40Ar/39Ar record of late Pan–African exhumation of a granulite facies terrain, central Dronning Maud Land, East Antarctica

⁴⁰Ar/³⁹Ar geochronological data on hornblende, biotite and K-feldspar provide constraints on the cooling path experienced by a high-grade metamorphic complex from the Mühlig–Hofmannfjella and Filchnerfjella (6–8°E), central Dronning Maud Land, Antarctica, during the late Neoproterozoic-early Palaeozoic Pan–African orogeny. Hornblende ages yield c. 481 Ma, biotite ages range from c. 466 Ma to c. 435 Ma, whereas K-feldspar ages of the gneisses are c. 437 Ma. The ⁴⁰Ar/³⁹Ar data suggest initial cooling at a rate of ~10 °C/Myr between 481 and 465 Ma, followed by a lower cooling rate of ~6 °C/Myr during the subsequent c. 30 million years. The K-feldspar ⁴⁰Ar/³⁹Ar ages place a lower time limit on the duration of the exhumation, by the time of thermal relaxation to a stable continental geotherm. The ⁴⁰Ar/³⁹Ar data reflecting cooling indicate tectonic exhumation related to orogenic collapse during a later phase of the Pan–African orogeny.     

The 40Ar/39Ar and Rb–Sr chronology of the Precambrian Aksu blueschists in western China

Three metapelite samples from the Aksu blueschist terrane, Xinjiang, China, were dated by the ⁴⁰Ar/³⁹Ar method on separated phengite grains, obtaining plateau ages in the range of 741−757 Ma. In contrast, the measured Rb and Sr isotope data for the three samples yielded isochron ages ranging from 630 Ma to 900 Ma, suggesting large heterogeneity in the blueschist protolith and suppression of diffusional exchange owing to the low-temperature metamorphic conditions. Because the protolith of Aksu blueschist is composed of oceanic materials that formed ⁴⁰Ar-free phengite during HP and UHP metamorphism and the apparent ⁴⁰Ar/³⁹Ar plateaus ages in this study are similar to previous K–Ar and Rb–Sr ages, the existence of excess argon in these rocks is considered to be insignificant. As a result, the ⁴⁰Ar/³⁹Ar plateau ages in this study (ca. 750 Ma) likely represent the approximate time for peak metamorphism, given the low peak metamorphic temperatures for the Aksu blueschist terrane (300−400 °C). This strongly implies that modern style, cold subduction tectonics operated along the margin of the Aksu terrane no later than 750 Ma, in Neoproterozoic time.     

Pyrite Re–Os and muscovite 40Ar/39Ar dating of the Beizhan iron deposit in the Chinese Tianshan Orogen and its geological significance

The Awulale iron metallogenic belt (AIMB) hosts the majority of rich iron ores in Tianshan Orogen and has attracted much attention. However, a hot debate exists about the genesis of these iron deposits. Geochronological data are among the few critical evidences to solve the dispute. This study chooses the Beizhan iron deposit to carry out a geochronological research. The Beizhan magnetite deposit, with total iron ore reserves of 468 Mt at an average grade of 41% TFe, is the largest iron deposit in the AIMB. The orebodies of the Beizhan deposit are hosted in Carboniferous dacite and crystal tuff. Four stages of mineral formation can be recognized: an early skarn mineral stage, followed by the magnetite stage, the sulphide stage, and the carbonate stage in order. Pyrite separated from pyrite-rich ore samples yields an isochron age of 302.5 ± 8.2 Ma. Muscovite separated from muscovite-rich ore samples yields ⁴⁰Ar/³⁹Ar plateau ages of 304.7 ± 1.8 Ma, 304.5 ± 1.9 Ma, 308.1 ± 1.9 Ma, and 307.2 ± 1.8 Ma, and isochron ages of 306.1 ± 3.5Ma, 304.0 ± 3.0Ma, 308.2 ± 3.1Ma, and 308.7 ± 3.1Ma, respectively. These ages are consistent within the error range and are interpreted as the age of the Beizhan iron deposit. The results, combined with the other latest precise dating and geologically inferred ages, demonstrate that the iron deposits in the AIMB were formed in the Late Carboniferous. These iron deposits are considered to be iron skarn or medium–low -temperature hydrothermal origin and have genetic linkages between each other. They may be different mineralizing manifestations proximal to or distal from a pluton. The Late Carboniferous iron ores and the related magmatic rocks in the AIMB were produced when upwelling of the asthenosphere causes the partial melting of various sources and the formation of a narrow linear extension in the upper crust. The upwelling of the asthenosphere may be triggered by the detachment of an orogenic root zone.     

Jurassic cooling ages in Paleozoic to early Mesozoic granitoids of northeastern Patagonia: 40Ar/39Ar, 40K–40Ar mica and U–Pb zircon evidence

International Journal of Earth Sciences - U–Pb SHRIMP zircon crystallization ages and Ar–Ar and K–Ar mica cooling ages for basement rocks of the Yaminué and Nahuel Niyeu...     

Pre-metamorphic 40Ar/39Ar and U–Pb ages in HP metagranitoids from the Hercynian belt (France)

In most orogenic belts, the age of HP metamorphism and subsequent exhumation events still remain open to debate since geochronology can yield results which appear to conflict with the closure temperature concept [Dodson, M.H., 1973. Contrib. Mineral. Petrol. 40, 259–274], and because the behaviour of daughter radiogenic isotopes under HP to UHP conditions is poorly constrained. To obtain new data on isotope migration under high-pressure conditions, two undeformed HP metagranites with partially preserved magmatic assemblages from the French Variscan belt were investigated using the ⁴⁰Ar/³⁹Ar laser probe and U–Pb ion probe methods. In both cases, ⁴⁰Ar/³⁹Ar biotite and U–Pb zircon ages are consistent and could be related to the emplacement of pre-orogenic granites, despite petrological evidence of a strong metamorphic overprint during the Variscan orogeny. Temperatures experienced by the granites during subduction and exhumation events were more than 400 °C above that normally estimated for argon retention in biotite, but failed to cause significant resetting of the mica ⁴⁰Ar/³⁹Ar chronometer. Only a weak intragrain redistribution of argon is evidenced with the laser probe up to the contact with metamorphic garnet fringing biotite. By contrast, a complete resetting of biotite ages occurs in gneisses enclosing the metagranites. These results suggest that, in these dry undeformed HP metagranites, the thermally activated diffusion was relatively ineffective and that recrystallisation is the main process which controlled isotopic exchanges of Ar and Pb. It is likely that the absence of pervasive deformation and fluid circulation has also exercised some control on the preservation of pre-metamorphic isotopic signature in the studied rocks. The possible influence of several other parameters is also discussed. This example reveals that recovering thermochronological information in high-pressure metamorphic rocks is not straightforward and that great caution must be paid in the use of ages for the reconstruction of P–T–t paths.     

The Deccan Trap – Cretaceous–Paleogene boundary connection; new 40Ar/39Ar ages and critical assessment of existing argon data pertinent to this hypothesis

The Cretaceous–Paleogene boundary (KPgB) was dated by the ⁴⁰Ar/³⁹Ar method herein from the western interior of North America at 65.48 ± 0.12 Ma (1σ), in good agreement with other recent published estimates. For the Deccan Traps, India, new argon ages as well as others available in the literature, are assessed for reliability based on (a) statistical reliability of plateau/isochron sections and (b) freshness of material dated utilizing the alteration index method. From tholeiitic lavas from the Composite Western Ghats Section (CWGS), only six ages are found to be reliable estimates of the time of crystallization. These ages along with the magnetic polarity of the lavas agree with the geomagnetic polarity time scale (GPTS) at ∼67–64 Ma. Alkaline rocks from the Anjar area of Kutch, provide three reliable ages that suggest a hiatus in lava extrusion around KPgB. For the Rajahmundry basalts, the upper flow’s age defines its formation during chron 29n; a single age from the lower reversed polarity flow appears somewhat dichotomous when plotted against the GPTS. The reliable lava ages indicate the most voluminous (reversed polarity) sections of the CWGS were extruded at a time statistically indistinguishable from that of the KPgB. The Deccan Trap – KPgB faunal extinction hypothesis remains plausible, but must compete with the latest report, favoring a very close temporal connection (∼0.03 m.y.) between the Chixculub (Impact) Crater and the KPgB.     

Metagranitoids from the eastern part of the Central Rhodopean Dome (Bulgaria): U–Pb, Rb–Sr and 40Ar/39Ar timing of emplacement and exhumation and isotope-geochemical features

Summary Geochronological data (U–Pb, Rb–Sr and ⁴⁰Ar/³⁹Ar) are used to unravel the Late Alpine high-grade metamorphism, migmatisation and exhumation of Variscan granitoids within the core of the Central Rhodopean dome, Bulgaria. The age of the granitoid protolith is 300±11Ma, as determined by U–Pb analyses on single zircons selected from the core of the dome structure.Rb–Sr whole rock data define an errorchron with a large scatter of the data points due to the Late Alpine metamorphic overprint. The slope of the reference line indicates a Variscan magmatic event. Strontium characteristics are used to discriminate the samples most influenced by metamorphism from those, which reflect possible differences in the protolith age of the granitoids.Petrological-geochemical data, the initial strontium ratio of 0.708±0.001, and _Hf zircon values ranging from –2.58 to –3.82 point to a mixed, but crust-dominated origin of the Variscan magmas; young crustal material and mantle fragments were sources for the I-type metagranitoids.The exhumation of the granitoids from depths greater than 20–25km to about 5km below the surface was a rapid geological process. It started with the formation of granitic eutectic minimum melts at the temperature peak of metamorphism. Monazite crystallisation at about 650°C continued during isothermal decompression to possible depths of about 10–12km. An age of 35.83±0.40Ma was determined using conventional U–Pb isotope methods on four multigrain monazite fractions. A maximum average age of 36.6–37.5Ma (assuming same error uncertainties) for crystallisation of the metamorphic monazites was calculated assuming 10 to 20% monazite resetting during the subsequent Oligocene volcanism and hydrothermal activity in the region of the Central Rhodopean Dome. The rocks were then cooled to about 350–300°C at 35.35±0.22Ma according to ⁴⁰Ar/³⁹Ar ages of biotites and below 300°C at 35.31±0.25Ma (Rb–Sr data), as indicated by crystallisation of adularia in an open vein subsequent to pegmatite intrusion. A minimum exhumation rate of 3–5km per 1 million years can therefore be calculated for the exhumation of the metagranitoids during the period from 38–35Ma.     

New 40Ar/39Ar alunite ages from the Colquijirca district, Peru: evidence of a long period of magmatic SO2 degassing during formation of epithermal Au–Ag and Cordilleran polymetallic ores

We present ⁴⁰Ar/³⁹Ar data acquired by infra-red (CO₂) laser step-heating of alunite crystals from the large Miocene Colquijirca district in central Peru. Combined with previously published data, our results show that a long (at least 1.3 My) and complex period of magmatic-hydrothermal activity associated with epithermal Au–(Ag) mineralization and base metal, Cordilleran ores took place at Colquijirca. The new data indicate that incursion of magmatic SO₂-bearing vapor into the Colquijirca epithermal system began at least as early as ∼11.9 Ma and lasted until ∼10.6 Ma. Four alunite samples associated with high-sulfidation epithermal Au–(Ag) ore gave ⁴⁰Ar/³⁹Ar plateau ages between ∼11.9 and ∼11.1 Ma (compared to the previously documented ∼11.6 to ∼11.3 Ma). By combining individually these new ages with crosscutting relationships, the duration of the Au–(Ag) deposition period can be estimated to at least 0.4 My. Three new ⁴⁰Ar/³⁹Ar plateau ages on alunite associated with the base-metal Cordilleran ores are consistent with previously obtained ages, all of them between 10.83 ± 0.06 and 10.56 ± 0.06 Ma, suggesting that most of the sulfide-rich polymetallic deposits of Smelter and Colquijirca formed during this short period. The recognition of consecutive alunite-bearing and alunite-free mineral assemblages within both the Au–(Ag) and the base-metal Cordilleran ores may suggest that SO₂-bearing magmatic vapor entered the epithermal environment as multiple discontinuous pulses, a number of which was not necessarily associated in time with ore fluids. It is likely that a period of SO₂-bearing vapor degassing longer than 11.9 to 10.6 Ma may be recognized with further more detailed work. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

40Ar/39Ar mineral ages of eclogites from North Shahrekord in the Sanandaj–Sirjan Zone,Iran: Implications for the tectonic evolution of Zagros orogen

Eclogites are high-pressure/low-temperature metamorphic rocks and are regularly considered as an indicator of ancient subduction zones. Eclogites have recently been found in the North Shahrekord metamorphic complex (NSMC) of the Sanandaj–Sirjan zone and represent the only ones within the Zagros orogen. Their occurrence and timing are important for the reconstruction of convergence history and geodynamic evolution of the Neo-Tethys Ocean and Zagros orogen. White mica from the eclogites and an associated paragneiss give ⁴⁰Ar/³⁹Ar ages ranging from 184.3 ± 0.9 to 172.5 ± 0.8 Ma and represent the age of cooling through the closure temperature for phengitic white mica. The NSMC also comprises the ductile NW–SE trending North Shahrekord Shear Zone (NSSZ), which is located in the northeast of the Main Zagros Reverse Fault. The NSMC consists mainly of various metasedimentary rocks, orthogneiss and small-sized bodies of metabasic rocks containing also the eclogites. Furthermore, pre-metamorphic granitoids represent part of the NSMC. The North Shahrekord eclogites are composed of garnet, omphacite, zoisite, Ca–Na amphibole, phengite and rutile. The highly deformed and metamorphosed granitoids yield hornblende and biotite ⁴⁰Ar/³⁹Ar ages 170.1 ± 0.9 Ma and 110.7 ± 0.3 Ma, respectively. According to the new age dating results of eclogites, the rocks are the oldest high-pressure metamorphic rocks in the Zagros orogenic belt testifying the Neo-Tethys Ocean subduction. Our new data indicate that the eclogites formed during Early Jurassic subduction of a Panafrican microcontinental piece from the northern margin of the Neo-Tethyan Ocean under the Central Iranian microplate. We suggest that initiation of subduction in Neo-Tethyan Ocean occurred a few million years prior to 184 Ma (Pliensbachian stage).     

Timing and Processes of Ore Formation in the Qingchengzi Polymetallic Orefield, Northeast China: Evidence from 40Ar/39Ar Geochronology

The Qingchengzi orefield is a large polymetallic ore concentration area in the Liaodong peninsula,northeastern China,that includes twelve Pb-Zn deposits and five Au-Ag deposits along its periphery.The ore-forming age remains much disputed,which prevents the identification of the relationship between the mineralization and the associated magmatism.In this paper,we quantitatively present the feasibility of making ore mineral ~(40)Ar/~(39)Ar dating and report reliable ~(40)Ar/~(39)Ar ages of lamprophyre groundmass,K-feldspar and sphalerite from the Zhenzigou deposit.Direct and indirect methods are applied to constrain the timing of mineralization,which plays a vital role in discussing the contribution of multistage magmatism to ore formation.The low-potassium sphalerite yielded an inverse isochron age of 232.8±41.5 Ma,which features a relatively large uncertainty.Two lamprophyre groundmasses got reliable inverse isochron ages of 193.2±1.3 Ma and 152.3±1.5 Ma,respectively.K-feldspar yielded a precise inverse isochron age of 134.9±0.9 Ma.These four ages indicate that the mineralization is closely associated with Mesozoic magmatism.Consequently,regarding the cooling age of the earliest Mesozoic Shuangdinggou intrusion(224.2±1.2 Ma)as the initial time of mineralization,we can further constrain the age of the sphalerite to 224–191 Ma.These new and existing geochronological data,combined with the interaction cutting or symbiotic relationship between the lamprophyre veins and ore veins,suggest that the Pb-Zn-Au-Ag mineralization in the Qingchengzi orefield mainly occurred during three periods:the late Triassic(ca.224–193 Ma),the late Jurassic(ca.167–152 Ma)and the early Cretaceous(ca.138–134 Ma).This polymetallic deposits are shown to have been formed during multiple events coinciding with periods of the Mesozoic magmatic activity.In contrast,the Proterozoic magmatism and submarine exhalative and hydrothermal sedimentation in the Liaolaomo paleorift served mainly to transport and concentrate the ore-forming substances at the Liaohe Group with no associated Pb-Zn-Au-Ag mineralization.     

The effect of chlorite interlayering on <Superscript>40</Superscript>Ar–<Superscript>39</Superscript>Ar biotite dating: an <Superscript>40</Superscript>Ar–<Superscript>39</Superscript>Ar laser-probe and TEM investigations of variably chloritised biotites

Biotite is one of the most common minerals dated by the ⁴⁰Ar–³⁹Ar method. It frequently shows K contents below the expected stoichiometric value, suggesting the presence of low-K impurities. The most common low-K alteration product of biotite is chlorite. Therefore, it is important to understand the effects of chlorite interlayering on ⁴⁰Ar–³⁹Ar ages in order to correctly interpret ⁴⁰Ar–³⁹Ar data. This study examines the outcome of ⁴⁰Ar–³⁹Ar dating analyses on variably chloritised biotites from Ordovician intrusive rocks. The infrared (IR) laser-probe technique and different gas extraction methods were adopted. Incremental laser-heating data on bulk samples yielded hump-shaped age profiles with meaningless young and old age steps. Both the extent of anomalous old age steps and the degree of discordance of the age spectra were much more pronounced in the more chloritised biotite samples. In contrast, in situ data on rock chips and total-fusion ages on single biotite flakes yielded ages concordant with, or younger than, the inferred emplacement ages. Transmission electron microscopy (TEM) was used to texturally characterise biotite samples at the nanometre scale. It was also used to document the complex decomposition-transformation process affecting interlayered biotite–chlorite during in-vacuo IR-laser heating to temperatures ranging from ~600 to >1,000 °C. TEM results suggest that hump-shaped age profiles result from an interplay between ³⁹Ar_K redistribution by recoil during sample irradiation and differential release of argon isotopes hosted in three main reservoirs. These reservoirs are (from least to most retentive): extended defects, chlorite and biotite. The final descending age segment is attributed to the progressive release of argon with increasing temperature from large biotite domains for which ³⁹Ar_K recoil loss was less important. ⁴⁰Ar–³⁹Ar data support previous findings, which suggest that ⁴⁰Ar–³⁹Ar ages when recoil effects are minimised, provide minimum estimates that approach the true biotite age, when the pristine domains are analysed. The most effective approach for obtaining meaningful ⁴⁰Ar–³⁹Ar ages was using individual total-fusion analyses on carefully selected, single flakes previously split along the basal cleavage by wet-grinding and corresponding to a sample mass of a few micrograms.Editorial responsibility: I Parsons     

Kimberlite age in the Arkhangelsk Province,Russia: Isotopic geochronologic Rb–Sr and <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar and mineralogical data on phlogopite

The paper reports detailed data on phlogopite from kimberlite of three facies types in the Arkhangelsk Diamondiferous Province (ADP): (i) massive magmatic kimberlite (Ermakovskaya-7 Pipe), (ii) transitional type between massive volcaniclastic and magmatic kimberlite (Grib Pipe), and (iii) volcanic kimberlite (Karpinskii-1 and Karpinskii-2 pipes). Kimberlite from the Ermakovskaya-7 Pipe contains only groundmass phlogopite. Kimberlite from the Grib Pipe contains a number of phlogopite populations: megacrysts, macrocrysts, matrix phlogopite, and this mineral in xenoliths. Phlogopite macrocrysts and matrix phlogopite define a single compositional trend reflecting the evolution of the kimberlite melt. The composition points of phlogopite from the xenoliths lie on a single crystallization trend, i.e., the mineral also crystallized from kimberlite melt, which likely actively metasomatized the host rocks from which the xenoliths were captured. Phlogopite from volcaniclastic kimberlite from the Karpinskii-1 and Karpinskii-2 pipes does not show either any clearly distinct petrographic setting or compositional differentiation. The kimberlite was dated by the Rb–Sr technique on phlogopite and additionally by the ⁴⁰Ar/³⁹Ar method. Because it is highly probable that phlogopite from all pipes crystallized from kimberlite melt, the crystallization age of the kimberlite can be defined as 376 ± 3 Ma for the Grib Pipe, 380 ± 2 Ma for the Karpinskii-1 pipe, 375 ± 2 Ma for the Karpinskii-2 Pipe, and 377 ± 0.4 Ma for the Ermakovskaya-7 Pipe. The age of the pipes coincides within the error and suggests that the melts of the pipes were emplaced almost simultaneously. Our geochronologic data on kimberlite emplacement in ADP lie within the range of 380 ± 2 to 375 ± Ma and coincide with most age values for Devonian alkaline–ultramafic complexes in the Kola Province: 379 ± 5 Ma; Arzamastsev and Wu, 2014). These data indicate that the kimberlite was formed during the early evolution of the Kola Province, when alkaline–ultramafic complexes (including those with carbonatite) were emplaced.     

Re–Os and <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar ages of the Jiguanshan porphyry Mo deposit,Xilamulun metallogenic belt,NE China,and constraints on mineralization events

The large Jiguanshan porphyry Mo deposit, with more than 100 Mt of ore and grades ranging from 0.08% to 0.11%, is located in the newly identified Xilamulun metallogenic belt along the northern margin of the North China Craton. The Mo mineralization is predominantly disseminated in the host granite porphyry, but locally occurs as stockworks in lithic tuff and rhyolitic rocks. ⁴⁰Ar/³⁹Ar dates of samples from groundmass material in the host granite porphyry, post-ore diabase, and quartz porphyry dikes show plateau ages of 155.1 ± 1.9, 149.4 ± 0.9, and 147.6 ± 0.9 Ma, with inverse isochron ages of 156.0 ± 1.8, 149.3 ± 1.3, and 148.3 ± 1.2 Ma, respectively. Seven samples of disseminated molybdenite yielded a weighted average ¹⁸⁷Re-¹⁸⁷Os age of 155.3 ± 0.9 Ma, whereas six veinlet-type molybdenite samples yielded a weighted average ¹⁸⁷Re-¹⁸⁷Os age of 153.0 ± 0.9 Ma, providing direct timing constraints for the Mo mineralization at 153–155 Ma. The regional geological setting together with the emplacement of post-ore diabase and quartz porphyry dikes in the Jiguanshan deposit, are indicative of an extensional regime in Late Jurassic, which was probably linked to lithospheric extension in northeast China.     

The origin of the world class tin-polymetallic deposits in the Gejiu district,SW China: Constraints from metal zoning characteristics and 40Ar–39Ar geochronology

The Gejiu tin-polymetallic deposits in the Western Cathaysia Block of South China comprise the world's largest primary tin district, with a total resource of approximately 300 million metric ton ores, at an average grade of 1 wt percent Sn. Tin polymetallic mineralization occurs in five deposits and has four ore types, i.e., greisen, skarn, stratabound cassiterite-sulfide (mostly oxidized) and vein type ore. In each deposit the orebodies typically occur in an extensive hydrothermal system centered on a shallow Late Cretaceous granitoid cupola. Metal zoning is well developed both vertically and horizontally over the entire district, from W + Be + Bi ± Mo ± Sn ores inside granite intrusions, to Sn + Cu-dominated ores at intrusion margins and farther out to Pb + Zn deposits in the surrounding host carbonate. This zoning pattern is similar to that of other hydrothermal deposits in other parts of the world, indicating a close genetic relationship between magmatism and mineralization. In this paper, we dated thirteen mica samples from all types of mineralization and from the five deposits in the Gejiu district. The ages range from 77.4 ± 0.6 Ma to 95.3 ± 0.7 Ma and are similar to the existing zircon U–Pb age of the granitic intrusions (77.4 ± 2.5–85.8 ± 0.6), indicating a genetic relationship between the mineralization and the intrusions. Geological characteristics, metal zoning patterns and new geochronological data all indicate that the tin-polymetallic ores in the Gejiu district are hydrothermal in origin and are genetically related to the nearby granitic intrusions. It is unlikely that the deposits are syngenetic, as has been proposed in recent years.     

How continuous and precise is the record of P–T paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics (Schistes Lustrés,W. Alps)

Pressure–temperature (P–T) paths as complete as possible and with a precision on the km‐scale or less are needed to further improve the knowledge of deformation, re‐equilibration processes and element/fluid transfer, in particular along subduction zones. This contribution attempts to (i) critically evaluate the precision and continuity with which metamorphic P–T histories are retrieved today and (ii) discuss implications for regional‐scale accretionary processes in subduction zones, through application to the Schistes Lustrés complex (Haute Maurienne, W. Alps). P–T estimates are compared and combined using several independent approaches: (i) from minerals assumed to be in textural equilibrium; (ii) from electron microprobe compositional maps; and (iii) from pseudosection modelling predictions. Multi‐equilibrium calculations were performed with tweequ and thermocalc, and pseudosections were built with Perple_X and Theriak/Domino. These P–T estimates were also compared with maximum temperatures (T_max) deduced from the Raman spectroscopy of carbonaceous matter. The different methods used here yield the peak of pressure for the lower structural unit of the Schistes Lustrés at 480 °C and 23 kbar and document the retrograde path for both the Median and Lower Units. The results show that P–T conditions are recorded almost continuously and can be determined with a precision of ±1 kbar and ±30 °C at best. This study underlines the complementarity of the various thermobarometric methods and demonstrates that precision could be increased by improving solid solution models for chlorite. Observed tectonic patterns, major lithological boundaries, pressure–temperature and T_max data suggest that underplating processes and early structural development played a key role in the Schistes Lustrés accretionary complex.     

The alteration, mineralogy and geochronology (SHRIMP U–Pb and 40Ar/39Ar) of copper-bearing Anjerd skarn, north of the Shayvar Mountain, NW Iran

A Cu-bearing skarn zone occurs north of the Shayvar Mountain in northwestern Iran. Skarn-type metasomatic alteration and mineralization occur along the contact between Upper Cretaceous impure carbonates and a Miocene Cu-bearing granitic stock. Both endoskarn and exoskarn developed in the rocks. Exoskarn is the principal skarn zone and is enclosed by a skarnoid–hornfelsic zone. Skarn formation occured during stages: (1) prograde, (2) middle stage and (3) late stage. In the prograde stage, there were two main processes: (a) metamorphic–bimetasomatic and (b) prograde metasomatic. The metamorphic process began immediately after intrusion of the pluton into the enclosing impure carbonates. The prograde metasomatic stage commenced with segregation and evolution of a fluid phase in the pluton and movement into fractures and micro-fractures in the skarnoid–hornfelsic rocks developed in a metamorphic zone. The introduction of considerable amounts of Fe, Si and Mg led to the development of voluminous medium- to coarse-grained anhydrous calc-silicates. During the middle stage, the previously formed skarn zones were affected by intense multiple hydrofracturing in the Cu-bearing stock. In addition to Fe, Si and Mg, substantial amounts of Cu, Pb and Zn, along with volatile components such as H₂S and CO₂ were added to the skarn system. Consequently, substantial amounts of hydrous calc-silicates (epidote, tremolite–actinolite), sulfides (pyrite, chalcopyrite and molybdenite), oxides (magnetite, hematite) and carbonates (calcite) replaced the anhydrous calc-silicates. The retrograde stage was synchronous with the incursion of relatively low-temperature, more oxidized fluids into skarn system, resulting in partial alteration of the early-formed calc-silicates and development of a series of very fine-grained aggregates of chlorite, clay, hematite and calcite. Zircon grains from the endoskarn zone provide constraints on the timing of solidification of the granite stock (9.91 ± 0.31 Ma) that caused mineralization in the Anjerd area. One sample of primary hornblende from the monzogranitic Shayvar batholith has an ⁴⁰Ar/³⁹Ar age of 26.54 ± 0.65 Ma and indicates that intrusion of the Miocene stock and associated Cu skarn formation occurred a considerable time after intrusion of the batholith.     

The Meso-Cenozoic thermo-tectonic evolution of the Eastern Pyrenees: an <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar fission track and (U–Th)/He thermochronological study of the Canigou and Mont-Louis massifs

Vertical displacements on the SW–NE Têt fault (Eastern Pyrenees Axial Zone, France), which separates the Variscan Canigou-Carança and Mont-Louis massifs, were constrained using a thermochronologic multi-method approach. ⁴⁰Ar/³⁹Ar data from the granitic Mont-Louis massif record its Variscan cooling history and reveal no ages younger than Early Cretaceous, while the Canigou-Carança gneiss massif records systematically younger ⁴⁰Ar/³⁹Ar ages. These younger ⁴⁰Ar/³⁹Ar ages in the Canigou-Carança gneiss massif are the result of partial to total rejuvenation of argon isotopic systems related to a thermal flow coeval with the Cretaceous HT-BP metamorphism in the North Pyrenean Zone. Only the deepest rocks from the Canigou-Carança suffered this extensive Mid-Cretaceous thermal overprint probably due to differential burial around 4 km at that time. The post Mid-Cretaceous vertical displacements along the Têt fault are recorded by “low” temperature thermochronology using K-feldspar ⁴⁰Ar/³⁹Ar, zircon and apatite fission track and (U–Th)/He datings. The Mont-Louis granite samples experienced a long period of protracted cooling reflecting a lack of thermo-tectonic activity in this area from Late Palaeozoic to Early Cenozoic, followed by cooling from 55–60 Ma to Late Eocene at a mean rate of 15–20°C/Ma in the final stage. This cooling stage corresponds to Têt fault reactivation with a reversed component, promoting exhumation of the Mont-Louis roof zone contemporaneously with the south-vergent Pyrenean thrusting. In the Canigou-Carança massif, the main cooling event occurred from 32 to 18 Ma at a maximum rate of 30°C/Ma during Early Oligocene followed by a more moderate rate of 3°C/Ma from Late Oligocene to Early Burdigalian, coeval with the normal reactivation of the Têt fault in brittle conditions that accommodated the final exhumation of the massif during the opening of the Gulf of Lion.     

The First Data of U/Pb and 40Ar/39Ar Dating of the Pre-Dzhugdzhur Volcanics: New Evidence of Time Diversity in the Formation of Individual Sectors of the Okhotsk–Chukotka Volcanogenic Belt

Doklady Earth Sciences - The first data of U/Pb and 40Ar/39Ar dating of volcanogenic rocks of the Pre-Dzhugdzhur volcano-tectonic trough (PDVT) of the Okhotsk–Chukotka volcanogenic belt...     

Accessory mineral U–Th–Pb ages and <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar eruption chronology,and their bearing on rhyolitic magma evolution in the Pleistocene Coso volcanic field,California

We determined Ar/Ar eruption ages of eight extrusions from the Pleistocene Coso volcanic field, a long-lived series of small volume rhyolitic domes in eastern California. Combined with ion-microprobe dating of crystal ages of zircon and allanite from these lavas and from granophyre geothermal well cuttings, we were able to track the range of magma-production rates over the past 650 ka at Coso. In ≤230 ka rhyolites we find no evidence of protracted magma residence or recycled zircon (or allanite) from Pleistocene predecessors. A significant subset of zircon in the ~85 ka rhyolites yielded ages between ~100 and 200 Ma, requiring that generation of at least some rhyolites involves material from Mesozoic basement. Similar zircon xenocrysts are found in an ~200 ka granophyre. The new age constraints imply that magma evolution at Coso can occur rapidly as demonstrated by significant changes in rhyolite composition over short time intervals (≤10’s to 100’s ka). In conjunction with radioisotopic age constraints from other young silicic volcanic fields, dating of Coso rhyolites highlights the fact that at least some (and often the more voluminous) rhyolites are produced relatively rapidly, but that many small-volume rhyolites likely represent separation from long-lived mushy magma bodies.