Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
In the Oman ophiolite crustal section, a contact zone between the gabbro unit and the volcanics and diabase sheeted dikes, called the root zone of the sheeted dike complex, has been recently mapped at a fine scale in a selected area. The Oman ophiolite is derived from a fast spreading ridge which had a melt lens located between the main gabbro unit and the root zone of the sheeted dike complex. With a few exceptions accounted for, this horizon has a fairly constant thickness, similar to 100 m, and a crude internal pseudo-stratigraphy. At the base of the root zone are isotropic ophitic gabbros interpreted as a thermal boundary layer. This layer is transitional between the magmatic system of the melt lens, convecting at 1200 degrees C, and a high-temperature (< 1100 degrees C) hydrothermal system, convecting within the root zone. Above this level, the isotropic gabbros have been, locally, largely molten due to an influx of seawater, at similar to 1100 degrees C, thus generating varitextured ophitic and pegmatitic gabbros. These latter gabbros constitute the upper part of the root zone and are associated with trondjhemitic intrusions as screens in the lower sheeted dikes. Diorites and trondjhemites were also generated by hydrous melting, at temperatures below 1000 degrees C. The whole root zone is a domain of very sharp average thermal gradient (similar to 7 degrees C/m). At the top of the root zone, a new thermal boundary layer, with diabase dikes hydrated in amphibolite facies conditions, separates the preceding high- temperature convective system from the well-known greenschist facies (< 450 degrees C) hydrothermal system operating throughout the sheeted dike complex, up to the seafloor. The isotropic gabbros near the base of the root zone are intruded by protodikes with distinctive microgranular margins and an ophitic center. Protodike swarms are exceptional because, intruding a medium at similar to 1100 degrees C, they are largely destroyed by dike-in-dike intrusions and by hydrous melting. However, they demonstrate that this zone was generated by melt conduits issued from the underlying melt lens. Each dike of the sheeted dike complex is thus fed by one protodike. As this zone has been recently drilled by IODP in the eastern Pacific Ocean, a brief comparison is proposed.
ISSN: 1525-2027

hal-00411331
https://hal.archives-ouvertes.fr/hal-00411331
DOI : 10.1029/2007GC001918

Éditeur(s) : HAL CCSD
Résumé : The northern Mirdita ophiolite massifs in Albanian Dinarides formed at a slow-spreading ridge, active during the Jurassic (160–165 Ma). They share a common horizontal Jurassic–Lower Cretaceous sedimentary cover showing that they were not deeply and intrinsically affected by later Alpine thrusting. The western massifs of Mirdita, first oceanic core complex (OCC) and detachment shear zone described in ophiolites, compare with OCCs in slow-spreading ridges and provide continuous exposure of the deep internal structure of this system, revealing its kinematics, thanks to detailed structural mapping in peridotites and gabbros. The Mirdita detachments root in the Moho transition zone (MTZ), a weak zone at the top of asthenospheric mantle, where basaltic melts impregnate dunites. The OCC domes are plagioclase-amphibole–bearing mylonitic peridotites, ∼400 m thick, grading downward within 200 m to harzburgitic mantle. The mylonitic detachments crossed Moho beneath a NNE-SSW–trending ridge. On the western side of OCC domes, the hanging wall of the ridge, crustal gabbros, and basalts are still preserved, despite being deeply affected by hydrothermal alteration. From there, the partially molten MTZ was detached as a shear zone, mixing with lower gabbros. The OCC emerged, migrating upsection and eastward over 5 km. Finally, the OCC front is observed in hornblende-rich syntectonic mylonites derived from upper gabbros and from the overlying former lid. Serpentinization is static within these mylonites. A low-temperature detachment fault is expressed as a sheared antigoritic mélange at the margin of the mylonitic shear zone. Asthenospheric flow in the harzburgitic mantle beneath the ridge of origin has been preserved below the OCC rooting. The dominant asthenospheric flow direction trends parallel to the ridge axis. This mantle flow rotates over 200 m into the low-temperature mylonitic detachments, where OCC motion turns transversal to the ridge. Crystal preferred orientation measurements on six samples point to brown hornblende crystal growth during mylonitic flow and illustrate the change of olivine intra-crystalline slip system in mylonites compared to porphyroclastic harzburgite.
ISSN: 1553-040X

hal-01504440
https://hal.archives-ouvertes.fr/hal-01504440
DOI : 10.1130/GES01383.1