Agates from Paleoproterozoic Volcanic Rocks of the Onega Structure,Central Karelia

Geology of Ore Deposits - Agate mineralization in Central Karelia, Northwest Russia is related to the Paleoproterozoic volcanic rocks of the Ludicovian Superhorizon (2.05–1.95 Ga) within the...     

Nd and Sr isotopic geochemistry of mafic layered intrusions in the eastern Baltic shield: implications for the evolution of Paleoproterozoic continental mafic magmas

 Nd and Sr isotopic data are presented for the 2449–2441 Ma Olanga and Burakovka layered mafic complexes in the eastern Baltic Shield. These complexes have similar tectonic position, but differ in two aspects: the age of the enclosing crust and the post-crystallization metamorphic history. The Sm–Nd isotopic results for the Kivakka and Lukkulaisvaara intrusions, Olanga Complex, are consistent with the model of closed-system crystallization of a single magma without significant wallrock assimilation. The Rb–Sr systems of minerals were disturbed by late Rb addition during 1.75–1.50 Ga metamorphism. The Nd and Sr isotopic systems in the Burakovka complex show no metamorphic disturbance and indicate mixing of at least four isotopically distinct components. Isotopic variations in the Burakovka Complex can be explained by a 4–20 per cent contamination of a primary komatiitic or picritic magma with a Mesoarchean crust, similar to that exposed in the region. A similar model, applied to the Olanga Complex using a Neoarchean crustal isotopic composition, cannot reproduce the observed isotopic signature. The nearly uniform initial ɛ_Nd values between −1 and −2.3, observed in the Kivakka and Lukkulaisvaara intrusions of the Olanga Complex, as well as in the other 2.50–2.44 Ga layered mafic intrusions throughout the eastern Baltic Shield, are better explained by a mantle plume model with small amounts of crustal contamination and minor involvement of asthenospheric material. This model is also consistent with the geological observations and the temporal distribution of the Paleoproterozoic mafic magmatism in the eastern Baltic Shield. As an alternative, the enriched isotopic characteristics may be explained by melting of a metasomatically modified lithospheric mantle source. Received: 4 August 1994/Accepted: 5 April 1996     

Geophysical structures and tectonic evolution of the southern Guyana shield,Brazil

Aerogeophysical data of an area located on the southern portion of the Guyana shield in Brazil was processed using a fine interpolating mesh, and a corresponding spatial data integration strategy which included the stacking of different high-resolution images, and interpretation following quality control of these. The selected images were correlated to the local known surface geologic units, and to the spatial distribution of the main geochronological provinces of the Amazonian craton. The interpretation of the results also included the available geophysical information for the region, related to Moho depth values, and previously determined SKS shear-wave splitting direction. The observed magnetic regional trends may be strongly influenced by the Proterozoic crustal structure in the area, while radiometric anomalies correlate with the more detailed geologic features. Based on the parallelism among mapped geochronological provinces of the Amazonian craton, and observed geophysical structures on the study area, a geotectonic model is proposed for southern Guyana shield at Proterozoic age.     

Paleoproterozoic tectonic evolution of the Losevo Suture Zone in the Voronezh Crystalline Massif

A new model of the tectonic evolution of the Losevo Suture Zone is advanced based on the composition of the rocks occurring therein, the U/Pb age of the Voronezh Formation, and the specified age of collision. The model comprises the destruction of the Sarmatia continent; the formation of active continental margins of the West Pacific and Andean types, replacing one another en echelon from north to south; and the collision of Sarmatia and Volgo-Uralia with a distinctly expressed late orogenic stage marked by deposition of the Voronezh Formation crowned by the Baigora volcanic-plutonic structure.     

Tertiary tectonic evolution along the Arne-Elin Trend in the Danish Central Trough

Tertiary sequence boundaries are mapped on reflection seismic sections in the northern part of the Danish Central Trough, North Sea. The geometry of the sequence boundaries and the isopach maps of the sequences are used as indicators of differential subsidence. The Tertiary tectonic evolution is subdivided into: (i) Late Palaeocene and Early Oligocene episodes of inversion along the NNW–SSE striking Arne–Elin Trend (ii) Early Oligocene to Pliocene episodes of differential subsidence across pre-Tertiary fault trends. The observed differential subsidence and inversion is related to pre-Tertiary faults and fault trends, and dating of the sequences shows that the tectonic phases were contemporaneous with tectonic events in NW Europe. It is suggested that the differential subsidence-inversion is controlled by reactivation of pre-Tertiary faults/fault-trends by block movements due to an interaction of the Alpine collision and the opening of the Atlantic.     

A model for the tectonic evolution of the Benue Trough of Nigeria

Based on the results of a detailed magnetic study of the Lower and Middle Benue Trough of Nigeria carried out by the author and results of previous geophysical studies of the Benue Trough and of similar structures elsewhere in the world, a possible model for the tectonic evolution of the Benue Trough has been developed. It is here suggested that the tectonic evolution of the Benue Trough of Nigeria involved the rise of a mantle plume or mantle upwelling, emplacement of intrusive igneous material in the crust, crustal stretching and thinning and consequently rifting. It is thought that this sequence of events may have been repeated in a cyclic manner with intercyclic structural deformations. The basic ideas of the model is shown to be in agreement with acceptable ideas on riftogenesis.

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Zusammenfassung Es wird eine mögliche Modellvorstellung für die tektonische Entwicklung des Benue-Grabens aufgezeigt, die auf Ergebnissen einer detaillierten magnetischen Untersuchung des Autors im unteren und mittleren Benue-Graben fußt, sowie auf Ergebnissen aus vorangegangenen geophysikalischen Untersuchungen im Benue-Graben und in anderen, vergleichbaren Strukturen. Die tektonische Entwicklung des Benue-Grabens in Nigeria wird mit dem Aufstieg eines mantle plume, einer Mantelaufwölbung oder mit der Platznahme von intrusivem Magma in der Kruste, Krusten-Dehnung und -Ausdünnen in Verbindung gebracht. Es wird angenommen, daß diese Folge von Ereignissen sich zyklisch wiederholte mit strukturellen Deformationen zwischen den Zyklen. Die Grundgedanken dieses Modells stimmen mit den gängigen Vorstellungen über die Entstehung von Riffsystemen überein.

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Résumé L-auteur présente un modèle possible de l'évolution tectonique du graben de Benue (Nigéria). Cette étude se base: d'une part sur une analyse magnétique détaillée, effectuée par l'auteur, des régions inférieure et centrale de la dépression, d'autre part sur des données géophysiques antérieures recueillies dans la même région, ainsi que dans d'autres structures analogues, ailleurs dans le monde. Le modèle propose pour l'évolution tectonique du graben, l'intervention de la montée d'une «mantle plume» ou d'un bombement du manteau, la mise en place de magmas intrusifs dans la croûte, l'étirement et l'amincissement de la croûte, et la fracturation en graben. Cette suite d'événements a pu se répéter de manière cyclique, avec déformations structurales entre les cycles. Les conceptions de base de ce modèle sont en accord avec les idées admises au sujet de la genèse des rifts.

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Benue . Benue , , . , . .

     

Rock salt mass in the Paleoproterozoic sequence of the Onega trough in Karelia (from the Onega parametric well data)

A thick (200 m) rock salt mass covering Late Archean granitoids was exposed for the first time in the Early Proterozoic volcanogenic-sedimentary formations in the Onega trough of the east Baltic Shield by the Onega parametric well. The mineral composition of salts, their geochemical features, and the isotopic composition of carbonate carbon and oxygen have been studied. After fluid inclusions present in salts, their metamorphism temperature and isotopic composition of helium and argon were determined. The obtained results give evidence of the fact that rock salts and magnesites associated with them were formed in an evaporate basin with participation of deep crustal processes. The age of the underlying granitoids (2.716 ± 9 Ma) is determined using the Pb—Pb method.     

Near-surface earthquakes in the Baltic shield, notably Sweden

A recent series of Swedish earthquakes at a focal depth not exceeding 2–3 km, the largest with I_O = V + (MSK scale) andM_L = 3.2 shows that relatively strong seismic activity can occur in the uppermost part of the Baltic Shield. During the last 15 years several near-surface earthquakes have occurred in this region, as indicated by recorded Rg-waves and/or macroseismic data. Many events are located along the coast of central Sweden, suggesting a seismic belt of minor, near-surface activity, which should be considered in connection with the radioactive waste storage in the Swedish bedrock. The appearance of Rg, common in seismic records of explosions and rockbursts, is not a sufficient discriminator between artificial events and earthquakes.     

Proterozoic tectonic evolution and late Svecokarelian plate deformation of the Central Baltic Shield

The continental crust of the Central Baltic Shield evolved by accretion towards the west during the Svecokarelian orogeny 1700–2200 Ma ago. The following features are consistent with a plate tectonic mechanism involving subduction of oceanic crust below an Archean craton in the east: flysch-sediments with serpentinite masses and pillow lavas, linear high-grade metamorphic zones, island-arc type volcanic belts and late tectonic batholiths with porphyry type Cu-Mo deposits.Semi-consolidated new crust was affected by late Svecokarelian deformation (D_n) after 1850 Ma; NNE-trending folds with crenulation cleavage were overprinted on older structures together with associated NW trending ductile transcurrent shear zones that curve the F_n folds into gentle S and Z shapes. The late tectonic batholiths intruded partly at the same time as and partly after the D_n deformation.

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Zusammenfassung Die kontinentale Kruste des zentralen Baltischen Schildes entwickelte sich durch nach Westen gerichtetes Anwachsen während der Svecokarelischen Orogenese vor 1700 bis 2200 Ma. Die folgenden Erscheinungsformen lassen sich mit einem plattentektonischen Mechanismus in Einklang bringen, der Subduktion von ozeanischer Kruste unter einen Archaischen Kraton im Osten einschließt: Flysch-Sedimente mit Serpentinit-Massen und Kissenlaven, lineare hochmetamorphe Zonen, vulkanische Gürtel vom Inselbogen-Typ und spättektonische Batholithe mit porphyrischen Cu-Mo-Lagerstätten.Die halbkonsolidierte neue Kruste wurde durch späte Svecokarelische Deformation (D_n) nach 1850 Ma erfaßt; NNE-orientierte Falten mit Krenulationsschieferung wurden älteren Strukturen aufgeprägt in Verbindung mit NW-streichenden, plastischen Transcurrent-Scherzonen, die die F_n-Falten in sanfte S- und Z-Formen verbiegen. Die spättektonischen Batholithe intrudierten teils während, teils nach der D_n-Deformation.

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Résumé La croûte continentale du Boucher baltique central a évolué par voie d'accrétion vers l'ouest durant l'orogénie svécocarélienne 1700–2200 Ma. Les événements suivants sont en accord avec un mécanisme de tectonique de plaques impliquant la subduction d'une croûte océanique sous un craton archéen à l'est: sédiments flyschoïdes avec masses de serpentinite et de laves en coussins, zones linéaires à haut degré de métamorphisme, ceintures volcaniques du type guirlande d'îles et batholithes tectoniques tardifs avec gisements porphyriques de type Cu-Mo.La nouvelle croûte à semi-consolidée fut affectée par une déformation svécocarélienne tardive (D_n) postérieure à 1850 Ma. Des plis de direction NNE avec clivage de crénulation ont été superposés sur des structures plus anciennes, associés à des zones de cisaillement transcurrentes de direction NW qui ont incurvé les plis F_n suivant des formes en S et Z. Le batholithe tectonique tardif s'est mis en place en partie au même moment que, et en partie après, la déformation D_n.

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, , 1500–2200 . , , : ; ; - . , 1850 ; NNE- , , NW , , F_n S Z. , .

     

Early tectonic zones in the baltic shield

A pattern of mainly NW-striking tectonic zones is described in the western part of the Northeastern European Platform.     

Neodymium isotope systematics of the mantle beneath the Baltic shield: Evidence for depleted mantle evolution since the Archaean

More than 300 published and unpublished Nd isotopic analyses of mantle derived rocks from the Baltic shield have been compiled. The rocks range in age from Archaean to Phanerozoic. Within any given age-interval, the mantle derived rocks range in ε_Nd(t) from depleted mantle values at or above the growth curves of the global depleted mantle reservoirs of DePaolo (1981) and DePaolo et al. (1991) to negative values. Initial neodymium isotopic compositions below the De Paolo curve are best explained by interaction between depleted mantle derived magmas and local crustal contaminants. The data now available lend no support to the existence of isolated, less depleted or undepleted mantle domains beneath the Baltic Shield, as was suggested by Mearns et al. (1986) and Valbracht (1991a, b).     

苏鲁造山带古元古代超高温麻粒岩及其构造意义

本文报道了在苏鲁超高压变质带北端威海地区识别出的古元古代超高温泥质麻粒岩。泥质麻粒岩与大理岩、钙硅酸盐岩和片麻岩等变质表壳岩伴生。这些表壳岩以透镜体的形式产于经历了三叠纪超高压变质的新元古代正片麻岩中。泥质麻粒岩由石榴石、夕线石、斜长石、钾长石、反条纹长石、石英、黑云母、白云母和金红石组成。黑云母和白云母呈石榴石和夕线石的冠状体产出,为退变质矿物。相平衡模拟结果表明,麻粒岩峰期变质的温度和压力条件为~940℃和~1.2GPa。泥质麻粒岩中的锆石近等粒状或椭圆形,具有扇状或冷杉状分带,大多数具有平坦甚至亏损的重稀土配分模式,并有Eu负异常,为典型的麻粒岩相变质锆石。锆石的LA-ICP-MS U-Pb定年结果给出了1845±9Ma的207Pb/206Pb加权平均年龄,表明超高温麻粒岩形成于古元古代。结合现有的其他研究结果,我们推测扬子板块北缘经历了古元古代的增生造山作用。     

Mineralogy, petrography, geochemistry and genesis of the Paleoproterozoic Birimian manganese-formation of Nsuta/Ghana

The manganese deposit of Nsuta, in the Ashanti Belt of Southern Ghana, is sandwiched between Birimian metasedimentary rocks. The metasedimentary rocks contain interbedded carbonate-rich layers, which exhibit a characteristic banded appearance near the contact with the orebody. The orebody is a carbonate-type manganese-formation and in terms of origin is considered here as a Mn-analogue of the volcanogenic-exhalative Algoma type iron-formation. The protolith of the orebody (chemical sediment including Fe-bearing rhodochrosite and alabandite) is envisioned to have been formed in a marine basin with relatively high CO₂ activity and Eh-pH conditions were extremely low (Eh 1 to −0.6 Volt and pH 8 to 11) during Birimian times (2170–2180 Ma). These conditions occurred immediately below the shelf break in a shallow-marine environment. Subsequent submarine weathering (halmyrolysis) followed later by metamorphism of Eburnian age (2100 Ma) led to the formation of Mg-Ca-Fe-bearing rhodochrosite, the dominant mineral in the orebody. Other minerals of the orebody are: sulfides (e.g. two generations of alabandite sphalerite, pyrite, millerite, niccolite, gersdorffite, and molybdenite), oxides and hydroxides (vanadium-bearing jacobsite, galaxite; brucite, Mn²⁺-todorokite), Mn-silicates and an unknown boron mineral. Pyrochroite, possibly preceded by manganosite, occurs as a retrograde mineral. This mineral assemblage forms the protore of the Nsuta deposit. Opaque Mn⁴⁺-todorokite replacing Mn²⁺-todorokite, manganite, manganomelane, pyrolusite and nsutite which formed at the expense of rhodochrosite, are of supergene origin and represent the economic part of the deposit. The orebody is interleaved between the associated pelitic-psammitic metasedimentary rocks suggesting that its protoliths was deposited over a time interval during the sedimentation of the latter. Both units underwent subsequent processes (submarine weathering and metamorphism) together. The compositional differences between the orebody with high Mn and CO₂ and low Si and Al contents relative to the metasedimentary rocks are explained by a model involving the continuous sedimentation of continent-derived materials (protolith of the metasedimentary rocks). During this time a pulsatory phase of submarine volcanism and consequent precipitation of materials of essentially volcanogenic-exhalative origin occurred (protolith of the orebody). From the exhalations, the carbonate minerals in both the manganese-rich sediments and the metasedimentary host-rocks (in the latter in the form of layers and disseminations leading to relatively high concentrations of Mn, Ca and CO₂) were precipitated. Received: 18 April 1997 / Accepted: 16 July 1998     

五台地区雁门关早元古代镁铁-超镁铁岩体的岩石地球化学特征、成因及其大地构造意义

本文对五台地区侵入于原北金岗库组的雁门关镁铁-超镁铁岩体进行了详细的岩石学、矿物学和岩石地球化学分析研究.雁门关岩体在矿物成份上,特别是单斜辉石、尖晶石和角闪石,具有阿拉斯加型侵入岩体和俯冲带或弧相关的岩浆特征,其元素地球化学特征也可与阿拉斯加型侵入杂岩体类比,具有明显的Nd (Ta),Zr (Hf),Y和Ti负异常.从构造环境判别图来看,雁门关岩体主要为与俯冲相关的成因,具大陆边缘弧的性质.因此,雁门关岩体是侵入在大陆边缘弧环境的岩浆经历了橄榄石和单斜辉石的分离结晶作用的产物.它与恒山-五台-阜平地区广泛分布的同时代花岗岩和火山岩一起,构成了华北克拉通中部造山带内一期重要的古元古代大陆边缘岛弧岩浆活动.     

Paleoproterozoic evolution of Fennoscandia and Greenland

The Paleoproterozoic evolution of Fennoscandia and Greenland can be divided into major rifting and orogenic stages. The Paleoproterozoic rifting of Fennoscandia started with 2.505-2.1 Ga, multiphase, southwest-prograding, intraplate rifting. Both Fennoscandia and Greenland experienced 2.1- 2.04 Ga drifting and separation of their Archean cratons by newly-formed oceans. The main Paleoproterozoic orogenic evolution of Fennoscandia resulted in the Lapland-Kola orogen （1.94-1.86 Ga） and the composite Svecofennian orogen （1.92-1.79 Ga）. The Paleoproterozoic orogens in Greenland, from north to south, are the lnglefield mobile belt （1.95-1.92 Ga）, the Rinkian .fold belt/Nagssugtoqidian orogen （1.88-1.83 Ga） and the Ketilidian orogen （c. 1.8 Ga）. The Lapland-Kola orogen, Inglefield mobile belt and the Rinkian fold belt/Nagssugtoqidian orogen are continent-continent collision zones with limited formation of new Paleoproterozoic crust, whereas the Ketilidian orogen displays a convergent plate-tectonic system, without subsequent collision. The composite Svecofennian orogen is responsible for the main Paleoproterozoic crustal growth of Fennoscandia.     

赣南古家营盆地英安岩时代、成因及对早古生代构造演化的制约

华南板块早古生代的构造属性是长期存在争议的重大地质问题。以赣南古家营盆地英安岩作为研究对象,通过岩石学、野外实测剖面的测量、SHRIMP锆石U-Pb年龄、主微量元素及Sr-Nd-Pb-O同位素分析,对其岩石类型、地层归属、成因及构造环境进行了研究。结果表明：该火山岩具有富硅、富钾、低钠、贫镁的特点,属于高钾钙碱性英安岩;轻重稀土元素分馏明显（La/Yb=5.52~15.08）,富集大离子亲石元素Rb、Th、K,亏损高场强元素Ta、Nb及呈现明显的负Eu异常,显示出火山弧的特征;SHRIMP锆石U-Pb年龄分析结果为439.0±4.0 Ma,与南迳英安岩、安山岩年龄一致,均为晚奥陶世末期—早志留世初期岩浆活动的产物,古家营剖面与南迳盆地乔子山标准剖面及中寨标准剖面相似,为一套灰白色英安岩、流纹岩、火山碎屑岩、浅变质岩系组合,在地层上归属于南迳组;负的εNd（t）值（-6.5~-6.3）、较小的（87Sr/86Sr）i值（0.70624~0.70704）、较年轻的TDM2（1685~1703 Ma）,以及较高的（206Pb/204Pb）i、（207Pb/204Pb）i、（208Pb/204Pb）i值,指示古家营英安岩为中元古代地壳物质部分熔融形成。古家营英安岩形成于与俯冲有关的岛弧环境,赣南—粤北地区在晚奥陶世—早志留世存在洋陆俯冲作用。