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1.
This paper presents the first data on the systematics of rare earth elements (REE), Th, Hf, Sc, Co, Cr, and Ni and the Nd model ages of fine-grained aluminosilicate clastic rocks of the Serebryanka and Sylvitsa groups of the Vendian from the Kvarkushsko-Kamennogorskii meganticlinorium (western slope of the Central Urals). It was found that the REE distribution patterns of shales and mudstones of the two groups are similar to those of the majority of post-Archean fine-grained terrigenous complexes. The presence of pelitic rocks with GdN/YbN > 2.0 in a number of Vendian levels suggests a contribution from an Archean component in the composition of the fine aluminosilicate clastic material. This is probably also indicated by the high degree of heavy REE depletion in some mudstone samples. The REE systematics allow us to suppose a heterogeneity of Vendian paleocatchments and variations in their composition with time. The eroded areas had the most mature composition in the beginning of Serebryanka. Starting from the second half of Serebryanka, mafic and/or ultramafic rocks started playing a significant role in the provenances. The rocks of the lower portion of the Serebryanka Group show TNd(DM) values of about 2.0 Ga, whereas the upper part of the section is dominated by rocks with TNd(DM) ? 1.77–1.73 Ga. This indicates that during the Taninskaya and Koiva time periods, fine aluminosilicate clastic material was supplied into the sedimentation region mainly from the west, from the eastern areas of the east European platform, where Archean and Early Proterozoic crystalline complexes dominated. A decrease in model ages was related to the addition of juvenile mantle material to the mature continental crust. Such processes can be illustrated by the mafic-ultramafic complexes (Dvoretskii, Shpalorezovskii, Vil’vinskii, etc.) located in the field of Vendian sedimentary sequences, which show TNd(DM) values from 824 to 707 Ma. It was concluded that the history of the formation of an Early Vendian rift in the western slope of the central Urals included only one rifting event (rather than three, as was previously supposed), which was supported by a variety of recent geological and isotope geochemical data.  相似文献   

2.
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   

3.
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   

4.
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   

5.
In the mid-1980s, it was concluded based on geochemical study that Th, Sc, La concentrations and ratios Th/Sc, La/Sc and Eu/Eu* did not wary significantly in the post-Archean time. It was impossible to judge about compositional variations of upper crust during the Riphean and Vendian, because data of that time characterized a limited number of samples from the post-Archean basins of Australia, New Zealand, and Antarctic. Considered in this work are variations of Eu/Eu*, LREE/HREE, Th/Sc, and La/Sc ratios in Upper Precambrian fine-grained siliciclastic rock of the Southern Urals western flank (Bashkirian meganticlinorium) and Uchur-Maya region (Uchur-Maya plate and Yudoma-Maya belt). As is established, only the Eu anomaly in the studied siliciclastic rocks is practically identical to this parameter of the average post-Archean shale. Three other parameters plot on the Riphean-Vendian variation curves with positive and negative excursions of diverse magnitude, which do not coincide always in time. It is assumed that these excursions likely mark stages of local geodynamic activity, destruction of pre-Riphean cratons, and progressing recycling of sedimentary material during the Riphean.  相似文献   

6.
Lithogeochemical features of the Vendian mudstones and silty mudstones taken from Borehole Keltma 1 in the southern part of the Vychegda trough of the Mezen syneclise are discussed. It is shown that fine-grained clastic rocks of the Ust-Pinega, Krasavino, and Mezen formations have similar chemical compositions, suggesting their accumulation in sufficiently similar settings. The main part of the studied samples has K2O/Al2O3 < 0.4. This fact, in combination with the absence of TM-FM and NPM-HM correlations, indicates a significant contribution of recycled aluminosiliciclastics in their composition. At the same time, the absence of correlation between CIA and indicator ratios of rock composition in the paleodrainage basins, such as Th/Cr and Th/Sc, indicates that CIA and some other lithochemical indicators appropriately reflect the paleoclimatic conditions in source areas surrounding a basin. The CIA value in most of the analyzed samples is no more than 70. Thus, the Keltma section is similar to Upper Vendian sequences of the Kvarkush-Kamennogorsk anticlinorium and the Shkapovo-Shikhany depression. It has been established that felsic and intermediate magmatic rocks coupled with a significant contribution of quartz-rich sediments served as the source of fine aluminosiliciclastics for the southern Vychegda trough during the Vendian. High Ce/Cr values in the mudstones and silty mudstones suggest that the geochemically primitive Archean protoliths were not involved in the washout. In the SiO2-K2O/Na2O diagram, the Vendian mudstones and silty mudstones are plotted in the field of sediments of active continental margins. Typical low values of Mo/Mn and some other redox indices in these rocks indicate that oxidizing environment predominated in bottom waters of the sedimentation basin during the entire Vendian. Analysis of variations of the lithochemical indicators upward the Vendian sedimentary successions in borehole Keltma 1 made it possible to divide the section into three sequences of different lithofacies and paleontological compositions.  相似文献   

7.
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   

8.
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   

9.
The geochemical features of basal fine-grained terrigenous rocks from the Riphean sedimentary megasequences of the Southern Urals, Uchur-Maya region, and Yenisei Range were compared in order to estimate the maturity of the continental crust that was formed by the beginning of the Riphean. It was shown that initial shales from the base of the Riphean sequence of the Yenisei Range and fine-grained aluminosiliciclastic rocks from the base of the Riphean sections of the Southern Urals were formed by the erosion of a rather mature continental crust. In contrast, fine-grained terrigenous rocks from the base of the Riphean of the Uchur-Maya region were derived from immature Late Archean protoliths or their Early Proterozoic analogs. The fine-grained terrigenous rocks of the three sedimentary megasequences show different variations in the (La/Yb)N ratio. In the Southern Urals, this ratio is high (12–15) in the Burzyan Group and decreases upsection to 6–10. In the shales of the Uchur-Maya region, the (La/Yb)N ratio decreases upsection, and the La/Sc ratio shows a sympathetic behavior. This is due to a decrease in the proportion of “primitive” tonalite-trondhjemite associations of the Archean granite-greenstone terranes in the provenance area with time and the appearance of intra-plate (riftogenic?) granitoids and significant amounts of basic and ultrabasic rocks. The latter marks the onset of large rift-forming events in the Uchur-Maya region at the beginning of the Late Riphean. The (La/Yb)N of the studied rocks from the Yenisei Range are mostly similar to the PAAS ratio, but higher values were found in the Upper Vorogovka and Chingasan groups, which was related to the contribution of strongly LREE-enriched granitoids and rift felsic and alkali basaltic volcanic associations to the formation of the terrigenous material. A comparison of Rb, Sr, Y, Zr, Ba, Hf, Th, U, Cr, and Ni contents and Zr/Y, (La/Yb)N, Ni/Co, Cr/Th, Cr/Sc, and La/Th ratios in the fine-grained terrigenous rocks of the Riphean megasequences of the Southern Urals, Uchur-Maya region, and the Yenisei Range with those in the model geochemical objects (PAAS, UCPR1, UCAR2, and others) showed that, in terms of most of the parameters, the Riphean fine-grained terrigenous rocks from the three regions are similar to each other, PAAS, and Proterozoic cratonic shales. This indicates a fairly high general maturity of the protoliths that were eroded during the Riphean in the eastern East European craton and in the southeastern and southwestern parts of the Siberian craton.  相似文献   

10.
华北克拉通南缘古-中元古代构造-沉积演化的俯冲说、裂解说之争由来已久,兵马沟组作为熊耳群形成后的第一套碎 屑沉积岩层,对解释上述过程有重要意义。该文通过对豫西伊川地区的兵马沟组泥质岩微量及稀土元素的地球化学特征分析, 探讨了其沉积环境、物源,进而揭示了其发育的大地构造背景。结果表明：（1）兵马沟组泥质岩稀土元素含量高,球粒陨石标准 化后轻重稀土分异明显,δEu负异常,δCe异常不明显,富集Rb、Th、La、Ce、Nd、Zr、Hf 等,较上地壳平均值富集Sc、V、Cr、Co、Ni、 Rb;（2）Ceanom指数、V/V + Ni反映了泥质岩沉积时的还原环境,Sr/Ba、B/Ga表明其为由陆相至海相的过渡型沉积;（3）Cr/Zr、Th/U 等元素比值关系反应其物源无深部物质加入,La/Yb-Ce、Co/Th-La/Sc图解表明物源为中酸性火山岩及少量稳定陆壳物质; （4）Th-Sc-Zr/10、Th-Co-Zr/10图解表明其物源主要来自大陆岛弧。综合上述分析结果,结合区域地质信息,可以得出中元古界 兵马沟组形成于大陆岛弧弧后盆地,熊耳群分布区为中元古代俯冲成因的大陆岛弧区。  相似文献   

11.
Low Sr and Ca contents in fine-grained terrigenous rocks of the Vendian sedimentary cover in the East European Platform may be related to the following reasons: (1) the absence of authigenic carbonates in Vendian mudstones owing to desalination of epiplatformal sedimentary basins; (2) significant contribution of the chemically weathered rocks in the formation of the Vendian cover of the East European Platform.  相似文献   

12.
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   

13.
何佳伟  谢渊  刘建清  何利 《沉积学报》2021,39(3):656-671
利用化学元素行为特征的差异性,以总有机碳（TOC）、主量元素和微量元素的含量或比值判别四川盆地西南缘盐津牛寨剖面下志留统龙马溪组细粒沉积岩形成时的初级生产力、氧化还原条件、陆源供应的输入和盆地水体滞留程度等环境特征,并对比川南沉降中心（长宁剖面）和南川非沉降中心（南川剖面）龙马溪组的古环境参数,综合探讨了早志留世上扬子海滞留海盆细粒沉积岩有机质富集的差异性。结果表明,盐津牛寨地区下志留统细粒沉积岩以黑色—灰黑色碳质页岩、页岩和泥岩为主,富有机质主要集中在龙马溪组下段,TOC平均含量为2.73%;Babio显示盆地西南缘龙马溪组早期具有较高的生产力;Ni/Co、V/（V+Ni）,U/Th和V/Cr揭示龙马溪组从底部缺氧环境逐步过渡到顶部的氧化环境;TiO2、Al2O3、Zr表明龙马溪组下段陆源碎屑供应量明显增加;Mo/TOC值和U-Mo协变模式图均说明四川盆地西南缘在早志留世水体呈中等滞留环境。与长宁剖面和南川剖面对比分析,发现盆地西南缘TOC与氧化还原条件呈现显著的正向相关关系;与盆地水体的滞留程度呈现微弱的正向相关关系;与陆源碎屑物质的注入和初级生产力无明显相关性,表明缺氧还原的沉积环境是影响该地区龙马溪组黑色页岩有机质富集的主要因素。  相似文献   

14.
沉积盆地中碎屑沉积岩的地球化学特征主要受物源区的制约。对北羌塘盆地三叠系康鲁组碎屑沉积岩进行全岩测试,分析表明,样品的成分变异指数(ICV)和化学蚀变指数(CIA)变化不大,反映其源区物质较新鲜,成熟度相对较低,化学风化作用较弱-中等。岩石SiO2含量普遍较高,TFe2O3+MgO含量相对较高,但K2O/Na2O比值变化较大。微量元素标准化蛛网图与后太古代页岩和上地壳相似,且REE配分模式图显示富集LREE,δEu明显负异常以及δCe呈弱负异常。此外,Th/U、La/Sc、Th/Sc、Ba/Sr等微量元素比值以及TiO2-Ni、La/Th-Hf、Co/Th-La/Sc和Th/Sc-La/Sc判别图表明沉积物物源具有浅源、长英质的特征。沉积岩的SiO2-K2O/Na2O、La-Th、La/Y-Sc/Cr双变量图解和La-Th-Sc、Th-Sc-Zr/10、Th-Co-Zr/10三变量图解,以及Th/U、Rb/Sr等比值特征表明北羌塘三叠系康鲁组沉积砂岩的物源区构造背景以大陆岛弧为主,兼有大洋岛弧和被动大陆边缘构造背景特征。  相似文献   

15.
由于受区域构造作用、海平面变化、上升洋流以及古气候等主控因素的制约,中上扬子地区纽芬兰世-第二世主要表现为浅海环境。早寒武世梅树村期,主要发育潮坪相碎屑岩、磷块岩、白云岩和硅质岩,陆棚相含磷结核硅质岩、泥岩以及陆棚深水盆地相碳质页岩、硅质岩等。筇竹寺期,海侵范围扩大,陆棚深水盆地向西北扩展,其余广大区域为碎屑岩浅海所覆盖。沧浪铺期,主要表现为碎屑岩和碳酸盐交互沉积的混积陆棚环境,盆地相则具有继承性,主要为碳质页岩沉积。早寒武世龙王庙期开始向碳酸盐台地演化,广大区域为碳酸盐沉积,盆地区范围有所缩小。早寒武世的岩相古地理演化表明,从早到晚沿着东南边缘始终存在一较深水沉积区,其沉积物质主要为碳质页岩和硅质岩,为烃源岩有利沉积区,奠定了油气生烃成藏的物质基础。同时在演化过程中还形成了相应的储层以及分布较广的膏盐层。根据这一油气地质条件组合及其空间分布,针对中上扬子地区早古生代海相地层的油气勘探应该首先围绕这一富烃沉积区来开展,集中对膏盐层下部目的层开展油气勘探工作。  相似文献   

16.
川西北中生代特提斯沉积物地球化学特征对陆源风化条件的指示意义顾雪祥（成都理工学院．成都６１００５９）关键词特提斯浊积岩，风化条件，元素分配型式，碱金属－碱土金属，化学变异指数地史时期的物源区现今许多已被剥蚀破坏，因而有关这类物源区的母岩性质、同化条件...  相似文献   

17.
内蒙古阿拉善北部杭乌拉地区位于中亚造山带的中段南缘,处于西伯利亚板块、塔里木板块和华北板块之间,是研究中亚造山带和古亚洲洋演化的重要区域。文中以杭乌拉地区硅质岩地球化学特征为主要研究内容,结合前人研究结果,对阿拉善北部地区早古生代构造背景进行分析。杭乌拉地区下古生界西双鹰山组和班定陶勒盖组硅质岩镜下可见黏土、陆源粉砂和放射虫等,表现为明显的沉积成因特征。硅质岩地球化学特征一致性较差,其中Fe、Mn、Al等主量元素、Sc/Th值、(La/Ce)n值和δCe值反映出硅质岩为大陆边缘沉积,(La/Yb)n值反映硅质岩接近于远洋沉积背景,δEu 值表现为无热液作用影响,V、V/Y、Ti/V值多接近于洋中脊和大洋盆地硅质岩特征,U/Th、Ba/Sr值也属于热水成因硅质岩特征。硅质岩稀土元素配分曲线呈平坦状,无明显的右倾(大陆边缘轻稀土元素富集配分模式)和左倾(开放洋盆重稀土元素富集配分模式)特征。研究区早古生代地质记录相对较少,但是鉴于该地区晚古生代复杂的构造演化特征及其对早古生代古地理格局的继承,并结合上述硅质岩地球化学特征,认为研究区在早古生代为一个多岛洋环境。  相似文献   

18.
The Derba block is one of the largest Precambrian terranes of the Sayan-Yenisei accretionary belt in the southwestern margin of the Siberian Platform. It is composed of metamorphosed terrigenous-carbonate rocks of the Sayan Group, injected by granitoids. The geochemical features of gneiss-schist associations indicate the low maturity of their sedimentary protoliths corresponding in composition mainly to graywackes and terrigenous-carbonate rocks (marls). According to the results of U-Pb (LA-ICP-MS) dating of detrital zircons from gneisses and schists, the sedimentary protolith formed in the Vendian. Neoproterozoic subduction complexes were probably the major provenance for terrigenous material, and Early Precambrian rocks made a limited contribution. The Ar-Ar and U-Pb isotope data testify to nearly coeval and multistage events of metamorphism (up to the amphibolite facies) and granitoid magmatism (~ 510-500 and 480-465 Ma) in the Derba block. These processes were reflective of the Early Caledonian orogenic processes in the structures of the Central Asian Orogenic Belt. The similarity in the composition, time of sedimentation, and provenances of metaterrigenous-carbonate complexes of the Derba block (Sayan Group), West Sangilen block of the Tuva-Mongolian massif (Erzin and Moren complexes), and the Khamar-Daban terrane (Slyudyanka Group) suggests that these structures were a single Vendian continental margin with lateral variations in depositional environments and the sources of terrigenous material.  相似文献   

19.
We consider the general and specific features of the evolution of the composition of fine-grained terrigenous rocks in the Riphean sedimentary megasequences of the Southern Urals, Uchur-Maya region, and Yenisei Ridge. It has been established that the crust on the southwestern (in the modern frame of references) periphery of the Siberian craton was geochemically the most mature segment of the Riphean continental crust. For example, the fine-grained clastic rocks and metapelites of all Riphean lithostratigraphic units of the Yenisei Ridge have higher median contents of Th than the most mature Paleoproterozoic crust, and in median contents of Y and Cr/Th values they are the most similar to it. In the Southern Urals and Uchur-Maya region, some units of the Riphean sedimentary sequences show median contents of Y and Th and Cr/Th values close to those of primitive Archean crust. Analysis of Cr/Th variations in the fine-grained terrigenous rocks of all three megasequences shows that the minimum Cr/Th values, evidencing a predominance or the abundance of felsic rocks in provenances, are typical of the Riphean argillaceous shales and metapelites of the Yenisei Ridge. The distinct Cr/Th and Cr/Sc increase in the fine-grained clastic rocks of the Chingasan Group of the ridge reflects the large-scale destruction of continental crust during the formation of rift troughs as a result of the Rodinia breakup in the second half of the Late Riphean. The Cr/Th variations in the Lower and Middle Riphean argillaceous shales and mudstones of the Bashkirian mega-anticlinorium and Uchur-Maya region are in agreement, which evidences the subglobal occurrence of rifting in the early Middle Riphean (so-called “Mashak rifting”).  相似文献   

20.
The geochemical and Sm–Nd isotope characteristics of Late Precambrian and Early Cambrian sandstones previously related to the sedimentary cover of the Dzabkhan continental block are reported. It is established that the Riphean and Vendian sedimentary rocks of the Ul’zitgol’skaya and Tsaganolomskaya Formations were accumulated within the Dzabkhan continental block as a result of recycling of the terrigenous deposits formed at the expense of destruction of basement rocks and younger granite. The formation of terrigenous rocks of the Bayangol’skaya Formation after a gap in sedimentation occurred in the sedimentary basin, where only the Late Riphean formations of the juvenile crust, probably of the Dzabkhan–Mandal block were the sources, without the contribution of the ancient crustal material. The Tsaganolomskaya and Bayangol’skaya Formations were formed in different sedimentary basins and cannot be related to the same complex.  相似文献   

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