Abstract
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.
Highlights
Sapphirine ([Mg,Fe,Al]8O2[Al,Si]6O18; Moore, 1969) is a relatively rare mineral which records either ultrahigh-temperature conditions in silica-rich systems (Harley & Motoyoshi, 2000; Wheller & Powell, 2014) or high temperature and intermediate pressure conditions in silica-poor systems
A large portion of the sapphirine-bearing clastic suite is composed of mono- and polymineralic debris that derive from unknown protolith(s)
– Type III was collected in Layers A, B and C at site 1 and corresponds to microbreccias devoid of any calcitic cement and made of either 100% of monomineralic clasts or of scarce polymineralic clasts associated with predominant monomineralic clasts
Summary
Sapphirine ([Mg,Fe,Al]8O2[Al,Si]6O18; Moore, 1969) is a relatively rare mineral which records either ultrahigh-temperature conditions in silica-rich systems (Harley & Motoyoshi, 2000; Wheller & Powell, 2014) or high temperature and intermediate pressure conditions in silica-poor systems. There, sapphirine crystals occur in layered clastic deposits characterized by uncommon mineral associations including Mgamphibole, aluminous orthopyroxene, spinel and kornerupine, which is a rare boron-bearing sorosilicate (Mg3Al6[Si,Al, B]5O21[OH]; Grew et al, 1990; Hawthorne et al, 2009). This clastic assemblage consists of polymictic breccias and sandstones exposed in a contact zone between the lherzolites and Mesozoic metasediments, mainly marbles (Monchoux, 1969). These marbles underwent the high-temperature, low-pressure (HT-LP) Mid-Cretaceous Pyrenean metamorphic event which is recognized all along the North Pyrenean Zone (NPZ; Ravier, 1959; Albarède and Michard-Vitrac, 1978; Montigny et al, 1986; Golberg and Maluski, 1988; Dauteuil and Ricou, 1989; Golberg and Leyreloup, 1990; Clerc and Lagabrielle, 2014; Clerc et al, 2015; Ducoux, 2017). Golberg and Leyreloup (1990) were the first to ascribe this metamorphic event to a crustal thinning episode in the future NPZ
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