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1.
The first results of U–Pb dating of detrital zircons from Upper Ordovician sandstones of the Bashkir uplift in the Southern Urals and U–Pb isotopic ages available for detrital zircons from six stratigraphic levels of the Riphean–Paleozoic section of this region are discussed. It is established that the long (approximately 1.5 Ga) depositional history of sedimentary sequences of the Bashkir uplift includes a peculiar period lasting from the Late Vendian to the Emsian Age of the Early Devonian (0.55–0.41 Ga). This period is characterized by the following features: (1) prevalence of material from eroded Mesoproterozoic and Early Neoproterozoic crystalline complexes among clastics with ages atypical of the Volga–Urals segment of the East European Platform basement; (2) similarity of age spectra obtained for detrital zircons from different rocks of the period: Upper Vendian–Lower Cambrian lithic sandstones and Middle Ordovician substantially quartzose sandstones.  相似文献   

2.
U–Pb dating of detrital zircons was performed on mélange-hosted lithic and basaltic sandstones from the Inthanon Zone in northern Thailand to determine the timing of accretion and arc activity associated with Paleo-Tethys subduction. The detrital zircons have peak ages at 3400–3200, 2600–2400, 1000–700, 600–400, and 300–250 Ma, similar to the peaks ages of detrital zircons associated with other circum-Paleo-Tethys subduction zones. We identified two types of sandstone in the study area based on the youngest detrital zircon ages: Type 1 sandstones have Late Carboniferous youngest zircon U–Pb ages of 308 ± 14 and 300 ± 16 Ma, older than associated radiolarian chert blocks within the same outcrop. In contrast, Type 2 sandstones have youngest zircon U–Pb ages of 238 ± 10 and 236 ± 15 Ma, suggesting a Middle Triassic maximum depositional age. The youngest detrital zircons in Type 1 sandstones were derived from a Late Carboniferous–Early Permian ‘missing’ arc, suggesting that the Sukhothai Arc was active during sedimentation. The data presented within this study provide information on the development of the Sukhothai Arc, and further suggest that subduction of the Paleo-Tethyan oceanic plate beneath the Indochina Block had already commenced by the Late Carboniferous. Significant Middle Triassic arc magmatism, following the Late Carboniferous–Early Permian arc activity, is inferred from the presence of conspicuous detrital zircon U–Pb age peaks in Type 2 sandstones and the igneous rock record of the Sukhothai Arc. In contrast, only minimal arc activity occurred during the Middle Permian–earliest Triassic. Type 1 sandstones were deposited between the Late Permian and the earliest Triassic, after the deposition of associated Middle–Late Permian cherts that occur in the same mélanges and during a hiatus in Sukhothai Arc magmatism. In contrast, Type 2 sandstones were deposited during the Middle Triassic, coincident with the timing of maximum magmatism in the Sukhothai Arc, as evidenced by the presence of abundant Middle Triassic detrital zircons. These two types of sandstone were probably derived from discrete accretionary units in an original accretionary prism that was located along the western margin of the Sukhothai Arc.  相似文献   

3.
The crystallochemical characteristics of Cr-spinels and tourmalines in combination with U–Pb isotope data on detrital zircons from the Upper Permian and Lower Triassic sandstones of the Belskii Depression showed that the main provenances of the molasse sequence in the southern part of the Cis-Uralian foredeep were Lower Paleozoic (Sakmara zone) and Precambrian (Uraltau zone) complexes. The absence of Late Paleozoic zircons in the Tatarian (Upper Permian) sandstones, as well as their small amount in the Lower Triassic psammites together with geochemical and petrographic data, suggest that granite massifs of the Main Granitic Axis of the Urals, as Middle–Upper Paleozoic magmatic complexes of the Magnitogorsk Megazone, were not involved in erosion. At the same time, the significant amount of Precambrian zircons is indicative of the presence of metamorphic complexes in the provenance of the Uraltau zone.  相似文献   

4.
Doklady Earth Sciences - The paper presents new petrographic and geochemical data from terrigenous rocks and first U–Pb ages of detrital zircons from sandstones of the Tekturmas zone of...  相似文献   

5.
This report presents the first results of U/Pb dating, isotope-geochemical, and geochemical studies of detrital zircons from the Neoproterozoic clastic rocks of the Southern Timan. Sixty-one zircon grains were treated, including 51 from red-colored sandstones and 10 grains from aleurosandstones of the Djejim Formation of the southern Chetlas-Djejim zone (Djejim-Parma Hill). It was found that the U/Pb-ages of zircons from the rocks of the Djejim Formation, varied from ∼2.97 to ∼1.20 Ga. The studies of microelement composition in 47 grains (of 61 U/Pb isotope ages obtained), on the basis of several empirical regularities found formerly, show that the detrital zircons had originated from “granites” (22 grains), “diorites” (12 grains), or their volcanic analogues, or more rarely, from “syenites” and “basites” (5 and 8 grains, respectively). The Lu/Hf isotope system of zircons allows one to estimate the model ages (T DMC) of the substrate magmatic rocks being parental to the zircons considered. In particular, Archean zircons are characterized by ∼2.84–3.36 Ga model ages of magmaforming rocks. For some of the grains, their model ages (∼2.84 Ga) are close to those of zircons as such (∼2.7–2.8 Ga), which points to the juvenile character of the substrate from which the parent magma of the zircons treated was fused. For Proterozoic (to Middle Riphean) zircons, the Lu/Hf isotope system allows one to estimate the model age of the substrate of their parental rocks within ∼2.00–3.36 Ga, which shows that these rocks were formed under the recycling of the Archean and Early-Proterozoic crust. The ages obtained for detrital zircons, as well as model ages of the substrate of the corresponding parental magmatic rocks, are quite comparable to the age of crystalline complexes of the ancient framework of the East European Platform (EEP), formed in the course of the Archean, Early-Proterozoic, and Early-Middle Riphean tectonomagmatic events. This permits us to conclude that the Neoproterozoic detrital complexes of the Timan were formed owing to the erosion of earlier Neoproterozoic and Early Precambrian complexes constituting the Neoproterozoic Baltica continent, presenting complexes of the passive margin of this continent. A variety of ages of detrital zircons from sandstones and aleurosandstones from the Djejim Formation of Djejim-Parma Hill, and of the estimates of magmatic rocks parental to these zircons, may be characterized as a Baltic Provenance signal.  相似文献   

6.
The first LA–ICP–MS U–Pb isotopic ages of detrital zircons from the Ordovician sandstones of the Sol–Iletsk Block (well 2–Ordovician), located at junction of the East European Platform with the Pre-Caspian Basin and the Pre-Uralian foredeep, are presented. Two detrital zircons with well-defined ages of 561 ± 4 and 570 ± 5 Ma were found in sample K15–501. They confirm the Ordovician age of the sandstones, which earlier had been defined on the basis of seismic–stratigraphic and lithological correlations. The age distribution of the detrital zircons indicates the significant role of Late Precambrian rocks as provenance sources. However, those rocks still remain unknown in the Early Precambrian basement of the Volga–Ural part of the EEP.  相似文献   

7.
The late Carboniferous to Triassic tectonic history of eastern Australia includes important periods of regional-scale crustal extension and contraction. Evidence for these periods of tectonism is recorded by the extensive Pennsylvanian (late Carboniferous) to Triassic basin system of eastern Australia. In this study, we investigate the use of U–Pb dating of detrital zircons in reconstructing the tectonic development of one of these basins, the eastern Galilee Basin of Queensland. U–Pb detrital zircon ages were obtained from samples of stratigraphically well-constrained Cisuralian and Lopingian (early and late Permian, respectively) sandstone in the Galilee Basin. Detrital zircons in these sandstones are dominated by a population with ages in the range of 300–250 Ma, and ages from the youngest detrital zircons closely approximate depositional ages. We attribute these two fundamental findings to (1) appreciable derivation of detrital zircons in the Galilee Basin from the New England Orogen of easternmost Australia and (2) syndepositional magmatism. Furthermore, Cisuralian sandstone of the Galilee Basin contains significantly more >300 Ma detrital zircons than Lopingian sandstone. The transition in detrital zircon population, which is bracketed between 296 and 252 Ma based on previous high-precision U–Pb zircon ages from Permian ash beds in the Galilee Basin, corresponds with the Hunter–Bowen Orogeny and reflects a change in the Galilee Basin from an earlier extensional setting to a later foreland basin environment. During the Lopingian foreland basin phase, the individual depocentres of the Galilee and Bowen basins were linked to form a single and enormous foreland basin that covered >300 000 km2 in central and eastern Queensland.  相似文献   

8.
Graptolite‐bearing Middle and Upper Ordovician siliciclastic facies of the Argentine Precordillera fold‐thrust belt record the disintegration of a long‐lived Cambro‐Mid Ordovician carbonate platform into a series of tectonically partitioned basins. A combination of stratigraphic, petrographic, U‐Pb detrital zircon, and Nd‐Pb whole‐rock isotopic data provide evidence for a variety of clastic sediment sources. Four Upper Ordovician quartzo‐lithic sandstones collected in the eastern and central Precordillera yield complex U‐Pb zircon age spectra dominated by 1·05–1·10 Ga zircons, secondary populations of 1·22, 1·30, and 1·46 Ga, rare 2·2 and 1·8 Ga zircons, and a minor population (<2%) of concordant zircons in the 600–700 Ma range. Archaean‐age grains comprise <1% of all zircons analysed from these rocks. In contrast, a feldspathic arenite from the Middle Ordovician Estancia San Isidro Formation of the central Precordillera has two well‐defined peaks at 1·41 and 1·43 Ga, with no grains in the 600–1200 Ma range and none older than 1·70 Ga. The zircon age spectrum in this unit is similar to that of a Middle Cambrian quartz arenite from the La Laja Formation, suggesting that local basement rocks were a regional source of ca 1·4 Ga detrital zircons in the Precordillera Terrane from the Cambrian onwards. The lack of grains younger than 600 Ma in Upper Ordovician units reinforces petrographic data indicating that Ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins. Nd isotopic data (n = 32) for Middle and Upper Ordovician graptolitic shales from six localities define a poorly mixed signal [ɛNd(450 Ma) = −9·6 to −4·5] that becomes more regionally homogenized in Upper Ordovician rocks (−6·2 ± 1·0; TDM = 1·51 ± 0·15 Ga; n = 17), a trend reinforced by the U‐Pb detrital zircon data. It is concluded that proximal, recycled orogenic sources dominated the siliciclastic sediment supply for these basins, consistent with rapid unroofing of the Precordillera Terrane platform succession and basement starting in Mid Ordovician time. Common Pb data for Middle and Upper Ordovician shales from the western and eastern Precordillera (n = 15) provide evidence for a minor (<30%) component that was likely derived from a high‐μ (U/Pb) terrane.  相似文献   

9.
The first U–Pb dating of detrital zircons from the Lower Carboniferous sandstones in the frontal part of the northern Verkhoyansk fold-and-thrust belt showed that detrital zircon age spectra for the Lower Visean (Krestyakh Formation) and the Upper Visean–Serpukhovian (Tiksi Formation) rocks are quite different. The Early Visean sandstones contain up to 95% detrital zircons of Precambrian age, while those of Late Visean–Serpukhovian age, only 55%. The shape of age distribution plots of Precambrian zircons for both samples is similar, indicating that reworking of terrigenous sediments of the Krestyakh Formation or the same sources dominated in Early Visean time (crystalline basement of the craton, eroded Meso- and Neoproterozoic sedimentary complexes, and igneous rocks of Central Taimyr) contributed significantly to the accumulation of the Late Visean–Serpukhovian deposits. In the rocks of the Tiksi Formation, 45% of detrital zircons are of Paleozoic age, while 24% are Early Paleozoic, with prevailing Cambrian and Ordovician ages. Possible provenance areas with abundant igneous rocks of this age could be the Taimyr–Severnaya Zemlya and Central Asian fold belts extending along the northern, western or southwestern margins of the Siberia. The presence of Middle–Late Devonian zircons is thought to be related to the erosion of granitoids of the Yenisei Ridge and the Altai–Sayan region. Early Carboniferous detrital zircons probably had a provenance in igneous rocks of the Taimyr–Severnaya Zemlya fold belt, on the assumption that collision between the Kara block and the northern margin of the Siberian continent had already occurred by that time. In Early Visean time, sedimentation occurred in small deltaic fans, likely along steep fault scarps that formed as a result of Middle Paleozoic (Devonian–Carboniferous) rifting. The clastic material came from small rivers that eroded the nearby area. Late Visean–Serpukhovian time was marked by a sharp increase in the amount of clastic material and by the appearance of detrital zircons coming from new provenance regions, such as fold belts extending along the northern and southwestern margins of the Siberian continent. A large river system, which was able to transport clastic material over large distances to deposit it in submarine fans on the northern Verkhoyansk passive continental margin, had already existed by that time.  相似文献   

10.
It is shown that the Riphean sediments of the Bashkir and Isherim anticlinoria of the Ural mobile belt were formed in different settings. Sediments of the Bashkir Anticlinorium are autochthonous and were formed in the eastern (in present-day coordinates) Baltica from proximal siliciclastics, whereas the complexes of the Isherim Anticlinorium did not belong to it. Geochemical features of the Middle and lower Upper Riphean metapelites of the Isherim Anticlinorium and U–Pb isotope ages of detrital zircons from sandstones suggest that their provenances were the northern and northwestern parts of Baltica. In the Late Riphean (?)–Vendian, the Isherim block migrated along the Timan margin of Baltica into the present-day position.  相似文献   

11.
U–Pb (SHRIMP) determinations on detrital zircons from the Early Paleozoic Gelnica Terrane metasandstones and their Permian overlap sediments of the Inner Western Carpathian Southern Gemeric Unit define five age populations based on age-probability plots. The metasandstones were sampled for detrital zircons from six stratigraphic levels, four of them in the Late Cambrian/Ordovician Gelnica Terrane metasandstones and the two in Permian envelope sequence. The data set includes 84 U–Pb ages for individual detrital zircons. These ages are combined with the previously dated inherited zircons from the associated metavolcanites (n?=?31). The majority of the pre-Permian detrital and inherited zircons (95%) belong to the three main populations: population A—the Paleoproterozoic/Neoarchean ages ranging from 1.75 to 2.6?Ga; population B—the Mesoproterozoic ages with the range of 0.9 to 1.1?Ga; population C—the Neoproterozoic ages, ranging from 560 to 807?Ma. The detrital zircon age spectrum from the basal Permian sediments reflects the strong recycling from the underlying Gelnica Terrane, with the presence of the dominant Precambrian C and B populations (94% of total), including the minor populations A. The range of the detrital zircon ages from the Late Permian sandstones is wider, with additional population D, ranging from 497 to 450?Ma and population E with a time span from 369 to 301?Ma. Within the Late Permian detrital zircon assemblage, the Proterozoic population A?+?B?+?C form only 25% of total. The detrital zircon data suggest that the Gelnica Terrane belongs to the peri-Gondwanan terrane with a source area located on the northwestern margin of Gondwana close to Amazonia. This terrane should have travelled a long distance in the Phanerozoic times.  相似文献   

12.
The first U?Pb dates are obtained for detrital zircons from Upper Precambrian deposits of the Sredni (Zemlepakhtinskaya and Kuyakan formations) and Rybachi (Lonskii Formation) peninsulas. The spectra of ages of detrital zircons in sandstone samples from the Zemlepakhtinskaya and Kuyakan formations are similar to a significant extent to each other, which implies the dominant role of the same provenances. Most zircon grains are the Paleoproterozoic and Mesoproterozoic in age; some of them are characterized by Mesoarchean and Neoarchean ages. Zircons dated back to 1.0?2.0 Ga with maxima at approximately 1.8, 1.5, 1.3, and 1.1 Ga are the most abundant. The youngest zircon grains are the Mesoproterozoic in age: 1050 ± 21Ma (i.e., close to the Mesoproterozoic?Neoproterozoic boundary) and 1028 ± 21 Ma from the Zemlepakhtinskaya and Kuyakan formations, respectively. The distribution spectrum of ages obtained for zircons from sandstones of the Lonskii Formation significantly differs from that characteristic of zircons from sandstones of the Zemlepakhtinskaya and Kuyakan formations. The zircon population from the Lonskii Formation is dominated by detrital zircons with Neoarchean and Paleoproterozoic ages (2.8?1.6 Ga); Paleoarchean and Mesoarchean grains are scarce. Their age maxima are registered at levels of approximately 2.7 and 1.8 Ga. The minimum age obtained for zircons from sandstones of the Lonskii Formation (1349 ± 35 Ma) allows the Rybachi block to be considered as being older as compared with the Sredni bock. Crystalline complexes of the Baltic Shield served as a main provenance for the Upper Precambrian deposits of the peninsulas under consideration. The dates obtained for detrital zircons from the Upper Precambrian deposits of the Sredni and Rybachi peninsulas are compared with similar data on the Upper Precambrian sequences of the Timan and Varanger Peninsula areas to reveal differences and similarities in the distribution of ages.  相似文献   

13.
In order to constrain the detrital provenance of the siliciclastic rocks, palaeogeographic variations, and crustal growth history of central China, we carried out simultaneously in situ U–Pb dating and trace element and Hf isotope analyses on 368 detrital zircons obtained from upper Permian–Triassic sandstones of the Songpan terrane, eastern Tibetan Plateau. Two groups of detrital zircons, i.e. magmatic and metamorphic in origin, have been identified based on cathodoluminescence images, zircon Ti-temperatures, and Th/U ratios. Our data suggest that the derivation of siliciclastic rocks in the Songpan terrane was mainly from the Qinling, Qilian, and Kunlun orogens, whereas the Yangtze and North China Cratons served as minor source areas during late Permian–Triassic times. The detrital zircons from Middle–Late Triassic siliciclastic rocks exhibit wide age spectra with two dominant populations of 230–600 Ma and >1600 Ma, peaking at ~1.8–1.9 Ga and ~2.4–2.5 Ga, suggestive of a derivation from the Qinling, Qilian, and Kunlun orogens and the Yangtze Craton being the minor source area. The proportions of detrital zircon populations from the northern Qinling, Qilian, and Kunlun orogens distinctly decreased during Middle–Late Triassic time, demonstrating that the initial uplift of the western Qinling occurred then and it could have blocked most of the detritus from the Qilian–northern Qinling orogens and North China Cratons into the main Songpan–Ganzi depositional basin. The relatively detrital zircon proportions of the Yangtze Craton source decreased during Early-Middle Late Triassic time, indicating that the Longmenshan orogen was probably being elevated, since the early Late Triassic and gradually formed a barrier between the Yangtze Craton and the Songpan terrane. In addition, our Lu–Hf isotopic results also reveal that the Phanerozoic magmatic rocks in central China had been the primary products of crustal reworking with insignificant formation of a juvenile crust.  相似文献   

14.
We present a synopsis of detrital zircon U–Pb ages of sandstones from North Africa and neighboring Israel and Jordan, which allows us to identify zones with characteristic sediment provenance along the northern Gondwana margin (in present-day coordinates) in Cambrian–Ordovician times, and helps us to unravel the peri-Gondwana jigsaw puzzle. A special feature of the early Paleozoic cover sequence of North Africa is the eastward increase of 1.1–0.95 Ga detrital zircons, which become ubiquitous in the early Paleozoic sandstones of the Saharan Metacraton. Detrital zircons aged about 2.7–2.5, 2.15–1.75 and 0.75–0.53 Ga are also present. Early Paleozoic sandstones with similar provenance are known from peri-Gondwana terranes in the Eastern and Western Mediterranean and from NW Iberia. These terranes need not be transported from western Gondwana (Amazonia) as suggested previously. They were likely located to the north of the Saharan Metacraton during the early Paleozoic before they rifted off from Gondwana. Furthermore, we recognize an increase, as stratigraphic ages get younger, of ca. 1.0 Ga detrital zircons at some point between the Late Cambrian and late Middle Ordovician. We speculate that this might be linked to far-field tectonics and regional uplift in central Gondwana related to plate-tectonic reorganization along the Gondwana margin, leading to erosion of ca. 1.0 Ga basement and country rocks of the Transgondwanan supermountain and fluvial dispersal of detritus toward the Gondwana margin.  相似文献   

15.
U–Pb–Hf of detrital zircons from diverse Cambrian units in Morocco and Sardinia were investigated in order to clarify the sandstone provenance and how it evolved with time, to assess whether the detrital spectra mirror basement crustal composition and whether they are a reliable pointer on the ancestry of peri-Gondwanan terranes. Coupled with Hf isotopes, the detrital age spectra allow a unique perspective on crustal growth and recycling in North Africa, much of which is concealed below Phanerozoic sediments.In Morocco, the detrital signal of Lower Cambrian arkose records local crustal evolution dominated by Ediacaran (0.54–0.63 Ga) and Late-Paleoproterozoic (1.9–2.2 Ga; Eburnian) igneous activity. A preponderance of the Neoproterozoic detrital zircons possess positive εHf(t) values and their respective Hf model ages (TDM) concentrate at 1.15 Ga. In contrast, rather than by Ediacaran, the Neoproterozoic detrital signal from the Moroccan Middle Cambrian quartz-rich sandstone is dominated by Cryogenian-aged detrital zircons peaking at 0.65 Ga alongside a noteworthy early Tonian (0.95 Ga) peak; a few Stenian-age (1.0–1.1 Ga) detrital zircons are also distinguished. The majority of the Neoproterozoic zircons displays negative εHf(t), indicating the provenance migrated onto distal Pan-African terranes dominated by crustal reworking. Terranes such as the Tuareg Shield were a likely provenance. The detrital signal of quartz–arenites from the Lower and Middle Cambrian of SW Sardinia resembles the Moroccan Middle Cambrian, but 1.0–1.1 Ga as well as ~ 2.5 Ga detrital zircons are more common. Therefore, Cambrian Sardinia may have been fed from different sources possibly located farther to the east along the north Gondwana margin. 1.0–1.1 Ga detrital zircons abundant in Sardinia generally display negative εHf(t) values while 0.99–0.95 Ga detrital zircons (abundant in Morocco) possess positive εHf(t), attesting for two petrologically-different Grenvillian sources. A paucity of detrital zircons younger than 0.6 Ga is a remarkable feature of the detrital spectra of the Moroccan and Sardinian quartz-rich sandstones. It indicates that late Cadomian orogens fringing the northern margin of North Africa were low-lying by the time the Cambrian platform was deposited. About a quarter of the Neoproterozoic-aged detrital zircons in the quartz-rich sandstones of Morocco (and a double proportion in Sardinia) display positive εHf(t) values indicating considerable juvenile crust addition in North Africa, likely via island arc magmatism. A substantial fraction of the remaining Neoproterozoic zircons which possess negative εHf(t) values bears evidence for mixing of old crust with juvenile magmas, implying crustal growth in an Andean-type setting was also significant in this region.  相似文献   

16.
张英利  王宗起  闫臻  王涛 《地质学报》2012,86(4):548-560
库鲁克塔格地区是土什布拉克组的命名地和典型剖面分布区,主要由灰绿色砂岩及粉砂岩组成。运用LA-ICP-MS U-Pb方法,对土什布拉克组3件砂岩碎屑锆石进行U-Pb年龄测定,共获得了183组U-Pb有效年龄,既限定了地层的最早形成时代,同时又获取研究区早古生代的演化资料。获得14个较年轻锆石年龄表明,土什布拉克组形成于中—晚志留世。碎屑锆石的谐和年龄表明,物源主要集中在422～537 Ma、559～999 Ma、1018～1574Ma和1604～2498Ma。碎屑锆石年龄394～537Ma和CL图像揭示,在早古生代时期发育大量岩浆岩,但目前地表仅有少量的岩浆岩记录。研究区新元古代的岩浆锆石年龄值,可能与罗迪尼亚超大陆的聚合-裂解有关。碎屑锆石也表明研究区发育区域变质作用,部分与哥伦比亚超大陆有关。  相似文献   

17.
The results of LA–ICP–MS U–Pb analyses of detrital zircons from the Precambrian deposits of Luga–Ladoga monocline are discussed. The age spectra of the zircons separated from the Riphean to Upper Vendian sandstones from the Shotkusa-1 well demonstrate dominance of the Paleo- and Mesoproterozoic grains while the Archaean zircons are subordinate. The Riphean debris sources were local swells of the Northern Ladoga basement. The sequence interval presumably corresponding to the Vasilieostrov Formation (Upper Vendian) has yielded not only Paleo- and Mesoproterozoic zircon ages, but Neoproterozoic as well, implying a Timanide provenance: these zircons (527 ± 9 and 516 ± 13 Ma) allow deposition of a significant part of the Shotkusa-1 sequence at the very beginning of the Cambrian.  相似文献   

18.
Abstract

U-Pb. systematics of detrital zircons carry a mineral-specific information summarizing important geologic events during the preelastic slate of the minerals. Comparisons with U/Pb isotope rati of zircons from potential provenances reveal relationships between source areas of the zircons and their final location of deposition in a sedimentary basin. The Palaeozoic zircon detritus accumulated in sedimentary basins on the Rhenohercynian crustal segment is taken as an example to elucidate the plate-tectonical induced changes of the source areas by significant changes of the 206Pb/238 vs. 207Pb/235ratios in the zircons.

The U-Pb systematic of detrital zircon- from the Cambrian sediments deposited in the Brabant Massif and in the Ardennes indicale two source areas. Part of the detritus derived from an area. where strong Cadomian-Panafrican events influenced the U-Pb systems of the zircons. The oilier part reflects a source, in which the U-Pb systems were able to preserve their Arehaean to Early Proterozoic age information. Zircons of the latter source record the most ancient ages so far observed in detrital zircons of the later Rhenohercynian crustal seg ment. The similarities with the U/Pb isotope ratios of zircons from the Armorican Massif, the Bohemian Massif. and certain regions of the Mps indicate a geotectonic position of the sedimentar) basin during Cambrian times in the periphery of the Condwana mega-continent.

The detrital zircons accumulated in the Variscan Rhenish basin during the Lower Devonian show a completely different summarizing age information. The majority of the zircons reflect a Laurussian-type origin. which suggests a palaeogeographic position of the Variscan Rhenohercynian basin close to Laurussia. Euhedral zircons crystallized during Caledonian times document the erosion of Caledonian granitoids from structural highs in the Mid European Caledonides.

Zircons of the Lower Carboniferous flysch sediments of todays Eastern Rhenish Massif originate from two source areas characterised by very different geologic histories. Euhedral zircons represent a rather young component of about 410 Ma in the detritus, whereas. in contrast. the well rounded crystals show a summarizing age-information identical to that of the zircons found in the Cambrian sandstones. The low ages resemble intrusion ages as recorded from the Mid-German-Crystalline-Rise, the high age reflect a Gondwana-type input into the Rhenohercynian sedimentary basin during Lower Carboniferous times. The detritus thus documents the Variscan collision and a renewed coherence of the Rhenohercynian crustal segment to Cnndwana.

The zircon population from Upper Carboniferous molasse deposits is comprised of Condwana-tуpc material and of mate rial with similar U/Pb ratios as recorded in the Lower Devonian zircons. In parts the Variscan molasse must have been derived from sediments once deposited in a southern part of the Rhenohercynian basin and in the Saxothurìngian basin. U/Pb ratios of euhedral and round diamond-like lustrous zircons indicate a major geologic event at the Namurian/Westphalian boundary (310-315 Ma). These zircons thus reflect an influx of detritus into the molasse from other source rocks, probably synsedimentary volcanics.  相似文献   

19.
Here we present new U–Pb and Hf isotopic data for detrital zircons obtained from six samples of late Palaeozoic units from central Jilin Province, Northeast China, and use these data and sedimentary formations to constrain the late Palaeozoic tectonic evolution of the eastern segment of the southern margin of the Central Asian Orogenic Belt. The majority of the detrital zircons from the six samples are euhedral–subhedral and exhibit oscillatory zoning, indicating a magmatic origin. Zircons from sandstones in the Devonian Wangjiajie and Xiaosuihe formations yield seven main age populations (399, 440, 921, 1648, 1864, 1911, and 2066 Ma) and two minor age populations (384 and 432 Ma), respectively. Zircons from a quartz sandstone in the Carboniferous Luquantun Formation yield four age populations (~332, 363, 402, and 428 Ma), and zircons from quartz sandstones of the Permian Shoushangou, Fanjiatun, and Yangjiagou formations yield age populations of 265, 369, 463, 503, and 963 Ma; 264, 310, 337, 486, and 529 Ma; and 262, 282, 312, 338, 380, 465, and 492 Ma, respectively. These data, together with the ages of magmatic zircons from interbedded volcanics and biostratigraphic evidence, as well as analysis of formations, give rise to the following conclusions. (1) The Wangjiajie and Xiaosuihe formations were deposited in an extensional environment during Middle and Middle–Late Devonian time, respectively. The former was sourced mainly from ancient continental material of the North China Craton with minor contributions from newly accreted crust, while the latter was sourced mainly from newly accreted crust. (2) The Luquantun Formation formed in an extensional environment during early–late Carboniferous time from material sourced mainly from newly accreted crust. (3) The Shoushangou, Fanjiatun, and Yangjiagou formations formed during a period of rapid uplift in the late Permian, from material sourced mainly from newly accreted crust.  相似文献   

20.
《地学前缘(英文版)》2019,10(2):683-704
In the eastern part of the Central Asian Orogenic Belt (CAOB) in northeastern (NE) China, scattered outcrops of molasse deposits mark the ending of an orogeny and are crucial for understanding the evolution of the Paleo-Asian Ocean (PAO). However, the timing of tectonic events and the relationships among these strata remain controversial. To better constrain these geologic events, a comprehensive study of the detrital zircon U-Pb geochronology and geochemistry of the sandstones of the Kaishantun (KST) Formation and Kedao (KD) Group in eastern Jilin Province, NE China, was conducted. The KST Formation is traditionally considered a molasse deposit. The sandstones display low CIA, PIA and high ICV values and low Th/U and Rb/Sr ratios, which suggest that the rocks were derived from an immature intermediate-felsic igneous source and experienced a simple sedimentary recycling history with relatively weak chemical weathering. LA-ICP-MS U-Pb dating of detrital zircons from two samples of the KST Formation yields ages of 748–252 Ma, suggesting that the KST Formation was deposited between 254.5 Ma and 252 Ma in Late Permian. The zircons were mainly derived from the continental northern part of the North China Craton (NCC). In contrast, the U-Pb dating of detrital zircons from five samples of the KD Group yields ages of 2611–230 Ma, suggesting that the KD samples were deposited in the Early to Middle Triassic (ca. 248–233 Ma). The detrital zircon ages for the KD samples can be divided into groups with peaks at 2.5 Ga, 1.8 Ga, 800–1000 Ma, 500 Ma and 440–360 Ma, which suggest that the samples were derived from bidirectional provenances in the Jiamusi-Khanka Block and the NCC. These new data, combined with previously published results, suggest that at least three orogenic events occurred in central-eastern Jilin Province during the Early Permian (270–262 Ma), Early Triassic (254–248 Ma) and Middle–Late Triassic (242–227 Ma). The final closure of the PAO occurred during 242–227 Ma in the Middle–Late Triassic along the Changchun-Yanji suture zone. The detrital zircon geochronological data clearly record plate convergence and the scissor-like closure of the PAO in the eastern CAOB.  相似文献   

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