Mantle exhumation at magma-poor passive continental margins. Part I. 3D architecture and metasomatic evolution of a fossil exhumed mantle domain (Urdach lherzolite, north-western Pyrenees, France)

Abstract

In two companion papers, we report the detailed geological and mineralogical study of two emblematic serpentinized ultramafic bodies of the western North Pyrenean Zone (NPZ), the Urdach massif (this paper) and the Saraillé massif (paper 2). The peridotites have been exhumed to lower crustal levels during the Cretaceous rifting period in the future NPZ. They are associated with Mesozoic pre-rift metamorphic sediments and small units of thinned Paleozoic basement that were deformed during the mantle exhumation event. Based on detailed geological cross-sections and microprobe mineralogical analyses, we describe the lithology of the two major extensional fault zones that accommodated: (i) the progressive exhumation of the lherzolites along the Cretaceous basin axis; (ii) the lateral extraction of the continental crust beneath the rift shoulders and; (iii) the decoupling of the pre-rift cover along the Upper Triassic (Keuper) evaporites and clays, allowing its gliding and conservation in the basin center. These two fault zones are the (lower) crust-mantle detachment and the (upper) cover décollement located respectively at the crust-mantle boundary and at the base of the detached pre-rift cover. The Urdach peridotites were exposed to the seafloor during the Late Albian and underwent local pervasive carbonation and crystallization of calcite in a network of orthogonal veins (ophicalcites). The carbonated serpentinized peridotites were partly covered by debris-flows carrying fragments of both the ultramafics and Paleozoic crustal rocks now forming the polymictic Urdach breccia. The mantle rocks are involved in a Pyrenean overturned fold together with thin units of crustal mylonites. Continent-derived and mantle-derived fluids that circulated along the Urdach crust-mantle detachment led to the crystallization of abundant metasomatic rocks containing quartz, calcite, Cr-rich chlorites, Cr-rich white micas and pyrite. Two samples of metasomatized material from the crust-mantle detachment yielded in situ zircon U/Pb ages of 112.9 ± 1.6 Ma and 109.4 ± 1.2 Ma, thus confirming the Late Albian age of the metasomatic event. The cover décollement is a 30-m thick fault zone which also includes metasomatic rocks of greenschist facies, such as serpentine-calcite association and listvenites, indicating large-scale fluid-rock interactions implying both ultramafic and continental material. The lowermost pre-rift cover is generally missing along the cover décollement due to tectonic disruption during mantle exhumation and continental crust elision. Locally, metasomatized and strongly tectonized Triassic remnants are found as witnesses of the sole at the base of the detached pre-rift cover. We also report the discovery of a spherulitic alkaline lava flow emplaced over the exhumed mantle. These data collectively allow to propose a reconstruction of the architecture and fluid-rock interaction history of the distal domain of the upper Cretaceous northern Iberia margin now inverted in the NPZ.

Mantle exhumation at magma-poor passive continental margins. Part II: Tectonic and metasomatic evolution of large-displacement detachment faults preserved in a fossil distal margin domain (Saraillé lherzolites, northwestern Pyrenees, France)

“…Comparison with photographs in Figure 7 shows that the lenticular fabric of the Saraillé crust-mantledetachment is very similar to fabrics observed in these massifs. A similar conclusion arises from the study of the Urdach body (see companion paper Lagabrielle et al, 2019). As a consequence, further studies would check that the development of a 20-50 m thick shear zone in the serpentinized mantle during its exhumation to upper crustal levels during the Albian-Cenomanian extension appears as an ubiquitous characteristic along the entire NPZ.…”

“…Our interpretation of the origin and main pathways of the cover décollement Saraillé fluids can be compared to the results obtained from the study of the Urdach cover décollement along the "ball trap" section (see companion paper Lagabrielle et al, 2019). The Urdach cover décollement is a preferential pathway for serpentinization and carbonation-related fluids during mantle exhumation, triggering the co-crystallization of serpentine and calcite and involving large amount of seawater.…”

“…Our observation of the internal fabric of the Saraillé body confirms that the early pervasive serpentinization is not associated with deforming zones. Static pervasive serpentinization is also documented in the core of the nearby Urdach massif (see companion paper Lagabrielle et al, 2019). In marked contrast, the Turon de la Tecouére massif located some tens of km eastward, is entirely composed of highly deformed lherzolites that suffered progressive mylonitization associated with the transformation from spinel-to plagioclase-bearing assemblages in the range of 800-600°C with pressure less than 800-600 Mpa, corresponding to depths of 30-20 km (Vissers et al, 1997).…”

“…To sum up, displacement along the cover décollement during Albian-Cenomanian rifting locally led to the complete elision of the Triassic sequence with Dogger dolomites directly overlying the mantle rocks, as observed along the Saraillé fold (upper limb) and in the col d'Urdach section (see companion paper Lagabrielle et al, 2019). Therefore, the truncation, boudinage and cataclastic brecciation of the Triassic sequence compose the major structural pattern of the cover décollement at the regional scale.…”

“…We finally envision the evolution of the entire distal domain during the rifting of the northern Iberia margin, taking into account the geological constraints provided by the Saraillé massif (this study) and the nearby Urdach massif (see companion paper, Lagabrielle et al, 2019).…”