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1.
As a result of studying the Vetreny Belt greenstone structure (the southeast of the Baltic Shield), zircons from terrigenous deposits of the Toksha Formation, underlying the section of the sedimentary-volcanogenic complex, and zircons of the Vetreny Belt Formation, deposits of which crown the section, were dated. The results of analysis of age data of detrital zircons from quartzites of the Toksha Formation indicate that Mesoarchean greenstone complexes and paleo-Archean granitogneisses of the Vodlozero Block (Karelia) were the provenance area from which these zircons were derived. The occurrence of the youngest zircons with age of 2654.3 ± 38.5 Ma is evidence that the formation of the Vetreny Belt, including the Toksha Formation, began no earlier than this time. Zircons from volcanic rocks of the Vetreny Belt yielded the age of 2405 ± 5 Ma. Thus, the age interval of the formation of the sedimentary-volcanogenic complex of the Vetreny Belt ranges from 2654.3 ± 38.5 to 2405 ± 5 Ma.  相似文献   

2.
The Keivy Terrane in the northeastern Baltic Shield appreciably differs from the adjacent tectonic blocks. In the northwestern part of this terrane (the Serpovidny Range), an outlier of Paleoproterozoic supracrustal rocks called the Serpovidny structure is surrounded by Archean (?) Keivy high-alumina paraschists. As follows from structural and magnetic data, the Paleoproterozoic rocks are deformed into a tight sheath fold 8 × 2 km in size at the surface and 5 km in length along the sheath axis. Faults parallel to the boundaries of the layers and locally cutting them off at an acute angle are involved in folding as well. The outer boundaries of the Serpovidny structure are tectonic. This structure is complementary to a larger tectonic lens composed of the Keivy mica schists. It is concluded that all of the supracrustal rocks of the Serpovidny Range are in fact tectonic sheets and lenses deformed into sheath folds. The literature data show that kilometer-scale sheath folds occur throughout the Keivy paraschist belt and most likely were formed owing to thrusting of the Murmansk Craton onto the Keivy Terrane in the south-southwestern direction. Foliation and lineation related to thrusting have been established in the Archean silicic metavolcanics and peralkaline granites occupying the most part of the terrane. In contrast, the granitoids and gabbroanorthosites of the Archean basement, which form a block 90 × 20 km in the southwestern Keivy Terrane, were not affected by Paleoproterozoic deformation. In other words, a detached assembly of tectonic sheets composed of the upper and middle crustal rocks that underwent deformation at the initial stage of the Paleoproterozoic Lapland-Kola Orogeny and the Archean basement, which is free of this deformation, are distinguished. The depth of detachment is estimated at 20–25 km. The detachment of the upper and middle crust in the Keivy Terrane and its position in the structure of the Baltic Shield are consistent with a spatiotemporal succession that resulted in the formation of a Paleoproterozoic supercontinent and the Baltic Shield as its fragment. This succession began with the amalgamation and deformation of the Archean terranes in the northeast of the Baltic Shield during the Lapland-Kola Orogeny, the Keivy Terrane showing a record of the earliest reworking (1.97–1.93 Ga). The succession completed in the southern and southwestern parts of the shield (1.80 Ga) after the Svecofennian Orogeny, expressed in the accretion of island-arc terranes composed of Paleoproterozoic juvenile crust to the continent.  相似文献   

3.
The components of deformation related to endogenic (tectonic) and exogenic (glaciotectonic, gravitational sliding) factors are considered in the sedimentary platform cover by a study of Andoma dislocations in the Upper Devonian sedimentary rocks of the Lake Onega district. These structures are localized in the eponymous segment of the southeastern margin of the Fennoscandian (Baltic) Shield; indications of high tectonic activity are noted along this margin. As has been shown by a structural study, the development of the Andoma structure is caused by long-term (Late Devonian–Quaternary) multistage deformations related to tectonic factors and, to a lesser degree, glaciotectonics. The consecutive stages of synsedimentation normal faulting and landsliding, bedding-plane gliding, various forms of shear flow in a regime of transpression, glaciotectonic thrusting, and neotectonic transpression are recognized in the deformation history.  相似文献   

4.
The Kola region in the northeastern Baltic Shield is characterized by diverse Paleoproterozoic collision processes. The Keivy Terrane is one of the major tectonic units in the northeastern foreland of the Paleoproterozoic Lapland-Kola Collisional Orogen, which markedly differs in a number of parameters from other tectonic units of the Kola region. The study of the Keivy Terrane allowed us to unravel one more basic difference: the large Paleoproterozoic sheath synform of the Serpovidny (Crescentic) Range localized in this terrane. Its core is occupied by volcanic and sedimentary rocks, which correlate with the fill of the Imandra-Varzuga Rift; the limbs are composed of metamorphosed mature sedimentary rocks known as Keivy paraschists of Neoarchean or Paleoproterozoic age. The lower limb of the Serpovidny Synform is strongly squeezed, whereas the upper limb consists of almost undeformed rocks. The deformed rocks underwent ductile flow under conditions of simple or general shear. In the degree of its asymmetry and main parameters, the Serpovidny Synform is similar to the plunging and recumbent anticlines in the Helvetic nappes of the Alps. It is concluded that the Paleoproterozoic core of the Serpovidny Sheath Synform, or plunging anticline, is a fragment of the almost completely eroded deep Serpovidny Nappe of the Helvetic type. During the collision related to the Lapland-Kola Orogeny (1.9–2.0 Ga), this nappe was pushed out northward from the Paleoproterozoic Imandra-Varzuga Rift, which is situated 50 km south of the Serpovidny structure, and thrust over the Keivy paraschists. The latter, together with underlying the Lebyazhka Gneiss, were folded in the process of thrusting and were involved in the structure of the Serpovidny Synform. The Keivy paraschists make up a para-autochthon or a separate nappe of the Pennine type. The Archean Lebyazhka metafelsic volcanics underlie the Keivy paraschists and overlie granitoids of the Archean basement that remained undeformed during thrusting. Most likely, they also belong to the para-autochthon; however, it cannot be ruled out that, like the Keivy paraschists, they occur as a Pennine-type nappe. The large sheath folds known in the Paleoproterozoic and Phanerozoic orogens are genetically related to deep-seated nappes or channel-flow tectonics. Paleoproterozoic and Phanerozoic orogens are similar in this respect.  相似文献   

5.
This paper presents the results of a study of the Paleoproterozoic basal garnet-kyanite-staurolite-two-mica paraschists from the Kukasozero structure of the Karelides of Northern Karelia, Baltic Shield, underlying Neoarchean acid metavolcanic rocks, and schists with quartz, phengite, kyanite, staurolite, garnet, and tschermakite located in the Paleoproterozoic rocks and considered to be metasomatic in origin. It was established that the sedimentary protolith of the Paleoproterozoic paraschists contains detritus of Neoarchean igneous rocks as follows from detrital 2737 ± 11 Ma zircons with oscillatory magmatic zoning. Metavolcanic 2757 ± 13 Ma rocks, close in age and composition, are known directly in the framework of the Kukasozero structure and are considered to be the most likely source of the sedimentary schist protolith. The coincidence of the Nd-model ages of paraschists (t DM is 2.73–2.76 Ga) with the age of detrital zircons indicates no contribution of older rocks to the protolith composition. The age of magmatic crystallization of metavolcanic rocks directly underlying the Paleoproterozoic paraschists is 2681 ± 18 Ma and coincides with the age of porphyry granites in the western framework of the structure (2680.3 ± 3.6 Ma). No detrital zircons of similar age were found in basal paraschists, but the restricted amount of dated zircons does not allow us to draw a final conclusion about the absence of detritus of the underlying metavolcanic rocks in the paraschist protolith. It was confirmed that phengite-bearing schists are the products of acid metasomatism of the Paleoproterozoic amphibolites and amphibole schists (metavolcanic rocks). The metasomatic features were revealed in garnet-kyanite-staurolite-two-mica paraschists, so the strict identification of their sedimentary protolith is impossible. The paraschists do not represent metamorphosed weathering crust, because acid metasomatism gives a false impression of the greater maturity of the primary sedimentary rocks.  相似文献   

6.
 In the central Vetreny Belt, southeastern Baltic Shield, an areally extensive 110 m deep lava lake is exposed consisting of remarkably fresh differentiated komatiitic basalt. During eruption, the liquid had a temperature of 1380–1400 °C and contained ∼15% MgO. The lava ponded in a large topographic depression soon after eruption. The differentiation of the lava lake was controlled by settling of transported olivine and chromite phenocrysts and caused the origin of prominent internal layering. The last portions of the trapped liquid crystallized at temperatures of 1250– 1070 °C. A Sm-Nd isochron of 2410±34 Ma for whole rock samples, olivine, augite and pigeonite separates from the lava lake provides a reliable estimate for the time of formation of the uppermost sequences in the Vetreny Belt. This age is in good agreement with the Sm-Nd and Pb-Pb isochron ages of 2449±35 and 2424±178 Ma for the volcanic rocks from the same stratigraphic level in the northwestern Vetreny Belt. Modeling of Nd-isotopes and major and trace elements shows that the komatiitic basalts at Lion Hills may have had a komatiite parent depleted in highly incompatible elements. It can be shown that this initial liquid was contaminated by 7–9% of Archaean upper crustal material from the adjacent Vodla and Belomorian Blocks en route to the surface thus acquiring the observed geochemical and isotope signatures including relative enrichment in Zr, Ba, and LREE, negative Nb- and Ti-anomalies and ɛNd(T) of −1. Received: 8 December 1995/Accepted: 26 March 1996  相似文献   

7.
Compositional peculiarities of the siliceous high-Mg series(SHMS)rocks formed at the Archean-Paleoproterozoic boundary as a function of plume activity are discussed using example of Early Paleoproterozoic mafic volcanic rocks of the Vodlozero Domain,Fennoscandian Shield.These rocks are characterized by wide variations in Mg#(33-67)and Cr contents(25-1123 ppm),LREE enrichment,and weakly negative_(εNd)(from-0.7 to-2.9).The high Gd/Yb ratio in the primitive high-Mg rocks of the Vodlozero Domain suggests their generation from a garnet-bearing source.At the same time,their negative _(εNd)in combination with LREE enrichment points to the crustal contamination.A new model was proposed to explain the remarkable global-scale similarity of SHMS.Such rocks can be generated by the contamination of a high-degree(30%)partial melt derived from a depleted mantle.The lower crustal sanukitoid-type rocks can be considered as a universal crustal contaminant.Modeling showed that such mixing can provide the observed narrow_(εNd)variations in Early Paleoproterozoic volcanics.The Neoarchean sanukitoid suites,which are widespread on all cratons,presumably composed the lower crust at the beginning of the Paleoproterozoic.Therefore,this mechanism can be considered universal for the genesis of the SHMS rocks.The high-to low-Cr rock series can be produced by the fractionation of the mafic melt coupled with an insignificant crustal assimilation of felsic end members of the sanukitoid suite of the Vodlozero Domain en route to the surface,as suggested by the positive correlation of_(εNd)with Cr and Mg#,negative correlation with Th,and slight decrease of_(εNd)in the more evolved varieties.  相似文献   

8.
The large (more than 200000 km2) Uchur-Maya Meso- and Neoproterozoic intracontinental basin is delineated for the first time in the southeastern Siberian Platform, including the territory overlapped by the Ediacaran to Cambrian plate complex. The tectonic elements of higher orders—uplifts, domes, and deeps formed during the Neoproterozoic and Late Mesozoic tectonomagmatic activity—are outlined. The tectonic nappes of Paleoproterozoic rocks from 200 to 400 m thick were established between the Calymmian and Ectasian to Stenian rocks in the southeastern portion of the basin. The thickness of sedimentary rocks increases toward the Yudoma-Maya Aulacogen. The considerable thickness of Meso- and Neoproterozoic rocks and bitumen contents in sedimentary rocks are favorable factors testifying to the petroleum resource potential of the basin. The southern part of the basin is promising for ore deposits (primarily, uranium and gold) in zones of structural and stratigraphic unconformities and within the sedimentary cover itself. This study may serve as a background for further, more detailed tectonic investigations and substantiation of strategic lines of geological exploration.  相似文献   

9.
The eastern margin of Sarmatia comprises the Paleoproterozoic (2.1–2.05 Ga) rock associations of the eastern Voronezh Crystalline Massif, including the Lipetsk-Losevo volcanic-plutonic belt and the adjacent East Voronezh lithotectonic zone composed of metasedimentary rocks of the Vorontsovka Group. The isotopic and geochemical study of the available drill cores that characterize the main rock associations of the Lipetsk-Losevo belt and its nearest framework allowed us to furnish evidence for the formation of this belt in the regime of an island arc at the active margin of the Archean continent above a low-angle subduction zone. The juvenile isotopic and geochemical signatures of metaturbidites of the Vorontsovka Group indicate that only a fast growing mountain edifice with the Lipetsk-Losevo Belt in its highest part (foreland) could have been a provenance of the flysch basin. It is proposed to name this Paleoproterozoic mountain system the East Sarmatian Orogen. The hinterland of this orogen embraced the megablock of the Kursk Magnetic Anomaly as a part of the Voronezh Massif and the Azov Block of the Ukrainian Shield. It has been shown that the East Sarmatian Orogen was formed in the same way as accretionary orogens of the Cordilleran type.  相似文献   

10.
The Ruiga differentiated mafic-ultramafic intrusion in the northwestern part of the Vetreny Belt paleorift was described for the first time based on geological, petrological, geochronological, and geochemical data. The massif (20 km2 in exposed area) is a typical example of shallow-facies peridotite-gabbro-komatiite-basalt associations and consists of three zones up to 810 m in total thickness (from bottom to top): melanogab-bronorite, peridotite, and gabbro. In spite of pervasive greenschist metamorphism, the rocks contain locally preserved primary minerals: olivine (Fo 75–86), bronzite, augite of variable composition, labradorite, and Cr-spinels. A mineral Sm-Nd isochron on olivine melanogabbronorite from the Ruiga Massif defines an age of 2.39 ± 0.05 Ga, while komatiitic basalts of the Vetreny Belt Formation were dated at 2.40–2.41 Ga (Puchtel et al., 1997). The rocks of the Ruiga intrusion and lava flows of Mt. Golets have similar major, rare-earth, and trace element composition, which suggests their derivation from a single deep-seated source. Their parent magma was presumably a high-Mg komatiitic basalt. In transitional crustal chambers, its composition was modified by olivine-controlled fractionation and crustal contamination, with the most contaminated first portions of the ejected melt. In terms of geology and geochemistry, the considered magmatic rocks of the Vetreny Belt are comparable with the Raglan Ni-PGE komatiite gabbro-peridotite complex in Canada (Naldrett, 2003).  相似文献   

11.
The Mid-Russian Dislocation Zone is a large within-plate structural element of the East European Platform, which extends for more than 1100 km from the Timan Foredeep to the Orsha Basin. This deep, long-lived zone was formed against a background of changeable geodynamic settings, including (1) Late Paleoproterozoic collision events, (2) Late Riphean-Early Vendian epicontinental rifting, (3) Late Vendian-Early Triassic intraplatform tectogenesis with formation of horst-like uplifts within the zone against the background of general subsidence, and (4) Mesozoic-Cenozoic within-plate reactivation. At the final Kimmerian-Alpine stage of its evolution, the Mid-Russian Zone developed as a left-lateral transpressional structure with penetrative dissipative shear deformation resulting in the general horizontal transfer of Phanerozoic sedimentary rocks. The dislocations were manifested as two dynamically conjugate structural forms: a zone of scattered shearing and a bedding-plane tectonic flow. The dynamic manifestation of the Mid-Russian and the conjugate Belomorian-Dvina zones, which make up a common arcuate structure (in plan view), allowed us to outline the Dvina-Sukhona plate-flow with horizontal mass transfer in the southeastern direction. The tectonics of the Mid-Russian Dislocation Zone is considered in this paper with particular emphasis on the structural and kinematic assemblies in sedimentary rocks of the Phanerozoic cover.  相似文献   

12.
A model of the evolution of the central Belomorian-Lapland Granulite-Gneiss Belt is proposed on the basis of analysis of the Paleoproterozoic structural-kinematic assemblages. It is shown that this tectonic zone is a long-lived mobile structural unit that evolved through several stages of tectonic transformation and metamorphism of rocks, including (1) the Reboly stage, which comprises subduction (2.88–2.82 Ga) and collision (2.74–2.53 Ga) substages; (2) the Selet stage of rifting and extension of the continental crust according to the model of simple shear (2.45–2.35 Ga); and (3) the Svecofennian stage, characterized by collision and general transpression (1.94–1.75 Ga). The results of structural-kinematic study indicate that tectonic flow in the Svecofennian time was nonuniform and related to the formation of the Kolvitsa-Umba near-horizontal protrusion. The propagation of this protrusion was caused by transpressional extrusion of plastic lower-crustal masses to the surface as a gently ascending tectonic flow directed to the northwest (in present-day coordinates). Thereby, a thrust-normal-fault kinematic effect was expressed in pushing-out of deep-seated complexes contemporaneously with tectonic erosion of the upper portions of the sequence owing to the development of lowangle normal faults.  相似文献   

13.
The geological setting, geochemistry, and Nd isotopic systematics of tonalite-trondhjemite-granodiortite (TTG) series in ancient cratons are considered. It is shown that the TTG series were formed from ∼4.2 to 2.6 Ga ago in the oldest continental cores; many TTG series do not reveal chronological links to greenstone belts. This follows from the evolution of the Slave Craton in the Canadian Shield, the Vodlozero Craton in the Baltic Shield, and the Pilbara and Yilgarn cratons in the Australian Shield, where greenstone associations postdated TTG series. As has been established at the Baltic Shield, the primary melts of the Mesoarchean TTG associations were formed at a shallower depth (P < 15 kbar) compared to the Neoarchean TTG, likely, due to the increasing thickness of the continental crust beneath the Baltic Shield over time.  相似文献   

14.
越南中部的Kontum 地块是印支地块中前寒武纪变质岩最重要的出露地区之一。该地块由不同时代不同变质程度的岩石组成。该研究通过对该地区主要变质岩的岩石学、地球化学和锆石U-Pb 年代学分析,探讨了Kontum 地块的物质组成以及其形成的构造背景。岩相学和化学成分分析结果表明,这些基底变质岩主要由变质沉积岩和少量的变质火成岩组成。变质沉积岩的原岩主要是硬砂岩和页岩。锆石U-Pb 定年结果显示,这些沉积岩形成于古元古代晚期到新元古代晚期,并大致可以划分为五期沉积盆地的形成和演化,分别对应岛弧环境、被动大陆边缘环境、活动大陆边缘或大陆弧环境、活动大陆边缘-被动大陆边缘过渡环境和活动大陆边缘的构造背景。二个变质火成岩的原岩分别是拉斑玄武岩和S 型花岗岩,它们分别形成于1424 Ma 和1485 Ma。拉斑玄武岩显示富集Rb、Ba、Sr 等大离子亲石元素,亏损Nb、Ta、Zr、Ti 等高场强元素,符合典型岛弧玄武质岩石的地球化学特征。其锆石的εHf(t)值主要为正值（-1.68~+14.2）, 表明其岩浆起源于亏损的岩石圈地幔。地球化学特征表明其形成于岛弧环境,与第三期沉积沉积作用环境相符。S 型花岗岩具有较高的SiO2、ALK和A/CNK（>1.1）,以及中等的稀土含量和中等的Eu 负异常。其原岩岩浆锆石的εHf (t)值变化于+5.97~+12.1,表明源区沉积岩的碎屑主要来自新生地壳。对比显示Kontum 地块与海南岛在中元古代时期很可能曾经相连,在Columbia 超大陆裂解时期处于超大陆的边缘。  相似文献   

15.
New paleomagnetic data on Paleoproterozoic complexes of the Central Karelian and Vodlozero terranes of the Karelian Craton were obtained. A new key paleomagnetic pole (1.98 Ga) was calculated for rocks of the Vodlozero terrane. The positions of Central Karelian and Vodlozero terranes 1.98 Ga ago in subtropical and moderate latitudes of the Northern Hemisphere, respectively, were reconstructed. The latitudinal difference (1.98 Ga) between the positions of Central Karelian and Vodlozero terranes supports the existence of oceanic basins between separate terranes of the Karelian Craton.  相似文献   

16.
Data on gold ore objects in the Strelna Greenstone Belt in the southeastern Kola Peninsula are presented in the paper. The studied Vorgovy and Sergozero ore occurrences are localized in the zone of tectonic contact of the Neoarchean complexes making up the greenstone belt and the volcanic–sedimentary sequences of the Paleoproterozoic Imandra–Varzuga Zone. The Vorgovy gold occurrence is related to stockwork of carbonate–quartz veins and veinlets hosted in a biotite gneiss transformed into chlorite–sericite–quartz metasomatic rock with pyrrhotite–arsenopyrite dissemination. The Sergozero occurrence is localized in amphibolites corresponding to komatiitic and tholeiitic basalts hosted in biotite gneiss (metapelite). Mineralization is confined to the zone of tectonized contact between komatiitic and tholeiitic basalts, where it is controlled by a strip of metasomatic biotite–calcite rock with gersdorffite–arsenopyrite dissemination. The native gold grains medium to high in fineness are up to 0.1 mm in size and mainly localized at the contact of arsenopyrite and gersdorffite with gangue minerals. Gold mineralization is of superimposed character, and, as indicated by isotopic geochronology, was formed at the retrograde stage of the Svecofennian regional metamorphism. Comparison of ore occurrences localized in the Strelna Greenstone Belt with gold deposits in greenstone belts of the western Fennoscandian Shield and the Superior Province in Canada allows us to suggest a high perspective of the entire Strelna Belt for gold.  相似文献   

17.
《Precambrian Research》2001,105(2-4):269-287
The Kolvitsa Belt in the south-western Kola Peninsula formed coeval with the earliest Palaeoproterozoic rift-belts in the Fennoscandian Shield. The Palaeoproterozoic history of this belt comprises the deposition of the 2.47 Ga Kandalaksha amphibolite (metabasalt) sequence onto 2.7 Ga granitoid gneisses, the intrusion of the 2.45–2.46 Ga Kolvitsa Massif of gabbro-anorthosite and the subsequent multiple injection of mafic dykes and magmatic brecciation, followed by the intrusion of 2.44 Ga dioritic dykes, and extensive shearing at 2.43–2.42 Ga. The gabbro-anorthosite and dykes contain high-pressure garnet-bearing assemblages that have previously been considered as evidence for metamorphism in a compressional setting of the Kolvitsa Belt at 2.45–2.42 Ga, i.e. coeval with the formation of the Imandra–Varzuga rift-belt and layered mafic intrusions in an extensional setting. The Kochinny Cape study area on the White Sea coast presents an unique remnant of a 2.44 Ga mafic dyke swarm that endured ca. 1.9 Ga collision but preserved its primary structural pattern well. All these dykes were intruded along numerous NW-trending shear zones within the Kolvitsa Massif and contain angular xenoliths of sheared gabbro-anorthosite. Every new batch of mafic melt underwent shearing during or immediately after solidification, and later dykes intruded into already sheared dykes. Thus, rocks of the Kolvitsa Massif and its dyke complex were successively injected into a large-scale shear zone which was active from ca. 2.46 to 2.42 Ga. Multiple injection of mafic melts, the presence of mutually intruding, composite, sheared mafic dykes, of magmatic breccias with gabbroic groundmass, and of host rocks fragments (showing no evidence of tectonic stacking at the time of brecciation), all indicate an extensional setting. Shearing was also extensional as it occurred simultaneously with the multistage magmatism. The asymmetric morphology of deformed dykes, and asymmetric flexures within weakly deformed lenses show that all these extensional shear zones, apart from a few exceptions, are dextral, were formed in a transtensional setting and are attributed to general W–E to WSW–ENE extension. Structural data available for 2.4–2.5 Ga magmatic rocks elsewhere in the Kola region suggest that the same kinematics operated on a regional scale. The presence of the garnet-bearing assemblages in gabbro-anorthosite and dykes may be explained by crystallisation and shearing of the magmatic rocks at deep crustal levels. Alternatively, corona development might have occurred much later as a result of tectonic loading due to the juxtaposition and overthrusting of the Umba Granulite Terrane onto the Kolvitsa Belt at ca. 1.9 Ga. In view of the field evidence and published ages, an overall extensional setting rather than a combination of compressional and extensional zones is preferable for Palaeoproterozoic tectonics in the north-eastern Fennoscandian Shield at 2.5–2.4 Ga.  相似文献   

18.
The Archean provinces and lithotectonic complexes of the Baltic (Fennoscandian) Shield are considered. The supracrustal complexes are classified by age: <3.2, 3.10–2.90, 2.90–2.82, 2.82–2.75, and 2.75–2.65 Ga. The data on Archean granitoid complexes and metamorphic events are mentioned briefly, whereas the recently found fragments of the Archean ophiolitic and eclogite-bearing associations are discussed in more detail. The Paleoarchean rocks and sporadic detrital grains of Paleoarchean zircons have been found in the Baltic Shield; however, the relatively large fragments of the continental crust likely began to form only in the Mesoarchean (3.2–3.1 Ga ago), when the first microcontinents, e.g., Vodlozero and Iisalmi, were created. The main body of the continental crust was formed 2.90–2.65 Ga ago. The available information on the Paleoarchean complexes of the Baltic Shield is thus far too scanty for judgment on their formation conditions. The geologic, petrologic, isotopic, and geochronological data on the Meso-and Neoarchean lithotectonic complexes testify to their formation in the geodynamic settings comparable with those known in Phanerozoic: subduction-related (ensialic and ensimatic), collisional, spreading-related, continental rifting, and the setting related to mantle plumes.  相似文献   

19.
The paper reports the results of lithogeochemical studies of the Upper Devonian rocks from the Andoma Hill zone of fold-and-fault dislocations (SE Onega region). The rocks are characterized by the negative Eu anomaly (from 0.4 to 0.65) that maks them different from modern sediments of the White Sea. The latter can be regarded as the average composition of mainly Archean (Karelian–Kola) part of the Baltic Shield. In terms of the contents of some trace elements, they also differ from the Vendian rocks of the Zimnii Bereg area. Since the considered rocks are geochemically similar to the Svecofennian metamorphic rocks and Paleoproterozoic granite rapakivi, they could be formed by the erosion of these complexes. The clastic material was transported via a channel confined to the Baltic Shield and Russian Platform junction known as the Polkanov geoflexure.  相似文献   

20.
位于喜马拉雅东构造结西北部的南迦巴瓦复合体,是构造应力最强、隆升和剥蚀最快、新生代变质和深熔作用最强的地区。为厘定该地区早期的变质岩浆作用,本文对南迦巴瓦复合体北部的花岗片麻岩和混合岩进行了岩石学和年代学研究。花岗片麻岩原岩为富钾的偏铝质花岗岩,具有岩浆弧花岗岩的成分特征。花岗片麻岩中的锆石具有岩浆锆石的环带结构,记录了487.9±1.6Ma的一期构造岩浆事件;混合岩的锆石具有明显的核-边结构,核部和边部的不协和线交点年龄分别为1559±13Ma、1154±12Ma。对比印度大陆东部的西隆高原、东高止造山带,发现三者都经历了拉布拉多期、格林威尔期以及泛非期的造山作用。因此,我们认为喜马拉雅东构造结与这两个地区密切相关,可能是他们向北的延伸,这三者可能组成统一的印度大陆东部造山带,一起经历了哥伦比亚超大陆、Rodinia和冈瓦纳超大陆的聚合与裂解过程。  相似文献   

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