulk earth. We suggest an earth model with about 1.35 times ordinary chondrite abundances of refractory lithophile elements and about 0.2 times carbonaceous type 1 chondrite abundances of moderately volatile elements (such as Na, Rb, K, Mn). P shows severe depletion in the model earth relative to carbonaceous chondrites, a feature either due to volatilization or core formation (preferred). Our data support the hypothesis of Ringwood that the source material for the earth is a carbonaceous chondrite-like material.The generation of mid-ocean ridge basalts (MORB) is examined in the light of the model earth composition and Al2O3/TiO2, CaO/TiO2 ratios. It is suggested that for primitive basalts, these values can be used to predict the residual phases in their source. Comparison of chemical characteristics of inferred sources for 2.7-b.y. Archaean basalts and modern “normal” MORB indicates that the MORB source is severely depleted in highly incompatible elements such as Cs, Ba, Rb, U, Th, K, La and Nb, but has comparable abundances of less incompatible elements such as Ti, Zr, Y, Yb. The cause of the depletion in the MORB source is examined in terms of crust formation and extraction of silica-undersaturated melts. The latter seems to be a more likely explanation, since the degree of enrichment of highly incompatible elements in the crust only accounts for up to 40% of their abundances in the bulk earth and cannot match the depletion pattern in normal MORB. A large volume of material, less depleted than the source for normal MORB must therefore exist in the mantle and can serve as the source for the ocean island basalts and “normal” MORB.Three different mantle evolution models are examined and each suggests that the mantle is stratified with respect to abundances of incompatible trace elements. We suggest that no satisfactory model is available to fully explain the spectrum of geochemical and geophysical data. In particular the Pb and Sr isotope data on oceanic basalts, the depletion patterns of MORB and the balance between lithophile abundances in the crust and mantle, are important geochemical constraints to mantle models. Further modelling of the mantle evolution will be dependent on firmer information on the role of subduction, mantle convection pattern, and basalt production through geologic time together with a better understanding of the nature of Archaean crustal genesis. 相似文献
13.
Mantle heat flow and thermal structure of the northern block of Southern Granulite Terrain, India 总被引：1，自引：0，他引：1
Continental shield regions are normally characterized by low-to-moderate mantle heat flow. Archaean Dharwar craton of the Indian continental shield also follows the similar global pattern. However, some recent studies have inferred significantly higher mantle heat flow for the Proterozoic northern block of Southern Granulite Terrain (SGT) in the immediate vicinity of the Dharwar craton by assuming that the radiogenic elements depleted exposed granulites constitute the 45-km-thick crust. In this study, we use four-layered model of the crustal structure revealed by integrated geophysical studies along a geo-transect in this region to estimate the mantle heat flow. The results indicate that: (i) the mantle heat flow of the northern block of SGT is 17 ± 2 mW/m2, supporting the global pattern, and (ii) the lateral variability of 10–12 mW/m2 in the surface heat flow within the block is of crustal origin. In terms of temperature, the Moho beneath the eastern Salem–Namakkal region appears to be at 80–100 °C higher temperature than that beneath the western Avinashi region. 相似文献
14.
Magma underplating and Hannuoba present crust-mantle transitional zone composition: Xenolith petrological and geochemical evidence 总被引：7，自引：0，他引：7
FAN Qicheng ZHANG Hongfu SUI Jianli ZHAI Mingguo SUN Qian & LI Ni . Institute of Geology Geophysics Chinese Academy of Sciences Beijing China . Institute of Geology China Seismological Bureau Beijing China 《中国科学D辑(英文版)》2005,48(8):1089-1105
Studies on the deep-seated xenoliths from global volcanoes reveal that the present petrological crust-mantle boundary between the lower crust and the upper mantle actually is a transitional layer from mainly mafic granulites to ultramafic spinel lher-zolites[1,2], i.e. a transitional zone distinctive from the seismological Moho[3]. Oceanic lithosphere crust- mantle transitional zone can be established from the study on the exposed ophiolites. However, as for the continental lithosphere, since … 相似文献
15.
The times of original fractionation of the Sm and Nd component of clastic sediments from a mantle source (≡ crustal residence age) have been estimated from Sm-Nd model ages calculated relative to a depleted mantle evolution. In this way the provenance and evolution of selected Precambrian and Phanerozoic sediments and metasediments from the British Isles have been examined. Whereas some Archaean and early Proterozoic sediments have Sm-Nd model ages that are close to their stratigraphic age, the Phanerozoic sediments analysed have model ages as much as 2.0 Ga in excess of their stratigraphic age.A more detailed study of Lower Palaeozoic sediments deposited on the northern margin of the Iapetus Ocean provides evidence for a marked change of provenance in the Ordovician after the deposition of the Dalradian Supergroup. A component with comparatively high143Nd/144Nd and Sm/Nd ratio (presumably basaltic) is present in the sediments throughout the accretionary prism. Crustal residence age estimates average about 1.5 Ga for both these Lower Palaeozoic sediments, and modern pelagic clays, and collectively fail to provide any evidence for significant continental growth during the Phanerozoic. 相似文献
16.
M.J. Bickle 《Earth and Planetary Science Letters》1986,80(3-4):314-324
The increased depth and volume of melting induced in a higher temperature Archaean mantle controls the stability of the lithosphere, heat loss rates and the thickness of the oceanic crust. The relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions. The density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200°C or more hotter than modern upper mantle. If Archaean continental crust formed initially above oceanic lithosphere, the compositional density gradients may be sufficient to preserve a thick Archaean continental lithosphere within which the Archaean age diamonds are preserved. The amount of heat advected by melts at mid-ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases. Archaean tectonics could have been dominated by spreading rates 2–3 times greater than now and with mantle temperatures between ca. 1600°C and 1800°C at the depth of the solidus. Mid-ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form, protected from copious volcanism and high mantle temperatures. 相似文献
17.
The lower portions of the volcanic sequence of some Archaean greenstone belts include members with crystallized from ultramafic liquids extruded at the earth's surface at 1600–1650°C. These liquids are interpreted as products of 60–80% melting of their mantle source composition which implies more catastrophic conditions of mantle melting than obtained in Palaeozoic, Mesozoic or Recent crust-mantle dynamics. Such conditions may be a consequence of major impacts on the surface of the primitive earth. It is suggested that the production of the lunar maria basins was accompanied by similar impacts on the earth and that such terrestrial maria played an important role in early stages of chemical differentiation of the crust and upper mantle. An hypothesis is presented in which some Archaean greenstone belts are interpreted as very large impact scars, initially filled with impact-triggered melts of ultramafic to mafic composition and thereafter evolving with further magmatism, deformation and metamorphism to the present Archaean greenstone belts. 相似文献
18.
The chronology and isotope geochemistry of a selection of Proterozoic Scourie dykes has been investigated in order to specify both their time of emplacement within the thermal history of the Archaean crust of N.W. Scotland, and to attempt to characterise the evolution of continental lithosphere. SmNd, RbSr and UPb isotope analyses are presented. Primary, major igneous minerals separated from four well preserved dykes yield SmNd ages of 2.031 ± 0.062Ga, 2.015 ± 0.042Ga, 1.982 ± 0.044Ga and 2.101 ± 0.078Ga, which are interpreted as crystallisation ages.The initial Nd isotope compositions in the dykes at their emplacement age of 2.0 Ga, range from +3.4 to −6.8, indicating the presence of an older lithospheric component. SmNd whole-rock isotope data for fifteen dykes, if interpreted to have age significance, yield an “age” of 3.05 ± 0.27 Ga. SmNd crustal residence ages for the same dykes average 2.95 Ga, which is interpreted as the time that small melts were added to the Lewisian lithosphere. The possibility that correlated147Sm/144Nd and143Nd/144Nd ratios are an artifact of mixing between depleted mantle melts generated at 2.0 Ga, and an older enriched lithospheric component is not eliminated by the data, but the relationship between 1/Nd and143Nd/144Nd ratios rules out any simple mixing. UPb isotope data for plagioclase feldspars and whole-rock samples of dykes provide useful estimates of initial Pb-isotope composition of the dykes at the time of their emplacement. Initial206Pb/204Pb and207Pb/204Pb ratios vary considerably and range from 13.98 to 15.78, and 14.72 to 15.56 respectively, and suggest that the UPb fractionation responsible must have occurred at least 2.5 Ga ago.The Scourie dykes have inherited a trace element enriched component from the Lewisian lithosphere, which has resided there since ca. 3 Ga ago. Whether the dykes inherited this material from the crust or the mantle portions of the lithosphere or both, it seems likely that small basaltic melts derived from asthenospheric mantle were ultimately responsible for the enrichment. The simplest view is that these small melt fractions had been resident in the mantle part of the Lewisian lithosphere. In this case the Archaean trace-element enrichment and element fractionation in the Lewisian lithospheric mantle sampled by the dykes was closely associated in time with the generation of the 2.9 Ga old crustal portion of the lithosphere [36,37]. 相似文献
19.
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
20.
W.L. Griffin V.R. McGregor A. Nutman P.N. Taylor D. Bridgwater 《Earth and Planetary Science Letters》1980,50(1):59-74
Relict high-pressure granulite-facies rocks have been found in the Ami?tsoq gneisses and inclusions of the older Akilia supracrustal association, on islands south of Godthåb. Only amphibolite-facies assemblages have been found in Ameralik dykes and younger rocks from this area. The Ami?tsoq gneisses are depleted in Rb and U relative to those of Ameralik and Isua. Well-fitted Pb/Pb and Rb-Sr isochrons on Ami?tsoq granulites indicate that this depletion, correlated with the granulite-facies metamorphism, occurred ca. 3600 Ma ago. Textural features suggest that the present cpx + opc + gnt + plag + qtz + hbl assemblages evolved from earlierintermediate-P assemblages (cpx + opx + plag), probably during cooling from the metamorphic peak. Re-equilibrium of olderintermediate-P assemblages in local environments of low ?H2O, during the ca. 2800-Ma metamorphism of the Malene supracrustals, is feasible but is considered unlikely. Either interpretation requires crustal thickness of at least 20 km and geothermal gradients of?30°C/km, ca.3600Ma ago. The higher heat production of early Archaean times was apparently dissipated through oceanic, rather than continental, areas. 相似文献