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1.
Analysis of the litho-geochemistry of fine-grained terrigenous rocks (metapelites, shales, and mudstones) of sedimentary megasequences in the Southern Urals, Uchur-Maya area, and the Yenisei Kryazh indicates that Riphean sequences in these regions are dominated by chlorite-hydromica rocks, with montmorillonite and potassic feldspar possibly occurring only in some of the lithostratigraphic units. According to the values of their hydrolysate modulus, most clay rocks from the three Riphean metamorphosed sedimentary sequences are normal or supersialites, with hydrosialites and hydrolysates playing subordinate roles. The most lithochemicaly mature rocks are Riphean clays in the Yenisei Kryazh (Yenisei Range). The median value of their CIA is 72, whereas this index is 70 for fine-grained aluminosilicate rocks from the Uchur-Maya area and 66 for fine-grained terrigenous rocks of the Riphean stratotype. Hence, at ancient water provenance areas from which aluminosilicate clastic material was transported in sedimentation basins in the southwestern (in modern coordinates) periphery of the Siberian Platform, the climate throughout the whole Riphean was predominantly humid. At the same time, the climate at the eastern part of the East European Platform was semiarid-semihumid. The K2O/Al2O3 ratio, which is employed as an indicator of the presence of petro-and lithogenic aluminosilicate clastic component in Riphean sedimentary megasequences, shows various tendencies. According to their Sc, Cr, Ni, Th, and La concentrations and the Th/Sc ratio, the overwhelming majority of Riphean shales and mudstones notably differ from the average Archean mudstone and approach the average values for post-Archean shales. This suggests that mafic Archean rock in the provenance areas did not play any significant role in the origin of Riphean sedimentary megasequences. The Co/Hf and Ce/Cr ratios of the terrigenous rocks of the three Riphean megaseqeunces and their (Gd/Yb) N and Eu/Eu* ratios place these rocks among those containing little (if any) erosion products of primitive Archean rocks. According to various geochemical data, the source of the great majority of fine-grained aluminosilicate clastic rocks in Riphean sediment megasequences in our study areas should have been mature sialic (felsic), with much lower contents of mafic and intermediate rocks as a source of the clastic material. The REE patterns of the Riphean shales and metapelites in the Bashkir Meganticlinorium, Uchur-Maya area, and Yenisei Kryazh show some features that can be regarded as resulting from the presence of mafic material in the ancient provenance areas. This is most clearly seen in the sedimentary sequences of the Uchur-Maya area, where the decrease in the (La/Yb) N ratio up the sequence of the fine-grained terrigenous rocks from 15–16.5 to 5.8–7.1 suggests that mantle mafic volcanics were brought to the upper crust in the earliest Late Riphean in relation to rifting. Analysis of the Sm-Nd systematics of the Riphean fine-grained rocks reveals the predominance of model age values in the range of 2.5–1.7 Ga, which can be interpreted as evidence that the rocks were formed of predominantly Early Proterozoic source material. At the same time, with regard for the significant role of recycling in the genesis of the upper continental crust, it seems to be quite possible that the ancient provenance areas contained Archean complexes strongly recycled in the Early Proterozoic and sediments formed of their material. An additional likely source of material in the Riphean was mafic rocks, whose variable contribution is reflected in a decrease in the model age values. Higher Th and U concentrations in the Riphean rocks of the Yenisei Kryazh compared to those in PAAS indicate that the sources of their material were notably more mature than the sources of fine-grained aluminosilicate clastic material for the sedimentary megaseqeunces in the Southern Urals and Uchur-Maya area.  相似文献   

2.
Based on the LA-ICP-MS data, detrital zircons from the tillite-type conglomerates of the Tanin Formation (Serebryanka Group) on the western slope of the Central Urals include approximately equal proportions of crystals with Neoarchean and Paleoproterozoic U-Pb ages. Therefore, we can assume that crystalline rocks of the basement beneath the eastern part of the East European Craton served as a provenance for aluminosilicate clastics in the initial Serebryanka period. Detrital zircons from sandstones of the Kernos Formation have the Meso-Neoarchean (∼15%), Paleoproterozoic (∼60%), and Mesoproterozoic (∼26%) age. Comparison of the obtained data with the results of the study of detrital zircons from Riphean and Vendian sandstones of the Southern Urals shows that the Riphean and Lower Vendian rocks are mainly represented by erosional products of Middle and Upper Paleoproterozoic crystalline rocks that constitute the basement of the East European Craton. In addition, a notable role belonged to older (Lower Proterozoic, Neoarchean and Mesoarchean) rock associations during the formation of the Serebryanka Group. The terminal Serebryanka time (Kernos Age) differed from its initial stage (Tanin Age) by the appearance of Mesoproterozoic complexes in provenances. According to available data, these complexes played an insignificant role in the formation of Riphean-Vendian rocks in the neighboring South Uralian segment. This implies a spatiotemporal diversity of clastic material sources for Upper Precambrian rocks in the western megazone of the Southern and Central Urals.  相似文献   

3.
Sedimentation environments were reconstructed for the Early Vendian successions of the western slope of the Central Urals, which comprises one of the most complete sections of the terminal Precambrian system in northern Eurasia. It was shown that, despite the presence of several diamictite levels in the sections of the Serebryanka Group, mature and multiply recycled fine-grained siliciclastic materials (CIA = 65–77) were delivered into the sedimentation basin over the whole Early Vendian. Based on the lithochemical characteristics of shales, the climate of Serebryanka time can be estimated as semiarid-semihumid, similar to that dominating in Late Vendian paleocatchments. Based on relatively high Mo/Mn values (0.011–0.024), it was suggested that anoxic or similar conditions existed in the basin of Buton time, whereas other sedimentary complexes of the Serebryanka Group were formed in well aerated environments. The systematics of Sr, Ba, Zr, Cu, and V in fine-grained siliciclastic rocks and Sr isotopic data for carbonate rocks indicate that the sediments were accumulated in a fresh-water basin. The values of trace-element indicator ratios, e.g., Th/Sc, La/Sc, Th/Cr and others, in the shales of the Serebryanka Group and Nd model age estimates indicate that a variety of mainly Early Proterozoic complexes, ranging from granitoids to basic rocks, occurred in the Early Vendian paleocatchments. The basic rocks were eroded most extensively probably in the end of Serebryanka time. Based on the Ce/Ce* values of shales, it was concluded that submarine volcanism had no significant influence on sedimentation processes in the Early Vendian. An exception is Koiva and Kernos time, when hematite-bearing shales were accumulated in association with pillow basalts in some zones of the basin. The distribution of the compositions of shales from various formations of the Serebryanka Group in discrimination diagrams suggests that the Early Vendian sedimentary sequences were formed in passive geodynamic settings.  相似文献   

4.
The results of application of the LA-ICP-MS method for analysis of U-Pb ages of detrital zircons from Serebryanka Group (Middle Urals) has been presented for the first time. It has been found that the detrital zircon population in tillite-like conglomerates of the Tany Formation is represented nearly in equal shares by grains with Neoarchean and Paleoproterozoic U-Pb ages. This allows the basement crystalline rocks from the eastern part of the East European Craton to be considered as the principal source of aluminosilicate clastic material in the beginning of the Serebryanka. The population of detrival zircons in sandstones from the Kernos Formation consists of Meso- and Neoarchean (approximately 15%), Paleoproterozoic (∼60%), and Mesoproterozoic (∼26%) zircon grains. Comparison of the obtained data with the materials on detrital zircons from Tonian and Ediacarian sandstones in the southern Urals has shown that the principal role in formation of Tonian and Ediacaran sedimentary sequences was played by the products of Mid- and Late Paleoproterozoic crystalline rock erosion in the craton socle. In addition to this, accumulation of the Serebryanka Group took place with visible participation of more ancient—Early Paleoproterozoic, Neo- and Mesoarchean—rock associations on the paleo-water catchment areas. The end of the Serebryanka period differed from its beginning by the additional appearance of Mesoproterozoic complexes in alimentation zones; for comparison, the role played by these complexes in formation of Ectasian-Ediacarian deposits of the adjacent southern Urals segment is insufficient, judging by the currently available data. All these facts may indicate that there are significant differences in the spatiotemporal distribution of clastic material sources for Upper Precambrian deposits in the West Megazone of the southern and middle Urals.  相似文献   

5.
The middle segment of the northern margin of the North China Craton （NCC） consists mainly of metamorphosed Archean Dantazi Complex, Paleoproterozoic Hongqiyingzi Complex and unmetamorphosed gabbro-anorthosite-meta-alkaline granite, as well as metamorphosed Late Paleozoic mafic to granitoid rocks in the Damiao-Changshaoying area. The -2.49 Ga Dantazi Complex comprises dioritic-trondhjemitic-granodoritic-monzogranitic gneisses metamorphosed in amphibolite to granulite facies. Petrochemical characteristics reveal that most of the rocks belong to a medium- to high-potassium calc-alkaline series, and display Mg^# less than 40, right-declined REE patterns with no to obviously positive Eu anomalies, evidently negative Th, Nb, Ta and Ti anomalies in primitive mantlenormalized spider diagrams, εNd（t）=＋0.65 to -0.03, and depleted mantle model ages TDM=2.78-2.71 Ga. Study in petrogenesis indicates that the rocks were formed from magmatic mixing between mafic magma from the depleted mantle and granitoid magma from partial melting of recycled crustal mafic rocks in a continental margin setting. The 2.44-2.41 Ga Hongqiyingzi Complex is dominated by metamorphic mafic-granodioritic-monzogranitic gneisses, displaying similar petrochemical features to the Dantazi Complex, namely medium to high potassium calc-alkaline series, and the mafic rocks show evident change in LILEs, negative Th, Nb, Ta, Zr anomalies and positive P anomalies. And the other granitiod samples also exhibit negative Th, Nb, Ta, P and Ti anomalies. All rocks in the Hongqiyingzi Complex show right-declined REE patterns without Eu anomaly. The metamorphic mafic rocks with εNd（t） = -1.64 may not be an identical magmatic evolution series with granitoids that have εNd（t） values of ＋3.19 to ＋1.94 and TDM ages of 2.55-2.52 Ga. These granitic rocks originated from hybrid between mafic magma from the depleted mantle and magma from partial melting of juvenile crustal mafic rocks in an island arc setting. All the -311 Ma Late Paleozoic metamorphic mafic rocks and related granitic rocks show a medium-potassium calc-alkaline magmatic evolution series, characterized by high Mg^#, obviously negative Th, Nb, Ta anomalies and positive Sr anomalies, from no to strongly negative Ti anomalies and flat REE patterns with εNd（t） = ＋8.42, implying that the maflc magma was derived from the depleted mantle. However the other granitic rocks are characterized by right-declined REE patterns with no to evidently positive Eu anomalies, significantly low εNd（t） = -13.37 to -14.04, and TDM=1.97-1.96 Ga, revealing that the granitoid magma was derived from hybrid between maflc magma that came from -311 Ma depleted mantle and granitoid magma from Archean to Early Paleoproterozoic ancient crustal recycling. The geochemistry and Nd isotopic characteristics as well as the above geological and geochronological results indicate that the middle segment of the northern margin of the NCC mainly experienced four crustal growth episodes from Archean to Late Paleozoic, which were dominated by three continental marginal arc accretions （-2.49, -2.44 and 311 Ma）, except the 1.76-1.68 Ga episode related to post-collisional extension, revealing that the crustal accretion of this segment was chiefly generated from arc accretion and amalgamation to the NCC continental block.  相似文献   

6.
Formation conditions of sedimentary successions in the Mezen Basin are considered on the basis of Cr, Th, Sc, Ni, Hf, and REE distribution and model Nd age of the Upper Vendian fine-grained terrigenous rocks. Geochemistry of mudstones and shales of the Lyamitsa, Verkhovka, Zimnie Gory, and Erga formations in the Belomorian-Kuloi Plateau, as well as the Ust-Pinega and Mezen formations in the Vychegda Trough, does not allow us to consider these stratigraphic units as erosion products of the primitive Archean basement of the Baltic Shield or the central segment of the East European Craton (EEC) basement. Taking into account sedimentological data on the direction of paleoflows in the basin and the model Nd age of the fine-grained terrigenous rocks, we suggest that the Mezen Basin was filled in the Late Vendian mainly with erosion products of the Riphean igneous and metasedimentary complexes of the Timan-Pechora region. These conclusions are consistent with the sequence-stratigraphic architecture of sediments in the basin. According to the new model proposed, the Late Vendian Mezen Basin was a foredeep formed as a result of subsidence of the northeastern margin of the EEC under the load of overthrusted rock masses of the Timan-Pechora Foldbelt. The clastic material was derived from the emerging orogen.  相似文献   

7.
Siliciclastic metasediments of the Ladoga Group that is the Kalevian stratotype in Karelia correlative with the Kalevian siliciclastic succession in Finland are studied in terms of geochemistry and Sm-Nd isotopic systematics. The results obtained show that rocks in the Ladoga Group lower part are enriched, as compared to rocks of the upper part, in TiO2, Fe2O3, MgO, Cr, Co, Ni, and Sc, being comparatively depleted in Al2O3 and Th that is a result of compositional changes in provenances. The Sm-Nd isotopic data evidence that siliciclastic sediments of the Ladoga Group have accumulated during the erosion of rocks, which originated at the time of the Archean and Early Proterozoic crust-forming processes. Siliciclastic material with the Archean and Early Proterozoic TNd(DM) values, which are characteristic of metasediments in the group lower part, was derived respectively from granite gneisses of the Archean basement in the Karelian megablock of the Baltic Shield and from volcanic rocks of the Sortavala Group. Volcanic rocks of island-arc terranes of the Svecofennian foldbelt represented main source of siliciclastic material that accumulated in upper part of the succession.  相似文献   

8.
The provenance composition and geological evolution of different segments in the distal zone of the Late Vendian foreland basin of the Timan orogen were deciphered on the basis of sequence stratigraphic reconstructions and precision geochemical data on the Upper Vendian fine-grained terrigenous rocks from the Southeast White Sea region, Vychegda, Verkhnekama, and Shkapovo-Shikhan basins, and the Kvarkush-Kamennogorsk and Bashkirian meganticlinoriums. The Upper Vendian of the Southeast White Sea region is subdivided into four sequences: Agma, Solza, Zimnie Gory, and Erga. The tracing of sequence boundaries and lateral facies associations from the Southeast White Sea Region to the South Urals made it possible to identify the main stages of the evolution of sedimentary filling of the foreland basin: (I) shallow water epiplatformal basin, (II) isolated basin, (III) lowstand system tract, and (IV) progradation of delta platforms. Position of data points of the Upper Vendian shales, silty mudstones, and mudstones plotted in the diagrams Th-La, Ni-Cr, GdN/YbN-Eu/Eu*, Sc-Th/Sc, La/Sm-Sc/Th, and Co/Hf-Ce/Cr, together with their REE systematics and data on Nd model ages indicates that the Proterozoic, including Neoproterozoic rocks of the Timan thrust-folded belt served as the main source for fine aluminosiliciclastic material in the distal zones of the Timan foreland basin. Obtained results are consistent with the concept that the Vendian stage of the evolution of the Eastern European platform is related to the intense input of allochthonous material in its peripheral area owing to the activation of orogenic processes in the adjacent mobile belts and allow us to significantly specify the paleogeographical reconstructions.  相似文献   

9.
Collisional granitoid magmatism caused by the Early Neoproterozoic orogeny in the west of the Siberian craton is considered. New data on the petrogeochemical composition, U-Pb (SHRIMP II), Ar-Ar, and Sm-Nd isotopic ages of the Middle Tyrada granitoid massif in the northwestern Yenisei Ridge are presented. Plagiogranites, granodiorites, and quartz diorites of the massif are of calcareous and calc-alkalic composition. The elevated alumina contents and presence of accessory garnet permit them to be assigned to S-type granitoids. Their spidergrams show Rb, Ba, and Th enrichment, minimum Nb, P, and Ti contents, and no Sr depletion. The granitoids formed through the melting of plagioclase-enriched graywacke source, obviously Paleoproterozoic metaterrigenous rocks of the Garevka Formation and Teya Group (TNd(DM) = 2.0-2.5 Ga), judging from the isotope composition of the granitoids (TNd(DM-2st) = 2200 Ma and 8Nd(T) = − 6.0) and the presence of ancient zircon cores (1.80-1.85 Ga). Formation of granitoids took place in the final epoch of the Grenville collision events in the late Early Neoproterozoic (U-Pb zircon age is 857.0 ± 9.5 Ma). In the Late Neoproterozoic, the granitoids underwent tectonothermal reworking caused by Vendian accretion and collision events on the southwestern margin of the Siberian craton, which explain the younger K-Ar biotite age, 615.5 ± 6.3 Ma.  相似文献   

10.
The Early Precambrian granulite-gneiss complex of the Irkut Block (Sharyzhalgai salient of the Siberian Craton basement) with the protoliths represented by a wide range of magmatic and sedimentary rocks, has a long-term history including several magmatic and metamorphic stages. To estimate the age of sedimentation and metamorphism of the terrigenous deposits, the composition of the garnet-biotite, hyper-sthene-biotite, and cordierite-bearing gneisses has been studied; their isotopic Sm-Nd values have been revealed; and the U-Pb zircon dating has been performed using the SHRIMP II ion microprobe. The protoliths of the terrigenous sediments metamorphosed under conditions of the granulite facies correspond to a rock series from siltstones and graywackes to pelites. The Nd model ages of paragneisses range from 2.4 to 3.1 Ga. Zircons of the cordierite-bearing and hypersthene—biotite gneisses show the presence of cores and rims. The clastic, smoothed, and irregular shape of the cores indicates their detrital character and relicts of oscillatory zoning suggest the magmatic origin of zircon. The rim’s metamorphic genesis is indicated by the lack of zoning and by the lower Th/U ratio compared to that of the cores. The age of the detrital cores (≥2.7, ~2.3, and 1.95—2.0 Ga) and metamorphic rims (1.85–1.86 Ga) defines the time of sedimentation at 1.85–1.95 Ga ago. Potential sources for the Archean detrital zircons were metamagmatic rocks of the granulite—gneiss complexes in the southwestern margin of the Siberian Craton. The age of the dominant detrital cores at 1.95–2.0 Ga ago, together with the minimal TNd(DM) values, indicates the contribution of the juvenile Paleoproterozoic crust to the formation of sediments. The juvenile Paleoproterozoic crust was likely represented by magmatic complexes similar to the volcanic and granitoid associations of the Aldan shield, which were formed 1.99–2.0 Ga ago and showthe model age of 2.0—2.4 Ga. The isotopic Sm-Nd data show that the Late Paleoproterozoic metasedimentary rocks occur not only in the Sharyzhalgai salient but in the Aldan and Anabar shields of the Siberian Craton as well.  相似文献   

11.
The climatic impact on the formation of fine-grained rocks from the Riphean stratotype and Vendian Asha Group on the western slope of the South Urals during the time interval lasting approximately 1200 Ma is considered. It is shown that these rocks are largely represented by “tectonosilicate-dominated” shales. This feature combined with changes in the average K2O/Al2O3 values disavows the hypothesis in (Kennedy et al., 2006), according to which the growth of free oxygen concentration in the Late Riphean and Vendian atmosphere was determined by gradual intensification of the organic carbon extraction from the biosphere by clays. The average values of the hydrolyzate module, chemical index of alteration (CIA), and several lithogeochemical parameters calculated for the Riphean and Vendian clayey rocks provide grounds for the conclusion that intensity of weathering in paleodrainage areas during the accumulation of the Upper Precambrian sedimentary successions was low. The curve reflecting changes of the average CIA values in the Upper Precambrian fine-grained siliciclastic rocks of the South Urals is similar to some extent with the “standard” CIAcorrect. curve (GonzalezAlvarez and Kerrich, 2012). It is assumed that changes in microand macrobiotic communities during the Late Precambrian were controlled to a variable extent by climate fluctuations as well. At the same time, these fluctuations most likely left the chemical composition of water in the ocean virtually unchanged, which is evident from analysis of the redox conditions in the ocean and the distribution of primary producers with the average CIAcorrect. and CIA values.  相似文献   

12.
The paper reports data on metapelites (gneisses and crystalline schists) from various Early Precambrian lithostratigraphic units of the Yenisei Range (Kan and Yenisei complexes, Garevka Unit, and Teya Group). A representative selection of 57 samples from the examined vertical section were analyzed for major oxides, LILE, transition elements, HFSE, REE, radioactive elements, and other trace and minor elements. The most important distinctive geochemical feature of metapelites composing most of the vertical section is their elevated concentrations of Al, which are higher than in the Post-Archean Australian Shale (PAAS), K, Rb, Ba, Ga, Sc, Pb, Th, Nb, Y, and REE. These data and the systematics of elements in certain discriminant diagrams testify that the metasedimentary complexes in the sedimentation basin and the rocks associations composing the erosion area (crystalline massifs of the ancient basement of the Siberian craton) were strongly geochemically differentiated and mature. The composition of the eroded crustal material was close to the average composition of the post-Archean continental crust and PAAS. The metapelites of the Kuzeeva Unit in the Kan Complex and Penchenginskaya Formation in the Teya Group were the only ones that could have basic rocks as an additional source of material, as also follows from the elevated Cr concentrations of the rocks and their REE systematics. The three types of REE patterns of the metapelites largely correspond to the composition of the rocks that composed the ancient drainage areas and the degrees of the differentiation and averaging of their terrigenous material. The first predominant type corresponds to PAAS and the averaged composition of the upper continental crust. The second and third types of the REE patterns with high and low LaN/YbN ratios, respectively, and with or without negative Eu anomalies provide evidence of the contribution of tonalite-trondhjemite-granodiorite (TTG) or basite sources typical of granite-greenstone provinces. In certain diagrams, the compositional fields of Early Precambrian metapelites in the Yenisei Range almost exactly coincide with the compositional fields of regional fine-grained Riphean terrigenous rocks. This suggests that the Late Precambrian sedimentary rock sequences inherited their geochemical features from more ancient rocks. The reproduced ancient geodynamic environments in which the Early Precambrian metaterrigenous complexes of the Yenisei Range were accumulated correspond to ensialic continental marginal basins. The rocks of the Kan and Yenisei complexes and the Garevka Unit were formed mostly on active continental margins and, less frequently, on passive margins. The protoliths of metasedimentary rocks of the Teya Group were accumulated in a subplatform environment at a passive margin.  相似文献   

13.
 Sittampundi and Bhavani Archean layered meta-anorthosite complexes occur as tectonic lenses within the Cauvery shear zone (CSZ), a crustal scale shear dividing the Precambrian granulite crust of south India into late Archean (> 2.5 Ga) and Proterozoic (c. 0.55 Ga) blocks. They and their host supracrustal-gneiss rocks record at least two stages of tectonometamorphic history. The first is seen as regional scale refolded isoclinal folds and granulite metamorphism (D1-M1) while the second stage is associated with dominantly E–W dextral transcurrent shearing and metamorphic recrystallisation (D2-MCSZ). Whole rock Sm-Nd isochrons for several comagmatic rocks of the layered complexes yield concordant ages: Sittampundi – 2935±60 Ma, ɛNd + 1.85±0.16 and Bhavani – 2899±28 Ma, ɛNd + 2.18±0.14 (2σ errors). Our Sm-Nd results suggest that: (1) the magmatic protoliths of the Sittampundi and Bhavani layered complexes were extracted from similar uniform and LREE depleted mantle sources; (2) M1 metamorphism occurred soon after emplacement at c.3.0 Ga ago. P-T estimates on garnet granulites from the Sittampundi complex characterise the MCSZ as a high-P event with metamorphic peak conditions of c. 11.8 kbar and 830°C (minimum). The MCSZ is associated with significant isothermal decompression of the order of 4.5–3.5 kbar followed by static high-temperature rehydration and retrogression around 600°C. The timing of MCSZ is inferred to be Neoproterozoic at c. 730 Ma based on a whole rock-garnet-plagioclase-hornblende Sm-Nd isochron age for a garnet granulite from the Sittampundi complex and its agreement with the 800–600 Ma published age data on post-kinematic plutonic rocks within the CSZ. These results demonstrate that the Cauvery shear zone is a zone of Neoproterozoic reworking of Archean crust broadly similar to the interface between the Napier and Rayner complexes of the East Antarctic shield in a model Proterozoic Gondwana supercontinent. Received: 5 December 1995 / Accepted: 3 May 1996  相似文献   

14.
The paper presents data on the mineralogy, petrography, and lithogeochemistry of Riphean and Vendian sandstones in the Volga-Ural region. The studied rocks generally differ in several parameters. The Zr/Sc and Th/Sc values typical of psammites suggest that the rocks are dominated by the petrogenic clastic material. This conclusion is supported by median K2O/Al2O3 values (from 0.39 to 0.45). The fine-grained clastic rocks associated with sandstones are also characterized by sufficiently high K2O/Al2O3 values, indicating the prevalence of the first cycle material therein. It is shown that the Prikamsk and Tukaevo sandstones include, in addition to the petrogenic quartz, a significant amount of lithogenic (multifold redeposited) quartz, whereas ratio of these rock types is approximately equal in the Leonidovo and Baikibashevo sandstones. Sandstones of the Kairovo and Shkapovo groups are dominated by the petrogenic quartz. Analysis of the ln(Q/L + CE)-ln(Q/F) diagram shows that the Vendian psammites are dominated by disintegration products of plutonic rocks, whereas the Riphean psammites contain a significant portion of clastic material (related to the erosion of metamorphic rocks) along with felsic and intermediate-felsic igneous rocks. Relationships of feldspars, rock clasts, polycrystalline quartz, and quartz, in general, reflected in the Qt/(F + R)-Qp/(F + R) diagram indicate that the Riphean psammites were deposited in a humid setting; the Vendian psammites, in a semihumid/semiarid setting.  相似文献   

15.
The geochemical and zircon geochronological (U-Pb, SHRIMP-II) study of Mesoarchean gabbros of the South Vygozersky and Kamennoozersky greenstone structures of Central Karelia made it possible to distinguish four gabbro types: (1) Fe–Ti gabbro, 2869 ± 12 Ma, (2) gabbro compositionally close to tholeiitic basalts, 2857 ± 7 Ma, (3) leucogbabbro, 2840 ± 5 Ma; and (4) melanogabbro, 2818 ± 14 Ma. From the early to late gabbros, the rocks are depleted in Ti, Fe, V, Y, Zr, Nb, Hf, REE and enriched in Mg, Ca, Cr, Ni. According to the systematics (Condie, 2005), the Nb/Y, Zr/Y, Zr/Nb ratios in the studied Late Archean gabbros are close to those of primitive mantle, while the gabbros in composition are similar to those of plumederived ocean-plateau basalts. Their magma sources were derived from different mantle reservoirs. The leucogabbro and melanogabbro with similar εNd = +4 were derived from a depleted mantle source (DM). The gabbro close in composition to tholeiitic basalts and having the elevated positive εNd (+4.9) was derived from a strongly depleted mantle source. Insignificant admixture of crustal material or lithospheric mantle is inferred in a source of the Fe–Ti gabbro (with lowest εNd = +2.1).  相似文献   

16.
The existence of the pre-Early Vendian hiatus in the Bashkir Meganticlinorium is supported by large incised valleys filled with Early Vendian sediments both on the western and eastern limbs of the meganticlinorium. It is suggested that a great glacioeustatic lowering of the sealevel occurred in the sedimentation basin. Taking into account the reconstruction of similar Early Vendian events for many provinces elsewhere, we can suppose that Early Vendian sedimentation basins in the southern and middle Urals were connected with the World Ocean. The comparative study of Lower Vendian sedimentary sequences in the Urals and Norway demonstrates similarities in rock associations. The correlation of Lower Vendian sections in the Urals with modern facies models of glacial sedimentation shows that glacimarine sequences of the southern Urals fit the shelf sedimentary associations proper. The character of section at the Serebryanka level in the middle Urals and the spatial distribution of mixtites therein correspond to sedimentation on the basin shelf and slope.  相似文献   

17.
Initial Nd isotopic ratios are reported for 23 samples representing magmatic crustal components in the Svecokarelian terrain of South Finland. U-Pb zircon ages are determined for all geologic units, involving 21 separate upper concordia intercept ages based on more than 100 UP-b analyses. The ages range for all the rocks from 1.90 Ga for primitve plutonic rocks to 1.79 Ga for post-tectonic intrusions. The well-known gabbro-diorite-tonalite-trondhjemite association of the Kalanti district appears to consist of components with different ages: trondhjemites are probably 1.90 Ga or older, diorites/tonalites belong to the main Svecokarelian plutonic eposide at 1.89-1.87 Ga, and at least some gabbro has a post-tectonic age of 1.80 Ga. Nd (T) values range between +2 to +3 for meta-andesites, large gabbros and primitive granitoids to –0.5 for more evolved granitoids. A magma source with Nd of at least +2 to +3 was available during 1.90 to 1.87 Ga, but evolved granitoids have Nd close to zero. The preferred interpretation is that depleted mantle with Nd=+4 to +5 was present beneath the Svecokarelian crust forming during 1.9-1.8 Ga, and that all rocks have been affected more or less by addition of an Archean crustal component with Nd=–9 to –13. The primitive rocks with Nd=+2 to + 3 were only slightly affected, while granitoids with Nd close to zero include a 10% Archean component. The widespread nature of the Archean addition and the distance of up to 500 km to actual exposed Archean crust make it most realistic that the Archean component was added to the form of sediments delivered by subducting Proterozoic ocean crust. The plutonic rocks of the Finnish Svecokarelian crust in areas away from Archean cratons consist of 90% newly mantle-derived material.  相似文献   

18.
《地学前缘(英文版)》2020,11(6):2221-2242
We present the first evidence of Archean oceanic crust submitted to Proterozoic high-pressure (HP) metamorphism in the South American Platform. Sm–Nd and Lu–Hf isotopic data combined with U–Pb geochronological data from the Campo Grande area, Rio Grande do Norte domain, in the Northern Borborema Province, reflect a complex Archean (2.9 ​Ga and 2.6 ​Ga) and Paleoproterozoic (2.0 ​Ga) evolution, culminating in the Neoproterozoic Brasiliano/Pan-African orogeny (ca. 600 Ma). The preserved mafic rocks contain massive poikiloblastic garnet and granoblastic amphibole with variable proportions of plagioclase ​+ ​diopside in symplectitic texture, typical of high-pressure rocks. These clinopyroxene-garnet amphibolites and the more common garnet amphibolites from the Campo Grande area are exposed as rare lenses within an Archean migmatite complex. The amphibolite lenses represent 2.65 ​Ga juvenile tholeiitic magmatism derived from depleted mantle sources (positive εHf(t) values of +3.81 to +30.66) later enriched by mantle metasomatism (negative εNd(t) values of –7.97). Chondrite and Primitive Mantle-normalized REE of analyzed samples and discriminant diagrams define two different oceanic affinities, with E-MORB and OIB signature. Negative Eu anomalies (Eu/Eu1 ​= ​0.75–0.95) indicate depletion of plagioclase in the source. Inherited zircon cores of 3.0–2.9 ​Ga in analyzed samples indicate that the Neoarchean tholeiitic magmatism was emplaced into 2923 ​± ​14 ​Ma old Mesoarchean crust (εNd(t) ​= ​–2.58 and Nd TDM ​= ​3.2 ​Ga) of the Rio Grande do Norte domain. The age of retro-eclogite facies metamorphism is not yet completely understood. We suggest that two high-grade metamorphic events are recognized in the mafic rocks: the first at 2.0 ​Ga, recorded in some samples, and the second, at ca. 600 Ma, stronger and more pervasive and recorded in several of the mafic rock samples. The Neoproterozoic zircon grains are found in symplectite texture as inclusions in the garnet grains and represent the age of HP conditions in the area. These zircon grains show a younger cluster of concordant analyses between 623 ​± ​3 ​Ma and 592 ​± ​5 ​Ma with εHf(t) values of +0.74 to –65.88. Thus, the Campo Grande rock assemblage is composed of Archean units that were amalgamated to West Gondwana during Neoproterozoic Brasiliano orogeny continent-continent collision and crustal reworking.  相似文献   

19.
《Gondwana Research》2008,13(4):489-508
U–Pb SHRIMP zircon geochronology, bulk-rock geochemistry, and Sr–Nd isotopic data are herein presented in order to better constrain the tectonic evolution of the Nico Pérez Terrane (NPT), Uruguay. The studies are focused on two key geological units located in the southern part of the NPT, namely Carapé Complex (CC) and Lavalleja Metamorphic Complex (LMC). The Carapé Complex, previously interpreted as part of a remnant magmatic arc, formed at 1.75 Ga mainly by partial melting of Archean crust (TDM model ages between 2.8 and 3.0 Ga with strong negative ɛNd (t) values). The Lavalleja Metamorphic Complex, previously interpreted as a back-arc basin, has volcanic units formed at 0.59 Ga during the Brasiliano Cycle, but may also contain older (Paleoproterozoic) units. The geochemistry of the LMC meta-igneous rocks indicates a complex scenario. Two distinct signatures have been recognized, one similar to MORBs and the other similar OIBs. Sm–Nd TDM model ages of basic rocks present a bimodal (Paleoproterozoic and Archean) distribution, which is also recognized in several other units of the NPT. In the light of new results, we conclude that the NPT represents a cratonic margin setting, characterized by the stacking of lithotectonic assemblages with Archean, Paleoproterozoic, and Neoproterozoic age at the margin of the Rio de la Plata Craton.  相似文献   

20.
The U–Pb isotope data and corresponding ages of detrital zircons from rocks of the basal complexes of the Uralides of different segments of the Ural Fold Belt are considered. It was established that complexes of ancient domains of the East European Platform (Volga-Uralia, Sarmatia, Kola, etc.) seem to have been the main provenance areas of the clastic material for the Southern, Middle, and Northern Urals. This means that there were relatively remote and local (igneous formations of the pre-Uralides) provenance areas. Rift rock associations of the Uralides of the Subpolar and Polar Urals were formed mainly through erosion of local provenance areas (predominantly, Late Riphean–Vendian island-arc and orogenic magmatic complexes of the Proto-Uralides–Timanides). Detrital zircons of Riphean age dominate in rocks of the basal complexes of the Uralides. A source for them could have been rock complexes of Svecofennian-Norwegian Orogen and Cadomides of the Scythian-Turan Plate, intraplate magmatic formations, and metamorphic complexes, as well as blocks accreted to the margin of the East European Platform in the Late Precambrian–Cambrian and later detached and displaced during the Ordovician rifting and spreading. In general, the basal complexes of Uralides were formed owing to supply of clastic material from both remote and local sources. Despite the appearance of information of a totally new level (U–Pb isotope ages of detrital zircons, their Lu–Hf systematics, and the distribution features of rare earth and trace elements), the contribution of these sources to the formation of the Late Cambrian–Early Ordovician clastic strata is hardly possible at present to evaluate.  相似文献   

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