Search

The effects of metamorphic reactions occurring during decompression were explored to understand their influence on the 40Ar-39Ar ages of micas. Monometamorphic metasediments from the Lepontine Alps (Switzerland) reached lower amphibolite facies during the Barrovian metamorphism related to the collision between European and African (Adria) continental plates. Mineral assemblages typically composed of garnet, plagioclase, biotite, muscovite and paragonite (or margarite) were screened for petrological equilibrium, to focus on samples that record a minimum degree of retrogression. X-ray diffraction data indicate that some mineral separates prepared for 40Ar-39Ar stepwise heating analysis are monomineralic, whereas others are composed of two white micas (muscovite with paragonite or margarite), or biotite and chlorite. In monomineralic samples 37Ar/39Ar and 38Ar/39Ar (proportional to Ca/K and Cl/K ratios) did not change and the resulting ages can be interpreted unambiguously. In mineral separates containing two white micas, Ca/K and Cl/K ratios were variable, reflecting non-simultaneous laboratory degassing of the two heterochemical Ar reservoirs. These ratios were used to identify each Ar reservoir and to unravel the age. In a chlorite-margarite-biotite calcschist equilibrated near 560°C and 0·65 GPa, biotite, margarite, and muscovite all yield ages around 18 Ma. At slightly higher grade (560-580°C, 0·8-0·9 GPa), the assemblage muscovite-paragonite-plagioclase is in equilibrium and remains stable during retrogression. In this case, muscovite and paragonite yield indistinguishable ages around 16·5 Ma. Above 590°C, paragonite was mostly consumed to form plagioclase >590°C, whereby the relict mica yields an age up to 5·6 Ma younger than muscovite. This partial or total resetting of the Ar clock in paragonite is interpreted to reflect plagioclase growth during decompression. Where biotite is present within this same assemblage, it systematically yields a younger age than muscovite, by 0·5-2 Ma. However, these biotites all show small amounts of retrograde chlorite formation. We conclude that even very minor chloritization of biotite is apparently a more effective process than temperature in resetting the Ar clock, as is the formation of plagioclase from paragonite decomposition. Multi-equilibrium thermobarometry is an excellent means to ensure that equilibrium in investigated samples is preserved, and this helps to obtain geologically meaningful metamorphic ages. However, even samples passing such equilibrium tests may still show retrograde effects that affect the Ar retention of micas. A more robust interpretation of such 40Ar-39Ar results may require use of a second geochronometer, such as U-Pb on monazite

The effects of metamorphic reactions occurring during decompression were explored to understand their influence on the 40Ar- 39Ar ages of micas. Monometamorphic metasediments from the Lepontine Alps (Switzerland) reached lower amphibolite facies during the Barrovian metamorphism related to the collision between European and African (Adria) continental plates. Mineral assemblages typically composed of garnet, plagioclase, biotite, muscovite and paragonite (or margarite) were screened for petrological equilibrium, to focus on samples that record a minimum degree of retrogression. X-ray diffraction data indicate that some mineral separates prepared for 40Ar- 39Ar stepwise heating analysis are monomineralic, whereas others are composed of two white micas (muscovite with paragonite or margarite), or biotite and chlorite. In monomineralic samples 37Ar/ 39Ar and 38Ar/ 39Ar (proportional to Ca/K and Cl/K ratios) did not change and the resulting ages can be interpreted unambiguously. In mineral separates containing two white micas, Ca/K and Cl/K ratios were variable, reflecting non-simultaneous laboratory degassing of the two heterochemical Ar reservoirs. These ratios were used to identify each Ar reservoir and to unravel the age. In a chlorite-margarite-biotite calcschist equilibrated near 560°C and 0·65 GPa, biotite, margarite, and muscovite all yield ages around 18 Ma. At slightly higher grade (560-580°C, 0·8-0·9 GPa), the assemblage muscovite-paragonite-plagioclase is in equilibrium and remains stable during retrogression. In this case, muscovite and paragonite yield indistinguishable ages around 16·5 Ma. Above 590°C, paragonite was mostly consumed to form plagioclase >590°C, whereby the relict mica yields an age up to 5·6 Ma younger than muscovite. This partial or total resetting of the Ar clock in paragonite is interpreted to reflect plagioclase growth during decompression. Where biotite is present within this same assemblage, it systematically yields a younger age than muscovite, by 0·5-2 Ma. However, these biotites all show small amounts of retrograde chlorite formation. We conclude that even very minor chloritization of biotite is apparently a more effective process than temperature in resetting the Ar clock, as is the formation of plagioclase from paragonite decomposition. Multi-equilibrium thermobarometry is an excellent means to ensure that equilibrium in investigated samples is preserved, and this helps to obtain geologically meaningful metamorphic ages. However, even samples passing such equilibrium tests may still show retrograde effects that affect the Ar retention of micas. A more robust interpretation of such 40Ar- 39Ar results may require use of a second geochronometer, such as U-Pb on monazite. © The Author 2011. Published by Oxford University Press. All rights reserved.

International audience

During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial melting in the lower crust. Permian metamorphism and magmatism has extensively been recorded and dated in the Central, Eastern, and Southern Alps. However, Permian metamorphic ages in the Western Alps so far are constrained by very few and sparsely distributed data. The present study fills this gap. We present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266Ma. Trace element thermometry yields temperatures of 580–890°C from Ti-in-zircon and 630–850°C from Zr-in-rutile for Permian metamorphic rims. These temperature estimates, together with preserved mineral assemblages (garnet–prismatic sillimanite–biotite–plagioclase–quartz–K-feldspar–rutile), define pervasive upper-amphibolite to granulite facies conditions for Permian metamorphism. U/Pb ages from this study are similar to Permian ages reported for the Ivrea Zone in the Southern Alps and Austroalpine units in the Central and Eastern Alps. Regional comparison across the former Adriatic and European margin reveals a complex pattern of ages reported from late Palaeozoic magmatic and metamorphic rocks (and relics thereof): two late Variscan age groups (~330 and ~300Ma) are followed seamlessly by a broad range of Permian ages (300–250Ma). The former are associated with late-orogenic collapse; in samples from this study these are weakly represented. Clearly, dominant is the Permian group, which is related to crustal thinning, hinting to a possible initiation of continental rifting along a passive margin.

Subducted continental terranes commonly comprise an assembly of subunits that reflect the different tectono-metamorphic histories they experienced in the subduction zone. Our challenge is to unravel how, when, and in which part of the subduction zone these subunits were juxtaposed. Petrochronology offers powerful tools to decipher pressure–temperature–time (P–T–t) histories of metamorphic rocks that preserve a record of several stages of transformation. A major issue is that the driving forces for re-equilibration at high pressure are not well understood. For example, continental granulite terrains subducted to mantle depths frequently show only partial and localized eclogitization. The Sesia Zone (NW Italy) is exceptional because it comprises several continental subunits in which eclogitic rocks predominate and high-pressure (HP) assemblages almost completely replaced the Permian granulite protoliths. This field-based study comprises both main complexes of the Sesia terrane, covering some of the recently recognized tectonic subunits involved in its assembly; hence our data constrain the HP tectonics that formed the Sesia Zone. We used a petrochronological approach consisting of petrographic and microstructural analysis linked with thermodynamic modelling and U–Th–Pb age dating to reconstruct the P–T–t trajectories of these tectonic subunits. Our study documents when and under what conditions re-equilibration took place. Results constrain the main stages of mineral growth and deformation, associated with fluid influx that occurred in the subduction channel. In the Internal Complex (IC), pulses of fluid percolated at eclogite facies conditions between 77 and 55 Ma with the HP conditions reaching ∼ 2 GPa and 600–670 °C. By contrast, the External Complex (EC) records a lower pressure peak of ∼ 0.8 GPa for 500 °C at ∼ 63 Ma. The juxtaposition of the two complexes occurred during exhumation, probably at ∼ 0.8 GPa and 350 °C; the timing is constrained between 46 and 38 Ma. Mean vertical exhumation velocities are constrained between 0.9 and 5.1 mm year−1 for the IC, up to its juxtaposition with the EC. Exhumation to the surface occurred before 32 Ma, as constrained by the overlying Biella Volcanic Suite, at a mean vertical velocity between 1.6 and 4 mm year−1. These findings constrain the processes responsible for the assembly and exhumation of HP continental subunits, thus adding to our understanding of how continental terranes behave during subduction.

Contiguous continental high-pressure terranes in orogens offer insight into deep recycling and transformation processes that occur in subduction zones. These remain poorly understood, and currently debated ideas need testing. The approach we chose is to investigate, in detail, the record in suitable rock samples that preserve textures and robust mineral assemblages that withstood overprinting during exhumation. We document complex garnet zoning in eclogitic mica schists from the Sesia Zone (western Italian Alps). These retain evidence of two orogenic cycles and provide detailed insight into resorption, growth, and diffusion processes induced by fluid pulses in high-pressure conditions. We analysed local textures and garnet compositional patterns, which turned out remarkably complex. By combining these with thermodynamic modelling, we could unravel and quantify repeated fluid–rock interaction processes. Garnet shows low-Ca porphyroclastic cores that were stable under (Permian) granulite facies conditions. The series of rims that surround these cores provide insight into the subsequent evolution: the first garnet rim that surrounds the pre-Alpine granulite facies core in one sample indicates that pre-Alpine amphibolite facies metamorphism followed the granulite facies event. In all samples documented, cores show lobate edges and preserve inner fractures, which are sealed by high-Ca garnet that reflects high-pressure Alpine conditions. These observations suggest that during early stages of subduction, before hydration of the granulites, brittle failure of garnet occurred, indicating high strain rates that may be due to seismic failure. Several Alpine rims show conspicuous textures indicative of interaction with hydrous fluid: (a) resorption-dominated textures produced lobate edges, at the expense of the outer part of the granulite core; (b) peninsulas and atoll garnet are the result of replacement reactions; and (c) spatially limited resorption and enhanced transport of elements due to the fluid phase are evident along brittle fractures and in their immediate proximity. Thermodynamic modelling shows that all of these Alpine rims formed under eclogite facies conditions. Structurally controlled samples allow these fluid–garnet interaction phenomena to be traced across a portion of the Sesia Zone, with a general decrease in fluid–garnet interaction observed towards the external, structurally lower parts of the terrane. Replacement of the Permian HT assemblages by hydrate-rich Alpine assemblages can reach nearly 100 % of the rock volume. Since we found no clear relationship between discrete deformation structures (e.g. shear zones) observed in the field and the fluid pulses that triggered the transformation to eclogite facies assemblages, we conclude that disperse fluid flow was responsible for the hydration.

The buoyancy of continental crust opposes its subduction to mantle depths, except where mineral reactions substantially increase rock density. Sluggish kinetics limit such densification, especially in dry rocks, unless deformation and hydrous fluids intervene. Here we document how hydrous fluids in the subduction channel invaded lower crustal granulites at 50–60 km depth through a dense network of probably seismically induced fractures. We combine analyses of textures and mineral composition with thermodynamic modeling to reconstruct repeated stages of interaction, with pulses of high-pressure (HP) fluid at 650–670°C, rehydrating the initially dry rocks to micaschists. SIMS oxygen isotopic data of quartz indicate fluids of crustal composition. HP growth rims in allanite and zircon show uniform U-Th-Pb ages of ∼65 Ma and indicate that hydration occurred during subduction, at eclogite facies conditions. Based on this case study in the Sesia Zone (Western Italian Alps), we conclude that continental crust, and in particular deep basement fragments, during subduction can behave as substantial fluid sinks, not sources. Density modeling indicates a bifurcation in continental recycling: Chiefly mafic crust, once it is eclogitized to >60%, are prone to end up in a subduction graveyard, such as is tomographically evident beneath the Alps at ∼550 km depth. By contrast, dominantly felsic HP fragments and mafic granulites remain positively buoyant and tend be incorporated into an orogen and be exhumed with it. Felsic and intermediate lithotypes remain positively buoyant even where deformation and fluid percolation allowed them to equilibrate at HP.

International audience; This contribution presents an approach and a computer program (GRTMOD) for numerical simulation of garnet evolution based on compositions of successive growth zones in natural samples. For each garnet growth stage, a new local effective bulk composition is optimized, allowing for resorption and/or fractionation of previously crystallized garnet. The successive minimizations are performed using the Nelder – Mead algorithm; a heuristic search method. An automated strategy including two optimization stages and one refinement stage is described and tested. This program is used to calculate pressure – temperature (P–T) conditions of crystal growth as archived in garnet from the Sesia Zone (Western Alps). The compositional variability of successive growth zones is characterized using standardized X-ray maps and the program XMapTools. The model suggests that Permian garnet cores crystallized under granulite-facies conditions at T>800°C and P = 6 kbar. During Alpine times, a first garnet rim grew at eclogite-facies conditions (650°C, 16 kbar) at the expense of the garnet core. A second rim was added at lower P (~1 kbar) and 630°C. In total, garnet resorption is modeled to amount to ~9 vol% during the Alpine evolution; this value is supported by our observations in X-ray compositional maps.