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1.
The comprehensive study of sections of the Shatak Complex has revealed that conglomerates at the base of Middle Riphean rocks are not basal but intraformational rocks. Previously described angular unconformities between shales of the Sukhin Subformation (Yusha Formation, R1) and conglomerates of the Kuz”elga Subformation (Mashak Formation, R2) are related to late tectonic movements. Magmatic rocks developed at the base of the Middle Riphean section are represented by sheet intrusions formed in the course of emplacement of a fluid-saturated magmatic melt into partially or completely lithified terrigenous rocks at the graben formation stage during the origination of synkinematic faults that served as magma conduits. It is inferred that distribution of provenances of clastic materials and sedimentation basins in the Burzyanian and Yurmatian should be scrutinized in the study region, because the normal regressive sequence of rocks from the uppermost Yusha Formation to the lowermost Mashak Formation, which was established in the Shatak Ridge, eliminates a clear boundary distinguished between them at present. The idea about an older age of the Mashak conglomerates is substantiated.  相似文献   

2.
The oldest crystalline complexes of the Early Caledonian superterrane of Central Asia were formed in the Early Precambrian. They are exposed in the basement of microcontinents, which represent old cratonic fragments. Among the latters are the crystalline complexes of the Tarbagatai block previously ascribed to the Dzabkhan microcontinent. It was shown that the crystalline complexes of the Tarbagatai block have a heterogeneous structure, consisting of the Early Precambrian and later Riphean lithotectonic complexes. Structurally, the Early Precambrian complexes are made up of tectonic sheets of gneisses, migmatites, and gneiss granites of the Ider Complex that are cut by gabbroanorthosite massif. The Riphean Jargalant Complex comprises alternating hornblende crystalline schists and biotite (sometimes sillimanite-bearing) gneisses with marble horizons. The upper age boundary of the Riphean Complex is determined by the subautochthonous granitoids with age about 810 Ma. The presence of the Riphean high-grade rocks indicates that structures with newly formed crust were formed in the paleooceanic framing of the Early Precambrian blocks of the Rodinia supercontinent by the Mid-Late Riphean. Divergence that began at that time within old Rodinian cratons and caused rifting and subsequent break-up of the supercontinent was presumably changed by convergence in the paleooceanic area.  相似文献   

3.
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   

4.
Mineral transformation of host rocks and localization of orebodies at the unconformity-type uranium deposits are considered for the Karku deposit in the northern Ladoga region. It is shown that the great depth of uranium mineral formation and the peculiar composition of host rocks, along with temperature and chemistry of fluids, played a critical role in variation of lithostatic and fluid pressure, porosity, and permeability. The compaction of quartz sandstone and gravelstone, which are typical host rocks at unconformity-type deposits, the development of microstylolithic sutures, conformal structures, pressure solution and deposition of quartz in free pores gave rise to the closure or constraint of pore space and to increase in pore pressure of fluids in the deep part of the Riphean troughs with approaching lithostatic loading. A transitional zone between hydrostatic and lithostatic pressure controlled localization of orebodies and was decisive for uranium mineral formation. This zone coincided with the Riphean-Paleoproterozoic unconformity and sank somewhat into the crystalline basement. Below this transitional zone, the intergranular fluid was under a pressure that was close to the pressure on solid phases, i.e., P tot ≈ P fl. The reliability of this phenomenon is confirmed by cessation of pressure solution-redeposition of quartz and distinct deceleration of dehydration of hydrous minerals. As is shown for the Karku deposit, the highly hydrated clay minerals of the illite-smectite series are widespread in its subore portion and lacking at the supraore levels along with termination of quartz regeneration. It is suggested that a zone of superhigh fluid pressure in deep parts of sedimentary basins constrains localization of uranium orebodies by structural and stratigraphic unconformity between Riphean and Paleoproterozoic rocks. It is stated that altered wall rocks at the unconformity-type uranium deposits cannot be identified with products of hydrothermal phyllic and argillic alteration of host rocks at the medium- and low-temperature endogenic deposits. The main distinctions consist in lack of wall-rock metasomatic reaction zoning and acid-alkaline evolution of solutions. All transformations of host rocks should be classified as diverse manifestations of deep catagenesis of sedimentary sequences and buried regolith. The carbon and oxygen isotopic compositions of calcite from host rocks at the Karku deposit are far from those of magmatic and hydrothermal carbonates. They are characterized by a high δ18O = +17 to +25‰ and a high dispersion of δ13C = −1.5 to almost −15‰. No granitoid magmatism is known in the regions, where the unconformity-type uranium deposits occur. Therefore, the rocks of the crystalline basement are the most probable source of uranium, which precipitated on the reductive barrier as a product of interaction with bitumen contained in the Riphean basal beds.  相似文献   

5.
The Early Caledonian folded area of Central Asia comprises a variety of continental crust fragments with Early to Late Precambrian crystalline basement. Crystalline rocks, which form part of the Songino block, outcrop at the junction between the Dzabkhan and Tuva-Mongolian terranes. The Bayannur zone in the southern part of the Songino block contains the Bayannur migmatite-gneiss and Kholbonur terrigenous-metavolcanic metamorphic complexes. Previous studies provide the 802 ± 6 Ma age for the regional metamorphism and folding within the Bayannur complex. On the basis of the minimum Nd model age of 1.5 Ga, gneisses from this complex cannot be regarded as Early Precambrian. Two main rock associations were distinguished in the Kholbonur complex. Mafic metavolcanics compose the dominant lithology of the first rock association, whereas the second association comprises terrigenous-volcanic and predominantly terrigenous suites. The rocks of the predominantly terrigenous suite, including mudstones, sandstones, and conglomerates, are interpreted to derive from the Late Riphean accretionary prism. The lithology and composition of metaterrigenous rocks suggest that they were possibly derived from erosion of a volcanic arc. The upper age limit of this suite is constrained by postkinematic granites (790 ± 3 Ma; U-Pb zircon), the lower age is given by plagiogranite (874 ± 3 Ma; U-Pb zircon) from comglomerate pebbles. Therefore, the timing of deposition of this terrigenous suite can be bracketed by the 874–790 Ma time interval. These ages and compositional features of the Kholbonur complex terrigenous rocks suggest that the convergence took place at around 870–880 Ma and thus it can be correlated with the divergent processes between the blocks of continental crust composing the supercontinent Rodinia.  相似文献   

6.
The Early Caledonian folded area in Central Asia (Early Caledonian superterrane) hosts micro-continent fragments with an Early and Late Precambrian crystalline basement, the largest of them being the Dzabkhan and Tuva-Mongolian fragments. Their junction zone hosts exposures of crystalline rocks that were previously thought to be part of the Early Precambrian Dzabkhan microcontinent. The Bayannur zone in the southern part of the Songino block hosts the Baynnur gneiss-migmatite and Kholbonur metavolcanic-terrigenous metamorphic complexes. The former is believed to be the Early Proterozoic crystalline basement, and the latter is thought to unconformably overly the Late Riphean cover complex of the Songino block. Various rocks of the tectono-stratigraphic complexes in the Bayannur zone were studied geologically and geochronologically (by the U-Pb technique of zircon). Regional metamorphism and folding in the Bayannur Complex were dated at 802 ± 6 Ma. The Nd model ages lie within the range of 1.5–2.0 Ga and thus preclude the correlation of these rocks with those in the Archean and Early Proterozoic basement of the Dzabkhan microcontinent. The upper age limit for folding and metamorphism in the Bayannur zone is marked by postkinematic granites dated at 790 ± 3 Ma, and the lower limit of the volcano-sedimentary complex is determined by the Nd model age of the sandstone (1.3 Ga). The upper age limit of the volcano-plutonic rocks in this zone is set by the gabbroids and anorthosites: 783 ± 2 and 784 ± 3 Ma, respectively. The complex of island-arc granitoids in the Bayannur zone is dated at 859 ± 3 Ma. The age constraints make it possible to correlate crystalline rocks in the Bayannur Complex of the Sangino block and the Dzhargalant Complex in the Tarbagatai block. Currently available data testify that the Precambrian Khangai group of blocks in the Early Caledonian Central Asian superterrane includes continental crustal blocks related to the processes of Early Precambrian, Late Riphean, and Vendian tectonism.  相似文献   

7.
Geochemical and Nd isotope data are reported for Late Riphean metamorphic complexes and granitoids of the Bayannur zone of the Songino block in the Early Caledonian superterrane of Central Asia. Geological, geochronolgical, geochemical, and isotope data were integrated to discuss rock sources and main mechanisms responsible for the formation and evolution of the Late Riphean continental crust. It was established that lithotectonic complexes of the Bayannur zone were formed on a convergent plate during Late Proterozoic tectonogenesis (around 1.3–0.78 Ga). This period primarily produced a juvenile crust represented by paleooceanic (N- and E-MORB types) and island arc basalts. An interval of 800–880 Ma was marked by the formation of rocks of the Bayannur complex and metaterrigenous sequence (accretionary wedge) of the Kholbonur complex, and the emplacement of quartz diorites and granodiorites of the Gashunnur pluton due to erosion and melting of both Late Riphean juvenile sources and ancient possibly Early Precambrian crustal material in a setting of ensialic island arc. At the final stage of the Late Riphean evolution of the Bayannur zone, postkinematic granitoids of the Bayannur pluton, and gabbrodiorites and anorthosites of the Ontsula pluton were derived from mantle juvenile and crustal sources in a within-plate setting. In terms of isotope characteristics, the crystalline complexes of the Bayannur zone are comparable with the Japan-type modern island arc systems. A synthesis of geological, geochronological, and isotope-geochemical data indicates a much wider distribution of the Late Riphean juvenile crust-forming processes than considered previously and remobilization of continental crust in the eastern segment of the Central Asian Fold Belt. The Vendian-Paleozoic stage in the evolution of this segment was characterized by an intense growth of juvenile crust, while magmatism during Late Riphean stage was determined by mixing of Late Riphean juvenile and ancient Early Precambrian sources.  相似文献   

8.
Petrographic, geochemical, and isotopic data have been obtained for 33 samples selected to provide constraints on contamination models for the volcanic and intrusive components of the Late Permian to Early Triassic, Siberian flood-volcanic province. Twenty-one of these samples were carried from great depth in an explosive diatreme of Triassic age, whereas 12 were collected from drill core from depths of tens to 2000 m. The studied diatreme xenoliths are: (1) fragments of the crystalline basement; and (2) fragments of a basaltic-to-rhyolitic volcanic suite.

Prompted by an unexpected, Late Paleozoic, Rb-Sr isochron age for this compositionally diverse volcanic suite, a SHRIMP U-Pb zircon age of ～270 Ma was obtained for a rhyodacite xenolith. Previously, a SHRIMP zircon U-Pb age of ～910 Ma had been determined for a leucogranite xenolith from the crystalline basement; this sample also contains substantial amounts of inherited, Early Proterozoic and Archean zircon.

The presence of this volcanic suite, only ～20 m.y. older than the 251 Ma, flood-volcanic sequence, is an extremely provocative result, inasmuch as hundreds of exploration drill holes in the Noril'sk area, and throughout the Siberian platform, have encountered only Tungusskaya Series coal-bearing sedimentary rocks in this stratigraphic/time interval. These data support arguments that subduction/underthrusting from the West Siberian Lowland under the northwest margin of the Siberian craton took place in Late Permian time.

The isotopic data obtained for the xenolith suite indicate that the upper part of the crystalline basement under the northwest margin of the Siberian craton is composed of Late Proterozoic (Riphean) rocks-alkaline granites, trondhjemites, crystalline schists, gneisses, and amphibolites-with much in common with rocks of the Central zone of the Taymyr folded area, which has been interpreted as an accretionary block formed and joined to Siberia in Late Riphean to Vendian time.

Measured isotopic characteristics for the Precambrian crystalline basement, and the Paleozoic sedimentary rocks that host the ore-bearing intrusions in the Noril'sk region, provide parameters for quantitative modeling of crustal contamination during evolution of the Siberian flood-volcanic rocks and related intrusions, both while en route to the surface and at the site of intrusion emplacement.  相似文献   

9.
The Early Caledonian Central Asian Orogenic Belt hosts fragments of continental blocks with Early and Late Precambrian crystalline basement. One of the structures with an Early Precambrian basement was thought to be the Dzabkhan microcontinent, which was viewed as an Early Precambrian “cratonal terrane”. The first geochronologic data suggest that the basement of the Dzabkhan microcontinent includes a zone of crystalline rocks related to Late Riphean tectonism. Geological, geochronological (U-Pb zircon dates), and Nd isotopic-geochemical data were later obtained on the northwestern part of the Dzabkhan microcontinent. The territory hosts the most diverse metamorphic complexes thought to be typical of the Early Precambrian basement. The complexes were determined to comprise the Dzabkhan-Mandal and Urgamal zones of high-grade metamorphic rocks. Gabbrodiorites related to the early metamorphic episode and dated at 860 ± 3 Ma were found in the Dzabkhan-Mandal zone, and the gneiss-granites marking the termination of this episode were dated at 856 ± 2 Ma. The granitoids of the Dzabkhan batholith, whose emplacement was coeval with the termination of the late high-grade metamorphic episode in rocks of both zones, have an age of 786 ± 6 Ma. Similar age values were determined for the granitoids cutting across the Late Precambrian rocks of the Songino and Tarbagatai blocks, which mark the stage when the mature Late Riphean continental crust was formed. The Late Riphean magmatic and metamorphic rocks of the Dzabkhan microcontinent were found out to have Nd model ages mostly within the range of 1.1–1.4 Ga at ?Nd(T) from +1.9 to +5.5. The Nd model age of the metaterrigenous rocks is 2.2?1.3 Ga at ?Nd(T) from ?7.2 to +3.1. The results of our studies provide evidence of convergence processes, which resulted in the Late Riphean (880?780 Ma) continental crust in Central Asia. Simultaneously with these processes, divergence processes that were responsible for the breakup of Rodinia occurred in the structures of the ancient cratons. It is reasonable to suggest that divergence processes within ancient continental blocks and Rodinia shelf were counterbalanced by the development of the Late Riphean continental crust in the convergence zones of its surrounding within established interval.  相似文献   

10.
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   

11.
Bimodal metapelites of complex (sedimentary-epigenetic) origin were found at the base of the Upper Riphean Zhuya Group, which disconformably overlies the Lower-Middle Riphean Kadalikan Group. The substrate of these rocks is represented by relatively low-aluminous mudstones of the Valyukhta Formation, which bear evidence of physical disintegration replaced upsection by chemical decomposition. The altered mudstones compose a weathering crust with the typical fissure, clastic, rubble-grus (lithomarge), and fine-clastic (dispersed) zones. Rocks of the lithomarge zone are enriched in silica, which was removed from the dispersed zone during the formation of the chemical weathering crust. During partial erosion of this crust, detrital quartz and fine-dispersed (mature) clay material penetrated the fissured mudstones. The bulk of detrital quartz was precipitated in the lithomarge and clastic zones. Fine-dispersed clay matter was accumulated in lower parts of the fissure zone. Mudstones subjected to different-depth epigenetic alterations are transformed into sericite-bearing shales of in the fissure zone, quartz-bearing shales in the clastic zone, siliceous-quartz shales and clay-like siliceous-quartz rocks in the rubble-grus zone, and sericite phyllites in the fine-clastic zone. These rocks, developed after mudstones of the Valyukhta Formation, represent different-depth indicators of the weathering crust.  相似文献   

12.
The paper reports newly obtained geological, geochronological (U-Pb zircon method), Nd isotopic, and geochemical data on Middle and Late Paleozoic granitoids and metamorphic rocks from the southern slope of the Mongolian Altai and Gobi Altai and on granitoids from the Trans-Altai Gobi. Tectonically, the former rocks are hosted in the margin of a Caledonian paleocontinent, and the latter are localized among island-arc and oceanic complexes related to the development of the Hercynian Southern Mongolian Ocean. According to their geological setting, the intrusive complexes are subdivided into two major groups: (i) related to processes of regional metamorphism and (ii) separated from these processes. Geochemical data suggest that the source of most of the granitoids and metamorphic rocks contained island-arc rocks and their erosion products. Nd isotopic evidence indicates that practically all of the allochthonous granitoids, regardless of their composition, age, and structural setting, have positive ?Nd(T) values [i.e., belong to the ?(+) type] and could not be formed by the melting of metaterrigenous rocks widespread at the modern erosion level. These granitoids in both the Caledonian and the Hercynian structures have practically identical Late Riphean Nd model ages [TNd(DM) = 0.97–0.60 Ga], which become slightly younger in the granitoids of the Hercynides. The exception are ultrametamorphic subautochthonous ?(?) granites of the first group localized in the peripheral part of migmatite fields. The sources of these granitoids could be the host metaterrigenous rocks. The results obtained in the course of this research suggest, with regard for preexisting data on granitoids in the isotopic provinces in Central Asia, that the sources of the Paleozoic granitoids were the rocks of the “juvenile” Caledonian and Hercynian island-arc crust and of the older crust of cratonic blocks with a Early Precambrian and Late Riphean basement, respectively. The Late Riphean crustal material in Caledonian and Hercynian structures related to the development of the corresponding oceanic basins most probably consisted of clastic sediments or relatively small fragments of the Late Riphean crust. The occurrence of this crustal material in the sources of the granitoids can be explained by the involvement of sediments in subduction zones and the participation of these sediments and fragments of Late Riphean complexes in the accretionary-collision processes during the closure of the paleoceanic basins. Simultaneously, the subduction zones received juvenile material that could be later involved in the melting processes together with older rocks.  相似文献   

13.
New age and structural data are reported for the siliceous-volcanogenic complexes developed in the lower reaches of the Ussuri River. These complexes, which were previously treated as one stratigraphic unit, are subdivided into the Snarsky tectonostratigraphic complex (end of the Middle Jurassic-Middle Aptian) and the basaltic sequence (supposedly, Campanian-Maastrichtian). The Snarsky Complex is made up of basic volcanics, cherts, siliceous-clayey rocks, as well as subordinate limestones, sandstones, and conglomerates. Its distinctive features are the large amounts of genetically diverse basalts, the abundance of volcanomictic and pyroclastic material in siliceous-clayey rocks, the absence of fragmental rocks typical of the continental convergent zone, and the facies heterogeneity of the deposits. The complex is considered to be the southwestern continuation of the Kiselevka-Manoma terrane. Its origin is presumably related to the tectonic piling of genetically heterogeneous assemblages. The basaltic sequence includes basalts, basaltic andesites, their tuffs, and tuff conglomerates. The tuff conglomerates contain numerous fragments of granites and garnet-bearing felsic volcanics. The sequence was formed on the crystalline paleocontinental basement in the Late Cretaceous.  相似文献   

14.
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   

15.
The basement of the central Qilian fold belt exposed along the Minhe-Ledu highway consists of psammitic schists, metabasitic rocks, and crystalline limestone. Migmatitic rocks occur sporadically among psammitic schist and metabasitic rocks. The mineral assemblage of psammitic schist is muscovite + biotite + feldspar + quartz ± tourmaline ± titanite ± sillimanite and that of metabasitic rocks is amphibole + plagioclase + biotite ± apatite ± magnetite ± pyroxene ± garnet ± quartz. The migmatitic rock consists of leucosome and restite of various volume proportions; the former consists of muscovite + alkaline feldspar + quartz ± garnet ± plagioclase while the latter is either fragments of psammitic schist or those of metabasitic rock. The crystalline limestone consists of calcite that has been partly replaced by olivine. The olivine was subsequently altered to serpentine. Weak deformations as indicated by cleavages and fractures were imposed prominently on the psammitic schists, occasionally on me  相似文献   

16.
Microfabrics of detrital quartz grains and quartz cement of four gold- and uranium-bearing conglomerates of various goldfields of the Witwatersrand Basin, South Africa, were investigated by optical and cathodoluminescence microscopy. The study revealed that the vast majority of quartz grains (<5 mm) originate from felsic magmatic source rocks. Cherts and polymineralic mineral grains, pointing to greenschist to amphibolite facies metasedimentary rocks, follow in abundance, whereas diagenetic to weakly metamorphosed quartzites are subordinate. Material from hydrothermal vein quartz is almost completely lacking, both in the sand and in the pebble fraction. No obvious relationships are discernible between the varying proportions of detrital siliciclastic components of the different reefs and their ore grades. Assuming a sedimentological control of gold distribution, this finding supports multiple sources for the detrital components, which were thoroughly mixed during transport. The post-depositional history of the sediments is characterized by a complex polyphase succession of deformation, cementation and hydrothermal alteration events. Both fragmentation and pressure solution features within detrital quartz, quartz cement and round grains of pyrite, zircon, chromite and uraninite demonstrate that these materials were present in the conglomerates during diagenesis, and, thus, are true detrital grains with abraded, rounded grain morphologies. By analogy, it is assumed that gold is also a detrital component, although most of the gold grains display characteristics of hydrothermal overprinting. During subsequent metamorphism, micro-shear zones are developed, and brittle-ductile crystal-plastic deformation and limited quartz recrystallization occur. Maximum temperatures of about 350 °C were reached on the prograde metamorphic path. Recrystallization and redistribution of detrital siliciclastic and ore minerals took place, and various hydrothermal/metamorphic minerals including chlorite, sericite, pyrophyllite and chloritoid were formed. These redistribution processes involved existing detrital minerals only and were generally isochemical because little evidence exists for the development of a secondary porosity and permeability that would allow major external inputs into the Witwatersrand conglomerates. Most of the gold grains have hydrothermal characteristics, as evidenced by their authigenic, crystalline shapes and their chemical compositions. However, these features are regarded to result from overprinting. Most likely, the gold grains experienced more drastic modifications relative to other ore components because of the ductile and mobile nature of gold. The retrograde metamorphic path is characterized by percolating radioactive fluids at T<300 °C, recorded by radiation damage indicated by cathodoluminescence alteration rims along quartz grain boundaries and microcracks. The degrees of radiation damage observed are proportional to the uranium contents of the conglomerate ores. The collective evidence of our study supports the modified placer model for the genesis of the Witwatersrand ores. This model explains most of the observations on the detrital mineral assemblage and its post-depositional modification elegantly and in a satisfactory manner.  相似文献   

17.
The analysis of modal and normative composition of sedimentary rocks is widely used for studying their sources and tectonic settings. The normative calculation of the mineral composition of rocks in this study is formulated as a linear programming problem and is solved by means of the simplex method. This enables both simultaneous and successive subtraction of a set of basic minerals from a rock sample represented by its chemical composition {SiO2...LOI}. Such an approach provides a more exact calculation of the contribution of basic minerals in the rock. This mathematical approach is used to study two representative sets of sandstones and fine-grained rocks from a Meso- to Neoproterozoic marginal basin of southeastern Siberia (Uchur–Maya region, Yakutia) and a Pennsylvanian-Lower Permian uplifted continental block in Colorado, USA. The calculated normative mineral compositions of the Siberian sandstones are consistent with the observed modal compositions. These sandstones vary from K- Feldspar rich arkoses at the base of the sequence (the Uchur Group, lower Riphean) to quartz arenites or lithic sandstones and wacke in transgressive successions of the middle-upper Riphean. Arkoses and quartz arenites are dominant in Meso- to Neoproterozoic Siberia. These samples represent craton interior uplifted basement and quartzose, recycled orogen provenance of a stable craton in Rodinia. There are higher but consistent discrepancies between the calculated and observed compositions for the Pennsylvanian to Lower-Permian arkoses and quartz arenites (Sangre de Cristo, Belden, and Maroon Formations). The differences between the predicted and observed mineralogy may be due to uncertainties in the modes in the matrix and cement of the sandstones. This normative program should supplement modal calculations and provide better genetic constructions, especially in case of matrix-rich sandstones.  相似文献   

18.
Types and lateral and vertical distribution of silicification in Riphean (largely dolomitic) rocks of the Yurubcha-Tokhomo zone of the Siberian Craton are discussed. It is shown that quartz and pyroclastic material in sediments were subjected to intense dissolution in a highly alkaline Riphean basin with the release of silica. Rapid and abrupt decrease in alkalinity during hiatus and desiccation periods resulted in the precipitation of dissolved silica and silicification of near-surface sediments. Lateral distribution of silicification was controlled by the redistribution of silica during the pre-Vendian hiatus, when surface waters were filtered through a carbonate massif with the simultaneous karst formation and silica dissolution. In the water discharge area, secondary silica was precipitated owing to changes in pH values and other physicochemical conditions.  相似文献   

19.
The available geological data on the Meso-and Neoproterozoic rocks in the north of the East European Platform are considered, involving the results of a comprehensive study along regional seismic profile I-I that extends for 460 km and crosses the main structural units of the Mezen Syneclise from SW to NE. Many previously unknown structural features of aulacogens filled with thick (up to 4–8 km) sequences of the Meso-and Neoproterozoic sedimentary rocks that make up the preplate complex are demonstrated in this profile. The Riphean rocks are subdivided into three seismostratigraphic sequences: the lower part of the Lower Riphean, the Lower-Middle Riphean, and the Upper Riphean. The geodynamic events in the north of the East European Platform are correlated with those that occurred in its central part and the adjacent foldbelts.  相似文献   

20.
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   

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