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1.
Hydrothermal treatment of closely sized muscovite aggregates in a piston-cylinder apparatus induced 40Ar∗ loss that is revealed in 40Ar/39Ar step heating spectra. Age spectra and Arrhenius data, however, differ from that expected from a single diffusion length scale. A numerical model of episodic loss assuming the presence of multiple diffusion domains yields excellent fits between synthetic and actual degassing spectra. We used this model to isolate 40Ar∗ loss from the grains that remained intact during hydrothermal treatment at 10 kbar permitting calculation of diffusion coefficients in the temperature range 730-600 °C. Diffusion data generated in this manner yield an activation energy (E) of 63 ± 7 kcal/mol and frequency factor (Do) of 2.3  cm2/s. Experiments at 20 kbar yield diffusivities lower by about an order of magnitude and correspond to an activation volume of ∼14 cm3/mol. Together, these parameters predict substantially greater retentivity of Ar in muscovite than previously assumed and correspond to a closure temperature (Tc) of 425 °C for a 100 μm radius grain cooling at 10 °C/Ma at 10 kbar (Tc = 405 °C at 5 kbar. Age and log (r/ro) spectra for the run products show strong correlations indicating that muscovites can retain Ar diffusion boundaries and mechanisms that define their natural retentivity during vacuum step heating. This may permit the application of high resolution, continuous 40Ar/39Ar thermochronology to low grade, regionally metamorphosed terranes.  相似文献   

2.
40Ar/39Ar dating was conducted on the Da Lien granite related to greisen‐skarn type polymetallic (W‐CaF2‐Cu‐Bi‐Au) mineralization in Nui Phao, northern part of Vietnam in the South China Plate. Biotite and muscovite separates from the biotite‐muscovite granite and greisenized granite indicate four plateau ages: 82.2 ± 0.4 Ma, 82.8 ± 0.3 Ma, 81.5 ± 0.3 Ma and 82.5 ± 0.4 Ma. The plateau ages were not significantly influenced by excess 40Ar in dated minerals or by loss of radiogenic 40Ar due to hydrothermal activities. The results indicate that solidification of granite related to the polymetallic mineralization occurred in the Late Cretaceous between 82.8 Ma and 81.5 Ma.  相似文献   

3.
Muscovite and biotite from a crustal-scale mylonite zone (Pogallo Shear Zone, southern Alps) were investigated using furnace step-heating and in-situ UV-laser ablation 40Ar/39Ar geochronology. Undeformed muscovite porphyroclasts yield 40Ar/39Ar plateau ages of 182.0ǃ.6 Ma, whereas in-situ UV-laser ablation 40Ar/39Ar dating and furnace step-heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger. The range of 40Ar/39Ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra-grain scale. These microstructurally controlled segments are bound by either first-order lattice discontinuities, sub-microscopic structural defects and/or zones of high defect density, which create variable length-scales for intragranular argon diffusion. The observed deformational microstructures within muscovite and biotite acted as intra-grain fast diffusion pathways in the slowly cooled mylonitic rocks. Therefore, the high-spatial resolution 40Ar/39Ar data record the initial and final closure to argon diffusion over a time span of about 60 Ma.  相似文献   

4.
Linking ages to metamorphic stages in rocks that have experienced low‐ to medium‐grade metamorphism can be particularly tricky due to the rarity of index minerals and the preservation of mineral or compositional relicts. The timing of metamorphism and the Mesozoic exhumation of the metasedimentary units and crystalline basement that form the internal part of the Longmen Shan (eastern Tibet, Sichuan, China), are, for these reasons, still largely unconstrained, but crucial for understanding the regional tectonic evolution of eastern Tibet. In situ core‐rim 40Ar/39Ar biotite and U–Th/Pb allanite data show that amphibolite facies conditions (~10–11 kbar, 530°C to 6–7 kbar, 580°C) were reached at 210–180 Ma and that biotite records crystallization, rather than cooling, ages. These conditions are mainly recorded in the metasedimentary cover. The 40Ar/39Ar ages obtained from matrix muscovite that partially re‐equilibrated during the post peak‐P metamorphic history comprise a mixture of ages between that of early prograde muscovite relicts and the timing of late muscovite recrystallization at c. 140–120 Ma. This event marks a previously poorly documented greenschist facies metamorphic overprint. This latest stage is also recorded in the crystalline basement, and defines the timing of the greenschist overprint (7 ± 1 kbar, 370 ± 35°C). Numerical models of Ar diffusion show that the difference between 40Ar/39Ar biotite and muscovite ages cannot be explained by a slow and protracted cooling in an open system. The model and petrological results rather suggest that biotite and muscovite experienced different Ar retention and resetting histories. The Ar record in mica of the studied low‐ to medium‐grade rocks seems to be mainly controlled by dissolution–reprecipitation processes rather than by diffusive loss, and by different microstructural positions in the sample. Together, our data show that the metasedimentary cover was thickened and cooled independently from the basement prior to c. 140 Ma (with a relatively fast cooling at 4.5 ± 0.5°C/Ma between 185 and 140 Ma). Since the Lower Cretaceous, the metasedimentary cover and the crystalline basement experienced a coherent history during which both were partially exhumed. The Mesozoic history of the Eastern border of the Tibetan plateau is therefore complex and polyphase, and the basement was actively involved at least since the Early Cretaceous, changing our perspective on the contribution of the Cenozoic geology.  相似文献   

5.
The role of internal deformation in resetting argon ages of micas has been investigated by measuring 40Ar/39Ar ratios of biotite and muscovite, before and after experimentally deforming them. Neither mica crushed cataclastically at room temperature on-line with a mass spectrometer showed any measurable change in 40Ar/39Ar age. Muscovite crystals either sheared along the K-interlayer and/or kinked at 400 °C and 100–200 MPa confining pressure, exhibit small (0.7–1.0%) reductions in age and marked increases in bulk diffusion coefficients, as determined from argon release during the initial stages of step-heating between 550 and 810 °C. We conclude that the relatively young ages of fine-grained mica populations in naturally deformed mylonites result primarily from grain size refinement and reductions in length scale for volume diffusion and/or by syntectonic neocrystallization. Internal deformation involving dislocation slip and kinking may contribute to some argon loss by pipe diffusion, but reductions in closure temperature associated with multipath diffusion are small.  相似文献   

6.
In the case of volume diffusion, the closure temperature of a mineral is function of, among other factors, the characteristic diffusion dimension, which can be approximated by the grain size of the mineral analysed for grains smaller than or similar in size to the diffusion domains. The theoretical possibility of single mineral grain size thermochronology had been demonstrated empirically in earlier studies, mostly using biotite. In order to examine the potential of this method, it was tested alongside the widely used multi-mineral 40Ar/39Ar thermochronology. The sample comes from the granitic McLean pluton, in the south section of the Grenville orogeny. Seven grain size separates of biotite (ranging between 90 and 1000 μm), eight size fractions of amphibole (between 63 and 1000 μm), and three size fractions of K-feldspar (250-600 μm) were extracted and dated by the laser step-heating 40Ar/39Ar method. The total gas ages obtained behave as theoretically predicted, with increasing ages for increasing grain sizes, including for K-feldspar, but with the exception of the smallest and the largest grains for biotite and amphibole. The calculated cooling rates are ca. 0.7 °C/Ma for K-feldspar, ca. 2.5 °C/Ma for biotite, and ca. 11 °C/Ma for amphibole, corresponding very well to a monotonic cooling of the McLean pluton. A quick initial thermal re-equilibration with the cooler host-rocks is followed by a much slower cooling on a thermal path parallel to that of the Frontenac Terrain situated immediately to the southeast. The validity of the single mineral grain size thermochronology is demonstrated by comparison with the thermal evolution of the adjacent units and with the cooling history derived from a multi-mineral thermochronology, suggesting that it can be routinely used. The application of this method can be hampered by insufficiently low analytical uncertainties.  相似文献   

7.
The 40Ar/39Ar dating technique is based on the knowledge of the age of neutron fluence monitors (standards). Recent investigations have improved the accuracy and precision of the ages of most of the Phanerozoic-aged standards (e.g. Fish Canyon Tuff sanidine (FCs), Alder Creek sanidine, GA1550 biotite and LP-6 biotite); however, no specific study has been undertaken on the older standards (i.e. Hb3gr hornblende and NL-25 hornblende) generally used to date Precambrian, high Ca/K, and/or meteoritic rocks.In this study, we show that Hb3gr hornblende is relatively homogenous in age, composition (Ca/K) and atmospheric contamination at the single grain level. The mean standard deviation of the 40Ar?/39ArK (F-value) derived from this study is 0.49%, comparable to the most homogeneous standards. The intercalibration factor (which allows direct comparison between standards) between Hb3gr and FCs is RFCsHb3gr = 51.945 ± 0.167. Using an age of 28.02 Ma for FCs, the age of Hb3gr derived from the R-value is 1073.6 ± 5.3 Ma (1σ; internal error only) and ± 8.8 Ma (including all sources of error). This age is indistinguishable within uncertainty from the K/Ar age previously reported at 1072 ± 11 Ma [Turner G., Huneke, J.C., Podosek, F.A., Wasserburg, G.J., 1971. 40Ar-39Ar ages and cosmic ray exposure ages of Apollo 14 samples. Earth Planet. Sci. Lett. 12, 19-35].The R-value determined in this study can also be used to intercalibrate FCs if we consider the K/Ar date of 1072 Ma as a reference age for Hb3gr. We derive an age of 27.95 ± 0.19 Ma (1σ; internal error only) for FCs which is in agreement with the previous determinations. Altogether, this shows that Hb3gr is a suitable standard for 40Ar/39Ar geochronology.  相似文献   

8.
Improved precision of radiometric dating of ore deposits can provide information about the thermal history of hydrothermal circulation in cooling plutons. In Jales a Hercynian porphyritic two-mica granite and pre-Ordovician mica schists are cut and intensely altered by the Campo gold-quartz vein. The unaltered granite must be younger than 320 ± 6 Ma, and gives mica Rb-Sr ages of 308.5 ± 2.4 (1) Ma (muscovite) and 294.5 ± 1.1 Ma (biotite). Alteration muscovites from the granite give a weighted mean Rb-Sr age of 308.1 ± 1.5 Ma, and a mean 39Ar-40Ar age of 300.7 ± 2.8 Ma. Alteration muscovites from the mica schists give similar 39Ar-40Ar ages, averaging 303.0 ± 2.8 Ma. The results suggest that circulation of the Campo mineralising fluids took place no more than 2–4 Ma after the granite cooled through the muscovite Rb-Sr closure temperature, about 500 °C, and that subsequent cooling to biotite closure at about 300 °C took place at less than 14°C/Ma. The mean cooling rate following emplacement was 15 to 25 °C/Ma. The most detailed comparable published data, for the Cornubian ore field, imply much faster cooling rates.  相似文献   

9.
Muscovites from the Blond granite (West French Massif Central) were dated by the 40Ar/39Ar single-grain method. The 40Ar/39Ar ages obtained vary from 305.5 ± 0.3 Ma to 311.3 ± 0.5 Ma, and most of the age spectra are slightly saddle-shaped. The analyzed muscovites show phengitic recrystallization under optical microscope observations, SEM images, and electron microprobe chemical analyses. It is proposed that the saddle-shaped age spectra result from a partial recrystallization, which produced three different isotopic reservoirs in the analyzed white mica single grains: domains of early muscovite, domains of neocrystallized muscovite formed by phengitic and Al-Fe substitutions, and “low-activation energy sites.”  相似文献   

10.
New 40Ar/39Ar thermochronology results and thermal modeling support the hypothesis of Hollister et al. (2004), that reheating of the mid-Cretaceous Ecstall pluton by intrusion of the Coast Mountains Batholith (CMB) was responsible for spatially variable remagnetization of the Ecstall pluton. 40Ar/39Ar ages from hornblende and biotite from 12 locations along the Skeena River across the northern part of the Ecstall pluton decrease with proximity to the Quottoon plutonic complex, the nearest member of the CMB to the Ecstall pluton. The oldest 40Ar/39Ar ages are found farthest from the Quottoon plutonic complex, and are 90 ± 3 Ma for hornblende, and 77.9 ± 1.2 Ma for biotite. The youngest 40Ar/39Ar ages are found closest to the Quottoon plutonic complex, and are 51.6 ± 1.2 Ma for hornblende, and 45.3 ± 1.7 Ma for biotite. No obvious relationship between grain size and age is seen in the Ecstall pluton biotites. Spatial trends in 40Ar/39Ar ages are consistent with model results for reheating by a thermal wall at the location of the Quottoon plutonic complex. Although no unique solution is suggested, our results indicate that the most appropriate thermal history for the Ecstall pluton includes both reheating and northeast side up tilting of the Ecstall pluton associated with intrusion of the Quottoon plutonic complex. Estimates of northward translation from shallow paleomagnetic inclinations in the western part of the Ecstall pluton are reduced to ∼3000 km, consistent with the Baja-BC hypothesis, when northeast side up tilting is accounted for.  相似文献   

11.
Studies of meteorites are based mostly on samples that fell to Earth in the recent past (i.e., a few million years at most). The Morokweng LL-chondrite meteorite is a particularly interesting specimen as its fall is much older (ca. 145 Ma) than most other meteorites and because it is the only macro-meteorite clast (width intersected in drill core: 25 cm) found in a melt sheet of a large impact structure. When applied to the Morokweng meteorite, 40Ar/39Ar thermochronology provides an opportunity to study (1) effects associated with pre-impact and post-impact processes and (2) collision events within a potentially distinct and as yet unsampled asteroid population.A single multi-grain aliquot yielded an inverse isochron age of 625 ± 163 Ma. This suggests a major in-space collisional event at this time. We have modeled the diffusion of 40Ar∗ within the meteorite and plagioclase during and after the ∼145 Ma impact on Earth to tentatively explain why pre-terrestrial impact 40Ar∗ has been preserved within the plagioclase grains. The ∼145 Ma terrestrial impact age is recorded in the low-retentivity sites of the meteorite plagioclase grains that yielded a composite inverse isochron age at 141 ± 15 Ma and thus, confirms that age information about major (terrestrial or extraterrestrial) impacts can be recorded in the K-rich mineral phases of a meteorite and measured by the 40Ar/39Ar technique. More studies on fossil meteorites need to be carried out to understand if the rough 0.6 Ga age proposed here corresponds to major LL-chondrite asteroid population destructions or, rather, to an isolated collision event.  相似文献   

12.
The argon isotope systematics of vein-quartz samples with two different K-reservoirs have been evaluated in detail. Potassium is hosted by ultra-high-salinity fluid inclusions in quartz samples from the Eloise and Osborne iron-oxide-copper-gold (IOCG) deposits of the Mt Isa Inlier, Australia. In contrast, K is hosted by accidentally trapped mica within lower-salinity fluid inclusions of a sample selected from the Railway Fault, 13 km south of the Mt Isa copper mine, Australia. Imprecise apparent ages have been obtained for all of the samples studied and conclusively demonstrate that quartz fluid inclusions are retentive to Ar and have not leaked over billions of years. IOCG samples that host K in fluid inclusions only, have K/Cl values of <1 and the ages obtained represent the maximum ages for mineralization. In contrast, the Railway Fault samples that include accidentally trapped mica have K/Cl values of ?1. Excess 40ArE plus Cl hosted by fluid inclusions, and radiogenic 40ArR plus K, are strongly correlated in these samples and define a plane in 3D 40Ar-36Ar-K-Cl space. In this case, the plane yields an ‘excess 40ArE’ corrected age of ∼1030 Ma that is 100’s of Ma younger than nearby Cu-mineralization at Mt Isa. The age is interpreted to reflect 40Ar-loss from the accidentally trapped mica into the surrounding fluid inclusions, and is not related to the samples’ age of formation. The initial 40Ar/36Ar value of fluid inclusions is widely used to provide information on fluid origin. For the IOCG samples that host K in fluid inclusions only, the initial 40Ar/36Ar values are close to the measured values at every temperature of stepped heating experiments. For samples that include accidentally trapped mica, the correction for post-entrapment radiogenic 40ArR production is significant. Furthermore, because 39ArK present in accidentally trapped mica crystals is released at different temperatures to radiogenic 40ArR lost to the surrounding fluid inclusions, intra-sample 40Ar/36Ar variation cannot be reliably documented. The results demonstrate that noble gas analysis is readily applicable to Proterozoic, or older, samples but that if K-mineral impurities are present within quartz the abundance of K must be determined before calculation of mean 40Ar/36Ar values that are representative of the samples’ initial composition.  相似文献   

13.
In a recent study, Qiu and Wijbrans (2006) [Qiu, H.-N. and Wijbrans, J. R. (2006). Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from 40Ar/39Ar dating by stepwise crushing. Geochimica et Cosmochimica Acta70, 2354-2370.] analyzed Ar-isotopes extracted by crushing garnets from the Dabie Shan Bixiling eclogite and claimed to have constrained a pre-Triassic (∼450 Ma) episode of UHP metamorphism from primary fluid inclusions.However, in the absence of careful sample characterization and stepped heating analyses, the reported ages are more easily explained as experimental artifacts related to Ar extracted from either mineral inclusions or the interface sites between mineral inclusions and the garnet matrix: Dabie Shan garnets commonly contain mineral impurities such as K-rich omphacite and/or K-feldspar.If Dabie Shan UHP metamorphism is of the generally accepted Triassic age (210-240 Ma), the apparent age of a phengite sample in equilibrium with the garnet can be explained by the presence of an extraneous 40Ar∗ component with mean 40Ar/36Ar value of ∼5000. This value is similar to the composition of extraneous 40Ar∗ in other eclogite facies terrane.  相似文献   

14.
The 40Ar/39Ar dating technique requires the activation of 39Ar via neutron irradiation. The energy produced by the reaction is transferred to the daughter atom as kinetic energy and triggers its displacement, known as the recoil effect. Significant amounts of 39Ar and 37Ar can be lost from minerals leading to spurious ages and biased age spectra. Through two experiments, we present direct measurement of the recoil-induced 39Ar and 37Ar losses on Fish Canyon sanidine and plagioclase. We use multi-grain populations with discrete sizes ranging from 210 to <5 μm. One population consists of a mixture between sanidine and plagioclase, and the other includes pure sanidine.We show that 39Ar loss (depletion factor) for sanidine is ∼3% for the smallest fraction. Age spectra of fractions smaller than ∼50 μm show slight departure from flat plateau-age spectrum usually observed for large sanidine. This departure is roughly proportional to the size of the grain but does not show typical 39Ar loss age spectra. The calculated thickness of the total depletion layer d0(sanidine) is 0.035 ± 0.012 (2σ). This is equivalent to a mean depth of the partial depletion layer (x0) of 0.070 ± 0.024 μm. The latter value is indistinguishable from previous values of ∼0.07-0.09 μm obtained by argon implantation experiments and simulation results.We show that it is possible to adequately correct ages from 39Ar ejection loss provided that the d0-value and the size range of the minerals are sufficiently constrained. As exemplified by similar calculations performed on results obtained in a similar study of GA1550 biotite [Paine J. H., Nomade S., and Renne P. R. (2006) Quantification of 39Ar recoil ejection from GA1550 biotite during neutron irradiation as a function of grain dimensions. Geochim. Cosmochim. Acta70, 1507-1517.], the d0(biotite) is 0.46 ± 0.06 μm. The significant difference between empirical results on biotite and sanidine, along with different simulation results, suggests that for biotite, crystal structures and lattice defects of the stopping medium and possibly subsequent thermal degassing (due to ∼150-200 °C temperature in the reactor or extraction line bake out) must play an important role in 39Ar loss.The second experiment suggests that 37Ar recoil can substantially affect the age via the interference corrections with results that suggest up to ∼98% of 37Ar can be ejected from the ∼5 μm grain dimension.Further investigation of silicates of various compositions and structures are required to better understand (and correct) the recoil and recoil-induced effects on both 39Ar and 37Ar and their influences on 40Ar/39Ar dating.  相似文献   

15.
This study presents the first measurement of 39Ar recoil ejection loss from individual, dimensionally characterized mineral grains due to neutron irradiation, and reveals the extent to which this recoil loss is problematic for 40Ar/39Ar dating. Using the well-characterized biotite standard GA1550, known to have between grain reproducibility of 40Ar*/39ArK of order 0.1%, we measured the thicknesses (3-210 μm) and surface areas (0.07-0.90 mm2) of 159 grains selected to span the dimensional range represented in the aliquot. Thinner grains with high surface area/volume (SA/V) reveal elevated 40Ar/39Ar, as much as 26% higher than thicker grains expected to suffer proportionately negligible depletion. Although the thinner grains yield intrinsically less precise measurements due to small 39Ar ion beams, a regular decrease in net recoil loss with increasing biotite grain thickness is clear for grains thinner than ca. 50 μm. Grains thicker than 50 μm reveal essentially no 39Ar loss within analytical uncertainties. The measured 39Ar loss spectrum is significantly higher than predicted by previous modeling approaches. These results suggest a practical threshold of ca. 50 μm grain thickness for biotites, and probably other phyllosilicates, irradiated with 235U fission spectrum neutrons in order to avoid recoil artifacts. Poor agreement between our data and simulation results indicates that recoil displacement models should be revisited in order to resolve the discrepancy. Further empirical work to determine the recoil loss of 39Ar in other minerals is important not only for routine age measurements, but also to shed more light on the role of recoil in multi-diffusion domain theory and other thermochronologic applications exploiting variable diffusion radii and/or grain size effects.  相似文献   

16.
Mica porphyroblasts, a common metamorphic microstructure, are analyzed in the slate belt of northern Taiwan where large fish-like growths are found within a meta-pyroclastics. With constraints on the time-temperature history from deposition through peak metamorphic state to exhumation, in-situ 40Ar/39Ar laser microprobe dating was carried out on muscovite and corrensite fibers of mm-scale mica porphyroblasts grown on a pressure-solution seam. Because the peak metamorphic temperature and the porphyroblast formation condition (∼250 °C) remained far below the closure temperature of the K-Ar radioisotope system in muscovite, and the absence of muscovite in the mafic protolith, the dating results likely document the growth of the mica porphyroblast fabrics. The syn-kinematic nature of the analyzed porphyroblasts is confirmed by the ∼6 to ∼2.5 Ma growth ages, suggesting that the host rock was continuously deformed during the earlier two-thirds of the Taiwan Orogeny. The pattern of fiber growth, in contrast to outward-decreasing ages normally observed in peripheral recrystallization, appears random and resembles void fills in boudin openings. We postulate that syntaxial crack-seal following tensile micro-boudinage, along with slips on sub-grain boundaries, as a viable mechanism for the development of mica porphyroblasts and fish especially in lower-grade metamorphic rocks.  相似文献   

17.
Forty-four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for 40Ar/39Ar and demonstrate the importance of bulk Ar diffusivity and system geometry—factors not usually considered in the interpretation and collection of 40Ar/39Ar age data. The resulting 40Ar/39Ar apparent ages range from 11.30 ± 0.05 Ma to 17.90 ± 0.10 Ma. The ages (and excess argon contents) are spatially and lithologically correlated. The pelite samples all yield ages clustering around ∼12 Ma, the age expected for cooling through biotite closure (∼360°C) in this region of the Alps. Ages in the amphibolite biotites are older, showing a smooth trend between 15 Ma at the contact with the pelite to 18 Ma, 34 cm from the contact. This data shows that characterization of the Ar closure age for biotite in a given system should not rest on a single sample, as otherwise irresolvable differences in age between samples within the same outcrop can exist. A generalized mechanistic model for excess argon is presented. The presence (or absence) of excess Ar depends on an intrinsic system parameter, τT, the transmissive timescale, which is the characteristic time for 40Ar to escape through the local intergranular transporting medium (ITM) to some sink for argon. To prevent buildup of geochronologically significant excess 40Ar, τT must be very short relative to the true closure age of the mineral. A FORTRAN code including radiogenic Ar production, diffusive loss of Ar from biotite, and bulk Ar diffusion through the ITM has been developed. Application of numerical modeling suggests that the time-averaged effective bulk diffusivity, DeffAr, in the biotite-amphibolite rock during early retrograde cooling is 2.2 ± 1.0 × 10−8 m2/yr (assuming steady state conditions) - the first such measurement available. Numerical modeling also provides information about the transmissivity and geologic history specific to the field site, including a drop in DeffAr at 15.5 ± 1.0 Ma. The timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack.  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.
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   

20.
Detailed field and microstructural studies have been combined with high spatial resolution ultraviolet laser 40Ar/39Ar dating of naturally deformed K-feldspar to investigate the direct relationship between deformation-related microstructure and Ar isotope systematics. The sample studied is a ~1,000 Ma Torridonian arkose from Skye, Scotland, that contains detrital feldspars previously metamorphosed at amphibolite-facies conditions ~1,700 Ma. The sample was subsequently deformed ~430 Ma ago during Caledonian orogenesis. The form and distribution of deformation-induced microstructures within three different feldspar clasts has been mapped using atomic number contrast and orientation contrast imaging, at a range of scales, to identify intragrain variations in composition and lattice orientation. These variations have been related to thin section and regional structural data to provide a well-constrained deformation history for the feldspar clasts. One hundred and forty-three in-situ 40Ar/39Ar analyses measured using ultraviolet laser ablation record a range of apparent ages (317-1030 Ma). The K-feldspar showing the least strain records the greatest range of apparent ages from 420-1,030 Ma, with the oldest apparent ages being found close to the centre of the feldspar away from fractures and the detrital grain boundary. The most deformed K-feldspar yields the youngest apparent ages (317-453 Ma) but there is no spatial relationship between apparent age and the detrital grain boundary. Within this feldspar, the oldest apparent ages are recorded from orientation domain boundaries and fracture surfaces where an excess or trapped 40Ar component resides. Orientation contrast images at a similar scale to the Ar analyses illustrate a significant deformation-related microstructural difference between the feldspars and we conclude that deformation plays a significant role in controlling Ar systematics of feldspars at both the inter- and intragrain scales even at relatively low 'bulk' strains. The data show that Ar loss and trapping within the deformed K-feldspars reflects the presence of a deformation-induced population of small diffusion domains in combination with 'short-circuit' diffusion along deformation-induced defects. The complex history of microstructures induced in the K-feldspars during their cooling, alteration, erosion and sedimentation do not appear to be as significant as deformation-induced microstructures in controlling the distribution of apparent ages at the grain scale.  相似文献   

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