ALBERT

Description: Terra Nova, 00, 1–10, 2012 Abstract In situ U–Pb geochronology of detrital zircons from various Palaeozoic sedimentary rocks of West Junggar accretionary complexes (Central Asia) suggests two distinct episodes of arc magmatism, and an evolution in three steps: (i) Ordovician-Silurian subduction generating juvenile arc crust; (ii) Late Silurian subduction jamming, erosion and intraplate magmatism; (iii) development of two new opposed Devonian-Carboniferous subductions recycling the Early Palaeozoic crust. Zircon Hf isotopes document three pre-Permian episodes of mantle-derived magmatic input into the crust: (1) Neoproterozoic (850–550 Ma), (2) Early Palaeozoic (530–450 Ma) and (3) Late Palaeozoic (380–320 Ma). Zircons also record the recycling of Neoproterozoic and Early Palaeozoic juvenile crusts during the Early and Late Palaeozoic. These data support a model of episodic continental crust growth in Central Asia.

Description: Terra Nova, 00, 1–11, 2012 Abstract The northern mountains of Beijing are the type locality of the Mesozoic Yanshanian orogen. Our structural study emphasizes the importance of dextral strike-slip for the formation of this intracontinental belt. The South- and North-directed thrusts are positive flower structures rooted in strike-slip faults. This transpressional tectonics developed from Late Triassic to Late Jurassic-Early Cretaceous through three deformation phases coeval with syntectonic sedimentation, separated by two transtensional episodes coeval with magmatism. The Late Jurassic–Early Cretaceous tectonic event is also recognized in several places of North China. Strike-slip faulting controls the deformation of the northern border of the North China block in the Yanshan-Yinshan belt. Simultaneously, East-directed thrusts and folds develop along N-S elongated ranges in Helanshan, Shanxi highlands and Taihangshan. The Jurassic tectonics of the North China Block is interpreted in a unitary way as the result of the westward underthrusting of the North China block below the Alashan block.

Description: The current structure of the South Armorican Domain in the French Variscan Belt, provides an excellent record of the late orogenic evolution, as documented by the complex interactions between extensional structures and partial melting. In the Golfe du Morbihan, the South Brittany Migmatite Belt (SBMB), tectonically overlain by non-melted metamorphic units, was studied by combined structural and 40Ar/39Ar methods. The recognition of successive deformation stages points to a strain continuum initiated at depth at the end of a partial melting event, and continued to near-surface brittle faulting, encompassing ductile strain localization over a major top-to-the-ESE extensional shear zone (SSZ) consistent with an overall WNW-ESE orogen-parallel stretching. This multidisciplinary study making the link between structure, texture, microchemistry and 40Ar/39Ar geochronology allow distinguishing between cooling, recrystallization and syn-tectonic crystallization ages. The first extensional insights, witnessed by dyke intrusion were dated at ∼319 ± 6 Ma immediately after the SBMB partial melting. Strain localization along the SSZ, occurred until ∼302–298 Ma when SBMB crossed the ductile-brittle transition. Moreover, in situ 40Ar/39Ar analysis, of syn-tectonic white micas sampled in the ductile and brittle structures yield similar ∼300 Ma. Formation or reactivation of the SSZ may have occurred during the rapid cooling of the SBMB settled at ∼305 Ma by conventional 40Ar/39Ar analysis. Large-scale late orogenic extensional event of the previously overthickened Variscan crust thus lasted over ∼20 ± 3 Myr in the same extensional tectonic regime guided by the South Armorican Shear Zone.

Description: In November 2007 we conducted a water column and seafloor mapping study of the submarine volcanoes of the Aeolian Arc in the southern Tyrrhenian Sea aboard the R/V Urania. On 26 conductivity-temperature-depth casts and tows we measured temperature, conductivity, pressure, and light scattering and also collected discrete samples for helium isotopes, methane, and pH. The 3He/4He isotope ratio, an unambiguous indicator of hydrothermal input, showed a clear excess above background at 6 of the 10 submarine volcanoes surveyed. Marsili seamount had the highest anomaly, where the 3He/4He ratio reached a δ3He value of 23% at 610 m depth compared with background values of ∼5%. Smaller but distinct δ3He anomalies occurred over Palinuro, Enarete, Eolo, Sisifo, and Secca del Capo. Although hydrothermal emissions are known to occur offshore of some Aeolian subaerial volcanoes, and hydrothermal deposits have been sampled throughout the arc, our results are the first to confirm active discharge on Marsili, Enarete, Eolo, Sisifo, and Secca del Capo. Samples collected over Lametini, Filicudi North, Alicudi North, and Alcione had δ3He near the regional background values, suggesting either absence of, or very weak, hydrothermal activity on these seamounts. Hydrocasts between the volcanoes revealed a consistent δ3He maximum between 11% and 13% at 2000 m depth throughout the SE Tyrrhenian Sea. The volcanoes of the Aeolian arc and the Marsili back arc, all

Description: The Reynolds–Anmatjira Ranges, central Australia, form part of a high-grade basement terrane dissected by intensely metasomatized transpressional shear zones active during the Ordovician–Carboniferous Alice Springs Orogeny. Unlike typical retrograde structures associated with discrete fluid flow, the mid-crustal setting and intracontinental nature of these shear systems present significant problems for the source and ingress mechanism of the fluid involved in their rehydration. To address these issues, we describe two detailed traverses across deformed and metasomatized basement rocks in this region, and interrogate their record of fluid–rock interaction from various perspectives. Both traverses combine structural and petrological observations with Zr-in-rutile and Ti-in-quartz thermobarometry, oxygen and hydrogen stable isotope analysis, and major, trace and rare earth element mobility trends. Each technique is critically evaluated for its utility in this study and its more widespread applicability to alternative field areas, providing a strategic framework for the general investigation of fluid-affected shear zones. Ultimately, the integrated data sets specify pressure–temperature conditions of ∼530 °C and 4–5 kbar, implying average apparent thermal gradients of 29–36 °C km −1 and depths of 14–18 km. Other characteristic features to emerge include strongly variable element mobilities and pronounced isotopic depletion fronts consistent with the alteration effects of an externally derived, non-equilibrium fluid. This is confirmed by calculated fluid compositions indicative of contributions from a fluid of meteoric origin, with estimated δ 18 O and δ D values as low as 2.3‰ and −59.8‰, respectively. We propose that these surficial fluid signatures are imposed on the mid-crust by the prograde burial and dehydration of hydrothermally altered fault panels produced during pre-orogenic basin formation. Progressive fluid release with continued subsidence then leads to the accumulation of increasing fluid volumes in the vicinity of the brittle–ductile transition, promoting extensive hydration, metasomatism and reaction softening at the locus of stress transmission from plate-boundary sources. The sustained injection of externally derived fluids into refractory crustal material may thus stimulate a critical reduction in the long-term strength of the lithosphere, providing strong impetus for the initiation and advancement of intracontinental orogenesis.

Description: The ODP/IODP multileg campaign at ODP Site 1256 (Cocos plate, eastern equatorial Pacific) provides the first continuous in situ sampling of fast spreading ocean crust from the extrusive lavas, through the sheeted dikes and down into the uppermost gabbros. This paper focuses on a detailed petrographic and microanalytical investigation of the gabbro section drilled during IODP Expedition 312. The marked patchy and spotty features that can be observed in many Hole 1256D gabbros is mostly due to a close association of two different lithological domains in variable amounts: (1) subophitic domains and (2) a granular matrix. Major and trace element mineral compositions, geothermometry, and petrological modeling suggest that subophitic and granular domains follow one single magma evolution trend formed by in situ fractionation. The subophitic domains correspond to the relative primitive, high-temperature end-member, compositionally similar to the basalts and dikes from the extrusive unit upsection, while the granular domains fit with a magma evolution by crystal fractionation to lower temperatures, up to a degree of crystallization of ∼80%. Our results support the following scenario for the fossilization of the axial melt lens at ODP Site 1256: relatively primitive MORB melts under near-liquidus conditions fill the melt lens and feed the upper, extrusive crust. Near the melt lens–sheeted dike boundary at lower temperatures, crystallization starts with first plagioclase before clinopyroxene in a mushy zone forming the subophitic domains. At decreasing temperatures, the subophitic domains continue to crystallize, finally forming a well-connected framework. Evolved, residual melt is finally trapped within the subophitic network, crystallizing at near-solidus conditions to the granular matrix. Another important textural feature in Hole 1256D gabbros is the presence of microgranular domains which are interpreted as relics of stoped/assimilated sheeted dikes (transformed to “granoblastic dikes” by contact metamorphism). All these different domains can be observed in close association, often at the thin section scale, demonstrating the extremely complex petrological record of combined crystallization/assimilation processes ongoing in the axial melt lens. Very similar gabbros with a marked spotty/patchy appearance, and bearing the same close association of lithological domains as observed at Site 1256, are known in the so-called “varitextured gabbro” unit from the Oman Ophiolite located at the same structural level, between cumulate gabbros and granoblastic dikes. The close petrological similarity of the gabbro/dike transition between both IODP Hole 1256D and the Oman ophiolite suggests that in situ fractionation and dike assimilation/contamination are major magmatic processes controlling the dynamics and fossilization of the axial melt lens at fast spreading oceanic ridges.

Description: Terra Nova, 00, 1–10, 2012 Abstract In situ U–Pb geochronology of detrital zircons from various Palaeozoic sedimentary rocks of West Junggar accretionary complexes (Central Asia) suggests two distinct episodes of arc magmatism, and an evolution in three steps: (i) Ordovician-Silurian subduction generating juvenile arc crust; (ii) Late Silurian subduction jamming, erosion and intraplate magmatism; (iii) development of two new opposed Devonian-Carboniferous subductions recycling the Early Palaeozoic crust. Zircon Hf isotopes document three pre-Permian episodes of mantle-derived magmatic input into the crust: (1) Neoproterozoic (850–550 Ma), (2) Early Palaeozoic (530–450 Ma) and (3) Late Palaeozoic (380–320 Ma). Zircons also record the recycling of Neoproterozoic and Early Palaeozoic juvenile crusts during the Early and Late Palaeozoic. These data support a model of episodic continental crust growth in Central Asia.

Description: The Rhone River provides the largest inputs of terrestrial freshwater and nutrients into the Mediterranean Sea. The Rhone River diluted water intrusions into the Bay of Marseille were investigated, examining their physical generation processes and associated biogeochemical impact by using in situ observations, remote sensing data, and a three-dimensional physical/biogeochemical coupled model. During our study period from 2007 to 2011, Rhone River intrusions occurred on average 7.6 times per year and affected more frequently the northern part of the bay. A classification of intrusion events in three categories is proposed (short-lived, big and small) as a function of their duration and spatial extent. The intrusions appeared to be driven by: (i) wind forcing, (ii) the presence of a mesoscale eddy, (iii) the Rhone River discharge volume, and (iv) the variation in thermocline depth. Typically, a combination of these favorable factors was necessary to induce an intrusion. An intrusion strongly impacts the biogeochemical functioning of the Bay of Marseille by bringing large quantities of nutrients into the bay. Mass balances were computed allowing us to quantify this impact on the Bay of Marseille. The results show that the ecological impact depends very much on the type of intrusion, with big intrusions having the highest impact.

Description: [1]  The tectonic evolution of the Paleo-Asian Ocean between the North China Block (NCB) and the Mongolia Block (MOB) is a contentious issue, and geodynamic models remain speculative. In an effort to puzzle out this controversy, a paleomagnetic study was carried out on the Silurian to Permian formations in central- eastern Inner Mongolia (China). More than 680 sedimentary and volcanic samples were collected from 86 sites. We have established titanium-poor magnetite and hematite as the principal magnetic carriers. AMS measurements demonstrate negligible deformation of the majority of study rocks with sedimentary fabrics. From primary magnetizations, a Late Devonian and a Permian pole are calculated for IMB at: λ  = 46.8°N, φ  = 349.1°E, dp = 14.6°, dm = 27.3° with N = 3 and λ  = 48.7°N, φ  = 3.7°E, dp = 5.2°, dm = 9.1° with N = 6, respectively. Two stages of secondary magnetization are also identified probably due to Early Permian and Early Cretaceous magmatic events. As preliminary results, the comparison of our new paleomagnetic poles with available data from NCB, MOB and Siberia indicates that (1) the paleolatitude of IMB, NCB and MOB are consistent between Late Devonian and Permian, suggesting pre-Late Devonian closure of the Paleo-Asian Ocean and further evaluation of these three blocks as a single entity; (2) post-Permian intracontinental deformation was significant and characterized by block rotations, which are due to strike-slip faulting within the welded NCB-IMB-MOB block.