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The sapphirine-bearing rocks in contact with the Lherz peridotite body; new mineralogical data, age and interpretation

Jessica Uzel, Yves Lagabrielle, Serge Fourcade, Christian Chopin, Pierre Monchoux, Camille Clerc and Marc Poujol
The sapphirine-bearing rocks in contact with the Lherz peridotite body; new mineralogical data, age and interpretation
Bulletin de la Societe Geologique de France (2020) 191 (1)


Sapphirine-bearing rocks are described in the Aulus Basin (Ariege, 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.

ISSN: 0037-9409
EISSN: 1777-5817
Serial Title: Bulletin de la Societe Geologique de France
Serial Volume: 191
Serial Issue: 1
Title: The sapphirine-bearing rocks in contact with the Lherz peridotite body; new mineralogical data, age and interpretation
Affiliation: Universite de Rennes I, Geosciences Rennes, Rennes, France
Pages: Article 5
Published: 2020
Text Language: English
Summary Language: French
Publisher: Societe Geologique de France, Paris, France
References: 119
Accession Number: 2020-041610
Categories: Igneous and metamorphic petrologyMineralogy of silicatesGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables, geol. sketch map
N42°47'60" - N42°49'00", E01°22'00" - E01°22'60"
N42°45'00" - N42°49'00", E01°19'60" - E01°25'00"
Secondary Affiliation: PSL Research University, FRA, FranceUniversite de Toulouse, FRA, FranceUniversite de la Nouvelle Caledonie, NCL, New Caledonia
Country of Publication: France
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Abstract, Copyright, Societe Geologique de France. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2020
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