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The exhumation of crustal ultra-high-pressure (UHP) material depends on temporal and spatial variations in its detachment within the subduction channel. This dependence is investigated using numerical models with variable initial crustal strengths, representing a range of initial crustal compositions, and parameterized strain weakening, representing a range of processes that reduce effective crustal viscosity during deformation. Competition between down-channel shear traction, favoring subduction, and up-channel buoyancy, favoring exhumation, is expressed as the exhumation number, E, which can vary with time and position along the channel. Exhumed lower strength crust, which resists subduction owing to weak down-channel traction, records peak conditions 38 kbar. Given sufficient strain weakening, exhumation proceeds at >60 km [Ma.sup.-1], indicating that buoyancy (E [much greater than] 1) drives exhumation in these models. In all models, exhuming UHP material forms a deforming ductile plume, with a range of possible structural relationships predicted between exhumed UHP and HP materials.

The presence of high-pressure (P) eclogites along the northeast margin of the Arabian continental plate beneath the Semail ophiolite demonstrates that subduction of thinned continental crust to pressures of 20 kbar (~80 km depth) has occurred. Current debate centers on whether this high-P metamorphism occurred prior to, during, or following ophiolite emplacement, which is known to have started ca. 95-93 Ma. Five concordant U-Pb zircon ages from a garnet-clinopyroxene-phengite-crossite eclogite from As Sifah, northeast Oman, precisely constrain the age of high-P metamorphism at 79.1 [+ or -] 0.3 Ma. This age demonstrates that high-P metamorphism occurred during continental subduction 15 m.y. after formation of the ocean floor that subsequently became the Semail ophiolite. Keywords: Oman ophiolite, subduction, Arabian margin, eclogites, U-Pb geochronology

Tectonic models are commonly underpinned by metamorphic cooling rates derived from diffusive‐loss thermochronology data. Such cooling ages are usually linked to temperature via Dodson's closure temperature (TC) formulation, which specifies a 1/time‐shaped cooling path (Contributions to Mineralogy and Petrology, 1973, 40, 259). Geologists, however, commonly discuss cooling rates as a linear temperature/time shape. We present the results of a series of simple finite‐difference diffusion models for Ar diffusion in muscovite and biotite that show that the difference in recorded age between 1/t and linear cooling paths increases significantly with hotter starting temperatures, slower cooling rates and smaller grain sizes. Our results show that it is essential to constrain the cooling path shape in order to make meaningful interpretations of the measured data. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Abstract: 40Ar/39Ar dating of metamorphic white mica is commonly used to elucidate the timing of cooling and/or exhumation of metamorphic terranes. In theory, so long as the dated mineral originally crystallised above its closure temperature, the yielded 40Ar/39Ar ages should not vary significantly within or between samples, and should vary only due to grain size (~3Ma variation depending on the estimated cooling rate). Infra-red laser single grain fusion and ultra-violet laser spot muscovite and/or phengite data from mafic eclogite bodies and their host gneisses in the Nordfjord region of the Western Gneiss Complex (WGC), Norway, show that the ages yielded both within and between white micas from the same mafic eclogite sample vary by up to 284Ma. Ages yielded within the same pelitic gneiss sample vary by up to 28Ma. Overall the 40Ar/39Ar phengite dates from mafic eclogites range from ~750 to 440Ma and are significantly older than the previously reported 405–400Ma U–Pb zircon age constraining peak metamorphism in the Nordfjord WGC. The felsic host gneiss samples, which recrystalised during a post-peak, exhumation-related, amphibolite-facies overprint, yield single grain fusion and laser spot 40Ar/39Ar white mica dates spanning ~420 to 385Ma, encompassing previous estimates of the timing of the entire metamorphic cycle. These data suggest that Ar is heterogeneously distributed both within grains and between grains on the cm to sub-mm scale in all samples, and imply restricted Ar transport and inefficient removal during the metamorphic cycle. Open system (zero grain boundary Ar concentration) behaviour is an inherent assumption in the commonly applied closure temperature formulation, but unless this can be demonstrated, linking 40Ar/39Ar ages to temperature is no longer trivial. The high spatial and chronological precision afforded by UV laser ablation mean that intra-grain Ar concentration distributions may be used to assess open vs. closed system behaviour. The data presented here suggest that open system behaviour cannot be assumed in metamorphic systems, even those in which the pressure, temperature and time conditions in theory allow for efficient diffusion during the metamorphic cycle. High spatial resolution 40Ar/39Ar data and robust comparison with numerical models of open system behaviour provide the greatest future potential for unravelling the complexities of metamorphic exhumation timescales. [Copyright &y& Elsevier]

Abstract: Metamorphic cooling rates determined using muscovite 40Ar/39Ar ages underpin many tectonic models, but are based on several simplifying approximations. We demonstrate that a number of these approximations, especially those relating to the assignment of a single “closure temperature”, might not hold for certain metamorphic scenarios, and particularly for metamorphism during rapid orogenic cycles. Using numerical diffusion models that include a recently reported significant pressure dependence of Ar diffusion in muscovite, we systematically interrogate the approximations associated with linking 40Ar/39Ar dates to pressure–temperature histories. The results of the simulations are presented in a simple graphic form allowing evaluation of the pressure–temperature (PT) regions in which Ar diffusion is efficient, and hence those in which the muscovite may yield true 40Ar/39Ar cooling ages. We suggest that a robust method for determining whether metamorphic muscovite 40Ar/39Ar ages relate to the timing of cooling involves: (1) the determination of the PT conditions and relative timing of muscovite crystallisation during the metamorphic cycle, (2) the collection of high precision and high spatial resolution 40Ar/39Ar data from muscovite grains and (3) the comparison between analytical data and numerical diffusion models which test different post-crystallisation pressure–temperature histories. [Copyright &y& Elsevier]

During continental collision, crustal rocks are buried, deformed, transformed and exhumed. The rates and timescales of these processes can be constrained by linking geochemical and geochronological data in metamorphic rock-forming and accessory minerals. Many studies of metamorphic timescales focus on single lithologies to shed light on particular stages of the terrane evolution. Mafic rocks typically only record geochronological evidence for their high-grade history. Felsic rocks, on the other hand, tend to yield ages of exhumation processes. In NW Bhutan, eastern Himalaya, metabasaltic layers and boudins record a complex history involving an early high pressure (eclogite facies, ca 600C, >1.6 GPa) stage, a later high temperature (granulite facies, 800C, ~1 GPa) stage and a final amphibolite facies (~600C, 0.7-0.8 GPa) overprint. In their metasedimentary hosts, rock-forming mineral assemblages record granulite facies metamorphism, exhumation, and anatexis, with little to no evidence for high-pressure prograde metamorphism. This terrane provides a rare glimpse into the evolution and exhumation of the deep eastern Himalayan crust, and a detailed case study for deciphering the rates and timescales of deep-crustal processes in orogenic settings. Our new petrochronological dataset suggests that the metabasites and their host metasediments record different and conflicting tectonic evolutions. Monazite in the metasediments yields 20-15 Ma ages, with trace element chemistry suggesting crystallisation during anatexis and exhumation. Rare hints of an earlier prograde history up to ~34 Ma old are also preserved. On the other hand, allanite and zircon in the metabasites suggests that eclogite facies conditions still prevailed at ~ 15 Ma, well after their host metasediments were apparently melting and decompressing. The age of the granulite facies overprint is still unclear in both lithologies. Detailed interrogation of conventional assumptions about how to link 'isotopic age' and 'trace-element-fingerprint stage' in different chronometers is therefore necessary to reconcile the two records.The dataset highlights the importance of collecting evidence from a variety of different geochronometers in different lithologies in order to provide the most detailed picture of the evolution and exhumation of deep orogenic crust. [ABSTRACT FROM AUTHOR]