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1.
This paper presents the first results of a 14C and optical luminescence dating project on extensive alluvial silt formations in the southern Anti Atlas Mountains in southern Morocco. The silts comprise three members: basal silts, an inter-silt fluvial gravel member and a top silt member and are incised by the rivers. Top silt dates are: 11 340–11 940, 12 670–13 130 cal. yr BP, and 10.6±0.7 ka (OSL). OSL dates for the inter-silt gravels are 29.6±2.0 ka and for the bottom silts is 45.0±3.7 ka. The sediments correlate with Soltanian stratigraphies known elsewhere in Morocco. To cite this article: M. Thorp et al., C. R. Geoscience 334 (2002) 903–908.  相似文献   

2.
Metamorphic study on Triassic schists in the internal zone of the Indo-Burma range, essentially based on chlorite–mica equilibrium in metapelites, allows a P–T path to be quantified. During the prograde metamorphism, the geothermic gradient evolves from that of a ‘normal’ crust (30 °C km?1) to that of a thickened crust (18 °C km?1). The peak conditions are around 8 kbar and 450 °C. This thickening (25–30 km) is probably made in a wedge set up between the Late Cretaceous and the Eocene, in front of the obduction. The obtained cold retrograde path requires a mechanism allowing thermal re-equilibration, implying slow exhumation. It occurred along a shear zone that put into contact the micaschists of the core with the Triassic schists of the roof. To cite this article: A. Socquet et al., C. R. Geoscience 334 (2002) 573–580.  相似文献   

3.
Temperate latitude salt marshes are a proven environment from which high precision (±0.10–0.20 m) relative sea level (RSL) records can be developed over recent decades and centuries. Such records provide an important link between millennial histories of RSL change and instrumental records. The high latitude salt marshes in Greenland have not previously been explored as potential archives of recent RSL change. Here we develop four diatom-based transfer functions using contemporary diatom data collected from three salt marshes located 40 km south of the coastal town of Sisimiut, West Greenland. Our preferred model has a good fit between observed and predicted elevations (r2 = 0.94) and a root mean square error prediction of ±0.19 m. We apply the four models to a short sediment profile collected from one of the marshes that formed between c. 600 cal. year BP and the present. Three of the four models predict the same trend in which RSL rose from ?0.55 ±0.19 m mean tide level (MTL) to ?0.05 ± 0.19 m MTL between c. 600 and 400 cal. year BP at a rate of c. 2.7 mm year?1. After 400 cal. year BP RSL slowed and remained stable until the present day. The results of this study demonstrate that Greenland salt marshes are potentially valuable archives of data for developing quantitative estimates of RSL change during the last few centuries, thereby bridging the gap between existing millennial-scale approaches and more recent direct observations of ice sheet behaviour and associated vertical land motions.  相似文献   

4.
Hydrothermally altered rocks are products of fluid–rock interactions, and typically preserve numerous quartz veins that formed as chemical precipitates from fluids that fill up cracks. Thus, quartz veins are the record of the fluid system that involved fracture flow in the direction of changing temperature or pressure. In order to decipher the fluid activity in the Sulu ultrahigh-pressure (UHP) terrane in eastern China, quartz veins together with an adjacent eclogite lens and the host gneiss were studied. In one location a deformed quartz vein is located at the boundary between the host gneiss and the eclogite lens. The amphibolite-facies overprinting of the eclogite lens decreases from the rim to the core of the lens, with fresh eclogite preserved in the core. The foliated biotite gneiss contains felsic veins and residual phengites. Zircon rims from the gneiss are characterized by melt-related signatures with steep HREE patterns, high Hf contents and negative Eu anomalies, and a pool of weighted average 206Pb/238U analyses reveal an age of 219 ± 3 Ma (2σ), which is younger than the UHP metamorphic age (236 ± 2 Ma, 2σ) recorded by zircons from the eclogite lens. This suggests that the gneiss in the Sulu UHP terrane could have suffered from partial melting due to phengite dehydration during the “hot” exhumation stage.The formation age of the quartz vein (219 ± 2 Ma, 2σ) defined by zircon rims agrees well with the partial melting time (219 ± 3 Ma, 2σ) of the host gneiss. The initial 176Hf/177Hf ratios of zircon rims from the quartz vein are obviously lower than zircons from the eclogite lens, but overlap with the coeval zircon domains from the nearby granite dikes produced by partial melting of orthogneiss. These observations suggest that the quartz vein and corresponding fluid flow could be associated with partial melting of the host gneiss. On the other hand, amphibole-bearing and HREE-rich zircon rims from the amphibolite pool an amphibolite-facies metamorphic age of 217 ± 5 Ma (2σ), overlap with the formation age of the quartz vein. This implies that retrogression of the eclogite lens could have been caused by melting-induced fluid flow. Based on the above observations, we speculate that partial melting of the gneiss in the continental subduction-related UHP belt could have induced a significant fluid flow during the exhumation stage, and thus contributed significantly to the extensive retrogression of eclogites in the Sulu UHP terrane.  相似文献   

5.
This study seeks to quantify the rate and timing of regolith generation in the Critical Zone at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Meteoric 10Be depth profiles were determined using measurements from 30 hillslope soil and bedrock core samples in an effort to constrain 10Be inventories. The SSHO is located in the temperate climate zone of central Pennsylvania and comprises a first-order watershed developed entirely on a Fe-rich, organic-poor, Silurian-aged shale. Two major perturbations to the landscape have occurred at SSHO in the geologically recent past, including significant and sustained periglacial activity until after the retreat of the Laurentide ice sheet (～21 ka) and deforestation during early colonial land-use. Bulk soil samples (n = 16) were collected at three locations along a planar hillslope on the southern ridge of the catchment, representing the ridge top, mid-slope and valley floor. Rock chip samples (n = 14) were also collected from a 24 m deep core drilled into the northern ridge top. All meteoric 10Be concentration profiles show a declining trend with depth, with most of the 10Be retained in the uppermost decimeters of the soil. Meteoric 10Be inventories are higher at the mid-slope and valley floor sample sites, at 3.71 ± 0.02 × 1010 at/cm2 and 3.69 ± 0.02 × 1010 at/cm2, than at the ridge top site (1.90 ± 0.01 × 1010 at/cm2). The 10Be inventory at the convex ridge top site implies a minimum residence time of ～10.6 ka, or if erosion is steady, an erosion rate of 19.4 ± 0.2 m/My.  相似文献   

6.
Many earlier studies have shown the very large spatial variability of rainfall in the Sahel at all time steps, from the event to the season. Often, the meteorological network in these countries is sparse, with one to five rain gauges per 10 000 km2. It is thus difficult to calculate accurate estimates of the mean rainfall over such a large area. To improve the knowledge of Sahelian systems and the spatial distribution of rainfall, a dense network was set up in an area of 16 000 km2 in southwestern Niger between 1991 and 1996. The aim was to calculate accurate rainfall spatial means over an area of 12 000 km2 at different time steps (from the season to the ten-day period). With the spatialisation method used (kriging), it was possible to calculate curves of estimation errors of mean rainfall versus the rain-gauge network density. Operational abacuses of the standard estimation error as a function of the spatial mean of rainfall and the network density are proposed.  相似文献   

7.
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   

8.
We have synthesized new and existing relative sea-level (RSL) data to produce a quality-controlled, spatially comprehensive database from the North Carolina coastline. The RSL database consists of 54 sea-level index points that are quantitatively related to an appropriate tide level and assigned an error estimate, and a further 33 limiting dates that confine the maximum and minimum elevations of RSL. The temporal distribution of the index points is very uneven with only five index points older than 4000 cal a BP, but the form of the Holocene sea-level trend is constrained by both terrestrial and marine limiting dates. The data illustrate RSL rapidly rising during the early and mid Holocene from an observed elevation of ?35.7 ± 1.1 m MSL at 11062–10576 cal a BP to ?4.2 m ± 0.4 m MSL at 4240–3592 cal a BP.We restricted comparisons between observations and predictions from the ICE-5G(VM2) with rotational feedback Glacial Isostatic Adjustment (GIA) model to the Late Holocene RSL (last 4000 cal a BP) because of the wealth of sea-level data during this time interval. The ICE-5G(VM2) model predicts significant spatial variations in RSL across North Carolina, thus we subdivided the observations into two regions. The model forecasts an increase in the rate of sea-level rise in Region 1 (Albemarle, Currituck, Roanoke, Croatan, and northern Pamlico sounds) compared to Region 2 (southern Pamlico, Core and Bogue sounds, and farther south to Wilmington). The observations show Late Holocene sea-level rising at 1.14 ± 0.03 mm year?1 and 0.82 ± 0.02 mm year?1 in Regions 1 and 2, respectively. The ICE-5G(VM2) predictions capture the general temporal trend of the observations, although there is an apparent misfit for index points older than 2000 cal a BP. It is presently unknown whether these misfits are caused by possible tectonic uplift associated with the mid-Carolina Platform High or a flaw in the GIA model. A comparison of local tide gauge data with the Late Holocene RSL trends from Regions 1 and 2 support the spatial variation in RSL across North Carolina, and imply an additional increase of mean sea level of greater than 2 mm year?1 during the latter half of the 20th century; this is in general agreement with historical tide gauge and satellite altimetry data.  相似文献   

9.
From a large collection (more than 300 oriented cores) of Baja California Mio-Pliocene volcanic units, sampled for magnetostratigraphy and tectonics, 46 samples were selected for Thellier paleointensity experiments because of their low viscosity index, stable remanent magnetization and close to reversible continuous thermomagnetic curves. 19 samples, coming from 4 individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 3.6 to 6.2 ×1022 A m2. Our results, although not numerous, are of high technical quality and comparable to other paleointensity data recently obtained on younger lava flows. The NRM fractions used for paleointensity determination range from 38 to 79% and the quality factors vary between 4.8 and 16.7, being normally greater than 5. The combination of Baja California data with the available comparable quality Plio-Plesitocene paleointensity results yields a mean VDM of 6.3 ×1022 A m2, which is almost 80% of the present geomagnetic axial dipole. Reliable paleointensity results for the last 5 Ma are still scarce and of dissimilar quality, which makes it hard to draw any firm conclusions regarding the Pliocene and Early/Middle Pleistocene evolution of the geomagnetic field. To cite this article: J. Morales et al., C. R. Geoscience 335 (2003).  相似文献   

10.
In 2001, a special issue of the Bulletin of the Seismological Society of America (BSSA) featured seismological research for the 1999 Chi–Chi Taiwan earthquake. This study uses source parameters suggested by the first author in this special issue to estimate static stress drop associated with the Chi–Chi earthquake. The waveform simulation method was used to carefully examine these source parameters. The simulation results indicate that source parameters, inferred from near-fault observations, are well determined. According to the rupture area and slip, the static stress drops (Δσs) obtained were distributed between a small value of 47 bars near the epicentral region and a much larger value (>200 bars) to the north. Similar trends in dynamic stress drop (Δσd) were also recognized by the first author in his paper published in 2001 BSSA special issue. Comparing the Δσs with Δσd, satisfies the relation Δσs/Δσd ≈ 1. This relation suggests that fault motion is mostly spent releasing seismic wave energy during the rupture process of the Chi–Chi earthquake. The consistency between static and dynamic stress drops thus provides a measure of energy-moment (Es/M0) ratios, which range from 9.0 × 10−5 to 6.5 × 10−4. The average Es/M0 ratio estimated for the northern portions of the fault is 3.4 × 10−4, which is about 3 times that of the south. Such a high Es/M0 ratio can be interpreted as having low strength in the rupture for the northern portions of the fault, where the fault would release less energy per unit rupture surface to create the new rupture.  相似文献   

11.
The Ranger 1 unconformity-related uranium deposit in the Northern Territory of Australia is one of the world's largest uranium deposits and has ranked in the top two Australian producers of uranium in recent years. Mineralisation at the Ranger, Jabiluka and other major unconformity-related deposits in the Alligator Rivers Uranium Field (ARUF) occurs in Paleoproterozoic metamorphic basement rocks immediately beneath the unconformity with the Paleo- to Mesoproterozoic McArthur Basin.The sites of uranium mineralisation and associated alteration at the Ranger 1 deposit (Number 3 orebody) were fundamentally controlled by reactivated shear zones that were initiated during the regional Nimbuwah tectonothermal event. The timing of shearing at medium metamorphic grade was constrained by ion microprobe U–Pb dating of zircons in two pegmatites, one weakly foliated (1867.0 ± 3.5 Ma) and another that is unfoliated and cuts the shear fabric (1862.8 ± 3.4 Ma). The younger age of ~ 1863 Ma represents the minimum age of D1 shearing during the Nimbuwah event at the Ranger 1 deposit (Number 3 orebody). Titanite within veins of amphibole-plagioclase-apatite yielded an ion microprobe U–Pb age of 1845.4 ± 4.2 Ma, which represents a previously unrecognised hydrothermal event in the ARUF. Based on previous data, retrograde hydrothermal alteration during D2 reactivation of D1 shear zones is interpreted to have occurred at ~ 1800 Ma during the regional Shoobridge tectonothermal event.Detailed paragenetic observations supported by whole-rock geochemical data from the Ranger 1 deposit (Number 3 orebody) reveal a sequence of post-D2 hydrothermal events, as follows. (1) Intense magnesium-rich chlorite alteration and brecciation, focussed within schists of the Upper Mine Sequence in the Cahill Formation. (2) Silicification of Lower Mine Sequence carbonate rock units and overlying schist units, comprising quartz ± Mg-foitite (tourmaline) ± muscovite ± pyrite ± marcasite, and rare uraninite (early U1). (3) Formation of main stage uranium ore and heterolithic breccias including clasts of olivine–phyric dolerite, with breccia matrix composed of uraninite (U1), Mg-chlorite ± Mg-foitite and minor pyrite and chalcopyrite. (4) A second generation of uraninite (U2) veinlets with disordered graphitic carbon and quartz of hydrothermal origin. (5) Late-stage veinlets of massive uraninite (U3). As inferred in a previous study and confirmed herein, olivine–phyric dolerite dykes at Ranger are mineralised and chloritised, and are geochemically similar to the regional Oenpelli Dolerite. A maximum age for uranium mineralisation at the Ranger 1 deposit is therefore set by the age of the Oenpelli Dolerite (~ 1723 Ma).In-situ ion microprobe U–Pb analysis of texturally oldest U1 uraninite yielded a discordia array with a 206Pb/238U-207Pb/235U upper intercept age of 1688 ± 46 Ma. The oldest individual ion microprobe 207Pb–206Pb age is 1684 ± 7 Ma whereas the oldest age determined by in-situ electron microprobe chemical dating of U1 uraninite is ~ 1646 Ma. Another sample containing both U1 and U2 uraninite yielded discordant data with a 206Pb/238U–207Pb/235U upper intercept age of 1421 ± 68 Ma. When the 207Pb/206Pb ages are considered the data are suggestive of U2 uraninite formation and possible resetting of the U1 age between ~ 1420 Ma and ~ 1040 Ma. All ion microprobe analyses of U1 and U2 uraninite indicate variable and possibly repeated lead loss. In contrast ion microprobe U–Pb dating of the third generation of uraninite (U3) yielded several near-concordant analyses and a 206Pb/238U–207Pb/235U upper intercept age of 474 ± 6 Ma. This age is supported by electron microprobe chemical ages of U3 uraninite between 515 Ma and 385 Ma.The new results constrain the timing of initial uranium mineralisation at the Ranger 1 deposit (Number 3 orebody) to the period ~ 1720 Ma to ~ 1680 Ma, which just overlaps with a previous U–Pb age of 1737 ± 20 Ma for uraninite-rich whole-rock samples. Our results are consistent with individual laser-ICPMS 207Pb/206Pb and chemical ages of uraninite as old as 1690–1680 Ma reported from other deposits and prospects in the ARUF.Whole-rock geochemical data in this study of the Ranger 1 deposit (Number 3 orebody) and in other studies in the ARUF demonstrate that zones of intense chloritisation associated with uranium mineralisation experienced large metasomatic gains of Mg, U, Co, Ni, Cu and S and losses of Si, Na, Ca, Sr, Ba, K, Rb, Y and the light REE. More broadly in the ARUF, a regionally extensive illite–hematite ± kaolinite-bearing ‘paleoregolith’ zone in basement beneath the McArthur Basin exhibits depletion of about half of its uranium as well as major losses in Na, Sr, Pb, Ba and minor losses of Mg. These features together with new petrographic observations suggest this zone is a regional sub-McArthur Basin alteration zone produced by interaction with diagenetic or hydrothermal fluids of primary basinal origin, rather than representing a low-temperature paleo-weathering zone before the deposition of the McArthur Basin, as previously suggested.Based on these results and a synthesis of previous work, a new multi-stage model is proposed for the Ranger 1 ore-forming mineral system that may apply to other major unconformity-related uranium deposits in the ARUF and which may be used for targeting new deposits in the region. As in most recent models, oxidised diagenetic brines within the McArthur Basin are envisaged as crucial in mobilising uranium. However, a different architecture of fluid flow is proposed involving the sub-unconformity regional basement alteration zone as a preferential source of leached uranium. Possibly driven by convection during regional magmatism at ~ 1725–1705 Ma, oxidised basinal brines were drawn downwards and laterally through fault networks and fractures in the regional sub-unconformity alteration zone, leaching uranium from hematite-altered basement rocks. Simultaneously within deeper and lateral parts of the hydrothermal system, Mg-metasomatism produced chloritic alteration and brines with increased acidity and silica content (from the desilicification of the basement rock), analogous to processes described in sub-seafloor hydrothermal systems. Silicification occurred locally (e.g., Ranger deposit) within upflow zones of convective systems due to decreases in temperature and/or pressure of the brines and/or CO2 generation during carbonate dissolution. Interruptions to convection during transient regional extensional or strike-slip tectonic events resulted in generalised lateral and downwards flow of fluids from the McArthur Basin through deepened zones of sub-unconformity alteration, transferring leached uranium into reactivated shear zones within the basement. The main stage of uraninite precipitation at the Ranger deposit and elsewhere in the ARUF is proposed to have occurred between ~ 1720 Ma and ~ 1680 Ma as a result of reduction of oxidised and evolved basin-derived ore fluids during reaction with pre-existing Fe2 +-bearing minerals and/or mixing of the ore fluids with basement-reacted silica-rich brines.A second, volumetrically minor but locally high-grade, stage of uraninite mineralisation was associated with hydrothermal disordered carbon and quartz of presently unknown origin. Available data suggest formation between ~ 1420 Ma and ~ 1040 Ma. Almost a billion years later at ~ 475 Ma, fluids capable of mobilising uranium again resulted in uraninite (U3) deposition as sparse veinlets in the Ranger deposit, representing the first documentation of uranium mineralisation of this age in the region.  相似文献   

12.
This study presents the crustal shear wave velocity structure and radial anisotropy along two linear seismic arrays across the North China Craton (NCC) from ambient noise tomography. About a half to one year long ambient noise data from 87 stations were used for obtaining the inter-station surface wave empirical Green's functions (EGFs) from cross-correlation. Rayleigh and Love dispersion curves within the period band 5–30 s were measured from the EGFs of the vertical and transverse components, respectively. These dispersion data were then used to determine the crustal shear wave velocity structure (VSV and VSH) and radial anisotropy (2(VSH ? VSV) / (VSH + VSV)) from point-wise linear inversion with constraints from receiver function analysis. Our results reveal substantial structural variations among different parts of the NCC. The Bohai Bay Basin in the eastern NCC is underlain by a thin crust (~ 30 km) with relatively low velocities (particularly VSV) and large positive radial anisotropy in the middle to lower crust. Such a crustal structure is no longer of a cratonic type and may have resulted from the widespread tectonic extension and intensive magmatism in this region since late Mesozoic. Beneath the Ordos Basin in the western NCC, the crust is relatively thicker (≥ 40 km) and well stratified, and presents a large-scale low velocity zone in the middle to lower crust and overall weak radial anisotropy except for a localized lower crust anomaly. The overall structural features of this region resemble those of typical Precambrian shields, in agreement with the long-term stability of the region. The crustal structure under the Trans North China Orogen (TNCO, central NCC) is more complicated and characterized by smaller scale velocity variations, strong positive radial anisotropy in the middle crust and rapid change to weak-to-negative anisotropy in the lower crust. These features may reflect complex deformations and crust–mantle interactions, probably associated with tectonic extension and magmatic underplating during the Mesozoic to Cenozoic evolution of the region. Our structural images in combination with previous seismic, geological and geochemical observations suggest that the Phanerozoic lithospheric reactivation and destruction processes may have affected the crust (especially the middle and lower crust) of the eastern NCC, and the effect probably extended to the TNCO, but may have minor influence on the crust of the western part of the craton.  相似文献   

13.
We present new helium isotope data from the Macdonald seamount (Austral chain). The helium isotopic ratio varies from 4He/3He=45 000 (R/Ra=16.0) to 200 170 (R/Ra=3.6). The helium content is between 1.5×10?8 and 1.1×10?5 ccSTP/g. These helium results show clearly the presence of primitive mantle material in the source of the Austral chain. Macdonald has the lowest 4He/3He ratio among the Polynesian submarine volcanoes, except Hawaii (Loihi). The simplest explanation for the primitive helium signature is the presence under Macdonald of a mantle plume that derives either from the 670 km or 2900 km boundary layers, or, eventually, from the top of a large mantle dome resulting from a stratified two-layer convection. This plume contains less-degassed material with low 4He/3He ratio. To cite this article: M. Moreira, C. Allègre, C. R. Geoscience 336 (2004).  相似文献   

14.
The evolution and the internal architecture of an estuary type sand ridge has been studied with a set of bathymetric data recorded during the last two centuries and with a dense grid of recent very high resolution seismic profiles. Bathymetric data of the so-called, Longe de Boyard sand ridge, displays sand losts due to wave and tide erosion. Internal geometry, through seismic profile analysis, indicates two main phases of deposition recording both, a recent high energy environment and an older low energy one, respectively. Such an evolution is believed to record changes in sedimentation processes mainly related to the end of the Holocene transgression (8 000–5 000 yr BP). To cite this article: É. Chaumillon et al., C. R. Geoscience 334 (2002) 119–126.  相似文献   

15.
The large low-grade Piaotang W–Sn deposit in the southern Jiangxi tungsten district of the eastern Nanling Range, South China, is related to a hidden granite pluton of Jurassic age. The magmatic-hydrothermal system displays a zonation from an inner greisen zone to quartz veins and to peripheral veinlets/stringers (Five-floor zonation model). Most mineralization is in quartz veins with wolframite > cassiterite. The hidden granite pluton in underground exposures comprises three intrusive units, i.e. biotite granite, two-mica granite and muscovite granite. The latter unit is spatially associated with the W–Sn deposit.Combined LA-MC-ICP-MS U–Pb dating of igneous zircon and LA-ICP-MS U–Pb dating of hydrothermal cassiterite are used to constrain the timing of granitic magmatism and hydrothermal mineralization. Zircon from the three granite units has a weighted average 206Pb/238U age of 159.8 ± 0.3 Ma (2 σ, MSWD = 0.3). The cathodoluminescence (CL) textures indicate that some of the cassiterite crystals from the wolframite-cassiterite quartz vein system have growth zonations, i.e. zone I in the core and zone II in the rim. Dating on cassiterite (zone II) yields a weighted average 206Pb/238U age of 159.5 ± 1.5 Ma (2 σ, MSWD = 0.4), i.e. the magmatic and hydrothermal systems are synchronous. This confirms the classical model of granite-related tin–tungsten mineralization, and is against the view of a broader time gap of >6 Myr between granite magmatism and W–Sn mineralization which has been previously proposed for the southern Jiangxi tungsten district. The elevated trace element concentrations of Zr, U, Nb, Ta, W and Ti suggest that cassiterite (zone II) formed in a high-temperature quartz vein system related to the Piaotang granite pluton.  相似文献   

16.
《Quaternary Science Reviews》2007,26(19-21):2316-2321
Traditional ice sheet reconstructions have suggested two distinctly different ice sheet regimes along the East Greenland continental margin during the Last Glacial Maximum (LGM): ice to the shelf break south of Scoresby Sund and ice extending no further than to the inner shelf at and north of Scoresby Sund. We report new 10Be ages from erratic boulders perched at 250 m a.s.l. on the Kap Brewster peninsula at the mouth of Scoresby Sund. The average 10Be ages, calculated with an assumed maximum erosion rate of 1 cm/ka and no erosion (respectively, 17.3±2.3 ka and 15.1±1.7 ka) overlap with a period of increased sediment input to the Scoresby Sund fan (19–15 ka). The results presented here suggest that ice reached at least 250 m a.s.l. at the mouth of Scoresby Sund during the LGM and add to a growing body of evidence indicating that LGM ice extended onto the outer shelf in northeast Greenland.  相似文献   

17.
We carried out a detailed rock-magnetic and paleointensity study of the ～187-Ma volcanic succession from northern Chile. A total of 32 consecutive lava flows (about 280 oriented standard paleomagnetic cores) were collected at the Tocopilla locality. Only 26 samples with apparently preserved primary magnetic mineralogy and without secondary magnetization components were pre-selected for Thellier paleointensity determination. Eleven samples coming from four lava flows yielded reliable paleointensity estimates. The flow-mean virtual dipole moments range from 3.7±0.9 to 7.1±0.5(1022 A m2). This corresponds to a mean value of (5.0±1.8)×1022 A m2, which is in reasonably good agreement with other comparable quality paleointensity determinations from the Middle Jurassic. Given the large dispersion and the very poor distribution of reliable absolute intensity data, it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field. To cite this article: J. Morales et al., C. R. Geoscience 335 (2003).  相似文献   

18.
The amphibolite facies grade North Qinling metamorphic unit forms the centre of the Qinling orogenic belt. Results of LA-ICP-MS U-Pb zircon, 40Ar/39Ar amphibole and biotite dating reveal its Palaeozoic tectonic history. U-Pb zircon dating of migmatitic orthogneiss and granite dykes constrains the age of two possible stages of migmatization at 517 ± 14 Ma and 445 ± 4.6 Ma. A subsequent granite intrusion occurred at 417 ± 1.6 Ma. The 40Ar/39Ar plateau ages of amphibole ranging from 397 ± 33 Ma to 432 ± 3.4 Ma constrain the cooling of the Qinling complex below ca. 540 °C and biotite 40Ar/39Ar ages at about 330–368 Ma below ca. 300 °C. The ages are used to construct a cooling history with slow/non-exhumation during 517– 445 Ma, a time-integrated cooling at a rate < 2.5 °C/Ma during the period of 445–410 Ma, an acceleration of cooling at a rate of 8 °C/Ma from 397 Ma to 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. The data show a significant delay in exhumation after peak metamorphic conditions and a long period of tectonic quiescence after the suturing of the North China and South China blocks along the Shangdan suture. These relationships exclude classical exhumation models of formation and exhumation of metamorphic cores in orogens, which all imply rapid cooling after peak conditions of metamorphism.  相似文献   

19.
The combination of ion microprobe dating and cathodoluminescence (CL) imaging of zircons from a high-grade rock from the Central Zone of the Limpopo Belt were used to constrain the age of metamorphic events in the area. Zircon grains extracted from an orthopyroxene-gedrite-bearing granulite were prepared for single crystal CL-imaging and ion microprobe dating. The grains display complex zoning when using SEM-based CL-imaging. A common feature in most grains is the presence of a distinct core with a broken oscillatory zoned structure, which clearly appears to be the remnant of an original grain of igneous origin. This core is overgrown by an unzoned thin rim measuring about 10–30 μm in diameter, which is considered as new zircon growth during a single metamorphic event. Selected domains of the zircon grains were analysed for U, Pb and Th isotopic composition using a CAMECA IMS 1270 ion microprobe (Nordsim facility). Most of the grains define a near-concordant cluster with some evidence of Pb loss. The most concordant ages of the cores yielded a weighted mean 207Pb/206Pb age of 2689 ± 15 (2σ) Ma, interpreted as the age of the protolith of an igneous origin. The unzoned overgrowths of the zircon grains yielded a considerably younger weighted mean 207Pb/206Pb age of ∼2006.5 ± 8.0 Ma (2σ), and these data are interpreted to reflect closely the age of the ubiquitous high-grade metamorphic event in the Central Zone. This study shows clearly, based on both the internal structure of the zircons and the data obtained by ion microprobe dating, that only a single metamorphic event is recorded by the studied 2.69 Ga old rocks, and we found no evidence of an earlier metamorphic event at ∼2.5 Ga as postulated earlier by some workers.  相似文献   

20.
Parasound profiles across the Shaban Deep in the Red Sea indicate turbiditic transport of surface sediments from the topographic height (basalt ridge) into the interior of the deep. This is supported by petrographical and (isotope-) geochemical evidence in the East Basin of the Shaban Deep where the presence of variable mixtures of authochtonous and allochthonous sediment compounds had been found.The uppermost 170 cm of both sediment cores 17008-1 and 17009-3 reveal “normal” stable oxygen isotope values for the planktonic foraminifera Globigerinoides ruber near ?1‰ which is indicative for carbonate formation in Red Sea surface water around 27 °C. However, below 182 cm in core 17008-1 highly variable δ 18O values for G. ruber between 0.26 and ?10.68‰ occur which are not the result of temperature-controlled oxygen isotope fractionation between foraminiferal carbonate and Red Sea surface water. The lowest δ18O values of ?10.68‰ measured for highly-altered foraminifera shells suggests carbonate precipitation higher than 90 °C.Organic petrographical observations show a great diversity of marine-derived macerals and terrigenous organic particles. Based on petrographical investigations sediment core 17008-1 can be subdivided in intervals predominantly of authochtonous character (i.e. 1, 3, 5 corresponding to core depths 0–170 cm, 370–415 cm, 69–136 cm), and allochthonous/thermally altered character (e.g. 2 and 4 corresponding to core depths 189–353 cm and 515–671 cm). Allochthonous/thermally altered material displays a wide to an extremely wide range of maturities (0.38–1.42% Rr) and also natural coke particles were found.Similarly, the organic geochemical and pyrolysis data indicate the predominance of well-preserved, immature algal and bacterial remains with a minor contribution of land plant material. Sediments below 170 cm (core 17008-1) contain contributions of re-sedimented pre-heated material most likely from the area of the basaltic ridge. This is documented by individual coke particles reduced hydrogen indices and elevated Tmax values up to 440 °C.An “oil-type” contribution (evidenced by mature biomarkers, hopene/hopane ratios, elevated background fluorescence, n-alkane distribution) is also present in the sediments which most likely originated at greater depth and impregnated the surface sediments.The heat source responsible for recrystallisation of foraminiferal carbonate and maturation of organic particles in Shaban Deep sediments most likely is attributed to modern basalt extrusions which now separate the Shaban Deep subbasins.  相似文献   

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