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1.
Jurassic to Cretaceous red sandstones were sampled at 33 sites from the Khlong Min and Lam Thap formations of the Trang Syncline (7.6°N, 99.6°E), the Peninsular Thailand. Rock magnetic experiments generally revealed hematite as a carrier of natural remanent magnetization. Stepwise thermal demagnetization isolates remanent components with unblocking temperatures of 620–690 °C. An easterly deflected declination (D = 31.1°, I = 12.2°, α95 = 13.9°, N = 9, in stratigraphic coordinates) is observed as pre-folding remanent magnetization from North Trang Syncline, whereas westerly deflected declination (D = 342.8°, I = 22.3°, α95 = 12.7°, N = 13 in geographic coordinates) appears in the post-folding remanent magnetization from West Trang Syncline. These observations suggest an occurrence of two opposite tectonic rotations in the Trang area, which as a part of Thai–Malay Peninsula received clockwise rotation after Jurassic together with Shan-Thai and Indochina blocks. Between the Late Cretaceous and Middle Miocene, this area as a part of southern Sundaland Block experienced up to 24.5° ± 11.5° counter-clockwise rotation with respect to South China Block. This post-Cretaceous tectonic rotation in Trang area is considered as a part of large scale counter-clockwise rotation experienced by the southern Sundaland Block (including the Peninsular Malaysia, Borneo and south Sulawesi areas) as a result of Australian Plate collision with southeast Asia. Within the framework of Sundaland Block, the northern boundary of counter-clockwise rotated zone lies between the Trang area and the Khorat Basin.  相似文献   

2.
Porphyry Cu deposits occurred in the southern West Junggar of Xinjiang, NW China and are represented by the Baogutu and newly-discovered Jiamantieliek porphyry Cu deposits. Petrographical and geochemical studies show that both Jiamantieliek and Baogutu ore-bearing intrusions comprise main-stage diorite stock and minor late-stage diorite porphyry dikes and are the calc-alkaline intermediate intrusions. Based on U–Pb zircon SHRIMP analyses, the Jiamantieliek intrusion formed in 313 ± 4 Ma and 310 ± 5 Ma, while, based on U–Pb zircon SIMS analyses, the Baogutu intrusion formed in 313 ± 2 Ma and 312 ± 2 Ma. Rocks in the Jiamantieliek intrusion are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) with negative Nb anomaly. Their isotopic compositions (εNd(t) = +1.6 to +3.4, (87Sr/86Sr)i = 0.70369–0.70401, (207Pb/204Pb)i = 15.31–5.41) suggest a mixing origin from depleted to enriched mantle sources. In the Baogutu intrusion, the rocks are similar to those of the Jiamantieliek intrusion. Their Sr-Nd-Pb isotopic composition (εNd(t) = +4.4 to +6.0, (87Sr/86Sr)i = 0.70368–0.70385, (207Pb/204Pb)i = 15.34–5.42) shows a more depleted mantle source. These features suggest generation in an island arc. The Jiamantieliek and Baogutu intrusions have similar characteristics, indicating that a relatively uniform and integrated source region has existed in the southern West Junggar since the Palaeozoic. A larger contribution of calc-alkaline magma would be required to generate the Jiamantieliek intrusion, which may reflect the development of magma arc maturation towards the western section of the southern West Junggar.  相似文献   

3.
《Applied Geochemistry》2005,20(11):2017-2037
The Tertiary Thrace Basin located in NW Turkey comprises 9 km of clastic-sedimentary column ranging in age from Early Eocene to Recent in age. Fifteen natural gas and 10 associated condensate samples collected from the 11 different gas fields along the NW–SE extending zone of the northern portion of the basin were evaluated on the basis of their chemical and individual C isotopic compositions. For the purpose of the study, the genesis of CH4, thermogenic C2+ gases, and associated condensates were evaluated separately.Methane appears to have 3 origins: Group-1 CH4 is bacteriogenic (Calculated δ13CC1–C = −61.48‰; Silivri Field) and found in Oligocene reservoirs and mixed with the thermogenic Group-2 CH4. They probably formed in the Upper Oligocene coal and shales deposited in a marshy-swamp environment of fluvio-deltaic settings. Group-2 (δ13CC1–C = −35.80‰; Hamitabat Field) and Group-3 (δ13C1–C = −49.10‰; Değirmenköy Field) methanes are thermogenic and share the same origin with the Group-2 and Group-3 C2+ gases. The Group-2 C2+ gases include 63% of the gas fields. They are produced from both Eocene (overwhelmingly) and Oligocene reservoirs. These gases were almost certainly generated from isotopically heavy terrestrial kerogen (δ13C = −21‰) present in the Eocene deltaic Hamitabat shales. The Group-3 C2+ gases, produced from one field, were generated from isotopically light marine kerogen (δ13C = −29‰). Lower Oligoce ne Mezardere shales deposited in pro-deltaic settings are believed to be the source of these gases.The bulk and individual n-alkane isotopic relationships between the rock extracts, gases, condensates and oils from the basin differentiated two Groups of condensates, which can be genetically linked to the Group-2 and -3 thermogenic C2+ gases. However, it is crucial to note that condensates do not necessarily correlate to their associated gases.Maturity assessments on the Group-1 and -2 thermogenic gases based on their estimated initial kerogen isotope values (δ13C = −21‰; −29‰) and on the biomarkers present in the associated condensates reveal that all the hydrocarbons including gases, condensates and oils are the products of primary cracking at the early mature st age (Req = 0.55–0.81%). It is demonstrated that the open-system source conditions required for such an early-mature hydrocarbon expulsion exist and are supported by fault systems of the basin.  相似文献   

4.
The polymetallic Mykonos vein system in the Cyclades, Greece, consists of 15 tension-gashes filled with barite, quartz, pyrite, sphalerite, chalcopyrite and galena in ca. 13.5 Ma, I-type, Mykonos monzogranite. Zones of silica and chlorite–muscovite alteration are associated with the veins and overprint pervasive silicification, phyllic and argillic alteration that affected large parts of the monzogranite. The mineralization cements breccias and consists of an early barite–silica–pyrite–sphalerite–chalcopyrite assemblage followed by later argentiferous galena. A combination of fluid inclusion and stable isotope data suggests that the barite and associated mineralization were deposited from fluids containing 2 to 17 wt.% NaCl equivalent, at temperatures of ~ 225° to 370 °C, under a hydrostatic pressure of ≤ 100 bars. The mineralizing fluids boiled and were saturated in H2S and SO2.Calculated δ18OH2O and δDH2O, initial 87Sr/86Sr isotope compositions and the trace and REEs elements contents are consistent with a model in which the mineralizing fluids were derived during alteration of the Mykonos intrusion and subsequently mixed with Miocene seawater. Heterogeneities in the calculated δ34SSO4− 2 and δ34SH2S compositions of the ore fluids indicate two distinct sources for sulfur, namely of magmatic and seawater origin, and precipitation due to reduction of the SO4− 2 during fluid mixing. The physicochemical conditions of the fluids were pH = 5.0 to 6.2, logfS2 = − 13.8 to − 12.5, logfO2 = − 31.9 to − 30.9, logfH2S(g) = − 1.9 to − 1.7, logfTe2 = − 7.9 and logα(SO4− 2(aq)/H2S(aq)) = + 2.6 to + 5.5. We propose that retrograde mesothermal hydrothermal alteration of the Mykonos monzogranite released barium and silica from the alkali feldspars. Barite was precipitated due to mixing of SO4− 2-rich Miocene seawater with the ascending Ba-rich magmatic fluid venting upwards in the pluton.  相似文献   

5.
A combined magnetic fabric and paleomagnetic study has been carried out on the siliciclastic rocks gathered from a stratigraphic cross-section through the Nanpanjiang Basin, South China, in an attempt to extract the paleoflow information preserved in and, thus, constrain the possible origins of these clastic rocks. The sediments used for this study were formed by sediment-gravity flows along the southern margin of the South China block in the Middle Triassic time (ca. 245–228 Ma). The results show a normal distribution of both low field magnetic susceptibility values and natural remanent magnetization intensities, which along with the monotonic detrital framework mode, mainly comprising quartz and lithic particles, may suggest a single provenance involved in deposition of these clastic deposits. Anisotropy of magnetic susceptibility (AMS) analysis acquires primarily the sedimentary magnetic fabrics, which, in this study, reveal paleoflow directions ranging from NNW to ENE with an overall mean orientation of NE. Demagnetization on a part of samples isolates a characteristic remanent component averaged at D = 44.8°, I = 16.9°, κ = 9.7, α95 = 6. 5°, n = 55, corresponding to a paleolatitude N8.6° and a clockwise rotation of ca. 45° since the Middle Triassic for the studied cross-section. This mean direction passes fold tests and is consistent with the reference direction expected from the South China block at the 95% confidence level. Restoring this ∼45° declination renders an overall northward paleoflow, which, combined with other evidence, suggests a southern provenance for these sediments during deposition in the Middle Triassic time. In terms of the early Mesozoic plate framework of southeastern Asia, a tectonic scenario is proposed here, whereby the nearly N–S convergence of the Indochina and South China blocks and its related Indosinian orogeny in the Middle Triassic caused the formation of the Nanpanjiang foreland basin, which was filled by voluminous detritus shed from the uplifted orogenic belt on its southern side.  相似文献   

6.
The mid-late Eocene “Valley of Whales” in the Fayum province of Egypt contains hundreds of marine-mammals’ skeletons. Given its paleontological importance, we carried out a paleomagnetic study of the fossil-bearing formations. A sequence of basalts directly overlying the upper Eocene rocks in three distant clusters within a 25 km-long NW–SE graben in the southwestern part of the area was also studied. Thermal demagnetization of three-axis IRM was used to identify and eliminate sites dominated by hematite and/or goethite as potential remanence carriers. Progressive thermal demagnetization of the NRM isolated a characteristic NNE–SSW dual-polarity direction with a shallow inclination that passes both tilt and reversal tests. The mean tilt-corrected direction of the sedimentary formations is D/I = 16°/30° (k = 50, α95 = 3°) yielding a paleomagnetic pole at 70°N/159°E. The anisotropy of magnetic susceptibility (AMS) indicated that the observed inclinations were free from inclination shallowing, as did the nearly identical characteristic remanence of the overlying basalt flows (with a tilt-corrected reversed-polarity direction of D/I = 198°/−28° (k = 38, α95 = 7°) and a pole at 68°N/158°E). The new paleopoles place the Fayum province at a lower paleolatitude (15–17°N) than today (29.5°N), and point to the possible prevalence of tropical climate in northeast Africa during mid-late Eocene times. This tropical position is nearly identical to the paleolatitudes extrapolated from the mean of 36 coeval poles rotated from the other major cratons and from Africa itself. The declinations show a minor easterly deviation from those predicted by extrapolation from other continents. This is interpreted as due to a small clockwise rotation internal to NE Africa, possibly related to Red Sea/Gulf of Suez rifting after the late Eocene. The alternative explanation that the geomagnetic field had a non-zonal non-dipole field contribution is not favored.  相似文献   

7.
The Betsimisaraka Suture (B.S.) of Madagascar is an important structural zone defining the collision between Eastern and Western Gondwana. It is represented by highly deformed high-grade metamorphic rocks with mineral assemblages typical of ophiolitic material, including chromitite and nickel bearing rocks consistent with a suture zone setting. Analysis of satellite imagery coupled with field investigations has helped to elucidate the structure and evolution of the B.S. Digital image processing of Landsat ETM+ data was integrated with Synthetic Aperture Radar (SAR) to reduce the effects of the dense vegetation cover. Enhanced false color composite (7-4-2 and 4-2-3), single Landsat bands and band ratio composites including 2/7-1/7-2/5, 5/3-5/1-7/3, 5/1-7/1-4/1 and 5/7-5/1-5/4 × 3/4 (in RGB) improve the lithologic contrast, reduce the effects of topography and enhance the structural lineaments. Ductile deformation deduced from structural features mapped on Landsat enhanced images indicates three generations of folding (F1, F2, and F3) coupled with shearing: (1) F1 folds with NE striking axial surfaces; (2) F2 related with N–S striking axial surfaces, (3) and F3 associated with ENE–WSW axial surfaces, indicating NNW and SSE contractional strain similar to the deformation in the southern B.S. Mapping these structures enables three types of shearing to be delineated: (1) NW–SE dextral shearing as seen in the Befandriana region; (2) NW–SE sinistral shearing defined by sigmoidal bodies in Mandritsara and Ankijanilava-Marotandrano regions, (3) and NE–SW striking dextral shears recorded in the Lake Alaotra region. Several faults, joints and fractures represent brittle deformation events. Lineaments analyzed within the B.S. are divisible into two groups of brittle structures: (1) N–S trending lineaments correlated with the Gondwanan collision events and (2) much younger NE and NW trending lineaments that are mainly found in the Antananarivo block. The latter may represent an active tectonic event in the central plateau that bounds the western part of the B.S.  相似文献   

8.
Pure-iron end-member hibbingite, Fe2(OH)3Cl(s), may be important to geological repositories in salt formations, as it may be a dominant corrosion product of steel waste canisters in an anoxic environment in Na–Cl- and Na–Mg–Cl-dominated brines. In this study, the solubility of Fe2(OH)3Cl(s), the pure-iron end-member of hibbingite (FeII, Mg)2(OH)3Cl(s), and Fe(OH)2(s) in 0.04 m to 6 m NaCl brines has been determined. For the reactionFe2(OH)3Cl(s) + 3H+ ? 3 H2O + 2 Fe2+ + Cl?,the solubility constant of Fe2(OH)3Cl(s) at infinite dilution and 25 °C has been found to be log10 K = 17.12 ± 0.15 (95% confidence interval using F statistics for 36 data points and 3 parameters). For the reactionFe(OH)2(s) + 2H+ ? 2 H2O + Fe2+,the solubility constant of Fe(OH)2 at infinite dilution and 25 °C has been found to be log10 K = 12.95 ± 0.13 (95 % confidence interval using F statistics for 36 data points and 3 parameters). For the combined set of solubility data for Fe2(OH)3Cl(s) and Fe(OH)2(s), the Na+–Fe2+ pair Pitzer interaction parameter θNa+/Fe2+ has been found to be 0.08 ± 0.03 (95% confidence interval using F statistics for 36 data points and 3 parameters). In nearly saturated NaCl brine we observed evidence for the conversion of Fe(OH)2(s) to Fe2(OH)3Cl(s). Additionally, when Fe2(OH)3Cl(s) was added to sodium sulfate brines, the formation of green rust(II) sulfate was observed, along with the generation of hydrogen gas. The results presented here provide insight into understanding and modeling the geochemistry and performance assessment of nuclear waste repositories in salt formations.  相似文献   

9.
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10.
The study area lies between latitude 18–26°N and longitude 73–83°E, and mainly covers the Central India Tectonic Zone (CITZ). The frequency-dependent shear wave quality factor (Qs) has been estimated over the CITZ and its surroundings using Double Spectral Ratio (DSR) method. We have considered 25 local earthquakes with magnitude (ML) varies from 3.0 to 4.7 recorded at 11 stations running under national seismic network. The Fast Fourier Transformed (FFT) spectra were computed from the recorded waveform having time-window from onset of S-phase to 1.0 s and for a frequency-band of 0.1–10 Hz. Three different shear wave velocities (i.e., 3.87, 3.39 and 3.96 km/s) were obtained over the study area based on a pair of earthquakes recorded at a pair of stations. The low Qs values of 51–96 at 1 Hz (i.e., Qs = 51f0.49; Qs = 90f0.488 and Qs = 96f0.53) were found in the area covering the Son–Narmada–Tapti (SONATA) lineament, CITZ, eastern part of the Satpura fold belt, Vindhyan and Gondwana basins, Godavari and Mahanadi grabens, and southern part of Gangetic plain. Intermediate Qs values of the order of 204–277 (i.e., Qs = 204f0.56 and Qs = 277f0.55) were noted in the cartonic areas, namely, Bundelkhand, Dharwar-Bhandara and Bastar. While the higher Qs values of 391–628 at 1 Hz (i.e., Qs = 391f0.49, Qs = 409f0.48, Qs = 417f0.48, Qs = 500f0.66, Qs = 585f0.65 and Qs = 628f0.69) were found in the eastern part of the SONATA, CITZ, and the northeastern part of the Satpura fold belt. The low Qs values might be attributing to the more heterogeneous SONATA rift system. Low Qs values further may presumably be associated with lower-level of seismicity and apparently account for higher tectonic stress accumulation over long duration. The long-term accumulated stress is generally released through occasional triggering of moderate magnitude earthquakes in the SONATA zone. Surrounding the SONATA region, the higher Qs values possibly accounts for a more homogeneous subsurface structure along the SONATA zone.  相似文献   

11.
NE Iran, including the Kopeh Dagh and Allah Dagh-Binalud deformation domains, comprises the northeastern boundary of the Arabia–Eurasia collision zone. This study focuses on the evolution of the Plio-Quaternary tectonic regimes of northeast Iran. We present evidence for drastic temporal changes in the stress state by inversion of both geologically and seismically determined fault slip vectors. The inversions of fault kinematics data reveal distinct temporal changes in states of stress during the Plio-Quaternary (since ～ 5 Ma). The paleostress state is characterized by a regional transpressional tectonic regime with a mean N140 ± 10°E trending horizontal maximum stress axis (σ1). The youngest (modern) state of stress shows two distinct strike-slip and compressional tectonic regimes with a regional mean of N030 ± 15°E trending horizontal σ1. The change from the paleostress to modern stress states has occurred through an intermediate stress field characterized by a mean regional N trending σ1. The inversion analysis of earthquake focal mechanisms reveals a homogeneous, transpressional tectonic regime with a regional N023 ± 5°E trending σ1. The modern stress state, deduced from the youngest fault kinematics data, is in close agreement with the present-day stress state given by the inversions of earthquake focal mechanisms. According to our data and the deduced results, in northeast Iran, the Arabia–Eurasia convergence is taken up by strike-slip faulting along NE trending left-lateral and NNW trending right-lateral faults, as well as reverse to oblique-slip reverse faulting along NW trending faults. Such a structural assemblage is involved in a mechanically compatible and homogeneous modern stress field. This implies that no strain and/or stress partitioning or systematic block rotations have occurred in the Kopeh Dagh and Allah Dagh-Binalud deformation domains. The Plio-Quaternary stress changes documented in this paper call into question the extrapolation of the present-day seismic and GPS-derived deformation rates over geological time intervals encompassing tens of millions of years.  相似文献   

12.
The distribution of glycerol dialkyl glycerol tetraethers (GDGTs) can reflect continental environmental changes. Recently, the distribution of branched GDGTs (bGDGTs) has been proposed as a novel tool for paleoelevation reconstructions. Here we report the variation in TEX86 (tetraether index of 86 carbon atoms) of isoprenoidal GDGTs (iGDGTs) and MBT (methylation of branched tetraether index) of bGDGTs along an altitudinal transect on Mt. Xiangpi, NE Qinghai-Tibetan Plateau. Both TEX86 and MBT values of surface soils showed significant linear decreases with altitude (TEX86: R2 = 0.65; n = 50; MBT: R2 = 0.69; n = 24). We suggest that the apparent relationships between the two indices and altitude may be related to temperature. Our preliminary investigation suggests that the TEX86 index can potentially be applied as a paleoelevation indicator in addition to the MBT index on the Qinghai-Tibetan Plateau.  相似文献   

13.
《Chemical Geology》2006,225(1-2):40-60
Fluorite is the most common fluoride mineral in magmatic silicic systems and its crystallization can moderate or buffer fluorine concentrations in these settings. We have experimentally determined fluorite solubility and speciation mechanisms in haplogranitic melts at 800–950 °C, 100 MPa and aqueous-fluid saturation. The starting haplogranite compositions: peraluminous (alumina saturation index, ASI = 1.2), subaluminous (ASI = 1.0) and peralkaline (ASI = 0.8) were variably doped with CaO or F2O−1 in the form of stoichiometric mineral or glass mixtures. The solubility of fluorite along the fluorite–hydrous haplogranite binaries is low: 1.054 ± 0.085 wt.% CaF2 (peralkaline), 0.822 ± 0.076 wt.% (subaluminous) and 1.92 ± 0.15 wt.% (peraluminous) at 800 °C, 100 MPa and 10 wt.% H2O, and exhibits a minimum at ASI ∼ 1. Fluorite saturation isotherms are strongly hyperbolic in the CaO–F2O−1 space, suggesting that fluorite saturation is controlled by the activity product of CaO and F2O−1, i.e., these components are partially decoupled in the melt structure. The form of fluorite liquidus isotherms implies distinct roles of fluorite crystallization: in Ca-dominant systems, fluorite crystallization is controlled by the fluorine concentration in the melt only and remains nearly independent of calcium contents; in F-rich systems, the crystallization of fluorite is determined by CaO contents and it does not buffer fluorine concentration in the melt. The apparent equilibrium constant, K, for the equilibrium CaO + cF2O−1 = CaF2 (+ associates) is log K= − (2.449 ± 0.085)·Al2O3exc + (4.902 ± 0.066); the reaction-stoichiometry parameter varies as follows: c= − (0.92 ± 0.11)·Al2O3exc + (1.042 ± 0.084) at 800 °C, 100 MPa and fluid saturation where Al2O3exc are molar percent alumina in excess over alkali oxides. The reaction stoichiometry, c, changes at subaluminous composition: in peralkaline melts, competition of other network modifiers for excess fluorine anions leads to the preferential alkali–F short-range order, whereas in peraluminous compositions, excess alumina associates with calcium cations to form calcioaluminate tetrahedra. The temperature dependence of fluorite solubility is described by the binary symmetric Margules parameter, W = 36.0 ± 1.4 kJ (peralkaline), 39.7 ± 0.5 kJ (subaluminous) and 32.8 ± 0.7 kJ (peraluminous). The strong positive deviations from ideal mixing imply the occurrence of CaF2–granite liquid–liquid immiscibility at temperatures above 1258 °C, which is consistent with previous experimental data. These experimental results suggest very low solubilities of fluorite in Ca-rich melts, consistent with the lack of fluorine enrichment in peralkaline rhyolites and calc-alkaline batholiths. On the other hand, high CaO concentrations necessary to crystallize fluorite in F-rich peraluminous melts are not observed in nature and thus magmatic crystallization of fluorite in topaz-bearing silicic suites is suppressed. A procedure for calculating fluorite solubility and the liquidus isotherms for a whole-rock composition and temperature of interest is provided.  相似文献   

14.
(Ni-Sb)-bearing Cu-arsenides are rare minerals within the Mlakva and Kram mining sectors (Boranja ore field) one of the less-known Serbian Cu deposits. (Ni-Sb)-bearing Cu-arsenides were collected from the Mlakva skarn-replacement Cu(Ag,Bi)-FeS polymetallic deposit. The identified phases include β-domeykite, Ni-bearing koutekite and (Ni-Sb)-bearing α-domeykite. (Ni-Sb)-bearing Cu-arsenides are associated with nickeline, arsenical breithauptite, chalcocite, native Ag, native Pb and litharge. Pyrrhotite, pyrite, chalcopyrite, cubanite, bismuthinite, molybdenite, sphalerite, galena, Pb(Cu)-Bi sulfosalts and native Bi, as well as minor magnetite, scheelite and powellite are associated with the sulfide paragenesis. The electron microprobe analyses of the (Ni-Sb)-bearing Cu-arsenides yielded the following average formulae: (Cu2.73,Ni0.17,Fe0.03,Ag0.01)∑ 2.94(As0.98,Sb0.05,S0.02)∑ 1.06–β-domeykite (simplified formula (Cu2.7,Ni0.2)∑ 2.9As1.1); (Cu3.40,Ni1.40,Fe0.11)∑ 4.91(As1.94,Sb0.13,S0.02)∑ 2.08–Ni-bearing koutekite (simplified formula (Cu3.4Ni1.5)∑ 4.9As2.1); and Cu1.97(Ni0.98,Fe0.03)∑ 1.01(As0.81,Sb0.22)∑ 1.03–(Ni–Sb)-bearing α-domeykite (simplified formula Cu2NiAs). The Rietveld refinement yielded the following unit-cell parameters for β-domeykite and Ni–bearing koutekite: a = 7.1331(4); c = 7.3042(5) Å; V = 321.86(2) Å3, and a = 5.922(4); b = 11.447(9); c = 5.480(4) Å; V = 371.48(5) Å3, respectively. Ore geology, paragenetic assemblages and genesis of the Mlakva deposit are discussed in detail and the Cu-As-Ni-Sb-Pb mineralization has been compared with similar well-known global deposits.  相似文献   

15.
《Comptes Rendus Geoscience》2018,350(8):476-486
In this study, the model H(i) = 109.6103 + C1 × F1(i) + C2 × F2(i) + … + C33 × F33(i) obtained from depth modelling based on 33 recent benthic foraminifer species distribution, has been applied to the fossil benthic foraminifers from the borehole GDEC-4-2 drilled at a water depth of 491 m, in the East-Corsica basin, covering the last 550,000 years. The obtained variations of the paleo-depths show a medium correlation with the oscillations of the relative sea level and also with the fluctuations of the oxygen isotopic ratio (δ18O G. bulloides and δ18O C. pachyderma–C. wuellerstorfi). This newly developed transfer function is accompanied by an error margin of ± 86 m, suggesting that this model will probably be more suitable for a time scale of the order of a million years where sea level variations are recorded with larger amplitudes. Without considering these problems related to amplitudes, it also turns out that the “eustatic” signal of the microfauna is accompanied by a “trophic” signal, which should not to be neglected, especially at a millennial scale time resolution. Thus, the application of this method would require taking into account the bottom trophic effects strongly controlling the distribution of benthic foraminifer assemblages.  相似文献   

16.
Physical, physicochemical, and mineralogical-petrographic methods have been applied to samples of ophiolite-hosted chromite ore from different deposits and occurrences in the Urals. Temperature dependences of dielectric loss obtained for nine chromite ore samples consisting of 95–98% Cr spinel show prominent peaks indicating a relaxation origin of the loss. The analyzed samples have the loss peaks at different temperatures depending mainly on H = (FeO/Fe2O3)? : (FeO/Fe2O3)??, where (FeO/Fe2O3)? and (FeO/Fe2O3)?? are, respectively, the ferrous/ferric oxide ratios in the samples before and after heating to 800 °C, and H is thus the heating-induced relative change in the FeO/Fe2O3 ratio. These peak temperatures vary from 550 °C (sample 1, high-Cr chromium spinel with more than 52% Cr2O3) to 750 °C (sample 2, aluminous and magnesian spinel with less than 30% Cr2O3), and H ranges correspondingly from 1.61 to 5.49. The temperature of the loss peaks is related with H as H = 34.30 ? 11.52N + 1.20N2, with an error of σ = 0.19 (N = T · 10?2, T is temperature in °C).  相似文献   

17.
The linkage between the iron and the carbon cycles is of paramount importance to understand and quantify the effect of increased CO2 concentrations in natural waters on the mobility of iron and associated trace elements. In this context, we have quantified the thermodynamic stability of mixed Fe(III) hydroxo-carbonate complexes and their effect on the solubility of Fe(III) oxihydroxides. We present the results of carefully performed solubility measurements of 2-line ferrihydrite in the slightly acidic to neutral–alkaline pH ranges (3.8–8.7) under constant pCO2 varying between (0.982–98.154 kPa) at 25 °C.The outcome of the work indicates the predominance of two Fe(III) hydroxo carbonate complexes FeOHCO3 and Fe(CO3)33−, with formation constants log*β°1,1,1 = 10.76 ± 0.38 and log β°1,0,3 = 24.24 ± 0.42, respectively.The solubility constant for the ferrihydrite used in this study was determined in acid conditions (pH: 1.8–3.2) in the absence of CO2 and at T = (25 ± 1) °C, as log*Ks,0 = 1.19 ± 0.41.The relative stability of the Fe(III)-carbonate complexes in alkaline pH conditions has implications for the solubility of Fe(III) in CO2-rich environments and the subsequent mobilisation of associated trace metals that will be explored in subsequent papers.  相似文献   

18.
Eocene is a critical time for the elevation of Tibetan Plateau and global climate change, and previous studies suggested that the Eocene elevation was caused by intra-continental subduction of the Songpan–Garze block beneath the Qiangtang block. This paper reports zircon U–Pb age and geochemistry of the Eocene volcanic rocks from the Zuerkenwula mountain area in the northern part of Qiangtang block, and proposes that both slab break-off of the Neo-Tethys oceanic slab along the Bangong–Nujiang suture and intra-continental subduction of the Songpan–Garze block beneath the Qiangtang block caused the extensive partial melting of lithospheric mantle and subducted Songpan–Garze continental crust, which resulted in the significant elevation of the Tibetan Plateau. The volcanic rocks have LA-ICP MS U–Pb zircon age of 40.25 ± 0.15 Ma (MSWD = 2.1, 2σ), which is contemporaneous with the Eocene eclogites in the Great Himalayan and K-rich lavas in the southeastern Tibet. They display some adakitic characteristics with SiO2 = 57.44 to 68.72%, TiO2 = 0.38 to 0.81%, Na2O = 2.89 to 4.35%, K2O = 2.77 to 4.48%, Al2O3 = 13.92 to 18.22%, A/CNK = 0.69 to 1.03, MgO = 0.27 to 5.86% with Mg# ranging from 13.2 to 72.0, strongly depleted in heavy rare earth elements (HREEs) (Yb = 0.92 to 1.51 ppm and Y = 10.1 to 24.1 ppm), in combination with their positive Sr anomalies, high Sr/Y ratios and no significant Eu anomalies, which suggest a garnet-in and plagioclase-free source residue. These volcanic rocks can be divided into high-Mg# (> 45) and low-Mg# (< 45) groups. Both of the two groups share evolved Sr–Nd–Pb isotopic compositions with 87Sr/86Sr = 0.707412–0.708284; εNd(t) = ? 4.0 to ? 5.7; 206Pb/204Pb = 18.7499–18.8189, 207Pb/204Pb = 15.7189–15.7384; 208Pb/204Pb = 39.166–39.262. The geophysical data and regional geological setting suggest that the low-Mg# adakitic rocks were derived from the decompression melting of a subducted lower continental crust, when low-Mg# adakitic melts in the overlying peridotite mantle wedge captured some olivine crystals, resulting in their elevated Mg# and MgO values.  相似文献   

19.
The origin of high-Mg adakitic granitoids in collisional orogens can provide important information about the nature of the lower crust and upper mantle during the orogenic process. Late-Triassic high-Mg adakitic granite and its mafic enclaves from the Dongjiangkou area, the Qinling orogenic belt, central China, were derived by partial melting of subducted continental crust and underwent interaction with the overlying mantle wedge peridotite. Adakitic affinity of the different facies of the Dongjiangkou granite body are: high Sr, Ba, high La/Yb and Sr/Y, low Y,Yb, Yb/Lu and Dy/Yb, and no significant Eu anomalies, suggesting amphibole + garnet and plagioclase-free restite in their source region. Evolved Sr-Nd-Pb isotopic compositions [(87Sr/86Sr)i = 0.7050 to 0.7055,εNd(t) = –6.6 to –3.3; (206Pb/204Pb)i = 17.599 to 17.799, (207Pb/204Pb)i = 15.507 to 15.526, (208Pb/204Pb)i = 37.775 to 37.795] and high K2O, Rb, together with a large variation in zircon Hf isotopic composition (εHf(t) = ?9.8 to + 5.0), suggest that the granite was derived from reworking of the ancient lower continental crust. CaO, P2O5, K2O/Na2O, Cr, Ni, Nb/Ta, Rb/Sr and Y increase, and SiO2, Sr/Y and Eu/Eu* decrease with increasing MgO, consistent with interaction of primitive adakitic melt and overlying mantle peridotite. Zircons separated from the host granites have U-Pb concordia ages of 214 ± 2 Ma to 222 ± 2 Ma, compatible with exhumation ages of Triassic UHP metamorphic rocks in the Dabie orogenic belt. Mafic microgranular enclaves and mafic dykes associated with the granite have identical zircon U-Pb ages of 220 Ma, and are characterized by lower SiO2, high TiO2, Mg# and similar evolved Sr-Nd-Pb isotopic composition. Zircons from mafic microgranular enclaves (MMEs) and mafic dykes also show a large variation in Hf isotopic composition with εHf(t) between ?11.3 and + 11.3. It is inferred that they were formed by partial melting of enriched mantle lithosphere and contaminated by the host adakitic granite magma.In combination with the regional geology, high-Mg# adakitic granitoid rocks in the Dongjiangkou area are considered to have resulted from interaction between subducted Yangtze continental crust and the overlying mantle wedge. Triassic continental collision caused detachment of the Yangtze continental lithosphere subducted beneath the North China Craton, at ca. 220 Ma causing asthenosphere upwelling and exhumation of the continental crust. Triassic clockwise rotation of the Yangtze Craton caused extension in the Dabie area which led to rapid exhumation of the subducted continental lithosphere, while compression in the Qinling area and high-P partial melting (amphibole ± garnet stability field) of the subducted continental crust produced adakitic granitic magma that reacted with peridotite to form Mg-rich hybrid magma.  相似文献   

20.
The latest hydraulic fracturing and stress relief measurement data in the Chinese mainland were collected. The total of 3856 data entries are measured at 1474 locations. The measured area covers 75–130°E and 18–47°N, and the depth range varies from surface to 4000 meters depth, which generally includes each active tectonic block of China and each segment of North–South seismic belt. We investigated the tectonic stress field by removing the effect of gravity. For this, we assume lateral constraints and Heim’s rule. The gravity contribution is removed by using the assumption of lateral constraint and Heim’s rule. Our results show: (1) the maximum and the minimum horizontal principal stress σH, σh and the vertical stress σV in the shallow crust of China all increase linearly with depth: σH = 0.0229D + 4.738, σh = 0.0171D + 1.829, σV = 0.0272D. Maximum and minimum horizontal tectonic stress varies as a function of depth D linearly 4.738 < σT < 0.0139D + 4.738 and 1.829 < σt < 0.0162D + 1.829. The horizontal tectonic differential stress is σT − σt = 0.0058D + 2.912. (2) The intermediate value of σT1 (regression value of tectonic stress inferred from the assumption of lateral constraint at 2000 m depth) changes in different areas, the maximum value of which is 45.6 MPa, while the minimum value of which is 26.8 MPa. Horizontal tectonic differential stress σT − σt increases linearly with depth and the maximum and minimum of σT − σt is 25.3 MPa and 13.0 MPa, respectively. In general, the stress magnitude is much higher in western than in eastern China. This indicates that the strong Indo-Eurasian collision dominates the present tectonic stress field in Chinese mainland. (3) Compared with other study regions, the northward crustal compression to the Qinghai-Tibet block is relatively lower in magnitude in the shallow subsurface and higher at deeper depth. (4) The orientations of σT in China mainland generally form a radial scattering pattern centered in Tibetan Plateau. From western to eastern China, they rotate gradually clockwise from NS to NNE, NE, NEE, and SE, which is consistent with the result of focal mechanism solutions.  相似文献   

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