Cold subduction and the formation of lawsonite eclogite - constraints from prograde evolution of eclogitized pillow lava from Corsica

International audienceA new discovery of lawsonite eclogite is presented from the Lancône glaucophanites within the Schistes Lustrés nappe at Défilé du Lancône in Alpine Corsica. The fine-grained eclogitized pillow lava and inter-pillow matrix are extremely fresh, showing very little evidence of retrograde alteration. Peak assemblages in both the massive pillows and weakly foliated inter-pillow matrix consist of zoned idiomorphic Mg-poor (<0.8 wt% MgO) garnet + omphacite + lawsonite + chlorite + titanite. A local overprint by the lower grade assemblage glaucophane + albite with partial resorption of omphacite and garnet is locally observed. Garnet porphyroblasts in the massive pillows are Mn rich, and show a regular prograde growth-type zoning with a Mn-rich core. In the inter-pillow matrix garnet is less manganiferous, and shows a mutual variation in Ca and Fe with Fe enrichment toward the rim. Some garnet from this rock type shows complex zoning patterns indicating a coalescence of several smaller crystallites. Matrix omphacite in both rock types is zoned with a rimward increase in XJd, locally with cores of relict augite. Numerous inclusions of clinopyroxene, lawsonite, chlorite and titanite are encapsulated within garnet in both rock types, and albite, quartz and hornblende are also found included in garnet from the inter-pillow matrix. Inclusions of clinopyroxene commonly have augitic cores and omphacitic rims. The inter-pillow matrix contains cross-cutting omphacite-rich veinlets with zoned omphacite, Si-rich phengite (Si = 3.54 apfu), ferroglaucophane, actinolite and hematite. These veinlets are seen fracturing idiomorphic garnet, apparently without any secondary effects. Pseudosections of matrix compositions for the massive pillows, the inter-pillow matrix and the cross-cutting veinlets indicate similar P-T conditions with maximum pressures of 1.9-2.6 GPa at temperatures of 335-420 °C. The inclusion suite found in garnet from the inter-pillow matrix apparently formed at pressures below 0.6-0.7 GPa. Retrogression during initial decompression of the studied rocks is only very local. Late veinlets of albite + glaucophane, without breakdown of lawsonite, indicate that the rocks remained in a cold environment during exhumation, resulting in a hairpin-shaped P-T path

Petrophysical properties across scales and compositions

The scales at which observations from geophysical imaging are made are orders of magnitude larger than those made in field-based studies of fossil subduction and collision zones. Even more so, the determination of petrophysical properties of rocks is typically based on millimeter to centimeter-scale samples, and the so-obtained information is then used to inform large-scale geophysical imaging studies. Information on how such properties can be up-scaled to geophysically relevant scales is rare, underlining the need to combine petrophysical properties with structural data, obtained from relevant field analogues. We provide results from three field analogues; (1) Tenda massif, Corsica, (2) Monte Mucrone, Sesia Zone, western Alps, and (3) Holsnøy, Lindås nappe, Scandinavian Caledonides. The bulk rock compositions cover a gradient from felsic (1-2) to mafic (3), as would be expected in the upper and lower continental crust, respectively. Petrophysical properties (P and S wave velocities and their ratios and anisotropies) were determined by direct measurement (ultrasonic pulse transmission technique) and calculated (based on texture data from neutron diffraction measurements). The data set is then used for numerical modeling (finite element method) of meter to kilometer-scale structural associations as mapped in the field (3). The obtained results show that high-pressure metamorphism of mafic rocks results in significant increase in both P and S wave velocities, that in principle would generate a sufficient impedance contrast to be imaged by seismic methods. While structures observed in the field are typically below the scale of geophysical imaging techniques, our considerations of bulk petrophysical properties indicate that significant anisotropy may still be detectable on the kilometer scale. On the other hand, the increase of P and S wave velocities of felsic rocks during high pressure metamorphism is much smaller, however, as such compositions have a higher potential to form rocks with high mica contents, they display a large variability in seismic anisotropy, hinting at the potential to link relatively low seismic velocities, combined with high anisotropy to fluid intake during metamorphism

Widening of Hydrous Shear Zones During Incipient Eclogitization of Metastable Dry and Rigid Lower Crust— Holsnøy, Western Norway

The partially eclogitized crustal rocks on Holsnøy in the Bergen Arcs, Norway, indicate that eclogitization is caused by the interplay of brittle and ductile deformation promoted by fluid infiltration and fluid-rock interaction. Eclogitization generated an interconnected network of millimeterto- kilometer-wide hydrous eclogite-facies shear zones, which presumably caused transient weakening of the mechanically strong lower crust. To decipher the development of those networks, we combine detailed lithological and structural mapping of two key outcrops with numerical modeling. Both outcrops are largely composed of preserved granulite with minor eclogite-facies shear zones, thus representing the beginning phases of eclogitization and ductile deformation. We suggest that deformation promoted fluidrock interaction and eclogitization, which gradually consumed the granulite until fluid-induced reactions were no longer significant. The shear zones widen during progressive deformation. To identify the key parameters that impact shear zone widening, we generated scale-independent numerical models, which focus on different processes affecting the shear zone evolution: (i) rotation of the shear zones caused by finite deformation, (ii) mechanical weakening due to a limited amount of available fluid, and (iii) weakening and further hydration of the shear zones as a result of continuous and unlimited fluid supply. A continuous diffusion-type fluid infiltration, with an effective diffusion coefficient around 2 10 16 m s D , coupled with deformation is prone to develop structures similar to the ones mapped in field. Our results suggest that the shear zones formed under a continuous fluid supply, causing shear zone widening, rather than localization, during progressive deformation

High-Temperature Deformation During Continental-Margin Subduction & Exhumation: The Ultrahigh-Pressure Western Gneiss Region of Norway

A new dataset for the high-pressure to ultrahigh-pressure Western Gneiss Region allows the definition of distinct structural and petrological domains. Much of the study area is an E-dipping homocline with E-plunging lineations that exposes progressively deeper, more strongly deformed, more eclogite-rich structural levels westward. Although eclogites crop out across the WGR, Scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area. The Scandian reworking increases westward, culminating in strong Scandian fabrics with only isolated pockets of older structures; the dominant Scandian deformation was coaxial E–W stretching. The sinistrally sheared Møre–Trøndelag Fault Complex and Nordfjord Mylonitic Shear Zone bound these rocks to the north and south. There was moderate top-E, amphibolite-facies deformation associated with translation of the allochthons over the basement along its eastern edge, and the Nordfjord–Sogn Detachment Zone underwent strong lower amphibolite-facies to greenschist-facies top-W shearing. A northwestward increase in exhumation-related melting is indicated by leucosomes with hornblende, plagioclase, and Scandian sphene. In the western 2/3 of the study area, exhumation-related, amphibolite-facies symplectite formation in quartzofeldspathic gneiss postdated most Scandian deformation; further deformation was restricted to slip along biotite-rich foliation planes and minor local folding. That the Western Gneiss Region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation, implies that continental crust in general need not undergo pervasive deformation during subduction

Recruitment methods in Alzheimer's disease research: general practice versus population based screening by mail

<p>Abstract</p> <p>Background</p> <p>In Alzheimer's disease (AD) research patients are usually recruited from clinical practice, memory clinics or nursing homes. Lack of standardised inclusion and diagnostic criteria is a major concern in current AD studies. The aim of the study was to explore whether patient characteristics differ between study samples recruited from general practice and from a population based screening by mail within the same geographic areas in rural Northern Norway.</p> <p>Methods</p> <p>An interventional study in nine municipalities with 70000 inhabitants was designed. Patients were recruited from general practice or by population based screening of cognitive function by mail. We sent a questionnaire to 11807 individuals ≥ 65 years of age of whom 3767 responded. Among these, 438 individuals whose answers raised a suspicion of cognitive impairment were invited to an extended cognitive and clinical examination. Descriptive statistics, chi-square, independent sample t-test and analyses of covariance adjusted for possible confounders were used.</p> <p>Results</p> <p>The final study samples included 100 patients recruited by screening and 87 from general practice. Screening through mail recruited younger and more self-reliant male patients with a higher MMSE sum score, whereas older women with more severe cognitive impairment were recruited from general practice. Adjustment for age did not alter the statistically significant differences of cognitive function, self-reliance and gender distribution between patients recruited by screening and from general practice.</p> <p>Conclusions</p> <p>Different recruitment procedures of individuals with cognitive impairment provided study samples with different demographic characteristics. Initial cognitive screening by mail, preceding extended cognitive testing and clinical examination may be a suitable recruitment strategy in studies of early stage AD.</p> <p>Clinical Registration</p> <p>ClinicalTrial.gov Identifier: NCT00443014</p

Co-morbidity and drug treatment in Alzheimer's disease. A cross sectional study of participants in the Dementia Study in Northern Norway

Inappropriate medical treatment of co-morbidities in Alzheimer’s disease (AD) is an increasing concern in geriatric medicine. The objective of this study was to compare current drug use related to co-morbidity between individuals with a recent diagnosis of AD and a cognitively healthy control group in a population based clinical trial in Northern Norway. Setting: Nine rural municipalities with 70 000 inhabitants in Northern Norway. Participants: Participants with and without AD recruited in general practice and by population based screening. 187 participants with a recent diagnosis of AD were recruited among community dwellers. Of 791 respondents without cognitive symptoms, 500 were randomly selected and invited to further clinical and cognitive testing. The final control group consisted of 200 cognitively healthy individuals from the same municipalities. Demographic characteristics, data on medical history and current medication were included, and a physical and cognitive examination was performed. The statistical analyses were carried out by independent sample t-test, chi-square, ANCOVA and logistic regression. A co-morbidity score was significantly higher in AD participants compared to controls. The mean number of drugs was higher for AD participants compared to controls (5.1 ± 3.6 and 2.9 ± 2.4 respectively, p < 0.001 age and gender adjusted), also when adjusted for co-morbidity. AD participants used significantly more anticholinergic, sedative and antidepressant drugs. For nursing home residents with AD the mean number of drugs was significantly higher compared to AD participants living at home (6.9 ± 3.9 and 4.5 ± 3.3, respectively, p < 0.001). AD participants were treated with a significantly higher number of drugs as compared to cognitively healthy controls, even after adjustment for co-morbidity. An inappropriate use of anticholinergic and sedative drugs was identified, especially among nursing home residents with AD. The drug burden and the increased risk of adverse reactions among individuals suffering from AD need more attention from prescribing doctors

Oxygen and carbon isotope compositions of carbonates in a prominent lithologically mixed unit in the central South Norwegian Caledonides

International audienceA prominent pre-Scandian lithologically mixed unit in the central South Norwegian Caledonides contains more than 100 partly carbonated and hydrated metaperidotite bodies and locally fossiliferous detrital serpentinites. The lateral consistency of this mixed unit was not fully appreciated in the past. Therefore, parts of the mixed unit along strike were interpreted to belong to several different tectonostratigraphic levels. Here, we present new carbonate stable isotope data that suggest that the carbonates of the mixed unit between Bergen and Otta (re-)equilibrated at unit-wide similar peak metamorphic conditions. The isotope compositions are characteristic for this unit and indicate that it represented one single tectonic unit during the Scandian Orogeny. The carbonates in the mélange are characterized by a narrow range of δ18O (SMOW) values between + 11 and + 15.5‰ and three groups of δ13C (PDB) values: (I) + 1.6 to + 0.3‰, (II) − 1.8 to − 3.9‰, and (III) − 6 to − 8.6‰. Carbonates of group III probably were affected by decarbonation or by a fluid containing organic carbon, whereas carbonates of group I and II overlap with δ13C values typical for Ediacaran–Silurian marine carbonates and may have retained their initial δ13C imprint. We suggest that the δ18O values (re-)equilibrated with unit-wide released metamorphic fluids during Scandian metamorphism. An outcrop-scale homogenisation of the δ13C values reflects the local carbon isotope signature of the released metamorphic fluids that circulated channelized through the mélange unit

Peak metamorphic temperatures from Raman Spectroscopy onCarbonaceous Matter (RSCM) and 18O and 13C (carb) isotopecomposition of a major mélange zone in the South Norwegian Caledonides

International audienceA mélange in southern Norway comprises a matrix of garnet, mica- and black carbonaceous schists and phyllites of abyssal origin, interlayered with originally coarser grained siliciclastic metasediments, serpentinite conglomerates and sandstones, solitary metaperidotites and thin slivers of gneisses. Several models for the formation of the mélange have been suggested, including formation as a) an ophiolitic mélange formed during ophiolite obduction, b) an unconformable post-obduction transgressive sequence or c) a mélange formed during hyperextension along the pre-Caledonian margin of Baltica. In the past, the mélange has therefore not been treated as one single tectonic unit, but has been assigned to various tectonic positions with both outboard Iapetus and inboard Baltican origins. In this study, we argue that the mélange occupies a tectonostratigraphic position below major Baltican basement nappe-complexes previously assigned to the Middle Allochthon. Furthermore, we present new consistent resultson the peak metamorphic temperatures (T ∼500 °C), based on RSCM, and a characteristic δ18Ocarb isotope composition (11-15.5 permille SMOW), both consistent for more than 250 km along strike of the mélange. δ13Ccarb values fall within three clusters around 1, −2, and −7 permille (PDB), respectively.The stable isotope investigation presented here was carried out in order to explore if pre-Caledonian isotope signatures in various generations of carbonate veins and the early Ordovician fossils at Otta, could have been preserved through a later Caledonian metamorphic overprint. The results presented here, however, suggest that reequilibration of the carbonates took place in the Silurian, most likely coeval with peak metamorphism of ~500 °C at ~420 Ma, and the main fabric development close to the base of the nappe stack. Reequilibration of the carbonates was assisted by the presence a pervasive static fluid, allowing for oxygen isotope exchange with the surrounding schists and resulting in a mélange-wide uniform δ18Ocarb signature. The carbon isotope composition was re-equilibrated only within each lithological body and notably not beyond lithological boundarie