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1.
Six samples of a single carbonate-rich unit of the Swiss Préalpes, progressively metamorphosed from diagenesis to deep anchizone, yield 40Ar/39Ar spectra with variably developed staircase patterns, consistent with mixtures of detrital mica and neocrystallized mixed-layer illite/smectite. The lowest temperature heating steps for different size fractions (2–6?μm and 6–20?μm) converge to ～40?Ma providing an imprecise, maximum age of regional metamorphism. A method is described for distinguishing and quantifying the amount of pre-existing detrital mica versus neoformed illite layer in the illite/smectite formed during Tertiary Alpine metamorphism by comparison of X-ray diffraction patterns with Newmod© simulations. In the least metamorphosed samples the illite/smectite contains ～65% neoformed illite, and this illite accounts for approximately 17% of all dioctahedral phyllosilicate minerals in the rock (e.g., detrital mica and illite/smectite). In contrast, the illite/smectite from the more strongly metamorphosed samples contains >97% neoformed illite, which accounts for ～70% to >90% of all dioctahedral phyllosilicate minerals. Phyllosilicate morphologies viewed by scanning electron microscopy are consistent with these estimates. A process of dissolution/reprecipitation is inferred as a mechanism for the growth of the neoformed phyllosilicates. A plot of neoformed illite content versus 40Ar/39Ar total fusion age yields a near-linear curve with an extrapolated age of 27?Ma for 100% neoformed dioctahedral phyllosilicates. This age is interpreted as the time of incipient metamorphism and is consistent with independent biostratigraphic constraints. Model 40Ar/39Ar age spectra constructed with the XRD simulation results correspond well to the experimental data and illustrate the changes in degassing properties of progressively metamorphosed mixtures of detrital mica and neoformed illite.  相似文献   

2.
New radiometric ages from the Subpenninic nappes (Eclogite Zone and Rote Wand – Modereck Nappe, Tauern Window) show that phengites formed under eclogite-facies metamorphic conditions retain their initial isotopic signature, even when associated lithologies were overprinted by greenschist- to amphibolite-facies metamorphism. Different stages of the eclogite-facies evolution can be dated provided 40Ar/39Ar dating is combined with micro-structural analyses. An age of 39 Ma from the Rote Wand – Modereck Nappe is interpreted to be close to the burial age of this unit. Eclogite deformation within the Eclogite Zone started at the pressure peak along distinct shear zones, and prevailed along the exhumation path. An age of ca. 38 Ma is only observed for eclogites not affected by subsequent deformation and is interpreted as maximum age due to the possible influence of homogenously distributed excess argon. During exhumation deformation was localised along distinct mylonitic shear zones. This stage is mainly characterised by the formation of dynamically recrystallized omphacite2 and phengite. Deformation resulted in the resetting of the Ar isotopic system within the recrystallized white mica. Flat argon release spectra showing ages of 32 Ma within mylonites record the timing of cooling along the exhumation path, and the emplacement onto the Venediger Nappe. Ar-release patterns and 36Ar/40Ar vs.39Ar/40Ar isotope correlation analyses indicate no significant 40Ar-loss after initial closure, and only a negligible incorporation of excess argon. From the pressure peak onwards, eclogitic conditions prevailed for almost 8–10 Ma.  相似文献   

3.
Noblesse multi-collector noble gas mass spectrometer is specially designed for multi-collection of Ar isotopes with different beam sizes, especially for small ion beams, precisely, and hence is perfectly suitable for 40Ar/39Ar geochronology. We have analyzed widely used sanidine, muscovite, and biotite standards with recommended ages of ~ 1.2–133 Ma, with the aim to assess the reliability of Noblesse for 40Ar/39Ar dating. An ESI MIR10 30W CO2 laser was used for total fusion or incremental heating samples. Extracted gases were routinely purified by four SAES NP10 getters (one at ~ 400 °C and others at room temperature). A GP50 getter and a metal cold finger cooled by liquid N (? 196 °C) were also attached for additional purification if necessary. The Ar isotopes were then measured by Noblesse using Faraday or multiplier according to the signal intensities. Over a period of 1.5 months 337 air calibrations produced a weighted mean 40Ar/36Ar of 296.50 ± 0.08 (2σ, MSWD = 4.77). Fish Canyon sanidine is used to calculate J-values, which show good linear relationship with position in irradiation. The age of four mineral standards (Alder Creek sanidine, Brione muscovite, Yabachi sanidine, and Fangshan biotite) are within error of the accepted ages. Five Alder Creek sanidine aliquots yielded an age range of 1.174–1.181 ± 0.013 Ma (2σ) which broadly overlaps the established age of the standard and the uncertainty approaches those of the foremost Ar/Ar laboratories in the world. The weighted mean ages of four Brione muscovite aliquots (18.75 ± 0.16 Ma, 2σ), five Yabachi sanidine aliquots (29.50 ± 0.19 Ma, 2σ), and three Fangshan biotite aliquots (133.0 ± 0.76 Ma, 2σ) are consistent with the recommended values of these standards, and the uncertainties are typical of modern Ar/Ar laboratories world-wide.  相似文献   

4.
Potassium-Ar and Rb-Sr dating of minerals was fundamental in early efforts to date magmatic and metamorphic processes and paved the way for geochronology to become an important discipline within the earth sciences. Although K-Ar and, in particular, 40Ar/39Ar dating of micas is still widely applied, Rb-Sr dating of micas has declined in use, even though numerous studies demonstrated that tri-octahedral mica yields geologically realistic, and more reliable and reproducible Rb-Sr ages than the K-Ar or 40Ar/39Ar system. Moreover, a reduction of uncertainties typically reported for Rb-Sr ages (ca. 1%) can now be achieved by application of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) rubidium isotope dilution measurements (<0.3%). Replicate Rb-Sr biotite ages from the Oslo rift, Norway, yield an external reproducibility of ±0.3% (n=4) and an analytical error of ±0.8 Ma for individual ages that vary between 276.9 and 275.5 Ma. Conventional thermal ionisation mass spectrometry (TIMS) Rb analysis on the same mineral separates yields ages between 276.1 and 271.7 Ma, three times the spread compared to Rb MC-ICPMS data. Biotite and phlogopite from the central Nagssugtoqidian orogen, West Greenland, yield 40Ar/39Ar plateau ages (ca. 1700 Ma) with a spread of ±150 Ma, while Rb-Sr ages on either biotite or phlogopite separates have a much narrower range of ±10 Ma. This comparison of Rb-Sr and 40Ar/39Ar ages demonstrates the robustness of the Rb-Sr system in tri-octahedral micas and cautions against the sole use of 40Ar/39Ar tri-octahedral mica ages to date geological events. Analytical errors of 16 Ma for these Rb-Sr mica ages determined by TIMS are reduced to <±5 Ma when the Rb concentration is determined by MC-ICPMS. All the TIMS and MC-ICPMS data from the Nagssugtoqidian orogen agree within assigned analytical uncertainties. However, high precision Rb-Sr dating by MC-ICPMS can resolve geological information obscured by TIMS age determinations. TIMS data for seven phlogopite samples form an isochron age of 1645±6 Ma, and thus, no differentiation in age between the different samples can be made. In contrast, MC-ICPMS Rb measurements on the same samples reveal two distinct populations with ages of 1633±3 or 1652±5 Ma.Combining the mica Rb-Sr geochronological data with the well-constrained thermal history of this ancient orogen, we estimate the closure temperature of the Rb-Sr system in 1-2 mm slowly cooled phlogopite crystals, occurring in a matrix of calcite and plagioclase to be ∼435 °C, and at least 50 °C above that of biotite.  相似文献   

5.
A combined 40Ar/39Ar, K/Ar, Rb/Sr and stable isotope study has been made of white micas from the Gummfluh klippe (Briançonnais domain of the Préalpes), Switzerland. The klippe consists mainly of Mesozoic to early Tertiary carbonate rocks metamorphosed from anchizonal to epizonal conditions. At the base of the klippe is a 10–50 m thick, ductilely deformed marble mylonite containing deformed authigenic quartz segregations. Stable isotope measurements of the coexisting calcite (18OSMOW=24.5) and quartz (18OSMOW=28.4) from the mylonite indicate relatively low temperatures (<300°C) during mylonitization. Analyses of white mica separates of varying size fractions from the mylonitic rocks by K/Ar and Rb/Sr techniques yield ages between 57 and 103 Ma. This variation is correlated with two parameters, the size of the mineral fraction, and the proportion of 2M1 (more phengitic) to 1M (more muscovitic) polytype in the sample. The K/Ar and Rb/Sr ages are generally younger in the smaller size fractions, which also containless 2M1 phengite. High precision 40Ar/39Ar age spectra from different size fractions of these micas record three distinct components, a small Hercynian component (ca. 200–300 Ma), a significant Eoalpine component (64–80 Ma) forming 40Ar/39Ar age plateaus, and a very minor Tertiary component (ca. 20–40 Ma). Characterization of the samples by SEM indicates the presence of two white mica populations, a coarser grained, deformed, detrital mica that probably corresponds to the 2M1 phengite and a finer grained neoformed 1M mica. Collectively these observations suggest that the Gummfluh samples contain a mixture of detrital phengites of Hercynian age together with neocrystallized muscovites grown during the late Eoalpine metamorphic event followed by minor argon loss during the Tertiary. The main geologic episode recorded in the 40Ar/39Ar age spectra of white micas in the mylonite is of Late Cretaceous/Early Tertiary age (64–80 Ma), representing the first reliable Eoalpine ages ever to be reported from the Préalpes. Contrary to tectonic models, the marble mylonite at the base of the Gummfluh klippe appears to be a Cretaceous thrust plane and not the thrust surface formed during transport of the klippe into its present position from the Penninic Alps during the Tertiary. The late Cretaceous thrust developed during marine sedimentation at a depth of 800 m below the seafloor at temperatures of 280°C, facilitated by warm fluids along the tectonic discontinuity.  相似文献   

6.
Five detrital white mica concentrates from very low-grade, metaclastic sequences within pre-Variscan basement and post-Variscan cover units of the Upper Austroalpine Nappe Complex (Eastern Alps) have been dated with 40Ar/39Ar incremental heating techniques to constrain the age of tectonothermal events in their respective source areas. Two samples from early Palaeozoic sandstone exposed within the same Alpine nappe record slightly discordant age spectra. The maximum age recorded in one is 562.2±0.7?Ma, whereas the other yielded a 40Ar/39Ar plateau age of 607.3±0.3?Ma. These results indicate a source area affected by Cadomian tectonothermal activity. Three detrital muscovite concentrates from post-Variscan, Late Carboniferous and Permian cover sequences exposed within three different Alpine nappes yielded 40Ar/39Ar plateau ages of 359.6?±?1.1?Ma, 310.5±1.2?Ma, and 303.3±0.2?Ma. The contrasting detrital white mica ages are interpreted to reflect different source areas. Detrital muscovite from a post-Variscan Carboniferous molasse-type sequence and from a Permian Verrucano-type sequence record ages which indicate “late” Variscan (e.g. 330–300?Ma) metamorphic sources. By contrast, detrital white mica from another Permian Verrucano-type sequence suggests a source area affected by “early” Variscan (e.g. 400–360?Ma) metamorphism. These results help clarify palinspastic relationships and tectonic correlations between pre-Late Carboniferous metamorphic basement sequences and Carboniferous to Permian cover sequences.  相似文献   

7.
The Attic‐Cycladic crystalline belt in the central Aegean region records a complex structural and metamorphic evolution that documents Cenozoic subduction zone processes and exhumation. A prerequisite to develop an improved tectono‐metamorphic understanding of this area is dating of distinct P–T–D stages. To evaluate the geological significance of phengite ages of variably overprinted rocks, 40Ar/39Ar and Rb–Sr analyses were undertaken on transitional blueschist–greenschist and greenschist facies samples from the islands of Syros and Sifnos. White mica geochronology indicates a large age variability (40Ar/39Ar: 41–27 Ma; Rb–Sr: 34–20 Ma). Petrologically similar samples have either experienced greenschist facies overprinting at different times or variations in ages record variable degrees of greenschist facies retrogression and incomplete resetting of isotopic systematics. The 40Ar/39Ar and Rb–Sr data for metamorphic rocks from both islands record only minor, localized evidence for Miocene ages (c. 21 Ma) that are well documented elsewhere in the Cyclades and interpreted to result from retrogression of high‐pressure mineral assemblages during lower pressure metamorphism. Field and textural evidence suggests that heterogeneous overprinting may be due to a lack of permeability and/or limited availability of fluids in some bulk compositions and that retrogression was more or less parallel to lithological layering and/or foliation as a result of, possibly deformation‐enhanced, channelized fluid ingress. Published and new 40Ar/39Ar and Rb–Sr data for both islands indicate apparent age variations that can be broadly linked to mineral assemblages documenting transitional blueschist‐to‐greenschist‐ and/or greenschist facies metamorphism. The data do not record the timing of peak HP metamorphism, but may accurately record continuous (partial) resetting of isotopic systematics and/or (re)crystallization of white mica during exhumation and greenschist facies retrogression. The form of 40Ar/39Ar phengite age spectra are complex with the lowest temperature steps yielding Middle to Late Miocene ages. The youngest Rb–Sr ages suggest maximum ages of 20.6 ± 0.8 Ma (Syros) and 22.5 ± 0.6 Ma (Sifnos) for the timing of greenschist facies overprinting. The results of this study further accentuate the challenges of interpreting isotopic data for white mica from polymetamorphic terranes, particularly when mixing of populations and/or incomplete resetting of isotopic systematics occurs during exhumation. These data capture the full range of isotopic age variations in retrogressed HP rocks documented in previous isotopic studies, and can be interpreted in terms of the geodynamic evolution of the Aegean.  相似文献   

8.
40Ar/39Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from 40Ar/39Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05–2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful 40Ar/39Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite 40Ar/39Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite 40Ar/39Ar data suggest a detrital muscovite component with an apparent age ≥967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite 40Ar/39Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling.  相似文献   

9.
New single‐grain‐fusion muscovite and paragonite 40Ar/39Ar data from eclogite and blueschist units exposed in the Tauern Window, Eastern Alps yield a range of apparent ages from 90 to 23 Ma. These apparent ages are generally older than expected for 40Ar/39Ar cooling ages, given constraints from other geochronological systems such as Rb–Sr and U–Pb. Numerical Ar‐in‐muscovite diffusion models for Tauern Window nappe P–T paths in an open system suggest that 40Ar/39Ar ages should lie between 29 and 24 Ma, and that they should constrain cooling and decompression following the post‐high pressure Barrovian overprint. The measured ranges of apparent 40Ar/39Ar dates suggest that the assumption of open system behaviour is not valid for this region. The local and/or regional generation of fluid during exhumation promoted pervasive recrystallization of high pressure lithologies throughout the Tauern Window to greenschist and amphibolite facies assemblages. The old apparent 40Ar/39Ar white mica dates in all lithologies are therefore interpreted as being due to inefficient removal of grain boundary Ar by the grain boundary fluids during the Barrovian overprint, due to high Ar concentrations or limited connectivity or both. This caused spatially (mm‐scale) and temporally variable fluxes of Ar out of, and probably into, white mica in both metasedimentary and metabasic lithologies.  相似文献   

10.
Seven eclogite facies samples from lithologically different units which structurally underlie the Semail ophiolite were dated by the 40Ar/39Ar and Rb–Sr methods. Despite extensive efforts, phengite dated by the 40Ar/39Ar method yielded saddle, hump or irregularly shaped spectra with uninterpretable isochrons. The total gas ages for the phengite ranged from 136 to 85 Ma. Clinopyroxene–phengite, epidote–phengite and whole‐rock–phengite Rb–Sr isochrons for the same samples yielded ages of 78 ± 2 Ma. We therefore conclude that the eclogite facies rocks cooled through 500 °C at c. 78 ± 2 Ma, and that the 40Ar/39Ar dates can only constrain maximum ages due to the occurrence of excess Ar inhomogeneously distributed in different sites. Our new results lead us to conclude that high‐pressure metamorphism of the Oman margin took place in the Late Cretaceous, contemporaneous with ophiolite emplacement. Previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a NE‐dipping zone. Choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c. 50 km to 10–15 km within c. 10 Ma, and led to their juxtaposition against overlying lower grade rocks. Final exhumation of all high‐pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate.  相似文献   

11.
A high spatial resolution Ar–Ar dating study of compositionally zoned micas using UV laser extraction has been used to investigate the effect of composition and compositional boundaries upon argon diffusion in mica. The crystals are characterised by muscovite cores and Li–F–Rb–Cs-rich lepidolite rims produced by the interaction of a residual melt-vapour phase with the original muscovite during the late stages of pegmatite crystallisation. Single crystals exhibit dramatic intra-grain age variations, with a maximum range of 2,880–2,117 Ma. Backscattered electron images combined with UV-laser analyses reveal that apparent ages younger than 2,600 Ma are directly associated with the compositionally distinct rims, suggesting that Ar diffusion was faster in the lepidolite than in muscovite. Although it is not possible to be fully quantitative using the present data, it seems that argon diffusion rates in lepidolite are similar to those in biotite. Major rearrangements of the crystal structure needed to accommodate high concentrations of lithium, fluorine and large interlayer cations (Rb+, Cs+) in muscovite account for lower argon retentivity of the crystal rims. In most cases the age profiles show no disturbance at the compositional boundary, despite the transition from a di-octahedral to tri-octahedral mica, indicating that in general the boundaries do not act as fast diffusion pathways. However, in some cases there is a clear drop in ages at the compositional boundary, which acts as an effective sub-grain boundary. The difference between the two types may be related to the internal structure of the compositional boundaries or their degree of contact with the grain boundary network of the rock.  相似文献   

12.
Pegmatites of the Emiytas basic-ultrabasic metamorphic complex have a granodiorite-granite composition. Their genetic relations with the host amphibolites follow from the low initial 87Sr/86Sr ratios of 0.7028 and from the P-T conditions (650°C and 10 kbar), which are close to those of the host rocks. Amphibole, biotite, and muscovite megacrysts analyzed by the 40Ar/39Ar method yielded plateau ages of 209.7 ± 0.9, 203.0 ± 0.9, and 178.5 ± 1.5 Ma, respectively. The former two minerals contain excess Ar, whereas the K-Ar system of the muscovite is undisturbed. The cooling of the complex to the closure temperature of this system was likely controlled by its exhumation to a shallower depth level. Zircons from the Emiytas pegmatites occur as polyhedral equant crystals with weakly contrasting sectorial zoning, very low concentrations of U (4–8 ppm in the enriched domains), and low Th/U ratios (0.002–0.003), which suggest that the mineral crystallized at significant depth in the presence of fluid. Tentative SHRIMP II measurements (five analyses) yielded a zircon age of 202 ± 17 Ma. The applying a specialized approach to the analysis of young low-U zircons on an ion probe is discussed. In spite of the small number of analyses, new geochronologic data leave no doubt that the crystallization age of the pegmatites is Late Triassic-Early Jurassic and invalidate earlier ideas that the Emiytas complex is Precambrian-Early Paleozoic. This conclusion makes the Emiytas amphibolites to be one of the various oceanic and suprasubduction complexes related to the Mesozoic South Anyui suture, which is important for reconstructions of the tectonic evolution of the East Siberian Arctic shelf.  相似文献   

13.
New single grain fusion and core-rim 40Ar/39Ar laserprobe phengite data from the Saih Hatat high-pressure terrane in NE Oman show that individual samples yield a range of apparent ages which is similar to that previously reported from across the entire terrane. The majority of the determined ages are older than the previously reported U-Pb zircon peak metamorphic age. Core to rim age variations within individual grains range from no discernible difference across the grain to grains with older cores, or, rarely, older rims; some samples manifest all three patterns. Numerical diffusion modelling shows that due to the peak temperature of ca. 550°C, the measured apparent ages cannot be explained by simple cooling or by partial retention of crystallisation or detrital ages in an open system. The age variability is better explained by spatially and temporally variable open or closed system behaviour at the mm-cm scale coupled with pervasive and heterogeneously distributed excess argon. Anomalously old eclogite phengite 40Ar/39Ar ages are due either to internally derived 40Ar inherited from a K-bearing precursor, or externally derived 40Ar distributed by grain boundary fluids. Mica-rich schists within the eclogite boudins yield younger phengite ages, suggesting excess argon was absent or diluted. Pelites hosting the eclogite appear to have been affected by later fluid ingress during deformation and greenschist-facies overprint and yield very variable ages commonly with apparently older rims on younger cores. The grain- and sample-scale age variations measured in Saih Hatat indicate that the grain boundary network in eclogite pods was not an efficient transfer pathway for argon transport, whereas the grain boundary network in the surrounding pelites acted as a more efficient pathway on the timescale of the metamorphic cycle.  相似文献   

14.
The Shihu gold deposit is situated in the central shear zone of the Meso-Cenozoic Fuping metamorphic core complex in the middle-northern part of the Taihang Mountains. This kind of gold deposits named after ‘Shihu type’ widely occurs in this region. Gold-bearing quartz veins are their most important industrial orebodies. Detailed laser 40Ar/39Ar geochronology of the mineralized quartz veins in the study area reveals details of its tectono-magmatic history. The 40Ar/39Ar ages for the quartz are between 134.4 Ma and 155.9 Ma with a mean age of 141±4 Ma on the isochron line and 139±7 Ma on the inverse isochron line, respectively, which is interpreted to be the best estimate of the crystalline age of the quartz veins. The gold mineralization was most likely related to an underplating event that took place in the northern part of the Taihang Mountain at ca. 140 Ma. The timing of gold mineralization in the area is similar to those observed in other major gold-producing provinces in the NCC. This episode is simultaneous with those in the eastern NCC, indicative of a widespread late Yanshanian metallogenic event that was a response either to the subduction of the Izanagi-Pacific plate beneath eastern China or to the removal of the Early Cretaceous lithosphere in the eastern NCC. The ore-forming material sources may be the Early Precambrian metamorphic basement, instead of intermediate-acid rock bodies and dykes.  相似文献   

15.
Four slate samples from subduction complex rocks exposed on the south coast of New South Wales, south of Batemans Bay, were analysed by K–Ar and 40Ar/39Ar step‐heating methods. One sample contains relatively abundant detrital muscovite flakes that are locally oblique to the regional cleavage in the rock, whereas the remaining samples appear to contain sparse detrital muscovite. Separates of detrital muscovite yielded plateau ages of 505 ± 3 Ma and 513 ± 3 Ma indicating that inheritance has not been eliminated by metamorphism and recrystallisation. Step‐heating analyses of whole‐rock chips from all four slate samples produced discordant apparent age spectra with ‘saddle shapes’ following young apparent ages at the lowest temperature increments. Elevated apparent ages associated with the highest temperature steps are attributed to the presence of variable quantities of detrital muscovite (<1–5%). Two whole‐rock slate samples yielded similar 40Ar/39Ar integrated ages of ca 455 Ma, which are some 15–30 million years older than K–Ar ages for the same samples. These discrepancies suggest that the slates have also been affected by recoil loss/redistribution of 39Ar, leading to anomalously old 40Ar/39Ar ages. Two other samples, from slaty tectonic mélange and intensely cleaved slate, yielded average 40Ar/39Ar integrated ages of ca 424 Ma, which are closer to associated mean K–Ar ages of 423 ± 4 Ma and 409 ± 16 Ma, respectively. Taking into account the potential influences of recoil loss/redistribution of 39Ar and inheritance, the results from the latter samples suggest a maximum age of ca 440 Ma for deformation/metamorphism. The current results indicate that recoil and inheritance problems may also have affected whole‐rock 40Ar/39Ar data reported from other regions of the Lachlan Fold Belt. Therefore, until these effects are adequately quantified, models for the evolution of the Lachlan Fold Belt, that are based on such whole‐rock 40Ar/39Ar data, should be treated with caution.  相似文献   

16.
Geotectonically the Fengyang and Zhangbaling regions belong to the North China craton and the Dabie-Sulu oragene, respectively. Neo-Archean gneiss and amphibolite and metamor-phosed sea-facies sodic volcanic rocks axe the main outcrops in the two regions, respectively. The Zhangbaling terrane strike-skipped along the Tancheng-Lujiang fault zone in Mesozoic and Cenozo-ic eras and got close to the Fengyang terrane. Mesozoic Yanshanian intrusions occur broadly in thetwo regions. Gold-beating quartz veins occur in the metamorphic rocks in the Fengyang region and in the granodiorite and metamorphosed sea-facies sodic volcanic rocks in the Zhanghaling region.Generally, the formation of the auriferous quartz veins involved three stages. At the first stage,gold-poor sulfide quartz veins were formed; at the second stage gold-rich quartz sulfide veins wereformed; and at the third stage gold-poor barite and/or carbonate veins were formed. The 40^Ar/29^Ar step-heating plateau ages of the first-stage and the second-stage quartz aggregates from the Zhuding, Maoshan and Shangeheng gold deposits range between 116.1 0.6 Ma and 118.3 0.5 Ma and are pretty close to their least apparent ages and isoehronal ages, respectively. All plat-eau, least apparent and isoehronal ages range between 113.4 0.4 Ma and 118.3 0.5 Ma,which are considered as the formation age range of the quartz. It is reasonable and reliable to takethe 40^Ar/39^Ar age range of the quartz as the formation age range of gold-bearing quartz veins onthe basis of spatial relationship between gold-bearing quartz veins and their country rocks. Thegold deposits in the two regions were formed in Aptian, Cretaceous, when the Tancheng-Lujiangfault zone moved as a normal fault with slightly right-lateral strike-skip, was extensional and expe-rienced very strong magnmtic process. It is shown that the magnmtic hydrothermal fluid is a veryimportant part of the gold ore-forming hydrothermal fluid in the Fengyang and Zhanghaling re-gions. The formation of the gold ore deposits in the Fengyang and Zhanghaling regions had genetic relations with the extensional movement of the Tancheng-Lujiang fault zone and magmatic activities and took place under the extensional dynamic condition in Late Cretaceous. Therefore, the exten-sional movement of the Tancheng-Lujiang fault zone presented the energy and space for magmatic and gold ore-forming processes.  相似文献   

17.
Supergene Mn-oxide deposits are widely distributed in Guangxi, Guangdong, Yunnan, and Hunan Provinces, South China, accounting for 18% of the total Mn reserves in the country. Direct dating of supergene Mn enrichment, however, is lacking. In this paper, we present high-resolution 40Ar/39Ar ages of Mn oxides from the Xinrong Mn deposit, western Guangdong, to place numerical constraints on the timing and duration of supergene Mn enrichment. A total of ten cryptomelane samples, spanning a vertical extent of 67 m, were dated using the 40Ar/39Ar laser incremental heating technique, with seven samples yielding well-defined plateau or pseudo-plateau ages ranging from 23.48 ± 0.91 to 2.06 ± 0.05 Ma (2σ). One sample yields a staircase spectrum that does not reach a plateau; the spectrum, however, indicates the presence of two or more generations of Mn oxides in the sample, whose ages are best estimated at 22.34 ± 0.31 and 10.2 ± 0.86 Ma, respectively. The remaining two samples gave meaningless or uninterpretable results due to significant 39Ar recoil and contamination by old phases. The 40Ar/39Ar data thus reveal a protracted history of weathering and supergene Mn enrichment that started at least in the end of the Oligocene or beginning of Miocene and extending into the latest Pliocene. Staircase-apparent age spectra, resulting from banded or botryoidal samples, yield an average growth rate of Mn oxides at 0.6–0.7 × 10−3 mm kyr−1. The values indicate that a 1-mm grain of Mn oxides may host minerals precipitated during a time span of ca. 1.5 m.y., and accumulation of Mn oxides to form economic deposits under weathering environments may take millions of years. The distribution of weathering ages shows that the oldest Mn oxides occur on the top of the profile, whereas the youngest minerals are found at the bottom, suggesting downward propagation of weathering fronts. However, two samples located at the intermediate depths of the profile yield ages comparable with those occurring at the highest elevations. Such a complexity of age distribution is interpreted in terms of preferential penetration of Mn-rich weathering solutions along more permeable fault zones, or as a result of multi stages of dissolution and re-precipitation of Mn oxides. A synthesis of geochronological and geological data suggests that formation of the Xinrong deposit was a consequence of a combination of favorable lithological, climatic, and structural conditions. Because the climatic and structural conditions are similar among the provinces of South China during the Cenozoic, the geochronological results obtained at Xinrong may also have implications for the timing of supergene Mn enrichment throughout South China.  相似文献   

18.
Laser-probe dating of mylonite whole-rock samples from the North Tianshan—Main Tianshan fault zone that cross-cuts the North Tianshan domain’s southern margin yielded 40Ar/39Ar spectra with 255–285 Ma ages. Biotite from an undeformed, Early Carboniferous granite, which cuts the steep mylonitic foliation in the Proterozoic basement of the Yili arcs’s southern margin, gave a 263.4 ± 0.6 Ma plateau age (1σ). Pre-Carboniferous metasediments overlying this basement yielded plateau ages (1σ) of 253.3 ± 0.3 (muscovite) and 252.3 ± 0.3 (biotite) Ma. The Permian ages of mylonites date movement on these ductile, dextral strike-slip shear zones, whereas the mica ages are interpreted by recrystallisation as a result of fluid flow around such transcurrent faults. We propose that the Tianshan’s Permian syn-tectonic bimodal magmatism was created in a non-plume-related Yellowstone-like extensional–transtensional tectonic regime. Gold mineralisation, tracing aqueous flow in the crust, peaked in Permian time and continued locally into the Triassic. The picture is emerging that a convective fluid system partly driven by magmatic heat, existed in a strongly fractured and weakened crust with an elevated heat flow, leading to regional-scale isotope resetting. We suggest that surprisingly young isotopic ages in the literature for early orogenic (ultra)high-pressure metamorphism are similarly due to fluid-mediated recrystallisation.  相似文献   

19.
The Northern Zagros Suture Zone (NZSZ), formed as a result of the collision between Arabian and Sanandaj-Sirjan microplate, is considered as part of the Zagros orogenic belt. NZSZ is marked by two allochthonous thrust sheets in upward stacking order: lower and upper allochthon. The Bulfat complex is a part of the upper allochthon or “Ophiolite-bearing terrane” of Albian-Cenomenion age (97–105 Ma). Voluminous highly sheared serpentinites associated with ophiolites occur within this upper allochthon. In addition, the Gemo-Qandil Group is characterized by gabbroic to dioritic Bulfat intrusion with a crystallization age spanning from ~45 to ~?40 Ma, as well as extensive metapelites with contact to the Walash-Naupurdam metavolcanic rocks. Due to the deformation in the Sanandaj-Sirjan Zone along the eastern side of the Iraqi segment of NZSZ, the Gemo-Qandil Group was regionally metamorphosed during late Cretaceous (~?80 Ma). This tectono-compressional dynamics ultimately caused an oscillatory deformation against Arabian continental margin deposits as well. During these events, gabbro-diorite intrusion with high-grade contact metamorphic aureoles occurred near Bulfat. Thus, there is an overlap between regional and contact metamorphic conditions in the area. The earlier metamorphic characteristic can be seen only in places where the latter contact influence was insignificant. Generally, this can only observed at a distance of more than 2.5 km from the contact. According to petrographic details and field observations, the thermally metamorphosed metapelitic units of the metasediment have been completely assimilated, with only some streaks of biotite and relicts of initial foliation. They strongly resemble amphibolite-grade slices from the regional metamorphic rocks in the region. Metapelitic samples far from the intrusion give similar biotite cooling ages as the intrusive rocks. Thus, they may be affected by the same thermal event. 40Ar/39Ar dating of biotite in metapelite rocks of Bulfat by step-wise heating with laser gave average weighted isotopic ages of 34.78?±?0.06 Ma. This is interpreted as crystallization/recrystallization age of biotite possibly representing the time of cooling and uplift history of the Bulfat intrusion. Cooling and exhumation rates for the Bulfat gabbro-diorite rocks were estimated as ~?400 °C/Ma and ~?3.3 mm/year respectively. According to petrographic details, field observations and Ar/Ar dating concerning the contact metamorphism near Bulfat due to the gabbro-diorite intrusion, no significant deformation is visible during exhumation processes after the Paleogene tectono-thermal event, indicating that isotopic ages of 34.78?±?0.06 Ma could mark the timing of termination of the island arc activity in the Ophiolite-bearing terrane (upper allochthon).  相似文献   

20.
Sized aggregates of glasses (47–84 wt% SiO2) were fused from igneous-derived cohesive fault rock and igneous rock, and step-heated from ~400 to >1,200 °C to obtain their 39Ar diffusion properties (average E=33,400 cal mol?1; D o=4.63×10?3 cm2 s?1). At T<~1,000 °C, glasses containing <~69 wt% SiO2 and abundant network-forming cations (Ca, Fe, Mg) reveal moderate to strong non-linear increases in D and E, reflecting structural modifications as the solid transitions to melt. Extrapolation of these Arrhenius properties down to typical geologic T-t conditions could result in a 1.5 log10 unit underestimation in the diffusion rate of Ar in similar materials. Numerical simulations based upon the diffusion results caution that some common geologic glasses will likely yield 40Ar/39Ar cooling ages rather than formation ages. However, if cooling rates are sufficiently high, ambient temperatures are sufficiently low (e.g., <65–175 °C), and coarse particles (e.g., radius (r) >~1 mm) are analyzed, glasses with compositions similar to ours may preserve their formation ages.  相似文献   

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