A Variscan slow-spreading ridge (MOR-LHOT) in Limousin (French Massif Central): magmatic evolution and tectonic setting inferred from mineral chemistry

2006 ◽  
Vol 70 (2) ◽  
pp. 175-185 ◽  
Author(s):  
J. Berger ◽  
O. Féménias ◽  
J.C.C. Mercier ◽  
D. Demaiffe
Keyword(s):  
Tectonic Setting ◽  
Mineral Chemistry ◽  
Spreading Ridge ◽  
French Massif Central ◽  
Massif Central ◽  
Mid Atlantic Ridge ◽  
Slow Spreading ◽  
Armorican Massif ◽  
Hydrothermal Metamorphism ◽  
Nearly Continuous

AbstractThe Limousin ophiolite (French Massif Central) occurs as elongate bodies forming a (nearly) continuous suture zone between two major lithotectonic units of the French Variscan belt. The mantle section of the ophiolite is made of diopside-bearing harzburgite, harzburgite and dunite characteristic of a lherzolite-harzburgite ophiolite type (LHOT). The plutonic section is essentially composed of troctolites, wehrlites and gabbros locally intruded by ilmenite-rich mafic dykes. All the rocks were strongly affected by an ocean-floor hydrothermal metamorphism. The composition and evolution of primary magmatic phases (olivine, clinopyroxene, plagioclase and spinel) throughout the lowermost magmatic sequence correspond to those described in oceanic cumulates (ODP data). The Limousin ophiolite is thus of MOR type instead of SSZ type. The whole lithological section, the mineral chemistry, the extensive hydrothermal oceanic alteration and the relatively thin crustal section are typical of a slow-spreading ridge ocean (i.e. Mid-Atlantic ridge). Comparison of the Limousin ophiolite with other ophiolites from European Variscides suggests that the oceanic domain was actively spreading during the Late Palaeozoic and extended from the Armorican massif to the Polish Sudetes.

scholarly journals Quantifying tectonic strain and magmatic accretion at a slow spreading ridge segment, Mid-Atlantic Ridge, 29°N

1999 ◽  
Vol 104 (B5) ◽  
pp. 10421-10437 ◽  
Author(s):  
J. Escartín ◽  
P. A. Cowie ◽  
R. C. Searle ◽  
S. Allerton ◽  
N. C. Mitchell ◽  
...  
Keyword(s):  
Spreading Ridge ◽  
Mid Atlantic Ridge ◽  
Slow Spreading

Hydrothermal activity on a 105-year scale at a slow-spreading ridge, TAG hydrothermal field, Mid-Atlantic Ridge 26°N

1995 ◽  
Vol 100 (B9) ◽  
pp. 17855-17862 ◽  
Author(s):  
Claude Lalou ◽  
Jean-Louis Reyss ◽  
Evelyne Brichet ◽  
Peter A. Rona ◽  
Geoffrey Thompson
Keyword(s):  
Hydrothermal Activity ◽  
Hydrothermal Field ◽  
Spreading Ridge ◽  
Mid Atlantic Ridge ◽  
Slow Spreading ◽  
Tag Hydrothermal Field

scholarly journals Pressure-temperature conditions and significance of Upper Devonian eclogite and amphibolite facies metamorphisms in southern French Massif central

2020 ◽  
Vol 191 ◽  
pp. 28
Author(s):  
Anissa Benmammar ◽  
Julien Berger ◽  
Antoine Triantafyllou ◽  
Stéphanie Duchene ◽  
Abderrahmane Bendaoud ◽  
...  
Keyword(s):  
Upper Devonian ◽  
Amphibolite Facies ◽  
Garnet Growth ◽  
French Massif Central ◽  
Massif Central ◽  
Metamorphic History ◽  
Zircon Crystallization ◽  
Intermediate Unit ◽  
Armorican Massif ◽  
Subduction System

The southwestern French Massif central in western Rouergue displays an inverted metamorphic sequence with eclogite and amphibolite facies units forming the top of the nappe stack. They are often grouped into the leptyno-amphibolite complex included, in this area, at the base of the Upper Gneiss Unit. We sampled garnet micaschists and amphibolites to investigate their metamorphic history with isochemical phase diagrams, thermobarometry and U-Pb zircon dating. Our results demonstrate that two different tectono-metamorphic units can be distinguished. The Najac unit consists of biotite-poor phengite-garnet micaschists, a basic-ultrabasic intrusion containing retrogressed eclogites and phengite orthogneisses. Pressure and temperature estimates on micaschists with syn-kinematic garnets yield a prograde with garnet growth starting at 380 °C/6–7 kbar, peak pressure at 16 kbar for 570 °C, followed by retrogression in the greenschist facies. The age of high pressure metamorphism has been constrained in a recent publication between ca. 383 and 369 Ma. The Laguépie unit comprises garnet-free and garnet-bearing amphibolites with isolated lenses, veins or dykes of leucotonalitic gneiss. Thermobarometry and phase diagram calculation on a garnet amphibolite yield suprasolidus peak P-T conditions at 710 °C, 10 kbar followed by retrogression and deformation under greenschist and amphibolite facies conditions. New U-Pb analyses obtained on igneous zircon rims from a leucotonalitic gneiss yield an age of 363 ± 3 Ma, interpreted as the timing of zircon crystallization after incipient partial melting of the host amphibolite. The eclogitic Najac unit records the subduction of a continental margin during Upper Devonian. It is tentatively correlated to a Middle Allochthon, sandwiched between the Lower Gneiss Unit and the Upper Gneiss Unit. Such an intermediate unit is still poorly defined in the French Massif central but it can be a lateral equivalent of the Groix blueschists in the south Armorican massif. The Uppermost Devonian, amphibolite facies Laguépie unit correlates in terms of P-T-t evolution to the Upper Gneiss Unit in the Western French Massif central. This Late Devonian metamorphism is contemporaneous with active margin magmatism and confirms that the French Massif central belonged to the continental upper plate of an ocean-continent subduction system just before the stacking of Mississippian nappes.

New occurrence of UHP eclogites in Limousin (French Massif Central): Age, tectonic setting and fluid–rock interactions

Lithos ◽  
2010 ◽  
Vol 118 (3-4) ◽  
pp. 365-382 ◽  
Author(s):  
Julien Berger ◽  
Olivier Féménias ◽  
Daniel Ohnenstetter ◽  
Olivier Bruguier ◽  
Gaëlle Plissart ◽  
...  
Keyword(s):  
Tectonic Setting ◽  
French Massif Central ◽  
Massif Central

scholarly journals Geochemical Variability & Implications for Magma Transport at 14°N on the Mid-Atlantic Ridge

2020 ◽  
Author(s):  
Emma Elizabeth McCully
Keyword(s):  
Trace Element ◽  
Fractional Crystallization ◽  
Major Element ◽  
Spreading Ridge ◽  
Magma Transport ◽  
Chemical Variability ◽  
Trace Element Contents ◽  
Mid Atlantic Ridge ◽  
Slow Spreading ◽  
Element Contents

Observations of seafloor bathymetry and gravity surveys indicate that magma focuses in the center of slow spreading Mid-Ocean Ridge (MOR) segments, however; it is not well constrained how magma is generated, stored, and transported to the segment ends. There are two end-member models for magma transport: 1) a focused magma model wherein the magma upwells beneath the entire ridge axis, is focused and pools beneath the center of the segment, and is then transported towards the segment ends via lateral diking in the shallow crust and 2) a distributed magma model wherein magma vertically upwells and is erupted on the seafloor along the entire segment, but there is enhanced focusing in the segment center. (Figure 1). Both models are supported by the bathymetric and geophysical observations but have different implications for the chemistry of lavas erupted along the segment. To test how lava chemistries vary along a slow-spreading MOR, we systematically sampled a segment of the Mid-Atlantic Ridge. The segment (~14°N) (Figure 2) is known to host Popping Rocks, gas-rich basalts which, upon reaching surface pressures, explode. Two expeditions to this region in 2016 and 2018 collected both ship-based bathymetry (75 m gridded resolution) aboard the R/V Atlantis and high-resolution bathymetry (1 m) from the Autonomous Underwater Vehicle (AUV) Sentry. 27 dives from the Human Occupied Vehicle (HOV) Alvin collected 382 lavas all of which have been analyzed for major element contents, and 162 have been analyzed for trace element contents. During these expeditions, samples were collected both along and across axis from the magmatically robust segment center, through a transition region, to a sparsely magmatic region. Analytical results show that there is significant chemical variability along this segment. For example, there is less variability at the segment center (K/Ti ratios from 0.24 to 0.46 and La/Sm from 2.58 to 3.59) compared to the sparsely magmatic region (K/Ti values from 0.06 to 0.42 and La/Sm). This suggests that magmas erupted at the segment center are more homogeneous compared to lavas erupting in the sparsely magmatic region. Major element contents in each region vary, but on average, become more mafic moving southward away from the magmatically robust segment center towards the sparsely magmatic region. Petrologic modeling of fractional crystallization and trace element contents show that fractional crystallization dominates the chemical variability in the sparsely magmatic region, while either extent of melting or differing mantle sources dominates the variability in the transition regions and the sparsely magmatic region. Reconciling these data with both physical and geophysical observations of a slow spreading ridge, we present a model of magma generation, storage, and transport that is a hybrid of the two proposed models.

A Permian underplating event in late- to post-orogenic tectonic setting. Evidence from the mafic–ultramafic layered xenoliths from Beaunit (French Massif Central)

2003 ◽  
Vol 199 (3-4) ◽  
pp. 293-315 ◽  
Author(s):  
Olivier Féménias ◽  
Nicolas Coussaert ◽  
Bernard Bingen ◽  
Martin Whitehouse ◽  
Jean-Claude C Mercier ◽  
...  
Keyword(s):  
Tectonic Setting ◽  
French Massif Central ◽  
Massif Central

A novel approach for oceanic spreading terrain classification at the Mid-Atlantic Ridge using Eigenvalues of high-resolution bathymetry

2020 ◽  
Author(s):  
Gabriella Alodia ◽  
Chris Green ◽  
Andrew McCaig ◽  
Douglas Paton
Keyword(s):  
High Resolution ◽  
Quantitative Description ◽  
Buffer Zone ◽  
Spreading Ridge ◽  
Terrain Classification ◽  
Spreading Axis ◽  
Oceanic Spreading ◽  
Mid Atlantic Ridge ◽  
Slow Spreading ◽  
High Resolution Bathymetry

<p>Terrain classification at slow-spreading ridges has been a topic of interest since the significant discovery of mantle rocks exhumed by detachment faults in various segments of the Mid-Atlantic spreading axis. These rocks commonly form domed massifs, so-called core complexes, in contrast to the linear fault-bounded abyssal hills of magmatic spreading terrains. However, there is still limited quantitative description of these two distinct structures. We present analysis of high-resolution bathymetry data 21-24 N over the Mid-Atlantic Ridge and its derivatives to highlight the shapes and directionality of the two oceanic crust features. We assign an optimized 8 arc-minute (~14.8 km) window, mimicking the average size of core complexes, in which we compute the Eigenvalues from each cell within the window based on its directionality and slope. We use the two most dominant Eigenvalues – representing the window’s overall horizontal directionality – to compute eccentricity values and weight them with the sine of the slope. From the computation, we found that areas with weighted eccentricity of 0-0.6 represent the omnidirectional terrains that result from tectonic activities; 0.6-0.9 represents the extended terrain or the buffer zone between the tectonic and magmatic terrains; values >0.9 highlight bidirectional magmatic terrains. Based on this classification, we found significantly more evidence of detachment faulting west of the spreading axis compared to the eastern side. This analysis also highlights neo-volcanic activity that started at around 2 Ma that propagates to the south, cutting a fracture zone before it became inactive. The result contributes to a new approach in mining information from high-resolution bathymetry data to assess oceanic spreading type and its symmetry at a slow-spreading ridge through time.</p>

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