Geochronology of the Precambrian crust in the Mozambique belt in NE Mozambique, and implications for Gondwana assembly   总被引：2，自引：0，他引：2  

B. Bingen  J. Jacobs  G. Viola  I.H.C. Henderson   . Skr  R. Boyd  R.J. Thomas  A. Solli  R.M. Key  E.X.F. Daudi 《Precambrian Research》2009,170(3-4):231-255

Zircon and monazite U–Pb data document the geochronology of the felsic crust in the Mozambique Belt in NE Mozambique. Immediately E of Lake Niassa and NW of the Karoo-aged Maniamba Graben, the Ponta Messuli Complex preserves Paleoproterozoic gneisses with granulite-facies metamorphism dated at 1950 ± 15 Ma, and intruded by granite at 1056 ± 11 Ma. This complex has only weak evidence for a Pan-African metamorphism. Between the Maniamba Graben and the WSW–ENE-trending Lurio (shear) Belt, the Unango and Marrupa Complexes consist mainly of felsic orthogneisses dated between 1062 ± 13 and 946 ± 11 Ma, and interlayered with minor paragneisses. In these complexes, an amphibolite- to granulite-facies metamorphism is dated at 953 ± 8 Ma and a nepheline syenite pluton is dated at 799 ± 8 Ma. Pan-African deformation and high-grade metamorphism are more intense and penetrative southwards, towards the Lurio Belt. Amphibolite-facies metamorphism is dated at 555 ± 11 Ma in the Marrupa Complex and amphibolite- to granulite-facies metamorphism between 569 ± 9 and 527 ± 8 Ma in the Unango Complex. Post-collisional felsic plutonism, dated between 549 ± 13 and 486 ± 27 Ma, is uncommon in the Marrupa Complex but common in the Unango Complex. To the south of the Lurio Belt, the Nampula Complex consists of felsic orthogneisses which gave ages ranging from 1123 ± 9 to 1042 ± 9 Ma, interlayered with paragneisses. The Nampula Complex underwent amphibolite-facies metamorphism in the period between 543 ± 23 to 493 ± 8 Ma, and was intruded by voluminous post-collisional granitoid plutons between 511 ± 12 and 508 ± 3 Ma. In a larger context, the Ponta Messuli Complex is regarded as part of the Palaeoproterozoic, Usagaran, Congo-Tanzania Craton foreland of the Pan-African orogen. The Unango, Marrupa and Nampula Complexes were probably formed in an active margin setting during the Mesoproterozoic. The Unango and Marrupa Complexes were assembled on the margin of the Congo-Tanzania Craton during the Irumidian orogeny (ca. 1020–950 Ma), together with terranes in the Southern Irumide Belt. The distinctly older Nampula Complex was more probably linked to the Maud Belt of Antarctica, and peripheral to the Kalahari Craton during the Neoproterozoic. During the Pan-African orogeny, the Marrupa Complex was overlain by NW-directed nappes of the Cabo Delgado Nappe Complex before peak metamorphism at ca. 555 Ma. The nappes include evidence for early Pan-African orogenic events older than 610 Ma, typical for the Eastern Granulites in Tanzania. Crustal thickening at 555 ± 11 Ma is coeval with high-pressure granulite-facies metamorphism along the Lurio Belt at 557 ± 16 Ma. Crustal thickening in NE Mozambique is part of the main Pan-African, Kuunga, orogeny peaking between 570 and 530 Ma, during which the Congo-Tanzania, Kalahari, East Antarctica and India Cratons welded to form Gondwana. Voluminous post-collisional magmatism and metamorphism younger than 530 Ma in the Lurio Belt and the Nampula Complex are taken as evidence of gravitational collapse of the extensive orogenic domain south of the Lurio Belt after ca. 530 Ma. The Lurio Belt may represent a Pan-African suture zone between the Kalahari and Congo-Tanzania Craton.  相似文献   

Saghro Group in the Ougnat Massif (Morocco), an evidence for a continuous Cadomian basin along the northern West African Craton     

André Michard  Abderrahmane Soulaimani  Hassan Ouanaimi  Youssef Raddi  Lahsen Aït Brahim  Ech-Cherki Rjimati  Lahssen Baidder  Omar Saddiqi 《Comptes Rendus Geoscience》2017,349(2):81-90

The Saghro Group (SG) is a folded, low-grade volcano-sedimentary series up to 8 km thick that crops out within and to the north of the Pan-African suture zone in the central and eastern Anti-Atlas. Here we describe the SG of the Ougnat inliers that are exposed in the easternmost Anti-Atlas beneath the unconformable, Late Ediacaran Ouarzazate Group (OZG) volcanic rocks. The Ougnat SG mostly consists of volcaniclastic greywackes accumulated in a peritidal-to-shallow basin. The basin infilling was deformed by NNE-trending, mostly upright folds with axial-planar slaty cleavage and low-grade metamorphism. The deformed SG rocks were intruded by the ～550 Ma Mellab hypovolcanic granodiorite. The latter also crosscuts the lowest OZG rocks that are dated to 574–571 Ma in the western Saghro region. The SG rocks that form the Siroua and Saghro inliers have an oldest age of 620–610 Ma and were folded at ～610–580 Ma at the onset of the Cadomian orogenic events. We show that the SG rocks are similar to the “Série verte” (SV) rocks that are exposed in the Ougarta and western Hoggar east of the Pan-African suture. We infer that the SG and SV rocks accumulated in a same, continuous basin that was bounding the West African Craton to the north and the east. This strongly subsiding basin formed close to a volcanic arc and was folded during the last Pan-African synmetamorphic events. Fold orientation and age of folding differ however along the edge of the West African Craton. The orogenic greywackes that form the remnants of the SG-SV basin thus constitute a precious record of the diachronic Cadomian event s.l. along the West African Craton northern margin.  相似文献   

Contemporaneous assembly of Western Gondwana and final Rodinia break-up: Implications for the supercontinent cycle   总被引：2，自引：0，他引：2  

Sebastián Oriolo  Pedro Oyhantçabal  Klaus Wemmer  Siegfried Siegesmund 《地学前缘(英文版)》2017,8(6):1431-1445

Geological, geochronological and isotopic data are integrated in order to present a revised model for the Neoproterozoic evolution of Western Gondwana. Although the classical geodynamic scenario assumed for the period 800–700 Ma is related to Rodinia break-up and the consequent opening of major oceanic basins, a significantly different tectonic evolution can be inferred for most Western Gondwana cratons. These cratons occupied a marginal position in the southern hemisphere with respect to Rodinia and recorded subduction with back-arc extension, island arc development and limited formation of oceanic crust in internal oceans. This period was thus characterized by increased crustal growth in Western Gondwana, resulting from addition of juvenile continental crust along convergent margins. In contrast, crustal reworking and metacratonization were dominant during the subsequent assembly of Gondwana. The Río de la Plata, Congo-São Francisco, West African and Amazonian cratons collided at ca. 630–600 Ma along the West Gondwana Orogen. These events overlap in time with the onset of the opening of the Iapetus Ocean at ca. 610–600 Ma, which gave rise to the separation of Baltica, Laurentia and Amazonia and resulted from the final Rodinia break-up. The East African/Antarctic Orogen recorded the subsequent amalgamation of Western and Eastern Gondwana after ca. 580 Ma, contemporaneously with the beginning of subduction in the Terra Australis Orogen along the southern Gondwana margin. However, the Kalahari Craton was lately incorporated during the Late Ediacaran–Early Cambrian. The proposed Gondwana evolution rules out the existence of Pannotia, as the final Gondwana amalgamation postdates latest connections between Laurentia and Amazonia. Additionally, a combination of introversion and extroversion is proposed for the assembly of Gondwana. The contemporaneous record of final Rodinia break-up and Gondwana assembly has major implications for the supercontinent cycle, as supercontinent amalgamation and break-up do not necessarily represent alternating episodic processes but overlap in time.  相似文献   

Rb-Sr Isotope Dating of Neoproterozoic (Malani Group) Magmatism from Southwest Rajasthan, India: Evidence of Younger Pan-African Thermal Event by Ar-Ar Studies     

S.S. Rathore  T.R. Venkatesan  R.K. Srivastava   《Gondwana Research》1999,2(2):271

This paper reports Rb-Sr isotope ages of the Neoproterozoic volcanics, and associated granitoids of the trans-Aravalli belt of northwestern India. All these rocks along with the earlier reported 779±10 Ma old felsic volcanics from Diri, and Gurapratap Singh of Pali district, Rajasthan, constitute the Malani Group. The study indicates that different rock suites belonging to the Malani Group represent a polyphase igneous activity which spanned for about 100 Ma ranging from 780 to 680 Ma. The granitoids of the Malani Group, i.e. peraluminous Jalore type, and peralkaline Siwana type, were emplaced around 730, and 700 Ma ago, respectively. These plutonic suites represent two different magmatic episodes within a short time interval. The initial Sr ratios of these granitoids suggest lower crustal derivation of the magma. The peralkaline granitoids, and the associated peralkaline rhyolites (pantellerites) are coeval, and cogenetic. The ultrapotassic rhyolite exposed at Manihari of Pali district represents the youngest magmatic activity at 681±20 Ma, having a very high initial Sr ratio of 0.7135±0.0033. The high initial Sr ratio of these rocks may be due to incorporation of radiogenic ⁸⁷Sr from the country rock, by assimilation or fusion, into the residual fraction of the magma in the crust which gave rise to other differentiated rocks of the Group.⁴⁰Ar³⁹Ar studies of two Jalore granite samples indicate presence of post crystallisation thermal disturbance between 500550 Ma ago. The timing of this thermal overprinting on the Malani rocks is related to the widespread Pan-African thermo-tectonic event which is witnessed, and magmatically manifested in different part of the Indian shield.  相似文献   

刚果盆地西南缘Inkisi组碎屑锆石U-Pb年代学: 对地层时代及物源的约束     

靳立杰 《地质与勘探》2024,60(3):515-529

Inkisi组是泛非运动之后刚果盆地最早的沉积地层之一。确定Inkisi组地层时代、物源及恢复构造演化过程,对研究刚果盆地的演化具有重要地质意义。本文通过对Inkisi组碎屑锆石进行U-Pb年代学研究,深入探讨了其形成时代,对泛非运动进行了约束,并进一步探讨了其物源。研究发现,Inkisi组下段与上段碎屑锆石最小年龄分别为526±6 Ma和534±14 Ma,最年轻的4颗锆石的加权平均年龄为532±9.4 Ma,由此限定了Inkisi组的最大沉积时代。因此将其时代划归早寒武世,并将其从新元古界Xisto-Gresoso群解体。另外,Inkisi组岩石并未发生变质变形,也说明其形成于泛非运动之后。碎屑锆石年龄谱结果显示,Inkisi组碎屑锆石的年龄变化范围较广,整体呈现五个年龄峰值：＞2800 Ma、2800~2400 Ma、2100~1750 Ma、1500~900 Ma、850~500 Ma。通过将年龄峰值及周围地质体进行对比,认为西刚果造山带、卡赛克拉通、安哥拉克拉通是其主要的物源,卢弗里安构造带等是其次要物源。  相似文献   

U-Pb and Pb-Pb zircon dating of the older orthogneiss suite in the Silvretta nappe,eastern Alps: Cadomian magmatism in the upper Austro-Alpine realm     

Bernhard Müller  Urs Stefan Klötzli  Markus Flisch 《International Journal of Earth Sciences》1995,84(3):457-465

Two orthogneiss suites dominate the Silvretta nappe. Primary crystallization of the larger suite (younger orthogneisses) is assumed to be Ordovician in age. The second, adjacent magmatic suite consists of older, alkaline to calc-alkaline, ultrabasic, basic to intermediate and granitic rocks known as older orthogneisses. U-Pb data of multigrain zircon fractions, as well as single zircon stepwise evaporation ²⁰⁷Pb/²⁰⁶Pb results suggest a latest Proterozoic to early Cambrian intrusion age for the protoliths of the older orthogneisses as both dating methods yield early Cambrian crystallization ages of 526±7 and 519±7 Ma for an alkaline granite gneiss; similar results were obtained for two neighbouring calc-alkaline orthogneisses (²⁰⁷Pb/²⁰⁶Pb ages of 533 ± 4 and 568 ± 6 Ma, respectively). The crystal habitus corresponds to P5, S19 and S9 zircons of magmatic origin. Whole-rock initial Sr isotope ratios indicate a primitive source. The igneous protoliths of these older orthogneisses represent a fragment of a Cadomian (Pan-African) crust found in places within the basement of the European Hercynides.  相似文献   

Isothermal decompression history in the “Western Granulite” terrain, central Tanzania: Evidence from reaction textures and trapped fluids in metapelites     

Holger Sommer  Christoph Hauzenberger  Alfred Krner  Sospeter Muhongo 《Journal of African Earth Sciences》2008,51(3):123-144

The Mozambique Belt (MB) of the East Africa Orogen contains large areas of granulite-facies migmatitic gneisses with Archaean and Palaeoproterozoic protolith ages and that were recycled during the Neoproterozoic Pan-African orogeny. The study area is situated along the Great Ruaha River and within the Mikumi National Park in central Tanzania where migmatitic gneisses and mafic to intermediate granulites are interlayered with Neoproterozoic granulite-facies migmatitic metapelites. Mineral textures suggest isothermal decompression, with the peak mineral assemblage comprising Grt–Bt–Ky–Kfs–Pl–Qtz ± Phn ± Ti-Oxide ± melt and amphibolite-facies retrograde assemblage Grt–Bt–Sil–Ms–Kfs–Pl–Qtz ± Fe–Ti-Oxide. The near isothermal retrograde overprint is seen in well-developed formation of pseudomorphs after garnet. The HP granulite-facies assemblages record P–T conditions of 13–14 kbar at 760–800 °C. Retrogression and the release of fluids from crystallizing melts occurred at 7 kbar and 650–700 °C. A fluid inclusion study shows three types of fluid inclusion consisting of nearly pure CO₂, as well as H₂O–NaCl and H₂O–CO₂ mixtures. We suggest that a immiscible CO₂-bearing brine represents the fluid composition during high-grade peak metamorphism, and that the fluid inclusions containing H₂O–NaCl or nearly pure CO₂ represent trapped fluids from in situ crystallised melt. The results suggest strong isothermal decompression, which is probably related to a fast exhumation after crustal thickening in the central part of the Mozambique Belt in Tanzania.  相似文献   

High-pressure granulite facies metamorphism in the Pan-African belt of eastern Tanzania: P–T–t evidence against granulite formation by continent collision     

P. APPEL  A. MÖLLER  & V. SCHENK 《Journal of Metamorphic Geology》1998,16(4):491-509

To constrain the tectonic history of the Pan-African belt in Tanzania, we have studied the P–T evolution of granulites from northern and eastern Tanzania representative for a large part of the southern Pan-African belt of East Africa (e.g. Pare, Usambara, Ukaguru and Uluguru Mountains). Thermobarometry (conventional and multireaction equilibria) on enderbites and metapelites gives 9.5–11 kbar and 810±40 °C during peak metamorphism at 650–620 Ma. This is consistent with the occurrence of both sillimanite and kyanite in metapelites and of the high-P granulite facies assemblage garnet–clinopyroxene–quartz in mafic rocks. Peak metamorphic conditions are surprisingly similar over a very large area with N-S and E-W extents of about 700 and 200 km respectively. The prograde metamorphic evolution in the entire area started in the kyanite field but evolved mainly within the sillimanite stability field. The retrograde P–T evolution is characterized by late-stage kyanite in metapelites and garnet–clinopyroxene coronas around orthopyroxene in meta-igneous rocks. This is in agreement with thermobarometric results and isotopic dating, indicating a period of nearly isobaric and slow cooling prior to tectonic uplift. The anticlockwise P–T path could have resulted from magmatic underplating and loading of the lower continental crust which caused heating and thickening of the crust. Substantial postmetamorphic crustal thickening of yet unknown age (presumably after 550 Ma) led subsequently to the exhumation of high-P granulites over a large area. The results are consistent with formation of the Pan-African granulites at an active continental margin where tonalitic intrusions caused crustal growth and heating 70–100 Ma prior to continental collision. The P–T–t path contradicts recent geodynamic models which proposed tectonic crustal thickening due to continental collision between East and West Gondwana as the cause of granulite formation in the southern part of the Pan-African belt.  相似文献   

中国东北地区西部~500Ma泛非期孔兹岩系地球化学特征及其地质意义   总被引：4，自引：3，他引：1  

董策  周建波 《岩石学报》2012,28(9):2866-2878

最近在兴安、额尔古纳地块均发现原岩为中基性-中酸性火山岩、陆源碎屑沉积岩及碳酸盐岩组合组成的孔兹岩系,岩石组合主要分为富铝片岩-片麻岩类和斜长角闪岩两类。岩石地球化学特征表明,富铝片岩-片麻岩类的原岩为泥岩和砂岩,主量和微量元素的分配形式接近澳大利亚后太古宙页岩(PAAS)和北美页岩(NASC),稀土元素的分布特征与NASC相似,显示轻稀土元素富集及Eu负异常,(La/Yb)N=4～17,δEu=0.32～0.79。其Cr/Ti(0.01～0.03)、Zr/Y(1.2～13.3)、Cr/Th(0.1～8.63)、Cr/Zr(0.02～0.47)、Th/U(1.5～7.7)、CIW(68～99)表明物源区以花岗岩类为主,并可能含有碎屑沉积岩。其Ceanom(-0.04～-0.01)、Ce/Ce#(0.90～1.30)表明富铝片岩-片麻岩类原岩沉积环境为亚稳定的大陆边缘沉积。斜长角闪岩原岩为玄武岩或安山质玄武岩,其富Fe2O3(1.83%～12.37%)、MgO(4.28%～7.29%)、贫SiO2(47.06%～56.13%),呈低碱性。轻稀土元素相对富集,MORB标准化的微量元素配分型式为大隆起型,Zr/Nb、Hf/Th值均类似于板内拉斑玄武岩的特征。因此,研究区孔兹岩系应形成于裂谷沉积背景。这与佳木斯地块麻山群孔兹岩系形成环境一致,表明东北地区各地块的变质基底均为孔兹岩系,其原岩均形成于裂谷沉积环境,并统一经历～500Ma泛非期变质事件,这为研究我国前寒武纪的大陆演化、再造我国东北地区的构造格局及其在Gondwana大陆重建中的位置都将具有重大的理论意义。  相似文献   

Geochemical features of the Matomb alluvial rutile from the Neoproterozoic Pan-African belt,Southern Cameroon     

《Chemie der Erde / Geochemistry》2014,74(4):557-570

The Matomb region constitutes an important deposit of detrital rutile. The rutile grains are essentially coarse (> 3 mm), tabular and elongated, due to the short sorting of highly weathered detritus. This study reports the major, trace, and rare-earth element distribution in the bulk and rutile concentrated fractions. The bulk sediments contain minor TiO₂ concentrations (1–2 wt%), high SiO₂ contents (∼77–95 wt%) and variable contents in Al₂O₃, Fe₂O₃, Zr, Y, Ba, Nb, Cr, V, and Zn. The total REE content is low to moderate (86–372 ppm) marked by high LREE-enrichment (LREE/HREE ∼5–25.72) and negative Eu anomalies (Eu/Eu^* ∼0.51–0.69). The chemical index of alteration (CIA) shows that the source rocks are highly weathered, characteristic of humid tropical zone with the development of ferrallitic soils. In the concentrated fractions, TiO₂ abundances exceed 94 wt%. Trace elements with high contents include V, Nb, Cr, Sn, and W. These data associated with several binary diagrams show that rutile is the main carrier of Ti, V, Nb, Cr, Sn, and W in the alluvia. The REE content is very low (1–9 ppm) in spite of the LREE-abundance (LREE/HREE ∼4–40). The rutile concentrated fractions exhibit anomalies in Ce (Ce/Ce^* ∼0.58 to 0.83; ∼1.41–2.50) and Eu (Eu/Eu^* ∼0.42; 1.20–1.64). The high (La/Sm)_N, (La/Yb)_N and (Gd/Yb)_N ratios indicate high REE fractionation.  相似文献