Exhumation of a migmatitic unit through self-enhanced magmatic weakening enabled by tectonic contact metamorphism (Gruf complex, Central European Alps)

The Central Alpine lower crustal migmatitic Gruf complex was exhumed in contact to the greenschist-grade Chiavenna ophiolite and gneissic Tambo nappe leading to a lateral gradient of similar to 70 degrees C/km within the ophiolite. The 14 km long, E-W striking subvertical contact now bridges metamorphic conditions of similar to 730 degrees C, 6.6 kbar in the migmatitic gneisses and similar to 500 degrees C, 4.2 kbar in the serpentinites and Tambo schists 2-4 km north of the contact. An obvious fault, mylonite or highly sheared rock that could accommodate the similar to 8.5 km vertical displacement is not present. Instead, more than half of the movement was accommodated in a 0.2-1.2 km thick orthogneiss of the Gruf complex that was heterogeneously molten. Discrete bands with high melt fractions (45-65%) now contain variably stretched enclaves of the adjacent MOR-derived amphibolite. In turn, the adjacent amphibolites exhibit tonalitic in-situ leucosomes and dikes i.e., were partially molten. The H2O necessary for fluid-assisted melting of the orthogneiss and amphibolites was likely derived from the tectonic contact metamorphism of the Chiavenna serpentinites, at the contact now in enstatite + olivine-grade. U-Pb dating of zircons shows that partial melting and diking occurred at 29.0-31.5 Ma, concomitant with the calc-alkaline Bergell batholith that intruded the Gruf. The major driving forces of exhumation were hence the strong regional North-South shortening in the Alpine collisional belt and the buoyancy provided by the Bergell magma. The fluids available through tectonic contact metamorphism led to self-enhanced magmatic weakening and concentration of movement in an orthogneiss, where melt-rich bands provided a low friction environment. Continuous heating of the originally greenschist Chiavenna ophiolite and Tambo gneisses + schists by the migmatitic Gruf complex during differential uplift explains the skewed temperature profile, with intensive contact heating in the ophiolite but little cooling in the portion of the now-exposed Gruf complex.ISSN:0010-7999ISSN:1432-096

Quantifying frozen melt in crustal rocks: A new melt-o-meter based on zircon rim volumes

International audienceQuantifying the distribution of granitic melt at all scales in mid-to lower crustal migmatitic terranes is critical to understand crustal melting processes, chemical differentiation of the crust and its rheological behavior during deformation. We propose a new method to determine the fraction of frozen granitic melt on a hand specimen scale based on the relative volumes of newly precipitated to total zircon (FPZ = Fraction of newly Precipitated Zircon) as obtained by image analysis on dated zircon cores and rims. Using the calculated Zr-solubility [Zr] sat in the melt at the inferred melting temperature and the Zr concentration in the bulk sample [Zr] bulk , the fraction of melt F melt can be determined through F melt = FPZ × [Zr] bulk / [Zr] sat. The such obtained F melt corresponds to the melt fraction in the hand specimen at the time the system closed for melt mobility. Thermodynamic modelling further allows estimation of H 2 O-contents required to maintain the melt fraction obtained from the melt-o-meter in a molten stage. The applicability of this method has been tested on eight migmatitic samples with peak temperatures between 725 and 925°C. Most of the lower temperature migmatites (800°C) retained F melt of 0.35-0.50 (±0.07-0.10). At these melt fractions, melt extraction and melt migration from and within the source should be efficient. Consequently, these samples are likely open-system migmatites affected by melt accumulation or depletion processes. The melt-o-meter requires that the rock types under consideration produced a granitic melt that remained zircon-saturated and is therefore restricted to migmatitic meta-sediments and meta-granitoids. When applied carefully, this melt-o-meter offers a new and powerful tool to not only quantify melt distribution but also evaluate the extent of melt mobility in migmatites

Plutons and domes: the consequences of anatectic magma extraction—example from the southeastern French Massif Central

International audienceAnatectic magmas form plutons or accumulate in the core of anatectic domes. Both scenarios have distinct implications on the behaviour of the continental crust during orogenic evolution from collision to collapse. Considering a stepwise extraction of melt, we simulate the evolution of anatectic melt and of solid residues produced in the crust from collision to collapse using thermodynamic modelling. We also simulate the effect of entrainment of source material (restite-unmixing and peritectic assemblage entrainment) on the compositional range of the resulting magmas. The results are then compared to the compositions of lower crustal xenoliths and of peraluminous granites in both plutons and anatectic dome in the southeastern French Massif Central (SE-FMC). From our calculations, we identify two type of anatectic melts (1) cool-and-wet produced at low-temperature ( 750 °C) which only release fluids at the end of crystallisation. When emplaced around 0.4 GPa, cold-and-wet melts are produced by muscovite-dehydration melting reactions; hot-and-dry are produced by biotite-dehydration melting. In the SE-FMC, the Velay dome is cored by the Velay granite, intruded by small bodies of Velay leucogranite and surrounded by plutons made of either two mica leucogranite (MPG) or cordierite-bearing granite (CPG). MPG and Velay leucogranite compositions are best reproduced by cool-and-wet magmas. CPG and Velay granite compositions are best reproduced by hot-and-dry magmas. Melt extraction after biotite dehydration melting leaves residues that are similar in composition to lower-crustal xenoliths. Magmas forming plutons migrate freely toward the upper crust forming plutons with distinct compositions. On the contrary, to form a dome, magmas are retained on the way up. The emplacement and accumulation of magma at deeper level enhances (or trigger) melting due to the addition of heat (from hot-and-dry) and fluids (from cool-and-wet). The accumulation of magma and the in situ melting increases melt fraction and has consequence to weaken the middle crust and leads to the formation of an anatectic dome. We suggest that magmas are retained due to lithological heterogeneities in the crust. In the case of the Velay dome, a large orthogneiss formation similar to the Velay orthogneiss formation may have played that role

The efficacy of Botulinum toxin A in reducing muscle spasticity by different volumes of dilution

Objective: Spasticity is a speed-dependent increase in muscle tone associated with stiffness, enhanced osteotendinous reflexes, paresis, loss of fine motor control, and increased fatigue. The neural insult it self causes paresis, and the consequent immobilization of the paretic part of the body can cause adaptive shortening ofthe muscles and joint contractures. Infiltrative treatment with botulinum toxin typeA (BTX-A ) is effective, safe and with few side effects. Methods: 10 patients after different years of treatment with BTX-A of the brachial biceps (BB100U) and superficial flexor ofthe fingers (FSD 100U), show ed no clinical and instrumental efficacy; The functional benefit obtained in patients. is related to the evaluation of muscle changes evolved in spasticity and to the study of a treatment to prevent these changes

Gait Analysis in Parkinsonian Subjects with PISA Syndrome Subjected to Treatment with Botulinum Toxin

The aim was to evaluate the activation characteristics of the muscles involved in patients with Pisa Syndrome with surface electromy- ography (EMG) during walking after Botulinum Toxin TypeA treatment. Methods: 7 patients with Pisa Syndrome underwent Gait Analysis, according to the modified Davis protocol; Subsequently, the patients underwent myometric evaluation of the cervical dorsal and lumbar paravertebra lmuscles and were treated with Onabotulinum toxin typeA (BoTN/A); Conclusions: Our analysis highlights continuous muscle activity in the paravertebral muscles, especially contralateral to the flexion curve