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1.
T. A. Myskova R. I. Mil’kevich P. A. L’vov 《Stratigraphy and Geological Correlation》2012,20(2):166-178
During our study we obtained the first age datings of detrital zircons from metasandstones of the Ladoga Group (North Ladoga
Region, Russia) with the U-Pb isotopic method using the SHRIMP-II ion microprobe. The data obtained made it possible to clarify
the lower age limit of sedimentation and to obtain additional age data for evaluating the chemical composition and an age
of source areas. This work presents the results of isotopic-geochemical (Sm-Nd) and geochemical studies of metasediments.
High LREE concentrations, a high La/Sc ratio and a low Cr/h ratio, and the presence of a distinct Eu-minimum (Eu/Eu* = 0.54–0.72)
indicate a significant role of acidic terrigenous material in source areas. In addition, the data of the isotope analysis
of detrital zircons show that sediments of the Ladoga Group accumulated from the destruction of Proterozoic rocks (1.9–2.0
Ga; a proportion in sediments is 60–70%) and, to a lesser extent, Archean rocks (2.54–2.74 and 2.9–3.01 Ga; the proportion
in sediments is 30–40%). One of the Archean source areas could be granite-gneisses of the Pitkyaranta-Koirinoya dome structure
with the U-Pb zircon age of 2659 ± 15 Ma. We have established the lower age limit of sedimentation as 1.9 Ga within the measurement
error. The Sm-Nd model ages obtained (2.5–2.6 Ga for sediments of the Ladoga Group and over 3.4 Ga for granite-gneisses of
the dome structure) suggest a significant contribution of ancient crustal source area into source rocks. Our age data agree
well with those for svecofennides of Finland. 相似文献
2.
Preliminary isotopic data for Late Proterozoic (～ 1100 Ma) granulite-facies metamorphics of the Prydz Bay coast indicate only very minor reworking (i.e., remetamorphism) of Archaean continental crustal rocks. Only two orthopyroxene—quartz—feldspar gneisses from the Rauer Group of islands, immediately adjacent to the Archaean Vestfold Block, show evidence for an Early Archaean origin (～ 3700—3800 Ma), whereas the vast majority of samples have Middle Proterozoic crustal formation ages (～ 1600–1800 Ma). The Prydz Bay rocks consist largely of garnet-bearing felsic gneisses and interlayered aluminous metasediments, although orthopyroxene-bearing gneisses are common in the Rauer Group; in contrast, Vestfold Block gneisses are predominantly orthopyroxene-bearing orthogneisses. The extensive Prydz Bay metasediments may have been derived by erosion of Middle Proterozoic rocks, such as the predominantly orthogneiss terrain of the Rauer Group, and deposited not long before the Late Proterozoic metamorphism. Data from nearby parts of the East Antarctic shield also suggest only limited Proterozoic reworking of the margins of the Archaean cratons.As in the Prydz Bay area, high-grade metamorphies in nearby parts of the East Antarctic shield show a secular increase in the sedimentary component. Archaean terrains like the Vestfold Block consist mainly of granitic orthogneisses derived by partial melting of igneous protoliths (I-type), whereas Late Proterozoic terrains (such as the Prydz Bay coast) include a much higher proportion of rocks derived either directly or by partial melting (S-type granitic orthogneisses) from sedimentary protoliths. Related chemical trends include increases in K2O2, Rb, Pb, and Th, and decreases in CaO, Na2O2 and Sr with decreasing age, essentially reflecting changes in the proportions of plagioclase and K-feldspar. 相似文献
3.
4.
The early Proterozoic metasedimentary sequence of the Alligator Rivers Region (a part of the Pine Creek Geosyncline) in the Northern Territory, Australia, overlies an Archaean granitoid basement. Early Proterozoic sedimentary sequences, in general, record important changes in the composition of the upper continental crust about the Archaean-Proterozoic boundary. However, the geochemistry of only a few of these sequences has been documented. The geochemistry of the early Proterozoic succession in the Alligator Rivers Region is reported here and the results are interpreted in terms of differences between the stratigraphic units, their provenance—particularly in relation to crustal evolution, and their subsequent metamorphism and weathering.Clastic metasedimentary rocks throughout the Alligator Rivers Region have a remarkably uniform major and trace element geochemistry. The Kakadu Group and upper member of the Cahill Formation are relatively more enriched in SiO2 and correspondingly more depleted in Al2O3 than the rest of the sequence, reflecting the greater dominance of metapsammitic assemblages. The lower member of the Cahill Formation, which hosts the major U deposits of the Alligator Rivers Region, and the metasedimentary sequence in general, exhibit no significant enrichment in U above normal background values. Rare earth element (REE) concentrations in the metasedimentary units within the Alligator Rivers Region are uniform, though in detail there are some important differences within and between formations.The composition of the early Proterozoic clastic metasediments in the Alligator Rivers Region is consistent with the composition of similar material of the same age from other areas, and supports current ideas on crustal evolution. The Alligator Rivers metasediments are enriched in Si and K, and depleted in Mg, Ca, and Na relative to the Archaean average for clastic sedimentary rocks, and their REE geochemistry resembles typical post-Archaean sedimentary rocks having a light REE enriched pattern and a distinct Eu/Eu1 depletion compared to typical Archaean sediments. However, the REE data indicate that two compositionally distinct sources are involved in the provenance of the Kakadu Group, and possibly the lower member of the Cahill Formation, where two types of REE patterns can be distinguished on their HREE concentration and Eu/Eu1 anomaly. 相似文献
5.
《Geochimica et cosmochimica acta》1999,63(13-14):2071-2088
Twenty-three clastic metasediments from the Kongling high-grade terrain of the Yangtze craton, South China were analyzed for major, trace and rare earth elements and Sm-Nd isotopic ratios. Associated dioritic-tonalitic-trondhjemitic (DTT) and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance. The results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical, geochemical and Sm-Nd isotopic compositions. Group A is characterized by having no to slight negative Eu anomalies (Eu/Eu1 = 0.82–1.07), being high in Cr (191–396 ppm) and Ni (68–137 ppm), and low in Th (3.3–7.8 ppm) and REE (ΣREE = 99–156 ppm). These characteristics are similar to those of metasediments from Archean greenstone belts. In addition, the Group A metasediments have the value of the Chemical Index of Alteration (CIW) close to felsic gneisses. Their Sm-Nd isotopic, REE and trace element compositions can be interpreted by mixtures of the DTT gneisses and amphibolites. Dating of detrital zircons from 2 Group A samples by SHRIMP reveals a major concordant age group of 2.87–3.0 Ga, which is identical to the age of the trondhjemitic gneiss. These results strongly suggest that Group A was principally the first-cycle erosion product of the local Kongling DTT gneiss and amphibolite. Moreover, the higher than amphibolite Cr content and slight Eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3.0–3.3 Ga in age also played a role.Group B is characterized by the presence of graphite and shows a more evolved composition similar to post-Archean shales with a prominent negative Eu anomaly (Eu/Eu1 = 0.48–0.77) and high CIW. On paired Cr/Th vs La/Co and Co/Th plots, Group B samples conform to a two-end member mixing line of the Kongling granitic gneiss and amphibolite. However, data on Nd model age and CIW suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source.Both Groups A and B exhibit a clear positive correlation between CIW and TDM and a negative one between CIW and Eu/Eu1. These correlations point to the crustal evolution of the Yangtze craton towards coupled increasing CIW and Eu depletion with decreasing age. This in turn reflects the change of granitoid magmatism from local Na-rich dioritic-tonalitic-trondhjemitic rocks to widespread K-feldspar granite. The change led to the intracrustal differentiation, stabilization and growth of the craton.Group C is restite and contains abundant sillimanite and garnet and unusually high ilmenite (7–11vol%), which can be seen to be dehydration melting products of biotite under the microscope. This group shows extremely varied REE distributions from LREE enriched to depleted and from negative to strong positive (Eu/Eu1 = 1.63) Eu anomalies. Compared to Groups A and B, Group C is severely depleted in Na2O, K2O, LREE, Rb and Ba, whereas TiO2, Co, V, Sc and HREE and Y are considerably enriched. This is accompanied by anomalous high Sm/Nd (0.21–0.28), 147Sm/144Nd (0.1361–0.1738) and 143Nd/144Nd (0.511589–0.511958) ratios. TDM correlates clearly with Sm/Nd ratio and 2 out of 3 samples give significantly older to unrealistic TDM (3.9–4.9 Ga). The results document redistribution of REE and an open behavior of the Sm-Nd isotope system during the biotite dehydration melting of metasediments. 相似文献
6.
Major and trace element compositions of the Paleoproterozoic metaterrigenous rocks (Neroi Group) formed in a large sedimentation
basin in the southwestern Siberian Craton (Biryusa Block) were determined to reconstruct the protoliths of metasediments,
degree of their recycling, and maturity of source rocks. Primary rocks from the lower part of the sequence (Alkhadyr Formation)
are represented by both petrogenic (“first cycle”) and recycled sediments of the graywacke to siltstone and aluminous pelite
series. Protoliths of the micaceous and carbonaceous schists from the upper part of the sequence (Tumanshet Formation) correspond
to silty pelites and pelites. As the micaceous schists of the Alkhadyr Formation, these rocks have K2O/Al2O3 < 0.3 and elevated Th concentrations, indicating the contribution of recycling in the formation of the fine-grained rocks.
Distribution of trace and rare earth elements (REE) in metaterrigenous rocks of the Neroi Group testifies to the predominance
of felsic rocks in the source area, while the prominent Eu minimum indicates the presence of granitoids—the products of crustal
melting. Rocks of the Alkhadyr Formation also show elevated contents of Cr, Co, Ni, Sc, and Fe, indicating the development
of mafic rocks in the source area. Comparison of the trace element contents and their ratios in rocks of the Neroi Group with
those in the Archean (3.5–2.5 Ga) and Paleoproterozoic (2.5–1.6 Ga) upper continental crust made it possible to establish
that metasedimentary rocks of the Neroi Group were formed by the erosion of sufficiently mature (geochemically differentiated)
protoliths, which are similar to the Paleoproterozic crust. Judging from the Sm-Nd isotope data, one of the components of
source areas for the terrigenous rocks of the Neroi Group were Archean rocks similar to basement rocks of the Biryusa block
with the Nd model ages within 2.8–2.6 Ga. The second component in the source area could be juvenile Paleoproterozoic crust
(Nd model age ∼1.9 Ga), which was probably represented by the metavolcanic associations of grabens surrounding the Biryusa
block. The minimum Nd model ages for metaterrigenous rocks of the Neroi Group define the lowermost sedimentation boundary
at 1.9 Ga. 相似文献
7.
Sh. K. Baltybaev G. V. Ovchinnikova V. A. Glebovitskii I. A. Alekseev I. M. Vasil’eva N. G. Risvanova 《Doklady Earth Sciences》2017,476(1):992-996
We have studied Pb isotopic systems of K-feldspar, pyrite, and pyrrhotine from gabbroids and ore of the Velimyaki Early Proterozoic massif in the northern Ladoga region in the southeastern part of the Fennoscandian Shield. The isochronous Pb–Pb age of sulfides has been determined as ～450 Ma, which corresponds to intersection of the regression line with the lead accumulation curve with μ = 10.4–10.8; the model Pb age of sulfides is close to isochronous under the condition that the composition of lead evolved from a geochemical reservoir with an age of 1.9 Ga. The isotopic parameters of the lead in sulfides and K-feldspar indicate their formation in upper crust conditions (μ = 238U/204Pb > 10). From the obtained data, it follows that the isotopic composition of lead in K-feldspar corresponds to a Proterozoic age (1890 Ma) of magmatic crystallization of the rocks in the massif, and strongly radiogenic lead sulfides testify, with the greatest probability, to the later (Caledonian) formation of sulfide ores. 相似文献
8.
A. V. Kuptsova A. K. Khudoley W. Davis R. H. Rainbird V. P. Kovach N. Yu. Zagornaya 《Stratigraphy and Geological Correlation》2011,19(2):125-140
The data on the age of sediments and their sources were first obtained by isotopic methods and lithological-petrographic observations
for the Priozersk and Salmi formations in the northeastern Pasha-Ladoga basin. Wide development of coarse-grained terrigenous
rocks and peculiar structures point to the proximity of provenances and dominant sedimentation in terrestrial settings. U-Pb
dating of 168 detrital zircons revealed that approximately 87% of all grains are Early Riphean in age, while other grains
are dated back to the Early Proterozoic. Archean age is documented for only two zircon grains. It is established that detrital
material of sandstones from the Priozersk and Salmi formations largely originate from the Salmi Massif (1.55–1.53 Ga) and
secondarily from the Svecofennian intrusions of the northern Ladoga region aged approximately 1.88 Ga, which is confirmed
also by Sm-Nd data. The occurrence of detrital zircons dated at approximately 1480–1490 Ma indicates development of coeval
magmatic bodies in the immediate proximity to the sedimentation basin, which remain undiscovered. The concordant age obtained
for the youngest detrital zircon grain points to the onset of sedimentation in the basin after 1477 ± 8 Ma ago. Thus, Riphean
sedimentation on the eastern slope of the trough commenced only 20 Ma prior to the emplacement of the Valaam Sill. 相似文献
9.
《International Geology Review》2012,54(9):825-839
The mafic-ultramafic metavolcanics of the Volotsk sequence are the earliest formations of the Karelian gneiss-greenstone region. They are fragments of Early Archean komatiite-basalt crust, constituting the base of the section of the Vodla block, on the periphery of which greenstone belts formed in the Late Archean. The komatiites of the Volotsk sequence are of the Al-nondepleted geochemical type [(Al2O3/TiO2)N, (Gd/Yb)N = 1.0], which may be related to relatively low temperatures existing in the Early Archean mantle of the Karelian gneiss-greenstone region. The parent magmas of the komatiites were generated in a depth range of 450-250 km under conditions of high (>50%) degrees of partial melting in equilibrium with an olivine-orthopyroxene restite. Differentiation of the komatiite melts after eruption on the surface was controlled by fractionation of olivine (Fo93) and a small amount of liquidus chromite. Sm-Nd isotopic dating of the komatiites and metabasalts to 3391±76 Ma reflects the time the Volotsk sequence accumulated. The positive value of εNd(T) = +1.2±0.21 suggests the existence in the Early Archean mantle of a region of reservoirs depleted in highly incongruent elements [(La/Zr)N = 0.78] produced during early stages of melting and extraction of basaltic magmas during a 100-200-m.y. interval before formation of the Volotsk sequence. The 2764±150 Ma Pb-Pb isotopic age of the metavolcanics corresponds to the time of metamorphism and granite formation, extensively manifested in the region. 相似文献
10.
Liu Jianzhong WANG Lili ZHANG FuQin LI Chunlai ZOU Yongliao OUYANG Ziyuan XU Lin 《中国地球化学学报》2005,24(1)
This paper deals with the meta-mafic volcanic rocks of the Gongyiming iron deposit at Baotou, Inner Mongolia. The major and trace elements and REE data indicate that the meta-mafic volcanic rocks occurred in the environment similar to a modern continental rift. Sm-Nd and Rb-Sr isotopic studies indicated that the meta-basic rocks were formed during the Early Neoarchean from 2800 Ma to 2900 Ma and reworked during the Late Neoarchean (2500 Ma) by metamorphism. Because of the separation of the North China Craton from the Siberia Craton during the Middle Proterozoic ( 1600 Ma), the Rb-Sr systematics of the rocks has been changed. The Se' ertengshan greenstone seems to occur during the Middle Archean. A stable continental crust may have existed during the Paleoarchean. 相似文献
11.
辽东地区构造上位于辽中—吉南元古宙活动带西南部,为胶辽克拉通金矿床分布比较集中的地区。金矿化作用发生于古元古宙变质沉积岩和花岗片麻岩的韧性及脆-韧性剪切带之中,剪切活动与成金事件具同时性关系。矿化剪切带分为3大系统,其中近东—西向和北东—南西向两大系统发育于辽河群分布区,其受控于岩石非均匀性,分别借助于早(吕梁)期S1(≈S0)和S2或D3剪切带而形成,剪切活动(构造事件)与成金事件主要发生于印支运动;丹东韧性剪切带及其相关构造为本区最重要的含矿系统,矿床均顺主构造线分布,并规律性地受控于其次级构造,剪切带的形成和成金事件均在燕山运动时期。本区提供了一个特殊的克拉通成矿作用格局:容矿岩石时代老,成金事件时代新。 相似文献
12.
This paper deals with the meta-mafic volcanic rocks of the Gongyiming iron deposit at Baotou, Inner Mongolia. The major and trace elements and REE data indicate that the meta-mafic volcanic rocks occurred in the environment similar to a modern continental rift. Sm-Nd and Rb-Sr isotopic studies indicated that the meta-basic rocks were formed during the Early Neoarchean from 2800 Ma to 2900 Ma and reworked during the Late Neoarchean (2500 Ma) by metamorphism. Because of the separation of the North China Craton from the Siberia Craton during the Middle Proterozoic (1600 Ma), the Rb-Sr systematics of the rocks has been changed. The Se‘ ertengshan greenstone seems to occur during the Middle Archean. A stable continental crust may have existed during the Paleoarchean. 相似文献
13.
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
14.
On the basis of petrochemical data, orthorocks are defined among highly metamorphosed sequences of the Landenpokh’ya Group attributed to the Kalevian and Svecofennian systems in the Ladoga region. The contents of major and trace elements in orthorocks, which constitute some volcanogenic sedimentary complexes in the western Ladoga region, are discussed. The volcanics of the first complex (Kukhka Island area) belong to the calc-alkaline series and are characterized by the presence of OIB-type basaltic andesites. The basites of the second complex (Kil’pola Island area) are attributed to the tuffaceous rocks of the WPB-type tholeiitic series. The volcanics of the third complex (Kuznechnoe-Khiitola area) are largely represented by dacites referred to the Svecofennian mature island arc. The presented geological and geochemical data imply that the first and second complexes, which include intraplate volcanics, could be formed on the basite Jatulian-Ludikovian basement (protolith). These Kalevian rocks of the western Ladoga region are correlated with the volcanogenic sedimentary complexes in the southern part of the Savo belt in Finland and belong to the Karelian province. 相似文献
15.
Sh. K. Baltybaev O. A. Levchenkov V. M. Savatenkov 《Stratigraphy and Geological Correlation》2010,18(3):234-251
The petrologic and isotopic-geochronologic study of basement rocks that were penetrated by a deep borehole in the marginal
part of the East European Platform revealed that its section overlain by the Vendian-Paleozoic sedimentary cover is Early
Proterozoic in age and largely consists of aluminous migmatized biotite, biotite-cordierite, and biotite-cordierite-sillimanite
gneisses, which are intruded by granites, plagiogranites, and metatonalites. The lower part of the section is dominated by
amphibole schists and amphibolites with subordinate nonmetamorphosed dolerite dikes and pegmatite veins. By metamorphism parameters
(T = 630–680°C, P = 2–4 kbar), the metamorphic complex may be considered as the shallowest one, compared with other Early Proterozoic complexes,
developed at least in the southwestern part of Fennoscandia. The progressive decrease in the mineral-formation pressure observed
in the Lower Proterozoic metamorphic rocks southward, away from the Karelian Craton is likely explained by the tectonic transport
(thrusting) of the Svecofennides over the margin of the Karelian Craton and their subsequent deeper erosion near the craton.
The magmatic crystallization of metamorphic palgiogranites, penetrated at depths of 925–928 and 1004 m, is estimated by the
U-Pb ID-TIMS method on zircons to occur 1860 ± 9 Ma ago. It is shown that by their age, the REE composition, and isotopic-geochemical
characteristics, these rocks are close to the plagiogranites formed in the southeastern extremity of the Svecofennnian belt
in the present-day northern Ladoga region and the Karelian Isthmus. No rocks, which could be correlated by their lithology
with the Archean rocks of the Karelian Craton, are found. 相似文献
16.
A. V. Maslov A. D. Nozhkin V. N. Podkovyrov E. F. Letnikova O. M. Turkina Yu. L. Ronkin N. V. Dmitrieva E. Z. Gareev O. P. Lepikhina O. Yu. Popova 《Geochemistry International》2008,46(11):1117-1144
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
17.
《Russian Geology and Geophysics》2007,48(1):61-70
We studied geology and main rock assemblages of the Precambrian Kan, Arzybei, and Derba terranes of the Central Asian Fold Belt which border the Siberian craton in the southwest. The Precambrian terranes include three isotopic provinces (Paleoproterozoic, Mesoproterozoic, and Neoproterozoic) distinguished from the Sm-Nd isotope compositions of granitoids, felsic metavolcanics, and metasediments. The terranes formed in three stages of crustal evolution: 2.3–2.5, 0.9–1.1, and 0.8–0.9 Ga. Proterozoic juvenile crust was produced by subduction-related magmatism; it was originally of transitional composition and transformed into continental crust by potassic plutonism as late as the Late Vendian-Cambrian. Terrigenous sediments in the Arzybei and Derba terranes vary in T(DM) Nd model ages from 1.0 to 2.0 Ga. The Nd ages of the underlying metavolcanics and lowest T(DM) of metasediments indicate that terrigenous sedimentation started in the Neoproterozoic. It was maintained by erosion of Mesoproterozoic-Neoproterozoic crust and, to a lesser extent, of Early Precambrian rocks on the craton margin or in Paleoproterozoic terranes. Ar-Ar dating of amphiboles and biotites from metamorphic rocks and U-Pb dating of zircons from granitoids yielded 600–555 and 500–440 Ma, respectively, corresponding to the Vendian and Early Paleozoic stages of nearly synchronous metamorphism and plutonism. Accretion and collision events caused amalgamation of the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic terranes in the Vendian and their collision with the Siberian craton. The lateral growth of the paleocontinent completed in the Late Ordovician. 相似文献
18.
The oxygen isotopic compositions of over 100 Archean clastic metasedimentary, felsic metavolcanic and gneissic rocks from selected areas within the Superior Province have been determined. δ18O values of low grade, immature clastic metasediments range from 8.0 to 13.3%. and hence are somewhat depleted in 18O relative to other clastic metasediments. The lower 18O content is attributed to the large proportion of unaltered rock fragments in the Archean metasediments. Felsic metavolcanics have a similar range of δ18O values (7.3 to 13.0%..δ18O values for the middle amphibolite facies Pakwash paragneiss (8.8–11.7%.) are higher than those determined for the amphibolite to granulite facies Twilight paragneiss (7.3–9.2%.). Archean orthogneisses such as the Clay Lake gneiss (7.0–9.0%.), the Cedar Lake gneiss (7.4–8.7%.) and the Footprint gneiss (5.9-8.8%.) have δ18O values similar to those of the Twilight paragneiss. Therefore, at least at metamorphic grades higher than middle amphibolite facies, Archean metasedimentary gneisses may have δ18O values that are indistinguishable from those of Archean orthogneisses, and hence are no longer isotopically recognizable as metasedimentary rocks. 相似文献
19.
The Re-Os isotopic systematics of two ca. 2.7-Ga komatiite flows from Belingwe, Zimbabwe are examined. Rhenium and Os concentrations in these rocks are similar to concentrations in other Archean, Proterozoic, and Phanerozoic komatiites. Despite the excellent preservation of primary magmatic minerals, the Re-Os systematics of whole-rock samples of the komatiites show open-system behavior. Consistent model ages for several whole-rock samples suggest a disturbance to the system during the Proterozoic. Despite the open-system behavior in the whole rocks, Re-Os systematics for concentrates of primary magmatic olivine and spinel indicate generally closed-system behavior since the magmatic event that produced the rocks. Regression of the data for the mineral concentrates yields an age of 2721 ± 21 Ga, which is consistent with Pb-Pb and Sm-Nd ages that have been previously reported for the komatiites (Chauvel et al., 1993), and an initial 187Os/188Os ratio of 0.11140 ± 84 (γOs = +2.8 ± 0.8).The 2 to 3% enrichment in 187Os/188Os ratio of the mantle source of the komatiites, relative to the chondritic composition of the contemporaneous convecting upper mantle, most likely reflects either the incorporation of substantially older (≥ 4.2 Ga), Re-rich recycled mafic crust into the mantle source of the komatiites or the contribution of suprachondritic Os to the source from the putative 187Os-enriched outer core. The former interpretation would indicate the Hadean formation and recycling of mafic crust. The latter interpretation would require early formation of a substantial inner core followed by upwelling of a mantle plume from the core-mantle boundary, at least as far back as the Late Archean. Either interpretation requires large-scale mantle convection during the first half of Earth history. 相似文献
20.
《Geochimica et cosmochimica acta》1999,63(11-12):1787-1804
Rift-related lavas of the North Shore Volcanic Group (NSVG) are intruded by plutonic rocks of the Duluth Complex along the unconformity between the NSVG and the underlying Proterozoic metasedimentary rocks (Animikie Group) and Archean volcano-sedimentary and plutonic rocks. Heat associated with the emplacement of the mafic intrusions generated fluid flow in the overlying plateau lavas. δ18O values for whole rocks from the NSVG and hypabyssal sills range from 5.5 to 17.7‰ and 5.3 to 11.5‰, respectively, and most values are higher than those considered “normal” for basaltic rocks (5.4 to 6.0‰). In general, there is a positive correlation between whole rock δ18O and water content, which suggests that elevated δ18O values are related primarily to secondary mineral growth and isotopic exchange during hydrothermal alteration and metamorphism. δ18OH2O values computed from amygdule-filling minerals such as smectite, chlorite, and epidote found in low- to high-temperature metamorphic zones range from ∼−1 to 6‰ with an average value of ∼3‰. Smectite in the lower-grade zones gives computed δDH2O values between −26 and −83‰, whereas epidote in the higher-grade zones gives δDH2O values of −15 to 6‰. Fluid isotopic compositions computed from epidote and smectite values are suggestive of the involvement of at least two fluids during the early stages of amygdule filling. Fluid δD and δ18O values determined from epidote at the higher metamorphic grades indicate that seawater dominated the deeper portions of the system where greenschist facies assemblages and elevated δ18O values were produced in flow interiors, as well as margins. Smectite isotopic compositions suggest that meteoric water was predominant in the shallower portions of the system. The increase in δ18O values of massive flow interiors with depth is interpreted as a result of rock interaction with a fluid of constant oxygen isotopic composition with increasing temperature. The stable isotopic data are supportive of previous suggestions that seawater was involved in the hydrothermal system associated with the Midcontinent Rift. Although the origin of the seawater remains problematic, it appears that marine incursions may have occurred during the late stages of Portage Lake volcanism, and periodically thereafter. 相似文献