Regional patterns in the paragenesis and age of inclusions in diamond, diamond composition, and the lithospheric seismic structure of Southern Africa   总被引：1，自引：0，他引：1  

Steven B. Shirey  Jeffrey W. Harris  Stephen H. Richardson  Matthew Fouch  David E. James  Pierre Cartigny  Peter Deines  Fanus Viljoen 《Lithos》2003,71(2-4):243-258

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   

Re–Os systematics of diamond-bearing eclogites from the Newlands kimberlite     

A. H. Menzies  R. W. Carlson  S. B. Shirey  J. J. Gurney 《Lithos》2003,71(2-4):323-336

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 (γ_Os=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   

Geodynamic significance of the Raspas Metamorphic Complex (SW Ecuador): geochemical and isotopic constraints     

Delphine Bosch  Piercarlo Gabriele  Henriette Lapierre  Jean-Louis Malfere  Etienne Jaillard   《Tectonophysics》2002,345(1-4)

The Raspas Metamorphic Complex of southwestern Ecuador is regarded as the southernmost remnant of oceanic and continental terranes accreted in the latest Jurassic–Early Cretaceous. It consists of variably metamorphosed rock types. (1) Mafic and ultramafic rocks metamorphosed under high-pressure (HP) conditions (eclogite facies) show oceanic plateau affinities with flat REE chondrite-normalized patterns, Nd_{150 Ma} ranging from +4.6 to 9.8 and initial Pb isotopic ratios intermediate between MORB and OIB. (2) Sedimentary rocks metamorphosed under eclogitic conditions exhibit LREE enriched patterns, strong negative Eu anomalies, Rb, Nb, U, Th, Pb enrichments, low Nd_{150 Ma} values (from −6.4 to −9.5), and high initial ⁸⁷Sr/⁸⁶Sr and ^206,207,208Pb/²⁰⁴Pb isotopic ratios suggesting they were originally sediments derived from the erosion of an old continental crust. (3) Epidote-bearing amphibolites show N-MORB affinities with LREE depleted patterns, LILE, Zr, Hf and Th depletion, high Nd_{150 Ma} (>+10) and low initial Pb isotopic ratios.The present-day well defined internal structure of the Raspas Metamorphic Complex seems to be inconsistent with the formerly proposed interpretation of a “tectonic mélange”. The association of oceanic plateau rocks and continent-derived sediments both metamorphosed in HP conditions suggests that the thin edge of the oceanic plateau first entered the subduction zone and dragged sediments downward of the accretionary wedge along the Wadatti–Benioff zone. Subsequently, when its thickest part arrived into the subduction zone, the oceanic plateau jammed the subduction processes, due to its high buoyancy.In Ecuador and Colombia, the latest Jurassic–Early Cretaceous suture involves HP oceanic plateau rocks and N-MORB rocks metamorphosed under lower grades, suggesting a composite or polyphase nature for the latest Jurassic–Early Cretaceous accretionary event.  相似文献   

苏鲁超高压变质带中国大陆科学钻探主孔(CCSD-MH)榴辉岩中发现金刚石   总被引：2，自引：4，他引：2  

张仲明  杨经绥戎合　HU JinZhu　SU JinFu　MAO HoKuang 《岩石学报》2007,23(12):3201-3206

苏鲁榴辉岩中除了早前发现的一粒金刚石外,最近又发现了6粒,它们具有基本相似的外形和大小,但其晶体结构与普通自然界金刚石和人工合成金刚石存在一定差别.金刚石具有异常X-射线衍射峰以及有磁铁矿的包裹体等特征,表明可以排除这些金刚石是人造或被外来样品混染的可能性.金刚石的再发现,证实苏鲁榴辉岩的形成压力可以达到4GPa以上,并且为大别和苏鲁两个超高压变质带的对比提供了一个重要标志.金刚石的异常晶胞参数和X-Ray异常峰的出现有可能与超高压变质作用有关,但需要实验岩石学研究证实.  相似文献   

Forward modeling corona growth in a partially eclogitized leucogabbro, Bourbon Island, North-East Greenland     

C. Sartini-Rideout  J.A. Gilotti  C.T. Foster Jr. 《Lithos》2007,95(3-4):279-297

Coronitic textures are common in partially eclogitized igneous bodies, such as gabbros, leucogabbros, and anorthosites, east of the Germania Land Deformation Zone in North-East Greenland. Coronas formed by prograde metamorphic processes that transformed the gabbroic bodies to eclogite facies, and record frozen stages of the prograde metamorphic evolution of these rocks. A metaleucogabbro-norite body on Bourbon Island in Jøkelbugt is characterized by three concentric areas: a coronitic core, a mottled inner rim with areas of completely eclogitized material surrounded by a matrix of coronitic metaleucogabbro, and an outer rim of strongly foliated and completely retrogressed amphibolite. The Bourbon body preserves four stages of the prograde metamorphic history: Stage I, Stage II, Stage III, and Eclogite Stage. Stage I coronas are found only in the core of the body, which is the least reacted part of the leucogabbro-norite and the closest to the protolith, and is characterized by the corona sequence Pl_rim/Grt + Kfs + Amp/Grt + Amp/Cpx_rim. The typical corona sequence for Stage II is Pl_rim/Grt + Pl + Zo/Cpx_rim/Amp_rim. Stage III samples show a Pl_rim + Ky + Scp/Grt + Pl + Qtz/Qtz + Pl sequence, with the relict clinopyroxene being replaced in part by microcrystalline aggregates of Cpx + Amp + Pl. The Eclogite Stage shows relict Pl completely replaced by Grt, and the relict Cpx completely replaced by aggregates of Omp + Pl + Kfs + Amp. We tested open-system grain boundary diffusion (OSGBD) theories to model the prograde Stage I symplectitic coronas. The observed ratio of the thickness of the different layers is Pl_rim:Grt + Kfs + Amp:Grt + Amp:Cpx_rim equal to 3:1.3:0.95:0.5. These ratios are very close to the modeled ones of 2.7:1.1:1:0.5. Furthermore, subtle textural changes within the Grt + Kfs + Amp corona were also reproduced by the model. The model gave us insight into the conditions of the metamorphic system in which the coronas formed. The sequence Pl_rim/Grt + Kfs + Amp/Grt + Amp/Cpx_rim formed by diffusion driven reactions in an open system involving gain of Fe, K, and Na, and loss of Ca and Mg at the original clinopyroxene–plagioclase boundary. Relative mobilities of the different components within the corona layers were L_MgMg > L_AlAl > L_SiSi > L_CaCa > L_KK > L_FeFe > L_NaNa. Fluid circulation was active to some degree during the transformation to eclogite. The differences between core, inner rim, and the two domains within the inner rim of the metaleucogabbro-norite can be explained by different degrees of fluid circulation in different portions of the rock. The presence of phases containing Cl and P, such as scapolite, in completely eclogitized samples supports the presence of fluid circulation in the system. Another possible explanation for the mottled appearance of the inner rim is protolith heterogeneity.  相似文献   

非保守元素在苏鲁超高压榴辉岩退变质作用中有限迁移和近原地重新分布     

曾令森张泽明  刘福来梁凤华 《岩石学报》2007,23(12):3215-3220

中国大陆科学钻探工程主孔揭露的基性岩石包括新鲜榴辉岩和角闪岩.角闪岩是榴辉岩在折返过程中不同程度退变质作用的产物.全岩主量和痕量元素地球化学数据表明它们不仅在高场强元素(Ti,Zr,Nb,Ta),而且在高度活动元素(Rb,Cs,Sr,Ba)上化学特征相似.尽管从榴辉岩向角闪岩的退变质作用需要流体的参与,上述地球化学特征表明在榴辉岩快速折返过程中,流体仅仅有限地存在,在流体中高度活动的碱性元素仅作有限迁移和近乎原地重新分布.流体可能以高度局域化的管道流形式出现,是导致榴辉岩退变质作用和剪切变形高度局域化的主要因素.  相似文献   

New constraints on the origin and age of Variscan eclogitic rocks (Ligurian Alps,Italy)     

Folco Giacomini  Roberto Braga  Massimo Tiepolo  Riccardo Tribuzio 《Contributions to Mineralogy and Petrology》2007,153(1):29-53

Gabbro and eclogite boudins are preserved within the amphibolites of the composite para- and ortho-gneiss Variscan basement of the Savona Crystalline Massif (Ligurian Briançonnais, Italy). Whole rock trace element patterns, low initial εNd (+5.4 to +8.8) data and trace element analyses on relict igneous clinopyroxene revealed that the mafic rocks were derived from depleted mantle melts, which most likely underwent crustal contamination during emplacement. Gabbros have a cumulus origin controlled by clinopyroxene and plagioclase segregation, whereas the eclogites represent evolved melts. U-Pb and trace element micro-analyses on zircons separated from one amphibolitised gabbro and one eclogite help to constrain coeval ages at ~468 Ma for their igneous protoliths. The occurrence of a few inherited zircons confirms the involvement of a crustal component in the petrogenesis of the mafic rocks. In the eclogite, concordant zircon ages younger than the protolith age testify to metamorphic re-crystallisation (or new growth) from about 420 to 305 Ma. Zircon textures and trace element compositions indicate that eclogite facies metamorphism occurred 392–376 Ma ago. The younger zircon portions yielding a mean Concordia age of 333 ± 7 Ma are related to equilibration or new growth during the post-eclogite, amphibolite-facies equilibration.  相似文献   

Crystallization and very rapid exhumation of the youngest Alpine eclogites (Tauern Window,Eastern Alps) from Rb/Sr mineral assemblage analysis   总被引：1，自引：0，他引：1  

Johannes?GlodnyEmail author  Uwe?Ring  Alexander?Kühn  Philipp?Gleissner  Gerhard?Franz 《Contributions to Mineralogy and Petrology》2005,149(6):699-712

Multimineral Rb/Sr internal isochrons from eclogite facies rocks of the Eclogite Zone (Tauern Window, Eastern Alps) consistently yield an Early Oligocene age of 31.5±0.7 Ma. This age has been obtained both for late-prograde, dehydration-related eclogitic veins, and for rocks variably deformed and recrystallized under eclogite facies conditions (2.0–2.5 GPa, 600°C). Initial Sr-isotopic equilibria among all phases indicate absence of significant post-eclogitic isotope redistribution processes, therefore the ages date eclogite facies assemblage crystallization. Equilibria also prove that no prolonged pre-eclogite facies history is recorded in the rocks. Instead, subduction, prograde mineral reactions, and eclogitization proceeded rapidly. Fast exhumation immediately after eclogitization, with minimum rates >36 mm/a is inferred from a 31.5±0.5 Ma internal mineral isochron age of a post-eclogitic greenschist facies vein assemblage. Such rates equal typical subduction rates. Late Eocene to Early Oligocene subduction of the European continental margin, with subsequent rapid exhumation of high-pressure nappe complexes has previously been recognized only in the Western Alps. The new data signify synchronous continental collision all along the Alpine belt. Our results demonstrate the unique potential of Rb/Sr assemblage system analysis for precise dating of both eclogite facies and post-eclogitic events, thus for precisely constraining exhumation rates of deep-seated rocks, and for straightforward linkage of petrologic evidence with isotopic ages.  相似文献   

榴辉岩的地震波性质：对苏鲁超高压变质带地壳成分和折返机制的探讨   总被引：5，自引：3，他引：5  

王勤嵇少丞  许志琴MatthewH.SALISBURY夏斌  潘明宝 《岩石学报》2005,21(2):451-464

本文总结了榴辉岩的高温高压弹性波速测量结果,并将其应用于苏鲁超高压变质带地震资料的解释。由于榴辉岩具有高密度和高波速,它们和长英质片麻岩、大理岩、石英岩、角闪岩、麻粒岩、蛇纹石化橄榄岩的界面可以产生强反射。如果俯冲的陆壳物质以榴辉岩与围岩互层的形式在上地幔保留下来,就可能在造山带的上地幔产生地震反射。根据CCSD孔区地震剖面所建立的地壳成分模型表明:苏鲁超高压带地壳浅部的高速层可归因于夹在花岗质片麻岩、副片麻岩、角闪岩等岩石中的榴辉岩和超基性岩;中地壳主要由中酸性片麻岩、斜长角闪岩和副片麻岩组成;下地壳以中基性麻粒岩为主。在该超高压变质带现今的深部地壳,榴辉岩含量很少或几乎没有。因此,折返的超高压变质岩是以构造岩片的形式沿一系列剪切带逆冲并覆盖在正常的中下地壳之上,深部榴辉岩的缺乏可能与下地壳拆沉作用无关。  相似文献   

苏鲁超高压榴辉岩的石榴石生长成分环带及变质作用P—T轨迹   总被引：3，自引：6，他引：3  

张泽明  肖益林沈昆 高勇军 《岩石学报》2005,21(3):809-818

在南苏鲁东海地区，部分超高压榴辉岩中的变斑晶石榴石具有复杂的生长成分环带和多期矿物包体组合，它们记录了超高压变质岩的多阶段变质演化过程，即绿帘角闪岩相进变质、柯石英榴辉岩相峰期变质、石英榴辉岩相和角闪岩相退变质作用。运用相关的地质温、压计，使用代表最高变质温度的变斑晶石榴石慢部（具最低的Fe／Mg比值）和与其平衡的绿辉石包体成分，获得了〉900℃和4．1～4．5GPa的超高压变质务件。联合其他变质阶段的温、压条件，一个顺时针的变质作用P—T轨迹得以建立。它的特征是进变质与退变质路径近于平行，早期退变质作用为降温、降压过程．榴辉岩石榴石生长成分环带的保存说明超高压变质岩在峰期变质阶段有非常短暂的停留时间，并以很快的折返速率抬升到地壳浅部，超高压变质岩折返过程中的明显降温是石榴石生长环带得以保存的另一个有利条件，  相似文献