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1.
In this paper we present new zircon U–Pb ages, Hf isotope data, and whole-rock major and trace element data for Early Mesozoic intrusive rocks in the Erguna Massif of NE China, and we use these data to constrain the history of southward subduction of the Mongol–Okhotsk oceanic plate, and its influence on NE China as a whole. The zircon U–Pb dating indicates that Early Mesozoic magmatic activity in the Erguna Massif can be subdivided into four stages at ~ 246 Ma, ~ 225 Ma, ~ 205 Ma, and ~ 185 Ma. The ~ 246 Ma intrusive rocks comprise a suite of high-K calc-alkaline diorites, quartz diorites, granodiorites, monzogranites, and syenogranites, with I-type affinities. The ~ 225 Ma intrusive rocks consist of gabbro–diorites and granitoids, and they constitute a bimodal igneous association. The ~ 205 Ma intrusive rocks are dominated by calc-alkaline I-type granitoids that are accompanied by subordinate intermediate–mafic rocks. The ~ 185 Ma intrusive rocks are dominated by I-type granitoids, accompanied by minor amounts of A-types. These Early Mesozoic granitoids mainly originated by partial melting of a depleted and heterogeneous lower crust, whereas the coeval mafic rocks were probably derived from partial melting of a depleted mantle modified by subduction-related fluids. The rock associations and their geochemical features indicate that the ~ 246 Ma, ~ 205 Ma, and ~ 185 Ma intrusive rocks formed in an active continental margin setting related to the southward subduction of the Mongol–Okhotsk oceanic plate. The ~ 225 Ma bimodal igneous rock association formed within an extensional environment in a pause during the subduction process of the Mongol–Okhotsk oceanic plate. Every magmatic stage has its own corresponding set of porphyry deposits in the southeast of the Mongol–Okhotsk suture belt. Taking all this into account, we conclude the following: (1) during the Early Mesozoic, the Mongol–Okhotsk oceanic plate was subducted towards the south beneath the Erguna Massif, but with a pause in subduction at ~ 225 Ma; and (2) the southward subduction of the Mongol–Okhotsk oceanic plate not only caused the intense magmatic activity, but was also favorable to the formation of porphyry deposits. 相似文献
2.
The Hengshan massif is an exhumed, mid-crustal, plutonic–metamorphic dome formed during Cretaceous crustal extension in the Jiangnan orogenic belt, central South China. Multiple thermochronometers (mica 40Ar/39Ar, apatite fission track and zircon (U–Th)/He) are applied to its footwall along a slip-parallel transect to quantify its thermal history and cooling rate, and the slip magnitude, rate, initial geometry and kinematic evolution of the low-angle Hengshan detachment fault. Our thermochronological data, in conjunction with previous ages, indicate that (1) footwall rocks cooled from ~ 700 °C to ~ 60 °C in less than 60 Myr (136–80 Ma) at variable rates ranging from ~ 50 °C/Myr to ~ 13 °C/Myr, (2) the Hengshan detachment fault accommodated ~ 8–12 km of total slip at variable slip rates from 0.14 to 1 mm/yr during tectonic exhumation, (3) the footwall has been tilted ~ 26°–50° to the east since slip began, indicating that the low-angle Hengshan detachment fault initiated at a steep dip and was passively rotated to a more gentle orientation during subsequent normal slip. This study provides compelling evidence supporting that the low-angle detachment fault in the extensional dome can be generated by the reactivation and passive rotation of an initially steep reverse fault during normal slip. In addition, our thermochronological data constrain the time of extension in the Hengshan dome between 136 and 80 Ma, which implies that the back-arc extension within South China associated with the rollback of the Paleo-Pacific slab might have lasted until at least 80 Ma. 相似文献
3.
《Russian Geology and Geophysics》2014,55(11):1264-1277
On the northeastern slope of the Kuznetsk Alatau, small differentiated alkaline basic intrusive massifs form an isometric area ~ 100 km across. They are composed of subalkalic and alkali gabbroids, basic and ultrabasic foidolites, nepheline and alkali syenites, and carbonatites. Results of complex (U–Pb, Sm–Nd, and Rb–Sr) isotope dating suggest that alkaline basic magmatism developed at two stages, in the Middle Cambrian–Early Ordovician (~ 510–480 Ma) and in the Early–Middle Devonian (~ 410–385 Ma). Finding of accessory zircons (age 1.3–2.0 Ga) in alkaline rocks suggests that the ascent of mantle plume was accompanied by the melting of fragments of Proterozoic mature continental crust composing the basement of the Caledonian orogen of the Kuznetsk Alatau. Probably, parental Cambrian–Ordovician alkaline mafic melts initiated metasomatism and lithosphere erosion. During the next melting of lithosphere substrate in ~ 100 Myr, this caused the generation of magmas of similar composition with inherited isotope parameters (εNd(T) ≈ + 4.8 to + 5.7, TNd(DM) ≈ 0.8–0.9 Ga) pointing to the similar nature of their matter sources in the moderately depleted mantle. 相似文献
4.
The Palaeozoic to Mesozoic igneous and metamorphic basement rocks exposed in the Mérida Andes of Venezuela and the Santander Massif of Colombia are generally considered to define allochthonous terranes that accreted to the margin of Gondwana during the Ordovician and the Carboniferous. However, terrane sutures have not been identified and there are no published isotopic data that support the existence of separate crustal domains. A general paucity of geochronological data led to published tectonic reconstructions for the evolution of the northwestern corner of Gondwana that do not account for the magmatic and metamorphic histories of the basement rocks of the Mérida Andes and the Santander Massif. We present new zircon U–Pb (ICP-MS) data from 52 igneous and metamorphic rocks, which we combine with whole rock geochemical and Pb isotopic data to constrain the tectonic history of the Precambrian to Mesozoic basement of the Mérida Andes and the Santander Massif. These data show that the basement rocks of these massifs are autochthonous to Gondwana and share a similar tectono-magmatic history with the Gondwanan margin of Peru, Chile and Argentina, which evolved during the subduction of oceanic lithosphere of the Iapetus Ocean. The oldest Palaeozoic arc magmatism is recorded at ~ 500 Ma, and was followed shortly by Barrovian metamorphism. Peak metamorphic conditions at upper amphibolite facies are recorded by anatexis at ~ 477 Ma and the intrusion of synkinematic granitoids until ~ 472 Ma. Subsequent retrogression resulted from localised back-arc or intra-arc extension at ~ 453 Ma, when volcanic tuffs and interfingered sedimentary rocks were deposited over the amphibolite facies basement. Continental arc magmatism dwindled after ~ 430 Ma and terminated at ~ 415 Ma, coevally with most of the western margin of Gondwana. After Pangaea amalgamation in the Late Carboniferous to Early Permian, a magmatic arc developed on its western margin at ~ 294 Ma as a result of subduction of oceanic crust of the palaeo-Pacific ocean. Intermittent arc magmatism recorded between ~ 294 and ~ 225 Ma was followed by the onset of the Andean subduction cycle at ~ 213 Ma, in an extensional regime. Extension was accompanied by slab roll-back which led to the migration of the arc axis into the Central Cordillera of Colombia in the Early Jurassic. 相似文献
5.
Nicolas Thibault Micha Ruhl Clemens V. Ullmann Christoph Korte David B. Kemp Darren R. Gröcke Stephen P. Hesselbo 《Proceedings of the Geologists' Association. Geologists' Association》2018,129(3):372-391
The Toarcian Oceanic Anoxic Event (T-OAE, ～183 Ma) was characterized by enhanced carbon burial, a prominent negative carbon-isotope excursion (CIE) in marine carbonate and organic matter, and numerous geochemical anomalies. A precursor excursion has also been documented at the Pliensbachian/Toarcian boundary, but its possible causes are less constrained. The T-OAE is intensively studied in the Cleveland Basin, Yorkshire, UK, whose sedimentary deposits have been litho-, bio- and chemostratigraphically characterised. Here, we present new elemental data produced by hand-held X-ray fluorescence analysis to test the expression of redox-sensitive trace metals and detrital elements across the upper Pliensbachian to mid-Toarcian of the Cleveland Basin. Detrital elemental concentrations (Al, Si, Ti, Zr) are used as proxies for siliciclastic grain content and thus, sea-level change, which match previous sequence stratigraphic interpretations from the Cleveland Basin. The timescale of the event is debated, though our new elemental proxies of relative sea level change show evidence for a cyclicity of 350 cm that may be indicative of ～405 kyr eccentricity cycles in Yorkshire. Trends in total organic carbon and redox-sensitive elements (S, Fe, Mo, As) confirm scenarios of widespread ocean deoxygenation across the T-OAE. The correlation of comparable trends in Mo across the T-OAE in Yorkshire and the Paris Basin suggests a similar oceanic drawdown of this element accompanying widespread anoxia in the two basins. Data from Yorkshire point to a transgressive trend at the time of the Mo drawdown, which contradicts the “basin restriction” model for the euxinic conditions that characterise the CIE interval. 相似文献
6.
《Gondwana Research》2014,26(4):1484-1500
The southern Rheinisches Schiefergebirge, which is part of the Rhenohercynian zone of the Central European Variscides, exhibits several allochthonous units: the Gießen-, and the Hörre nappe, and parts of the Frankenbach imbrication zone. These units were thrust over autochthonous and par-autochthonous volcano-sedimentary complexes of the Lahn and Dill–Eder synclines. This paper reports a representative data set of U–Pb LA–SF–ICP–MS ages of 1067 detrital zircon grains from Devonian and Lower Carboniferous siliciclastic sediments of the autochthonous and the allochthonous areas, respectively. The cluster of U–Pb ages from the allochthonous units points to a provenance in the Saxothuringian zone. Zircon populations from the Saxothuringian zone are representative of a Gondwanan hinterland and are characterized by age clusters of ~ 530–700 Ma, ~ 1.8–2.2 Ga, ~ 2.5–2.7 Ga, and ~ 3.0–3.4 Ga. Further samples were taken from the autochthonous and par-autochthonous units of the Lahn–Dill and Kellerwald areas. A Lower Devonian sandstone sample from the Siegen anticline provides a reference for siliciclastic sediments derived from the Old Red Continent. These samples show a provenance representative of Laurussia with debris primarily derived from Baltica and Avalonia. U–Pb zircon age clusters occur at ~ 400–450 Ma, 540–650 Ma, 1.0–1.2 Ga, ~ 1.4–1.5 Ga, ~ 1.7–2.2 Ga, and 2.3–2.9 Ga. Provenance analysis and geochemical data of the Rhenohercynian zone provide new information on the evolution of magmatic arcs in the Mid-Paleozoic. The data set constrains top-SE and top-NW directed subduction of the oceanic crust of the Rheic Ocean. Subduction-related volcanism lasted from the Early Devonian to the Early Carboniferous and thus confirms the existence of the Rheic Ocean until the Early Carboniferous. The tectonic model outlined for the Rhenohercynian zone suggests a wide Rheic Ocean. 相似文献
7.
The large, newly discovered Sharang porphyry Mo deposit and nearby Yaguila skarn Pb–Zn–Ag (–Mo) deposit reside in the central Lhasa terrane, northern Gangdese metallogenic belt, Tibet. Multiple mineral chronometers (zircon U–Pb, sericite 40Ar–39Ar, and zircon and apatite (U–Th)/He) reveal that ore-forming porphyritic intrusions experienced rapid cooling (> 100 °C/Ma) during a monotonic magmatic–hydrothermal evolution. The magmatic–hydrothermal ore-forming event at Sharang lasted ~ 6.0 Myr (~ 1.8 Myr for cooling from > 900 to 350 °C and ~ 4.0 Myr for cooling from 350 to 200 °C) whereas cooling was more prolonged during ore formation at Yaguila (~ 1.8 Myr from > 900 to 500 °C and a maximum of ~ 16 Myr from > 900 to 350 °C). All porphyritic intrusions in the ore district experienced exhumation at a rate of 0.07–0.09 mm/yr (apatite He ages between ~ 37 and 30 Ma). Combined with previous studies, this work implies that uplift of the eastern section of the Lhasa terrane expanded from central Lhasa (37–30 Ma) to southern Lhasa (15–12 Ma) at an increasing exhumation rate. All available geochronologic data reveal that magmatic–hydrothermal–exhumation activities in the Sharang–Yaguila ore district occurred within four periods of magmatism with related mineralization. Significant porphyry-type Mo mineralization was associated with Late Cretaceous–Eocene felsic porphyritic intrusions in the central Lhasa terrane, resulting from Neotethyan oceanic subduction and India–Asia continental collision. 相似文献
8.
《Gondwana Research》2014,25(3):1202-1215
The South China Block, consisting of the Yangtze and the Cathaysia blocks, is one of the largest Precambrian blocks in eastern Asia. However, the early history of the Cathaysia Block is poorly understood due largely to intensive and extensive reworking by Phanerozoic polyphase orogenesis and magmatism which strongly overprinted and obscured much of the Precambrian geological record. In this paper, we use the detrital zircon U–Pb age and Hf isotope datasets as an alternative approach to delineate the early history of the Cathaysia Block. Compilation of published 4041 Precambrian detrital zircon ages from a number of (meta)sedimentary samples and river sands exhibits a broad age spectrum, with three major peaks at ~ 2485 Ma, ~ 1853 Ma and ~ 970 Ma (counting for ~ 10%, ~ 16% and ~ 24% of all analyses, respectively), and four subordinate peaks at ~ 1426 Ma, ~ 1074 Ma, ~ 780 Ma and ~ 588 Ma. Five of seven detrital zircon age peaks are broadly coincident with the crystallisation ages of ~ 1.89–1.83 Ga, ~ 1.43 Ga, ~ 1.0–0.98 Ga and ~ 0.82–0.72 Ga for known igneous rocks exposed in Cathaysia, whereas, igneous rocks with ages of ~ 2.49 Ga and ~ 0.59 Ga have not yet been found. The Hf isotopic data from 1085 detrital zircons yield Hf model ages (TDMC) between ~ 4.19 Ga and ~ 0.81 Ga, and the calculated εHf(t) values between − 40.2 and 14.4. The Archean detrital zircons are exclusively oval in shape with complicated internal textures, indicating that they were sourced by long distance transportations and strong abrasion from an exotic Archean continent. In contrast, the majority of detrital zircons in age between ~ 1.9 and ~ 0.8 Ga are euhedral to subhedral crystals, indicative of local derivation by short distance transportations from their sources. The oldest crustal basement rocks in Cathaysia were most likely formed by generation of juvenile crust and reworking of recycled Archean components in Late Paleoproterozoic at ~ 1.9–1.8 Ga, rather than in the Archean as previously speculated. Reworking and recycling of the continental crust are likely the dominant processes for the crustal evolution of Cathaysia during the Mesoproterozoic to Neoproterozoic time, with an intervenient period of significant generation of juvenile crust at ~ 1.0 Ga.Precambrian crustal evolutions of the Cathaysia Block are genetically related to the supercontinent cycles. By comparing detrital zircon data from Cathaysia with those for other continents, and integrating multiple lines of geological evidence, we interpret the Cathaysia Block as an orogenic belt located between East Antarctica, Laurentia and Australia during the assembly of supercontinent Columbia/Nuna at ~ 1.9–1.8 Ga. The Cathaysia Block amalgamated with the Yangtze Block to form the united South China Block during the Sibao Orogeny at ~ 1.0–0.89 Ga. The Laurentia–Cathaysia–Yangtze–Australia–East Antarctica connection gives the best solution to the paleo-position of Cathaysia in supercontinent Rodinia. The significant amount of ~ 0.6–0.55 Ga detrital zircons in Cathaysia and West Yangtze have exclusively high crustal incubation time of > 300 Ma, indicating crystallisation from magmas generated dominantly by crustal reworking. This detrital zircon population compares well with the similar-aged zircon populations from a number of Gondwana-derived terranes including Tethyan Himalaya, High Himalaya, Qiangtang and Indochina. The united South China–Indochina continent was likely once an integral part of Gondwanaland, connected to northern India by a “Pan-African” collisional orogen. 相似文献
9.
The Qinling Orogenic Belt marks the link between the South China and North China Blocks and is an important region to understand the geological evolution of the Chinese mainland as well as the Asian tectonic collage. However, the tectonic affinity and geodynamic evolution of the South Qinling Tectonic Belt (SQTB), a main unit of the Qinling Orogenic Belt, remains debated. Here we present detailed geological, geochemical and zircon U–Pb–Hf isotopic studies on the Zhangjiaba, Xinyuan, Jiangjiaping, Guangtoushan and Huoshaodian plutons from the Guangtoushan granitoid suite (GGS) in the western segment of the SQTB. Combining geology, geochronology and whole-rock geochemistry, we identify four distinct episodes of magmatism as: (1) ~ 230–228 Ma quartz diorites and granodiorites, (2) ~ 224 Ma fine-grained granodiorites and monzogranites, (3) ~ 218 Ma porphyritic monzogranites and (4) ~ 215 Ma high-Mg# quartz diorites and granodiorites as well as coeval muscovite monzogranites. The ~ 230–228 Ma quartz diorites and granodiorites were generated by magma mixing between a mafic melt from mantle source and a granodioritic melt derived from partial melting of Neoproterozoic rocks in the lower continental crust related to a continental arc regime. The ~ 224 Ma fine-grained granodiorites and monzogranites were formed through partial melting of a transitional source with interlayers of basaltic rocks and greywackes in the deep zones of the continental arc. The ~ 218 Ma porphyritic monzogranites originated from partial melting of metamorphosed greywackes in lower crustal levels, suggesting underthrusting of middle or upper crustal materials into lower crustal depths. The ~ 215 Ma high-Mg# quartz diorites and granodiorites (with Mg# values higher than 60) were derived from an enriched mantle altered by sediment-derived melts. Injection of hot mantle-derived magmas led to the emergence of the ~ 215 Ma S-type granites at the final stage.Integrating our studies with previous data, we propose that the Mianlue oceanic crust was still subducting beneath the SQTB during ~ 248–224 Ma, and final closure of the Mianlue oceanic basin occurred between ~ 223 Ma and ~ 218 Ma. After continental collision between the South China Block and the SQTB, slab break-off occurred, following which the SQTB transformed into post-collisional extension setting. 相似文献
10.
《Gondwana Research》2015,28(4):1487-1493
Receiver function imaging along a temporary seismic array (ANTILOPE-2) reveals detailed information of the underthrusting of the Indian crust in southern Tibet. The Moho dips northward from ~ 50 km to 80 km beneath the Himalaya terrane, and locally reaches ~ 85 km beneath the Indus–Yalung suture. It remains at ~ 80 km depth across the Lhasa terrane, and shallows to ~ 70 km depth under the Qiangtang terrane. An intra-crustal interface at ~ 60 km beneath the Lhasa terrane can be clearly followed southward through the Main Himalaya Thrust and connects the Main Boundary Thrust at the surface, which represents the border of the Indian crust that is underthrusting until south of the Bangong–Nujiang Suture. A mid-crustal low velocity zone is observed at depths of 14–30 km beneath the Lhasa and Himalaya terranes probably formed by partial melt and/or aqueous fluids. 相似文献
11.
The Tibetan Plateau (TP) is the highest plateau in the world, which has been the focus of Cenozoic geological studies. The Northeast Tibetan Plateau (NETP) is a key location to decipher the Cenozoic evolution history of the TP. Understanding the building of the Qimen Tagh Mountains located in NETP will help to constrain the development of the northern boundary of the main TP, test the existence of a Paleo-Qaidam Basin and test the eastward growth model of the TP. In this study, granite samples from the Qimen Tagh Mountains were dated by LA-ICPMS and apatite fission track (AFT). The LA-ICPMS zircon U–Pb ages give two magmatic events around ~ 405 and ~ 255 Ma from two different sites. AFT modeling shows that the initial uplift took place at ~ 40–30 Ma in these mountains, which should be controlled by the Altyn Tagh Fault. Compiling previously low-temperature thermochronometry results, it reveals that the initial Cenozoic uplift of the northern boundary of the TP (Qimen Tagh and East Kunlun mountains), soon after the India–Eurasia collision in the southern TP, has divided the Paleo-Qaidam Basin into several sub-basins. The approximate NE–E growth process occurred along the lithospheric Altyn Tagh and Kunlun faults. The current basin and range morphology of the NETP took place around ~ 8 Ma. 相似文献
12.
《Quaternary Science Reviews》2007,26(1-2):56-69
We describe the evolution of climate system dynamics by examining the climate response to changes in obliquity and precession over the last 5.3 Myr. In particular, we examine changes in the shape of glacial cycles and the power of obliquity and precession response in benthic δ18O. When the exponential trend in δ18O variance is removed, its spectral power exhibits strong, proportional responses to amplitude modulations in orbital forcing over most of the Plio–Pleistocene. Precession responses correlate with modulations in forcing for the last 5 Myr, but 41-kyr response is sensitive to obliquity modulation only before 1.4 Myr. Where responses are sensitive to modulations in forcing, we demonstrate that glacial cycles are orbitally forced rather than being self-sustained or paced by orbital changes. The shapes of glacial cycles have several nonlinear properties, which may be indicative of glacial–interglacial differences in climate sensitivity or response time. The “saw-tooth” asymmetry of glacial cycles first appears shortly after the onset of major northern hemisphere glaciation, and the relative duration of interglacial stages decreases at 1.4 Myr. Collectively, trends in the shape of glacial cycles and the sensitivity of δ18O to obliquity and precession are suggestive of major transitions in climate dynamics at approximately 2.5 and 1.4 Myr but show no significant change associated with the appearance of strong 100-kyr cycles during the mid-Pleistocene transition. 相似文献
13.
《Gondwana Research》2013,23(3-4):828-842
Whether any Grenvillian magmatic records are preserved in the North China Craton (NCC) is a key issue to understand the Proterozoic tectonic evolution of the NCC and its correlation to the supercontinent Rodinia. Meso- to Neo-proterozoic sedimentary series is well exposed in the NCC, but magmatic events in this period, especially of 1.3–1.0 Ga, have seldom been reported. New U–Pb isotopic dating and Hf isotopic composition analyses have been carried out in this study using SIMS and LA–ICP-MS methods on detrital zircons from sandstones of the Tumen Group in the Shandong Peninsula and quartz sandstones of the Sangwon System in the Phyongnam Basin, North Korea. The age populations of the detrital zircons of the Tumen Group are at ~ 2.5 Ga, ~ 1.85 Ga, ~ 1.7 Ga, ~ 1.58 Ga, ~ 1.5 Ga, ~ 1.36 Ga and ~ 1.2 Ga and those of the Sangwon System are at 1.88–1.86 Ga, ~ 1.78 Ga, 1.62–1.58 Ga, 1.46–1.41 Ga, ~ 1.32 Ga, ~ 1.17 Ga and ~ 980 Ma. Most of the age peaks of Neoarchean and Proterozoic correspond to the significant tectonic-magmatic-thermal events previously recognized in the NCC, revealing that the main provenances of the Tumen Group and the Sangwon System are Early Precambrian basement and Late Paleo- to Meso-proterozoic magmatic rocks of the NCC. Furthermore, the youngest detrital zircon ages of ~ 1.1 Ga from the Tumen Group and 984 Ma from the Sangwon System, as well as 910 Ma Rb–Sr whole rock isochron age of a limestone from the Tumen Group and 899 Ma mafic sills intruding the Sangwon System suggest that both groups were deposited in the Neoproterozoic, coevally with the Qingbaikou System in the Yanliao Aulacogen. The common zircon ages of 1.3–1.0 Ga from the Tumen Group and the Sangwon System, as well as the contemporaneous Penglai and Yushulazi Group in the eastern margin of the NCC, indicate that during the deposition of these sediments there have been significant contributions from Grenvillian magmatic rocks in the eastern NCC. This may provide clues to understand the possible relationship of the NCC and the supercontinent Rodinia. Moreover, the positive εHf (t) and ~ 2.8 Ga crust model ages of detrital magmatic zircons of 2.8–2.4 Ga suggest that there have been significant crustal growth at ~ 2.8 Ga in the eastern margin of the NCC, same as in other areas of the NCC. 相似文献
14.
The end-Triassic mass extinction and the transition and explosive diversification of fauna over the Triassic-Jurassic boundary is poorly understood and poorly represented in the rock record of the Southern Hemisphere. This is despite the rich diversity in both body and trace fossils of Triassic-Jurassic age in southern Africa, which is not found in coeval Northern Hemisphere localities. We report here the first palaeomagnetic polarity zonation of the Upper Triassic-Lower Jurassic continental red bed succession (Elliot Formation; Stormberg Group) in southern Africa. The results from 10 partially overlapping sections, with a composite thickness of ~ 280 m, provide a magnetic polarity chronology of the main Karoo Basin in South Africa and Lesotho. Palaeomagnetic analyses reveal that heating samples to between 150 °C and ~ 300 °C removes the secondary, moderately inclined (~ 48°) normal-polarity component of remanent magnetization. This component overlaps with the present-day field and is comparable to the overprint direction expected from Lower Jurassic Karoo dolerite intrusions. In contrast, a likely primary, high unblocking temperature component, of dual polarity, consistently is of steeper inclination (~ 63°). This characteristic remanence passes the reversals test, except where means are based on small sample populations. There are only two resulting polarity zones for the ~ 200 m thick lower Elliot Formation (LEF) with potential for a thin 3rd magnetozone in the uppermost part. The upper Elliot Formation (UEF), in contrast, which was sampled over a thickness of ~ 80 m, has five polarity zones. The failure of the reversal test for the UEF and combined Elliot Formation (LEF + UEF) indicates that the normal polarity samples may be biased by a younger overprint of either the Jurassic normal polarity of the Karoo Large Igneous Province or present day field. The separate poles calculated for the four sites in the LEF and ten sites in the UEF overlap with the Late Triassic and Early to Middle Jurassic Gondwana poles, respectively. The combined Elliot Formation and UEF pole positions are better constrained than the LEF and therefore considered more reliable. Overall the LEF shows considerable overlap with the Late Triassic Apparent Polar Wander Paths (APWP) poles. 相似文献
15.
Sedimentological and geochronological analyses were performed on Carboniferous strata from central Inner Mongolia (China) to determine the tectonic setting of the southeastern Central Asian Orogenic Belt (CAOB). Sedimentological analyses indicate that the widespread Late Carboniferous strata in central Inner Mongolia were dominated by shallow marine clastic-carbonate deposition with basal conglomerate above the Precambrian basement and Early Paleozoic orogenic belts. Based on lithological comparison and fossil similarity, five sedimentary stages were used to represent the Carboniferous deposition. The depositional stages include, from bottom to top, 1) basal molassic, 2) first carbonate platform, 3) terrigenous with coeval intraplate volcanism, 4) second carbonate platform, and 5) post-carbonate terrigenous. These five stages provide evidence for an extensive transgression in central Inner Mongolia during the Late Carboniferous. Detrital zircon geochronological studies from five samples yielded five main age populations: ~ 310 Ma, ~ 350 Ma, 400–450 Ma, 800–1200 Ma and some Meso-Proterozoic to Neoarchean grains. The detrital zircon geochronological studies indicate that the provenances for these Late Carboniferous strata were mainly local magmatic rocks (Early Paleozoic arc magmatic rocks and Carboniferous intrusions) with subordinate input of Precambrian basement. Combining our sedimentological and provenance analyses with previous fossil comparison and paleomagnetic reconstruction, an inland sea was perceived to be the main paleogeographic feature for central Inner Mongolia during the Late Carboniferous. The inland sea developed on a welded continent after the collision between North China Craton and its northern blocks. 相似文献
16.
The Central Asian Orogenic Belt (CAOB), as one of the largest accretionary orogens in the world, was built up through protracted accretion and collision of a variety of terranes due to the subduction and closure of the Paleo-Asian Ocean in the Neoproterozoic to Early Mesozoic. Located in the Uliastai continental margin of the southeastern CAOB, the Chagan Obo Temple area is essential for understanding the tectonic evolution of the southeastern part of the CAOB and its relation with the “Hegenshan Ocean”. In this study, detrital zircon U-Pb geochronology coupled with Hf isotopic analysis was performed on Paleozoic sedimentary strata in this area. Most detrital zircons from the studied samples possess oscillatory zoning and have Th/U ratios of 0.4-1.73, indicative of an igneous origin. Detrital zircons from the Ordovician to Devonian sedimentary strata yield a predominant age group at 511-490 Ma and subordinate age groups at 982-891 Ma, 834-790 Ma and ~ 574 Ma, and have a large spread of εHf(t) values (-20.77 to + 16.94). Carboniferous and Early Permian samples yield zircon U-Pb ages peaking at ~ 410 Ma and ~ 336 Ma, and have dominantly positive εHf(t) values (+ 1.30 to + 14.86). Such age populations and Hf isotopic signatures match those of magmatic rocks in the Northern Accretionary Orogen and the Mongolian arcs. A marked shift of provenance terranes from multiple sources to a single source and Hf isotope compositions from mixed to positive values occurred at some time in the Carboniferous. Such a shift implies that the Northern Accretionary Orogen was no longer a contributor of detritus in the Carboniferous to Early Permian, due to the opening of the “Hegenshan Ocean” possibly induced by the slab rollback of the subducting Paleo-Asian Ocean. 相似文献
17.
《Quaternary Science Reviews》2007,26(3-4):279-286
A stable isotope record from a stalagmite collected from Antro del Corchia cave (Apuan Alps, Central Italy), supported by 17 uranium-series ages, indicates enhanced regional rainfall between ca 8.9 and 7.3 kyr cal. BP at the time of sapropel S1 deposition. Within this phase, the highest rainfall occurred between 7.9 and 7.4 kyr cal. BP. Comparison with different marine and lake records, and in particular with the Soreq Cave record (Israel), suggests substantial in-phase occurrence of enhanced rainfall between the Western and Eastern Mediterranean basins. There is no convincing evidence for major climatic change at the time of the “8.2 kyr event”. 相似文献
18.
Paleomagnetism has played an important role in quantifying the Mesozoic evolution of “Proto-Tibet”. In this paper, we present new paleomagnetic data from five Middle-Upper Jurassic sedimentary sequences (Quemo Co, Buqu, Xiali, Suowa and Xueshan Fms.) of the eastern North Qiangtang Terrane (QT) at Yanshiping (33.6°N, 92.1°E). The new paleomagnetic results form a large dataset (99 sites, 1702 samples) and reveal a paleopole at 79.1°N/306.9°E (dp = 3.9°, dm = 6.3°) for the Quemo Co Fm., at 68.9°N/313.8°E (dp = 2.1°, dm = 3.7°) for the Buqu Fm., at 66.1°N/332.1°E (dp = 2.7°, dm = 4.6°) for the Xiali Fm., at 72.4°N/318.6°E (dp = 3.9°, dm = 6.7°) for the Suowa Fm., and at 76.9°N/301.1°E (dp = 7.9°, dm = 13.2°) for the Xueshan Fm. These results indicate clockwise (CW) rotations of ~ 19.8 ± 9.4° between ~ 171.2 and 161.7 Ma and counterclockwise (CCW) rotations of ~ 15.4 ± 13.4° between ~ 161.7 and < 157.2 Ma for Yanshiping. We attribute the change in rotation sense at approximately ~ 161.7 Ma to the initial collision of the Lhasa and Qiangtang terranes. Using this and other paleomagnetic data from the Lhasa, Qiangtang and Tarim terranes, as well as other geological evidence (e.g., tectonism-related sedimentary sequences, volcanism, and HP metamorphism), we propose a new conceptual evolution model for the Mesozoic QT and Tethyan Oceans. The Longmo Co-Shuanghu oceanic slab was subducted before 248 Ma, followed by continental collision of the North-South Qiangtang subterranes between ~ 245 and 237 Ma. The Qiangtang Terrane experienced post-collisional exhumation between ~ 237 and 230 Ma during subduction of the Jinsha oceanic slab. The collision of the Qiangtang and Songpan-Ganzi terranes occurred between ~ 230 and 225 Ma. The QT experienced post-collisional relaxation from ~ 225 to ~ 200 Ma, followed by subsidence and extension-related exhumation between ~ 200 and 162 Ma in association with subduction of the Bangong-Nujiang oceanic slab. Finally, these events were followed by the scissor-like diachronous collisions of the Lhasa and Qiangtang terranes between ~ 162 Ma and the mid-Cretaceous. 相似文献
19.
Lindsay L. Vare Guillaume Massé Thomas R. Gregory Christopher W. Smart Simon T. Belt 《Quaternary Science Reviews》2009,28(13-14):1354-1366
A sea ice record for Barrow Strait in the Canadian Arctic Archipelago (CAA) is presented for the interval 10.0–0.4 cal. kyr BP. This Holocene record is based primarily on the occurrence of a sea ice biomarker chemical, IP25, isolated from a marine sediment core obtained from Barrow Strait in 2005. A core chronology is based on 14C AMS dating of mollusc shells obtained from ten horizons within the core. The primary IP25 data are compared with complementary proxy data obtained from analysis of other organic biomarkers, stable isotope composition of bulk organic matter, benthic foraminifera, particle size distributions and ratios of inorganic elements. The combined proxy data show that the palaeo-sea ice record can be grouped according to four intervals, and these can be contextualised further with respect to the Holocene Thermal Maximum (HTM). Spring sea ice occurrence was lowest during the early–mid Holocene (10.0–6.0 cal. kyr BP) and this was followed by a second phase (6.0–4.0 cal. kyr BP) where spring sea ice occurrence showed a small increase. Between 4.0 and 3.0 cal. kyr BP, spring sea ice occurrence increased abruptly to above the median and we associate this interval with the termination of the HTM. Elevated spring sea ice occurrences continued from 3.0 to 0.4 cal. kyr BP, although they were more variable on shorter timescales. Within this fourth interval, we also provide evidence for slightly lower and subsequently higher spring sea ice occurrence during the Mediaeval Warm Period and the Little Ice Age respectively. Comparisons are made between our proxy data with those obtained from other palaeo-climate and sea ice studies for the CAA. 相似文献
20.
We have used geodetic techniques to improve constraints on the crustal motion of the Pamir Plateau. Three campaigns of Global Position System data acquisition between 2011 and 2015 demonstrate that, in association with the India–Asia collision, a complex pattern of crustal motion exists in the Pamir Plateau. In a north–south direction from the Indian Plate to the Hazak Block, the crust has absorbed ~ 35 mm/yr of shortening, of which ~ 35% is distributed around the Hindu Kush region (~ 12 mm/yr), and another ~ 35% is taken up around the Alai Valley (also ~ 12 mm/yr). Global Position System measurements also show ~ 5 mm/yr of shortening between the Pamir Plateau and the Tajik Basin, whereas between the Pamir and the Tarim Basin, an ~ 10 mm/yr extension rate is observed. With respect to the stable Eurasian Plate, the Pamir rotates counterclockwise at a rate of ~ 1.822°Myr− 1, with an Euler pole positioned about the west end of the Tajik Basin (37.03 ± 0.74°N, 65.89 ± 0.12°E). The strain rate field calculated from Global Position System velocities reveals that the crustal motion is consistent with localized deformation around the Hindu Kush and the Alai Valley, the latter representing a zone with strong shallow seismic activity. 相似文献