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1.
Nature and origin of eclogite xenoliths from kimberlites   总被引：16，自引：0，他引：16  
D.E. Jacob   《Lithos》2004,77(1-4):295-316
Eclogites from the Earth's mantle found in kimberlites provide important information on craton formation and ancient geodynamic processes because such eclogites are mostly Archean in age. They have equilibrated over a range of temperatures and pressures throughout the subcratonic mantle and some are diamond-bearing. Most mantle eclogites are bimineralic (omphacite and garnet) rarely with accessory rutiles. Contrary to their overall mineralogical simplicity, their broadly basaltic-picritic bulk compositions cover a large range and overlap with (but are not identical to) much younger lower grade eclogites from orogenic massifs. The majority of mantle eclogites have trace element geochemical features that require an origin from plagioclase-bearing protoliths and oxygen isotopic characteristics consistent with seawater alteration of oceanic crust. Therefore, most suites of eclogite xenoliths from kimberlites can be satisfactorily explained as samples of subducted oceanic crust. In contrast, eclogite xenoliths from Kuruman, South Africa and Koidu, Sierra Leone stem from protoliths that were picritic cumulates from intermediate pressures (1–2 Ga) and were subsequently transposed to higher pressures within the subcratonic mantle, consistent with craton growth via island arc collisions. None of the eclogite suites can be satisfactorily explained by an origin as high pressure cumulates from primary melts from garnet peridotite.  相似文献   

2.
We present results of geochemical studies and isotope dating of eclogites and associated rocks from the Kuru-Vaara quarry, Belomorian Belt, Northeastern Baltic Shield. The southern and northern eclogites are similar in geochemical features. Their protoliths were primitive, mainly high-Mg basalts of oceanic affinity derived from a primitive mantle source rather than from a depleted mantle source characteristic of modern MORB. The post-eclogitic intrusive rocks show obvious evidence for crustal contamination. The eclogite-hosting tonalitetrondhjemite-granodiorite (TTG) gneisses form a coherent series including high-Al and low-Al varieties. The trace element data show that the TTG series formed through the hydrous partial melting of the southern eclogites in the presence of garnet and amphibole in the field of the rutile stability (>15 kbar). Zircons from the southern eclogites exhibit features of their strong re-equilibration by coupled dissolution–repre-cipitation processes but have locally preserved patches with a primary magmatic zoning. The geochemistry of the patches points to the oceanic provenance of protolithic zircons; their isotope dating (SHRIMP-II) yielded a concordant age of 2821 ± 21 Ma. Zircons from the trondhjemite gneiss with geochemical features of Archean adakite were dated at 2805 ± 11 Ma, which suggests the syneclogitic facies origin of the TTG melts. The concordant age of high-pressure zircons from the northern eclogites is 2722 ± 21 Ma, close to the age of the earlier described Gridino eclogites. The overview of the isotopically dated eclogite bodies show the presence of at least three temporally distinct groups of eclogites in the Belomorian Belt, ～2.86–2.87, ～2.82–2.80, and ～2.72 Ga, which is in a good accordance with the known isotopic ages for major crust-forming events in the belt. This, in turn, implies a close genetic relationship between the eclogites and the TTG origin, which might be consistent with the model of the short intermitted events of subduction of the thickened Archean oceanic crust. The presence of HP/UHP eclogites of different ages and the structural style of the Belomorian Belt permit it to be assigned to megamélange belts.  相似文献   

3.
The Origins of Yakutian Eclogite Xenoliths   总被引：2，自引：2，他引：2       下载免费PDF全文
Owing to the association with diamonds, eclogite xenoliths havereceived disproportionate attention given their low abundancein kimberlites. Several hypotheses have been advanced for theorigin of eclogite xenoliths, from the subduction and high-pressuremelting of oceanic crust, to cumulates and liquids derived fromthe upper mantle. We have amassed a comprehensive data set,including major- and trace-element mineral chemistry, carbonisotopes in diamonds, and Rb–Sr, Sm–Nd, Re–Os,and oxygen isotopes in ultrapure mineral and whole-rock splitsfrom eclogites of the Udachnaya kimberlite pipe, Yakutia, Russia.Furthermore, eclogites from two other Yakutian kimberlite pipes,Mir and Obnazhennaya, have been studied in detail and offercontrasting images of eclogite protoliths. Relative to eclogitesfrom southern Africa and other Yakutian localities, Udachnayaeclogites are notable in the absence of chemical zoning in mineralgrains, as well as the degree of light rare earth element (LREE)depletion and unradiogenic Sr; lack of significant oxygen, sulfur,and carbon isotopic variation relative to the mantle; and intermineralradiogenic isotopic equilibration. Several of these eclogitescould be derived from ancient, recycled, oceanic crust, butmany others exhibit no evidence for an oceanic crustal protolith.The apparent lack of stable-isotope variation in the Udachnayaeclogites could be due to the antiquity of the samples and consequentlack of deep oceanic and biogenically diverse environments atthat time. Those eclogites that are interpreted to be non-recycledhave compositions characteristic of Group A eclogites from otherlocalities that also have been interpreted as being directlyfrom the mantle. At least two separate and diverse isotopicreservoirs are suggested by Nd isotopic whole-rock reconstructions.Most samples were derived from typical depleted mantle. However,two groups of three samples each indicate both enriched mantleand possible ultra-depleted mantle present beneath Yakutia duringthe late Archean and early Proterozoic. The vast majority ofeclogites studied from the Obnazhennaya pipe also exhibit characteristicsof Group A eclogites and are probably derived directly fromthe mantle. However, the eclogites from the Mir kimberlite aremore typical of other eclogites world-wide and show convincingevidence of a recycled, oceanic crustal affinity. We concurwith the late Ted Ringwood that eclogites can be formed in avariety of ways, both within the mantle and from oceanic crustalresidues. KEY WORDS: diamonds; eclogite xenoliths; isotopic composition; REE; Yakutia  相似文献   

4.
Late Archean (2.57 Ga) diamond-bearing eclogite xenoliths from Udachnaya, Siberia, exhibit geochemical characteristics including variation in oxygen isotope values, and correlations of δ18O with major elements and radiogenic isotopes which can be explained by an origin as subducted oceanic crust. Trace element analyses of constituent garnet and clinopyroxene by Laser-ICPMS are used to reconstruct whole-rock trace element compositions, which indicate that the eclogites have very low high field strength element (HFSE) concentrations and Zr/Hf and Nb/Ta ratios most similar to modern island arcs or ultradepleted mantle. Although hydrothermal alteration on the Archean sea floor had enough geochemical effect to allow the recognition of its effects in the eclogites and thus diagnose them as former oceanic crust, it was not severe enough to erase many other geochemical features of the original igneous rocks, particularly the relatively immobile HFSEs. Correlations of the trace element patterns with oxygen isotopes show that some, generally Mg-richer, eclogites originated as lavas, whereas others have lower δ18O and higher Sr and Eu contents indicating an origin as plagioclase-bearing intrusive rocks formed in magma chambers within the ocean crust. Major and trace element correlations demonstrate that the eclogites are residues after partial melting during the subduction process, and that their present compositions were enriched in MgO by this process. The original lava compositions were picritic, but not komatiitic, whereas the intrusives had lower, basaltic MgO contents. The HFSE signature of the eclogites may indicate that ocean floor basalts of the time were relatively close to island arcs and recycled material, which would be consistent with a larger number of smaller oceanic plates. Their composition appears to indicate that komatiitic ocean crust compositions were restricted to the early Archean which is not known to be represented among the eclogite xenolith population.  相似文献   

5.
The paper focuses on the metamorphic geology of the oldest crustal eclogites discovered in the Late Archean tonalite-trondhjemite-granodiorite (TTG) complex of the Belomorian Mobile Belt on the Kola Peninsula. Eclogite bodies are, most likely, widespread. We studied one of the key objects, the Kuru-Vaara quarry, where several tens of retrogressed eclogite blocks randomly embedded in the TTG gneisses were stripped at the benches. Based on the field observations, two visually different types of eclogites have been recognized: “southern”, strongly retrogressed coarse-grained, and “northern”, well-preserved fine-grained. The southern eclogite blocks bear evidence of their partial melting with the formation of veins and melt percolation channels. The northern eclogite blocks show no evidence of melting. Despite the significant mineralogic difference, both types of eclogites can be assigned to amphibole eclogite facies. The applied jadeite solubility geobarometers yielded the minimum pressures of ～12 kbar for the northern eclogites and ～14–14.5 kbar for the southern ones. The used geothermometers yielded ～700°C and ～750°C, respectively. But the presence of quartz lamellae in Na-clinopyroxenes in both types of eclogites and their bulk compositions corresponding to high-Mg basalts suggest that the Kuru-Vaara eclogites might have reached the field of ultrahigh-pressure metamorphism. Analysis of the tentative P-T paths of metamorphic evolution for both types of eclogites showed that their burial–exhumation cycle might have taken as short as a few million years. The set of presented data suggests that the formation of the Kuru-Vaara eclogites was related to the subduction of the Archean oceanic crust, which should have differed in composition and structure from the modern oceanic crust.  相似文献   

6.
Mafic granulite and pyroxenite xenoliths from Cenozoic alkaline basalts at Hannuoba, Hebei Province, North China have been selected for a systematic geochemical and Sr–Nd–Pb isotopic study, which provides a unique opportunity to explore nature of the lower crust and the interaction between the continental crust and lithospheric mantle beneath an Archean craton. The major, compatible and incompatible elements and radiogenic isotopes of these xenoliths suggest great chemical heterogeneity of the lower crust beneath the Hannuoba region. Petrological and geochemical evidences indicate a clear cumulate origin, and most likely, they are related to basaltic underplating in different geological episodes. However, the Sr–Nd–Pb isotopic compositions of the xenoliths reveal a profound enriched source signature (EM I) with some influence of EM II, which implies that some portion of pre-existing, old metasomatized subcontinental lithospheric mantle could have played an important role in their genesis. It is suggested that the interaction between continental crust and subcontinental mantle as manifested by basaltic underplating would be closely related to regional tectonic episodes and geodynamic processes in the deep part of subcontinental lithospheric mantle.  相似文献   

7.
There is no consensus on the processes responsible for near-coeval formation of Archaean continental crust (dominantly tonalite-trondhjemite-granodiorite: TTG), greenstone belts dominated by komatiitic to tholeiitic lavas (KT), and sub-continental lithospheric mantle (SCLM). The Douglas Harbour domain (2.7-2.9 Ga) of the Minto Block, northeastern Superior Province, has two TTG suites, the western and eastern Faribault-Thury (WFT and EFT), with embedded KT greenstones. Tonalites of both suites have high light/heavy rare-earth element ratios (L/HREE), high large ion lithophile element (U-Th-Rb-Cs-La: LILE) contents, positive Sr-Pb anomalies, and negative Nb-Ta-Ti anomalies. Such typical Archaean TTG signatures are commonly explained by melting of subducted oceanic crust, but could also originate by melting the base of thick basaltic plateaux formed above mantle upwellings (plumes), leaving behind restites containing pyroxene, garnet, and rutile. Field relationships (in situ segregation veins), phase equilibria (hornblende stabilized at lower crustal pressure), petrography (corroded epidote and muscovite phenocrysts, rare plagioclase phenocrysts), and trace element models, all imply that FT tonalite to trondhjemite evolution reflects hornblende-dominated fractional crystallization, not partial melting of subducted crust. The geochemistry of parental FT tonalites can be modeled by 15-30% melting of FT tholeiitic metabasalts, with residues of eclogite, garnet-websterite, or hornblende-garnet websterite. A minor residual Ti-phase such as rutile is also needed to generate negative Ti-Nb-Ta troughs in the TTGs. However, large volumes of eclogitic restites complementary to TTG are not observed either at the base of Archaean crustal sections, or in the SCLM. Additional problems with slab-melting models include: (a) the rarity of lithologies and associations characteristic of active margins (ophiolites, andesites, blueschists, accretionary mélanges, molasse, flysch, high-pressure belts, and thrust-and-fold belts); (b) the need to deliver plume-derived KT melt through the slab; and (c) extracting enough TTG melt from a subducting slab in the time available (200-300 my). In the plateau-melting model, heat for crustal anatexis is supplied by ongoing KT magma derived from mantle upwellings. However, SCLM rocks differ from predicted 1-stage mantle melting residua; and the voluminous residual eclogites complementary to TTG generation somehow need to be removed. These two problems might solve one another if the dense crustal restites disaggregated and mixed into the underlying depleted mantle. Mantle melting slows upon exhaustion of Ca-Al-rich phases, with large temperature increases needed to extract more melt from harzburgite residua. Physical addition of delaminated crustal restites would refertilize the refractory mantle, allowing extraction of additional melt increments, and might explain the ultra-depleted and orthopyroxene-rich nature of the SCLM. A hybrid source composed of 10% eclogitic restite of EFT tonalite generation, mixed with harzburgitic residues from 25% melting of primitive mantle, yields model melts with trace element signatures resembling typical Munro komatiites. Variations in the mineralogy and geochemistry of the delaminated component might account for the diversity of komatiite types. Degassing of hornblende-rich delaminated restites would transfer LILE to surrounding depleted mantle and could generate boninites. Fusion of undepleted metabasalt sandwiched among denser restites could generate sanukitoids. Mantle melt pulses generated by catastrophic delamination events would underplate nascent TTG crust and trigger renewed crustal melting, followed by delamination of newly formed eclogitic restites, triggering additional mantle melting, and so on. I posit that delamination of crustal restites catalyzed multi-stage melting of the SCLM and maturation of the Archaean continental crust. Thus, Archaean crust and SCLM are genetically inter-linked, and both form above major mantle upwellings.  相似文献   

8.
俄罗斯白海活动带中的太古宙榴辉岩   总被引：1，自引：0，他引：1  
在俄罗斯白海活动区发现的迄今为止最古老的太古宙榴辉岩的出露,对整个地质学领域是一次革命性事件。白海活动带位于芬诺斯干地亚地盾东北部太古宙陆核,处于科拉半岛大陆和卡累利阿克拉通之间的太古宙增生碰撞带中,在新太古代和古元古代期间多次受到中高压变质和构造变形作用。榴辉岩出露包括Gridino和Salma两大地区。Gridino榴辉岩区的榴辉岩产状可分为TTG片麻岩围岩中具有复杂成因的太古宙榴辉岩包裹镶体(2.72 Ga),组成强烈构造变形的混合混杂岩体(mélange),以及众多古元古代侵入岩墙岩脉状基性榴辉岩。Salma榴辉岩区的榴辉岩年龄应该晚于2.87 Ga,其中的Fe Ti 榴辉岩年龄测定为约2.80 Ga。两大榴辉岩区的p T演化轨迹比较类似,Gridino榴辉岩的峰期变质温压值(T=740~865 ℃,p=1.4~1.8 GPa)比Salma榴辉岩(T≈700 ℃,p=1.3~1.4 GPa)要高。Salma榴辉岩原岩可能与大洋环境有关。  相似文献   

9.
Eclogites were relatively recently found in the Belomorian Mobile Belt (BMB) (Volodichev et al., 2004; Shchipanskii et al., 2005; Konilov et al., 2004). The very first isotopic dates (Volodichev et al., 2004; Mints et al., 2010) were obtained for these rocks in the northwestern (in the Salma and Kuru-Vaara areas) and central (Gridino area) portions of BMB and corresponded to the Archean: approximately 2.72–2.87 Ga. Because no crustal eclogites older that 2.0 Ga (Möller et al., 1995) had been known before these dates were obtained, these eclogites were regarded as unique. It is commonly believed that no crustal eclogites could be formed in the Archean because the crust was then relatively thin (Kröner, 2010), and hence, the find of crustal eclogites of Archean age in BMB called for a fundamental revision of geodynamic reconstructions of the crustal evolution and was one of the main arguments invoked to support the hypothesis that currently operating geodynamic mechanisms of plate tectonic can be extrapolated to the Early Precambrian (Rozen et al., 2008). However, these finds were practically immediately followed by serious doubts that the primary estimates of the timing of the eclogite metamorphism in the Belomorian Belt may be incorrect (Mitrofanov et al., 2009; and others).  相似文献   

10.
The Komsomolskaya kimberlite is one of numerous (>1,000) kimberlite pipes that host eclogite xenoliths on the Siberian craton. Eclogite xenoliths from the adjacent Udachnaya kimberlite pipe have previously been geochemically well characterized; however, data from surrounding diamond-bearing kimberlite pipes from the center of the craton are relatively sparse. Here, we report major- and trace-element data, as well as oxygen isotope systematics, for mineral separates of diamondiferous eclogite xenoliths from the Komsomolskaya kimberlite, suggesting two distinct subgroups of a metamorphosed, subducted oceanic crustal protolith. Using almandine contents, this suite can be divided into two subgroups: group B1, with a high almandine component (>20 mol%) and group B2, with a low almandine component (<20 mol%). Reconstructed REE profiles for B1 eclogites overlap with typical oceanic basalts and lack distinct Eu anomalies. In addition, elevated oxygen isotope values, which are interpreted to reflect isotopic exchange with seawater at low temperatures (<350 °C), are consistent with an upper-oceanic crustal protolith. Reconstructed REE profiles for B2 eclogites are consistent with oceanic gabbros and display distinct Eu anomalies, suggesting a plagioclase-rich cumulate protolith. In contrast to B1, B2 eclogites do not display elevated oxygen isotope values, suggesting an origin deep within the crustal pile, where little-to-no interaction with hydrothermal fluids has occurred. Major-element systematics were reconstructed based on mineral modes; group B1 eclogites have higher MgO wt% and lower SiO2 wt%, with respect to typical oceanic basalts, reflecting a partial melting event during slab subduction. Calculated residues from batch partial melt modeling of a range of Precambrian basalts overlap with group B1 trace-element chemistry. When taken together with the respective partial melt trajectories, these melting events are clearly linked to the formation of Tonalite–Trondhjemite–Granodiorite (TTG) complexes. As a result, we propose that many, if not all, diamondiferous eclogite xenoliths from Komsomolskaya represent mantle ‘restites’ that preserve chemical signatures of Precambrian oceanic crust.  相似文献   

11.
Trace element characteristics of seven coesite-bearing eclogitic xenoliths from the Roberts Victor kimberlite demonstrate that this suite of eclogites originated as gabbroic cumulates in oceanic crust that was subsequently subducted. All but one of the garnets show positive Eu anomalies, accompanied by a flat heavy rare earth pattern, which is atypical of garnet, but characteristic of plagioclase, arguing for a considerable amount of plagioclase in the protoliths. Forward modelling of the accumulation of liquidus minerals from primitive komatiitic, picritic, and basaltic liquids suggests that at least some of the eclogite protoliths were not derived from basaltic parental liquids, whereas derivation from either komatiitic or picritic liquids is possible. The reconstructed eclogite bulk rocks compare favourably with oceanic gabbros from ODP hole 735B (SW Indian Ridge), even to the extent that oxygen isotopic systematics show signs of low-temperature seawater alteration. However, the oxygen isotope trends are the reverse of what is expected for cumulates in the lower section of the oceanic crust. These new findings show that δ18O values in eclogitic xenoliths, despite being sound indicators for their interaction with hydrothermal fluids at low pressure, do not necessarily bear a simple relationship with the inferred oceanic crustal stratigraphy of the protoliths.  相似文献   

12.
The ages of subcontinental lithospheric mantle beneath the North China and South China cratons are less well-constrained than the overlying crust. We report Re–Os isotope systematics of mantle xenoliths entrained in Paleozoic kimberlites and Mesozoic basalts from eastern China. Peridotite xenoliths from the Fuxian and Mengyin Paleozoic diamondiferous kimberlites in the North China Craton give Archean Re depletion ages of 2.6–3.2 Ga and melt depletion ages of 2.9–3.4 Ga. No obvious differences in Re and Os abundances, Os isotopic ratios and model ages are observed between spinel-facies and garnet-facies peridotites from both kimberlite localities. The Re–Os isotopic data, together with the PGE concentrations, demonstrate that beneath the Archean continental crust of the eastern North China Craton, Archean lithospheric mantle of spinel- to diamond-facies existed without apparent compositional stratification during the Paleozoic. The Mesozoic and Cenozoic basalt-borne peridotite and pyroxenite xenoliths, on the other hand, show geochemical features indicating metasomatic enrichment, along with a large range of the Re–Os isotopic model ages from Proterozoic to Phanerozoic. These features indicate that lithospheric transformation or refertilization through melt-peridotite interaction could be the primary mechanism for compositional changes during the Phanerozoic, rather than delamination or thermal-mechanical erosion, despite the potential of these latter processes to play an important role for the loss of garnet-facies mantle. A fresh garnet lherzolite xenolith from the Yangtze Block has a Re depletion age of ∼1.04 Ga, much younger than overlying Archean crustal rocks but the same Re depletion ages as spinel lherzolite xenoliths from adjacent Mesozoic basalts, indicating Neoproterozoic resetting of the Re–Os system in the South China Craton.  相似文献   

13.
The Archean lithospheric mantle beneath the Kaapvaal–Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150–250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2–3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely distributed younger Archean generation of eclogitic diamonds in the lithospheric mantle. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.  相似文献   

14.
A suite of 14 diamond-bearing and 3 diamond-free eclogite xenoliths from the Newlands kimberlite, South Africa, have been studied using the Re–Os isotopic system to provide constraints on the age and possible protoliths of eclogites and diamonds. Re concentrations in diamond-bearing eclogites are variable (0.03–1.34 ppb), while Os concentrations show a much more limited range (0.26–0.59 ppb). The three diamond-free eclogites have Re and Os concentrations that are at the extremes of the range of their diamond-bearing counterparts. 187Os/188Os ranges from 0.1579 to 1.4877, while 187Re/188Os varies from 0.54 to 26.2 in the diamond-bearing eclogites. The highly radiogenic Os in the diamond-bearing eclogites (γOs=23–1056) is consistent with their high 187Re/188Os and requires long-term isolation from the convecting mantle. Re–Os model ages for 9 out of 14 diamond-bearing samples lie between 3.08 and 4.54 Ga, in agreement with FTIR spectra of Newlands diamonds that show nitrogen aggregation states consistent with diamond formation in the Archean. Re–Os isochron systematics for the Newlands samples do not define a precise isochron relationship, but lines drawn between subsets of the data provide ages ranging from 2.9 to 4.1 Ga, all of which are suggestive of formation in the Archean. The Re–Os systematics combined with mineral chemistry and stable isotopic composition of the diamond-bearing eclogites are consistent with a protolith that has interacted with surficial environments. Therefore, the favored model for the origin of the Newlands diamond-bearing eclogites is via subduction. The most likely precursors for the Kaapvaal eclogites include komatiitic ocean ridge products or primitive portions of oceanic plateaus or ocean islands.  相似文献   

15.
高山  刘勇胜 《地学前缘》2003,10(3):61-67
测定了辽宁复县奥陶纪金伯利岩和河北汉诺坝与山东栖霞第三纪碱性玄武岩中产出的地幔包体的Re Os同位素组成。金伯利岩中地幔包体的Re贫化Os同位素模式年龄 (TRD)为 2 .5～ 2 .8Ga ,从Re Os同位素定年角度证明了华北克拉通确实存在太古宙岩石圈地幔。对汉诺坝二辉橄榄岩包体获得了 (1.9± 0 .18)Ga的Re Os同位素等时线年龄 ,表明现今保存在那里的地幔主要是古元古代时形成的。汉诺坝地区出露有大量新太古代岩石 ,表明曾存在太古宙地幔。由于缺乏太古宙年龄 ,说明由汉诺坝所代表的克拉通中部曾存在的太古宙地幔在古元古代时已被减薄 ,并被 1.9Ga的新生岩石圈地幔置换。该事件与华北克拉通中部广泛的古元古代碰撞造山过程导致的麻粒岩相变质作用的时代相同 ,说明有关的岩石圈置换作用可能主要与拆沉作用有关。栖霞地幔包体具有与现代对流地幔相同的Os同位素组成 ,且Os同位素组成与Re/Os比值没有明显相关性 ,表明年龄很新。结合其它地质地球化学证据 ,说明克拉通东部的太古宙岩石圈地幔的置换作用主要发生在中生代 ,且可能与三叠纪华北和扬子陆块的陆陆碰撞造山导致的岩石圈地幔和下地壳的拆沉作用有关。本研究表明华北克拉通岩石圈地幔置换作用在时空上的分布是十分不均一的。 2 .5～ 2 .8Ga与 1.9Ga不仅?  相似文献   

16.
下地壳拆沉作用及大陆地壳演化   总被引：13，自引：0，他引：13  
讨论了下地壳拆沉作用的地球化学示踪方法,并以近来对秦岭－大别造山带和美国西部内华达岩基地区的研究成果为例,说明了下地壳拆沉在两地区壳－幔演化方面可能起的重要作用。  相似文献   

17.
Previous studies of samples of subcontinental lithospheric mantle (SCLM) that underlay the North China Craton (NCC) during the Paleozoic have documented the presence of thick Archean SCLM at this time. In contrast, samples of SCLM underlying the NCC during the Cenozoic are characterized by evidence for melt depletion during the Proterozoic, and relatively recent juvenile additions to the lithosphere. These observations, coupled with geophysical evidence for relatively thin lithosphere at present, have led to the conclusion that the SCLM underlying the NCC was thinned and modified subsequent to the late Paleozoic. In order to extend the view into both the Paleozoic and modern SCLM underlying the NCC, we examine mantle xenoliths and xenocrystic chromites extracted from three Paleozoic kimberlites (Tieling, Fuxian and Mengyin), and mantle xenoliths extracted from one Cenozoic basaltic center (Kuandian). Geochemical data suggest that most of the Kuandian xenoliths are residues of small degrees of partial melting from chemically primitive mantle. Sr-Nd-Hf isotopic analyses indicate that the samples were removed from long-term depleted SCLM that had later been variably enriched in incompatible elements. Osmium isotopic compositions of the two most refractory xenoliths are depleted relative to the modern convecting upper mantle and have model melt depletion ages that indicate melt depletion during Paleoproterozoic. Other relatively depleted xenoliths have Os isotopic compositions consistent with the modern convecting upper mantle. This observation is generally consistent with earlier data for xenoliths from other Cenozoic volcanic systems in the NCC and surrounding cratons. Thus, the present SCLM underlying the NCC has a complex age structure, but does not appear to retain materials with Archean melt depletion ages. Results for what are presumed to be early Paleozoic xenoliths from Teiling are generally highly depleted in melt components, e.g. have low Al2O3, but have also been metasomatically altered. Enrichment in light rare earth elements, low εNd values (∼−10), and relatively high 87Sr/86Sr (0.707-0.710) are consistent with a past episode of metasomatism. Despite the metasomatic event, 187Os/188Os ratios are low and consistent with a late Archean melt depletion event. Thus, like results for xenoliths from other early Paleozoic volcanic centers within the NCC, these rocks sample dominantly Archean SCLM. The mechanism for lithospheric thinning is still uncertain. The complex age structure currently underlying the NCC requires either variable melt depletion over the entire history of this SCLM, or the present lithospheric material was partly or wholly extruded under the NCC from elsewhere by the plate collisions (collision with the Yangtze Craton and/or NNW subduction of the Pacific plate) that may have caused the thinning to take place.  相似文献   

18.
早中生代的扬子大陆向华北大陆的深俯冲碰撞作用以及中、新生代的华北东部岩石圈减薄作用是国际大陆动力学问题研究的两个热点。然而,把它们有机联系起来,探讨深部岩石圈演化动力学过程的研究还很少。报道了中国大陆科学钻探先导孔(CCSD-PP1)橄榄岩的矿物岩石化学分析结果。这些橄榄岩亏损玄武质组分(如低w(CaO+Al2O3)、高Mg#等),并经历了中元古代来自软流圈的碳酸岩熔体的交代作用和早中生代的超高压再平衡过程。结合已发表的华北捕虏体橄榄岩资料,对这一橄榄岩的原始地幔属性进行了分析。发现CCSD-PP1橄榄岩与华北古老岩石圈地幔组成相似,是早中生代来自华北岩石圈的构造侵入体。早中生代华北岩石圈的伸展减薄与苏鲁超高压变质岩石的折返提供了早期软流圈物质上涌的构造体制。  相似文献   

19.
华北克拉通破坏的物理、化学过程:地幔橄榄岩证据   总被引：1，自引：0，他引：1  
本文对比了华北东部不同时代、不同位置捕虏体/地质体橄榄岩的地幔属性,讨论了华北克拉通破坏的物理、化学过程。结果表明,拆沉作用不能很好地解释古老难熔、过渡型和新生饱满地幔并存的事实;单纯的熔体-橄榄岩相互作用也不易解释中、新生代岩石圈巨大减薄和新生饱满地幔Cpx中LREE亏损现象,即具复杂演化历史的克拉通地幔向演化历史简单的"大洋型"岩石圈的转换。华北东部克拉通破坏作用包括地幔伸展、熔-岩作用、侵蚀置换等复杂的物理、化学过程。岩石圈先大幅减薄、后小幅增厚实现了最终的地幔置换和岩石圈整体减薄过程。喷发时代为100 Ma的阜新玄武岩所捕获的橄榄岩主体是饱满的,说明华北东部部分地区在此之前有过地幔置换作用。  相似文献   

20.
Suites of mantle-derived xenoliths in volcanic rocks provide estimates of the geothermal gradient and composition of the subcontinental lithospheric mantle (SCLM) at the time of the volcanic eruption. The development of single-grain thermometry and barometry, applied to xenocryst minerals in volcanic rocks, has greatly expanded the number of localities for which such data can be obtained and made it feasible to map the geology of the SCLM on a broader scale, both vertically and laterally. From garnet xenocrysts, it is possible to derive profiles showing mean values of olivine composition, bulk-rock composition, density and seismic velocities, as well as geotherm parameters and constraints on the thickness of the SCLM. Geochemical profiles, coupled with Re–Os dating of peridotites and their enclosed sulfide minerals, show that Archean or Proterozoic SCLM is preserved at shallow levels beneath many areas of younger tectonothermal age; this implies rapid vertical variations in Vs and Vp with depth, which may affect seismic interpretations. Data from several hundred localities worldwide define a secular evolution in the composition of the SCLM, related to the tectonothermal age of the overlying crust. Archean SCLM is typically strongly depleted in basaltic components, highly magnesian and thick (160–250 km), and has low geotherms; Phanerozoic SCLM is typically fertile (rich in basaltic components), Fe-rich, thin (50–100 km) and has a range of high geotherms; Proterozoic SCLM (much of which may be reworked Archean mantle) tends to be intermediate in all respects. The correlated variations in SCLM fertility, lithospheric thickness and geotherm reinforce the effects of each on seismic velocity, and produce more rapid lateral variations in seismic response than would result from thermal effects alone. These correlations are the key to using seismic tomography images to map the lateral extent of different types of SCLM.  相似文献   

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