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1.
High-resolution pollen analyses ( 50 yr) from sediment cores retrieved at Chernyshov Bay in the NW Large Aral Sea record shifts in vegetational development from subdesertic to steppe vegetation in the Aral Sea basin during the late Holocene. Using pollen data to quantify climatic parameters, we reconstruct and date for the first time significant changes in moisture conditions in Central Asia during the past 2000 yr. Cold and arid conditions prevailed between ca. AD 0 and 400, AD 900 and 1150, and AD 1500 and 1650 with the extension of xeric vegetation dominated by steppe elements. These intervals are characterized by low winter and summer mean temperatures and low mean annual precipitation (Pmm < 250 mm/yr). Conversely, the most suitable climate conditions occurred between ca. AD 400 and 900, and AD 1150 and 1450, when steppe vegetation was enriched by plants requiring moister conditions (Pmm  250–500 mm/yr) and some trees developed. Our results are fairly consistent with other late Holocene records from the eastern Mediterranean region and the Middle East, showing that regional rainfall in Central Asia is predominantly controlled by the eastern Mediterranean cyclonic system when the North Atlantic Oscillation (NAO) is in a negative phase.  相似文献   

2.
Changing content of detrital input in laminated sediments traced by XRF scanning and microfacies analyses reflect prominent variations in sedimentation processes in the Aral Sea. A high-resolution record of titanium from a core retrieved in the northwestern Large Aral Sea allows a continuous reconstruction of wind strength and frequency in western Central Asia for the past 1500 yr. During AD 450–700, AD 1210–1265, AD 1350–1750 and AD 1800–1975, detrital inputs (bearing titanium) are high, documenting an enhanced early spring atmospheric circulation associated with an increase in intensity of the Siberian High pressure system over Central Asia. In contrast, lower titanium content during AD 1750–1800 and AD 1980–1985 reflects a diminished influence of the Siberian High during early spring with a reduced atmospheric circulation. A moderate circulation characterizes the time period AD 700–1150. Unprecedented weakened atmospheric circulation over western Central Asia are inferred during ca. AD 1180–1210 and AD 1265–1310 with a considerable decrease in dust storm frequency, sedimentation rates, lamination thickness and detrital inputs (screened at 40-μm resolution). Our results are concurrent with changes in the intensity of the Siberian High during the past 1400 yr as reported in the GISP2 Ice Core from Greenland.  相似文献   

3.
We have studied the paleomagnetism of the middle Cretaceous Iritono granite of the Abukuma massif in northeast Japan together with 40Ar–39Ar dating. Paleomagnetic samples were collected from ten sites of the Iritono granite (102 Ma 40Ar–39Ar age) and two sites of its associated gabbroic dikes. The samples were carefully subjected to alternating field and thermal demagnetizations and to rock magnetic analyses. Most of natural remanent magnetizations show mixtures of two components: (1) H component, high coercivity (Bc > 50–90 mT) or high blocking temperature (Tb > 350–560 °C) component and (2) L component, relatively low Bc or low Tb component. H component was obtained from all the 12 sites to give a mean direction of shallow inclination and northwesterly declination (I = 29.9°, D = 311.0°, α95 = 2.7°, N = 12). This direction is different from the geocentric axial dipole field at the present latitude (I = 56.5°) and the typical direction of the Cenozoic remagnetization in northeast Japan. Since rock magnetic properties indicate that the H component of the Iritono granite is carried mainly by magnetite inclusions in plagioclase, this component probably retains a primary one. Thus the shallow inclination indicates that the Abukuma massif was located at a low latitude (16.1 ± 1.6°N) about 100 Ma and then drifted northward by about 20° in latitude. The northwesterly deflection is attributed mostly to the counterclockwise rotation of northeast Japan due to Miocene opening of the Japan Sea. According to this model, the low-pressure and high-temperature (low-P/high-T) metamorphism of the Abukuma massif, which has been well known as a typical location, would have not occurred in the present location. On the other hand, the L component is carried mainly by pyrrhotite and its mean direction shows a moderate inclination and a northwesterly declination (I = 42.8°, D = 311.5°, α95 = 3.3°, N = 9). Since this direction is intermediate between the H component and early Cenozoic remagnetization in northeast Japan, some thermal event would have occurred at lower temperature than pyrrhotite Curie point ( 320 °C) during the middle Cretaceous to early Cenozoic time to have resulted in partial remagnetization.  相似文献   

4.
Systematic geochronologic, geochemical, and Nd isotopic analyses were carried out for an early Paleoproterozoic high-K intrusive complex exposed in southwestern Tarim, NW China. The results provide a better understanding of the Paleoproterozoic tectonic evolution of the Tarim Block. Zircon U–Pb age dating indicates two Paleoproterozoic magmatic episodes occurring at ca. 2.41 Ga and ca. 2.34 Ga respectively, which were followed by a ca. 1.9 Ga metamorphic event. The 2.41 Ga granodiorite–adamellite suite shares characteristics of late to post-orogenic metaluminous A-type granites in its high alkalinity (Na2O + K2O = 7.6–9.3%), total REE (410–788 ppm), Zr (370–660 ppm), and Y (21.7–58.4 ppm) contents. εNd(t) values for the suite range from − 3.22 to − 4.71 and accordingly the Nd modal ages (T2DM) vary between 3.05 Ga and 3.17 Ga. Based on geochemical data, the 2.34 Ga suite can be subdivided into two sub-suites, namely A-type and S-type. However, both types have comparable Nd isotope compositions (εNd(t) ≈ − 0.41 to − 2.08) and similar narrow T2DM ranges (2.76–2.91 Ga).Geochemical and Nd isotopic data for the high-K intrusive complex, in conjunction with the regional geological setting, suggest that both the 2.41 Ga suite and the 2.34 Ga A-type sub-suite might have been produced by partial melting of the Archean mafic crust in a continental rift environment. The S-type sub-suite is thought to have formed by partial melting of felsic pelites and/or metagreywackes recycled from Archean crust (TTG?). Gabbro enclaves with positive εNd(t) value (2.15) have been found to be intermingling within the 2.34 Ga suite; ca. 2.34–2.36 Ga gabbroic dykes and adamellites have previously been documented in eastern Tarim. These observations indicate that the high-K intrusions may reflect the emergence of depleted mantle upwelling beneath the Tarim Block at that time. We suggest a three-stages model for the Precambrian crustal evolution in the Tarim Block: (1) the formation of proto-crust (TTG) by ca. 2.5 Ga, (2) episodes of felsic magmatism possibly occurring in continental rift environments at ca. 2.41 Ga and ca. 2.34–2.36 Ga, and (3) ca. 1.9 Ga metamorphism that may represent the solidification of the Precambrian basement of the Tarim Block.  相似文献   

5.
A strategy to neutralize acidic pit lakes was tested in an upscaling process using field mesocosms of 26 to ca. 4500 m3 volume in the acidic pit Mining Lake 111 in Germany. After addition of the substrates Carbokalk and straw a neutral sediment layer formed, in which microbial sulfate and iron reduction as well as sulfide precipitation occurred. The net rate of neutralization was limited by the precipitation of iron sulfides rather than by microbial reactions. Oxidation of H2S by ferric iron in the anoxic sediment lowered the net sulfate reduction rate. Seasonal fluctuations of iron sulfides in the sediment showed that the reaction products were not necessarily stable. The long-term success of the approach depends on the net partition of the precipitated iron-(mono-/di-) sulfide that is permanently buried in the anoxic sediment. It could be shown by field experiments that the long-term success of the neutralization depends on the spatial scale and duration of the experiments. Volumes from 26 to 4500 m3, exposition times from 4 months to 5 years, and increasingly thick coverings of the sediments with straw, from zero to 40 cm, were used. Net neutralization rates decreased from 41 meq m− 2 d− 1 in laboratory microcosms to a mean rate of 2.3 meq m− 2 d− 1 in the 4500 m3 field experiment. The results show that the success of the microbial treatment of acid pit lakes lastly depends on the limnological conditions in the lake that cannot be simulated by upscaling of simple laboratory experiments.  相似文献   

6.
Twenty-eight samples of peat, peaty lignites and lignites (of both matrix and xylite-rich lithotypes) and subbituminous coals have been physically activated by pyrolysis. The results show that the surface area of the activated coal samples increases substantially and the higher the carbon content of the samples the higher the surface area.The adsorption capacity of the activated coals for NO, SO2, C3H6 and a mixture of light hydrocarbons (CH4, C2H6, C3H8 and C4H10) at various temperatures was measured on selected samples. The result shows a positive correlation between the surface area and the gas adsorption. In contrast, the gas adsorption is inversely correlated with the temperature. The maximum recorded adsorption values are: NO = 8.22 × 10− 5 mol/g at 35 °C; SO2 = 38.65 × 10− 5 mol/g at 60 °C; C3H6 = 38.9 × 10− 5 mol/g at 35 °C; and light hydrocarbons = 19.24 × 10− 5 mol/g at 35 °C. Adsorption of C3H6 cannot be correlated with either NO or SO2. However, there is a significant positive correlation between NO and SO2 adsorptions. The long chain hydrocarbons are preferentially adsorbed on activated lignites as compared to the short chain hydrocarbons.The results also suggest a positive correlation between surface area and the content of telohuminite maceral sub-group above the level of 45%.  相似文献   

7.
This paper describes the geomechanical back-analysis of the flexural failure of an overhanging limestone slab (volume = 68 m3) that collapsed on 26/01/1999 on the abandoned Cellina Valley state road SS 251 (western Friuli, NE Italy). The survey carried out on site examining the contact surfaces between the block and the slope has ascertained that the rock slab was connected to the stable rock mass by means of a single great rock bridge. The detailed reconstruction of the slab geometry and of the actual restraint conditions acting before the rockfall permitted an estimation of the stress state by using a 3D finite element code (Strand7). The finite element numerical model, referred to the restrained cross-section measured on site (Ares = 2.82 m2), gives a maximum value of σt = 5.19 MPa for the tensile stress, which is in good agreement with the estimated value for the characteristic strength mobilized at rupture by the intact rock material (T0 = 3.5–5.5 MPa). Considerably lower maximum tensile stress (σt = 2.25 MPa), smaller than one half, is obtained if the whole contact surface is assumed as resisting surface (Ares = 10.50 m2) and the rock bridge is neglected.  相似文献   

8.
The Yenice–Gönen Fault (YGF) is one of the most important active tectonic structures in the Biga peninsula. On March 18, 1953, a destructive earthquake (Mw = 7.2) occurred on the YGF, which is considered to be a part of the southern branch of the North Anatolian Fault Zone (NAFZ). A 70 km-long dextral surface rupture formed during the Yenice–Gönen Earthquake (YGE).In this study, structural and palaeoseismological features of the YGF have been investigated. The YGF surface ruptures have been mapped and three trenches were excavated at Muratlar, Karaköy and Seyvan sites.According to the palaeoseismic interpretation and the results of 14C AMS dating, Seyvan trench shows that an earthquake of palaeoseismic age ca. 620 AD ruptured a different strand of the 1953 fault, producing rather significant surface rupture displacement, while there are indications that at least two older events occurred during the past millennia. Another set of trenches excavated near Gönen town (Muratlar village) revealed extensive liquefaction not only during the 1953 event, but also during a previous earthquake, dated at 1440 AD. The Karaköy trench shows no indications of recent reactivations.Based on the trenching results, we estimate a recurrence interval of 660 ± 160 years for large morphogenic earthquakes, creating linear surface ruptures. The maximum reported displacement during the 1953 earthquake was 4.2 m. Taking into account the palaeoseismologically determined earthquake recurrence interval and maximum displacement, slip-rate of the YGF has been calculated to be 6.3 mm/a, which is consistent with present-day velocities determined by GPS measurements. According to the geological investigations, cumulative displacement of the YGF is 2.3 km. This palaeoseismological study contributes to model the behaviour of large seismogenic faults in the Biga Peninsula.  相似文献   

9.
Several methods were evaluated and compared for the estimation of pyrite oxidation rates (POR) in waste rock at Mine Doyon, Quebec, Canada. Methods based on data collected in situ, such as the interpretation of temperature and oxygen concentration profiles (TOP) measured in the waste rock pile and pyrite mass balance (PMB) on solid phase samples were compared with the oxygen consumption measurements (OCM) in closed chamber in the laboratory. A 1-D analytical solution to a gas and heat transport equation used temperature and oxygen profiles (TOP) measured in the pile for the preliminary POR estimates at a site close to the slope of the pile (Site 6) and in the core of the pile (Site 7). Resulting POR values were 1.1 × 10− 9 mol(O2) kg− 1 s− 1 and 1.0 × 10− 10 mol(O2) kg− 1 s− 1 for the slope site and the core site, respectively. Oxidation rates based on pyrite mass balance (PMB) calculations for solid samples were 2.21 × 10− 9 mol(O2) kg− 1 s− 1 and 2.03 × 10− 9 mol(O2) kg− 1 s− 1, respectively, for the same slope and core sites, but the difference between sites was within the error margin. The OCM measurements in the laboratory on fresh waste rock samples yielded higher POR values than field methods, with average oxidation rate of 6.7 × 10− 8 mol(O2) kg− 1 s− 1. However, the OCM results on weathered and decomposed material from the rock stockpile (average oxidation rate 3.4 × 10− 9 mol(O2) kg− 1 s− 1) were consistent with results from the field-based estimates. When POR values based on fresh material are excluded, the remaining POR values for all methods range from 1.0 × 10− 10 to 3.4 × 10− 9 mol(O2) kg− 1 s− 1. The lowest estimated value (1.0 × 10− 10 mol(O2) kg− 1 s− 1) was based on TOP estimates in the interior of the pile where oxygen transport was limited by diffusion from the surface. These results suggest that small-scale OCM laboratory experiments may provide relatively representative values of POR in the zones of waste rock piles in which oxygen transport is not dominated by diffusion.  相似文献   

10.
Shoshonitic series volcanic rocks (SSVR) and adakites are widely distributed in the Permian terrestrial volcanic strata of the Yishijilike–Awulale range of west Tianshan, north Xinjiang, China. Isotopic dating yields Permian ages of 280–250 Ma. The SSVR include absarokite, shoshonite and banakite which are characterized by enrichment of alkalis, particularly in K, combined with lower Ti, higher Al (A/NKC = 0.70–0.99, metaluminous) and Fe2O3 > FeO. The SSVR that are rich in LILE with high REE contents and Eu/Eu range from 0.59 to 1.30. They are rich in LREE ((La/Yb)N 2.15–11.97) and depleted in Nb, Ta and Ti (TNT negative anomalies). The adakites are metaluminous to weakly peraluminous (A/NKC = 0.85-1.16) and belong to the high-SiO2 type of adakite (HSA, SiO2 = 62%–71%). They are characterized by lower ΣREE with strong LREE enrichment ((La/Yb)N 13–35). Pronounced positive Eu anomalies (Eu/Eu = 1.02–1.27), very low Yb contents and distinct TNT-negative anomalies are evident. The SSVR have εNd(t) (+ 1.28 to + 4.92) and (87Sr/86Sr)i (0.7041–0.7057) that are similar to adakites in the regions which are characterized by εNd(t) = 0.95 to + 5.69 and (87Sr/86Sr)i = 0.7050–0.7053. Trace element, REE and Sr/Nd isotopic compositions suggest that both SSVR and adakites possess similar source regions associated with underplated mantle-derived basaltic materials. Lithosphere extension driven by magmatic underplating was responsible for the generation of both the SSVR and adakites. This magmatism serves as a petrological indicator of underplating during the Permian. Obviously thickened crust (62–52 km), a complex Moho discontinuity, high heat flow (~ 100 mw·m− 2), widespread contemporary alkali-rich granites, basic dike swarms (K–Ar ages of 187–271 Ma, Ar–Ar ages of 174–270 Ma and Rb–Sr ages of 255 ± 28 Ma; εNd(t) + 1.84 to + 10.1; (87Sr/86Sr)i 0.7035 and 0.7065), and basic granulites (SHRIMP U–Pb age of 268–279 ± 5.6 Ma) provide additional evidences for the underplating event in this area during Permian.  相似文献   

11.
The Qingchengzi orefield in northeastern China, is a concentration of several Pb–Zn, Ag, and Au ore deposits. A combination of geochronological and Pb, Sr isotopic investigations was conducted. Zircon SHRIMP U–Pb ages of 225.3 ± 1.8 Ma and 184.5 ± 1.6 Ma were obtained for the Xinling and Yaojiagou granites, respectively. By step-dissolution Rb–Sr dating, ages of 221 ± 12 Ma and 138.7 ± 4.1 Ma were obtained for the sphalerite of the Zhenzigou Zn–Pb deposit and pyrargyrite of the Ag ore in the Gaojiabaozi Ag deposit, respectively. Pb isotopic ratios of the Ag ore at Gaojiabaozi (206Pb/204Pb = 18.38 to 18.53) are higher than those of the Pb–Zn ores (206Pb/204Pb = 17.66 to 17.96; Chen et al. [Chen, J.F., Yu, G., Xue, C.J., Qian, H., He, J.F., Xing, Z., Zhang, X., 2005. Pb isotope geochemistry of lead, zinc, gold and silver deposit clustered region, Liaodong rift zone, northeastern China. Science in China Series D 48, 467–476.]). Triassic granites show low Pb isotopic ratios (206Pb/204Pb = 17.12 to 17.41, 207Pb/204Pb = 15.47 to 15.54, 208Pb/204Pb = 37.51 to 37.89) and metamorphic rocks of the Liaohe Group have high ratios (206Pb/204Pb = 18.20 to 24.28 and 18.32 to 20.06, 207Pb/204Pb = 15.69 to 16.44 and 15.66 to 15.98, 208Pb/204Pb = 37.29 to 38.61 and 38.69 to 40.00 for the marble of the Dashiqiao Formation and schist of the Gaixian Formation, respectively).Magmatic activities at Qingchengzi and in adjacent regions took place in three stages, and each contained several magmatic pulses: ca. 220 to 225 Ma and 211 to 216 Ma in the Triassic; 179 to 185 Ma, 163 to 168 Ma, 155 Ma and 149 Ma in the Jurassic, as well as ca. 140 to 130 Ma in the Early Cretaceous. The Triassic magmatism was part of the Triassic magmatic belt along the northern margin of the North China Craton produced in a post-collisional extensional setting, and granites in it formed by crustal melting induced by mantle magma. The Jurassic and Early Cretaceous magmatism was related to the lithospheric delamination in eastern China. The Triassic is the most important metallogenic stage at Qingchengzi. The Pb–Zn deposits, the Pb–Zn–Ag ore at Gaojiabaozi, and the gold deposits were all formed in this stage. They are temporally and spatially associated with the Triassic magmatic activity. Mineralization is very weak in the Jurassic. Ag ore at Gaojiabaozi was formed in the Early Cretaceous, which is suggested by the young Rb–Sr isochron age, field relations, and significantly different Pb isotopic ratios between the Pb–Zn–Ag and Ag ores. Pb isotopic compositions of the Pb–Zn ores suggest binary mixing for the source of the deposits. The magmatic end-member is the Triassic granites and the other metamorphic rocks of the Liaohe Group. Slightly different proportions of the two end-members, or an involvement of materials from hidden Cretaceous granites with slightly different Pb isotopic ratios, is postulated to interpret the difference of Pb isotopic compositions between the Pb–Zn–(Ag) and Ag ores. Sr isotopic ratios support this conclusion. At the western part of the Qingchengzi orefield, hydrothermal fluid driven by the heat provided by the now exposed Triassic granites deposited ore-forming materials in the low and middle horizons of the marbles of the Dashiqiao Formation near the intrusions to form mesothermal Zn–Pb deposits. In the eastern part, hydrothermal fluids associated with deep, hidden Triassic intrusions moved upward along a regional fault over a long distance and then deposited the ore-forming materials to form epithermal Au and Pb–Zn–Ag ores. Young magmatic activities are all represented by dykes across the entire orefield, suggesting that the corresponding main intrusion bodies are situated in the deep part of the crust. Among these, only intrusions with age of ca. 140 Ma might have released sufficient amounts of fluid to be responsible for the formation of the Ag ore at Gaojiabaozi.Our age results support previous conclusions that sphalerite can provide a reliable Rb–Sr age as long as the fluid inclusion phase is effectively separated from the “sulfide” phase. Our work suggests that the separation can be achieved by a step-resolution technique. Moreover, we suggest that pyrargyrite is a promising mineral for Rb–Sr isochron dating.  相似文献   

12.
A novel one-step hydrothermal synthesis of 11 Å tobermorite, a cation exchanger, from a unique combination of waste materials is reported. 11 Å tobermorite was prepared from stoicheiometric quantities of cement bypass dust and waste container glass at 100 °C in water. The product also comprised 10 wt.% calcite and trace quartz as residual parent phases from the cement bypass dust. In a batch sorption study at 20 °C the uptakes of Cd2+ and Pb2+ by the waste-derived tobermorite product were found to be 171 mg g− 1 and 467 mg g− 1, respectively, and in both cases the removal process could be described using a simple pseudo-second-order rate model (k2 = 2.30 × 10− 5 g mg− 1 min− 1 and 5.09 × 10− 5 g mg− 1 min− 1, respectively). The sorption characteristics of the 11 Å tobermorite are compared with those of other waste-derived sorbents and potential applications are discussed.  相似文献   

13.
176 vertical-component, short period observations from aftershocks of the Mw 7.7, 26 January, 2001 Kachchh earthquake are used to estimate seismic wave attenuation in western India using uniform and two layer models. The magnitudes (Mw) of the earthquakes are less than 4.5, with depths less than 46 km and hypocentral distances up to 110 km. The studied frequencies are between 1 and 30 Hz. Two seismic wave attenuation factors, intrinsic absorption (Qi− 1) and scattering attenuation (Qs− 1) are estimated using the Multiple Lapse Time Window method which compares time integrated seismic wave energies with synthetic coda wave envelopes for a multiple isotropic scattering model. We first assume spatial uniformity of Qi− 1, Qs− 1 and S wave velocity (β). A second approach extends the multiple scattering hypothesis to media consisting of several layers characterized by vertically varying scattering coefficient (g), intrinsic absorption strength (h), density of the media (ρ) and shear wave velocity structure. The predicted coda envelopes are computed using Monte Carlo simulation. Results show that, under the assumption of spatial uniformity, scattering attenuation is greater than intrinsic absorption only for the lowest frequency band (1 to 2 Hz), whereas intrinsic absorption is predominant in the attenuation process at higher frequencies (2 to 30 Hz). The values of Q obtained range from Qt = 118, Qi = 246 and Qs = 227 at 1.5 Hz to Qt ≈ 4000, Qi ≈ 4600 and Qs ≈ 33,300 at 28 Hz center frequencies, being Qt− 1 a measure of total attenuation. Results also show that Qi− 1, Qs− 1 and Qt− 1 decrease proportional to f−ν. Two rates of decay are clearly observed for the low (1 to 6 Hz) and high (6 to 30 Hz) frequency ranges. Values of ν are estimated as 2.07 ± 0.05 and 0.44 ± 0.09 for total attenuation, 1.52 ± 0.21 and 0.48 ± 0.09 for intrinsic absorption and 3.63 ± 0.07 and 0.06 ± 0.08 for scattering attenuation for the low and high frequency ranges, respectively. Despite the lower resolution in deriving the attenuation parameters for a two layered crust, we find that scattering attenuation is comparable to or smaller than the intrinsic absorption in the crust whereas intrinsic absorption dominates in the mantle. Also, for a crustal layer of thickness 42 km, intrinsic absorption and scattering estimates in the crust are lower and greater than those of the mantle, respectively.  相似文献   

14.
The western Pacific region has been refrigerated by the subducting cold oceanic plates since 450 Ma. However, the region is also characterized by the presence of many oceanic microplates less than 1300 km across, as well as active magmatism; the Philippine Sea plate is representative. We have compiled and examined petrochemical characters of drilled basalts of DSDP from the Philippine Sea plate, and conclude that the source mantle for oceanic basalts is rich in water ca. 0.2 wt.%, and is 50–60 °C lower than that for MORB. The extensive melting is due to the high water content in the source mantle.It is well known that some marginal basins apparently have greater depths than the major oceans. We calculated the age–depth correlation based on a model of transient half-space cooling at given parameters of temperatures of mantle and surface, 1280 and 0 °C, and the thermal diffusivity, 1 mm2 s− 1. The calculation shows the correlation of age-residual depth from a mid-oceanic ridge is 367 for the Philippine Sea, consistent with the bathymetric data. Moreover, the mid-oceanic ridge may be relatively deep because this region is underlain by the cooler mantle.Addition of water to the mantle peridotite lowers the solidus temperature and viscosity. Melting experiments of hydrous peridotite show that addition of 0.2 wt.% H2O content lowers the solidus temperature by 150 °C. As a result, the mantle under the region may practically correspond to a ca. 90 °C hotter mantle than normal MORB-source mantle in terms of magmatism and rheology. Numerical simulation for a hotter mantle suggests that many small plates should be formed because of extensive heat release by active magmatism, consistent with many microplates in this region. The presence of many oceanic microplates may be analogous to Archean plate tectonics, characterized by a hotter mantle.  相似文献   

15.
A geochemical and isotopic study was carried out for the Mesozoic Yangxin, Tieshan and Echeng granitoid batholiths in the southeastern Hubei Province, eastern China, in order to constrain their petrogenesis and tectonic setting. These granitoids dominantly consist of quartz diorite, monzonite and granite. They are characterized by SiO2 and Na2O compositions of between 54.6 and 76.6 wt.%, and 2.9 to 5.6 wt.%, respectively, enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE), and relative depletion in Y (concentrations ranging from 5.17 to 29.3 ppm) and Yb (0.34–2.83 ppm), with the majority of the granitoids being geochemically similar to high-SiO2 adakites (HSA). Their initial Nd (εNd = − 12.5 to − 6.1) and Sr ((87Sr/86Sr)i = 0.7054–0.7085) isotopic compositions, however, distinguish them from adakites produced by partial melting of subducted slab and those produced by partial melting of the lower crust of the Yangtze Craton in the Late Mesozoic. The granitoid batholiths in the southeastern Hubei Province exhibit very low MgO ranging from 0.09 to 2.19 wt.% with an average of 0.96 wt.%, and large variations in negative to positive Eu anomalies (Eu/Eu = 0.22–1.4), especially the Tieshan granites and Yangxin granite porphyry (Eu/Eu = 0.22–0.73). Geochemical and Nd–Sr isotopic data demonstrate that these granitoids originated as partial melts of an enriched mantle source that experienced significant contamination of lower crust materials and fractional crystallization during magma ascent. Late Mesozoic granitoids in the southeastern Hubei Province of the Middle–Lower Yangtze River belt were dominantly emplaced in an extensional tectonic regime, in response to basaltic underplating, which was followed by lithospheric thinning during the early Cretaceous.  相似文献   

16.
We report analyses of noble gases and Nd–Sr isotopes in mineral separates and whole rocks of late Pleistocene (< 0.2 Ma) monzonites from Ulleungdo, South Korea, a volcanic island within the back arc basin of the Japan island arc. A Rb–Sr mineral isochron age for the monzonites is 0.12 ± 0.01 Ma. K–Ar biotite ages from the same samples gave relatively concordant ages of 0.19 ± 0.01and 0.22 ± 0.01 Ma. 40Ar/39Ar yields a similar age of 0.29 ± 0.09 Ma. Geochemical characteristics of the felsic plutonic rocks, which are silica oversaturated alkali felsic rocks (av., 12.5 wt% in K2O + Na2O), are similar to those of 30 alkali volcanics from Ulleungdo in terms of concentrations of major, trace and REE elements. The initial Nd–Sr isotopic ratios of the monzonites (87Sr/86Sr = 0.70454–0.71264, 143Nd/144Nd = 0.512528–0.512577) are comparable with those of the alkali volcanics (87Sr/86Sr = 0.70466–0.70892, 143Nd/144Nd = 0.512521–0.512615) erupted in Stage 3 of Ulleungdo volcanism (0.24–0.47 Ma). The high initial 87Sr/86Sr values of the monzonites imply that seawater and crustally contaminated pre-existing trachytes may have been melted or assimilated during differentiation of the alkali basaltic magma.A mantle helium component (3He/4He ratio of up to 6.5 RA) associated with excess argon was found in the monzonites. Feldspar and biotite have preferentially lost helium during slow cooling at depth and/or during their transportation to the surface in a hot host magma. The source magma noble gas isotopic features are well preserved in fluid inclusions in hornblende, and indicate that the magma may be directly derived from subcontinental lithospheric mantle metasomatized by an ancient subduction process, or may have formed as a mixture of MORB-like mantle and crustal components. The radiometric ages, geochemical and Nd–Sr isotopic signatures of the Ulleungdo monzonites as well as the presence of mantle-derived helium and argon, suggests that these felsic plutonic rocks evolved from alkali basaltic magma that formed by partial melting of subcontinental lithospheric mantle beneath the back arc basin located along the active continental margin of the southeastern part of the Eurasian plate.  相似文献   

17.
J.D.A. Piper   《Tectonophysics》2007,432(1-4):133-157
The Southern Uplands terrane is an Ordovician–Silurian back-arc/foreland basin emplaced at the northern margin of the Iapetus Ocean and intruded by granite complexes including Loch Doon (408.3 ± 1.5 Ma) during Early Devonian times. Protracted cooling of this 130 km3 intrusion recorded magnetic remanence comprising a predominant (‘A’) magnetisation linked to initial cooling with dual polarity and mean direction D / I = 237 / 64° (α95 = 4°, palaeopole at 316°E, 21°N). Subsidiary magnetisations include Mesozoic remanence correlating with extensional tectonism in the adjoining Irish Sea Basin (‘B’, D / I = 234/− 59°) and minority populations (‘C’, D / I = 106/− 2° and ‘D’, D / I = 199/1°) recording emplacement of younger ( 395 Ma) granites in adjoining terranes and the Variscan orogenic event. The ‘A’ directions have an arcuate distribution identifying anticlockwise rotation during cooling. A comparable rotation is identified in the Orthotectonic Caledonides to the north and the Paratectonic Caledonides to the south following closure of Iapetus. Continental motion from midsoutherly latitudes ( 40°S) at 408 Ma to equatorial palaeolatitudes by  395 Ma is identified and implies minimum rates of continental movement between 430 and 390 Ma of 30–70 cm/year, more than double maximum rates induced by plate forces and interpreted as a signature of true polar wander. Silurian–Devonian palaeomagnetic data from the British–Scandinavian Caledonides define a 430–385 Ma closed loop comparable to the distributed contemporaneous palaeomagnetic poles from Gondwana. They reconcile pre-430 Ma and post-380 Ma APW from this supercontinent and show that Laurentia–Baltica–Avalonia lay to the west of South America with a relict Rheic Ocean opening to the north which closed to produce Variscan orogeny by a combination of pivotal closure and right lateral transpression.  相似文献   

18.
Starting from 1961, the Aral Sea, a major saline lake in Central Asia, has been continuously shrinking because of deficiency in its water budget. Accordingly, the salinity of the once brackish lake increased by a factor of magnitude. During the desiccation, the salt composition of the Aral Sea has been subject to continuous changes because of chemical precipitation accompanying the salinity buildup. This paper provides a summary of these changes based on water samples collected from the so-called Large Aral Sea during the field surveys of 2002–2007. Once fully ventilated, the lake developed anoxic conditions and H2S contamination is frequently observed in the bottom layers. However, hydrogen sulfide is a variable rather than a permanent feature of the present Aral Sea. Because of the precipitation of calcium carbonate, gypsum, and, possibly, mirabilite, which successively occurred as the salinity increased, the relative content of SO4 ? and Ca2+ ions decreased. Accordingly, compared with the pre-desiccation period before 1960, the sulfate-to-chloride mass ratio decreased by 10–30%, while the relative content of calcium decreased almost 7-fold. The depletion in calcium is more pronounced in the shallow eastern part of the lake, where salinity is much higher. However, the reduction of the sulfate-to-chloride ratio in the eastern basin is smaller than that for the western basin of the Aral Sea. Hypothetically, this could be explained through precipitation of halite already taking place in the eastern basin, but not yet in the western basin. Vertical profiles of the ionic content in the relatively deep western part of the lake reveal a decrease of calcium content and relative increase of sulfate ion content toward the bottom, which is consistent with the previously published concept that the bottom layers of the western trench contain a significant admixture of the water advected from the eastern basin.  相似文献   

19.
The Mapocho river, which crosses downtown Santiago, is one of the most important rivers in contact with a population of about six million inhabitants. Anthropogenic activities, industrialization, farming activities, transport, urbanization, animal and human excretions, domestic wastes and copper mining have affected the river, contaminating it and its sediments with heavy metals. Concentration and distribution of Cu, Zn, Pb and Cd were studied with the purpose of determining their bioavailability and their relation with the characteristics of the sediments. Freshly deposited seasonal sediments were collected from 0–8 cm depths from 6 locations (S1 to S6) along the 30-km long channel length, in the four seasons of year on the following dates: May 2001 (D1, autumn); August 2001 (D2, winter); October 2001 (D3, spring) and January 2002 (D4, summer). The dried samples were sifted to obtain the < 63-μm sediment fraction, since it has been shown that large amounts of heavy metals are bound in the fine-grained fraction of the sediment. Cu and Zn were analyzed by atomic absorption spectrophotometry and Pb and Cd by square wave anodic stripping voltammetry. The highest concentrations of Cu (2850 μg g− 1) were found in the northern part of the river (S1, average D1–D4), near the mountains and a copper mine, and then decreased downstream to 209 μg g− 1 (S6). Total Zn showed an irregular variation, with higher values at S1 (1290 μg g− 1) and high values in some winter sampling (1384 μg g− 1 S4, S5–D2). Pb showed different trends, increasing from S1 to S6 (17 to 61 μg g− 1), with the highest values in the summer samples (83 μg g− 1, S4–S6, D4), and total Cd increased slightly from mean values of 0.2 and 0.5 μg g− 1. Partition into five fractions was made using Tessier's analytical sequential extraction technique; the residue was treated with aqua regia for recovery studies, although this step is not part of the Tessier procedure. The results show that Cu, Zn and Pb in the sediments were dependent on the sampling places along the river, and variation in two years was low (D1–D4). The highest values of total organic matter, carbonate and conductivity were found in S6, which has the smallest size particles, while at S1 the sediments were predominantly sand and contain larger amounts of silica. Cu associated with carbonate decreased gradually from 58% (1771 μg g− 1, S1) to 16% (32 μg g− 1, S6); Cu bonded to reducible fraction was almost constant (33% to 37%), and Cu associated with oxidizable fraction increased from 7% (S1) to 34% (S6), but copper content was lower (214 to 68 μg g− 1). Zn had a similar fractionation profile. However, Pb bound to oxidizable fraction did not show significant percent variation along the river (20% to 19%), but the amount bounded was 4 to 12 μg g− 1. The residual fraction increased from 24% to 41% (5 to 25 μg g− 1, S1 to S6). The distribution of Cd in the sediment was almost independent of the sampling stations and was bound to carbonate, reducible and residual fraction in similar proportion. Cu and Zn at S1 were mainly bound to carbonates and reducible phases with 91% and 73% (2779 and 965 μg g− 1, respectively), and with a change in the pH and/or the redox potential of the sediment–water system, these contaminants could easily enter the food chain. In S6 the amount of Cu and Zn in these phases was 50% and 53% (100 to 313 μg g− 1, respectively).  相似文献   

20.
Leucogranitic lenses are found within the Xiwan ophiolitic mélange in northeastern Jiangxi Province, South China. The leucogranites occur exclusively within the serpentinized peridotite unit of the ophiolite suite. SHRIMP U–Pb zircon dating results indicate that these granites were formed at 880 ± 19 Ma, and were overprinted by an Indosinian tectono-thermal event at ~ 230 Ma. The leucogranites are peraluminous (A/CNK = 1.0–1.24), characterized by high Al2O3 (14–18.33%) and Na2O (6.5–10%) and clearly low εNd(T) values of 0.8 to − 3.9 compared with the other rock units of the ophiolite suite. On the basis of their REE characters, the leucogranites can be divided into three groups. Group I leucogranites show the most fractionated LREE-enrichment patterns (with LaN/YbN and LaN/SmN ratios of 30.1–75.0 and 2.3–3.9, respectively). Group II leucogranites have moderately fractionated LREE-enrichment patterns (with LaN/YbN and LaN/SmN ratios of 13.1–26.5 and 0.8–1.9, respectively). Group III leucogranites are characterized by obviously low total REE contents and flat REE patterns with significant positive Eu anomalies, probably due to small degrees of partial melting. All these leucogranites were likely formed by partial melting of sedimentary rocks from a marginal basin at the Yangtze side of the orogen, beneath a major thrust fault during the obduction of the ophiolite onto the continental crust. They are broadly similar to obduction-related granites within ophiolites identified in many places worldwide. Identification of the ca. 880 Ma obduction-type granites in the NE Jiangxi ophiolite provides a petrological constraint on the timing of the ophiolite obduction onto the continental crust. In combination with the termination of the Shuangxiwu arc magmatism at ca. 890 Ma, we interpret that the close of the Neoproterozoic back-arc basin and the termination of the continental amalgamation between the Yangtze and Cathaysia Blocks occurred at ca. 880 Ma.  相似文献   

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