Repeated thermal resetting of phengites in the Mulhacen Complex (Betic Zone, southeastern Spain) shown by 40Ar/39Ar step heating and single grain laser probe dating (original) (raw)
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Earth and Planetary Science Letters
This study reports the results of the first 4°Ar/39mr combined induction furnace and laser probe dating of phengites from the Mulhacen HP/LT metamorphic complex in the Betic Cordilleras, southern Spain. Laser step heating and spot fusion analyses on different halves of a split single grain were made with a continuous laser probe. Spot fusion analysis resulted in ages of about 30-31 Ma in the core and ages as low as 25-26 Ma in the rim. Laser step heating on the other half of the grain gave a spectrum with apparent ages increasing from about 25 Ma to 29.5 Ma. The age spectrum and the decreasing ages towards the rim of the grain may imply that resetting essentially occurred by volume diffusion of radiogenic 4°Ar due to late stage reheating resulting from extensional tectonics. Ages around 30 Ma in the core of the grain are interpreted as minimum estimates of the cooling age of the main tectono-metamorphic phase D 2.
Chemical Geology, 1995
Bulk samples and single grains of high-pressure white micas and a biotite from two restricted areas (Mucrone and Marine) 10 km apart in the Sesia Zone (Western Alps, Italy) were analysed by the conventional (step-heating) and continuous laser probe (step-heating and spot fusion) 4°Ar/39Ar techniques, respectively. The analysed minerals crystallized during the Eoalpine, eclogitic facies metamorphism. The phengites from the Mucrone area display ages which scatter between ~ 104 and ~ 180 Ma. In the Marine area (a 50-m-wide outcrop) the phengites from four samples with different bulk-rock compositions display precise plateau ages, which scatter between 69.4___ 0.7 and 76.9 + 0.6 Ma, and homogeneous age maps, whereas two minerals (phengite and biotite) from the same rock sample provide two very "discordant" plateau ages, 69.4 + 0.7 and 140.5 __+ 0.6 Ma, respectively. In the light of the present knowledge of the history of the Alps, ages as old as 180 Ma are unrealistic and are related to excess argon, whereas ages in the range 70-80 Ma, perhaps even 140 Ma, could be significant. We suggest that excess argon, probably conveyed by a fluid phase and trapped in defects within the structure of the phengite grains at the time of their isotopic closure, is the best explanation for the observed spread in apparent ages on a regional to single grain scale, in spite of a nearly universal existence of plateau ages and homogeneous age maps. We propose that the age of closure of the phengites to argon loss is ~<69.4+ 0.7 Ma.
K–Ar phengite geochronology of HP–UHP metamorphic rocks –An in–depth review–
Journal of Mineralogical and Petrological Sciences, 2020
The reported discordant and anomalously old K-Ar (40 Ar/ 39 Ar) phengitic white mica ages from collisional orogenic belts are due to the fact that white micas in continental lithologies are not reset completely during high-to ultrahigh-pressure (HP-UHP) metamorphism because the closure temperature of white mica is much higher than the generally accepted value, approximately 600°C. On the other hand, phengites in HP-UHP schists experience deformation-induced recrystallization during exhumation of the host lithology. The radiogenic argon is released from the deformed phengite, as documented by comparison of the in situ 40 Ar/ 39 Ar dating of phengite included in rigid garnet and of stretched phengite in the matrix. These non-resetting and argon-release phenomena give inconsistent phengitic white mica ages in metamorphosed continental lithologies. The heterogeneity in the deformational process due to differences in lithological compositions, local domains or even within single mica crystals results in inconsistent ages, as documented from the in situ 40 Ar/ 39 Ar dating of the deformed micas. The Sanbagawa HP schist belt and Lago di Cignana HP-UHP units both consist of metamorphosed oceanic lithologies that usually record only a single metamorphic cycle and have phengites without any inherited excess argon. The duration of deformation during exhumation spans from the peak metamorphism to the end of deformation in the crust, making it possible to estimate the exhumation rates of the metamorphic sequences. The low exhumation rates (<6 mm/y) of the Sanbagawa belt suggest a slow strain rate during rock deformation, resulting in a 'slow schist' sequence with a recumbent fold structure. The rapid exhumation rates (<26 mm/y) of Lago di Cignana suggest a high strain rate during rock deformations, resulting in a 'fast schist' sequence consisting of several units with fault-bounded contacts. The Lago di Cignana UHP unit, which underwent the highest exhumation rate, could indicate a subsequent continental collision event, whereas the Sanbagawa belt did not experience a subsequent continental collision event.
Migmatitic paragneisses of the Valle Fértil–La Huerta Ranges at the Western margin of the Sierras Pampeanas are composed of garnet–cordierite–plagioclase–biotite–quartz-bearing units that experienced peak metamorphic conditions of ca. 800 °C at 6–7 kbar. Based on petrological studies, pseudosection modeling and petrographic observations, an anticlockwise P–T path with a small pressure increment is proposed. Rare earth element LA-ICP-MS patterns acquired from rutile bearing garnets suggest a single stage of garnet growth at high-T at pressures above the ilmenite–rutile transition. U–Pb dating of zircon rims from the migmatites indicates two distinct metamorphic U–Pb ages of 525±9 Ma and 478±9 Ma. The older age is suggested to record an amphibolite facies event of the Pampean orogeny. The younger metamorphic age is contemporary with igneous zircons from metatonalites and pegmatites that yield 478±4 Ma. We suggest that the prograde high-T metamorphic Famatinian event is associated with the emplacement of large magmatic bodies in which large-scale magmatic activity gave rise to an increased geothermal gradient of about 35 °C/km. Sm–Nd garnet ages of 447±3 Ma indicate a time span of around 30 Ma for which temperatures above the garnet closure temperature prevailed. Using U–Pb, Sm–Nd and Rb–Sr isotope systems, a cooling rate of 3 to 6 °C/Myr is inferred.
Earth and Planetary Science Letters, 2018
The preservation of 40 Ar/ 39 Ar ages of high pressure (HP) metamorphic white mica reflects an interplay of processes that mobilise 40 Ar, either through mica recrystallisation or by diffusive 40 Ar loss. The applicability of resulting ages for dating tectonic processes is critically dependent on whether either of these processes can be proven to be efficient and exclusively active in removing 40 Ar from mica. If not, preservation of an inherited or mixed age signal in a sample must be considered for interpretation. The Cycladic Blueschist Unit on Syros has become a new focal area in the discussion of the geological significance of argon age results from multi-grain step heating experiments. While some argue that age results can directly be linked to deformation or metamorphic growth events, others interpret age results to reflect the interplay of protracted recrystallisation and partial resetting, preserving a mixed age signal. Here, we demonstrate the potential of a new approach of multiple single grain fusion dating. Using the distribution of ages at the sample, section and regional scale, we show that in Northern Syros mica ages display systematic trends that can be understood as the result of three competing processes: 1) crystallisation along the prograde to peak metamorphic path, 2) a southward trend of increasing 40 Ar loss by diffusion and 3) localised and rock type dependent deformation or metamorphic reactions leading to an observed age spread typically limited to ∼10 Myr at the section scale. None of the sections yielded the anomalously old age results that would be diagnostic for significant excess 40 Ar. The recorded trends in ages for each of the studied sections reflect a range of P-T conditions and duration of metamorphism. Diffusion modelling shows that in a typical subduction metamorphic loop, subtle variations in P-T-t history can explain that age contrasts occur on a regional scale but are limited on the outcrop scale. Our new approach provides a comprehensive inventory of the range of ages present in different rocks and at different scales, which results in a more refined understanding of argon retention and isotopic closure of phengite and the geological significance of the ages. We verify the added value of our new approach by comparison with multi-grain step heating experiments on selected samples from the same sections.
Lithos, 2010
Migmatitic paragneisses of the Valle Fértil-La Huerta Ranges at the Western margin of the Sierras Pampeanas are composed of garnet-cordierite-plagioclase-biotite-quartz-bearing units that experienced peak metamorphic conditions of ca. 800°C at 6-7 kbar. Based on petrological studies, pseudosection modeling and petrographic observations, an anticlockwise P-T path with a small pressure increment is proposed. Rare earth element LA-ICP-MS patterns acquired from rutile bearing garnets suggest a single stage of garnet growth at high-T at pressures above the ilmenite-rutile transition. U-Pb dating of zircon rims from the migmatites indicates two distinct metamorphic U-Pb ages of 525 ± 9 Ma and 478 ± 9 Ma. The older age is suggested to record an amphibolite facies event of the Pampean orogeny. The younger metamorphic age is contemporary with igneous zircons from metatonalites and pegmatites that yield 478 ± 4 Ma. We suggest that the prograde high-T metamorphic Famatinian event is associated with the emplacement of large magmatic bodies in which large-scale magmatic activity gave rise to an increased geothermal gradient of about 35°C/km. Sm-Nd garnet ages of 447 ± 3 Ma indicate a time span of around 30 Ma for which temperatures above the garnet closure temperature prevailed. Using U-Pb, Sm-Nd and Rb-Sr isotope systems, a cooling rate of 3 to 6°C/Myr is inferred.
Lithos, 2011
In order to date low-temperature deformation, intensely strained muscovite porphyroclasts and neocrystallized shear band phengite from greenschist-facies shear zones have been dated by 40Ar/39Ar method in the Argentera-Mercantour massif. Shear zones are featured by gradual mylonitization of a Variscan granite, gneiss and Permian pelite protolith (300–315 Ma) during the Alpine orogenic event. Mineralogical and textural observations indicate that phengites and chlorites developed from biotite and plagioclase in fluid system during deformation following dissolution–transport–precipitation reactions of the type biotite + plagioclase + aqueous fluid = chlorite + albite + phengite + quartz + titanite + K-bearing fluid in the granite-gneiss mylonite. Contrariwise, phengite developed at the expense of clays following substitution reaction in pelite mylonite. Based on conventional thermobarometry on phengite and chlorite and Pressure–Temperature-aqueous fluid (P–T-MH2O) pseudosections calculated with shear zone bulk compositions, the conditions during shear deformation were estimated at 375 ± 30 °C and 4.8–7 ± 1 kbar in an H2O-satured system. In this low temperature environment, 40Ar/39Ar analysis of the Variscan muscovite for various grades of ductile strain intensity shows a limited 40Ar/39Ar isotopic resetting, all ages scattering between 296 and 315 Ma. Under conditions of intense ductile deformation and large-scale fluid circulation, muscovite grains formed during the Variscan retain their much older ages. 40Ar/39Ar dating of very fine grained synkinematic phengite grains, neoformed during the Alpine history, give consistent plateau ages (34–20 Ma) for each shear zone. In detail, 40Ar excess can be detected in the pelite mylonitic sample where phengites crystallized by substitution process while the other mylonitic samples where phengites grow from fluid-induced reactions do not evidence any 40Ar excess. These results demonstrate that the 40Ar/39Ar dating of neocrystallized synkinematic white mica allows the determination of precise ages of deformation and fluid activity. Together with precise thermobarometry undertaken on the basis of mineral chemistry and whole-rock composition, 40Ar/39Ar dating of white mica leads to the reconstitution of precise depth-deformation history of low-grade (< 400 °C) metamorphic units. At the Argentera-Mercantour massif scale, several stages of shear zone development at 15–21 km depth are dated between 33 and 20 Ma. In the SE part of the massif shear zone ages are well constrained to be either (1) 33.6 ± 0.6 Ma or in the range (2) 26.8 ± 0.7 Ma–26.3 ± 0.7 Ma. In the West of the massif, younger shear zone ages range between (3) 22.2 ± 0.3 Ma and (4) 20.5 ± 0.3 Ma.► Inherited deformed white mica preserve their initial 40Ar/39Ar crystallisation age. ► Deformed minerals cannot provide the time of deformation. ► Neocrystallisation argon age provide a reliable age of deformation-fluid activity. ► Si-phengite can be used as a good geobarometer in the water-saturated domain.
Very fast exhumation of high-pressure metamorphic rocks with excess 40Ar and inherited 87Sr, Betic Cordilleras, southern Spain, 2003
In order to attempt to further constrain the age of the early Alpine tectonic evolution of the Mulhacén Complex and to explore the influence of inherited isotopes, micas from a small number of well-characterised rocks from the Sierra de los Filábres, with a penetrative tectonic fabric related to the exhumation of eclogite-facies metamorphic rocks, were selected for 40Ar/39Ar and Rb–Sr dating. A single phengite grain from an amphibolite yielded an 40Ar/39 Ar laser step heating plateau age of 86.9 ± 1.2 Ma (2σ; 70% 39Ar released) and an inverse isochron age of 86.2 ± 2.4 Ma with an 36Ar/40Ar intercept within error of the atmospheric value. Induction furnace step heating of a biotite separate from a gabbro relic in an eclogite yielded a weighted mean age of 173.2 ± 6.3 Ma (2σ; 95% 39 Ar released). These ages are diagnostic of excess argon (40ArXS) incorporation, as they are older than independent age estimates for the timing of eclogite-facies metamorphism and intrusion of the gabbros. 40ArXS incorporation probably resulted from restricted fluid mobility in the magmatic rocks during their metamorphic recrystallisation. Rb–Sr whole-rock–phengite ages of graphite-bearing mica schists from Paleozoic rocks (Secano unit) show a dramatic variation (66.1 ± 3.2, 40.6 ± 2.6 and 14.1 ± 2.2 Ma). An albite chlorite mica schist from the Mesozoic series of the Nevado–Lubrín unit has a whole-rock–mica–albite age of 17.2 ± 1.9 Ma, which is within error of an 40Ar/39Ar plateau age published previously and of the youngest Rb–Sr age of the Paleozoic series obtained in this study. The significant spread in Rb–Sr ages implies that progressive partial resetting of an older isotopic system has occurred. The microstructure of the samples with pre-Miocene Rb–Sr ages reveals incomplete recrystallisation of white mica and inhibited grain growth due to the presence of graphite particles. This interpretation agrees with previously published, disturbed and slightly dome-shaped 40Ar/39Ar age spectra that may point similarly to the presence of an older isotope component. The progressively reset Rb–Sr system is a relic of Variscan metamorphism of the Paleozoic series of the Mulhacén Complex. In contrast, the origin of the ca. 17.2 Ma old sample from the Mesozoic series precludes any isotopic inheritance, in agreement with its pervasive tectono-metamorphic recrystallisation during the Miocene. Exhumation of the eclogite-facies Mulhacén Complex occurred in two stages with contrasting rates of about 22.5 mm/year during the early phase and 9–10 mm/year during the late phase; the latter with a cooling rate in the order of 330 ˚C/Ma.