Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Structural controls are critical during magmatic-to-hydrothermal transition in the formation of intrusion-related gold deposits. They may explain why only some parts of intrusions are mineralized and why only very few intrusions host large deposits. Moreover, most of these gold deposits postdate peak regional metamorphism and were formed in zones of relatively low strain. Indeed, the efficiency of structural gold traps is highest along favourably oriented intrusion/host-rock contacts where mechanical instability maintains high permeability in the cracked thermal aureole. This may be reinforced by melt injections from an underlying root zone. We present a structural analysis of the deformation features of a granite-pluton roof; large-scale dykes and a network of gold veins are intensively developed in this roof, recording a succession of mechanical instabilities. Our gravity survey underlines the presence of a pluton feeder zone located just beneath the mineralized network. It is argued that interferences between regional stress and melt injection in the feeder zone favoured the development of the network by strain located close to the granite roof. This stresses the role of mechanical instabilities triggered by the combined effects of regional stress and melt dynamics in determining the location and size of this type of gold deposit.
ISSN: 0191-8141

insu-00914780
https://hal-insu.archives-ouvertes.fr/insu-00914780
https://hal-insu.archives-ouvertes.fr/insu-00914780/document
https://hal-insu.archives-ouvertes.fr/insu-00914780/file/manuscript.pdf
DOI : 10.1016/j.jsg.2013.11.006

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : Vertical displacements on the SW-NE T (e) over capt fault (Eastern Pyrenees Axial Zone, France), which separates the Variscan Canigou-Caranca and Mont-Louis massifs, were constrained using a thermochronologic multi-method approach. Ar-40/Ar-39 data from the granitic Mont-Louis massif record its Variscan cooling history and reveal no ages younger than Early Cretaceous, while the Canigou-Caranca gneiss massif records systematically younger Ar-40/Ar-39 ages. These younger Ar-40/Ar-39 ages in the Canigou-Caranca gneiss massif are the result of partial to total rejuvenation of argon isotopic systems related to a thermal flow coeval with the Cretaceous HT-BP metamorphism in the North Pyrenean Zone. Only the deepest rocks from the Canigou-Caranca suffered this extensive Mid-Cretaceous thermal overprint probably due to differential burial around 4 km at that time. The post Mid-Cretaceous vertical displacements along the T (e) over capt fault are recorded by "low" temperature thermochronology using K-feldspar Ar-40/Ar-39, zircon and apatite fission track and (U-Th)/He datings. The Mont-Louis granite samples experienced a long period of protracted cooling reflecting a lack of thermo-tectonic activity in this area from Late Palaeozoic to Early Cenozoic, followed by cooling from 55-60 Ma to Late Eocene at a mean rate of 15-20 degrees C/Ma in the final stage. This cooling stage corresponds to T (e) over capt fault reactivation with a reversed component, promoting exhumation of the Mont-Louis roof zone contemporaneously with the south-vergent Pyrenean thrusting. In the Canigou-Caranca massif, the main cooling event occurred from 32 to 18 Ma at a maximum rate of 30 degrees C/Ma during Early Oligocene followed by a more moderate rate of 3 degrees C/Ma from Late Oligocene to Early Burdigalian, coeval with the normal reactivation of the T (e) over capt fault in brittle conditions that accommodated the final exhumation of the massif during the opening of the Gulf of Lion.
ISSN: 1437-3254

hal-00411442
https://hal.archives-ouvertes.fr/hal-00411442
DOI : 10.1007/s00531-007-0179-x

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
New U–Pb monazite, zircon, and xenotime ages date the Late Carboniferous crystallization of the anatectic Vialais granite in the Montagne Noire Axial Zone and the high-temperature deformation and metamorphism of the augen gneiss (Ordovician granite protolith) from which the granite was likely derived. The U–Pb monazite ages obtained from the augen gneiss (308 ± 3 Ma), late kinematic Vialais granite (303 ± 4 Ma), and post-kinematic leucogranite (298 ± 2 Ma), bracket the high-temperature deformation and metamorphism at ~ 310–300 Ma, clearly postdating regional contraction and nappe emplacement (> 320 Ma). The planar–linear and locally linear fabrics in the augen gneiss outline a regional-scale double dome structure (Caroux and Espinouse sub-domes) containing smaller (km-scale) upright folds. Compared to the Caroux sub-dome, the larger, migmatitic Espinouse sub-dome contains abundant leucosome, leucogranite bodies and late-kinematic intrusions (Vialais granite). The Vialais granite displays a weak magmatic foliation that defines the main layering of the Espinouse migmatite. Ellipsoidal quartz–sillimanite nodules concentrated above the roof of the granite recorded an increment of strain (vertical shortening, E–NE elongation) during granite emplacement and crystallization, consistent with the extension event that exhumed the domes. These new geochronological and structural data suggest that the Montagne Noire double dome formed in a pull-apart structure within a dextral strike–slip system. Upright folding in the sub-domes has been traditionally assigned to a regional contraction event; alternatively, we propose that local contraction is associated with the convergence of low-viscosity crust beneath the upper crust pull-apart. Dynamic models of extension of hot crust indicate that contraction at depth is generated by flow of low-viscosity orogenic crust converging and rising to fill the gap created by upper crust extension. This interpretation solves the long-standing problem of apparent coeval contraction and extension in the Montagne Noire double dome and establishes migmatite dome emplacement in strike–slip corridors as a style of late Variscan tectonics.
ISSN: 0040-1951

insu-01112009
https://hal-insu.archives-ouvertes.fr/insu-01112009
DOI : 10.1016/j.tecto.2014.12.002

Éditeur(s) : HAL CCSD Elsevier
Résumé : Hot collisional orogens are characterized by abundant syn-kinematic granitic magmatism that profoundly affects their tectono-thermal evolutions. Voluminous granitic magmas, emplaced between 360 and 270 Ma, played a visibly important role in the evolution of the Variscan Orogen. In the Limousin region (western Massif Central, France), syntectonic granite plutons are spatially associated with major strike-slip shear zones that merge to the northwest with the South Armorican Shear Zone. This region allowed us to assess the role of magmatism in a hot transpressional orogen. Microstructural data and U/Pb zircon and monazite ages from a mylonitic leucogranite indicate synkinematic emplacement in a dextral transpressional shear zone at 313 +/- 4 Ma. Leucogranites are coeval with cordierite-bearing migmatitic gneisses and vertical lenses of leucosome in strike-slip shear zones. We interpret U/Pb monazite ages of 315 +/- 4 Ma for the gneisses and 316 +/- 2 Ma for the leucosomes as the minimum age of high-grade metamorphism and migmatization respectively. These data suggest a spatial and temporal relationship between transpression, crustal melting, rapid exhumation and magma ascent, and cooling of high-grade metamorphic rocks.;Some granites emplaced in the strike-slip shear zone are bounded at their roof by low dip normal faults that strike N-S, perpendicular to the E-W trend of the belt. The abundant crustal magmatism provided a low-viscosity zone that enhanced Variscan orogenic collapse during continued transpression, inducing the development of normal faults in the transpression zone and thrust faults at the front of the collapsed orogen.
ISSN: 0040-1951

hal-00445260
https://hal.archives-ouvertes.fr/hal-00445260
DOI : 10.1016/j.tecto.2009.03.022