Search

Sapphirine-bearing rocks are described in the Aulus Basin (Ariège, France) in a contact zone between the Lherz peridotitic body and Mesozoic metasediments which underwent the Pyrenean Cretaceous high-temperature, low-pressure metamorphic event (Monchoux, 1970, 1972a, 1972b). Sapphirine crystals occur in layered clastic deposits characterized by an uncommon suite of Al-Mg-rich minerals. A detailed petrographic study of sixteen samples representative of the diversity of the Lherz sapphirine-bearing rocks is presented. These rocks include breccias and microbreccias with various compositions. Some samples are composed of polymineralic clasts and isolated minerals that derive from regionally well-known protoliths such as ultramafic rocks, meta-ophites, “micaceous hornfels”, and very scarce Paleozoic basement rocks. Nevertheless, a large portion of the sapphirine-bearing clastic suite is composed of mono- and polymineralic debris that derive from unknown protolith(s). We define a "sapphirine-bearing mineral suite” (SBMS) composed of monomineralic debris including: sapphirine + enstatite + aluminous spinel + Mg-amphiboles + Ca-amphiboles + kornerupine + accessory minerals (apatite, diopside, rutile, serpentine, smectite, tourmaline, vermiculite and a white mica). We highlight the dominance of metamorphic Keuper clastic materials in the studied rocks and the presence of inclusions of anhydrite and F-, Cl-, Sr-rich apatite in minerals of the Al-Mg-rich suite. The brecciated texture and the presence of unequivocal sedimentary features suggest that the sapphirine-bearing rocks were mechanically disaggregated and then experienced winnowing in underwater conditions with poor mixing between the different sources. We measured U-Pb rutile age data in order to provide constraints on the age of (one of) the protolith(s) of those clastic deposits. The obtained age (98.6 + 1.2 Ma) is interpreted as the age of metamorphism of this protolith of the SBMS. Previous works interpreted the Lherz sapphirine-bearing rocks as crustal protoliths modified at depth along the contact with the ultramafic rocks of the Lherz body during their ascent towards shallower depths. These new data imply: (i) an Upper Triassic to Lower Jurassic origin for the main protolith of the sapphirine-bearing rocks; (ii) the metamorphism of this protolith along an active hot crust–mantle detachment during Cenomanian times with the involvement of metasomatic, brine-type fluids; and (iii) its brecciation during the exhumation of the material due to the evolution of the detachment, followed by subsequent sedimentary reworking of the metamorphic material.

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.

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 (paper 1) and the Saraillé massif (this paper). The peridotites have been uplifted 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. In the Saraillé massif, both the pre-rift cover and the thin Paleozoic crustal lenses are involved in a Pyrenean recumbent fold having the serpentinized peridotites in its core. Based on detailed geological cross-sections microscopic observations and microprobe mineralogical analyses, we describe the lithology of the two major extensional fault zones that accommodated: (i) the progressive uplift of the lherzolites upward the Cretaceous basin axis, (ii) the lateral extraction of the continental crust beneath the rift margins 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 to the detached pre-rift cover. The Saraillé peridotites were never exposed to the seafloor of the Cretaceous NPZ basins and always remained under a thin layer of crustal mylonites. Field constraints allow to reconstruct the strain pattern of the mantle rocks in the crust-mantle detachment. A 20–50 m thick layer of serpentinized lherzolites tectonic lenses separated by anastomosed shear zones is capped by a thin upper damage zone made up of strongly sheared talc-chlorite schists invaded by pyrite crystallization. The cover décollement is a few decameter-thick fault zone resulting from the brecciation of Upper Triassic layers. It underwent strong metasomatic alteration in the greenschist facies, by multi-component fluids leading to the crystallization of quartz, dolomite, talc, Cr-rich chlorite, amphiboles, magnesite and pyrite. 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.