首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 375 毫秒
1.
We have identified iridium in an ~ 5 m-thick section of pelagic sediment cored in the deep sea floor at Site 886C, in addition to a distinct spike in iridium at the K–Pg boundary related to the Chicxulub asteroid impact. We distinguish the contribution of the extraterrestrial matter in the sediments from those of the terrestrial matter through a Co–Ir diagram, calling it the “extraterrestrial index” fEX. This new index reveals a broad iridium anomaly around the Chicxulub spike. Any mixtures of materials on the surface of the Earth cannot explain the broad iridium component. On the other hand, we find that an encounter of the solar system with a giant molecular cloud can aptly explain the component, especially if the molecular cloud has a size of ~ 100 pc and the central density of over 2000 protons/cm3. Kataoka et al. (2013, 2014) pointed that an encounter with a dark cloud would drive an environmental catastrophe leading to mass extinction. Solid particles from the hypothesized dark cloud would combine with the global environment of Earth, remaining in the stratosphere for at least several months or years. With a sunshield effect estimated to be as large as − 9.3 W m− 2, the dark cloud would have caused global climate cooling in the last 8 Myr of the Cretaceous period, consistent with the variations of stable isotope ratios in oxygen (Barrera and Huber, 1990; Li and Keller, 1998; Barrera and Savin, 1999; Li and Keller, 1999) and strontium (Barrera and Huber, 1990; Ingram, 1995; Sugarman et al., 1995). The resulting growth of the continental ice sheet also resulted in a regression of the sea level. The global cooling, which appears to be associated with a decrease in the diversity of fossils, eventually led to the mass extinction at the K–Pg boundary.  相似文献   

2.
A comparison of geochemical and Sr–Nd–Pb isotopic compositions for Deccan Continental Flood Basalts (CFBs) and Central Indian Ridge (CIR) Basalts is presented: these data permit assessment of possible parental linkages between the two regions, and comparison of their respective magmatic evolutionary trends in relation to rift-related tectonic events during Gondwana break-up. The present study reveals that Mid-Ocean Ridge Basalt (MORB) from the northern CIR and basalts of Deccan CFB are geochemically dissimilar because of: (1) the Deccan CFB basalts typically show a greater iron-enrichment as compared to the northern CIR MORB, (2) a multi-element spiderdiagram reveals that the Deccan CFBs reveal a more fractionated slope (Ba/YbN > 1), as compared to relatively flat northern CIR MORB (Ba/YbN < 1), (3) there is greater REE fractionation for Deccan CFB than for the northern CIR MORB (i.e., La/YbN ∼ 2.3 and 1 respectively) and (4) substantial variation of compatible–incompatible trace elements and their ratios among the two basalt groups suggests that partial melting is a dominant process for northern CIR MORB, while fractional crystallization was a more important control to the geochemical variation for Deccan CFB. Further, incompatible trace element ratios (Nb/U and Nb/Pb) and radiogenic isotopic data (Sr–Pb–Nd) indicate that the northern CIR MORBs are similar to depleted mantle [and/or normal (N)-MORB], and often lie on a mixing line between depleted mantle and upper continental crust. By contrast, Deccan CFB compositions lie between the lower continental crust and Ocean island basalt. Accordingly, we conclude that the basaltic suites of the northern CIR MORB and Deccan CFB do not share common parentage, and are therefore genetically unrelated to each other. Instead, we infer that the northern CIR MORB were derived from a depleted mantle source contaminated by upper continental crust, probably during the break up of Gondwanaland; the Deccan CFB are more similar to Ocean island basalt (Reunion-like) composition, and perhaps contaminated by lower continental crust during their evolution.  相似文献   

3.
《Gondwana Research》2014,25(2):585-613
The Belomorian eclogite province was repeatedly affected by multiple deformation episodes and metamorphism under moderate to high pressure. Within the Gridino area, high pressure processes developed in a continental crust of tonalite–trondhjemite–granodiorite (TTG) affinity that contains mafic pods and dykes, in which products of these processes are most clearly evident. New petrological, geochemical and geochronological data on mafic and felsic rocks, including PT-estimates, mineral chemistry, bulk rock chemistries, REE composition of the rocks and zircons and U–Pb and Lu–Hf geochronology presented in the paper make it possible to reproduce the magmatic and high-grade metamorphic evolution in the study area. In the framework of the extremely long-lasting geologic history recorded in the Belomorian province (3–1.7 Ga), new geochronological data enabled us to define the succession of events that includes mafic dyke emplacement between 2.87 and 2.82 Ga and eclogite facies metamorphism of the mafic dykes between ~ 2.82 and ~ 2.72 Ga (most probably in the time span of 2.79–2.73 Ga). The clockwise PT path of the Gridino association crosses the granulite- and amphibolite-facies PT fields during the time period of 2.72 Ga to 2.64 Ga. A special aspect of this work concerns the superposed subisobaric heating (thermal impact) with an increase in the temperature to granulite facies conditions at 2.4 Ga. Later amphibolite facies metamorphism occurred at 2.0–1.9 Ga. Our detailed geochronological and petrological studies reveal a complicated Mesoarchaean–Palaeoproterozoic history that involved deep subduction of the continental crust and a succession of plume-related events.  相似文献   

4.
The Cretaceous–Paleogene boundary (KPgB) was dated by the 40Ar/39Ar method herein from the western interior of North America at 65.48 ± 0.12 Ma (1σ), in good agreement with other recent published estimates. For the Deccan Traps, India, new argon ages as well as others available in the literature, are assessed for reliability based on (a) statistical reliability of plateau/isochron sections and (b) freshness of material dated utilizing the alteration index method. From tholeiitic lavas from the Composite Western Ghats Section (CWGS), only six ages are found to be reliable estimates of the time of crystallization. These ages along with the magnetic polarity of the lavas agree with the geomagnetic polarity time scale (GPTS) at ∼67–64 Ma. Alkaline rocks from the Anjar area of Kutch, provide three reliable ages that suggest a hiatus in lava extrusion around KPgB. For the Rajahmundry basalts, the upper flow’s age defines its formation during chron 29n; a single age from the lower reversed polarity flow appears somewhat dichotomous when plotted against the GPTS. The reliable lava ages indicate the most voluminous (reversed polarity) sections of the CWGS were extruded at a time statistically indistinguishable from that of the KPgB. The Deccan Trap – KPgB faunal extinction hypothesis remains plausible, but must compete with the latest report, favoring a very close temporal connection (∼0.03 m.y.) between the Chixculub (Impact) Crater and the KPgB.  相似文献   

5.
Increased seismicity and occurrences of hot springs having surface temperature of 36–58 °C are observed in the central part of India (74–81° E, 20–25° N), where the NE trending Middle Proterozoic Aravalli Mobile Belt meets the ENE trending Satpura Mobile Belt. Earlier Deep Seismic Sounding (DSS) studies along Thuadara-Sendhwa-Sindad profile in the area has showed Mesozoic Sediments up to around 4 km depth covered by Deccan Trap and the Moho depth with a boundary velocity (Pn) of 8.2 km/s. In the present study, surface heat flow of 48 ± 4 mW m?2 has been estimated based on Pn velocity, which agrees with the value of heat flow of 52 ± 4 mW m?2 based on Curie point isotherms estimates. The calculated temperature-depth profile shows temperature of 80–120 °C at the basement, which is equivalent to oil window temperature in Mesozoic sediments and around 570–635 °C at Moho depth of 38–43 km and the thermal lithosphere is about 110 km thick, which is comparatively higher than those of adjoining regions. The present study reveals the brittle–ductile transition zone at 14–41 km depth (temperature around 250–600 °C) where earthquake nucleation takes place.  相似文献   

6.
To study the crustal structure beneath the onshore–offshore transitional zone, a wide-angle onshore–offshore seismic experiment was carried out in northern South China Sea near Hong Kong, using large volume airgun sources at sea and seismic stations on land. The crustal velocity model constructed from traveltime fitting shows that the sedimentary thickness abruptly increases seaward of the Dangan Islands based on the characteristics of Pg and Multiple Pg, and the crustal structure beneath the sedimentary layer is relatively simple. The Moho depth is about 25–28 km along the profile and the P-wave velocity increases gradually with depth. The velocities in the upper crust range from 5.5 to 6.4 km/s, while that in the lower crust is 6.4–6.9 km/s. It also reveals a low velocity zone with a width of more than 10 km crossing the crust at about 75–90 km distance, which suggests that the Littoral Fault Zone (LFZ) exists beneath the onshore–offshore transitional zone. The magnetism anomalies, bouguer gravity anomalies and active seismic zone along the coastline imply the LFZ is a main tectonic fault in the onshore–offshore area. Combined with two previously published profiles in the continental South China (L–G profile) and in the northern margin of South China Sea (OBS1993) respectively, we constructed a land-sea super cross-section about 1000 km long. The results show the onshore–offshore transitional zone is a border separating the unstretched and the stretched continental crust. The low velocity layer (LVL) in the middle crust was imaged along L–G profile. However, the high velocity layer (HVL) in the lower crust was detected along OBS1993. By analyzing the mechanisms of the LVL in the middle crust and HVL in the base of crust, we believe the crustal structures had distinctly different attributes in the continental South China and in the northern SCS, which indicates that the LFZ could be the boundary fault between them.  相似文献   

7.
A high-resolution passive seismic experiment in the Kachchh rift zone of the western India has produced an excellent dataset of several thousands teleseismic events. From this network, 500 good teleseismic events recorded at 14 mobile broadband sites are used to estimate receiver functions (for the 30–310° back-azimuth ranges), which show a positive phase at 4.5–6.1 s delay time and a strong negative phase at 8.0–11.0 s. These phases have been modeled by a velocity increase at Moho (i.e. 34–43 km) and a velocity decrease at 62–92 km depth. The estimation of crustal and lithospheric thicknesses using the inversion of stacked radial receiver functions led to the delineation of a marked thinning of 3–7 km in crustal thickness and 6–14 km in lithospheric thickness beneath the central rift zone relative to the surrounding un-rifted parts of the Kachchh rift zone. On an average, the Kachchh region is characterized by a thin lithosphere of 75.9 ± 5.9 km. The marked velocity decrease associated with the lithosphere–asthenoshere boundary (LAB), observed over an area of 120 km × 80 km, and the isotropic study of xenoliths from Kachchh provides evidence for local asthenospheric updoming with pockets of partial melts of CO2 rich lherzolite beneath the Kachchh seismic zone that might have caused by rifting episode (at 88 Ma) and the associated Deccan thermal-plume interaction (at 65 Ma) episodes. Thus, the coincidence of the area of the major aftershock activity and the Moho as well as asthenospheric upwarping beneath the central Kachchh rift zone suggests that these pockets of CO2-rich lherzolite partial melts could perhaps provide a high input of volatiles containing CO2 into the lower crust, which might contribute significantly in the seismo-genesis of continued aftershock activity in the region. It is also inferred that large stresses in the denser and stronger lower crust (at 14–34 km depths) induced by ongoing Banni upliftment, crustal intrusive, marked lateral variation in crustal thickness and related sub-crustal thermal anomaly play a key role in nucleating the lower crustal earthquakes beneath the Kachchh seismic zone.  相似文献   

8.
The Mesoproterozoic successions in the North China Craton (NCC) and the Qinling–Qilian–Kunlun Orogens have been revised using the new and highly reliable age data. Many Proterozoic strata in the Qinling–Qilian–Kunlun Orogens, such as the Qinling, Jinshuikou and Beidahe groups that have been ascribed to be Paleoproterozoic are actually of Mesoproterozoic Era. The most significant advances are recent geochronological studies on the Mesoproterozoic stratigraphy and magmatic events in the NCC. The boundary age between the Dahongyu Formation and the overlying Gaoyuzhuang Formation is well constrained to be ∼1600 Ma, corresponding to the boundary age between Statherian and Calymmian. The boundary between the Tieling Formation and the overlying Xiamaling Formation is best positioned at ∼1400 Ma, which is coeval with the boundary between Calymmian and Ectasian, and is about 400 Myrs older than the conventional value of 1000 Ma originally defined by the All China Commission of Stratigraphy. Hence the Jixianian System, including the Gaoyuzhuang, Yangzhuang, Wumishan, Hongshuizhuang and Tieling formations in ascending order, is comparable with the Calymmian System in the International Stratigraphic Chart. The lower boundary of the Changchengian System, the first system of the Mesoproterozoic in China Regional Stratigraphic Chart, also needs revision from the conventional 1800 Ma to ∼1650 Ma well constrained by the zircon U–Pb ages 1673 ± 10 Ma (LA-MC-ICP-MS) and 1669 ± 20 Ma (SHRIMP) of a granite-porphyry dike that was overlain unconformably by the basal conglomerate of the Changzhougou Formation, the first formation of the Changchengian System. Therefore, the earliest Mesoproterozoic sequence in the NCC represented by the Changchengian and Jixianian Systems in the Yanliao Aulacogen is identical to that of the Vindhyan Supergroup in Central Indian and the Riphean Series in Russia. On the other hand, a series of 1.8–1.6 Ga anorogenic magmatic records were well-preserved around the NCC, which marked the initial rifting of the Columbia Supercontinent in the NCC. The magmatic events can thus be subdivided into three phases with peaks at ca 1.77 Ga, ca 1.70 Ga and ca 1.63 Ga, respectively. In addition to 1.8–1.6 Ga magmatic events, some minor volcanic eruptions at ca 1.56 Ga and 1.44 Ga, and wide-spread bi-modal magmatic intrusions at 1.35–1.32 Ga have been recognized in the northern NCC, marking the continued rifting of the Columbia Supercontinent since ∼1.8 Ga.  相似文献   

9.
A continuous shallow marine 10 m thick succession within the Langpar Formation in the Um Sohryngkew river section of Meghalaya, containing late Maastrichtian through early Danian planktonic foraminiferal zones – CF4, CF3, CF2, CF1, P0, Pα and P1a and the K/Pg boundary (between CF1 and P0) that makes unique of its kind. The section has been re-studied and sampled for clay mineralogy to understand the palaeoenvironmental conditions prevalent in the region and to assess the K/Pg transition. The relative abundances of the clay mineral phases permitted a threefold sub-division of the studied section with a illite, illite/ smectite dominated lower part, illite, kaolinite and abundance of montmorillonite dominated middle part and kaolinite–montmorillonite dominated upper part. Enriched HREEs in the lower part of the succession suggest variations in the pH of alteration solutions. Most of the samples show positive cerium (δCe) and europium (δEu) anomalies, the former reflecting oxidizing conditions at the time of clay formation. Illite dominated clays present a positive Eu anomaly, formed at relatively higher temperatures than the clays with less illite and without Eu anomalies, whereas clays occurring in the lower and upper parts exhibit a prominent negative Eu anomaly. Shifts in the redox condition found in this section are more or less similar to the foraminiferal changes and Au, Pt, Pd anomalies. Clay mineralogical attributes and REE patterns, comparable to those of the known K/Pg boundaries, appeared within the CF3 and CF2 zones in the Um Sohryngkew river section. The sample at the boundary between CF3 and CF2 is marked by a negative <delta>Ce anomaly, high La/Yb and TOC values, suggesting that sea level rise during the upper part of CF3 was caused by tectonism rather than warming. The similar characteristics of clay minerals and REE patterns, attributed to the initiation of tectonic events during the CF3 zone, indicate environmental changes that affected the shelf area and the provenance of these sediments.  相似文献   

10.
Comparing the early Earth to the present day, geological–geochemical evidence points towards higher mantle potential temperature and a different type of tectonics. In order to investigate possible changes in Precambrian tectonic styles, we conduct 3D high-resolution petrological–thermomechanical numerical modelling experiments for oceanic plate subduction under an active continental margin at a wide range of mantle potential temperature TP (∆ TP = 0 − 250 K, compared to present day conditions). At present day mantle temperatures (∆ TP = 0 K), results of numerical experiments correspond to modern-style subduction, whereas at higher temperature conditions important systematic changes in the styles of both lithospheric deformation and mantle convection occur. For ∆ TP = 50 − 100 K a regime of dripping subduction emerges which is still very similar to present day subduction but is characterised by frequent dripping from the slab tip and a loss of coherence of the slab, which suggests a close relationship between dripping subduction and episodic subduction. At further increasing ∆ TP = 150 − 200 K dripping subduction is observed together with unstable dripping lithosphere, which corresponds to a transitional regime. For ∆ TP = 250 K, presumably equivalent to early Archean, the dominating tectonic style is characterised by small-scale mantle convection, unstable dripping lithosphere, thick basaltic crust and small plates. Even though the initial setup is still defined by present day subduction, this final regime shows many characteristics of plume-lid tectonics. Transition between the two end-members, plume-lid tectonics and plate tectonics, happens gradually and at intermediate temperatures elements of both tectonic regimes are present. We conclude, therefore, that most likely no abrupt geodynamic regime transition point can be specified in the Earth's history and its global geodynamic regime gradually evolved over time from plume-lid tectonics into modern style plate tectonics.  相似文献   

11.
Whole rock major and trace element data from granitoids adjacent to the Kalahari Craton–Mozambique–Maud Belt boundary are described. The data from ～1140 Ma old granodioritic and ～1110 Ma old granitic bodies in the Mozambique Belt show that they are typical of calc-alkaline and A-type granitoids respectively. Radiogenic Rb/Sr and Sm/Nd isotope data from the two granitoid bodies suggest significant older crustal contributions during their genesis. The granodioritic gneisses show TDM model ages of ～2100–3500 Ma whereas megacrystic granitic gneisses have TDM model ages of ～1600–3100 Ma. Granite from the Archaean-age Kalahari Craton has TDM model ages of ～3000–3500 Ma.The data from Mozambique are compared with whole rock major and trace element chemistry and U/Pb zircon SHRIMP data from the Maud Belt in western Dronning Maud Land. These show that ～1140 Ma old granodioritic gneisses in Sverdrupfjella and Kirwanveggan have similar ages and chemical compositions to similar rocks in central Mozambique. Radiogenic isotope characteristics of the gneisses from central Mozambique and Sverdrupfjella are similar and suggest older crustal contributions in contrast to the juvenile nature of the gneisses from Kirwanveggan.Similarly, ～1090 Ma old granitic gneisses from central Mozambique, Sverdrupfjella and Kirwanveggan have similar ages and A-type chemical compositions. In contrast the radiogenic isotope compositions from Kirwanveggan are juvenile whereas those from central Mozambique show a significant older crustal contribution.The whole rock radiogenic isotope data can be interpreted to suggest that the Mesoproterozoic Mozambique Belt rocks were generated by partial melting which probably involved mixing of Archaean/Paleoproterozoic crust and younger Mesoproterozoic juvenile magma at ～1100 Ma and suggest that the Kalahari Craton probably extends eastwards at depths for more than 30 km from its exposure at surface.The data support correlations between the Mozambique Belt and the Maud Belt in Antarctica in general and more specifically show similarities between the Kalahari Craton boundary and the Mozambique–Maud Belt in lithologies immediately adjacent to that boundary.Two episodes of anatectic migmatisation are recognized in rocks from the Mozambique Belt in central Mozambique. These show an earlier migmatitic vein phase oriented parallel to the planar foliation in the granitic and tonalitic gneisses and a later discordant vein phase which is oriented parallel to localized but intense N–S oriented shearing along the Kalahari Craton/Mozambique Belt boundary zone. SHRIMP zircon data from the younger migmatitic vein phase suggests a crystallization age of 997 ± 4 Ma. Small numbers of inherited zircons have ages of ～2700 Ma and ～1100–1200 Ma. Younger discordant analyses suggesting metamorphic disturbance between ～400 Ma and 550 Ma are seen. The data imply the high strain along the eastern margin of the Kalahari Craton in the Manica area, occurred at ～1000 Ma and not at ～450 Ma as was previously thought. The data suggest the Pan African deformation and metamorphism in the area involved minor reworking. The undeformed to weakly deformed Tchinadzandze Granodiorite intruded into the Kalahari Craton has an age of 2617 ± 16 Ma.  相似文献   

12.
The pollen record of the long succession of marine and continental deposits filling the subsident north-Adriatic foredeep basin (NE Italy) documents the history of vegetation, the landscape evolution and the climate forcing during the last 215 ka at the south-eastern Alpine foreland. The chronology relies on several 14C determinations as well as on estimated ages of pollen-stratigraphical and sea-level event tie-points derived from comparison with high-resolution marine records, speleothemes and ice cores.Mixed temperate rainforests persisted throughout MIS 7a–7c, being replaced by conifer forests after the local glacioeustatic regression during early MIS 6. The Alpine piedmont facing the Adriatic foredeeep was glaciated at the culmination of the penultimate glaciation, as directly testified by in situ fluvioglacial aggradation related to the building of a large morainic amphitheatre. The pollen record allows correlation with other European records and with the IRD from N-Atlantic and off Iberia, thus the duration of the penultimate glacial culmination at the southalpine fringe is estimated less than 13 ka between 148 ± 1 and >135 ka. The site was not reached by the Last Interglacial maximum sea transgression and enregistered a typical, though incomplete, Eemian forest record, lacking Mediterranean evergreen trees. A complex sequence of stadial–interstadial episodes is reconstructed during the Early and Middle Würm: major xerophyte peaks match IRD maxima occurred during Heinrich events in deep-sea cores offshore Iberia and in the N-Atlantic and allows to frame lumps of interstadial phases, marked by Picea peaks, each one including several DO warm events. Broad-leaved thermophilous forests disappeared from the north-eastern plain of Italy at the end of the Early Würm, whereas reduced populations of Abies and Fagus probably sheltered even during the Last Glacial Maximum. A renewed fluvioglacial in situ deposition between 30.4 ± 0.4 and 21.6 ± 0.5 ka cal BP sets the time and duration of the last glacial culmination in the pedemontane morainic amphitheatre. Palynomorphs from Plio-Pleistocene marine successions were reworked by glacier erosion and deposited in the lowland during both the penultimate and the last deglaciation phases. This explains a bias affecting previous pollen records from the region.  相似文献   

13.
The aim of this study was to investigate the dissolution and transformation characteristics of phyllosilicate under low molecular weight organic acids in the farmland environment (pH 4.0–8.0). Changes of dissolution and morphology of biotite were evaluated using chemical extraction experiments and in situ/ex situ atomic force microscopy (AFM) with fluids of citric acid (CA) solution at pH 4.0, 6.0, and 8.0. Results of extracting experiments show that CA solutions contributed to the release rate of potassium (K), silicon (Si), and aluminum (Al) from biotite relative to a control aqueous solution. In situ AFM observations indicate that the dissolution of biotite from the biotite (0 0 1) surface occurred on the terrace, segment, and fringe of pits, while new etch pits did not readily form on biotite (0 0 1) surfaces in aqueous solutions. However, dissolution rates of terraces can be greatly accelerated with the help of citrate. In pH 4.0 CA solution, 70 min dissolution reactions of biotite (0 0 1) surfaces result in more etch pits than in pH 6.0 and 8.0 solutions. In addition, the transformation of biotite occurred simultaneously with the dissolution process. Secondary coating was observed on the biotite (0 0 1) surface after 140 h of immersion in a weak acid environment. Thus, the protons have a dominant role in the dissolution process of biotite with organic (carboxyl) acting as a catalyst under acidic condition. Based on the theory of interactions on a water–mineral interface in a weak acid environment, dissolution of biotite starts from defect/kink sites on the surface, one layer by one layer, and develops along the [h k 0] direction. A secondary coating that forms on the biotite (0 0 1) surface may restrain the formation and growth of etch pits, whereas this process may have a positive role on the stability of soil structure during long-term soil management.  相似文献   

14.
The Inner Mongolia Highland (IMH), along the northern edge of the North China Craton, was considered to be a long-standing topographic highland, whose exhumation history remains elusive. The aim of this study is to reveal Late Paleozoic exhumation processes of the IMH based on an integrated analysis of stratigraphy, petrography of clastic rocks, and U–Pb ages and Hf isotopes of detrital zircons from Permian–Triassic succession in the middle Yanshan belt. The results of the study show that the Benxi Formation, which was originally regarded as a Late Carboniferous unit, proves to be Early Permian in age because it contains detrital zircons as young as ∼298 Ma. The Lower Shihezi Formation is demonstrated to be a unit whose age spans the boundary of the Middle and Upper Permian, constrained by a U–Pb age of 260 ± 2 Ma from a dacite layer. Clastic compositions of conglomerate and sandstone change markedly, characterised by the predominance of sedimentary components in the Benxi–Shanxi Formations, by large amounts of volcanic clastics in the Lower and Upper Shihezi Formations, and by the presence of both metamorphic and igneous clastics in the Sunjiagou–Ermaying Formations. Sedimentary clastics include chert, carbonate, sandstone and quartzite, which may have been derived from Proterozoic to Lower Paleozoic sedimentary covers. Volcanic clasts were directly related to volcanic eruptions, while granite and gneiss grains were sourced from exhumed Late Paleozoic intrusive rocks and basement rocks. Detrital zircon U–Pb ages can be divided into five populations: 2.6–2.4 Ga, 1.9–1.7 Ga, 400–360 Ma, 325–290 Ma and 270–250 Ma. Precambrian detrital zircons are typically subrounded to rounded in shape, implying a recycling origin. Late Paleozoic zircons show oscillatory zones and their Th/U ratios >0.4, suggesting a magmatic origin. Most Phanerozoic zircons have negative εHf(T) values of −3.2 to −25.5, which are compatible with those of Late Paleozoic plutons in the IMH. The results indicate that the IMH may have been covered with Proterozoic to Lower Paleozoic sedimentary strata, which then underwent subsequent erosion and served as provenances for adjacent Late Paleozoic basins. Vertical changes in both clastic compositions and detrital zircon ages in Permian–Triassic strata imply an unroofing process of the IMH. Three phases of the IMH uplift are distinguished. The first-phase uplift commenced 325–312 Ma and resulted from magmatic intrusion related to southward subduction of the Paleo-Asian Ocean. The second-phase uplift took place in the Middle Permian and may be attributed to crustal contraction related to the collision of the North China Craton and the Southern Mongolia terrane. The third-phase uplift happened at the end of the Permian, and may have been induced by upwelling of calc-alkali magma under an extensional setting.  相似文献   

15.
The paper summarizes data on the Pleistocene combustion metamorphic complexes of the Kuznetsk Coal Basin. Paralava and clinker samples are dated by 40Ar/39Ar incremental heating. The 40Ar/39Ar ages of the combustion metamorphic rocks permit reconstructions of the succession of renewed activity of ancient faults in the Salair zone and age estimates for the evolution of the present-day drainage network. Cross sections of burned rocks from the western margin and center of the Kuznetsk Basin are compared. The geologic factors of coal ignition risks are analyzed. On the western margin of the Kuznetsk Basin, paleofires occurred in steeply dipping thick seams with predominant crushed vitrain–clarain coal, which has a high oxygen and methane adsorption capacity. Highly denuded high-temperature combustion metamorphic complexes are most often localized in the arches of slightly broken anticlines. Oxygen was supplied to the coals during the Late Cenozoic renewed fault activity and the subsequent erosion of the sediments. The natural fires in the area were a result of external rather than spontaneous ignition. The depths of the paleofires (up to 200 m) indicate that they occurred in a warm and dry climate. In the center of the Kuznetsk Basin, dispersed fire foci appeared in seams of self-igniting coals with the erosion propagation of the current drainage network. The combustion metamorphic complexes here are partly eroded and consist mostly of clinkers with a low degree of alteration. The 40Ar/39Ar ages and geological data indicate that the earliest large-scale combustion events on the western periphery of the basin occurred in the Eopleistocene (1.3–0.9 Ma).The oldest 40Ar/39Ar age of a coal fire episode (1.7 ± 0.3 Ma) might be the upper age boundary of the altitude differentiation of topography, which corresponds to the renewed activity of the Tyrgan and Afonino–Kiselevsk faults. The later synchronous combustion events on the western margin (0.2 ± 0.1 Ma) and in the center of the basin (0.13–0.02 Ma), most probably, occurred during the Kazantsevian interglacial, which gave rise to the present-day drainage network.  相似文献   

16.
The crystalline basement of the Sierra de San Luis, which belongs to the Eastern Sierras Pampeanas in central Argentina, consists of three main units: (1) Conlara, (2) Pringles, and (3) Nogolí metamorphic complexes. In the Pringles Metamorphic Complex, mafic–ultramafic bodies occur as discontinuous lenses along a narrow central belt concordant with the general NNE–SSW structural trend. A metamorphic gradient from granulite to greenschist facies is apparent on both sides of the mafic–ultramafic bodies. This work focuses on the characteristics of the mylonitization overprinted on the mafic–ultramafic intrusives in the Pringles Metamorphic Complex and their gneissic–migmatitic surroundings, both previously metamorphosed within the granulite facies. Petrogenetic grid and geothermobarometry applied to the paragenesis equilibrated during the mylonitic event, together with mineral deformation mechanisms, indicate that mafic and adjacent basement mylonites developed under upper amphibolite transitional to granulite facies metamorphic conditions at intermediate pressures (668–764 °C, 6.3–6.9 kbar, 0.3 < XCO2 < 0.7). However, the following mylonitic assemblages can be distinguished from the external limits of the Pringles Metamorphic Complex to its center: lower amphibolite facies ⇒ middle amphibolite facies ⇒ upper amphibolite transitional to granulite facies. Geothermobarometry applied to mylonitic assemblages indicate a temperature gradient from 555 °C to 764 °C and pressures of 6–7 kbar for the mylonitic event. This event is considered to have developed on a preexisting temperature gradient attributed to the intrusion of mafic–ultramafic bodies. The concentration of sulfides in mylonitic bands and textural relationships provide evidence of remobilization of primary magmatic sulfides of the mafic–ultramafic rocks (+PGM) during the mylonitic event. A lower-temperature final overprint produced brittle fracturing and localized retrogression on mafic–ultramafic minerals and ores by means of a water-rich fluid phase, which gave rise to a serpentine + magnetite ± actinolite association. Concordantly in the adjacent country rocks, fluids channeled along preexisting mylonitic foliation planes produced local obliteration of the mylonitic texture by a randomly oriented replacement of the mylonite mineralogy by a chlorite + sericite/muscovite + magnetite assemblage. Observed mineral reactions combined with structural data and geothermobarometry suggest a succession of tectonometamorphic events for the evolution of the Pringles Metamorphic Complex of Sierra de San Luis, developed in association with a counterclockwise P–T–d path. The most likely geological setting for this type of evolution is a backarc basin, associated with east-directed Famatinian subduction initiated in Mid-Cambrian times and closed during the collision of the allochthonous Precordillera terrane in Mid-Ordovician times.  相似文献   

17.
The Sylhet Basin of Bangladesh is a sub-basin of the Bengal Basin. It contains a very thick (up to 22 km) Tertiary stratigraphic succession consisting mainly of sandstones and mudstones. The Sylhet succession is divided into the Jaintia (Paleocene–late Eocene), Barail (late Eocene–early Miocene), Surma (middle–late Miocene), Tipam (late Miocene–Pliocene) and Dupitila Groups (Pliocene–Pleistocene), in ascending order. The origin of the organic matter (OM) and paleoenvironment of deposition have been evaluated on the basis of C, N, S elemental analysis, Rock-Eval pyrolysis and gas chromatography–mass spectrometry (GC–MS) analysis of 60 mudstone samples collected from drill core and surface outcrops. Total organic carbon (TOC) content ranges from 0.11% to 1.56%. Sulfur content is low in most samples. TOC content in the Sylhet succession varies systematically with sedimentation rate, with low TOC caused by clastic dilution produced by high sedimentation rates arising from rapid uplift and erosion of the Himalaya.The OM in the succession is characterized by systematic variations in pristane/phytane (Pr/Ph), oleanane/C30 hopane, n-C29/n-C19 alkane, Tm/Ts [17α(H)-22,29,30-trisnorhopane/18α(H)-22,29,30-trisnorhopane] and sterane C29/(C27 + C28 + C29) ratios during the middle Eocene to Pleistocene. Based on biomarker proxies, the depositional environment of the Sylhet succession can be divided into three phases. In the first (middle Eocene to early Miocene), deposition occurred completely in seawater-dominated oxic conditions, with abundant input of terrestrial higher plants, including angiosperms. The second phase (middle to late Miocene) consisted of mainly freshwater anoxic conditions along with a small seawater influence according to eustasic sea level change, with diluted OM derived from phytoplankton and a lesser influence from terrestrial higher plants. Oxygen-poor freshwater conditions prevailed in the third phase (post-late Miocene). Planktonic OM was relatively abundant in this stage, while a high angiosperm influx prevailed at times. Tmax values of ca. 450 °C, vitrinite reflectance (Ro) of ca. 0.66% and methylphenanthrene index (MPI 3) of ca. 1 indicate the OM to be mature. The lower part (middle Eocene to early Miocene) of the succession with moderate TOC content and predominantly terrestrial OM could have generated some condensates and oils in and around the study area.  相似文献   

18.
We present a new set of contour maps of the seismic structure of South America and the surrounding ocean basins. These maps include new data, helping to constrain crustal thickness, whole-crustal average P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle (Pn and Sn). We find that: (1) The weighted average thickness of the crust under South America is 38.17 km (standard deviation, s.d. ±8.7 km), which is ∼1 km thinner than the global average of 39.2 km (s.d. ±8.5 km) for continental crust. (2) Histograms of whole-crustal P-wave velocities for the South American crust are bi-modal, with the lower peak occurring for crust that appears to be missing a high-velocity (6.9–7.3 km/s) lower crustal layer. (3) The average P-wave velocity of the crystalline crust (Pcc) is 6.47 km/s (s.d. ±0.25 km/s). This is essentially identical to the global average of 6.45 km/s. (4) The average Pn velocity beneath South America is 8.00 km/s (s.d. ±0.23 km/s), slightly lower than the global average of 8.07 km/s. (5) A region across northern Chile and northeast Argentina has anomalously low P- and S-wave velocities in the crust. Geographically, this corresponds to the shallowly-subducted portion of the Nazca plate (the Pampean flat slab first described by Isacks et al., 1968), which is also a region of crustal extension. (6) The thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. (7) The crust in the Amazon basin and along the western edge of the Brazilian craton may be thinned by extension. (8) The average crustal P-wave velocity under the eastern Pacific seafloor is higher than under the western Atlantic seafloor, most likely due to the thicker sediment layer on the older Atlantic seafloor.  相似文献   

19.
Distribution and possible sources of polycyclic aromatic hydrocarbons (PAHs) have been investigated in 23 late Eocene to early Pleistocene mudstones from the Sylhet succession of the northeastern Bengal Basin, Bangladesh. Paleoclimatic conditions in the southern Himalaya region throughout the Himalayan uplift were reconstructed, based on combustion derived PAHs and aromatic land plant derived biomarkers. Phenanthrene, fluoranthene (Fla), pyrene (Py), benz[a]anthracene (BaAn), chrysene/triphenylene (Chry + Tpn), benzofluoranthenes (Bflas), benzo[e]pyrene (BePy), benzo[a]pyrene (BaPy), perylene (Pery), indeno[1,2,3-cd]pyrene (InPy), benzo[ghi]perylene (BghiP), coronene (Cor) and retene (Ret) were the identified PAHs. Fla/(Fla + Py) ratios > 0.5 and InPy/(InPy + BghiP) > 0.2 from almost all Sylhet samples suggest occurrence of natural wildfires. Low contents of BaAn and BaPy indicate decomposition by long exposure to sunlight before sedimentation, or early diagenetic weathering. Increased Cor, InPy and BghiP contents suggest occurrence of larger, high temperature wildfires. Correlation coefficients of the PAHs and p-values for statistical hypothesis testing showed that the positive and negative correlations within the PAHs may be indicative of high or low temperatures in wildfires. Fungi derived Pery showed negative correlations with Py (r = −0.67, p = 4.6 × 10−4) and Fla (r = −0.56, p = 5.0 × 10−3), but not with Cor, Bflas, InPy and BghiP. Based on the correlation coefficients for all PAHs and their p-values, five statistical groups ([Py, Fla], [Cor, Bflas, InPy, BghiP], [BaAn, Chry + Tpn, BaPy], [Pery] and [Ret]) were recognized. These groups are probably correlated with origins and depositional processes. According to the results, the Sylhet succession was deposited in three differing paleoclimatic regimes: (1) First phase (late Eocene to early Miocene, early to middle stage of Himalayan uplift): High contents of combustion derived PAHs (Fla, Py and BePy), significant gymnosperm derived Ret, and low Pery abundances in the Jaintia and Barail groups indicate arid climatic conditions. Although wildfires could often occur, 5- or 6-ring combustion PAHs (Cor, InPy and BghiP) contents are low, suggesting that the wildfires were relatively low temperature. (2) Second phase (middle to late Miocene: middle to late stage of Himalayan uplift): Combustion derived PAHs and fungi derived Pery were dominant in the Surma Group. The climate was arid to humid and seasonal, with a dry season giving conditions suitable for combustion. Abundant Fla, Bflas, BePy, Cor, InPy and BghiP imply high temperatures in large wildfires. However, frequency of the wildfires decreased because of wet climate. (3) Third phase (late Miocene to Pleistocene: late stage of Himalayan uplift): Moderate to high Pery contents and low Fla, Py and BePy abundances in the Tipam and Dupitila groups indicate establishment of more humid climate. InPy, BghiP, Cor, Bflas and BaPy were predominant. Intensified humid and seasonal climate arising from the Himalayan monsoon decreased the incidence and frequency of general wildfires, but increased the ratio of large to small wildfires.  相似文献   

20.
《Chemical Geology》2006,225(3-4):222-229
First principles phase diagram calculations were performed for the system NaCl–KCl. Plane-wave pseudopotential calculations of formation energies were used as a basis for fitting cluster expansion Hamiltonians, both with and without an approximation for the excess vibrational entropy (SVIB). Including SVIB dramatically improves the agreement between calculated and experimental phase diagrams: experimentally, the consolute point is {XC = 0.348, TC = 765 K}Exp; without SVIB, it is {XC = 0.46, TC ≈ 1630 K}Calc; with SVIB, it is {XC = 0.43, TC ≈ 930 K}Calc.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号