40Ar-39Ar and Rb-Sr geochronology of high-pressure metamorphism and exhumation history of the Tavsanli Zone, NW Turkey

Geochronological investigations in high- and ultra-high-pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages, and secondly the problem of “excess argon” affects ⁴⁰Ar-³⁹Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of HP-LT metamorphism, raising the question of whether they are geologically significant. This study presents new data from the Tavsanli Zone; ⁴⁰Ar-³⁹Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, confirming the presence of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger ⁴⁰Ar-³⁹Ar ages in the cores of coarser white micas, which in conjunction with ⁴⁰Ar-³⁹Ar ages from the finest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of syn-continent-continent collision at a rate of approximately 1.5 mma⁻¹ and exhumation to the surface via thrusting. The ⁴⁰Ar-³⁹Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological approach when attempting to determine the history of HP-LT rocks. Received: 12 May 1998 / Accepted: 21 April 1999     

Eocene high pressure metamorphism in the Penninic units of the Tauern Window (Eastern Alps): evidence from40Ar−39Ar dating and petrological investigations

The⁴⁰Ar-³⁹Ar degassing spectra of white micas and amphiboles from three tectonic units of the central Tauern Window (Pennine basement and cover in the Eastern Alps) have been measured. White micas are classified as (1) pre-Alpine low-Si relic micas with an age value of 292 Ma, variously disturbed by the Alpine metamorphism; (2) Alpine phengitic micas of variable composition with an age between 32 and 36 Ma; (3) Alpine low-Si micas with a maximum age of 27 Ma. We attribute the higher Alpine ages to a blueschist facies event, whereas the lower age reflects the late cooling of the nappe pile. Blueschist facies phengites from the basement (Lower Schieferhülle) and the tectonic cover (Upper Schieferhülle) crystallized at a temperature below the closure temperature (T _c) for argon diffusion in white mica and record ages of 32 to 36 Ma. At the same time a thin, eclogite facies unit (Eclogite Zone) was thrust between the Lower and the Upper Schieferhülle and cooled from eclogite facies conditions at about 600°C at 20 kbar to blueschist facies conditions at 450°C or even 300°C at >10 kbar. Eclogite facies phengites closed for argon diffusion and record cooling ages, coinciding with the crystallization ages in the hanging and the footwall unit. Amphibole age spectra (actinolite, glaucophane, barroisite) are not interpretable in terms of geologically meaningful ages because of excess argon.     

Ar-Ar age of mylonites along the Merens Fault, central Pyrenees

Seven mylonitic samples and two coarse muscovites from the central Pyrenees have been dated by the ⁴⁰Ar-³⁹Ar method. Whole rock specimens of mylonite were cut out of thin-section chips allowing complete characterisation of mineralogy and texture. Several specimens showed rising staircase patterns in the range 50–90 Ma, with much higher ages in the highest temperature steps. This is believed to reflect mixing of argon released from micas with excess argon contained in plagioclase and released mainly at high temperatures. One biotite-quartz mylonite gave a plateau age of 93 ± 2 Ma. Other inferred mica ages are about 60–73 Ma for biotite and 50–60 Ma for muscovite; it is probable that biotite contains excess argon and that 50 Ma approximates to the cooling age in the mylonites. One coarse muscovite collected immediately below the major Mérens shear zone gave a Hercynian plateau age, while another collected within the Mérens zone gave a partially reset Hercynian age.Taken together, the data indicate that the shear zones were active in Alpine times < 100 Ma and probably about 50 Ma ago. They are believed to have formed during the early stages of Eocene compression in the Pyrenees. Deformation and resultant uplift probably terminated an important thermal event in this part of the Pyrenean basement, which may have begun at the time of the mid-Cretaceous North Pyrenean metamorphism (90–100 Ma).     

Spatially correlated anomalous Ar/Ar “age” variations in biotites about a lithologic contact near Simplon Pass, Switzerland: a mechanistic explanation for excess Ar

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, τ_T, 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, τ_T 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, D_eff^Ar, 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 D_eff^Ar 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.     

An 40Ar–39Ar investigation of high-pressure metamorphism and the retrogressive history of mafic eclogites from the Lanterman Range (Antarctica): evidence against a simple temperature control on argon transport in amphibole

Mafic eclogites sampled from a restricted area in the Lanterman Range (Antarctica) retrogressed variably under amphibolite facies metamorphism. Assemblages range from well-preserved eclogite, with minor growth of Na-Ca amphibole, to strongly retrogressed ones with extensive development of Ca amphibole. ⁴⁰Ar-³⁹Ar furnace step-heating experiments on the different amphiboles yield results varying from plateau ages of ~498 Ma to a near-plateau age of ~490 Ma, and the greater the amphibolite retrogression, the younger the age. ⁴⁰Ar-³⁹Ar infrared laser-probe analyses on rock chips from a well-preserved eclogite and a slightly retrogressed one reveal the presence of an excess argon component. Whereas excess argon is invariably present in garnet and clinopyroxene developed under high-pressure metamorphism, it is heterogeneously distributed in amphibole on a millimetre scale. Results indicate that excess argon was incorporated during high-pressure metamorphism; this component was then lost during retrogression, while a change in composition of ambient argon to atmospheric argon occurred. New ⁴⁰Ar-³⁹Ar data and previously published Sm-Nd garnet and U-Pb rutile ages obtained from the same well-preserved eclogite sample suggest that the oldest Na-Ca amphibole age is reliable and not an artefact due to the incorporation of excess argon. The variably retrogressed eclogites are thought to derive from different parts of the enclosing metasedimentary rocks that were variably invaded by fluids during amphibolite facies metamorphism. Thus the circulation of fluids promoting (re)crystallisation, and not temperature, was the main process controlling the rate of argon transport in the studied eclogites. The different ⁴⁰Ar-³⁹Ar ages are interpreted to record diachronous amphibole growth at different crustal levels during exhumation. Data indicate that there was about a 10-Ma interval between the eclogite facies stage (at ̿.5 GPa) and the Ca amphibole-hydration forming reaction (at 0.3-0.5 GPa); this translates into an average exhumation rate of 3-4 km/Ma.     

A unique magnesiochloritoid-bearing,high-pressure assemblage from the Monte Rosa,Western Alps: petrologic and 40Ar-39Ar radiometric study

A phengite-talc-chloritoid-chlorite-kyanite-quartz assemblage is reported from a nearly undeformed quartz-rich metapelite found in the Monte Rosa massif (Western Alps). Chloritoid contains up to 74 mol % of the Mg end member and is the most magnesian ever reported. Textural relationships and mineral compositions suggest equilibrium and therefore a low-variance assemblage which represents the high-pressure stability limit of chlorite+quartz according to the terminal reaction $${\text{chlorite + quartz }} \rightleftarrows {\text{ talc + chloritoid + kyanite + H}}_{\text{2}} {\text{O}}{\text{.}}$$ Mineral compositions combined with new experimental data on the stability of the Mg-chloritoid end member lead, for a temperature close to 500° C, to a pressure estimate of 16 kbar and a water activity of 0.6 which is supported by fluid inclusions study. Chloritoid composition is in fact a fine metamorphic indicator which opens new ways for barometry in high-grade blueschists. It demonstrates here the existence of a high-pressure metamorphism in the Monte Rosa massif. The assemblage remained mineralogically unaffected during the subsequent lower-pressure evolution. Two size fractions of the single phengite generation were analysed by the ³⁹Ar-⁴⁰Ar incremental release method. Both spectra are identical with a plateau at 110±3 Ma representing over 96% of the ³⁹Ar degassed. The ages of the first heating steps are discordant and increase with increasing temperature from values near 70 Ma to the plateau age. Isotope correlation diagrams show two ³⁶Ar components, one released at high temperature and correlated with ⁴⁰Ar and ³⁹Ar, the other released at low temperature in a mixture of atmospheric argon and of a loosely held argon of 70 Ma apparent age. The 110 Ma plateau age may reflect the presence of homogeneously incorporated excess argon, the 70 Ma value might then be a true age. However we favour the alternative hypothesis that the 110 Ma plateau age is a true age, implying that the internal crystalline massifs of the Western Alps have endured high-pressure metamorphism as early as mid-Cretaceous. Whatever the interpretation chosen, the preserved high-pressure mineral assemblage remained isotopically unaffected during the low-pressure mid-Tertiary event which is recorded by the 37 Ma plateau age of phengite from a foliated, recrystallised quartzite collected in the same, westernmost part of the massif. The contrasting behaviour of the two samples shows that even at temperatures as high as 400–450° C deformation and recrystallisation are also major controlling factors of isotope mobility.     

Tracing the exhumation of the Eclogite Zone (Tauern Window,Eastern Alps) by <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of white mica in eclogites

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.     

Excess and Radiogenic Argon in High-Pressure Mylonites of the Tan-Lu Fault Zone, Eastern China

Bulk separates of porphyroclastic phengite, neoformed phengite and their mixtures from the Tan-Lu HP mylonites overprinted on the Sulu UHP rocks were analyzed with the 40Ar/39Ar step heating method. Two samples of the neoformed phengite from ultramylonite give 40Ar/39Ar plateau ages of 209.9±1.8 Ma and 214.3±1.8 Ma, which are interpreted as representing cooling times of the TanLu sinistral faulting, and provide geochronological evidence for the syn-orogenic faulting of the Tan-Lu fault zone. The results show that the phengite formed during the retrograde eclogite-facies mylonitization was not contaminated with excess argon and can be used for dating the deformation. Argon closure in previous K-bearing minerals with excess argon under a retrograde HP dry condition is considered to be the reason for lack of excess argon incorporation in the neoformed phengite. Five porphyroclastic phengite samples yield 40Ar/39Ar plateau ages ranging from 666±12 Ma to 307.1±3.3 Ma, which are interpreted as being contaminated with excess argon. Two mixture samples with plateau ages of 239.4±2.1 Ma and 239.3±2.0 Ma show upward-convex age spectra caused by the mixture of older porphyroclastic phengite with excess argon incorporation and younger neoformed phengite without excess argon incorporation. It is demonstrated that excess argon introduced from the previous UHP metamorphism is still preserved in the pre-existing phengite after the Tan-Lu eclogite-facies mylonitization. The intense deformation under HT and HP conditions cannot erase excess argon in the previous phengite totally due to restricted fluid activities. These porphyroclastic phengite previously contaminated with excess argon cannot be used for dating the later HP deformation. This indicates that deformation under a HP dry condition does not play an important role in removing previous 40Are in phengite.     

Metamorphic evolution of the Sesia-Lanzo Zone,Western Alps: time constraints from multi-system geochronology

 The Sesia-Lanzo Zone is a polymetamorphic unit containing Hercynian granulite relics overprinted by eclogite and greenschist facies metamorphism and deformation during the Alpine orogeny. Different parts of the unit record different stages on the P-T-deformation evolution, allowing multi-system isotopic studies to unravel the precise timing of the metamorphic history. New Rb–Sr white mica and U–Pb sphene data constrain the age of eclogite facies metamorphism and deformation to 60–70 Ma. This substantially alters the common view of early- to mid-Cretaceous eclogite facies metamorphism in this unit. The new results are more consistent with the established geotectonic framework for the Alpine orogeny, since they do not require a prolonged period of depressed geothermal gradient at a time when the region was in extension. It is also more concordant with recent studies of other units that demonstrate post-Cretaceous high-pressure metamorphism. Step-heated ⁴⁰Ar–³⁹Ar analysis of phengites yields good plateaux giving ages older than the corresponding Rb–Sr age. Such anomalously high ages indicate the presence of radiogenic argon-rich fluids in the grain boundary network under the fluid/pressure conditions acting during this high-pressure metamorphic event. The U–Pb sphene ages are variable in polymetamorphic rocks, and show inheritance of older Pb or sphene crystals into the high-pressure event. Two monometamorphic assemblages yield concordant ages at 66±1 Ma, reflecting crystallisation of the eclogite facies assemblage. The Gneiss Minuti Complex (GMC) lies structurally below the Eclogitic Micaschists, and its pervasive greenschist facies fabric yields tightly clustered Rb–Sr white mica ages at 38–39 Ma. This greenschist event did not affect the majority of the EMC. The ⁴⁰Ar–³⁹Ar ages of micas formed at this time were very disturbed, whereas micas surviving from an earlier higher pressure assemblage had their ⁴⁰Ar–³⁹Ar system reset. The greenschist event did not strongly affect U–Pb systematics in Hercynian age sphenes, suggesting that the GMC did not uniformly suffer an eclogite facies metamorphism during the Alpine cycle, but was juxtaposed against the EMC later in the orogeny. This model still requires that the locus of deformation and metamorphism (and possibly fluid flux) moved outboard with time, leaving the Sesia-Lanzo basement as a shear-bounded unreactive block within the orogenic wedge. Received: 12 October 1995/Accepted:25 June 1996     

Argon retentivity and argon excess in amphiboles from the garbenschists of the Western Tauern Window,Eastern Alps

22 hornblende K-Ar ages and 10 ³⁹Ar-⁴⁰Ar spectra were obtained for hornblende garbenschists from the Western Tauern Window. The post-kinematic amphiboles were produced during the late Alpine prograde metamorphism (6–10 kb and 500–570° C). Two nearly potassiumfree cummingtonites rimming hornblende yield K-Ar ages of 120 Ma, while the 20 tschermakitic hornblendes scatter between 17 and 37 Ma. The reason for this scatter is excess Ar, possibly incorporated into amphiboles during healing of fractures, now traceable by trails of fluid inclusions. Excess Ar is semiquantitatively corrected for by combining cogenetic hornblende and cummingtonite with K-Ar isochrons. It can be quantified in 4 out of 10 hornblendes by ³⁹Ar-⁴⁰Ar stepwise heating experiments. Ages of 18–20 Ma result for corrected hornblendes. The retentivity of ⁴⁰Ar, after correction for excess, shows no correlation with chemistry within the narrow compositional range observed; rather, it shows intriguing correlations with irregularities in Ca/K spectra, pointing to a microstructurally controlled mechanism for Ar loss. This observation leads to a critical evaluation of the closure temperature constant, which apparently depends on an incompletely known number of mineralogical and environmental parameters. In particular those ³⁹Ar-⁴⁰Ar release spectra which yield low temperature steps with younger ages than the plateaus are not interpretable in terms of a synchronous closure. This gives evidence that loss of radiogenic isotopes proceeds by a more complex mechanism than simple volume diffusion through isotropic media.     

Radiometric constraints on hydrothermal circulation in cooling granite plutons

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.     

Amphibole Ar/Ar Geochronology from the Okcheon Metamorphic Belt, South Korea and its Tectonic Implications

An understanding of the Okcheon Metamorphic Belt (OMB) in South Korea is central to unraveling the tectono-metamorphic evolution of East Asia. Amphibole-bearing rocks in the OMB occur as calcsilicate layers and lenses in psammitic rocks, in the psammitic rocks themselves, and in the mafic volcanic layers and intrusives. Most amphiboles fail to show ⁴⁰Ar/³⁹Ar plateau ages; those that do have ages ranging from 132 to 975 Ma. The disturbed age pattern and wide variation in ⁴⁰Ar/³⁹Ar ages can be related to metamorphic grade, retrograde chemical reactions, excess Ar and amphibole composition. The oldest age (975 Ma) can be interpreted either as an old igneous or metamorphic age predating sedimentation or a false age caused by excess Ar. The youngest age of 132 Ma and the disturbed age pattern found in amphiboles from rocks located close to Jurassic granitoids are the result of retrograde thermal metamorphic effects accompanying intrusion of the granitoids. Some medium- or coarse-grained amphiboles in the calcsilicates are aggregates of fine-grained crystals. As a result, they are heterogeneous and prove to be readily affected by excess Ar. A disturbed age pattern in amphiboles from the calcsilicates occurring in the high-grade metamorphic zone may also be the product of excess Ar. On the other hand, the disturbed pattern of amphiboles present in the calcsilicates from the low-grade metamorphic zone could arise from both excess Ar and mixed ages. However, amphiboles from psammitic rocks and some calcsilicates in the high-grade metamorphic zone and in intrusive metabasites display real plateau ages of 237 to 261 Ma. The temperature conditions in the high-grade metamorphic zone were higher than the argon closing temperature for amphibole, and the amphiboles in this zone give plateau ages only when they are homogeneous in composition, lack excess Ar, and have not been thermally affected by intrusion of the granitoids. The unmodified ⁴⁰Ar/³⁹Ar ages prove rather younger than the age of the Late Paleozoic metamorphic event of 280 to 300 Ma, but they are close to muscovite K-Ar ages of 263 to 277 Ma. These ⁴⁰Ar/³⁹Ar amphibole ages are interpreted as the time of cooling that followed the main regional, intermediate-P/T metamorphic climax. The results demonstrate that interpretation of ⁴⁰Ar/³⁹Ar amphibole ages in an area subjected to several metamorphic events can be accomplished only by undertaking a thorough tectono-metamorphic study, accompanied by detailed chemical analysis of the amphiboles.     

内蒙古呼和浩特变质核杂岩韧性拆离带40Ar-39Ar定年及其构造含义

晚中生代,内蒙古大青山依次经历晚侏罗世盘羊山逆冲推覆、早白垩世呼和浩特变质核杂岩伸展、早白垩世大青山逆冲推覆断层及早白垩世以来高角度正断层复杂构造演化。其中,呼和浩特变质核杂岩韧性剪切带的冷却时间和抬升机制的制约尚不明确。本文在野外考察和显微构造分析基础上,采用逐步加热40Ar-39Ar定年法对韧性剪切带内不同单矿物的冷却年龄进行了测定。角闪石、白云母、黑云母和钾长石单矿物40Ar-39Ar冷却年龄处于120~116Ma之间。结合已有年龄数据及单矿物封闭温度,构建了韧性剪切带的冷却曲线。结果表明,韧性剪切带在122~115Ma期间存在一个明显的快速冷却过程。这一阶段快速冷却是与变质核杂岩拆离断层相关核部杂岩拆离折返作为大青山逆冲推覆断层上盘抬升的结果。     

Time calibration of a PT-path from the Western Tauern Window,Eastern Alps: the problem of closure temperatures

New Hornblende K-Ar and ³⁹Ar-⁴⁰Ar and mica Rb-Sr and K-Ar ages are used to place specific timemarks on a well-constrained pressure-temperature path for the late Alpine metamorphism in the Western Tauern Window. After identification of excess ⁴⁰Ar, the closure behavior of Ar in hornblende is compared with that of Sr and Ar in phengite and biotite. Samples were collected in three locations, whose maximum temperatures were 570° C (Zemmgrund), 550° C (Pfitscher Joch), and 500–540° C (Landshuter Hütte).The average undisturbed age sequence found is: Phengite Rb-Sr (20 Ma)>hornblende K-Ar (18 Ma)>phengite K-Ar (15 Ma)>biotite Rb-Sr, K-Ar (13.3 Ma)>apatite FT (7 Ma). Except for the phengite Rb-Sr age, the significance of which is debatable, all ages are cooling ages. No compositional effects are seen for closure in biotite. Additionally, Rb-Sr phengite ages from shearzones possibly indicate continuous shearing from 20 to 15 Ma, with reservations regarding the validity of the initial Sr correction and possible variations of the closure temperatures. The obviously lower closure temperature (T _c) for Ar in these hornblendes than for Sr in the unsheared phengites indicates that the T _c sequence in the Western Tauern Window is different from those observed in other terrains. In spite of this discrepancy, valuable geological conclusions can be drawn if the application of closure temperatures is limited to this restricted area with similar T, P and : (1) All ages of samples located on equal metamorphic isotherms decrease from east to west by about 1 Ma which is the result of a westward tilting of the Tauern Window during uplift. (2) In a PT-path, the undisturbed cooling ages yield constantly decreasing uplift rates from 3.6 mm/a to 0.1 mm/a. (3) Use of recently published diffusion data for Ar in hornblende (T _c=520° C) and biotite (T _c=320° C) suggests an extrapolated phengite closure temperature for Sr at 550° C. This suggests that the prograde thermal metamorphism at this tectonic level of the Tauern Window lasted until some 20 Ma ago.     

Variscan vs Cadomian tectonothermal activity in northwestern sectors of the Teplá-Barrandian zone, Czech Republic: constraints from 40Ar/39Ar ages

Five muscovite concentrates from high-grade, pelitic metasedimentary basement rocks exposed in northwestern sectors of the Teplá-Barrandian zone (Czech Republic) record ⁴⁰Ar/³⁹Ar mineral plateau ages which range between ca. 376 and 362?Ma. Hornblende concentrates from metagabbro (Mariánské Lánzě complex) and fine-grained basement amphibolite display plateaux which define ³⁶Ar/⁴⁰Ar vs ³⁹Ar/⁴⁰Ar isotope-correlation ages of ca. 370 and ca. 375 Ma. The mineral ages are interpreted to date relatively rapid cooling through appropriate argon retention temperatures following early phases of Variscan (Early Devonian) regional metamorphism. A slate/phyllite basement sample collected within lower-grade metasedimentary rocks in southeastern portions of the Teplá-Barrandian zone is characterized by an internally discordant ⁴⁰Ar/³⁹Ar whole-rock age spectrum which suggests partial Variscan rejuvenation of intracrystalline argon systems which had cooled through appropriate argon retention temperatures following an initial regional metamorphism at or prior to ca. 500 Ma (Cadomian). Hornblende from undeformed diorite of the Kdyn? complex records a well-defined ⁴⁰Ar/³⁹Ar age plateau which corresponds to an isotope-correlation age of ca. 516?Ma. This is interpreted to date post-magmatic cooling following emplacement.     

Using white mica 40Ar/39Ar data as a tracer for fluid flow and permeability under high‐P conditions: Tauern Window,Eastern Alps

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.     

Argon and halogen geochemistry of hydrothermal fluids in the Loch Ainort granite,Isle of Skye,Scotland

Highly turbid alkali feldspars from the Loch Ainort granite (59 Ma), Isle of Skye, have been analysed using the ⁴⁰Ar-³⁹Ar method to obtain chronological and chemical (K, Cl, Br, I) information concerning their origin and hydrothermal alteration. Three methods of gas extraction have been applied to neutron-irradiated samples: laser probe spot fusion of feldspars, in vacuo crushing of a feldspar/quartz separate, and laser stepped heating of the crushed residue. Apparent ages obtained by laser probe spot fusion are mostly similar to the 59 Ma intrusion age. Analyses of relatively pristine regions give some high apparent ages (>59 Ma) indicating the presence of small amounts of ⁴⁰Ar_E (excess ⁴⁰Ar). Crushing releases significant amounts of ⁴⁰Ar_E, but is dominated by an ⁴⁰Ar_A (atmospheric ⁴⁰Ar) component. ⁸⁴Kr/³⁶Ar values obtained by crushing are higher than air and are consistent with air equilibration with fresh water at low temperature 20°C). Therefore, ⁴⁰Ar_A was most probably introduced as palaeoatmospheric argon dissolved in the circulating hydrothermal fluids that interacted with the granite, thus supporting a meteoric origin for the fluids. Stepped heating gives a flat age spectrum and an age of 56±4 Ma. Crushing and stepped heating both released significant amounts of halogens with high Br/Cl and I/Cl ratios; excess Xe is also present as indicated by the high ¹³²Xe/³⁶Ar values. It seems likely that the halogen (and possibly Xe) enrichments resulted from interaction of the meteoric fluids with Jurassic sedimentary country rocks.     

Phlogopite40Ar/39Ar geochronology of mantle xenoliths from the North China Craton: Constraints on the eruption ages of Cenozoic basalts

Mantle xenoliths provide direct information about lithospheric evolution and asthenosphere–lithosphere interaction, and therefore precise dating of the host basalts which carried the xenoliths is important. Here we report ⁴⁰Ar/³⁹Ar geochronology of phlogopite separates from five spinel lherzolite xenoliths collected from the North China Craton (Hannuoba of Hebei Province, Sanyitang of Inner Mongolia Autonomous Region and Hebi of Henan Province), as well as the groundmass of the host basalts. Argon extraction was performed by conventional step heating technique and ultra-violet laser ablation microprobe (UVLAMP) technique. ⁴⁰Ar/³⁹Ar incremental heating results on groundmass yielded geologically meaningless ages. However, conventional step heating on phlogopites produced Miocene cooling ages, identical to the eruption ages obtained from the K–Ar dating methods of the Hannuoba and Sanyitang basalts. Adopting procedures to exclude potential influence of excess radiogenic Ar from a deep fluid source on a phlogopite separate from lherzolite yielded results with a good agreement of ages suggesting that the argon isotopes are distributed homogenously in this mineral, with no influence of excess argon. Phlogopites from Hebi yield ages between 6.43 and 6.44 Ma which are slightly older than those obtained from K–Ar method on whole-rocks. The discrepancy in the K–Ar ages obtained from the altered whole-rock samples suggests partial loss of ⁴⁰Ar. As a consequence, phlogopite Ar–Ar ages are considered more accurate than that of the whole-rocks. These results suggest that ⁴⁰Ar/³⁹Ar chronology of phlogopite provides reliable and precise ⁴⁰Ar/³⁹Ar ages of host basalts.     

柴北缘锡铁山榴辉岩退变质成因角闪石40Ar/39Ar年代学研究

采用激光阶段加热⁴⁰Ar/³⁹Ar技术,对柴达木盆地北缘锡铁山榴辉岩退变质作用形成的榴闪岩和斜长角闪岩之角闪石进行了定年分析。09NQ44Amp来自榴闪岩,各阶段表观年龄(以现代空气氩⁴⁰Ar/³⁶Ar比值295.5扣除非放射性成因⁴⁰Ar)构成了单调下降的阶梯状年龄谱。在反等时线图解上,2~4阶段数据点和5~18阶段数据点分别构成了两条等时线,等时年龄分别为427.6±10Ma和425.1±2.6Ma,对应的初始⁴⁰Ar/³⁶Ar比值则分别为435.2±6.1和705.3±13。角闪石09NQ43Amp来自榴辉岩强烈退变质作用形成的斜长角闪岩,⁴⁰Ar/³⁹Ar阶段加热分析也获得单调下降的年龄谱,在反等时线图解上其数据点3~6阶段和7~16阶段分别构成了两条等时线,等时年龄分别为418.9±2.9Ma和418.1±2.1Ma,对应的初始⁴⁰Ar/³⁶Ar比值则分别为493.7±2.8和685.8±34.3。等时线截距值高于现代大气⁴⁰Ar/³⁶Ar比值,表明角闪石中含过剩⁴⁰Ar。同时,由低温和中-高温阶段加热数据点分别构成两条等时年龄基本一致,截距值却明显不同的等时线,表明在角闪石热力学性质不同的源区,存在两期明显不同且未混合的初始捕获Ar组分。等时年龄425~418Ma代表的是锡铁山榴辉岩角闪岩相退变质作用发生的时间。等时线图解法虽然有效的校正了角闪石中的过剩⁴⁰Ar,但仅根据表观年龄图谱和等时线图谱还无法清晰判断过剩⁴⁰Ar在角闪石中的赋存状态,有待进一步探讨。     

Constraints on retrograde metamorphism of UHP eclogites in North Qinling,Central China,from 40Ar/39Ar dating of amphibole and phengite

Coesite- and microdiamond- bearing ultra-high pressure (UHP) eclogites in the North Qinling terrane have been widely retrogressed to amphibolites. Previous geochronological studies on these UHP rocks mainly focused on the timing of peak eclogite facies metamorphism. The Kanfenggou UHP metamorphic domain is one of the best-preserved coesite-bearing eclogite occurrences in the North Qinling terrane. In this study, mafic amphibolites and host schists from this domain were collected for ⁴⁰Ar/³⁹Ar dating to constrain their retrograde evolution. Two generations of amphibole are recognized based on their mineral parageneses and ⁴⁰Ar/³⁹Ar ages. A first generation of amphibole from garnet amphibolites yielded irregularly-shaped age spectra with anomalously old apparent ages. Isochron ages of 484–473 Ma and initial ⁴⁰Ar/³⁶Ar ratios of 3695–774 are obtained from this generation of amphibole, indicating incorporation of excess argon. Second generation amphibole occurs in epidote amphibolites yielded flat age spectra with plateau ages of 464–462 Ma without evidence for excess argon. These ages suggest that the amphibolite-facies metamorphism has taken place as early as 484 Ma and lasted until 462 Ma for the North Qinling UHP metamorphic rocks. Phengite from the country-rock schists yielded ⁴⁰Ar/³⁹Ar plateau ages of 426–396 Ma, with higher phengite Si contents associated with the older the plateau ages. Based on our new ⁴⁰Ar/³⁹Ar ages and previous zircon UPb geochronological data, we construct a new detailed pressure-temperature-time (P-T-t) path illustrating the retrograde metamorphism and exhumation rate of the North Qinling eclogites and host schists. The P-T-t path suggests that these UHP metamorphic rocks experienced initial medium-to-high exhumation rates (ca. 8.7 mm/yr) during the Early Ordovician (489–484 Ma), which was mainly derived from buoyancy forces. Subsequently, the exhumation rate decreased gradually from ~0.8 to 0.3 mm/yr from 484 to 426 Ma, which was probably governed by extension and/or erosion.