L'unité de la Boghen (Nouvelle-Calédonie, Pacifique SW): un complexe d'accrétion jurassique. Données radiochronologiques préliminaires U-Pb sur les zircons détritiques.

La datation des zircons détritiques de l'unité de la Boghen permet d'attribuer un âge post-liasique à cet ensemble, autrefois considéré comme le " socle anté-permien " de la Nouvelle-Calédonie. Ce complexe est formé d'un ensemble volcanosédimentaire et terrigène à caractère distal, reposant directement sur une croûte océanique ou de bassin marginal incomplète. Compte tenu de ses caractères sédimentologiques, de l'existence d'un métamorphisme de haute pression d'âge Jurassique (environ 150 Ma) et de son association au complexe volcanosédimentaire d'arc de la Chaîne centrale, il est possible d'interpréter cette unité comme un prisme d'accrétion mis en place au Jurassique le long de la marge active Est-Gondwanienne. Le spectre d'âge des zircons détritiques permet d'assigner le système d'arcs permo-mésozoïques Sud-Est-Gondwanien et l'Antarctique comme sources principales de la fraction détritique. Abstract Radiochronological dating of detrital zircon extracted from the Boghen terrane metasediments allows a Jurassic age to be assigned. This terrane was formerly considered as the "pre-Permian basement" of New Caledonia. Its sedimentological features, its Late Jurassic high-pressure metamorphism (ca 150 Ma) and its association with the arc-related volcano-sedimentary complex of the Central Chain Terrane indicate that the Boghen terrane was an accretionary complex formed during the Jurassic period along the East-Gondwana active margin. The age spectrum of detrital zircons is consistent with a derivation from the Permian-Mesozoic Southeast-Gondwana arc system and the Antarctic continent

Detrital zircon records of Late Cretaceous syn-rift sedimentary sequences of New Caledonia: an Australian provenance questioned

International audienceThe Late Cretaceous clastic coastal sediments of New Caledonia are contemporaneous with the latest stages of the eastern Australian marginal rifting. As such, they record the erosion of basement terranes located on uplifted and tilted blocks and a contemporaneous volcanic activity. Detrital zircon populations contain two major components, the younger of which is Early Cretaceous, and the older Early Paleozoic and Precambrian. Following recent advances in the knowledge of detrital zircon content of basement terranes, and at variance with previous interpretations, that hypothesised a possible direct Australian provenance for Precambrian zircons, the detrital zircon record of these syn-rift sediments allows a local recycled provenance to be established. In consequence, this new evidence confirms that New Caledonia was already isolated from Australia as early as Coniacian time (ca. 89-85 Ma) a fact consistent with the development of faunal and floral endemism at that period. The prominent abundance of Early Cretaceous detrital zircons also establishes the importance of a previously unrecorded Early Cretaceous magmatism in the area

120 to 0 Ma tectonic evolution of the southwest Pacific and analogous geological evolution of the 600 to 220 Ma Tasman Fold Belt System

We review the tectonic evolution of the SW Pacific east of Australia from ca 120 Ma until the present. A key factor that developed early in this interval and played a major role in the subsequent geodynamic history of this region was the calving off from eastern Australia of several elongate microcontinental ribbons, including the Lord Howe Rise and Norfolk-New Caledonia Ridge. These microcontinental ribbons were isolated from Australia and from each other during a protracted extension episode from ca 120 to 52 Ma, with oceanic crust accretion occurring from 85 to 52 Ma and producing the Tasman Sea and the South Loyalty Basin. Generation of these microcontinental ribbons and intervening basins was assisted by emplacement of a major mantle plume at 100 Ma beneath the southern part of the Lord Howe Rise, which in turn contributed to rapid and efficient eastward trench rollback. A major change in Pacific plate motion at ca 55 Ma initiated east-directed subduction along the recently extinct spreading centre in the South Loyalty Basin, generating boninitic lithosphere along probably more than 1000 km of plate boundary in this region, and growth of the Loyalty-Entrecasteaux arc. Continued subduction of South Loyalty Basin crust led to the arrival at about 38 Ma of the 70-60 million years old western volcanic passive margin of the Norfolk Ridge at the trench, and west-directed emplacement of the New Caledonia ophiolite. Lowermost allochthons of this ophiolite are Maastrichtian and Paleocene rift tholeiites derived from the underthrusting passive margin. Higher allochthonous sheets include a poorly exposed boninitic lava slice, which itself was overridden by the massive ultramafic sheets that cover large parts of New Caledonia and are derived from the colliding forearc of the Loyalty-Entrecasteaux arc. Post-collisional extensional tectonism exhumed the underthrust passive margin, parts of which have blueschist and eclogite facies metamorphic assemblages. Following locking of this subduction zone at 38-34 Ma, subduction jumped eastward, to form a new west-dipping subduction zone above which formed the Vitiaz arc, that contained elements which today are located in the Tongan, Fijian, Vanuatu and Solomons arcs. Several episodes of arc splitting fragmented the Vitiaz arc and produced first the South Fiji Basin (31-25 Ma) and later (10 Ma to present) the North Fiji Basin. Collision of the Ontong Java Plateau, a large igneous province, with the Solomons section of the Vitiaz arc resulted in a reversal of subduction polarity, and growth of the Vanuatu arc on clockwise-rotating, older Vitiaz arc and South Fiji Basin crust. Continued rollback of the trench fronting the Tongan arc since 6 Ma has split this arc and produced the Lau Basin-Havre Trough. This southwest Pacific style of crustal growth above a rolling-back slab is applied to the 600-220 Ma tectonic development of the Tasman Fold Belt System in southeastern Australia, and explains key aspects of the geological evolution of eastern Australia. In particular, collision between a plume-triggered 600 Ma volcanic passive margin and a 510-515 Ma boninitic forearc of an intra-oceanic arc had the same relative orientation and geological effects as that which produced New Caledonia. A new subduction system formed probably at least several hundred kilometres east of the collision zone and produced the Macquarie arc, in which the oldest lavas were erupted ca 480 Ma. Continued slab rollback induced regional extension and the growth of narrow linear troughs in the Macquarie arc, which persisted until terminal deformation of this fold belt in the late-Middle to Late Devonian. A similar pattern of tectonic development generated the New England Fold Belt between the Late Devonian and Late Triassic. Parts of the New England Fold Belt have been broken from Australia and moved oceanward to locations in New Zealand, and on the Lord Howe Rise and Norfolk-New Caledonia Rise, during the post- 120 Ma breakup. Given that the Tasman Fold Belt System grew between 600 and 220 Ma by crustal accretion like the southwest Pacific since 120 Ma, facing the open Pacific Ocean, we question whether the eastern (Australia-Antarctica) part of the Neoproterozoic Rodinian supercontinent was joined to Laurentia

Magnetic anisotropy determination and magnetic hyperthermia properties of small Fe nanoparticles in the superparamagnetic regime

We report on the magnetic and hyperthermia properties of iron nanoparticles synthesized by organometallic chemistry. They are 5.5 nm in diameter and display a saturation magnetization close to the bulk one. Magnetic properties are dominated by the contribution of aggregates of nanoparticles with respect to individual isolated nanoparticles. Alternative susceptibility measurements are been performed on a low interacting system obtained after eliminating the aggregates by centrifugation. A quantitative analysis using the Gittleman s model allow a determination of the effective anisotropy Keff = 1.3 * 10^5 J.m^{-3}, more than two times the magnetocristalline value of bulk iron. Hyperthermia measurements are performed on agglomerates of nanoparticles at a magnetic field up to 66 mT and at frequencies in the range 5-300 kHz. Maximum measured SAR is 280 W/g at 300 kHz and 66 mT. Specific absorption rate (SAR) displays a square dependence with the magnetic field below 30 mT but deviates from this power law at higher value. SAR is linear with the applied frequency for mu_0H=19 mT. The deviations from the linear response theory are discussed. A refined estimation of the optimal size of iron nanoparticles for hyperthermia applications is provided using the determined effective anisotropy value

The subthalamic nucleus keeps you high on emotion: behavioral consequences of its inactivation

International audienceThe subthalamic nucleus (STN) belongs to the basal ganglia and is the current target for the surgical treatment of neurological and psychiatric disorders such as Parkinson's Disease (PD) and obsessive compulsive disorders (OCD), but also a proposed site for the treatment of addiction. It is therefore very important to understand its functions in order to anticipate and prevent possible side-effects in the patients. Although the involvement of the STN is well documented in motor, cognitive and motivational processes, less is known regarding emotional processes. Here we have investigated the direct consequences of STN inactivation by excitotoxic lesions on emotional processing and reinforcement in the rat. We have used various behavioral procedures to assess affect for neutral, positive and negative reinforcers in STN lesioned rats. STN lesions reduced affective responses for positive (sweet solutions) and negative (electric foot shock, Lithium Chloride-induced sickness) reinforcers while they had no effect on responses for a more neutral reinforcer (novelty induced place preference (NIPP)). Furthermore, when given the choice between saccharine, a sweet but non caloric solution, and glucose, a more bland but caloric solution, in contrast to sham animals that preferred saccharine, STN lesioned animals preferred glucose over saccharine. Taken altogether these results reveal that STN plays a critical role in emotional processing. These results, in line with some clinical observations in PD patients subjected to STN surgery, suggest possible emotional side-effects of treatments targeting the STN. They also suggest that the increased motivation for sucrose previously reported cannot be due to increased pleasure, but could be responsible for the decreased motivation for cocaine reported after STN inactivation

Palaeozoic collision between the North and South China Blocks, Triassic intracontinental tectonics, and the problem of the ultrahigh-pressure metamorphism

International audienceThe Qinling–Dabie Belt represents the boundary between the North and South China Blocks (NCB, SCB, respectively), where ultrahigh-pressure (UHP) rocks are widespread. A structural study in eastern Qinling and zircon LA ICPMS dating of the migmatite that form the core of the Central Qinling Unit allows us to argue that continental collision occurred in Silurian, before 400 Ma. In the Late Palaeozoic, from Devonian to Permian, the northern margin of SCB experienced a continental rifting. From Late Permian to Middle Triassic, northward continental subduction of SCB is responsible for the development of a high-pressure metamorphism. The age of the UHP metamorphism remains unsettled yet. A two-time genesis, Early Palaeozoic and Early Triassic, is often preferred, but a single Palaeozoic age followed by a Triassic resetting cannot be ruled out

The Zanhuang Massif, the second and eastern suture zone of the Paleoproterozoic Trans-North China Orogen

International audienceThis paper presents a reappraisal of the tectonic evolution of the Zanhuang Massif that lies at the eastern margin of the Trans-North China Orogen, a continent–continent collision belt that marked the amalgamation of the North China Craton in Late Paleoproterozoic. Detailed field work with focus on geometries of structures and kinematics was performed. This was completed with LA-ICP-MS U–Pb analyses on zircon, EPMA U–Th/Pb dating on monazite and 40Ar/39Ar dating on amphibole. These studies led us to propose a new three-fold litho-tectonic subdivision of the massif: The Western Zanhuang Domain (WZD) made of TTG, migmatite and pink anatectic granite is correlated to the Fuping Massif that crops out to the north-west. Both areas represent a continental block, called the Fuping Block, which acquired most of its architecture around 2100 Ma ago. The Eastern Zanhuang Domain (EZD) made of TTG and migmatite represents the western edge of an Eastern Neoarchean Block. In between, the Central Zanhuang Domain (CZD) is a NE–SW trending stack of supracrustal, gneiss and mafic magmatic rocks thrust sheets displaced toward the ESE upon the Eastern Block. The lithological features suggest that the CZD represents the remnant of an oceanic basin, called the Taihang Ocean that closed during the amalgamation of the Eastern Block and the Fuping Block around 1880–1850 Ma. In agreement with recent work done along the western margin of the belt, in the Lüliang Massif, this study documents the amalgamation of the North China Craton in response to the closure of two oceanic basins, namely the Lüliang Ocean and the Taihang Ocean. West-dipping subductions and collisions involving three distinct continental blocks, called the Western, the Fuping and the Eastern Blocks, took place around 1880–1850 Ma